diff --git a/CoreFoundation/Base.subproj/ForSwiftFoundationOnly.h b/CoreFoundation/Base.subproj/ForSwiftFoundationOnly.h
index a3063c2..2d20856 100644
--- a/CoreFoundation/Base.subproj/ForSwiftFoundationOnly.h
+++ b/CoreFoundation/Base.subproj/ForSwiftFoundationOnly.h
@@ -29,6 +29,14 @@
 #include <pthread.h>
 #include <execinfo.h>
 
+#if __has_include(<malloc/malloc.h>)
+#include <malloc/malloc.h>
+#endif
+
+#if __has_include(<malloc/malloc.h>)
+#include <malloc/malloc.h>
+#endif
+
 _CF_EXPORT_SCOPE_BEGIN
 
 struct __CFSwiftObject {
@@ -345,6 +353,54 @@
 /// a single base directory relative to which user-specific runtime files and other file objects should be placed. This directory is defined by the environment variable $XDG_RUNTIME_DIR.
 CF_EXPORT CFStringRef _CFXDGCreateRuntimeDirectoryPath(void);
 
+
+typedef struct {
+    void *_Nonnull memory;
+    size_t capacity;
+    _Bool onStack;
+} _ConditionalAllocationBuffer;
+
+static inline _Bool _resizeConditionalAllocationBuffer(_ConditionalAllocationBuffer *_Nonnull buffer, size_t amt) {
+#if TARGET_OS_MAC
+    size_t amount = malloc_good_size(amt);
+#else
+    size_t amount = amt;
+#endif
+    if (amount <= buffer->capacity) { return true; }
+    void *newMemory;
+    if (buffer->onStack) {
+        newMemory = malloc(amount);
+        if (newMemory == NULL) { return false; }
+        memcpy(newMemory, buffer->memory, buffer->capacity);
+        buffer->onStack = false;
+    } else {
+        newMemory = realloc(buffer->memory, amount);
+        if (newMemory == NULL) { return false; }
+    }
+    if (newMemory == NULL) { return false; }
+    buffer->memory = newMemory;
+    buffer->capacity = amount;
+    return true;
+}
+
+static inline _Bool _withStackOrHeapBuffer(size_t amount, void (__attribute__((noescape)) ^ _Nonnull applier)(_ConditionalAllocationBuffer *_Nonnull)) {
+    _ConditionalAllocationBuffer buffer;
+#if TARGET_OS_MAC
+    buffer.capacity = malloc_good_size(amount);
+#else
+    buffer.capacity = amount;
+#endif
+    buffer.onStack = (_CFIsMainThread() != 0 ? buffer.capacity < 2048 : buffer.capacity < 512);
+    buffer.memory = buffer.onStack ? alloca(buffer.capacity) : malloc(buffer.capacity);
+    if (buffer.memory == NULL) { return false; }
+    applier(&buffer);
+    if (!buffer.onStack) {
+        free(buffer.memory);
+    }
+    return true;
+}
+
+
 _CF_EXPORT_SCOPE_END
 
 #endif /* __COREFOUNDATION_FORSWIFTFOUNDATIONONLY__ */
diff --git a/CoreFoundation/Preferences.subproj/CFApplicationPreferences.c b/CoreFoundation/Preferences.subproj/CFApplicationPreferences.c
index 1029622..6366a5b 100644
--- a/CoreFoundation/Preferences.subproj/CFApplicationPreferences.c
+++ b/CoreFoundation/Preferences.subproj/CFApplicationPreferences.c
@@ -17,7 +17,7 @@
 #include <CoreFoundation/CFNumberFormatter.h>
 #include <CoreFoundation/CFDateFormatter.h>
 #include <sys/types.h>
-#if DEPLOYMENT_TARGET_MACOSX || DEPLOYMENT_TARGET_EMBEDDED
+#if DEPLOYMENT_TARGET_MACOSX || DEPLOYMENT_TARGET_EMBEDDED || DEPLOYMENT_TARGET_LINUX
 #include <unistd.h>
 #endif
 
diff --git a/CoreFoundation/Preferences.subproj/CFPreferences.c b/CoreFoundation/Preferences.subproj/CFPreferences.c
index 8f00af0..18723c6 100644
--- a/CoreFoundation/Preferences.subproj/CFPreferences.c
+++ b/CoreFoundation/Preferences.subproj/CFPreferences.c
@@ -445,7 +445,7 @@
 static CFURLRef _CFPreferencesURLForStandardDomainWithSafetyLevel(CFStringRef domainName, CFStringRef userName, CFStringRef hostName, unsigned long safeLevel) {
     CFURLRef theURL = NULL;
     CFAllocatorRef prefAlloc = __CFPreferencesAllocator();
-#if DEPLOYMENT_TARGET_MACOSX || DEPLOYMENT_TARGET_WINDOWS
+#if DEPLOYMENT_TARGET_MACOSX || DEPLOYMENT_TARGET_LINUX || DEPLOYMENT_TARGET_WINDOWS
     CFURLRef prefDir = _preferencesDirectoryForUserHostSafetyLevel(userName, hostName, safeLevel);
     CFStringRef  appName;
     CFStringRef  fileName;
@@ -479,7 +479,7 @@
 	CFRelease(appName);
     }
     if (fileName) {
-#if DEPLOYMENT_TARGET_MACOSX || DEPLOYMENT_TARGET_EMBEDDED
+#if DEPLOYMENT_TARGET_MACOSX || DEPLOYMENT_TARGET_EMBEDDED || DEPLOYMENT_TARGET_LINUX
         theURL = CFURLCreateWithFileSystemPathRelativeToBase(prefAlloc, fileName, kCFURLPOSIXPathStyle, false, prefDir);
 #elif DEPLOYMENT_TARGET_WINDOWS
 		theURL = CFURLCreateWithFileSystemPathRelativeToBase(prefAlloc, fileName, kCFURLWindowsPathStyle, false, prefDir);
@@ -497,10 +497,6 @@
     return _CFPreferencesURLForStandardDomainWithSafetyLevel(domainName, userName, hostName, __CFSafeLaunchLevel);
 }
 
-const _CFPreferencesDomainCallBacks __kCFXMLPropertyListDomainCallBacks = {
-    
-};
-
 CFPreferencesDomainRef _CFPreferencesStandardDomain(CFStringRef  domainName, CFStringRef  userName, CFStringRef  hostName) {
     CFPreferencesDomainRef domain;
     CFStringRef  domainKey;
diff --git a/CoreFoundation/Preferences.subproj/CFXMLPreferencesDomain.c b/CoreFoundation/Preferences.subproj/CFXMLPreferencesDomain.c
index 5941118..8073848 100644
--- a/CoreFoundation/Preferences.subproj/CFXMLPreferencesDomain.c
+++ b/CoreFoundation/Preferences.subproj/CFXMLPreferencesDomain.c
@@ -16,10 +16,12 @@
 #include <CoreFoundation/CFDate.h>
 #include "CFInternal.h"
 #include <time.h>
-#if DEPLOYMENT_TARGET_MACOSX
+#if DEPLOYMENT_TARGET_MACOSX || DEPLOYMENT_TARGET_LINUX
 #include <unistd.h>
 #include <stdio.h>
 #include <sys/stat.h>
+#endif
+#if DEPLOYMENT_TARGET_MACOSX
 #include <mach/mach.h>
 #include <mach/mach_syscalls.h>
 #endif
@@ -102,7 +104,7 @@
     if (parentURL) CFRelease(parentURL);
     if (!parentExists) return false;
 
-#if DEPLOYMENT_TARGET_MACOSX
+#if DEPLOYMENT_TARGET_MACOSX || DEPLOYMENT_TARGET_LINUX
     mode = worldReadable ? S_IRWXU|S_IRWXG|S_IROTH|S_IXOTH : S_IRWXU;
 #else
     mode = 0666;
@@ -306,7 +308,7 @@
         CFDataRef data = CFPropertyListCreateData(alloc, dict, desiredFormat, 0, NULL);
         if (data) {
             SInt32 mode;
-#if DEPLOYMENT_TARGET_MACOSX
+#if DEPLOYMENT_TARGET_MACOSX || DEPLOYMENT_TARGET_LINUX
             mode = isWorldReadable ? S_IRUSR|S_IWUSR|S_IRGRP|S_IROTH : S_IRUSR|S_IWUSR;
 #else
 	    mode = 0666;
diff --git a/Foundation.xcodeproj/project.pbxproj b/Foundation.xcodeproj/project.pbxproj
index 424e3e4..eeace9e 100644
--- a/Foundation.xcodeproj/project.pbxproj
+++ b/Foundation.xcodeproj/project.pbxproj
@@ -21,6 +21,7 @@
 		528776141BF2629700CB0090 /* FoundationErrors.swift in Sources */ = {isa = PBXBuildFile; fileRef = 522C253A1BF16E1600804FC6 /* FoundationErrors.swift */; };
 		528776191BF27D9500CB0090 /* Test.plist in Resources */ = {isa = PBXBuildFile; fileRef = 528776181BF27D9500CB0090 /* Test.plist */; };
 		555683BD1C1250E70041D4C6 /* TestUserDefaults.swift in Sources */ = {isa = PBXBuildFile; fileRef = 555683BC1C1250E70041D4C6 /* TestUserDefaults.swift */; };
+		559451EC1F706BFA002807FB /* CFXMLPreferencesDomain.c in Sources */ = {isa = PBXBuildFile; fileRef = 559451EA1F706BF5002807FB /* CFXMLPreferencesDomain.c */; };
 		5B0163BB1D024EB7003CCD96 /* DateComponents.swift in Sources */ = {isa = PBXBuildFile; fileRef = 5B0163BA1D024EB7003CCD96 /* DateComponents.swift */; };
 		5B13B3251C582D4700651CE2 /* main.swift in Sources */ = {isa = PBXBuildFile; fileRef = EA66F6381BF1619600136161 /* main.swift */; };
 		5B13B3261C582D4C00651CE2 /* TestAffineTransform.swift in Sources */ = {isa = PBXBuildFile; fileRef = C93559281C12C49F009FD6A9 /* TestAffineTransform.swift */; };
@@ -504,6 +505,7 @@
 		52829AD61C160D64003BC4EF /* TestCalendar.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = TestCalendar.swift; sourceTree = "<group>"; };
 		528776181BF27D9500CB0090 /* Test.plist */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text.plist.xml; path = Test.plist; sourceTree = "<group>"; };
 		555683BC1C1250E70041D4C6 /* TestUserDefaults.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = TestUserDefaults.swift; sourceTree = "<group>"; usesTabs = 1; };
+		559451EA1F706BF5002807FB /* CFXMLPreferencesDomain.c */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.c; path = CFXMLPreferencesDomain.c; sourceTree = "<group>"; };
 		5B0163BA1D024EB7003CCD96 /* DateComponents.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = DateComponents.swift; sourceTree = "<group>"; };
 		5B0C6C211C1E07E600705A0E /* TestNSRegularExpression.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = TestNSRegularExpression.swift; sourceTree = "<group>"; };
 		5B1FD9C11D6D160F0080E83C /* CFURLSessionInterface.c */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.c; path = CFURLSessionInterface.c; sourceTree = "<group>"; };
@@ -1034,6 +1036,7 @@
 			isa = PBXGroup;
 			children = (
 				5B5D886A1BBC948300234F36 /* CFApplicationPreferences.c */,
+				559451EA1F706BF5002807FB /* CFXMLPreferencesDomain.c */,
 				5B5D898B1BBDBF6500234F36 /* CFPreferences.c */,
 				5B5D886C1BBC94C400234F36 /* CFPreferences.h */,
 			);
@@ -2027,7 +2030,7 @@
 				TargetAttributes = {
 					5B5D885C1BBC938800234F36 = {
 						CreatedOnToolsVersion = 7.1;
-						LastSwiftMigration = 0800;
+						LastSwiftMigration = 0900;
 						LastSwiftUpdateCheck = 0800;
 						ProvisioningStyle = Manual;
 					};
@@ -2306,6 +2309,7 @@
 				5B7C8A8B1BEA7FE200C5B690 /* CFBigNumber.c in Sources */,
 				5B7C8A7D1BEA7FCE00C5B690 /* CFBinaryHeap.c in Sources */,
 				5B7C8A7E1BEA7FCE00C5B690 /* CFBitVector.c in Sources */,
+				559451EC1F706BFA002807FB /* CFXMLPreferencesDomain.c in Sources */,
 				5B7C8A8F1BEA7FEC00C5B690 /* CFBinaryPList.c in Sources */,
 				5B7C8A911BEA7FEC00C5B690 /* CFPropertyList.c in Sources */,
 				5B7C8AB41BEA801700C5B690 /* CFStringEncodingDatabase.c in Sources */,
diff --git a/Foundation/AffineTransform.swift b/Foundation/AffineTransform.swift
index cfd0504..3f1923f 100644
--- a/Foundation/AffineTransform.swift
+++ b/Foundation/AffineTransform.swift
@@ -338,6 +338,10 @@
         return other === self
             || (other.transformStruct == self.transformStruct)
     }
+
+    open override var hashValue: Int {
+        return transformStruct.hashValue
+    }
     
     public static var supportsSecureCoding: Bool {
         return true
diff --git a/Foundation/ByteCountFormatter.swift b/Foundation/ByteCountFormatter.swift
index e268880..3234e4e 100644
--- a/Foundation/ByteCountFormatter.swift
+++ b/Foundation/ByteCountFormatter.swift
@@ -13,8 +13,6 @@
         public let rawValue : UInt
         public init(rawValue: UInt) { self.rawValue = rawValue }
         
-        // This causes default units appropriate for the platform to be used. Specifying any units explicitly causes just those units to be used in showing the number.
-        public static let useDefault = Units(rawValue: 0)
         //  Specifying any of the following causes the specified units to be used in showing the number.
         public static let useBytes = Units(rawValue: 1 << 0)
         public static let useKB = Units(rawValue: 1 << 1)
@@ -50,9 +48,9 @@
         NSUnimplemented()
     }
     
-    /* Specify the units that can be used in the output. If ByteCountFormatter.Units.useDefault, uses platform-appropriate settings; otherwise will only use the specified units. This is the default value. Note that ZB and YB cannot be covered by the range of possible values, but you can still choose to use these units to get fractional display ("0.0035 ZB" for instance).
+    /* Specify the units that can be used in the output. If ByteCountFormatter.Units is empty, uses platform-appropriate settings; otherwise will only use the specified units. This is the default value. Note that ZB and YB cannot be covered by the range of possible values, but you can still choose to use these units to get fractional display ("0.0035 ZB" for instance).
      */
-    open var allowedUnits: Units = .useDefault
+    open var allowedUnits: Units = []
     
     /* Specify how the count is displayed by indicating the number of bytes to be used for kilobyte. The default setting is ByteCountFormatter.CountStyle.fileCount, which is the system specific value for file and storage sizes.
      */
@@ -127,52 +125,52 @@
      */
     private func convertValue(fromByteCount byteCount: Int64, for byteSize: [Unit: Double]) -> String {
         let byte = Double(byteCount)
-        if byte == 0, allowsNonnumericFormatting, allowedUnits == .useDefault, includesUnit, includesCount {
+        if byte == 0, allowsNonnumericFormatting, allowedUnits == [], includesUnit, includesCount {
             return partsToIncludeFor(value: "Zero", unit: Unit.KB)
         } else if byte == 1 || byte == -1 {
-            if allowedUnits.contains(.useAll) || allowedUnits == .useDefault {
+            if allowedUnits.contains(.useAll) || allowedUnits == [] {
                 return formatNumberFor(bytes: byte, unit: Unit.byte)
             } else {
                 return valueToUseFor(byteCount: byte, unit: allowedUnits)
             }
         } else if  byte < byteSize[Unit.KB]! && byte > -byteSize[Unit.KB]!{
-            if allowedUnits.contains(.useAll) || allowedUnits == .useDefault {
+            if allowedUnits.contains(.useAll) || allowedUnits == [] {
                 return formatNumberFor(bytes: byte, unit: Unit.bytes)
             } else {
                 return valueToUseFor(byteCount: byte, unit: allowedUnits)
             }
         } else if byte < byteSize[Unit.MB]! && byte > -byteSize[Unit.MB]! {
-            if allowedUnits.contains(.useAll) || allowedUnits == .useDefault {
+            if allowedUnits.contains(.useAll) || allowedUnits == [] {
                 return divide(byte, by: byteSize, for: .KB)
             }
             return valueToUseFor(byteCount: byte, unit: allowedUnits)
             
         } else if byte < byteSize[Unit.GB]! && byte > -byteSize[Unit.GB]! {
-            if allowedUnits.contains(.useAll) || allowedUnits == .useDefault {
+            if allowedUnits.contains(.useAll) || allowedUnits == [] {
                 return divide(byte, by: byteSize, for: .MB)
             }
             return valueToUseFor(byteCount: byte, unit: allowedUnits)
             
         } else if byte < byteSize[Unit.TB]! && byte > -byteSize[Unit.TB]! {
-            if allowedUnits.contains(.useAll) || allowedUnits == .useDefault {
+            if allowedUnits.contains(.useAll) || allowedUnits == [] {
                 return divide(byte, by: byteSize, for: .GB)
             }
             return valueToUseFor(byteCount: byte, unit: allowedUnits)
             
         } else if byte < byteSize[Unit.PB]! && byte > -byteSize[Unit.PB]! {
-            if allowedUnits.contains(.useAll) || allowedUnits == .useDefault {
+            if allowedUnits.contains(.useAll) || allowedUnits == [] {
                 return divide(byte, by: byteSize, for: .TB)
             }
             return valueToUseFor(byteCount: byte, unit: allowedUnits)
             
         } else if byte < byteSize[Unit.EB]! && byte > -byteSize[Unit.EB]! {
-            if allowedUnits.contains(.useAll) || allowedUnits == .useDefault {
+            if allowedUnits.contains(.useAll) || allowedUnits == [] {
                 return divide(byte, by: byteSize, for: .PB)
             }
             return valueToUseFor(byteCount: byte, unit: allowedUnits)
             
         } else {
-            if allowedUnits.contains(.useAll) || allowedUnits == .useDefault {
+            if allowedUnits.contains(.useAll) || allowedUnits == [] {
                 return divide(byte, by: byteSize, for: .EB)
             }
             return valueToUseFor(byteCount: byte, unit: allowedUnits)
@@ -297,7 +295,7 @@
             } else {
                 if lengthOfInt(number: Int(bytes)) == 3 {
                     numberFormatter.usesSignificantDigits = false
-                    numberFormatter.maximumFractionDigits = 1
+                    numberFormatter.maximumFractionDigits = 0
                 } else {
                     numberFormatter.maximumSignificantDigits = 3
                     numberFormatter.minimumSignificantDigits = 3
@@ -376,7 +374,7 @@
         } else if includesCount, includesUnit {
             return "\(value) \(unit)"
         } else if includesCount, !includesUnit {
-            if value == "Zero", allowedUnits == .useDefault {
+            if value == "Zero", allowedUnits == [] {
                 return "0"
             } else {
                 return value
diff --git a/Foundation/Data.swift b/Foundation/Data.swift
index 1436c88..5398567 100644
--- a/Foundation/Data.swift
+++ b/Foundation/Data.swift
@@ -27,6 +27,10 @@
 internal func __NSDataInvokeDeallocatorFree(_ mem: UnsafeMutableRawPointer, _ length: Int) {
     free(mem)
 }
+
+internal func __NSDataIsCompact(_ data: NSData) -> Bool {
+    return data._isCompact()
+}
     
 #else
     
@@ -98,47 +102,140 @@
     public var _needToZero: Bool
     public var _deallocator: ((UnsafeMutableRawPointer, Int) -> Void)?
     public var _backing: Backing = .swift
+    public var _offset: Int
     
     public var bytes: UnsafeRawPointer? {
         @inline(__always)
         get {
             switch _backing {
             case .swift:
-                return UnsafeRawPointer(_bytes)
+                return UnsafeRawPointer(_bytes)?.advanced(by: -_offset)
             case .immutable:
-                return UnsafeRawPointer(_bytes)
+                return UnsafeRawPointer(_bytes)?.advanced(by: -_offset)
             case .mutable:
-                return UnsafeRawPointer(_bytes)
+                return UnsafeRawPointer(_bytes)?.advanced(by: -_offset)
             case .customReference(let d):
-                return d.bytes
+                return d.bytes.advanced(by: -_offset)
             case .customMutableReference(let d):
-                return d.bytes
+                return d.bytes.advanced(by: -_offset)
+            }
+        }
+    }
+
+    @discardableResult
+    public func withUnsafeBytes<Result>(in range: Range<Int>, apply: (UnsafeRawBufferPointer) throws -> Result) rethrows -> Result {
+        switch _backing {
+        case .swift: fallthrough
+        case .immutable: fallthrough
+        case .mutable:
+            return try apply(UnsafeRawBufferPointer(start: _bytes?.advanced(by: range.lowerBound - _offset), count: Swift.min(range.count, _length)))
+        case .customReference(let d):
+            if d._isCompact() {
+                let len = d.length
+                guard len > 0 else {
+                    return try apply(UnsafeRawBufferPointer(start: nil, count: 0))
+                }
+                return try apply(UnsafeRawBufferPointer(start: d.bytes.advanced(by: range.lowerBound - _offset), count: Swift.min(range.count, len)))
+            } else {
+                var buffer = UnsafeMutableRawBufferPointer.allocate(count: range.count)
+                defer { buffer.deallocate() }
+                let sliceRange = NSRange(location: range.lowerBound - _offset, length: range.count)
+                var enumerated = 0
+                d.enumerateBytes { (ptr, byteRange, stop) in
+                    if NSIntersectionRange(sliceRange, byteRange).length > 0 {
+                        let lower = Swift.max(byteRange.location, sliceRange.location)
+                        let upper = Swift.min(byteRange.location + byteRange.length, sliceRange.location + sliceRange.length)
+                        let offset = lower - byteRange.location
+                        let effectiveRange = NSRange(location: lower, length: upper - lower)
+                        if effectiveRange == sliceRange {
+                            memcpy(buffer.baseAddress!, ptr, effectiveRange.length)
+                            stop.pointee = true
+                        } else {
+                            memcpy(buffer.baseAddress!.advanced(by: enumerated), ptr, effectiveRange.length)
+                        }
+                        enumerated += byteRange.length
+                    } else if sliceRange.location + sliceRange.length < byteRange.location {
+                        stop.pointee = true
+                    }
+                }
+                return try apply(UnsafeRawBufferPointer(buffer))
+            }
+        case .customMutableReference(let d):
+            if d._isCompact() {
+                let len = d.length
+                guard len > 0 else {
+                    return try apply(UnsafeRawBufferPointer(start: nil, count: 0))
+                }
+                return try apply(UnsafeRawBufferPointer(start: d.bytes.advanced(by: range.lowerBound - _offset), count: Swift.min(range.count, len)))
+            } else {
+                var buffer = UnsafeMutableRawBufferPointer.allocate(count: range.count)
+                defer { buffer.deallocate() }
+                let sliceRange = NSRange(location: range.lowerBound - _offset, length: range.count)
+                var enumerated = 0
+                d.enumerateBytes { (ptr, byteRange, stop) in
+                    if NSIntersectionRange(sliceRange, byteRange).length > 0 {
+                        let lower = Swift.max(byteRange.location, sliceRange.location)
+                        let upper = Swift.min(byteRange.location + byteRange.length, sliceRange.location + sliceRange.length)
+                        let effectiveRange = NSRange(location: lower, length: upper - lower)
+                        if effectiveRange == sliceRange {
+                            memcpy(buffer.baseAddress!, ptr, effectiveRange.length)
+                            stop.pointee = true
+                        } else {
+                            memcpy(buffer.baseAddress!.advanced(by: enumerated), ptr, effectiveRange.length)
+                        }
+                        enumerated += byteRange.length
+                    } else if sliceRange.location + sliceRange.length < byteRange.location {
+                        stop.pointee = true
+                    }
+                }
+                return try apply(UnsafeRawBufferPointer(buffer))
             }
         }
     }
     
+    @discardableResult
+    public func withUnsafeMutableBytes<Result>(in range: Range<Int>, apply: (UnsafeMutableRawBufferPointer) throws -> Result) rethrows -> Result {
+        switch _backing {
+        case .swift: fallthrough
+        case .mutable:
+            return try apply(UnsafeMutableRawBufferPointer(start: _bytes!.advanced(by:range.lowerBound - _offset), count: Swift.min(range.count, _length - range.lowerBound)))
+        case .customMutableReference(let d):
+            let len = d.length
+            return try apply(UnsafeMutableRawBufferPointer(start: d.mutableBytes.advanced(by:range.lowerBound - _offset), count: Swift.min(range.count, len - range.lowerBound)))
+        case .immutable(let d):
+            let data = d.mutableCopy() as! NSMutableData
+            _backing = .mutable(data)
+            _bytes = data.mutableBytes
+            return try apply(UnsafeMutableRawBufferPointer(start: _bytes!.advanced(by:range.lowerBound - _offset), count: Swift.min(range.count, _length - range.lowerBound)))
+        case .customReference(let d):
+            let data = d.mutableCopy() as! NSMutableData
+            _backing = .customMutableReference(data)
+            let len = data.length
+            return try apply(UnsafeMutableRawBufferPointer(start: data.mutableBytes.advanced(by:range.lowerBound - _offset), count: Swift.min(range.count, len - range.lowerBound)))
+        }
+    }
+
     public var mutableBytes: UnsafeMutableRawPointer? {
         @inline(__always)
         get {
             switch _backing {
             case .swift:
-                return _bytes
+                return _bytes?.advanced(by: -_offset)
             case .immutable(let d):
                 let data = d.mutableCopy() as! NSMutableData
                 data.length = length
                 _backing = .mutable(data)
                 _bytes = data.mutableBytes
-                return data.mutableBytes
+                return _bytes?.advanced(by: -_offset)
             case .mutable:
-                return _bytes
+                return _bytes?.advanced(by: -_offset)
             case .customReference(let d):
                 let data = d.mutableCopy() as! NSMutableData
                 data.length = length
                 _backing = .customMutableReference(data)
-                _bytes = data.mutableBytes
-                return data.mutableBytes
+                return data.mutableBytes.advanced(by: -_offset)
             case .customMutableReference(let d):
-                return d.mutableBytes
+                return d.mutableBytes.advanced(by: -_offset)
             }
         }
     }
@@ -176,32 +273,34 @@
         }
     }
     
-    public func enumerateBytes(_ block: (_ buffer: UnsafeBufferPointer<UInt8>, _ byteIndex: Data.Index, _ stop: inout Bool) -> Void) {
-        var stop: Bool = false
+    public func enumerateBytes(in range: Range<Int>, _ block: (_ buffer: UnsafeBufferPointer<UInt8>, _ byteIndex: Data.Index, _ stop: inout Bool) -> Void) {
+        var stopv: Bool = false
+        var data: NSData
         switch _backing {
-        case .swift:
-            block(UnsafeBufferPointer<UInt8>(start: _bytes?.assumingMemoryBound(to: UInt8.self), count: _length), 0, &stop)
-        case .immutable:
-            block(UnsafeBufferPointer<UInt8>(start: _bytes?.assumingMemoryBound(to: UInt8.self), count: _length), 0, &stop)
-        case .mutable:
-            block(UnsafeBufferPointer<UInt8>(start: _bytes?.assumingMemoryBound(to: UInt8.self), count: _length), 0, &stop)
+        case .swift: fallthrough
+        case .immutable: fallthrough
+        case .mutable: 
+            block(UnsafeBufferPointer<UInt8>(start: _bytes?.advanced(by: range.lowerBound - _offset).assumingMemoryBound(to: UInt8.self), count: Swift.min(range.count, _length)), 0, &stopv)
+            return
         case .customReference(let d):
-            d.enumerateBytes { (ptr, range, stop) in
-                var stopv = false
-                let bytePtr = ptr.bindMemory(to: UInt8.self, capacity: range.length)
-                block(UnsafeBufferPointer(start: bytePtr, count: range.length), range.location, &stopv)
-                if stopv {
-                    stop.pointee = true
-                }
-            }
+            data = d
+            break
         case .customMutableReference(let d):
-            d.enumerateBytes { (ptr, range, stop) in
-                var stopv = false
-                let bytePtr = ptr.bindMemory(to: UInt8.self, capacity: range.length)
-                block(UnsafeBufferPointer(start: bytePtr, count: range.length), range.location, &stopv)
-                if stopv {
-                    stop.pointee = true
-                }
+            data = d
+            break
+        }
+        data.enumerateBytes { (ptr, region, stop) in
+            // any region that is not in the range should be skipped
+            guard range.contains(region.lowerBound) || range.contains(region.upperBound) else { return }
+            var regionAdjustment = 0
+            if region.lowerBound < range.lowerBound {
+                regionAdjustment = range.lowerBound - (region.lowerBound - _offset)
+            }
+            let bytePtr  = ptr.advanced(by: regionAdjustment).assumingMemoryBound(to: UInt8.self)
+            let effectiveLength = Swift.min((region.location - _offset) + region.length, range.upperBound) - (region.location - _offset)
+            block(UnsafeBufferPointer(start: bytePtr, count: effectiveLength - regionAdjustment), region.location + regionAdjustment - _offset, &stopv)
+            if stopv {
+                stop.pointee = true
             }
         }
     }
@@ -319,7 +418,7 @@
     
     @inline(__always)
     public func append(_ bytes: UnsafeRawPointer, length: Int) {
-        precondition(length >= 0, "Length of appending bytes must be positive")
+        precondition(length >= 0, "Length of appending bytes must not be negative")
         switch _backing {
         case .swift:
             let origLength = _length
@@ -398,6 +497,43 @@
         }
         
     }
+
+    public func get(_ index: Int) -> UInt8 {
+        switch _backing {
+        case .swift: fallthrough
+        case .immutable: fallthrough
+        case .mutable:
+            return _bytes!.advanced(by: index - _offset).assumingMemoryBound(to: UInt8.self).pointee
+        case .customReference(let d):
+            if d._isCompact() {
+                return d.bytes.advanced(by: index - _offset).assumingMemoryBound(to: UInt8.self).pointee
+            } else {
+                var byte: UInt8 = 0
+                d.enumerateBytes { (ptr, range, stop) in
+                    if NSLocationInRange(index, range) {
+                        let offset = index - range.location - _offset
+                        byte = ptr.advanced(by: offset).assumingMemoryBound(to: UInt8.self).pointee
+                        stop.pointee = true
+                    }
+                }
+                return byte
+            }
+        case .customMutableReference(let d):
+            if d._isCompact() {
+                return d.bytes.advanced(by: index - _offset).assumingMemoryBound(to: UInt8.self).pointee
+            } else {
+                var byte: UInt8 = 0
+                d.enumerateBytes { (ptr, range, stop) in
+                    if NSLocationInRange(index, range) {
+                        let offset = index - range.location - _offset
+                        byte = ptr.advanced(by: offset).assumingMemoryBound(to: UInt8.self).pointee
+                        stop.pointee = true
+                    }
+                }
+                return byte
+            }
+        }
+    }
     
     @inline(__always)
     public func set(_ index: Int, to value: UInt8) {
@@ -405,7 +541,7 @@
         case .swift:
             fallthrough
         case .mutable:
-            _bytes!.advanced(by: index).assumingMemoryBound(to: UInt8.self).pointee = value
+            _bytes!.advanced(by: index - _offset).assumingMemoryBound(to: UInt8.self).pointee = value
         default:
             var theByte = value
             let range = NSRange(location: index, length: 1)
@@ -427,28 +563,29 @@
                 }
                 _length = newLength
             }
-            _DataStorage.move(_bytes!.advanced(by: range.location), bytes!, range.length)
+            _DataStorage.move(_bytes!.advanced(by: range.location - _offset), bytes!, range.length)
         case .immutable(let d):
             let data = d.mutableCopy() as! NSMutableData
-            data.replaceBytes(in: range, withBytes: bytes!)
+            data.replaceBytes(in: NSRange(location: range.location - _offset, length: range.length), withBytes: bytes!)
             _backing = .mutable(data)
             _length = data.length
             _bytes = data.mutableBytes
         case .mutable(let d):
-            d.replaceBytes(in: range, withBytes: bytes!)
+            d.replaceBytes(in: NSRange(location: range.location - _offset, length: range.length), withBytes: bytes!)
             _length = d.length
             _bytes = d.mutableBytes
         case .customReference(let d):
             let data = d.mutableCopy() as! NSMutableData
-            data.replaceBytes(in: range, withBytes: bytes!)
+            data.replaceBytes(in: NSRange(location: range.location - _offset, length: range.length), withBytes: bytes!)
             _backing = .customMutableReference(data)
         case .customMutableReference(let d):
-            d.replaceBytes(in: range, withBytes: bytes!)
+            d.replaceBytes(in: NSRange(location: range.location - _offset, length: range.length), withBytes: bytes!)
         }
     }
     
     @inline(__always)
-    public func replaceBytes(in range: NSRange, with replacementBytes: UnsafeRawPointer?, length replacementLength: Int) {
+    public func replaceBytes(in range_: NSRange, with replacementBytes: UnsafeRawPointer?, length replacementLength: Int) {
+        let range = NSRange(location: range_.location - _offset, length: range_.length)
         let currentLength = _length
         let resultingLength = currentLength - range.length + replacementLength
         switch _backing {
@@ -480,12 +617,12 @@
             let data = d.mutableCopy() as! NSMutableData
             data.replaceBytes(in: range, withBytes: replacementBytes, length: replacementLength)
             _backing = .mutable(data)
-            _length = replacementLength
+            _length = data.length
             _bytes = data.mutableBytes
         case .mutable(let d):
             d.replaceBytes(in: range, withBytes: replacementBytes, length: replacementLength)
             _backing = .mutable(d)
-            _length = replacementLength
+            _length = d.length
             _bytes = d.mutableBytes
         case .customReference(let d):
             let data = d.mutableCopy() as! NSMutableData
@@ -497,7 +634,8 @@
     }
     
     @inline(__always)
-    public func resetBytes(in range: NSRange) {
+    public func resetBytes(in range_: NSRange) {
+        let range = NSRange(location: range_.location - _offset, length: range_.length)
         if range.length == 0 { return }
         switch _backing {
         case .swift:
@@ -546,6 +684,7 @@
         _capacity = capacity
         _needToZero = !clear
         _length = 0
+        _offset = 0
         setLength(length)
     }
     
@@ -560,10 +699,12 @@
         _bytes = _DataStorage.allocate(capacity, false)!
         _capacity = capacity
         _needToZero = true
+        _offset = 0
     }
     
     public init(bytes: UnsafeRawPointer?, length: Int) {
         precondition(length < _DataStorage.maxSize)
+        _offset = 0
         if length == 0 {
             _capacity = 0
             _length = 0
@@ -588,8 +729,9 @@
         }
     }
     
-    public init(bytes: UnsafeMutableRawPointer?, length: Int, copy: Bool, deallocator: ((UnsafeMutableRawPointer, Int) -> Void)?) {
+    public init(bytes: UnsafeMutableRawPointer?, length: Int, copy: Bool, deallocator: ((UnsafeMutableRawPointer, Int) -> Void)?, offset: Int) {
         precondition(length < _DataStorage.maxSize)
+        _offset = offset
         if length == 0 {
             _capacity = 0
             _length = 0
@@ -630,7 +772,8 @@
         }
     }
     
-    public init(immutableReference: NSData) {
+    public init(immutableReference: NSData, offset: Int) {
+        _offset = offset
         _bytes = UnsafeMutableRawPointer(mutating: immutableReference.bytes)
         _capacity = 0
         _needToZero = false
@@ -638,7 +781,8 @@
         _backing = .immutable(immutableReference)
     }
     
-    public init(mutableReference: NSMutableData) {
+    public init(mutableReference: NSMutableData, offset: Int) {
+        _offset = offset
         _bytes = mutableReference.mutableBytes
         _capacity = 0
         _needToZero = false
@@ -646,7 +790,8 @@
         _backing = .mutable(mutableReference)
     }
     
-    public init(customReference: NSData) {
+    public init(customReference: NSData, offset: Int) {
+        _offset = offset
         _bytes = nil
         _capacity = 0
         _needToZero = false
@@ -654,7 +799,8 @@
         _backing = .customReference(customReference)
     }
     
-    public init(customMutableReference: NSMutableData) {
+    public init(customMutableReference: NSMutableData, offset: Int) {
+        _offset = offset
         _bytes = nil
         _capacity = 0
         _needToZero = false
@@ -675,30 +821,30 @@
     public func mutableCopy(_ range: Range<Int>) -> _DataStorage {
         switch _backing {
         case .swift:
-            return _DataStorage(bytes: _bytes?.advanced(by: range.lowerBound), length: range.count, copy: true, deallocator: nil)
+            return _DataStorage(bytes: _bytes?.advanced(by: range.lowerBound - _offset), length: range.count, copy: true, deallocator: nil, offset: range.lowerBound)
         case .immutable(let d):
             if range.lowerBound == 0 && range.upperBound == _length {
-                return _DataStorage(mutableReference: d.mutableCopy() as! NSMutableData)
+                return _DataStorage(mutableReference: d.mutableCopy() as! NSMutableData, offset: range.lowerBound)
             } else {
-                return _DataStorage(mutableReference: d.subdata(with: NSRange(location: range.lowerBound, length: range.count))._bridgeToObjectiveC().mutableCopy() as! NSMutableData)
+                return _DataStorage(mutableReference: d.subdata(with: NSRange(location: range.lowerBound, length: range.count))._bridgeToObjectiveC().mutableCopy() as! NSMutableData, offset: range.lowerBound)
             }
         case .mutable(let d):
             if range.lowerBound == 0 && range.upperBound == _length {
-                return _DataStorage(mutableReference: d.mutableCopy() as! NSMutableData)
+                return _DataStorage(mutableReference: d.mutableCopy() as! NSMutableData, offset: range.lowerBound)
             } else {
-                return _DataStorage(mutableReference: d.subdata(with: NSRange(location: range.lowerBound, length: range.count))._bridgeToObjectiveC().mutableCopy() as! NSMutableData)
+                return _DataStorage(mutableReference: d.subdata(with: NSRange(location: range.lowerBound, length: range.count))._bridgeToObjectiveC().mutableCopy() as! NSMutableData, offset: range.lowerBound)
             }
         case .customReference(let d):
             if range.lowerBound == 0 && range.upperBound == _length {
-                return _DataStorage(mutableReference: d.mutableCopy() as! NSMutableData)
+                return _DataStorage(mutableReference: d.mutableCopy() as! NSMutableData, offset: range.lowerBound)
             } else {
-                return _DataStorage(mutableReference: d.subdata(with: NSRange(location: range.lowerBound, length: range.count))._bridgeToObjectiveC().mutableCopy() as! NSMutableData)
+                return _DataStorage(mutableReference: d.subdata(with: NSRange(location: range.lowerBound, length: range.count))._bridgeToObjectiveC().mutableCopy() as! NSMutableData, offset: range.lowerBound)
             }
         case .customMutableReference(let d):
             if range.lowerBound == 0 && range.upperBound == _length {
-                return _DataStorage(mutableReference: d.mutableCopy() as! NSMutableData)
+                return _DataStorage(mutableReference: d.mutableCopy() as! NSMutableData, offset: range.lowerBound)
             } else {
-                return _DataStorage(mutableReference: d.subdata(with: NSRange(location: range.lowerBound, length: range.count))._bridgeToObjectiveC().mutableCopy() as! NSMutableData)
+                return _DataStorage(mutableReference: d.subdata(with: NSRange(location: range.lowerBound, length: range.count))._bridgeToObjectiveC().mutableCopy() as! NSMutableData, offset: range.lowerBound)
             }
         }
     }
@@ -710,26 +856,26 @@
         
         switch _backing {
         case .swift:
-            return try work(NSData(bytesNoCopy: _bytes!.advanced(by: range.lowerBound), length: range.count, freeWhenDone: false))
+            return try work(NSData(bytesNoCopy: _bytes!.advanced(by: range.lowerBound - _offset), length: range.count, freeWhenDone: false))
         case .immutable(let d):
             guard range.lowerBound == 0 && range.upperBound == _length else {
-                return try work(NSData(bytesNoCopy: _bytes!.advanced(by: range.lowerBound), length: range.count, freeWhenDone: false))
+                return try work(NSData(bytesNoCopy: _bytes!.advanced(by: range.lowerBound - _offset), length: range.count, freeWhenDone: false))
             }
             return try work(d)
         case .mutable(let d):
             guard range.lowerBound == 0 && range.upperBound == _length else {
-                return try work(NSData(bytesNoCopy: _bytes!.advanced(by: range.lowerBound), length: range.count, freeWhenDone: false))
+                return try work(NSData(bytesNoCopy: _bytes!.advanced(by: range.lowerBound - _offset), length: range.count, freeWhenDone: false))
             }
             return try work(d)
         case .customReference(let d):
             guard range.lowerBound == 0 && range.upperBound == _length else {
                 
-                return try work(NSData(bytesNoCopy: UnsafeMutableRawPointer(mutating: d.bytes.advanced(by: range.lowerBound)), length: range.count, freeWhenDone: false))
+                return try work(NSData(bytesNoCopy: UnsafeMutableRawPointer(mutating: d.bytes.advanced(by: range.lowerBound - _offset)), length: range.count, freeWhenDone: false))
             }
             return try work(d)
         case .customMutableReference(let d):
             guard range.lowerBound == 0 && range.upperBound == _length else {
-                return try work(NSData(bytesNoCopy: UnsafeMutableRawPointer(mutating: d.bytes.advanced(by: range.lowerBound)), length: range.count, freeWhenDone: false))
+                return try work(NSData(bytesNoCopy: UnsafeMutableRawPointer(mutating: d.bytes.advanced(by: range.lowerBound - _offset)), length: range.count, freeWhenDone: false))
             }
             return try work(d)
         }
@@ -766,31 +912,18 @@
         }
     }
     
-    public var hashValue: Int {
-        switch _backing {
-        case .customReference(let d):
-            return d.hash
-        case .customMutableReference(let d):
-            return d.hash
-        default:
-            let len = _length
-            return Int(bitPattern: CFHashBytes(_bytes?.assumingMemoryBound(to: UInt8.self), Swift.min(len, 80)))
-        }
-    }
-    
     public func subdata(in range: Range<Data.Index>) -> Data {
         switch _backing {
         case .customReference(let d):
-            return d.subdata(with: NSRange(location: range.lowerBound, length: range.count))
+            return d.subdata(with: NSRange(location: range.lowerBound - _offset, length: range.count))
         case .customMutableReference(let d):
-            return d.subdata(with: NSRange(location: range.lowerBound, length: range.count))
+            return d.subdata(with: NSRange(location: range.lowerBound - _offset, length: range.count))
         default:
-            return Data(bytes: _bytes!.advanced(by: range.lowerBound), count: range.count)
+            return Data(bytes: _bytes!.advanced(by: range.lowerBound - _offset), count: range.count)
         }
     }
 }
 
-@_versioned
 internal class _NSSwiftData : NSData {
     var _backing: _DataStorage!
     var _range: Range<Data.Index>!
@@ -828,7 +961,7 @@
     }
     
 #if !DEPLOYMENT_RUNTIME_SWIFT
-    @objc
+    @objc override
     func _isCompact() -> Bool {
         return true
     }
@@ -1016,7 +1149,7 @@
     /// - parameter deallocator: Specifies the mechanism to free the indicated buffer, or `.none`.
     public init(bytesNoCopy bytes: UnsafeMutableRawPointer, count: Int, deallocator: Deallocator) {
         let whichDeallocator = deallocator._deallocator
-        _backing = _DataStorage(bytes: bytes, length: count, copy: false, deallocator: whichDeallocator)
+        _backing = _DataStorage(bytes: bytes, length: count, copy: false, deallocator: whichDeallocator, offset: 0)
         _sliceRange = 0..<count
     }
     
@@ -1027,7 +1160,7 @@
     /// - throws: An error in the Cocoa domain, if `url` cannot be read.
     public init(contentsOf url: URL, options: Data.ReadingOptions = []) throws {
         let d = try NSData(contentsOf: url, options: ReadingOptions(rawValue: options.rawValue))
-        _backing = _DataStorage(immutableReference: d)
+        _backing = _DataStorage(immutableReference: d, offset: 0)
         _sliceRange = 0..<d.length
     }
     
@@ -1038,7 +1171,7 @@
     /// - parameter options: Encoding options. Default value is `[]`.
     public init?(base64Encoded base64String: String, options: Data.Base64DecodingOptions = []) {
         if let d = NSData(base64Encoded: base64String, options: Base64DecodingOptions(rawValue: options.rawValue)) {
-            _backing = _DataStorage(immutableReference: d)
+            _backing = _DataStorage(immutableReference: d, offset: 0)
             _sliceRange = 0..<d.length
         } else {
             return nil
@@ -1053,7 +1186,7 @@
     /// - parameter options: Decoding options. Default value is `[]`.
     public init?(base64Encoded base64Data: Data, options: Data.Base64DecodingOptions = []) {
         if let d = NSData(base64Encoded: base64Data, options: Base64DecodingOptions(rawValue: options.rawValue)) {
-            _backing = _DataStorage(immutableReference: d)
+            _backing = _DataStorage(immutableReference: d, offset: 0)
             _sliceRange = 0..<d.length
         } else {
             return nil
@@ -1074,10 +1207,10 @@
         let providesConcreteBacking = (reference as AnyObject)._providesConcreteBacking?() ?? false
 #endif
         if providesConcreteBacking {
-            _backing = _DataStorage(immutableReference: reference.copy() as! NSData)
+            _backing = _DataStorage(immutableReference: reference.copy() as! NSData, offset: 0)
             _sliceRange = 0..<reference.length
         } else {
-            _backing = _DataStorage(customReference: reference.copy() as! NSData)
+            _backing = _DataStorage(customReference: reference.copy() as! NSData, offset: 0)
             _sliceRange = 0..<reference.length
         }
         
@@ -1086,40 +1219,30 @@
     // slightly faster paths for common sequences
     
     public init<S: Sequence>(_ elements: S) where S.Iterator.Element == UInt8 {
-        let underestimatedCount = elements.underestimatedCount
-        self.init(count: underestimatedCount)
-        var idx = 0
-        for byte in elements {
-            if idx < underestimatedCount {
-                self[idx] = byte
-            } else {
-                self.append(byte)
+        if elements is Array<UInt8> {
+            self.init(bytes: _identityCast(elements, to: Array<UInt8>.self))
+        } else if elements is ArraySlice<UInt8> {
+            self.init(bytes: _identityCast(elements, to: ArraySlice<UInt8>.self))
+        } else if elements is UnsafeBufferPointer<UInt8> {
+            self.init(buffer: _identityCast(elements, to: UnsafeBufferPointer<UInt8>.self))
+        } else if let buffer = elements as? UnsafeMutableBufferPointer<UInt8> {
+            self.init(buffer: buffer)
+        } else if let data = elements as? Data {
+            let len = data.count
+            let backing = data.withUnsafeBytes { (bytes: UnsafePointer<UInt8>) in
+                return _DataStorage(bytes: bytes, length: len)
             }
-            idx += 1
+            self.init(backing: backing, range: 0..<len)
+        } else {
+            let underestimatedCount = elements.underestimatedCount
+            self.init(count: underestimatedCount)
+            
+            let (endIterator, _) = UnsafeMutableBufferPointer(start: _backing._bytes?.assumingMemoryBound(to: UInt8.self), count: underestimatedCount).initialize(from: elements)
+            var iter = endIterator
+            while let byte = iter.next() { self.append(byte) }
         }
     }
-    
-    public init(_ bytes: Array<UInt8>) {
-        self.init(bytes: bytes)
-    }
-    
-    public init(_ bytes: ArraySlice<UInt8>) {
-        self.init(bytes: bytes)
-    }
-    
-    public init(_ buffer: UnsafeBufferPointer<UInt8>) {
-        self.init(buffer: buffer)
-    }
-    
-    public init(_ buffer: UnsafeMutableBufferPointer<UInt8>) {
-        self.init(buffer: buffer)
-    }
-    
-    public init(_ data: Data) {
-        _sliceRange = 0..<data.count
-        _backing = data._backing.mutableCopy(data._sliceRange)
-    }
-    
+
     @_versioned
     internal init(backing: _DataStorage, range: Range<Index>) {
         _backing = backing
@@ -1152,7 +1275,7 @@
         }
         @inline(__always)
         set {
-            precondition(count >= 0, "count must be positive")
+            precondition(count >= 0, "count must not be negative")
             if !isKnownUniquelyReferenced(&_backing) {
                 _backing = _backing.mutableCopy(_sliceRange)
             }
@@ -1166,9 +1289,9 @@
     /// - warning: The byte pointer argument should not be stored and used outside of the lifetime of the call to the closure.
     @inline(__always)
     public func withUnsafeBytes<ResultType, ContentType>(_ body: (UnsafePointer<ContentType>) throws -> ResultType) rethrows -> ResultType {
-        let bytes =  _backing.bytes?.advanced(by: _sliceRange.lowerBound) ?? UnsafeRawPointer(bitPattern: 0xBAD0)!
-        let contentPtr = bytes.bindMemory(to: ContentType.self, capacity: count / MemoryLayout<ContentType>.stride)
-        return try body(contentPtr)
+        return try _backing.withUnsafeBytes(in: _sliceRange) {
+            return try body($0.baseAddress?.assumingMemoryBound(to: ContentType.self) ?? UnsafePointer<ContentType>(bitPattern: 0xBAD0)!)
+        }
     }
     
     
@@ -1181,9 +1304,9 @@
         if !isKnownUniquelyReferenced(&_backing) {
             _backing = _backing.mutableCopy(_sliceRange)
         }
-        let mutableBytes = _backing.mutableBytes?.advanced(by: _sliceRange.lowerBound) ?? UnsafeMutableRawPointer(bitPattern: 0xBAD0)!
-        let contentPtr = mutableBytes.bindMemory(to: ContentType.self, capacity: count / MemoryLayout<ContentType>.stride)
-        return try body(UnsafeMutablePointer(contentPtr))
+        return try _backing.withUnsafeMutableBytes(in: _sliceRange) {
+            return try body($0.baseAddress?.assumingMemoryBound(to: ContentType.self) ?? UnsafeMutablePointer<ContentType>(bitPattern: 0xBAD0)!)
+        }
     }
     
     // MARK: -
@@ -1196,15 +1319,19 @@
     /// - warning: This method does not verify that the contents at pointer have enough space to hold `count` bytes.
     @inline(__always)
     public func copyBytes(to pointer: UnsafeMutablePointer<UInt8>, count: Int) {
-        precondition(count >= 0, "count of bytes to copy must be positive")
+        precondition(count >= 0, "count of bytes to copy must not be negative")
         if count == 0 { return }
-        memcpy(UnsafeMutableRawPointer(pointer), _backing.bytes!.advanced(by: _sliceRange.lowerBound), Swift.min(count, _sliceRange.count))
+        _backing.withUnsafeBytes(in: _sliceRange) {
+            memcpy(UnsafeMutableRawPointer(pointer), $0.baseAddress!, Swift.min(count, $0.count))
+        }
     }
     
     @inline(__always)
     private func _copyBytesHelper(to pointer: UnsafeMutableRawPointer, from range: NSRange) {
         if range.length == 0 { return }
-        memcpy(UnsafeMutableRawPointer(pointer), _backing.bytes!.advanced(by: range.location), Swift.min(range.length, _sliceRange.count))
+        _backing.withUnsafeBytes(in: range.lowerBound..<range.upperBound) {
+            memcpy(UnsafeMutableRawPointer(pointer), $0.baseAddress!, Swift.min(range.length, $0.count))
+        }
     }
     
     /// Copy a subset of the contents of the data to a pointer.
@@ -1312,27 +1439,20 @@
     /// In some cases, (for example, a `Data` backed by a `dispatch_data_t`, the bytes may be stored discontiguously. In those cases, this function invokes the closure for each contiguous region of bytes.
     /// - parameter block: The closure to invoke for each region of data. You may stop the enumeration by setting the `stop` parameter to `true`.
     public func enumerateBytes(_ block: (_ buffer: UnsafeBufferPointer<UInt8>, _ byteIndex: Index, _ stop: inout Bool) -> Void) {
-        _backing.enumerateBytes(block)
+        _backing.enumerateBytes(in: _sliceRange, block)
     }
     
     @inline(__always)
     public mutating func append(_ bytes: UnsafePointer<UInt8>, count: Int) {
-        precondition(count >= 0, "count must be positive")
         if count == 0 { return }
-        if !isKnownUniquelyReferenced(&_backing) {
-            _backing = _backing.mutableCopy(_sliceRange)
-        }
-        _backing.append(bytes, length: count)
-        _sliceRange = _sliceRange.lowerBound..<(_sliceRange.upperBound + count)
+        append(UnsafeBufferPointer(start: bytes, count: count))
     }
     
     @inline(__always)
     public mutating func append(_ other: Data) {
-        if !isKnownUniquelyReferenced(&_backing) {
-            _backing = _backing.mutableCopy(_sliceRange)
+        other.enumerateBytes { (buffer, _, _) in
+            append(buffer)
         }
-        _backing.append(other._backing, startingAt: other._sliceRange.lowerBound, endingAt: other._sliceRange.upperBound)
-        _sliceRange = _sliceRange.lowerBound..<(_sliceRange.upperBound + other.count)
     }
     
     /// Append a buffer of bytes to the data.
@@ -1344,22 +1464,26 @@
         if !isKnownUniquelyReferenced(&_backing) {
             _backing = _backing.mutableCopy(_sliceRange)
         }
-        _backing.append(buffer.baseAddress!, length: buffer.count * MemoryLayout<SourceType>.stride)
+        _backing.replaceBytes(in: NSRange(location: _sliceRange.upperBound, length: _backing.length - (_sliceRange.upperBound - _backing._offset)), with: buffer.baseAddress, length: buffer.count * MemoryLayout<SourceType>.stride)
         _sliceRange = _sliceRange.lowerBound..<(_sliceRange.upperBound + buffer.count * MemoryLayout<SourceType>.stride)
     }
     
     @inline(__always)
     public mutating func append<S : Sequence>(contentsOf newElements: S) where S.Iterator.Element == Iterator.Element {
         let estimatedCount = newElements.underestimatedCount
-        var idx = count
-        count += estimatedCount
-        for byte in newElements {
-            let newIndex = idx + 1
-            if newIndex > count {
-                count = newIndex
+        guard estimatedCount > 0 else {
+            for byte in newElements {
+                append(byte)
             }
-            self[idx] = byte
-            idx = newIndex
+            return
+        }
+        _withStackOrHeapBuffer(estimatedCount) { allocation in
+            let buffer = UnsafeMutableBufferPointer(start: allocation.pointee.memory.assumingMemoryBound(to: UInt8.self), count: estimatedCount)
+            var (iterator, endPoint) = newElements._copyContents(initializing: buffer)
+            append(buffer.baseAddress!, count: endPoint - buffer.startIndex)
+            while let byte = iterator.next() {
+                append(byte)
+            }
         }
     }
     
@@ -1379,16 +1503,15 @@
     @inline(__always)
     public mutating func resetBytes(in range: Range<Index>) {
         // it is worth noting that the range here may be out of bounds of the Data itself (which triggers a growth)
-        precondition(range.lowerBound >= 0, "Ranges must be positive bounds")
-        precondition(range.upperBound >= 0, "Ranges must be positive bounds")
+        precondition(range.lowerBound >= 0, "Ranges must not be negative bounds")
+        precondition(range.upperBound >= 0, "Ranges must not be negative bounds")
         let range = NSMakeRange(range.lowerBound, range.upperBound - range.lowerBound)
         if !isKnownUniquelyReferenced(&_backing) {
             _backing = _backing.mutableCopy(_sliceRange)
         }
         _backing.resetBytes(in: range)
-        if _sliceRange.count < range.location + range.length {
-            let newLength = range.location + range.length
-            _sliceRange = _sliceRange.lowerBound..<(_sliceRange.lowerBound + newLength)
+        if _sliceRange.upperBound < range.upperBound {
+            _sliceRange = _sliceRange.lowerBound..<range.upperBound
         }
     }
     
@@ -1401,34 +1524,16 @@
     /// - parameter data: The replacement data.
     @inline(__always)
     public mutating func replaceSubrange(_ subrange: Range<Index>, with data: Data) {
-        _validateRange(subrange)
-        let nsRange = NSMakeRange(subrange.lowerBound, subrange.upperBound - subrange.lowerBound)
         let cnt = data.count
-        if !isKnownUniquelyReferenced(&_backing) {
-            _backing = _backing.mutableCopy(_sliceRange)
+        data.withUnsafeBytes {
+            replaceSubrange(subrange, with: $0, count: cnt)
         }
-        let resultingLength = data.withUnsafeBytes { (bytes: UnsafePointer<UInt8>) -> Int in
-            let currentLength = _backing.length
-            _backing.replaceBytes(in: nsRange, with: bytes, length: cnt)
-            return currentLength - nsRange.length + cnt
-        }
-        _sliceRange = _sliceRange.lowerBound..<(_sliceRange.lowerBound + resultingLength)
     }
     
     @inline(__always)
     public mutating func replaceSubrange(_ subrange: CountableRange<Index>, with data: Data) {
-        _validateRange(subrange)
-        let nsRange = NSMakeRange(subrange.lowerBound, subrange.upperBound - subrange.lowerBound)
-        let cnt = data.count
-        if !isKnownUniquelyReferenced(&_backing) {
-            _backing = _backing.mutableCopy(_sliceRange)
-        }
-        let resultingLength = data.withUnsafeBytes { (bytes: UnsafePointer<UInt8>) -> Int in
-            let currentLength = _backing.length
-            _backing.replaceBytes(in: nsRange, with: bytes, length: cnt)
-            return currentLength - nsRange.length + cnt
-        }
-        _sliceRange = _sliceRange.lowerBound..<(_sliceRange.lowerBound + resultingLength)
+        let range: Range<Int> = subrange.lowerBound..<subrange.upperBound
+        replaceSubrange(range, with: data)
     }
     
     /// Replace a region of bytes in the data with new bytes from a buffer.
@@ -1440,17 +1545,8 @@
     /// - parameter buffer: The replacement bytes.
     @inline(__always)
     public mutating func replaceSubrange<SourceType>(_ subrange: Range<Index>, with buffer: UnsafeBufferPointer<SourceType>) {
-        _validateRange(subrange)
-        let nsRange = NSMakeRange(subrange.lowerBound, subrange.upperBound - subrange.lowerBound)
-        let bufferCount = buffer.count * MemoryLayout<SourceType>.stride
-        
-        if !isKnownUniquelyReferenced(&_backing) {
-            _backing = _backing.mutableCopy(_sliceRange)
-        }
-        let currentLength = _backing.length
-        _backing.replaceBytes(in: nsRange, with: buffer.baseAddress, length: bufferCount)
-        let resultingLength = currentLength - nsRange.length + bufferCount
-        _sliceRange = _sliceRange.lowerBound..<(_sliceRange.lowerBound + resultingLength)
+        guard buffer.count > 0  else { return }
+        replaceSubrange(subrange, with: buffer.baseAddress!, count: buffer.count * MemoryLayout<SourceType>.stride)
     }
     
     /// Replace a region of bytes in the data with new bytes from a collection.
@@ -1461,39 +1557,18 @@
     /// - parameter subrange: The range in the data to replace.
     /// - parameter newElements: The replacement bytes.
     @inline(__always)
-    public mutating func replaceSubrange<ByteCollection : Collection>(_ subrange: Range<Index>, with newElements: ByteCollection)
-        where ByteCollection.Iterator.Element == Data.Iterator.Element {
-            _validateRange(subrange)
-            // Calculate this once, it may not be O(1)
-            let replacementCount: Int = numericCast(newElements.count)
-            let currentCount = self.count
-            let subrangeCount = subrange.count
-            
-            _validateRange(subrange)
-            
-            let resultCount = currentCount - subrangeCount + replacementCount
-            if resultCount != currentCount {
-                // This may realloc.
-                // In the future, if we keep the malloced pointer and count inside this struct/ref instead of deferring to NSData, we may be able to do this more efficiently.
-                self.count = resultCount
+    public mutating func replaceSubrange<ByteCollection : Collection>(_ subrange: Range<Index>, with newElements: ByteCollection) where ByteCollection.Iterator.Element == Data.Iterator.Element {
+        _validateRange(subrange)
+        let totalCount: Int = numericCast(newElements.count)
+        _withStackOrHeapBuffer(totalCount) { conditionalBuffer in
+            let buffer = UnsafeMutableBufferPointer(start: conditionalBuffer.pointee.memory.assumingMemoryBound(to: UInt8.self), count: totalCount)
+            var (iterator, index) = newElements._copyContents(initializing: buffer)
+            while let byte = iterator.next() {
+                buffer[index] = byte
+                index = buffer.index(after: index)
             }
-            
-            let shift = resultCount - currentCount
-            let start = subrange.lowerBound
-            
-            self.withUnsafeMutableBytes { (bytes : UnsafeMutablePointer<UInt8>) -> Void in
-                if shift != 0 {
-                    let destination = bytes + start + replacementCount
-                    let source = bytes + start + subrangeCount
-                    memmove(destination, source, currentCount - start - subrangeCount)
-                }
-                
-                if replacementCount != 0 {
-                    let buf = UnsafeMutableBufferPointer(start: bytes + start, count: replacementCount)
-                    var (it,idx) = newElements._copyContents(initializing: buf)
-                    precondition(it.next() == nil && idx == buf.endIndex, "newElements iterator returned different count to newElements.count")
-                }
-            }
+            replaceSubrange(subrange, with: conditionalBuffer.pointee.memory, count: totalCount)
+        }
     }
     
     @inline(__always)
@@ -1503,10 +1578,10 @@
         if !isKnownUniquelyReferenced(&_backing) {
             _backing = _backing.mutableCopy(_sliceRange)
         }
-        let currentLength = _backing.length
+        let upper = _sliceRange.upperBound
         _backing.replaceBytes(in: nsRange, with: bytes, length: cnt)
-        let resultingLength = currentLength - nsRange.length + cnt
-        _sliceRange = _sliceRange.lowerBound..<(_sliceRange.lowerBound + resultingLength)
+        let resultingUpper = upper - nsRange.length + cnt
+        _sliceRange = _sliceRange.lowerBound..<resultingUpper
     }
     
     /// Return a new copy of the data in a specified range.
@@ -1515,6 +1590,7 @@
     @inline(__always)
     public func subdata(in range: Range<Index>) -> Data {
         _validateRange(range)
+        let length = count
         if count == 0 {
             return Data()
         }
@@ -1549,7 +1625,15 @@
     
     /// The hash value for the data.
     public var hashValue: Int {
-        return _backing.hashValue
+        var hashValue = 0
+        let hashRange: Range<Int> = _sliceRange.lowerBound..<Swift.min(_sliceRange.lowerBound + 80, _sliceRange.upperBound)
+        _withStackOrHeapBuffer(hashRange.count) { buffer in
+            _backing.withUnsafeBytes(in: hashRange) {
+                memcpy(buffer.pointee.memory, $0.baseAddress!, hashRange.count)
+            }
+            hashValue = Int(bitPattern: CFHashBytes(buffer.pointee.memory.assumingMemoryBound(to: UInt8.self), hashRange.count))
+        }
+        return hashValue
     }
     
     @inline(__always)
@@ -1572,7 +1656,7 @@
         @inline(__always)
         get {
             _validateIndex(index)
-            return _backing.bytes!.advanced(by: index).assumingMemoryBound(to: UInt8.self).pointee
+            return _backing.get(index)
         }
         @inline(__always)
         set {
@@ -1658,11 +1742,24 @@
         }
     }
     
+    public func _copyContents(initializing buffer: UnsafeMutableBufferPointer<UInt8>) -> (Iterator, UnsafeMutableBufferPointer<UInt8>.Index) {
+        guard !isEmpty else { return (makeIterator(), buffer.startIndex) }
+        guard let p = buffer.baseAddress else {
+            preconditionFailure("Attempt to copy contents into nil buffer pointer")
+        }
+        let cnt = count
+        precondition(cnt <= buffer.count, "Insufficient space allocated to copy Data contents")
+        
+        withUnsafeBytes { p.initialize(from: $0, count: cnt) }
+        
+        return (Iterator(endOf: self), buffer.index(buffer.startIndex, offsetBy: cnt))
+    }
+    
     /// An iterator over the contents of the data.
     ///
     /// The iterator will increment byte-by-byte.
     public func makeIterator() -> Data.Iterator {
-        return Iterator(_data: self)
+        return Iterator(self)
     }
     
     public struct Iterator : IteratorProtocol {
@@ -1675,11 +1772,18 @@
         private var _idx: Data.Index
         private let _endIdx: Data.Index
         
-        fileprivate init(_data: Data) {
-            self._data = _data
+        fileprivate init(_ data: Data) {
+            _data = data
             _buffer = (0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0)
-            _idx = _data.startIndex
-            _endIdx = _data.endIndex
+            _idx = data.startIndex
+            _endIdx = data.endIndex
+        }
+        
+        fileprivate init(endOf data: Data) {
+            self._data = data
+            _buffer = (0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0)
+            _idx = data.endIndex
+            _endIdx = data.endIndex
         }
         
         public mutating func next() -> UInt8? {
@@ -1765,7 +1869,7 @@
         
         // Minimal size data is output as an array
         if nBytes < 64 {
-            children.append((label: "bytes", value: Array(self[0..<nBytes])))
+            children.append((label: "bytes", value: Array(self[startIndex..<Swift.min(nBytes + startIndex, endIndex)])))
         }
         
         let m = Mirror(self, children:children, displayStyle: Mirror.DisplayStyle.struct)
@@ -1822,11 +1926,27 @@
 
 extension Data : Codable {
     public init(from decoder: Decoder) throws {
+        // FIXME: This is a hook for bypassing a conditional conformance implementation to apply a strategy (see SR-5206). Remove this once conditional conformance is available.
+        do {
+            let singleValueContainer = try decoder.singleValueContainer()
+            if let decoder = singleValueContainer as? _JSONDecoder {
+                switch decoder.options.dataDecodingStrategy {
+                case .deferredToData:
+                    break /* fall back to default implementation below; this would recurse */
+
+                default:
+                    // _JSONDecoder has a hook for Datas; this won't recurse since we're not going to defer back to Data in _JSONDecoder.
+                    self = try singleValueContainer.decode(Data.self)
+                    return
+                }
+            }
+        } catch { /* fall back to default implementation below */ }
+
         var container = try decoder.unkeyedContainer()
         
         // It's more efficient to pre-allocate the buffer if we can.
         if let count = container.count {
-            self.init(count: count)
+            self = Data(count: count)
             
             // Loop only until count, not while !container.isAtEnd, in case count is underestimated (this is misbehavior) and we haven't allocated enough space.
             // We don't want to write past the end of what we allocated.
@@ -1835,7 +1955,7 @@
                 self[i] = byte
             }
         } else {
-            self.init()
+            self = Data()
         }
         
         while !container.isAtEnd {
@@ -1845,6 +1965,21 @@
     }
     
     public func encode(to encoder: Encoder) throws {
+        // FIXME: This is a hook for bypassing a conditional conformance implementation to apply a strategy (see SR-5206). Remove this once conditional conformance is available.
+        // We are allowed to request this container as long as we don't encode anything through it when we need the unkeyed container below.
+        var singleValueContainer = encoder.singleValueContainer()
+        if let encoder = singleValueContainer as? _JSONEncoder {
+            switch encoder.options.dataEncodingStrategy {
+            case .deferredToData:
+                break /* fall back to default implementation below; this would recurse */
+
+            default:
+                // _JSONEncoder has a hook for Datas; this won't recurse since we're not going to defer back to Data in _JSONEncoder.
+                try singleValueContainer.encode(self)
+                return
+            }
+        }
+
         var container = encoder.unkeyedContainer()
         
         // Since enumerateBytes does not rethrow, we need to catch the error, stow it away, and rethrow if we stopped.
@@ -1863,4 +1998,3 @@
         }
     }
 }
-
diff --git a/Foundation/Decimal.swift b/Foundation/Decimal.swift
index 3174ca4..36417b1 100644
--- a/Foundation/Decimal.swift
+++ b/Foundation/Decimal.swift
@@ -327,6 +327,7 @@
     }
 
     public mutating func negate() {
+        guard _length != 0 else { return }
         _isNegative = _isNegative == 0 ? 1 : 0
     }
 }
@@ -1282,9 +1283,7 @@
 
 public func NSDecimalSubtract(_ result: UnsafeMutablePointer<Decimal>, _ leftOperand: UnsafePointer<Decimal>, _ rightOperand: UnsafePointer<Decimal>, _ roundingMode: NSDecimalNumber.RoundingMode) -> NSDecimalNumber.CalculationError {
     var r = rightOperand.pointee
-    if r._length != 0 {
-        r.negate()
-    }
+    r.negate()
     return NSDecimalAdd(result, leftOperand, &r, roundingMode)
 }
 // Exact operations. result may be a pointer to same space as leftOperand or rightOperand
diff --git a/Foundation/FileManager.swift b/Foundation/FileManager.swift
index f7ca2e1..e5e2673 100644
--- a/Foundation/FileManager.swift
+++ b/Foundation/FileManager.swift
@@ -158,7 +158,7 @@
                 } else if let attr = attributes {
                     try self.setAttributes(attr, ofItemAtPath: path)
                 }
-            } else if isDir {
+            } else if isDir.boolValue {
                 return
             } else {
                 throw _NSErrorWithErrno(EEXIST, reading: false, path: path)
@@ -406,9 +406,9 @@
     }
     
     open func linkItem(atPath srcPath: String, toPath dstPath: String) throws {
-        var isDir = false
+        var isDir: ObjCBool = false
         if self.fileExists(atPath: srcPath, isDirectory: &isDir) {
-            if !isDir {
+            if !isDir.boolValue {
                 // TODO: Symlinks should be copied instead of hard-linked.
                 if link(srcPath, dstPath) == -1 {
                     throw _NSErrorWithErrno(errno, reading: false, path: srcPath)
@@ -532,12 +532,13 @@
             if let isDirectory = isDirectory {
                 if (s.st_mode & S_IFMT) == S_IFLNK {
                     if stat(path, &s) >= 0 {
-                        isDirectory.pointee = (s.st_mode & S_IFMT) == S_IFDIR
+                        isDirectory.pointee = ObjCBool((s.st_mode & S_IFMT) == S_IFDIR)
                     } else {
                         return false
                     }
                 } else {
-                    isDirectory.pointee = (s.st_mode & S_IFMT) == S_IFDIR
+                    let isDir = (s.st_mode & S_IFMT) == S_IFDIR
+                    isDirectory.pointee = ObjCBool(isDir)
                 }
             }
 
@@ -718,7 +719,11 @@
     open var homeDirectoryForCurrentUser: URL {
         return homeDirectory(forUser: CFCopyUserName().takeRetainedValue()._swiftObject)!
     }
-    open var temporaryDirectory: URL { NSUnimplemented() }
+    
+    open var temporaryDirectory: URL {
+        return URL(fileURLWithPath: NSTemporaryDirectory())
+    }
+    
     open func homeDirectory(forUser userName: String) -> URL? {
         guard !userName.isEmpty else { return nil }
         guard let url = CFCopyHomeDirectoryURLForUser(userName._cfObject) else { return nil }
diff --git a/Foundation/HTTPCookieStorage.swift b/Foundation/HTTPCookieStorage.swift
index 045b69d..2209818 100644
--- a/Foundation/HTTPCookieStorage.swift
+++ b/Foundation/HTTPCookieStorage.swift
@@ -36,14 +36,34 @@
 */
 open class HTTPCookieStorage: NSObject {
 
+    /* both sharedStorage and sharedCookieStorages are synchronized on sharedSyncQ */
     private static var sharedStorage: HTTPCookieStorage?
     private static var sharedCookieStorages: [String: HTTPCookieStorage] = [:] //for group storage containers
+    private static let sharedSyncQ = DispatchQueue(label: "org.swift.HTTPCookieStorage.sharedSyncQ")
+
+    /* only modified in init */
     private var cookieFilePath: String!
-    private let workQueue: DispatchQueue = DispatchQueue(label: "HTTPCookieStorage.workqueue")
-    var allCookies: [String: HTTPCookie]
+
+    /* synchronized on syncQ, please don't use _allCookies directly outside of init/deinit */
+    private var _allCookies: [String: HTTPCookie]
+    private var allCookies: [String: HTTPCookie] {
+        get {
+            if #available(OSX 10.12, iOS 10.0, tvOS 10.0, watchOS 3.0, *) {
+                dispatchPrecondition(condition: DispatchPredicate.onQueue(self.syncQ))
+            }
+            return self._allCookies
+        }
+        set {
+            if #available(OSX 10.12, iOS 10.0, tvOS 10.0, watchOS 3.0, *) {
+                dispatchPrecondition(condition: DispatchPredicate.onQueue(self.syncQ))
+            }
+            self._allCookies = newValue
+        }
+    }
+    private let syncQ = DispatchQueue(label: "org.swift.HTTPCookieStorage.syncQ")
 
     private init(cookieStorageName: String) {
-        allCookies = [:]
+        _allCookies = [:]
         cookieAcceptPolicy = .always
         super.init()
         let bundlePath = Bundle.main.bundlePath
@@ -59,9 +79,11 @@
     private func loadPersistedCookies() {
         guard let cookies = NSMutableDictionary(contentsOfFile: cookieFilePath) else { return }
         var cookies0 = _SwiftValue.fetch(cookies) as? [String: [String: Any]] ?? [:]
-        for key in cookies0.keys {
-            if let cookie = createCookie(cookies0[key]!) {
-                allCookies[key] = cookie
+        self.syncQ.sync {
+            for key in cookies0.keys {
+                if let cookie = createCookie(cookies0[key]!) {
+                    allCookies[key] = cookie
+                }
             }
         }
     }
@@ -87,11 +109,7 @@
     }
 
     open var cookies: [HTTPCookie]? {
-        var theCookies: [HTTPCookie]?
-        workQueue.sync {
-                theCookies = Array(self.allCookies.values)
-        }
-        return theCookies
+        return Array(self.syncQ.sync { self.allCookies.values })
     }
     
     /*!
@@ -103,10 +121,12 @@
     */
     open class var shared: HTTPCookieStorage {
         get {
-            if sharedStorage == nil {
-                sharedStorage = HTTPCookieStorage(cookieStorageName: "shared")
+            return sharedSyncQ.sync {
+                if sharedStorage == nil {
+                    sharedStorage = HTTPCookieStorage(cookieStorageName: "shared")
+                }
+                return sharedStorage!
             }
-            return sharedStorage!
         }
     }
     
@@ -122,12 +142,14 @@
         method with the same identifier will return the same cookie storage instance.
      */
     open class func sharedCookieStorage(forGroupContainerIdentifier identifier: String) -> HTTPCookieStorage {
-        guard let cookieStorage = sharedCookieStorages[identifier] else {
-            let newCookieStorage = HTTPCookieStorage(cookieStorageName: identifier)
-            sharedCookieStorages[identifier] = newCookieStorage
-            return newCookieStorage
+        return sharedSyncQ.sync {
+            guard let cookieStorage = sharedCookieStorages[identifier] else {
+                let newCookieStorage = HTTPCookieStorage(cookieStorageName: identifier)
+                sharedCookieStorages[identifier] = newCookieStorage
+                return newCookieStorage
+            }
+            return cookieStorage
         }
-        return cookieStorage
     }
 
     
@@ -138,7 +160,7 @@
         same name, domain and path, if any.
     */
     open func setCookie(_ cookie: HTTPCookie) {
-        workQueue.sync {
+        self.syncQ.sync {
             guard cookieAcceptPolicy != .never else { return }
 
             //add or override
@@ -173,9 +195,13 @@
     }
 
     private func updatePersistentStore() {
+        if #available(OSX 10.12, iOS 10.0, tvOS 10.0, watchOS 3.0, *) {
+            dispatchPrecondition(condition: DispatchPredicate.onQueue(self.syncQ))
+        }
+
         //persist cookies
         var persistDictionary: [String : [String : Any]] = [:]
-        let persistable = allCookies.filter { (_, value) in
+        let persistable = self.allCookies.filter { (_, value) in
             value.expiresDate != nil &&
             value.isSessionOnly == false &&
             value.expiresDate!.timeIntervalSinceNow > 0
@@ -190,14 +216,22 @@
     }
 
     /*!
+        @method lockedDeleteCookie:
+        @abstract Delete the specified cookie, for internal callers already on syncQ.
+    */
+    private func lockedDeleteCookie(_ cookie: HTTPCookie) {
+        let key = cookie.domain + cookie.path + cookie.name
+        self.allCookies.removeValue(forKey: key)
+        updatePersistentStore()
+    }
+
+    /*!
         @method deleteCookie:
         @abstract Delete the specified cookie
     */
     open func deleteCookie(_ cookie: HTTPCookie) {
-        let key = cookie.domain + cookie.path + cookie.name
-        workQueue.sync {
-            self.allCookies.removeValue(forKey: key)
-            updatePersistentStore()
+        self.syncQ.sync {
+            self.lockedDeleteCookie(cookie)
         }
     }
     
@@ -206,13 +240,15 @@
      @abstract Delete all cookies from the cookie storage since the provided date.
      */
     open func removeCookies(since date: Date) {
-        let cookiesSinceDate = allCookies.values.filter {
-            $0.properties![.created] as! Double >  date.timeIntervalSinceReferenceDate
+        self.syncQ.sync {
+            let cookiesSinceDate = self.allCookies.values.filter {
+                $0.properties![.created] as! Double >  date.timeIntervalSinceReferenceDate
+            }
+            for cookie in cookiesSinceDate {
+                lockedDeleteCookie(cookie)
+            }
+            updatePersistentStore()
         }
-        for cookie in cookiesSinceDate {
-            deleteCookie(cookie)
-        }
-        updatePersistentStore()
     }
 
     /*!
@@ -226,12 +262,8 @@
         into a set of header fields to add to a request.
     */
     open func cookies(for url: URL) -> [HTTPCookie]? {
-        var cookies: [HTTPCookie]?
         guard let host = url.host else { return nil }
-        workQueue.sync {
-            cookies = Array(allCookies.values.filter{ $0.domain == host })
-        }
-        return cookies
+        return Array(self.syncQ.sync(execute: {allCookies}).values.filter{ $0.domain == host })
     }
     
     /*!
diff --git a/Foundation/IndexSet.swift b/Foundation/IndexSet.swift
index ea103fa..1b4c7c4 100644
--- a/Foundation/IndexSet.swift
+++ b/Foundation/IndexSet.swift
@@ -407,7 +407,7 @@
     public func intersects(integersIn range: CountableClosedRange<Element>) -> Bool { return self.intersects(integersIn: Range(range)) }
     
     // MARK: -
-    // Indexable
+    // Collection
     
     public func index(after i: Index) -> Index {
         if i.value + 1 == i.extent.upperBound {
diff --git a/Foundation/JSONEncoder.swift b/Foundation/JSONEncoder.swift
index ecafeed..5c36e1f 100644
--- a/Foundation/JSONEncoder.swift
+++ b/Foundation/JSONEncoder.swift
@@ -64,6 +64,9 @@
 
     /// The strategy to use for encoding `Data` values.
     public enum DataEncodingStrategy {
+        /// Defer to `Data` for choosing an encoding.
+        case deferredToData
+        
         /// Encoded the `Data` as a Base64-encoded string. This is the default strategy.
         case base64
 
@@ -98,7 +101,7 @@
     open var userInfo: [CodingUserInfoKey : Any] = [:]
 
     /// Options set on the top-level encoder to pass down the encoding hierarchy.
-    fileprivate struct _Options {
+    internal struct _Options {
         let dateEncodingStrategy: DateEncodingStrategy
         let dataEncodingStrategy: DataEncodingStrategy
         let nonConformingFloatEncodingStrategy: NonConformingFloatEncodingStrategy
@@ -150,14 +153,14 @@
 
 // MARK: - _JSONEncoder
 
-fileprivate class _JSONEncoder : Encoder {
+internal class _JSONEncoder : Encoder {
     // MARK: Properties
 
     /// The encoder's storage.
     fileprivate var storage: _JSONEncodingStorage
 
     /// Options set on the top-level encoder.
-    fileprivate let options: JSONEncoder._Options
+    internal let options: JSONEncoder._Options
 
     /// The path to the current point in encoding.
     public var codingPath: [CodingKey]
@@ -672,6 +675,11 @@
 
     fileprivate func box(_ data: Data) throws -> NSObject {
         switch self.options.dataEncodingStrategy {
+        case .deferredToData:
+            // Must be called with a surrounding with(pushedKey:) call.
+            try data.encode(to: self)
+            return self.storage.popContainer()
+            
         case .base64:
             return NSString(string: data.base64EncodedString())
 
@@ -826,6 +834,9 @@
 
     /// The strategy to use for decoding `Data` values.
     public enum DataDecodingStrategy {
+        /// Defer to `Data` for decoding.
+        case deferredToData
+        
         /// Decode the `Data` from a Base64-encoded string. This is the default strategy.
         case base64
 
@@ -855,7 +866,7 @@
     open var userInfo: [CodingUserInfoKey : Any] = [:]
 
     /// Options set on the top-level encoder to pass down the decoding hierarchy.
-    fileprivate struct _Options {
+    internal struct _Options {
         let dateDecodingStrategy: DateDecodingStrategy
         let dataDecodingStrategy: DataDecodingStrategy
         let nonConformingFloatDecodingStrategy: NonConformingFloatDecodingStrategy
@@ -893,14 +904,14 @@
 
 // MARK: - _JSONDecoder
 
-fileprivate class _JSONDecoder : Decoder {
+internal class _JSONDecoder : Decoder {
     // MARK: Properties
 
     /// The decoder's storage.
     fileprivate var storage: _JSONDecodingStorage
 
     /// Options set on the top-level decoder.
-    fileprivate let options: JSONDecoder._Options
+    internal let options: JSONDecoder._Options
 
     /// The path to the current point in encoding.
     private(set) public var codingPath: [CodingKey]
@@ -2042,6 +2053,12 @@
         guard !(value is NSNull) else { return nil }
 
         switch self.options.dataDecodingStrategy {
+        case .deferredToData:
+            self.storage.push(container: value)
+            let data = try Data(from: self)
+            self.storage.popContainer()
+            return data
+            
         case .base64:
             guard let string = value as? String else {
                 throw DecodingError._typeMismatch(at: self.codingPath, expectation: type, reality: value)
diff --git a/Foundation/NSAttributedString.swift b/Foundation/NSAttributedString.swift
index 17dbe92..3877813 100644
--- a/Foundation/NSAttributedString.swift
+++ b/Foundation/NSAttributedString.swift
@@ -115,7 +115,7 @@
                 attributesInRange = attributes(at: currentIndex, longestEffectiveRange: &attributesEffectiveRange, in: enumerationRange)
             }
             
-            var shouldStop = false
+            var shouldStop: ObjCBool = false
             block(attributesInRange, attributesEffectiveRange, &shouldStop)
             stop.pointee = shouldStop
             
@@ -133,7 +133,7 @@
                 attributeInRange = attribute(attrName, at: currentIndex, longestEffectiveRange: &attributeEffectiveRange, in: enumerationRange)
             }
             
-            var shouldStop = false
+            var shouldStop: ObjCBool = false
             block(attributeInRange, attributeEffectiveRange, &shouldStop)
             stop.pointee = shouldStop
             
@@ -232,10 +232,10 @@
     func _enumerate(in enumerationRange: NSRange, reversed: Bool, using block: (Int, UnsafeMutablePointer<ObjCBool>) -> NSRange) {
         var attributeEnumerationRange = AttributeEnumerationRange(range: enumerationRange, reversed: reversed)
         while attributeEnumerationRange.hasMore {
-            var stop = false
+            var stop: ObjCBool = false
             let effectiveRange = block(attributeEnumerationRange.currentIndex, &stop)
             attributeEnumerationRange.advance(step: effectiveRange.length)
-            if stop {
+            if stop.boolValue {
                 break
             }
         }
diff --git a/Foundation/NSData.swift b/Foundation/NSData.swift
index 97b269d..3066406 100644
--- a/Foundation/NSData.swift
+++ b/Foundation/NSData.swift
@@ -991,6 +991,19 @@
     }
 }
 
+extension NSData {
+    internal func _isCompact() -> Bool {
+        var regions = 0
+        enumerateBytes { (_, _, stop) in
+            regions += 1
+            if regions > 1 {
+                stop.pointee = true
+            }
+        }
+        return regions <= 1
+    }
+}
+
 extension NSData : _StructTypeBridgeable {
     public typealias _StructType = Data
     public func _bridgeToSwift() -> Data {
diff --git a/Foundation/NSDictionary.swift b/Foundation/NSDictionary.swift
index f331104..18392f4 100644
--- a/Foundation/NSDictionary.swift
+++ b/Foundation/NSDictionary.swift
@@ -32,6 +32,10 @@
         return nil
     }
     
+    open func value(forKey key: String) -> Any? {
+        NSUnsupported()
+    }
+    
     open func keyEnumerator() -> NSEnumerator {
         guard type(of: self) === NSDictionary.self || type(of: self) === NSMutableDictionary.self else {
             NSRequiresConcreteImplementation()
@@ -40,6 +44,23 @@
         return NSGeneratorEnumerator(_storage.keys.map { _SwiftValue.fetch(nonOptional: $0) }.makeIterator())
     }
     
+    @available(*, deprecated)
+    public convenience init?(contentsOfFile path: String) {
+        self.init(contentsOf: URL(fileURLWithPath: path))
+    }
+    
+    @available(*, deprecated)
+    public convenience init?(contentsOf url: URL) {
+        do {
+            guard let plistDoc = try? Data(contentsOf: url) else { return nil }
+            let plistDict = try PropertyListSerialization.propertyList(from: plistDoc, options: [], format: nil) as? Dictionary<AnyHashable,Any>
+            guard let plistDictionary = plistDict else { return nil }
+            self.init(dictionary: plistDictionary)
+        } catch {
+            return nil
+        }
+    }
+    
     public override convenience init() {
         self.init(objects: [], forKeys: [], count: 0)
     }
@@ -467,11 +488,11 @@
                 lock.lock()
                 var stop = sharedStop
                 lock.unlock()
-                if stop { return }
+                if stop.boolValue { return }
                 
                 closure(keys[idx], objects[idx], &stop)
                 
-                if stop {
+                if stop.boolValue {
                     lock.lock()
                     sharedStop = stop
                     lock.unlock()
@@ -583,20 +604,6 @@
         super.init(objects: objects, forKeys: keys, count: cnt)
     }
     
-    public convenience init?(contentsOfFile path: String) {
-        self.init(contentsOfURL: URL(fileURLWithPath: path))
-    }
-    
-    public convenience init?(contentsOfURL url: URL) {
-        do {
-            guard let plistDoc = try? Data(contentsOf: url) else { return nil }
-            let plistDict = try PropertyListSerialization.propertyList(from: plistDoc, options: [], format: nil) as? Dictionary<AnyHashable,Any>
-            guard let plistDictionary = plistDict else { return nil }
-            self.init(dictionary: plistDictionary)
-        } catch {
-            return nil
-        }
-    }
 }
 
 extension NSMutableDictionary {
diff --git a/Foundation/NSIndexSet.swift b/Foundation/NSIndexSet.swift
index f621657..4a96e8a 100644
--- a/Foundation/NSIndexSet.swift
+++ b/Foundation/NSIndexSet.swift
@@ -396,9 +396,9 @@
         let iteration = withoutActuallyEscaping(block) { (closure: @escaping (P, UnsafeMutablePointer<ObjCBool>) -> R) -> (Int) -> Void in
             return { (rangeIdx) in
                 lock.lock()
-                var stop = sharedStop
+                var stop = ObjCBool(sharedStop)
                 lock.unlock()
-                if stop { return }
+                if stop.boolValue { return }
                 
                 let idx = rangeSequence.index(rangeSequence.startIndex, offsetBy: Int64(rangeIdx))
                 let curRange = rangeSequence[idx]
@@ -407,9 +407,9 @@
                     if intersection.length > 0 {
                         let _ = closure(intersection as! P, &stop)
                     }
-                    if stop {
+                    if stop.boolValue {
                         lock.lock()
-                        sharedStop = stop
+                        sharedStop = stop.boolValue
                         lock.unlock()
                         return
                     }
@@ -426,9 +426,9 @@
                         } else {
                             let _ = closure(idx as! P, &stop)
                         }
-                        if stop {
+                        if stop.boolValue {
                             lock.lock()
-                            sharedStop = stop
+                            sharedStop = stop.boolValue
                             lock.unlock()
                             return
                         }
diff --git a/Foundation/NSKeyedArchiver.swift b/Foundation/NSKeyedArchiver.swift
index 3790990..6df2ae1 100644
--- a/Foundation/NSKeyedArchiver.swift
+++ b/Foundation/NSKeyedArchiver.swift
@@ -553,9 +553,11 @@
         }
         
         // check replacement cache
-        objectToEncode = self._replacementMap[object as! AnyHashable]
-        if objectToEncode != nil {
-            return objectToEncode
+        if let hashable = object as? AnyHashable {
+            objectToEncode = self._replacementMap[hashable]
+            if objectToEncode != nil {
+                return objectToEncode
+            }
         }
         
         // object replaced by NSObject.replacementObject(for:)
diff --git a/Foundation/NSNotification.swift b/Foundation/NSNotification.swift
index 3926ee8..b848d6d 100644
--- a/Foundation/NSNotification.swift
+++ b/Foundation/NSNotification.swift
@@ -169,7 +169,7 @@
         }
     }
 
-    open func post(name aName: Notification.Name, object anObject: Any?, userInfo aUserInfo: [AnyHashable : Any]? = nil) {
+    open func post(name aName: NSNotification.Name, object anObject: Any?, userInfo aUserInfo: [AnyHashable : Any]? = nil) {
         let notification = Notification(name: aName, object: anObject, userInfo: aUserInfo)
         post(notification)
     }
@@ -189,11 +189,11 @@
     }
 
     @available(*,obsoleted:4.0,renamed:"addObserver(forName:object:queue:using:)")
-    open func addObserver(forName name: Notification.Name?, object obj: Any?, queue: OperationQueue?, usingBlock block: @escaping (Notification) -> Void) -> NSObjectProtocol {
+    open func addObserver(forName name: NSNotification.Name?, object obj: Any?, queue: OperationQueue?, usingBlock block: @escaping (Notification) -> Void) -> NSObjectProtocol {
         return addObserver(forName: name, object: obj, queue: queue, using: block)
     }
 
-    open func addObserver(forName name: Notification.Name?, object obj: Any?, queue: OperationQueue?, using block: @escaping (Notification) -> Void) -> NSObjectProtocol {
+    open func addObserver(forName name: NSNotification.Name?, object obj: Any?, queue: OperationQueue?, using block: @escaping (Notification) -> Void) -> NSObjectProtocol {
         let object = NSObject()
         
         let newObserver = NSNotificationReceiver()
diff --git a/Foundation/NSPathUtilities.swift b/Foundation/NSPathUtilities.swift
index 0126716..f0b65b4 100644
--- a/Foundation/NSPathUtilities.swift
+++ b/Foundation/NSPathUtilities.swift
@@ -408,9 +408,9 @@
             return false
         }
         
-        var isDirectory = false
+        var isDirectory: ObjCBool = false
         let exists = FileManager.default.fileExists(atPath: path, isDirectory: &isDirectory)
-        return exists && isDirectory
+        return exists && isDirectory.boolValue
     }
     
     internal typealias _FileNamePredicate = (String?) -> Bool
diff --git a/Foundation/NSSet.swift b/Foundation/NSSet.swift
index fdd5b99..939c358 100644
--- a/Foundation/NSSet.swift
+++ b/Foundation/NSSet.swift
@@ -255,7 +255,7 @@
             withUnsafeMutablePointer(to: &stop) { stop in
                 block(obj, stop)
             }
-            if stop {
+            if stop.boolValue {
                 break
             }
         }
diff --git a/Foundation/NSStringAPI.swift b/Foundation/NSStringAPI.swift
index 03ebc30..1b57164 100644
--- a/Foundation/NSStringAPI.swift
+++ b/Foundation/NSStringAPI.swift
@@ -42,12 +42,6 @@
   return result
 }
 
-func _toNSRange(_ r: Range<String.Index>) -> NSRange {
-  return NSRange(
-    location: r.lowerBound.encodedOffset,
-    length: r.upperBound.encodedOffset - r.lowerBound.encodedOffset)
-}
-
 #if !DEPLOYMENT_RUNTIME_SWIFT
 // We only need this for UnsafeMutablePointer, but there's not currently a way
 // to write that constraint.
@@ -439,10 +433,26 @@
     return self._ephemeralString._bridgeToObjectiveC()
   }
 
+  // self can be a Substring so we need to subtract/add this offset when
+  // passing _ns to the Foundation APIs. Will be 0 if self is String.
+  @_inlineable
+  @_versioned
+  internal var _substringOffset: Int {
+    return self.startIndex.encodedOffset
+  }
+
   /// Return an `Index` corresponding to the given offset in our UTF-16
   /// representation.
   func _index(_ utf16Index: Int) -> Index {
-    return Index(encodedOffset: utf16Index)
+    return Index(encodedOffset: utf16Index + _substringOffset)
+  }
+
+  @_inlineable
+  @_versioned
+  internal func _toRelativeNSRange(_ r: Range<String.Index>) -> NSRange {
+    return NSRange(
+      location: r.lowerBound.encodedOffset - _substringOffset,
+      length: r.upperBound.encodedOffset - r.lowerBound.encodedOffset)
   }
 
   /// Return a `Range<Index>` corresponding to the given `NSRange` of
@@ -603,7 +613,7 @@
     return locale != nil ? _ns.compare(
       aString,
       options: mask,
-      range: _toNSRange(
+      range: _toRelativeNSRange(
         range ?? startIndex..<endIndex
       ),
       locale: locale?._bridgeToObjectiveC()
@@ -612,7 +622,7 @@
     : range != nil ? _ns.compare(
       aString,
       options: mask,
-      range: _toNSRange(range!)
+      range: _toRelativeNSRange(range!)
     )
 
     : !mask.isEmpty ? _ns.compare(aString, options: mask)
@@ -1050,7 +1060,7 @@
     T : StringProtocol, R : RangeExpression
   >(in range: R, with replacement: T) -> String where R.Bound == Index {
     return _ns.replacingCharacters(
-      in: _toNSRange(range.relative(to: self)),
+      in: _toRelativeNSRange(range.relative(to: self)),
       with: replacement._ephemeralString)
   }
 
@@ -1083,7 +1093,7 @@
       of: target,
       with: replacement,
       options: options,
-      range: _toNSRange(
+      range: _toRelativeNSRange(
         searchRange ?? startIndex..<endIndex
       )
     )
@@ -1208,7 +1218,7 @@
   ) where R.Bound == Index {
     let range = range.relative(to: self)
     _ns.enumerateLinguisticTags(
-      in: _toNSRange(range),
+      in: _toRelativeNSRange(range),
       scheme: tagScheme._ephemeralString,
       options: opts,
       orthography: orthography != nil ? orthography! : nil
@@ -1273,7 +1283,7 @@
     ) -> Void
   ) where R.Bound == Index {
     _ns.enumerateSubstrings(
-      in: _toNSRange(range.relative(to: self)), options: opts) {
+      in: _toRelativeNSRange(range.relative(to: self)), options: opts) {
       var stop_ = false
 
       body($0,
@@ -1346,7 +1356,7 @@
         usedLength: usedBufferCount,
         encoding: encoding.rawValue,
         options: options,
-        range: _toNSRange(range.relative(to: self)),
+        range: _toRelativeNSRange(range.relative(to: self)),
         remaining: $0)
     }
   }
@@ -1373,7 +1383,7 @@
           contentsEnd in self._ns.getLineStart(
             start, end: end,
             contentsEnd: contentsEnd,
-            for: _toNSRange(range.relative(to: self)))
+            for: _toRelativeNSRange(range.relative(to: self)))
         }
       }
     }
@@ -1401,7 +1411,7 @@
           contentsEnd in self._ns.getParagraphStart(
             start, end: end,
             contentsEnd: contentsEnd,
-            for: _toNSRange(range.relative(to: self)))
+            for: _toRelativeNSRange(range.relative(to: self)))
         }
       }
     }
@@ -1428,7 +1438,8 @@
   public func lineRange<
     R : RangeExpression
   >(for aRange: R) -> Range<Index> where R.Bound == Index {
-    return _range(_ns.lineRange(for: _toNSRange(aRange.relative(to: self))))
+    return _range(_ns.lineRange(
+      for: _toRelativeNSRange(aRange.relative(to: self))))
   }
 
 #if !DEPLOYMENT_RUNTIME_SWIFT
@@ -1453,7 +1464,7 @@
     var nsTokenRanges: NSArray?
     let result = tokenRanges._withNilOrAddress(of: &nsTokenRanges) {
       self._ns.linguisticTags(
-        in: _toNSRange(range.relative(to: self)),
+        in: _toRelativeNSRange(range.relative(to: self)),
         scheme: tagScheme._ephemeralString,
         options: opts,
         orthography: orthography,
@@ -1477,7 +1488,7 @@
     R : RangeExpression
   >(for aRange: R) -> Range<Index> where R.Bound == Index {
     return _range(
-      _ns.paragraphRange(for: _toNSRange(aRange.relative(to: self))))
+      _ns.paragraphRange(for: _toRelativeNSRange(aRange.relative(to: self))))
   }
 #endif
 
@@ -1504,7 +1515,7 @@
       _ns.rangeOfCharacter(
         from: aSet,
         options: mask,
-        range: _toNSRange(
+        range: _toRelativeNSRange(
           aRange ?? startIndex..<endIndex
         )
       )
@@ -1535,7 +1546,7 @@
     // and output ranges due (if nothing else) to locale changes
     return _range(
       _ns.rangeOfComposedCharacterSequences(
-        for: _toNSRange(range.relative(to: self))))
+        for: _toRelativeNSRange(range.relative(to: self))))
   }
 
   // - (NSRange)rangeOfString:(NSString *)aString
@@ -1570,13 +1581,13 @@
       locale != nil ? _ns.range(
         of: aString,
         options: mask,
-        range: _toNSRange(
+        range: _toRelativeNSRange(
           searchRange ?? startIndex..<endIndex
         ),
         locale: locale
       )
       : searchRange != nil ? _ns.range(
-        of: aString, options: mask, range: _toNSRange(searchRange!)
+        of: aString, options: mask, range: _toRelativeNSRange(searchRange!)
       )
       : !mask.isEmpty ? _ns.range(of: aString, options: mask)
       : _ns.range(of: aString)
@@ -1687,7 +1698,7 @@
   @available(swift, deprecated: 4.0,
     message: "Please use String slicing subscript.")
   public func substring(with aRange: Range<Index>) -> String {
-    return _ns.substring(with: _toNSRange(aRange))
+    return _ns.substring(with: _toRelativeNSRange(aRange))
   }
 }
 
diff --git a/Foundation/NSSwiftRuntime.swift b/Foundation/NSSwiftRuntime.swift
index 58a7174..33112a8 100644
--- a/Foundation/NSSwiftRuntime.swift
+++ b/Foundation/NSSwiftRuntime.swift
@@ -18,13 +18,73 @@
 @_exported import Glibc
 #endif
 
+@_exported import Dispatch
+
 #if os(Android) // shim required for bzero
 @_transparent func bzero(_ ptr: UnsafeMutableRawPointer, _ size: size_t) {
     memset(ptr, 0, size)
 }
 #endif
 
-public typealias ObjCBool = Bool
+#if !_runtime(_ObjC)
+/// The Objective-C BOOL type.
+///
+/// On 64-bit iOS, the Objective-C BOOL type is a typedef of C/C++
+/// bool. Elsewhere, it is "signed char". The Clang importer imports it as
+/// ObjCBool.
+@_fixed_layout
+public struct ObjCBool : ExpressibleByBooleanLiteral {
+    #if os(OSX) || (os(iOS) && (arch(i386) || arch(arm)))
+    // On OS X and 32-bit iOS, Objective-C's BOOL type is a "signed char".
+    var _value: Int8
+
+    init(_ value: Int8) {
+        self._value = value
+    }
+
+    public init(_ value: Bool) {
+        self._value = value ? 1 : 0
+    }
+
+    #else
+    // Everywhere else it is C/C++'s "Bool"
+    var _value: Bool
+
+    public init(_ value: Bool) {
+        self._value = value
+    }
+    #endif
+
+    /// The value of `self`, expressed as a `Bool`.
+    public var boolValue: Bool {
+        #if os(OSX) || (os(iOS) && (arch(i386) || arch(arm)))
+        return _value != 0
+        #else
+        return _value
+        #endif
+    }
+
+    /// Create an instance initialized to `value`.
+    @_transparent
+    public init(booleanLiteral value: Bool) {
+        self.init(value)
+    }
+}
+
+extension ObjCBool : CustomReflectable {
+    /// Returns a mirror that reflects `self`.
+    public var customMirror: Mirror {
+        return Mirror(reflecting: boolValue)
+    }
+}
+
+extension ObjCBool : CustomStringConvertible {
+    /// A textual representation of `self`.
+    public var description: String {
+        return self.boolValue.description
+    }
+}
+#endif
 
 internal class __NSCFType : NSObject {
     private var _cfinfo : Int32
diff --git a/Foundation/NSTextCheckingResult.swift b/Foundation/NSTextCheckingResult.swift
index caed9a8..74dd8c0 100644
--- a/Foundation/NSTextCheckingResult.swift
+++ b/Foundation/NSTextCheckingResult.swift
@@ -87,7 +87,7 @@
 
 extension NSTextCheckingResult {
     
-    public func resultByAdjustingRangesWithOffset(_ offset: Int) -> NSTextCheckingResult {
+    public func adjustingRanges(offset: Int) -> NSTextCheckingResult {
         let count = self.numberOfRanges
         var newRanges = [NSRange]()
         for idx in 0..<count {
diff --git a/Foundation/NSURL.swift b/Foundation/NSURL.swift
index 697c488..a02f49c 100644
--- a/Foundation/NSURL.swift
+++ b/Foundation/NSURL.swift
@@ -309,7 +309,7 @@
 
         let thePath = _standardizedPath(path)
         
-        var isDir : Bool = false
+        var isDir: ObjCBool = false
         if thePath.hasSuffix("/") {
             isDir = true
         } else {
@@ -323,7 +323,7 @@
             let _ = FileManager.default.fileExists(atPath: absolutePath, isDirectory: &isDir)
         }
 
-        self.init(fileURLWithPath: thePath, isDirectory: isDir, relativeTo: baseURL)
+        self.init(fileURLWithPath: thePath, isDirectory: isDir.boolValue, relativeTo: baseURL)
     }
 
     public convenience init(fileURLWithPath path: String, isDirectory isDir: Bool) {
@@ -339,7 +339,7 @@
             thePath = path
         }
 
-        var isDir : Bool = false
+        var isDir: ObjCBool = false
         if thePath.hasSuffix("/") {
             isDir = true
         } else {
@@ -348,7 +348,7 @@
             }
         }
         super.init()
-        _CFURLInitWithFileSystemPathRelativeToBase(_cfObject, thePath._cfObject, kCFURLPOSIXPathStyle, isDir, nil)
+        _CFURLInitWithFileSystemPathRelativeToBase(_cfObject, thePath._cfObject, kCFURLPOSIXPathStyle, isDir.boolValue, nil)
     }
     
     public convenience init(fileURLWithFileSystemRepresentation path: UnsafePointer<Int8>, isDirectory isDir: Bool, relativeTo baseURL: URL?) {
@@ -779,8 +779,8 @@
         var result : URL? = appendingPathComponent(pathComponent, isDirectory: false)
         if !pathComponent.hasSuffix("/") && isFileURL {
             if let urlWithoutDirectory = result {
-                var isDir : Bool = false
-                if FileManager.default.fileExists(atPath: urlWithoutDirectory.path, isDirectory: &isDir) && isDir {
+                var isDir: ObjCBool = false
+                if FileManager.default.fileExists(atPath: urlWithoutDirectory.path, isDirectory: &isDir) && isDir.boolValue {
                     result = self.appendingPathComponent(pathComponent, isDirectory: true)
                 }
             }
@@ -859,14 +859,14 @@
         }
 
         // It might be a responsibility of NSURL(fileURLWithPath:). Check it.
-        var isExistingDirectory = false
+        var isExistingDirectory: ObjCBool = false
         let _ = FileManager.default.fileExists(atPath: resolvedPath, isDirectory: &isExistingDirectory)
 
         if excludeSystemDirs {
             resolvedPath = resolvedPath._tryToRemovePathPrefix("/private") ?? resolvedPath
         }
 
-        if isExistingDirectory && !resolvedPath.hasSuffix("/") {
+        if isExistingDirectory.boolValue && !resolvedPath.hasSuffix("/") {
             resolvedPath += "/"
         }
         
diff --git a/Foundation/Operation.swift b/Foundation/Operation.swift
index 9c17ad1..ab09c1f 100644
--- a/Foundation/Operation.swift
+++ b/Foundation/Operation.swift
@@ -43,13 +43,15 @@
     }
     
     open func start() {
-        lock.lock()
-        _executing = true
-        lock.unlock()
-        main()
-        lock.lock()
-        _executing = false
-        lock.unlock()
+        if !isCancelled {
+            lock.lock()
+            _executing = true
+            lock.unlock()
+            main()
+            lock.lock()
+            _executing = false
+            lock.unlock()
+        }
         finish()
     }
     
@@ -79,9 +81,13 @@
     }
     
     open func cancel() {
+        // Note that calling cancel() is advisory. It is up to the main() function to
+        // call isCancelled at appropriate points in its execution flow and to do the
+        // actual canceling work. Eventually main() will invoke finish() and this is
+        // where we then leave the groups and unblock other operations that might
+        // depend on us.
         lock.lock()
         _cancelled = true
-        _leaveGroups()
         lock.unlock()
     }
     
diff --git a/Foundation/URLSession/URLSession.swift b/Foundation/URLSession/URLSession.swift
index 7690cc4..8e707b3 100644
--- a/Foundation/URLSession/URLSession.swift
+++ b/Foundation/URLSession/URLSession.swift
@@ -137,11 +137,6 @@
 /// - SeeAlso: https://curl.haxx.se/libcurl/c/threadsafe.html
 /// - SeeAlso: URLSession+libcurl.swift
 ///
-/// The (publicly accessible) attributes of an `URLSessionTask` are made thread
-/// safe by using a concurrent libdispatch queue and only doing writes with a
-/// barrier while allowing concurrent reads. A single queue is shared for all
-/// tasks of a given session for this isolation. C.f. `taskAttributesIsolation`.
-///
 /// ## HTTP and RFC 2616
 ///
 /// Most of HTTP is defined in [RFC 2616](https://tools.ietf.org/html/rfc2616).
@@ -194,9 +189,6 @@
     fileprivate let multiHandle: _MultiHandle
     fileprivate var nextTaskIdentifier = 1
     internal let workQueue: DispatchQueue 
-    /// This queue is used to make public attributes on `URLSessionTask` instances thread safe.
-    /// - Note: It's a **concurrent** queue.
-    internal let taskAttributesIsolation: DispatchQueue 
     internal let taskRegistry = URLSession._TaskRegistry()
     fileprivate let identifier: Int32
     fileprivate var invalidated = false
@@ -222,7 +214,6 @@
         initializeLibcurl()
         identifier = nextSessionIdentifier()
         self.workQueue = DispatchQueue(label: "URLSession<\(identifier)>")
-        self.taskAttributesIsolation = DispatchQueue(label: "URLSession<\(identifier)>.taskAttributes", attributes: DispatchQueue.Attributes.concurrent)
         self.delegateQueue = OperationQueue()
         self.delegateQueue.maxConcurrentOperationCount = 1
         self.delegate = nil
@@ -245,7 +236,6 @@
         initializeLibcurl()
         identifier = nextSessionIdentifier()
         self.workQueue = DispatchQueue(label: "URLSession<\(identifier)>")
-        self.taskAttributesIsolation = DispatchQueue(label: "URLSession<\(identifier)>.taskAttributes", attributes: DispatchQueue.Attributes.concurrent)
         if let _queue = queue {
            self.delegateQueue = _queue
         } else {
diff --git a/Foundation/UserDefaults.swift b/Foundation/UserDefaults.swift
index 29d36fd..42cbd0d 100644
--- a/Foundation/UserDefaults.swift
+++ b/Foundation/UserDefaults.swift
@@ -45,37 +45,39 @@
         //Force the returned value to an NSObject
         switch CFGetTypeID(anObj) {
         case CFStringGetTypeID():
-            return (anObj as! CFString)._nsObject
+            return unsafeBitCast(anObj, to: NSString.self)
             
         case CFNumberGetTypeID():
-            return (anObj as! CFNumber)._nsObject
+            return unsafeBitCast(anObj, to: NSNumber.self)
             
         case CFURLGetTypeID():
-            return (anObj as! CFURL)._nsObject
+            return unsafeBitCast(anObj, to: NSURL.self)
             
         case CFArrayGetTypeID():
-            return (anObj as! CFArray)._nsObject
+            return unsafeBitCast(anObj, to: NSArray.self)
             
         case CFDictionaryGetTypeID():
-            return (anObj as! CFDictionary)._nsObject
-
+            return unsafeBitCast(anObj, to: NSDictionary.self)
+            
         case CFDataGetTypeID():
-            return (anObj as! CFData)._nsObject
+            return unsafeBitCast(anObj, to: NSData.self)
             
         default:
             return getFromRegistered()
         }
     }
+
     open func set(_ value: Any?, forKey defaultName: String) {
         guard let value = value else {
             CFPreferencesSetAppValue(defaultName._cfObject, nil, suite?._cfObject ?? kCFPreferencesCurrentApplication)
             return
         }
         
-        var cfType: CFTypeRef? = nil
+        let cfType: CFTypeRef
 		
-		//FIXME: is this needed? Am I overcomplicating things?
-        //Foundation types
+		// Convert the input value to the internal representation. All values are
+        // represented as CFTypeRef objects internally because we store the defaults
+        // in a CFPreferences type.
         if let bType = value as? NSNumber {
             cfType = bType._cfObject
         } else if let bType = value as? NSString {
@@ -84,11 +86,26 @@
             cfType = bType._cfObject
         } else if let bType = value as? NSDictionary {
             cfType = bType._cfObject
+        } else if let bType = value as? NSData {
+            cfType = bType._cfObject
+        } else if let bType = value as? NSURL {
+            set(URL(reference: bType), forKey: defaultName)
+            return
+        } else if let bType = value as? String {
+            cfType = bType._cfObject
         } else if let bType = value as? URL {
 			set(bType, forKey: defaultName)
 			return
+        } else if let bType = value as? Int {
+            var cfValue = Int64(bType)
+            cfType = CFNumberCreate(nil, kCFNumberSInt64Type, &cfValue)
+        } else if let bType = value as? Double {
+            var cfValue = bType
+            cfType = CFNumberCreate(nil, kCFNumberDoubleType, &cfValue)
         } else if let bType = value as? Data {
             cfType = bType._cfObject
+        } else {
+            fatalError("The type of 'value' passed to UserDefaults.set(forKey:) is not supported.")
         }
         
         CFPreferencesSetAppValue(defaultName._cfObject, cfType, suite?._cfObject ?? kCFPreferencesCurrentApplication)
@@ -140,10 +157,10 @@
     }
     open func data(forKey defaultName: String) -> Data? {
         guard let aVal = object(forKey: defaultName),
-              let bVal = aVal as? Data else {
+              let bVal = aVal as? NSData else {
             return nil
         }
-        return bVal
+        return Data(referencing: bVal)
     }
     open func stringArray(forKey defaultName: String) -> [String]? {
         guard let aVal = object(forKey: defaultName),
@@ -153,39 +170,61 @@
         return _SwiftValue.fetch(nonOptional: bVal) as? [String]
     }
     open func integer(forKey defaultName: String) -> Int {
-        guard let aVal = object(forKey: defaultName),
-              let bVal = aVal as? NSNumber else {
+        guard let aVal = object(forKey: defaultName) else {
             return 0
         }
-        return bVal.intValue
+        if let bVal = aVal as? NSNumber {
+            return bVal.intValue
+        }
+        if let bVal = aVal as? NSString {
+            return bVal.integerValue
+        }
+        return 0
     }
     open func float(forKey defaultName: String) -> Float {
-        guard let aVal = object(forKey: defaultName),
-              let bVal = aVal as? NSNumber else {
+        guard let aVal = object(forKey: defaultName) else {
             return 0
         }
-        return bVal.floatValue
+        if let bVal = aVal as? NSNumber {
+            return bVal.floatValue
+        }
+        if let bVal = aVal as? NSString {
+            return bVal.floatValue
+        }
+        return 0
     }
     open func double(forKey defaultName: String) -> Double {
-        guard let aVal = object(forKey: defaultName),
-              let bVal = aVal as? NSNumber else {
+        guard let aVal = object(forKey: defaultName) else {
             return 0
         }
-        return bVal.doubleValue
+        if let bVal = aVal as? NSNumber {
+            return bVal.doubleValue
+        }
+        if let bVal = aVal as? NSString {
+            return bVal.doubleValue
+        }
+        return 0
     }
     open func bool(forKey defaultName: String) -> Bool {
-        guard let aVal = object(forKey: defaultName),
-              let bVal = aVal as? NSNumber else {
+        guard let aVal = object(forKey: defaultName) else {
             return false
         }
-        return bVal.boolValue
+        if let bVal = aVal as? NSNumber {
+            return bVal.boolValue
+        }
+        if let bVal = aVal as? NSString {
+            return bVal.boolValue
+        }
+        return false
     }
     open func url(forKey defaultName: String) -> URL? {
         guard let aVal = object(forKey: defaultName) else {
             return nil
         }
         
-        if let bVal = aVal as? NSString {
+        if let bVal = aVal as? NSURL {
+            return URL(reference: bVal)
+        } else if let bVal = aVal as? NSString {
             let cVal = bVal.expandingTildeInPath
             
             return URL(fileURLWithPath: cVal)
@@ -227,10 +266,32 @@
     
     open func register(defaults registrationDictionary: [String : Any]) {
         for (key, value) in registrationDictionary {
-            registeredDefaults[key] = value
+            let nsValue: NSObject
+
+            // Converts a value to the internal representation. Internalized values are
+            // stored as NSObject derived objects in the registration dictionary.
+            if let val = value as? String {
+                nsValue = val._nsObject
+            } else if let val = value as? URL {
+                nsValue = val.path._nsObject
+            } else if let val = value as? Int {
+                nsValue = NSNumber(value: val)
+            } else if let val = value as? Double {
+                nsValue = NSNumber(value: val)
+            } else if let val = value as? Bool {
+                nsValue = NSNumber(value: val)
+            } else if let val = value as? Data {
+                nsValue = val._nsObject
+            } else if let val = value as? NSObject {
+                nsValue = val
+            } else {
+                fatalError("The type of 'value' passed to UserDefaults.register(defaults:) is not supported.")
+            }
+
+            registeredDefaults[key] = nsValue
         }
     }
-    
+
     open func addSuite(named suiteName: String) {
         CFPreferencesAddSuitePreferencesToApp(kCFPreferencesCurrentApplication, suiteName._cfObject)
     }
diff --git a/TestFoundation/TestAffineTransform.swift b/TestFoundation/TestAffineTransform.swift
index f1d1a51..9a897ad 100644
--- a/TestFoundation/TestAffineTransform.swift
+++ b/TestFoundation/TestAffineTransform.swift
@@ -38,6 +38,9 @@
             ("test_AppendTransform", test_AppendTransform),
             ("test_PrependTransform", test_PrependTransform),
             ("test_TransformComposition", test_TransformComposition),
+            ("test_hashing_identity", test_hashing_identity),
+            ("test_hashing_values", test_hashing_values),
+            ("test_rotation_compose", test_rotation_compose),
             ("test_Equal", test_Equal),
             ("test_NSCoding", test_NSCoding),
         ]
@@ -345,6 +348,16 @@
             XCTAssertEqual(ref.hashValue, val.hashValue)
         }
     }
+
+    func test_rotation_compose() {
+        var t = AffineTransform.identity
+        t.translate(x: 1.0, y: 1.0)
+        t.rotate(byDegrees: 90)
+        t.translate(x: -1.0, y: -1.0)
+        let result = t.transform(NSPoint(x: 1.0, y: 2.0))
+        XCTAssertEqual(0.0, Double(result.x), accuracy: accuracyThreshold)
+        XCTAssertEqual(1.0, Double(result.y), accuracy: accuracyThreshold)
+    }
     
     func test_Equal() {
         let transform = NSAffineTransform()
diff --git a/TestFoundation/TestByteCountFormatter.swift b/TestFoundation/TestByteCountFormatter.swift
index 8046a0f..c077266 100644
--- a/TestFoundation/TestByteCountFormatter.swift
+++ b/TestFoundation/TestByteCountFormatter.swift
@@ -49,7 +49,7 @@
     
     func test_DefaultValues() {
         let formatter = ByteCountFormatter()
-        XCTAssertEqual(formatter.allowedUnits, ByteCountFormatter.Units.useDefault)
+        XCTAssertEqual(formatter.allowedUnits, [])
         XCTAssertEqual(formatter.countStyle, ByteCountFormatter.CountStyle.file)
         XCTAssertEqual(formatter.allowsNonnumericFormatting, true)
         XCTAssertEqual(formatter.includesUnit, true)
@@ -68,7 +68,7 @@
         formatter.allowedUnits = .useGB
         XCTAssertEqual(formatter.string(fromByteCount: 0), "Zero KB")
         
-        formatter.allowedUnits = .useDefault
+        formatter.allowedUnits = []
         formatter.allowsNonnumericFormatting = false
         XCTAssertEqual(formatter.string(fromByteCount: 0), "0 bytes")
         
@@ -91,7 +91,7 @@
         formatter.allowedUnits = .useGB
         XCTAssertEqual(formatter.string(fromByteCount: 1), "0 GB")
         
-        formatter.allowedUnits = .useDefault
+        formatter.allowedUnits = []
         formatter.isAdaptive = false
         XCTAssertEqual(formatter.string(fromByteCount: 1), "1 byte")
         
@@ -365,7 +365,7 @@
         XCTAssertEqual(formatter.string(fromByteCount: 11999), "12.0 KB")
         XCTAssertEqual(formatter.string(fromByteCount: 900000), "900 KB")
         XCTAssertEqual(formatter.string(fromByteCount: 12345678), "12.3 MB")
-        XCTAssertEqual(formatter.string(fromByteCount: 123456789), "123.5 MB")
+        XCTAssertEqual(formatter.string(fromByteCount: 123456789), "123 MB")
         XCTAssertEqual(formatter.string(fromByteCount: 1234567898), "1.23 GB")
         XCTAssertEqual(formatter.string(fromByteCount: 12345678987), "12.3 GB")
     }
diff --git a/TestFoundation/TestDecimal.swift b/TestFoundation/TestDecimal.swift
index f626e1e..f34d9f7 100644
--- a/TestFoundation/TestDecimal.swift
+++ b/TestFoundation/TestDecimal.swift
@@ -289,6 +289,9 @@
         XCTAssertEqual(.minus, d.sign)
         d.negate()
         XCTAssertEqual(.plus, d.sign)
+        var e = Decimal(0)
+        e.negate()
+        XCTAssertEqual(e, 0)
         XCTAssertTrue(Decimal(3.5).isEqual(to: Decimal(3.5)))
         XCTAssertTrue(Decimal.nan.isEqual(to: Decimal.nan))
         XCTAssertTrue(Decimal(1.28).isLess(than: Decimal(2.24)))
diff --git a/TestFoundation/TestFileManager.swift b/TestFoundation/TestFileManager.swift
index a9cb932..90d1597 100644
--- a/TestFoundation/TestFileManager.swift
+++ b/TestFoundation/TestFileManager.swift
@@ -31,6 +31,7 @@
             ("test_subpathsOfDirectoryAtPath", test_subpathsOfDirectoryAtPath),
             ("test_copyItemAtPathToPath", test_copyItemAtPathToPath),
             ("test_homedirectoryForUser", test_homedirectoryForUser),
+            ("test_temporaryDirectoryForUser", test_temporaryDirectoryForUser),
         ]
     }
     
@@ -53,10 +54,10 @@
         // Ensure attempting to create the directory again fails gracefully.
         XCTAssertNil(try? fm.createDirectory(atPath: path, withIntermediateDirectories:false, attributes:nil))
 
-        var isDir = false
+        var isDir: ObjCBool = false
         let exists = fm.fileExists(atPath: path, isDirectory: &isDir)
         XCTAssertTrue(exists)
-        XCTAssertTrue(isDir)
+        XCTAssertTrue(isDir.boolValue)
 
         do {
             try fm.removeItem(atPath: path)
@@ -74,10 +75,10 @@
         
         XCTAssertTrue(fm.createFile(atPath: path, contents: Data(), attributes: nil))
         
-        var isDir = false
+        var isDir: ObjCBool = false
         let exists = fm.fileExists(atPath: path, isDirectory: &isDir)
         XCTAssertTrue(exists)
-        XCTAssertFalse(isDir)
+        XCTAssertFalse(isDir.boolValue)
         
         do {
             try fm.removeItem(atPath: path)
@@ -467,9 +468,9 @@
         }
         
         func directoryExists(atPath path: String) -> Bool {
-            var isDir = false
+            var isDir: ObjCBool = false
             let exists = fm.fileExists(atPath: path, isDirectory: &isDir)
-            return exists && isDir
+            return exists && isDir.boolValue
         }
         
         func createDirectory(atPath path: String) {
@@ -525,4 +526,25 @@
         XCTAssertNil(filemanger.homeDirectory(forUser: ""))
         XCTAssertNotNil(filemanger.homeDirectoryForCurrentUser)
     }
+    
+    func test_temporaryDirectoryForUser() {
+        let filemanger = FileManager.default
+        let tmpDir = filemanger.temporaryDirectory
+        XCTAssertNotNil(tmpDir)
+        let tmpFileUrl = tmpDir.appendingPathComponent("test.bin")
+        let tmpFilePath = tmpFileUrl.path
+        
+        do {
+            if filemanger.fileExists(atPath: tmpFilePath) {
+                try filemanger.removeItem(at: tmpFileUrl)
+            }
+            
+            try "hello world".write(to: tmpFileUrl, atomically: false, encoding: .utf8)
+            XCTAssert(filemanger.fileExists(atPath: tmpFilePath))
+
+            try filemanger.removeItem(at: tmpFileUrl)
+        } catch {
+            XCTFail("Unable to write a file to the temporary directory: \(tmpDir), err: \(error)")
+        }
+    }
 }
diff --git a/TestFoundation/TestHTTPCookieStorage.swift b/TestFoundation/TestHTTPCookieStorage.swift
index 472e34a..f65a01e 100644
--- a/TestFoundation/TestHTTPCookieStorage.swift
+++ b/TestFoundation/TestHTTPCookieStorage.swift
@@ -14,6 +14,7 @@
     import SwiftFoundation
     import SwiftXCTest
 #endif
+import Dispatch
 
 class TestHTTPCookieStorage: XCTestCase {
 
@@ -24,6 +25,7 @@
 
     static var allTests: [(String, (TestHTTPCookieStorage) -> () throws -> Void)] {
         return [
+            ("test_sharedCookieStorageAccessedFromMultipleThreads", test_sharedCookieStorageAccessedFromMultipleThreads),
             ("test_BasicStorageAndRetrieval", test_BasicStorageAndRetrieval),
             ("test_deleteCookie", test_deleteCookie),
             ("test_removeCookies", test_removeCookies),
@@ -34,6 +36,27 @@
         ]
     }
 
+    func test_sharedCookieStorageAccessedFromMultipleThreads() {
+        let q = DispatchQueue.global()
+        let syncQ = DispatchQueue(label: "TestHTTPCookieStorage.syncQ")
+        var allCookieStorages: [HTTPCookieStorage] = []
+        let g = DispatchGroup()
+        for _ in 0..<64 {
+            g.enter()
+            q.async {
+                let mySharedCookieStore = HTTPCookieStorage.shared
+                syncQ.async {
+                    allCookieStorages.append(mySharedCookieStore)
+                    g.leave()
+                }
+            }
+        }
+        g.wait()
+        let cookieStorages = syncQ.sync { allCookieStorages }
+        let mySharedCookieStore = HTTPCookieStorage.shared
+        XCTAssertTrue(cookieStorages.reduce(true, { $0 && $1 === mySharedCookieStore }), "\(cookieStorages)")
+    }
+
     func test_BasicStorageAndRetrieval() {
         basicStorageAndRetrieval(with: .shared)
         basicStorageAndRetrieval(with: .groupContainer("test"))
@@ -238,7 +261,7 @@
             destPath = NSHomeDirectory() + "/.local/share" + bundleName + "/.cookies.shared"
         }
         let fm = FileManager.default
-        var isDir = false
+        var isDir: ObjCBool = false
         let exists = fm.fileExists(atPath: destPath, isDirectory: &isDir)
         XCTAssertTrue(exists)
         //Test by setting the environmental variable
diff --git a/TestFoundation/TestNSData.swift b/TestFoundation/TestNSData.swift
index df0cd2e..3bacd6e 100644
--- a/TestFoundation/TestNSData.swift
+++ b/TestFoundation/TestNSData.swift
@@ -17,7 +17,171 @@
 
 class TestNSData: XCTestCase {
     
-
+    class AllOnesImmutableData : NSData {
+        private var _length : Int
+        var _pointer : UnsafeMutableBufferPointer<UInt8>? {
+            willSet {
+                if let p = _pointer { free(p.baseAddress) }
+            }
+        }
+        
+        init(length : Int) {
+            _length = length
+            super.init(bytes: nil, length: 0, copy: false, deallocator: nil)
+            _length = length
+        }
+        
+        required init?(coder aDecoder: NSCoder) {
+            // Not tested
+            fatalError()
+        }
+        
+        deinit {
+            if let p = _pointer {
+                free(p.baseAddress)
+            }
+        }
+        
+        override var length : Int {
+            get {
+                return _length
+            }
+        }
+        
+        override var bytes : UnsafeRawPointer {
+            if let d = _pointer {
+                return UnsafeRawPointer(d.baseAddress!)
+            } else {
+                // Need to allocate the buffer now.
+                // It doesn't matter if the buffer is uniquely referenced or not here.
+                let buffer = malloc(length)
+                memset(buffer!, 1, length)
+                let bytePtr = buffer!.bindMemory(to: UInt8.self, capacity: length)
+                let result = UnsafeMutableBufferPointer(start: bytePtr, count: length)
+                _pointer = result
+                return UnsafeRawPointer(result.baseAddress!)
+            }
+        }
+        
+        override func getBytes(_ buffer: UnsafeMutableRawPointer, length: Int) {
+            if let d = _pointer {
+                // Get the real data from the buffer
+                memmove(buffer, d.baseAddress!, length)
+            } else {
+                // A more efficient implementation of getBytes in the case where no one has asked for our backing bytes
+                memset(buffer, 1, length)
+            }
+        }
+        
+        override func copy(with zone: NSZone? = nil) -> Any {
+            return self
+        }
+        
+        override func mutableCopy(with zone: NSZone? = nil) -> Any {
+            return AllOnesData(length: _length)
+        }
+    }
+    
+    
+    class AllOnesData : NSMutableData {
+        
+        private var _length : Int
+        var _pointer : UnsafeMutableBufferPointer<UInt8>? {
+            willSet {
+                if let p = _pointer { free(p.baseAddress) }
+            }
+        }
+        
+        /*override*/ init(length : Int) {
+            _length = length
+            super.init(bytes: nil, length: 0, copy: false, deallocator: nil)
+            _length = length
+        }
+        
+        required init?(coder aDecoder: NSCoder) {
+            // Not tested
+            fatalError()
+        }
+        
+        deinit {
+            if let p = _pointer {
+                free(p.baseAddress)
+            }
+        }
+        
+        override var length : Int {
+            get {
+                return _length
+            }
+            set {
+                if let ptr = _pointer {
+                    // Copy the data to our new length buffer
+                    let newBuffer = malloc(newValue)!
+                    if newValue <= _length {
+                        memmove(newBuffer, ptr.baseAddress!, newValue)
+                    } else if newValue > _length {
+                        memmove(newBuffer, ptr.baseAddress!, _length)
+                        memset(newBuffer + _length, 1, newValue - _length)
+                    }
+                    let bytePtr = newBuffer.bindMemory(to: UInt8.self, capacity: newValue)
+                    _pointer = UnsafeMutableBufferPointer(start: bytePtr, count: newValue)
+                }
+                _length = newValue
+            }
+        }
+        
+        override var bytes : UnsafeRawPointer {
+            if let d = _pointer {
+                return UnsafeRawPointer(d.baseAddress!)
+            } else {
+                // Need to allocate the buffer now.
+                // It doesn't matter if the buffer is uniquely referenced or not here.
+                let buffer = malloc(length)
+                memset(buffer!, 1, length)
+                let bytePtr = buffer!.bindMemory(to: UInt8.self, capacity: length)
+                let result = UnsafeMutableBufferPointer(start: bytePtr, count: length)
+                _pointer = result
+                return UnsafeRawPointer(result.baseAddress!)
+            }
+        }
+        
+        override var mutableBytes: UnsafeMutableRawPointer {
+            let newBufferLength = _length
+            let newBuffer = malloc(newBufferLength)
+            if let ptr = _pointer {
+                // Copy the existing data to the new box, then return its pointer
+                memmove(newBuffer!, ptr.baseAddress!, newBufferLength)
+            } else {
+                // Set new data to 1s
+                memset(newBuffer!, 1, newBufferLength)
+            }
+            let bytePtr = newBuffer!.bindMemory(to: UInt8.self, capacity: newBufferLength)
+            let result = UnsafeMutableBufferPointer(start: bytePtr, count: newBufferLength)
+            _pointer = result
+            _length = newBufferLength
+            return UnsafeMutableRawPointer(result.baseAddress!)
+        }
+        
+        override func getBytes(_ buffer: UnsafeMutableRawPointer, length: Int) {
+            if let d = _pointer {
+                // Get the real data from the buffer
+                memmove(buffer, d.baseAddress!, length)
+            } else {
+                // A more efficient implementation of getBytes in the case where no one has asked for our backing bytes
+                memset(buffer, 1, length)
+            }
+        }
+    }
+    
+    var heldData: Data?
+    
+    // this holds a reference while applying the function which forces the internal ref type to become non-uniquely referenced
+    func holdReference(_ data: Data, apply: () -> Void) {
+        heldData = data
+        apply()
+        heldData = nil
+    }
+    
     // MARK: -
     
     // String of course has its own way to get data, but this way tests our own data struct
@@ -100,6 +264,250 @@
             ("test_replaceSubrange", test_replaceSubrange),
             ("test_sliceWithUnsafeBytes", test_sliceWithUnsafeBytes),
             ("test_sliceIteration", test_sliceIteration),
+
+            ("test_validateMutation_withUnsafeMutableBytes", test_validateMutation_withUnsafeMutableBytes),
+            ("test_validateMutation_appendBytes", test_validateMutation_appendBytes),
+            ("test_validateMutation_appendData", test_validateMutation_appendData),
+            ("test_validateMutation_appendBuffer", test_validateMutation_appendBuffer),
+            ("test_validateMutation_appendSequence", test_validateMutation_appendSequence),
+            ("test_validateMutation_appendContentsOf", test_validateMutation_appendContentsOf),
+            ("test_validateMutation_resetBytes", test_validateMutation_resetBytes),
+            ("test_validateMutation_replaceSubrange", test_validateMutation_replaceSubrange),
+            ("test_validateMutation_replaceSubrangeCountableRange", test_validateMutation_replaceSubrangeCountableRange),
+            ("test_validateMutation_replaceSubrangeWithBuffer", test_validateMutation_replaceSubrangeWithBuffer),
+            ("test_validateMutation_replaceSubrangeWithCollection", test_validateMutation_replaceSubrangeWithCollection),
+            ("test_validateMutation_replaceSubrangeWithBytes", test_validateMutation_replaceSubrangeWithBytes),
+            ("test_validateMutation_slice_withUnsafeMutableBytes", test_validateMutation_slice_withUnsafeMutableBytes),
+            ("test_validateMutation_slice_appendBytes", test_validateMutation_slice_appendBytes),
+            ("test_validateMutation_slice_appendData", test_validateMutation_slice_appendData),
+            ("test_validateMutation_slice_appendBuffer", test_validateMutation_slice_appendBuffer),
+            ("test_validateMutation_slice_appendSequence", test_validateMutation_slice_appendSequence),
+            ("test_validateMutation_slice_appendContentsOf", test_validateMutation_slice_appendContentsOf),
+            ("test_validateMutation_slice_resetBytes", test_validateMutation_slice_resetBytes),
+            ("test_validateMutation_slice_replaceSubrange", test_validateMutation_slice_replaceSubrange),
+            ("test_validateMutation_slice_replaceSubrangeCountableRange", test_validateMutation_slice_replaceSubrangeCountableRange),
+            ("test_validateMutation_slice_replaceSubrangeWithBuffer", test_validateMutation_slice_replaceSubrangeWithBuffer),
+            ("test_validateMutation_slice_replaceSubrangeWithCollection", test_validateMutation_slice_replaceSubrangeWithCollection),
+            ("test_validateMutation_slice_replaceSubrangeWithBytes", test_validateMutation_slice_replaceSubrangeWithBytes),
+            ("test_validateMutation_cow_withUnsafeMutableBytes", test_validateMutation_cow_withUnsafeMutableBytes),
+            ("test_validateMutation_cow_appendBytes", test_validateMutation_cow_appendBytes),
+            ("test_validateMutation_cow_appendData", test_validateMutation_cow_appendData),
+            ("test_validateMutation_cow_appendBuffer", test_validateMutation_cow_appendBuffer),
+            ("test_validateMutation_cow_appendSequence", test_validateMutation_cow_appendSequence),
+            ("test_validateMutation_cow_appendContentsOf", test_validateMutation_cow_appendContentsOf),
+            ("test_validateMutation_cow_resetBytes", test_validateMutation_cow_resetBytes),
+            ("test_validateMutation_cow_replaceSubrange", test_validateMutation_cow_replaceSubrange),
+            ("test_validateMutation_cow_replaceSubrangeCountableRange", test_validateMutation_cow_replaceSubrangeCountableRange),
+            ("test_validateMutation_cow_replaceSubrangeWithBuffer", test_validateMutation_cow_replaceSubrangeWithBuffer),
+            ("test_validateMutation_cow_replaceSubrangeWithCollection", test_validateMutation_cow_replaceSubrangeWithCollection),
+            ("test_validateMutation_cow_replaceSubrangeWithBytes", test_validateMutation_cow_replaceSubrangeWithBytes),
+            ("test_validateMutation_slice_cow_withUnsafeMutableBytes", test_validateMutation_slice_cow_withUnsafeMutableBytes),
+            ("test_validateMutation_slice_cow_appendBytes", test_validateMutation_slice_cow_appendBytes),
+            ("test_validateMutation_slice_cow_appendData", test_validateMutation_slice_cow_appendData),
+            ("test_validateMutation_slice_cow_appendBuffer", test_validateMutation_slice_cow_appendBuffer),
+            ("test_validateMutation_slice_cow_appendSequence", test_validateMutation_slice_cow_appendSequence),
+            ("test_validateMutation_slice_cow_appendContentsOf", test_validateMutation_slice_cow_appendContentsOf),
+            ("test_validateMutation_slice_cow_resetBytes", test_validateMutation_slice_cow_resetBytes),
+            ("test_validateMutation_slice_cow_replaceSubrange", test_validateMutation_slice_cow_replaceSubrange),
+            ("test_validateMutation_slice_cow_replaceSubrangeCountableRange", test_validateMutation_slice_cow_replaceSubrangeCountableRange),
+            ("test_validateMutation_slice_cow_replaceSubrangeWithBuffer", test_validateMutation_slice_cow_replaceSubrangeWithBuffer),
+            ("test_validateMutation_slice_cow_replaceSubrangeWithCollection", test_validateMutation_slice_cow_replaceSubrangeWithCollection),
+            ("test_validateMutation_slice_cow_replaceSubrangeWithBytes", test_validateMutation_slice_cow_replaceSubrangeWithBytes),
+            ("test_validateMutation_immutableBacking_withUnsafeMutableBytes", test_validateMutation_immutableBacking_withUnsafeMutableBytes),
+            ("test_validateMutation_immutableBacking_appendBytes", test_validateMutation_immutableBacking_appendBytes),
+            ("test_validateMutation_immutableBacking_appendData", test_validateMutation_immutableBacking_appendData),
+            ("test_validateMutation_immutableBacking_appendBuffer", test_validateMutation_immutableBacking_appendBuffer),
+            ("test_validateMutation_immutableBacking_appendSequence", test_validateMutation_immutableBacking_appendSequence),
+            ("test_validateMutation_immutableBacking_appendContentsOf", test_validateMutation_immutableBacking_appendContentsOf),
+            ("test_validateMutation_immutableBacking_resetBytes", test_validateMutation_immutableBacking_resetBytes),
+            ("test_validateMutation_immutableBacking_replaceSubrange", test_validateMutation_immutableBacking_replaceSubrange),
+            ("test_validateMutation_immutableBacking_replaceSubrangeCountableRange", test_validateMutation_immutableBacking_replaceSubrangeCountableRange),
+            ("test_validateMutation_immutableBacking_replaceSubrangeWithBuffer", test_validateMutation_immutableBacking_replaceSubrangeWithBuffer),
+            ("test_validateMutation_immutableBacking_replaceSubrangeWithCollection", test_validateMutation_immutableBacking_replaceSubrangeWithCollection),
+            ("test_validateMutation_immutableBacking_replaceSubrangeWithBytes", test_validateMutation_immutableBacking_replaceSubrangeWithBytes),
+            ("test_validateMutation_slice_immutableBacking_withUnsafeMutableBytes", test_validateMutation_slice_immutableBacking_withUnsafeMutableBytes),
+            ("test_validateMutation_slice_immutableBacking_appendBytes", test_validateMutation_slice_immutableBacking_appendBytes),
+            ("test_validateMutation_slice_immutableBacking_appendData", test_validateMutation_slice_immutableBacking_appendData),
+            ("test_validateMutation_slice_immutableBacking_appendBuffer", test_validateMutation_slice_immutableBacking_appendBuffer),
+            ("test_validateMutation_slice_immutableBacking_appendSequence", test_validateMutation_slice_immutableBacking_appendSequence),
+            ("test_validateMutation_slice_immutableBacking_appendContentsOf", test_validateMutation_slice_immutableBacking_appendContentsOf),
+            ("test_validateMutation_slice_immutableBacking_resetBytes", test_validateMutation_slice_immutableBacking_resetBytes),
+            ("test_validateMutation_slice_immutableBacking_replaceSubrange", test_validateMutation_slice_immutableBacking_replaceSubrange),
+            ("test_validateMutation_slice_immutableBacking_replaceSubrangeCountableRange", test_validateMutation_slice_immutableBacking_replaceSubrangeCountableRange),
+            ("test_validateMutation_slice_immutableBacking_replaceSubrangeWithBuffer", test_validateMutation_slice_immutableBacking_replaceSubrangeWithBuffer),
+            ("test_validateMutation_slice_immutableBacking_replaceSubrangeWithCollection", test_validateMutation_slice_immutableBacking_replaceSubrangeWithCollection),
+            ("test_validateMutation_slice_immutableBacking_replaceSubrangeWithBytes", test_validateMutation_slice_immutableBacking_replaceSubrangeWithBytes),
+            ("test_validateMutation_cow_immutableBacking_withUnsafeMutableBytes", test_validateMutation_cow_immutableBacking_withUnsafeMutableBytes),
+            ("test_validateMutation_cow_immutableBacking_appendBytes", test_validateMutation_cow_immutableBacking_appendBytes),
+            ("test_validateMutation_cow_immutableBacking_appendData", test_validateMutation_cow_immutableBacking_appendData),
+            ("test_validateMutation_cow_immutableBacking_appendBuffer", test_validateMutation_cow_immutableBacking_appendBuffer),
+            ("test_validateMutation_cow_immutableBacking_appendSequence", test_validateMutation_cow_immutableBacking_appendSequence),
+            ("test_validateMutation_cow_immutableBacking_appendContentsOf", test_validateMutation_cow_immutableBacking_appendContentsOf),
+            ("test_validateMutation_cow_immutableBacking_resetBytes", test_validateMutation_cow_immutableBacking_resetBytes),
+            ("test_validateMutation_cow_immutableBacking_replaceSubrange", test_validateMutation_cow_immutableBacking_replaceSubrange),
+            ("test_validateMutation_cow_immutableBacking_replaceSubrangeCountableRange", test_validateMutation_cow_immutableBacking_replaceSubrangeCountableRange),
+            ("test_validateMutation_cow_immutableBacking_replaceSubrangeWithBuffer", test_validateMutation_cow_immutableBacking_replaceSubrangeWithBuffer),
+            ("test_validateMutation_cow_immutableBacking_replaceSubrangeWithCollection", test_validateMutation_cow_immutableBacking_replaceSubrangeWithCollection),
+            ("test_validateMutation_cow_immutableBacking_replaceSubrangeWithBytes", test_validateMutation_cow_immutableBacking_replaceSubrangeWithBytes),
+            ("test_validateMutation_slice_cow_immutableBacking_withUnsafeMutableBytes", test_validateMutation_slice_cow_immutableBacking_withUnsafeMutableBytes),
+            ("test_validateMutation_slice_cow_immutableBacking_appendBytes", test_validateMutation_slice_cow_immutableBacking_appendBytes),
+            ("test_validateMutation_slice_cow_immutableBacking_appendData", test_validateMutation_slice_cow_immutableBacking_appendData),
+            ("test_validateMutation_slice_cow_immutableBacking_appendBuffer", test_validateMutation_slice_cow_immutableBacking_appendBuffer),
+            ("test_validateMutation_slice_cow_immutableBacking_appendSequence", test_validateMutation_slice_cow_immutableBacking_appendSequence),
+            ("test_validateMutation_slice_cow_immutableBacking_appendContentsOf", test_validateMutation_slice_cow_immutableBacking_appendContentsOf),
+            ("test_validateMutation_slice_cow_immutableBacking_resetBytes", test_validateMutation_slice_cow_immutableBacking_resetBytes),
+            ("test_validateMutation_slice_cow_immutableBacking_replaceSubrange", test_validateMutation_slice_cow_immutableBacking_replaceSubrange),
+            ("test_validateMutation_slice_cow_immutableBacking_replaceSubrangeCountableRange", test_validateMutation_slice_cow_immutableBacking_replaceSubrangeCountableRange),
+            ("test_validateMutation_slice_cow_immutableBacking_replaceSubrangeWithBuffer", test_validateMutation_slice_cow_immutableBacking_replaceSubrangeWithBuffer),
+            ("test_validateMutation_slice_cow_immutableBacking_replaceSubrangeWithCollection", test_validateMutation_slice_cow_immutableBacking_replaceSubrangeWithCollection),
+            ("test_validateMutation_slice_cow_immutableBacking_replaceSubrangeWithBytes", test_validateMutation_slice_cow_immutableBacking_replaceSubrangeWithBytes),
+            ("test_validateMutation_mutableBacking_withUnsafeMutableBytes", test_validateMutation_mutableBacking_withUnsafeMutableBytes),
+            ("test_validateMutation_mutableBacking_appendBytes", test_validateMutation_mutableBacking_appendBytes),
+            ("test_validateMutation_mutableBacking_appendData", test_validateMutation_mutableBacking_appendData),
+            ("test_validateMutation_mutableBacking_appendBuffer", test_validateMutation_mutableBacking_appendBuffer),
+            ("test_validateMutation_mutableBacking_appendSequence", test_validateMutation_mutableBacking_appendSequence),
+            ("test_validateMutation_mutableBacking_appendContentsOf", test_validateMutation_mutableBacking_appendContentsOf),
+            ("test_validateMutation_mutableBacking_resetBytes", test_validateMutation_mutableBacking_resetBytes),
+            ("test_validateMutation_mutableBacking_replaceSubrange", test_validateMutation_mutableBacking_replaceSubrange),
+            ("test_validateMutation_mutableBacking_replaceSubrangeCountableRange", test_validateMutation_mutableBacking_replaceSubrangeCountableRange),
+            ("test_validateMutation_mutableBacking_replaceSubrangeWithBuffer", test_validateMutation_mutableBacking_replaceSubrangeWithBuffer),
+            ("test_validateMutation_mutableBacking_replaceSubrangeWithCollection", test_validateMutation_mutableBacking_replaceSubrangeWithCollection),
+            ("test_validateMutation_mutableBacking_replaceSubrangeWithBytes", test_validateMutation_mutableBacking_replaceSubrangeWithBytes),
+            ("test_validateMutation_slice_mutableBacking_withUnsafeMutableBytes", test_validateMutation_slice_mutableBacking_withUnsafeMutableBytes),
+            ("test_validateMutation_slice_mutableBacking_appendBytes", test_validateMutation_slice_mutableBacking_appendBytes),
+            ("test_validateMutation_slice_mutableBacking_appendData", test_validateMutation_slice_mutableBacking_appendData),
+            ("test_validateMutation_slice_mutableBacking_appendBuffer", test_validateMutation_slice_mutableBacking_appendBuffer),
+            ("test_validateMutation_slice_mutableBacking_appendSequence", test_validateMutation_slice_mutableBacking_appendSequence),
+            ("test_validateMutation_slice_mutableBacking_appendContentsOf", test_validateMutation_slice_mutableBacking_appendContentsOf),
+            ("test_validateMutation_slice_mutableBacking_resetBytes", test_validateMutation_slice_mutableBacking_resetBytes),
+            ("test_validateMutation_slice_mutableBacking_replaceSubrange", test_validateMutation_slice_mutableBacking_replaceSubrange),
+            ("test_validateMutation_slice_mutableBacking_replaceSubrangeCountableRange", test_validateMutation_slice_mutableBacking_replaceSubrangeCountableRange),
+            ("test_validateMutation_slice_mutableBacking_replaceSubrangeWithBuffer", test_validateMutation_slice_mutableBacking_replaceSubrangeWithBuffer),
+            ("test_validateMutation_slice_mutableBacking_replaceSubrangeWithCollection", test_validateMutation_slice_mutableBacking_replaceSubrangeWithCollection),
+            ("test_validateMutation_slice_mutableBacking_replaceSubrangeWithBytes", test_validateMutation_slice_mutableBacking_replaceSubrangeWithBytes),
+            ("test_validateMutation_cow_mutableBacking_withUnsafeMutableBytes", test_validateMutation_cow_mutableBacking_withUnsafeMutableBytes),
+            ("test_validateMutation_cow_mutableBacking_appendBytes", test_validateMutation_cow_mutableBacking_appendBytes),
+            ("test_validateMutation_cow_mutableBacking_appendData", test_validateMutation_cow_mutableBacking_appendData),
+            ("test_validateMutation_cow_mutableBacking_appendBuffer", test_validateMutation_cow_mutableBacking_appendBuffer),
+            ("test_validateMutation_cow_mutableBacking_appendSequence", test_validateMutation_cow_mutableBacking_appendSequence),
+            ("test_validateMutation_cow_mutableBacking_appendContentsOf", test_validateMutation_cow_mutableBacking_appendContentsOf),
+            ("test_validateMutation_cow_mutableBacking_resetBytes", test_validateMutation_cow_mutableBacking_resetBytes),
+            ("test_validateMutation_cow_mutableBacking_replaceSubrange", test_validateMutation_cow_mutableBacking_replaceSubrange),
+            ("test_validateMutation_cow_mutableBacking_replaceSubrangeCountableRange", test_validateMutation_cow_mutableBacking_replaceSubrangeCountableRange),
+            ("test_validateMutation_cow_mutableBacking_replaceSubrangeWithBuffer", test_validateMutation_cow_mutableBacking_replaceSubrangeWithBuffer),
+            ("test_validateMutation_cow_mutableBacking_replaceSubrangeWithCollection", test_validateMutation_cow_mutableBacking_replaceSubrangeWithCollection),
+            ("test_validateMutation_cow_mutableBacking_replaceSubrangeWithBytes", test_validateMutation_cow_mutableBacking_replaceSubrangeWithBytes),
+            ("test_validateMutation_slice_cow_mutableBacking_withUnsafeMutableBytes", test_validateMutation_slice_cow_mutableBacking_withUnsafeMutableBytes),
+            ("test_validateMutation_slice_cow_mutableBacking_appendBytes", test_validateMutation_slice_cow_mutableBacking_appendBytes),
+            ("test_validateMutation_slice_cow_mutableBacking_appendData", test_validateMutation_slice_cow_mutableBacking_appendData),
+            ("test_validateMutation_slice_cow_mutableBacking_appendBuffer", test_validateMutation_slice_cow_mutableBacking_appendBuffer),
+            ("test_validateMutation_slice_cow_mutableBacking_appendSequence", test_validateMutation_slice_cow_mutableBacking_appendSequence),
+            ("test_validateMutation_slice_cow_mutableBacking_appendContentsOf", test_validateMutation_slice_cow_mutableBacking_appendContentsOf),
+            ("test_validateMutation_slice_cow_mutableBacking_resetBytes", test_validateMutation_slice_cow_mutableBacking_resetBytes),
+            ("test_validateMutation_slice_cow_mutableBacking_replaceSubrange", test_validateMutation_slice_cow_mutableBacking_replaceSubrange),
+            ("test_validateMutation_slice_cow_mutableBacking_replaceSubrangeCountableRange", test_validateMutation_slice_cow_mutableBacking_replaceSubrangeCountableRange),
+            ("test_validateMutation_slice_cow_mutableBacking_replaceSubrangeWithBuffer", test_validateMutation_slice_cow_mutableBacking_replaceSubrangeWithBuffer),
+            ("test_validateMutation_slice_cow_mutableBacking_replaceSubrangeWithCollection", test_validateMutation_slice_cow_mutableBacking_replaceSubrangeWithCollection),
+            ("test_validateMutation_slice_cow_mutableBacking_replaceSubrangeWithBytes", test_validateMutation_slice_cow_mutableBacking_replaceSubrangeWithBytes),
+            ("test_validateMutation_customBacking_withUnsafeMutableBytes", test_validateMutation_customBacking_withUnsafeMutableBytes),
+//            ("test_validateMutation_customBacking_appendBytes", test_validateMutation_customBacking_appendBytes),
+//            ("test_validateMutation_customBacking_appendData", test_validateMutation_customBacking_appendData),
+//            ("test_validateMutation_customBacking_appendBuffer", test_validateMutation_customBacking_appendBuffer),
+//            ("test_validateMutation_customBacking_appendSequence", test_validateMutation_customBacking_appendSequence),
+//            ("test_validateMutation_customBacking_appendContentsOf", test_validateMutation_customBacking_appendContentsOf),
+//            ("test_validateMutation_customBacking_resetBytes", test_validateMutation_customBacking_resetBytes),
+//            ("test_validateMutation_customBacking_replaceSubrange", test_validateMutation_customBacking_replaceSubrange),
+//            ("test_validateMutation_customBacking_replaceSubrangeCountableRange", test_validateMutation_customBacking_replaceSubrangeCountableRange),
+//            ("test_validateMutation_customBacking_replaceSubrangeWithBuffer", test_validateMutation_customBacking_replaceSubrangeWithBuffer),
+//            ("test_validateMutation_customBacking_replaceSubrangeWithCollection", test_validateMutation_customBacking_replaceSubrangeWithCollection),
+//            ("test_validateMutation_customBacking_replaceSubrangeWithBytes", test_validateMutation_customBacking_replaceSubrangeWithBytes),
+//            ("test_validateMutation_slice_customBacking_withUnsafeMutableBytes", test_validateMutation_slice_customBacking_withUnsafeMutableBytes),
+//            ("test_validateMutation_slice_customBacking_appendBytes", test_validateMutation_slice_customBacking_appendBytes),
+//            ("test_validateMutation_slice_customBacking_appendData", test_validateMutation_slice_customBacking_appendData),
+//            ("test_validateMutation_slice_customBacking_appendBuffer", test_validateMutation_slice_customBacking_appendBuffer),
+//            ("test_validateMutation_slice_customBacking_appendSequence", test_validateMutation_slice_customBacking_appendSequence),
+//            ("test_validateMutation_slice_customBacking_appendContentsOf", test_validateMutation_slice_customBacking_appendContentsOf),
+//            ("test_validateMutation_slice_customBacking_resetBytes", test_validateMutation_slice_customBacking_resetBytes),
+//            ("test_validateMutation_slice_customBacking_replaceSubrange", test_validateMutation_slice_customBacking_replaceSubrange),
+//            ("test_validateMutation_slice_customBacking_replaceSubrangeCountableRange", test_validateMutation_slice_customBacking_replaceSubrangeCountableRange),
+//            ("test_validateMutation_slice_customBacking_replaceSubrangeWithBuffer", test_validateMutation_slice_customBacking_replaceSubrangeWithBuffer),
+//            ("test_validateMutation_slice_customBacking_replaceSubrangeWithCollection", test_validateMutation_slice_customBacking_replaceSubrangeWithCollection),
+//            ("test_validateMutation_slice_customBacking_replaceSubrangeWithBytes", test_validateMutation_slice_customBacking_replaceSubrangeWithBytes),
+//            ("test_validateMutation_cow_customBacking_withUnsafeMutableBytes", test_validateMutation_cow_customBacking_withUnsafeMutableBytes),
+//            ("test_validateMutation_cow_customBacking_appendBytes", test_validateMutation_cow_customBacking_appendBytes),
+//            ("test_validateMutation_cow_customBacking_appendData", test_validateMutation_cow_customBacking_appendData),
+//            ("test_validateMutation_cow_customBacking_appendBuffer", test_validateMutation_cow_customBacking_appendBuffer),
+//            ("test_validateMutation_cow_customBacking_appendSequence", test_validateMutation_cow_customBacking_appendSequence),
+//            ("test_validateMutation_cow_customBacking_appendContentsOf", test_validateMutation_cow_customBacking_appendContentsOf),
+//            ("test_validateMutation_cow_customBacking_resetBytes", test_validateMutation_cow_customBacking_resetBytes),
+//            ("test_validateMutation_cow_customBacking_replaceSubrange", test_validateMutation_cow_customBacking_replaceSubrange),
+//            ("test_validateMutation_cow_customBacking_replaceSubrangeCountableRange", test_validateMutation_cow_customBacking_replaceSubrangeCountableRange),
+//            ("test_validateMutation_cow_customBacking_replaceSubrangeWithBuffer", test_validateMutation_cow_customBacking_replaceSubrangeWithBuffer),
+//            ("test_validateMutation_cow_customBacking_replaceSubrangeWithCollection", test_validateMutation_cow_customBacking_replaceSubrangeWithCollection),
+//            ("test_validateMutation_cow_customBacking_replaceSubrangeWithBytes", test_validateMutation_cow_customBacking_replaceSubrangeWithBytes),
+//            ("test_validateMutation_slice_cow_customBacking_withUnsafeMutableBytes", test_validateMutation_slice_cow_customBacking_withUnsafeMutableBytes),
+//            ("test_validateMutation_slice_cow_customBacking_appendBytes", test_validateMutation_slice_cow_customBacking_appendBytes),
+//            ("test_validateMutation_slice_cow_customBacking_appendData", test_validateMutation_slice_cow_customBacking_appendData),
+//            ("test_validateMutation_slice_cow_customBacking_appendBuffer", test_validateMutation_slice_cow_customBacking_appendBuffer),
+//            ("test_validateMutation_slice_cow_customBacking_appendSequence", test_validateMutation_slice_cow_customBacking_appendSequence),
+//            ("test_validateMutation_slice_cow_customBacking_appendContentsOf", test_validateMutation_slice_cow_customBacking_appendContentsOf),
+//            ("test_validateMutation_slice_cow_customBacking_resetBytes", test_validateMutation_slice_cow_customBacking_resetBytes),
+//            ("test_validateMutation_slice_cow_customBacking_replaceSubrange", test_validateMutation_slice_cow_customBacking_replaceSubrange),
+//            ("test_validateMutation_slice_cow_customBacking_replaceSubrangeCountableRange", test_validateMutation_slice_cow_customBacking_replaceSubrangeCountableRange),
+//            ("test_validateMutation_slice_cow_customBacking_replaceSubrangeWithBuffer", test_validateMutation_slice_cow_customBacking_replaceSubrangeWithBuffer),
+//            ("test_validateMutation_slice_cow_customBacking_replaceSubrangeWithCollection", test_validateMutation_slice_cow_customBacking_replaceSubrangeWithCollection),
+//            ("test_validateMutation_slice_cow_customBacking_replaceSubrangeWithBytes", test_validateMutation_slice_cow_customBacking_replaceSubrangeWithBytes),
+//            ("test_validateMutation_customMutableBacking_withUnsafeMutableBytes", test_validateMutation_customMutableBacking_withUnsafeMutableBytes),
+//            ("test_validateMutation_customMutableBacking_appendBytes", test_validateMutation_customMutableBacking_appendBytes),
+//            ("test_validateMutation_customMutableBacking_appendData", test_validateMutation_customMutableBacking_appendData),
+//            ("test_validateMutation_customMutableBacking_appendBuffer", test_validateMutation_customMutableBacking_appendBuffer),
+//            ("test_validateMutation_customMutableBacking_appendSequence", test_validateMutation_customMutableBacking_appendSequence),
+//            ("test_validateMutation_customMutableBacking_appendContentsOf", test_validateMutation_customMutableBacking_appendContentsOf),
+//            ("test_validateMutation_customMutableBacking_resetBytes", test_validateMutation_customMutableBacking_resetBytes),
+//            ("test_validateMutation_customMutableBacking_replaceSubrange", test_validateMutation_customMutableBacking_replaceSubrange),
+//            ("test_validateMutation_customMutableBacking_replaceSubrangeCountableRange", test_validateMutation_customMutableBacking_replaceSubrangeCountableRange),
+//            ("test_validateMutation_customMutableBacking_replaceSubrangeWithBuffer", test_validateMutation_customMutableBacking_replaceSubrangeWithBuffer),
+//            ("test_validateMutation_customMutableBacking_replaceSubrangeWithCollection", test_validateMutation_customMutableBacking_replaceSubrangeWithCollection),
+//            ("test_validateMutation_customMutableBacking_replaceSubrangeWithBytes", test_validateMutation_customMutableBacking_replaceSubrangeWithBytes),
+//            ("test_validateMutation_slice_customMutableBacking_withUnsafeMutableBytes", test_validateMutation_slice_customMutableBacking_withUnsafeMutableBytes),
+//            ("test_validateMutation_slice_customMutableBacking_appendBytes", test_validateMutation_slice_customMutableBacking_appendBytes),
+//            ("test_validateMutation_slice_customMutableBacking_appendData", test_validateMutation_slice_customMutableBacking_appendData),
+//            ("test_validateMutation_slice_customMutableBacking_appendBuffer", test_validateMutation_slice_customMutableBacking_appendBuffer),
+//            ("test_validateMutation_slice_customMutableBacking_appendSequence", test_validateMutation_slice_customMutableBacking_appendSequence),
+//            ("test_validateMutation_slice_customMutableBacking_appendContentsOf", test_validateMutation_slice_customMutableBacking_appendContentsOf),
+//            ("test_validateMutation_slice_customMutableBacking_resetBytes", test_validateMutation_slice_customMutableBacking_resetBytes),
+//            ("test_validateMutation_slice_customMutableBacking_replaceSubrange", test_validateMutation_slice_customMutableBacking_replaceSubrange),
+//            ("test_validateMutation_slice_customMutableBacking_replaceSubrangeCountableRange", test_validateMutation_slice_customMutableBacking_replaceSubrangeCountableRange),
+//            ("test_validateMutation_slice_customMutableBacking_replaceSubrangeWithBuffer", test_validateMutation_slice_customMutableBacking_replaceSubrangeWithBuffer),
+//            ("test_validateMutation_slice_customMutableBacking_replaceSubrangeWithCollection", test_validateMutation_slice_customMutableBacking_replaceSubrangeWithCollection),
+//            ("test_validateMutation_slice_customMutableBacking_replaceSubrangeWithBytes", test_validateMutation_slice_customMutableBacking_replaceSubrangeWithBytes),
+//            ("test_validateMutation_cow_customMutableBacking_withUnsafeMutableBytes", test_validateMutation_cow_customMutableBacking_withUnsafeMutableBytes),
+//            ("test_validateMutation_cow_customMutableBacking_appendBytes", test_validateMutation_cow_customMutableBacking_appendBytes),
+//            ("test_validateMutation_cow_customMutableBacking_appendData", test_validateMutation_cow_customMutableBacking_appendData),
+//            ("test_validateMutation_cow_customMutableBacking_appendBuffer", test_validateMutation_cow_customMutableBacking_appendBuffer),
+//            ("test_validateMutation_cow_customMutableBacking_appendSequence", test_validateMutation_cow_customMutableBacking_appendSequence),
+//            ("test_validateMutation_cow_customMutableBacking_appendContentsOf", test_validateMutation_cow_customMutableBacking_appendContentsOf),
+//            ("test_validateMutation_cow_customMutableBacking_resetBytes", test_validateMutation_cow_customMutableBacking_resetBytes),
+//            ("test_validateMutation_cow_customMutableBacking_replaceSubrange", test_validateMutation_cow_customMutableBacking_replaceSubrange),
+//            ("test_validateMutation_cow_customMutableBacking_replaceSubrangeCountableRange", test_validateMutation_cow_customMutableBacking_replaceSubrangeCountableRange),
+//            ("test_validateMutation_cow_customMutableBacking_replaceSubrangeWithBuffer", test_validateMutation_cow_customMutableBacking_replaceSubrangeWithBuffer),
+//            ("test_validateMutation_cow_customMutableBacking_replaceSubrangeWithCollection", test_validateMutation_cow_customMutableBacking_replaceSubrangeWithCollection),
+//            ("test_validateMutation_cow_customMutableBacking_replaceSubrangeWithBytes", test_validateMutation_cow_customMutableBacking_replaceSubrangeWithBytes),
+//            ("test_validateMutation_slice_cow_customMutableBacking_withUnsafeMutableBytes", test_validateMutation_slice_cow_customMutableBacking_withUnsafeMutableBytes),
+//            ("test_validateMutation_slice_cow_customMutableBacking_appendBytes", test_validateMutation_slice_cow_customMutableBacking_appendBytes),
+//            ("test_validateMutation_slice_cow_customMutableBacking_appendData", test_validateMutation_slice_cow_customMutableBacking_appendData),
+//            ("test_validateMutation_slice_cow_customMutableBacking_appendBuffer", test_validateMutation_slice_cow_customMutableBacking_appendBuffer),
+//            ("test_validateMutation_slice_cow_customMutableBacking_appendSequence", test_validateMutation_slice_cow_customMutableBacking_appendSequence),
+//            ("test_validateMutation_slice_cow_customMutableBacking_appendContentsOf", test_validateMutation_slice_cow_customMutableBacking_appendContentsOf),
+//            ("test_validateMutation_slice_cow_customMutableBacking_resetBytes", test_validateMutation_slice_cow_customMutableBacking_resetBytes),
+//            ("test_validateMutation_slice_cow_customMutableBacking_replaceSubrange", test_validateMutation_slice_cow_customMutableBacking_replaceSubrange),
+//            ("test_validateMutation_slice_cow_customMutableBacking_replaceSubrangeCountableRange", test_validateMutation_slice_cow_customMutableBacking_replaceSubrangeCountableRange),
+//            ("test_validateMutation_slice_cow_customMutableBacking_replaceSubrangeWithBuffer", test_validateMutation_slice_cow_customMutableBacking_replaceSubrangeWithBuffer),
+//            ("test_validateMutation_slice_cow_customMutableBacking_replaceSubrangeWithCollection", test_validateMutation_slice_cow_customMutableBacking_replaceSubrangeWithCollection),
+//            ("test_validateMutation_slice_cow_customMutableBacking_replaceSubrangeWithBytes", test_validateMutation_slice_cow_customMutableBacking_replaceSubrangeWithBytes),
+            ("test_sliceHash", test_sliceHash),
+            ("test_slice_resize_growth", test_slice_resize_growth),
+//            ("test_sliceEnumeration", test_sliceEnumeration),
         ]
     }
     
@@ -1164,5 +1572,2385 @@
         XCTAssertEqual(found[0], 2)
         XCTAssertEqual(found[1], 3)
     }
+
+        func test_validateMutation_withUnsafeMutableBytes() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+        data.withUnsafeMutableBytes { (ptr: UnsafeMutablePointer<UInt8>) in
+            ptr.advanced(by: 5).pointee = 0xFF
+        }
+        XCTAssertEqual(data, Data(bytes: [0, 1, 2, 3, 4, 0xFF, 6, 7, 8, 9]))
+    }
+    
+    func test_validateMutation_appendBytes() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+        data.append("hello", count: 5)
+        XCTAssertEqual(data[data.startIndex.advanced(by: 5)], 0x5)
+    }
+    
+    func test_validateMutation_appendData() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+        let other = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+        data.append(other)
+        XCTAssertEqual(data[data.startIndex.advanced(by: 9)], 9)
+        XCTAssertEqual(data[data.startIndex.advanced(by: 10)], 0)
+    }
+    
+    func test_validateMutation_appendBuffer() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+        let bytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+        bytes.withUnsafeBufferPointer { data.append($0) }
+        XCTAssertEqual(data[data.startIndex.advanced(by: 9)], 9)
+        XCTAssertEqual(data[data.startIndex.advanced(by: 10)], 0)
+    }
+    
+    func test_validateMutation_appendSequence() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+        let seq = repeatElement(UInt8(1), count: 10)
+        data.append(contentsOf: seq)
+        XCTAssertEqual(data[data.startIndex.advanced(by: 9)], 9)
+        XCTAssertEqual(data[data.startIndex.advanced(by: 10)], 1)
+    }
+    
+    func test_validateMutation_appendContentsOf() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+        let bytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+        data.append(contentsOf: bytes)
+        XCTAssertEqual(data[data.startIndex.advanced(by: 9)], 9)
+        XCTAssertEqual(data[data.startIndex.advanced(by: 10)], 0)
+    }
+    
+    func test_validateMutation_resetBytes() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+        data.resetBytes(in: 5..<8)
+        XCTAssertEqual(data, Data(bytes: [0, 1, 2, 3, 4, 0, 0, 0, 8, 9]))
+    }
+    
+    func test_validateMutation_replaceSubrange() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+        let range: Range<Data.Index> = data.startIndex.advanced(by: 4)..<data.startIndex.advanced(by: 9)
+        let replacement = Data(bytes: [0xFF, 0xFF])
+        data.replaceSubrange(range, with: replacement)
+        XCTAssertEqual(data, Data(bytes: [0, 1, 2, 3, 0xFF, 0xFF, 9]))
+    }
+    
+    func test_validateMutation_replaceSubrangeCountableRange() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+        let range: CountableRange<Data.Index> = data.startIndex.advanced(by: 4)..<data.startIndex.advanced(by: 9)
+        let replacement = Data(bytes: [0xFF, 0xFF])
+        data.replaceSubrange(range, with: replacement)
+        XCTAssertEqual(data, Data(bytes: [0, 1, 2, 3, 0xFF, 0xFF, 9]))
+    }
+    
+    func test_validateMutation_replaceSubrangeWithBuffer() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+        let range: Range<Data.Index> = data.startIndex.advanced(by: 4)..<data.startIndex.advanced(by: 9)
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBufferPointer {
+            data.replaceSubrange(range, with: $0)
+        }
+        XCTAssertEqual(data, Data(bytes: [0, 1, 2, 3, 0xFF, 0xFF, 9]))
+    }
+    
+    func test_validateMutation_replaceSubrangeWithCollection() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+        let range: Range<Data.Index> = data.startIndex.advanced(by: 4)..<data.startIndex.advanced(by: 9)
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        data.replaceSubrange(range, with: bytes)
+        XCTAssertEqual(data, Data(bytes: [0, 1, 2, 3, 0xFF, 0xFF, 9]))
+    }
+    
+    func test_validateMutation_replaceSubrangeWithBytes() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+        let range: Range<Data.Index> = data.startIndex.advanced(by: 4)..<data.startIndex.advanced(by: 9)
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBytes {
+            data.replaceSubrange(range, with: $0.baseAddress!, count: 2)
+        }
+        XCTAssertEqual(data, Data(bytes: [0, 1, 2, 3, 0xFF, 0xFF, 9]))
+    }
+    
+    func test_validateMutation_slice_withUnsafeMutableBytes() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])[4..<9]
+        data.withUnsafeMutableBytes { (ptr: UnsafeMutablePointer<UInt8>) in
+            ptr.advanced(by: 1).pointee = 0xFF
+        }
+        XCTAssertEqual(data, Data(bytes: [4, 0xFF, 6, 7, 8]))
+    }
+    
+    func test_validateMutation_slice_appendBytes() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])[4..<9]
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBufferPointer { data.append($0.baseAddress!, count: $0.count) }
+        XCTAssertEqual(data, Data(bytes: [4, 5, 6, 7, 8, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_slice_appendData() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])[4..<9]
+        let other = Data(bytes: [0xFF, 0xFF])
+        data.append(other)
+        XCTAssertEqual(data, Data(bytes: [4, 5, 6, 7, 8, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_slice_appendBuffer() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])[4..<9]
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBufferPointer { data.append($0) }
+        XCTAssertEqual(data, Data(bytes: [4, 5, 6, 7, 8, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_slice_appendSequence() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])[4..<9]
+        let seq = repeatElement(UInt8(0xFF), count: 2)
+        data.append(contentsOf: seq)
+        XCTAssertEqual(data, Data(bytes: [4, 5, 6, 7, 8, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_slice_appendContentsOf() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])[4..<9]
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        data.append(contentsOf: bytes)
+        XCTAssertEqual(data, Data(bytes: [4, 5, 6, 7, 8, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_slice_resetBytes() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])[4..<9]
+        data.resetBytes(in: 5..<8)
+        XCTAssertEqual(data, Data(bytes: [4, 0, 0, 0, 8]))
+    }
+    
+    func test_validateMutation_slice_replaceSubrange() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])[4..<9]
+        let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        let replacement = Data(bytes: [0xFF, 0xFF])
+        data.replaceSubrange(range, with: replacement)
+        XCTAssertEqual(data, Data(bytes: [4, 0xFF, 0xFF, 8]))
+    }
+    
+    func test_validateMutation_slice_replaceSubrangeCountableRange() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])[4..<9]
+        let range: CountableRange<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        let replacement = Data(bytes: [0xFF, 0xFF])
+        data.replaceSubrange(range, with: replacement)
+        XCTAssertEqual(data, Data(bytes: [4, 0xFF, 0xFF, 8]))
+    }
+    
+    func test_validateMutation_slice_replaceSubrangeWithBuffer() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])[4..<9]
+        let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBufferPointer {
+            data.replaceSubrange(range, with: $0)
+        }
+        XCTAssertEqual(data, Data(bytes: [4, 0xFF, 0xFF, 8]))
+    }
+    
+    func test_validateMutation_slice_replaceSubrangeWithCollection() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])[4..<9]
+        let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        data.replaceSubrange(range, with: bytes)
+        XCTAssertEqual(data, Data(bytes: [4, 0xFF, 0xFF, 8]))
+    }
+    
+    func test_validateMutation_slice_replaceSubrangeWithBytes() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])[4..<9]
+        let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBytes {
+            data.replaceSubrange(range, with: $0.baseAddress!, count: 2)
+        }
+        XCTAssertEqual(data, Data(bytes: [4, 0xFF, 0xFF, 8]))
+    }
+    
+    func test_validateMutation_cow_withUnsafeMutableBytes() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+        holdReference(data) {
+            data.withUnsafeMutableBytes { (ptr: UnsafeMutablePointer<UInt8>) in
+                ptr.advanced(by: 5).pointee = 0xFF
+            }
+            XCTAssertEqual(data, Data(bytes: [0, 1, 2, 3, 4, 0xFF, 6, 7, 8, 9]))
+        }
+    }
+    
+    func test_validateMutation_cow_appendBytes() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+        holdReference(data) {
+            data.append("hello", count: 5)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 9)], 0x9)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 10)], 0x68)
+        }
+    }
+    
+    func test_validateMutation_cow_appendData() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+        holdReference(data) {
+            let other = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+            data.append(other)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 9)], 9)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 10)], 0)
+        }
+    }
+    
+    func test_validateMutation_cow_appendBuffer() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+        holdReference(data) {
+            let bytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+            bytes.withUnsafeBufferPointer { data.append($0) }
+            XCTAssertEqual(data[data.startIndex.advanced(by: 9)], 9)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 10)], 0)
+        }
+    }
+    
+    func test_validateMutation_cow_appendSequence() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+        holdReference(data) {
+            let seq = repeatElement(UInt8(1), count: 10)
+            data.append(contentsOf: seq)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 9)], 9)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 10)], 1)
+        }
+    }
+    
+    func test_validateMutation_cow_appendContentsOf() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+        holdReference(data) {
+            let bytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+            data.append(contentsOf: bytes)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 9)], 9)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 10)], 0)
+        }
+    }
+    
+    func test_validateMutation_cow_resetBytes() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+        holdReference(data) {
+            data.resetBytes(in: 5..<8)
+            XCTAssertEqual(data, Data(bytes: [0, 1, 2, 3, 4, 0, 0, 0, 8, 9]))
+        }
+    }
+    
+    func test_validateMutation_cow_replaceSubrange() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+        holdReference(data) {
+            let range: Range<Data.Index> = data.startIndex.advanced(by: 4)..<data.startIndex.advanced(by: 9)
+            let replacement = Data(bytes: [0xFF, 0xFF])
+            data.replaceSubrange(range, with: replacement)
+            XCTAssertEqual(data, Data(bytes: [0, 1, 2, 3, 0xFF, 0xFF, 9]))
+        }
+    }
+    
+    func test_validateMutation_cow_replaceSubrangeCountableRange() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+        holdReference(data) {
+            let range: CountableRange<Data.Index> = data.startIndex.advanced(by: 4)..<data.startIndex.advanced(by: 9)
+            let replacement = Data(bytes: [0xFF, 0xFF])
+            data.replaceSubrange(range, with: replacement)
+            XCTAssertEqual(data, Data(bytes: [0, 1, 2, 3, 0xFF, 0xFF, 9]))
+        }
+    }
+    
+    func test_validateMutation_cow_replaceSubrangeWithBuffer() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+        holdReference(data) {
+            let range: Range<Data.Index> = data.startIndex.advanced(by: 4)..<data.startIndex.advanced(by: 9)
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            bytes.withUnsafeBufferPointer {
+                data.replaceSubrange(range, with: $0)
+            }
+            XCTAssertEqual(data, Data(bytes: [0, 1, 2, 3, 0xFF, 0xFF, 9]))
+        }
+    }
+    
+    func test_validateMutation_cow_replaceSubrangeWithCollection() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+        holdReference(data) {
+            let range: Range<Data.Index> = data.startIndex.advanced(by: 4)..<data.startIndex.advanced(by: 9)
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            data.replaceSubrange(range, with: bytes)
+            XCTAssertEqual(data, Data(bytes: [0, 1, 2, 3, 0xFF, 0xFF, 9]))
+        }
+    }
+    
+    func test_validateMutation_cow_replaceSubrangeWithBytes() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+        holdReference(data) {
+            let range: Range<Data.Index> = data.startIndex.advanced(by: 4)..<data.startIndex.advanced(by: 9)
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            bytes.withUnsafeBytes {
+                data.replaceSubrange(range, with: $0.baseAddress!, count: 2)
+            }
+            XCTAssertEqual(data, Data(bytes: [0, 1, 2, 3, 0xFF, 0xFF, 9]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_withUnsafeMutableBytes() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])[4..<9]
+        holdReference(data) {
+            data.withUnsafeMutableBytes { (ptr: UnsafeMutablePointer<UInt8>) in
+                ptr.advanced(by: 1).pointee = 0xFF
+            }
+            XCTAssertEqual(data, Data(bytes: [4, 0xFF, 6, 7, 8]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_appendBytes() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])[4..<9]
+        holdReference(data) {
+            data.append("hello", count: 5)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 4)], 0x8)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 5)], 0x68)
+        }
+    }
+    
+    func test_validateMutation_slice_cow_appendData() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])[4..<9]
+        holdReference(data) {
+            let other = Data(bytes: [0xFF, 0xFF])
+            data.append(other)
+            XCTAssertEqual(data, Data(bytes: [4, 5, 6, 7, 8, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_appendBuffer() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])[4..<9]
+        holdReference(data) {
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            bytes.withUnsafeBufferPointer { data.append($0) }
+            XCTAssertEqual(data, Data(bytes: [4, 5, 6, 7, 8, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_appendSequence() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])[4..<9]
+        holdReference(data) {
+            let seq = repeatElement(UInt8(0xFF), count: 2)
+            data.append(contentsOf: seq)
+            XCTAssertEqual(data, Data(bytes: [4, 5, 6, 7, 8, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_appendContentsOf() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])[4..<9]
+        holdReference(data) {
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            data.append(contentsOf: bytes)
+            XCTAssertEqual(data, Data(bytes: [4, 5, 6, 7, 8, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_resetBytes() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])[4..<9]
+        holdReference(data) {
+            data.resetBytes(in: 5..<8)
+            XCTAssertEqual(data, Data(bytes: [4, 0, 0, 0, 8]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_replaceSubrange() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])[4..<9]
+        holdReference(data) {
+            let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            let replacement = Data(bytes: [0xFF, 0xFF])
+            data.replaceSubrange(range, with: replacement)
+            XCTAssertEqual(data, Data(bytes: [4, 0xFF, 0xFF, 8]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_replaceSubrangeCountableRange() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])[4..<9]
+        holdReference(data) {
+            let range: CountableRange<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            let replacement = Data(bytes: [0xFF, 0xFF])
+            data.replaceSubrange(range, with: replacement)
+            XCTAssertEqual(data, Data(bytes: [4, 0xFF, 0xFF, 8]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_replaceSubrangeWithBuffer() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])[4..<9]
+        holdReference(data) {
+            let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            bytes.withUnsafeBufferPointer {
+                data.replaceSubrange(range, with: $0)
+            }
+            XCTAssertEqual(data, Data(bytes: [4, 0xFF, 0xFF, 8]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_replaceSubrangeWithCollection() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])[4..<9]
+        holdReference(data) {
+            let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            data.replaceSubrange(range, with: bytes)
+            XCTAssertEqual(data, Data(bytes: [4, 0xFF, 0xFF, 8]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_replaceSubrangeWithBytes() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])[4..<9]
+        holdReference(data) {
+            let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            bytes.withUnsafeBytes {
+                data.replaceSubrange(range, with: $0.baseAddress!, count: 2)
+            }
+            XCTAssertEqual(data, Data(bytes: [4, 0xFF, 0xFF, 8]))
+        }
+    }
+    
+    func test_validateMutation_immutableBacking_withUnsafeMutableBytes() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        data.withUnsafeMutableBytes { (ptr: UnsafeMutablePointer<UInt8>) in
+            ptr.advanced(by: 5).pointee = 0xFF
+        }
+        XCTAssertEqual(data[data.startIndex.advanced(by: 5)], 0xFF)
+    }
+    
+    func test_validateMutation_immutableBacking_appendBytes() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        data.append("hello", count: 5)
+        XCTAssertEqual(data[data.startIndex.advanced(by: 10)], 0x64)
+        XCTAssertEqual(data[data.startIndex.advanced(by: 11)], 0x68)
+    }
+    
+    func test_validateMutation_immutableBacking_appendData() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        let other = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+        data.append(other)
+        XCTAssertEqual(data[data.startIndex.advanced(by: 10)], 0x64)
+        XCTAssertEqual(data[data.startIndex.advanced(by: 11)], 0)
+    }
+    
+    func test_validateMutation_immutableBacking_appendBuffer() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        let bytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+        bytes.withUnsafeBufferPointer { data.append($0) }
+        XCTAssertEqual(data[data.startIndex.advanced(by: 10)], 0x64)
+        XCTAssertEqual(data[data.startIndex.advanced(by: 11)], 0)
+    }
+    
+    func test_validateMutation_immutableBacking_appendSequence() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        let seq = repeatElement(UInt8(1), count: 10)
+        data.append(contentsOf: seq)
+        XCTAssertEqual(data[data.startIndex.advanced(by: 10)], 0x64)
+        XCTAssertEqual(data[data.startIndex.advanced(by: 11)], 1)
+    }
+    
+    func test_validateMutation_immutableBacking_appendContentsOf() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        let bytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+        data.append(contentsOf: bytes)
+        XCTAssertEqual(data[data.startIndex.advanced(by: 10)], 0x64)
+        XCTAssertEqual(data[data.startIndex.advanced(by: 11)], 0)
+    }
+    
+    func test_validateMutation_immutableBacking_resetBytes() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        data.resetBytes(in: 5..<8)
+        XCTAssertEqual(data, Data(bytes: [0x68, 0x65, 0x6c, 0x6c, 0x6f, 0x00, 0x00, 0x00, 0x72, 0x6c, 0x64]))
+    }
+    
+    func test_validateMutation_immutableBacking_replaceSubrange() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        let range: Range<Data.Index> = data.startIndex.advanced(by: 4)..<data.startIndex.advanced(by: 9)
+        let replacement = Data(bytes: [0xFF, 0xFF])
+        data.replaceSubrange(range, with: replacement)
+        XCTAssertEqual(data, Data(bytes: [0x68, 0x65, 0x6c, 0x6c, 0xFF, 0xFF, 0x6c, 0x64]))
+    }
+    
+    func test_validateMutation_immutableBacking_replaceSubrangeCountableRange() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        let range: CountableRange<Data.Index> = data.startIndex.advanced(by: 4)..<data.startIndex.advanced(by: 9)
+        let replacement = Data(bytes: [0xFF, 0xFF])
+        data.replaceSubrange(range, with: replacement)
+        XCTAssertEqual(data, Data(bytes: [0x68, 0x65, 0x6c, 0x6c, 0xFF, 0xFF, 0x6c, 0x64]))
+    }
+    
+    func test_validateMutation_immutableBacking_replaceSubrangeWithBuffer() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        let range: Range<Data.Index> = data.startIndex.advanced(by: 4)..<data.startIndex.advanced(by: 9)
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBufferPointer {
+            data.replaceSubrange(range, with: $0)
+        }
+        XCTAssertEqual(data, Data(bytes: [0x68, 0x65, 0x6c, 0x6c, 0xFF, 0xFF, 0x6c, 0x64]))
+    }
+    
+    func test_validateMutation_immutableBacking_replaceSubrangeWithCollection() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        let range: Range<Data.Index> = data.startIndex.advanced(by: 4)..<data.startIndex.advanced(by: 9)
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        data.replaceSubrange(range, with: bytes)
+        XCTAssertEqual(data, Data(bytes: [0x68, 0x65, 0x6c, 0x6c, 0xFF, 0xFF, 0x6c, 0x64]))
+    }
+    
+    func test_validateMutation_immutableBacking_replaceSubrangeWithBytes() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        let range: Range<Data.Index> = data.startIndex.advanced(by: 4)..<data.startIndex.advanced(by: 9)
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        data.replaceSubrange(range, with: bytes)
+        XCTAssertEqual(data, Data(bytes: [0x68, 0x65, 0x6c, 0x6c, 0xFF, 0xFF, 0x6c, 0x64]))
+    }
+    
+    func test_validateMutation_slice_immutableBacking_withUnsafeMutableBytes() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))[4..<9]
+        data.withUnsafeMutableBytes { (ptr: UnsafeMutablePointer<UInt8>) in
+            ptr.advanced(by: 1).pointee = 0xFF
+        }
+        XCTAssertEqual(data[data.startIndex.advanced(by: 1)], 0xFF)
+    }
+    
+    func test_validateMutation_slice_immutableBacking_appendBytes() {
+        let base: [UInt8] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+        var data = base.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))[4..<9]
+        }
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBufferPointer { data.append($0.baseAddress!, count: $0.count) }
+        XCTAssertEqual(data, Data(bytes: [4, 5, 6, 7, 8, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_slice_immutableBacking_appendData() {
+        let base: [UInt8] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+        var data = base.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))[4..<9]
+        }
+        data.append(Data(bytes: [0xFF, 0xFF]))
+        XCTAssertEqual(data, Data(bytes: [4, 5, 6, 7, 8, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_slice_immutableBacking_appendBuffer() {
+        let base: [UInt8] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+        var data = base.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))[4..<9]
+        }
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBufferPointer { data.append($0) }
+        XCTAssertEqual(data, Data(bytes: [4, 5, 6, 7, 8, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_slice_immutableBacking_appendSequence() {
+        let base: [UInt8] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+        var data = base.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))[4..<9]
+        }
+        data.append(contentsOf: repeatElement(UInt8(0xFF), count: 2))
+        XCTAssertEqual(data, Data(bytes: [4, 5, 6, 7, 8, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_slice_immutableBacking_appendContentsOf() {
+        let base: [UInt8] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+        var data = base.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))[4..<9]
+        }
+        data.append(contentsOf: [0xFF, 0xFF])
+        XCTAssertEqual(data, Data(bytes: [4, 5, 6, 7, 8, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_slice_immutableBacking_resetBytes() {
+        let base: [UInt8] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+        var data = base.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))[4..<9]
+        }
+        data.resetBytes(in: 5..<8)
+        XCTAssertEqual(data, Data(bytes: [4, 0, 0, 0, 8]))
+    }
+    
+    func test_validateMutation_slice_immutableBacking_replaceSubrange() {
+        let base: [UInt8] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+        var data = base.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))[4..<9]
+        }
+        let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        data.replaceSubrange(range, with: Data(bytes: [0xFF, 0xFF]))
+        XCTAssertEqual(data, Data(bytes: [4, 0xFF, 0xFF, 8]))
+    }
+    
+    func test_validateMutation_slice_immutableBacking_replaceSubrangeCountableRange() {
+        let base: [UInt8] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+        var data = base.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))[4..<9]
+        }
+        let range: CountableRange<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        data.replaceSubrange(range, with: Data(bytes: [0xFF, 0xFF]))
+        XCTAssertEqual(data, Data(bytes: [4, 0xFF, 0xFF, 8]))
+    }
+    
+    func test_validateMutation_slice_immutableBacking_replaceSubrangeWithBuffer() {
+        let base: [UInt8] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+        var data = base.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))[4..<9]
+        }
+        let replacement: [UInt8] = [0xFF, 0xFF]
+        let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        replacement.withUnsafeBufferPointer { (buffer: UnsafeBufferPointer<UInt8>) in
+            data.replaceSubrange(range, with: buffer)
+        }
+        XCTAssertEqual(data, Data(bytes: [4, 0xFF, 0xFF, 8]))
+    }
+    
+    func test_validateMutation_slice_immutableBacking_replaceSubrangeWithCollection() {
+        let base: [UInt8] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+        var data = base.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))[4..<9]
+        }
+        let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        let replacement: [UInt8] = [0xFF, 0xFF]
+        data.replaceSubrange(range, with:replacement)
+        XCTAssertEqual(data, Data(bytes: [4, 0xFF, 0xFF, 8]))
+    }
+    
+    func test_validateMutation_slice_immutableBacking_replaceSubrangeWithBytes() {
+        let base: [UInt8] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+        var data = base.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))[4..<9]
+        }
+        let replacement: [UInt8] = [0xFF, 0xFF]
+        let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        replacement.withUnsafeBytes {
+            data.replaceSubrange(range, with: $0.baseAddress!, count: 2)
+        }
+        XCTAssertEqual(data, Data(bytes: [4, 0xFF, 0xFF, 8]))
+    }
+    
+    func test_validateMutation_cow_immutableBacking_withUnsafeMutableBytes() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        holdReference(data) {
+            data.withUnsafeMutableBytes { (ptr: UnsafeMutablePointer<UInt8>) in
+                ptr.advanced(by: 5).pointee = 0xFF
+            }
+            XCTAssertEqual(data[data.startIndex.advanced(by: 5)], 0xFF)
+        }
+    }
+    
+    func test_validateMutation_cow_immutableBacking_appendBytes() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        holdReference(data) {
+            data.append("hello", count: 5)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 10)], 0x64)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 11)], 0x68)
+        }
+    }
+    
+    func test_validateMutation_cow_immutableBacking_appendData() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        holdReference(data) {
+            let other = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+            data.append(other)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 10)], 0x64)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 11)], 0)
+        }
+    }
+    
+    func test_validateMutation_cow_immutableBacking_appendBuffer() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        holdReference(data) {
+            let bytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+            bytes.withUnsafeBufferPointer { data.append($0) }
+            XCTAssertEqual(data[data.startIndex.advanced(by: 10)], 0x64)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 11)], 0)
+        }
+    }
+    
+    func test_validateMutation_cow_immutableBacking_appendSequence() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        holdReference(data) {
+            let seq = repeatElement(UInt8(1), count: 10)
+            data.append(contentsOf: seq)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 10)], 0x64)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 11)], 1)
+        }
+    }
+    
+    func test_validateMutation_cow_immutableBacking_appendContentsOf() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        holdReference(data) {
+            let bytes: [UInt8] = [1, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+            data.append(contentsOf: bytes)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 10)], 0x64)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 11)], 1)
+        }
+    }
+    
+    func test_validateMutation_cow_immutableBacking_resetBytes() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        holdReference(data) {
+            data.resetBytes(in: 5..<8)
+            XCTAssertEqual(data, Data(bytes: [0x68, 0x65, 0x6c, 0x6c, 0x6f, 0x00, 0x00, 0x00, 0x72, 0x6c, 0x64]))
+        }
+    }
+    
+    func test_validateMutation_cow_immutableBacking_replaceSubrange() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        holdReference(data) {
+            let range: Range<Data.Index> = data.startIndex.advanced(by: 4)..<data.startIndex.advanced(by: 9)
+            let replacement = Data(bytes: [0xFF, 0xFF])
+            data.replaceSubrange(range, with: replacement)
+            XCTAssertEqual(data, Data(bytes: [0x68, 0x65, 0x6c, 0x6c, 0xff, 0xff, 0x6c, 0x64]))
+        }
+    }
+    
+    func test_validateMutation_cow_immutableBacking_replaceSubrangeCountableRange() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        holdReference(data) {
+            let range: CountableRange<Data.Index> = data.startIndex.advanced(by: 4)..<data.startIndex.advanced(by: 9)
+            let replacement = Data(bytes: [0xFF, 0xFF])
+            data.replaceSubrange(range, with: replacement)
+            XCTAssertEqual(data, Data(bytes: [0x68, 0x65, 0x6c, 0x6c, 0xff, 0xff, 0x6c, 0x64]))
+        }
+    }
+    
+    func test_validateMutation_cow_immutableBacking_replaceSubrangeWithBuffer() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        holdReference(data) {
+            let replacement: [UInt8] = [0xFF, 0xFF]
+            let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            replacement.withUnsafeBufferPointer { (buffer: UnsafeBufferPointer<UInt8>) in
+                data.replaceSubrange(range, with: buffer)
+            }
+            XCTAssertEqual(data, Data(bytes: [0x68, 0xff, 0xff, 0x64]))
+        }
+    }
+    
+    func test_validateMutation_cow_immutableBacking_replaceSubrangeWithCollection() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        holdReference(data) {
+            let replacement: [UInt8] = [0xFF, 0xFF]
+            let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            data.replaceSubrange(range, with: replacement)
+            XCTAssertEqual(data, Data(bytes: [0x68, 0xff, 0xff, 0x64]))
+        }
+    }
+    
+    func test_validateMutation_cow_immutableBacking_replaceSubrangeWithBytes() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        holdReference(data) {
+            let replacement: [UInt8] = [0xFF, 0xFF]
+            let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            replacement.withUnsafeBytes {
+                data.replaceSubrange(range, with: $0.baseAddress!, count: 2)
+            }
+            XCTAssertEqual(data, Data(bytes: [0x68, 0xff, 0xff, 0x64]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_immutableBacking_withUnsafeMutableBytes() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))[4..<9]
+        holdReference(data) {
+            data.withUnsafeMutableBytes { (ptr: UnsafeMutablePointer<UInt8>) in
+                ptr.advanced(by: 1).pointee = 0xFF
+            }
+            XCTAssertEqual(data[data.startIndex.advanced(by: 1)], 0xFF)
+        }
+    }
+    
+    func test_validateMutation_slice_cow_immutableBacking_appendBytes() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+        var data = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))[4..<9]
+        }
+        holdReference(data) {
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            bytes.withUnsafeBufferPointer { data.append($0.baseAddress!, count: $0.count) }
+            XCTAssertEqual(data, Data(bytes: [4, 5, 6, 7, 8, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_immutableBacking_appendData() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+        var data = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))[4..<9]
+        }
+        holdReference(data) {
+            data.append(Data(bytes: [0xFF, 0xFF]))
+            XCTAssertEqual(data, Data(bytes: [4, 5, 6, 7, 8, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_immutableBacking_appendBuffer() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))[4..<9]
+        holdReference(data) {
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            bytes.withUnsafeBufferPointer{ data.append($0) }
+            XCTAssertEqual(data, Data(bytes: [0x6f, 0x20, 0x77, 0x6f, 0x72, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_immutableBacking_appendSequence() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+        var data = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))[4..<9]
+        }
+        holdReference(data) {
+            let bytes = repeatElement(UInt8(0xFF), count: 2)
+            data.append(contentsOf: bytes)
+            XCTAssertEqual(data, Data(bytes: [4, 5, 6, 7, 8, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_immutableBacking_appendContentsOf() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+        var data = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))[4..<9]
+        }
+        holdReference(data) {
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            data.append(contentsOf: bytes)
+            XCTAssertEqual(data, Data(bytes: [4, 5, 6, 7, 8, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_immutableBacking_resetBytes() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+        var data = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))[4..<9]
+        }
+        holdReference(data) {
+            data.resetBytes(in: 5..<8)
+            XCTAssertEqual(data, Data(bytes: [4, 0, 0, 0, 8]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_immutableBacking_replaceSubrange() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+        var data = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))[4..<9]
+        }
+        holdReference(data) {
+            let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            data.replaceSubrange(range, with: Data(bytes: [0xFF, 0xFF]))
+            XCTAssertEqual(data, Data(bytes: [4, 0xFF, 0xFF, 8]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_immutableBacking_replaceSubrangeCountableRange() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+        var data = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))[4..<9]
+        }
+        holdReference(data) {
+            let range: CountableRange<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            data.replaceSubrange(range, with: Data(bytes: [0xFF, 0xFF]))
+            XCTAssertEqual(data, Data(bytes: [4, 0xFF, 0xFF, 8]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_immutableBacking_replaceSubrangeWithBuffer() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+        var data = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))[4..<9]
+        }
+        holdReference(data) {
+            let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            bytes.withUnsafeBufferPointer { data.replaceSubrange(range, with: $0) }
+            XCTAssertEqual(data, Data(bytes: [4, 0xFF, 0xFF, 8]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_immutableBacking_replaceSubrangeWithCollection() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+        var data = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))[4..<9]
+        }
+        holdReference(data) {
+            let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            data.replaceSubrange(range, with: bytes)
+            XCTAssertEqual(data, Data(bytes: [4, 0xFF, 0xFF, 8]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_immutableBacking_replaceSubrangeWithBytes() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+        var data = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))[4..<9]
+        }
+        holdReference(data) {
+            let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            bytes.withUnsafeBytes { data.replaceSubrange(range, with: $0.baseAddress!, count: 2) }
+            XCTAssertEqual(data, Data(bytes: [4, 0xFF, 0xFF, 8]))
+        }
+    }
+    
+    func test_validateMutation_mutableBacking_withUnsafeMutableBytes() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6]
+        var data = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))
+        }
+        data.append(contentsOf: [7, 8, 9])
+        data.withUnsafeMutableBytes { (ptr: UnsafeMutablePointer<UInt8>) in
+            ptr.advanced(by: 5).pointee = 0xFF
+        }
+        XCTAssertEqual(data[data.startIndex.advanced(by: 5)], 0xFF)
+    }
+    
+    func test_validateMutation_mutableBacking_appendBytes() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6]
+        var data = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))
+        }
+        data.append(contentsOf: [7, 8, 9])
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBufferPointer { data.append($0.baseAddress!, count: $0.count) }
+        XCTAssertEqual(data, Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_mutableBacking_appendData() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6]
+        var data = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))
+        }
+        data.append(contentsOf: [7, 8, 9])
+        data.append(Data(bytes: [0xFF, 0xFF]))
+        XCTAssertEqual(data, Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_mutableBacking_appendBuffer() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6]
+        var data = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))
+        }
+        data.append(contentsOf: [7, 8, 9])
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBufferPointer { data.append($0) }
+        XCTAssertEqual(data, Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_mutableBacking_appendSequence() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6]
+        var data = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))
+        }
+        data.append(contentsOf: [7, 8, 9])
+        data.append(contentsOf: repeatElement(UInt8(0xFF), count: 2))
+        XCTAssertEqual(data, Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_mutableBacking_appendContentsOf() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6]
+        var data = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))
+        }
+        data.append(contentsOf: [7, 8, 9])
+        data.append(contentsOf: [0xFF, 0xFF])
+        XCTAssertEqual(data, Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_mutableBacking_resetBytes() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6]
+        var data = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))
+        }
+        data.append(contentsOf: [7, 8, 9])
+        data.resetBytes(in: 5..<8)
+        XCTAssertEqual(data, Data(bytes: [0, 1, 2, 3, 4, 0, 0, 0, 8, 9]))
+    }
+    
+    func test_validateMutation_mutableBacking_replaceSubrange() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6]
+        var data = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))
+        }
+        data.append(contentsOf: [7, 8, 9])
+        let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        let replacement = Data(bytes: [0xFF, 0xFF])
+        data.replaceSubrange(range, with: replacement)
+        XCTAssertEqual(data, Data(bytes: [0, 0xFF, 0xFF, 9]))
+    }
+    
+    func test_validateMutation_mutableBacking_replaceSubrangeCountableRange() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6]
+        var data = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))
+        }
+        data.append(contentsOf: [7, 8, 9])
+        let range: CountableRange<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        let replacement = Data(bytes: [0xFF, 0xFF])
+        data.replaceSubrange(range, with: replacement)
+        XCTAssertEqual(data, Data(bytes: [0, 0xFF, 0xFF, 9]))
+    }
+    
+    func test_validateMutation_mutableBacking_replaceSubrangeWithBuffer() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6]
+        var data = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))
+        }
+        data.append(contentsOf: [7, 8, 9])
+        let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBufferPointer {
+            data.replaceSubrange(range, with: $0)
+        }
+        XCTAssertEqual(data, Data(bytes: [0, 0xFF, 0xFF, 9]))
+    }
+    
+    func test_validateMutation_mutableBacking_replaceSubrangeWithCollection() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6]
+        var data = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))
+        }
+        data.append(contentsOf: [7, 8, 9])
+        let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        data.replaceSubrange(range, with: [0xFF, 0xFF])
+        XCTAssertEqual(data, Data(bytes: [0, 0xFF, 0xFF, 9]))
+    }
+    
+    func test_validateMutation_mutableBacking_replaceSubrangeWithBytes() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6]
+        var data = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))
+        }
+        data.append(contentsOf: [7, 8, 9])
+        let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBytes {
+            data.replaceSubrange(range, with: $0.baseAddress!, count: $0.count)
+        }
+        XCTAssertEqual(data, Data(bytes: [0, 0xFF, 0xFF, 9]))
+    }
+    
+    func test_validateMutation_slice_mutableBacking_withUnsafeMutableBytes() {
+        var base = Data(referencing: NSData(bytes: "hello world", length: 11))
+        base.append(contentsOf: [1, 2, 3, 4, 5, 6])
+        var data = base[4..<9]
+        data.withUnsafeMutableBytes { (ptr: UnsafeMutablePointer<UInt8>) in
+            ptr.advanced(by: 1).pointee = 0xFF
+        }
+        XCTAssertEqual(data[data.startIndex.advanced(by: 1)], 0xFF)
+    }
+    
+    func test_validateMutation_slice_mutableBacking_appendBytes() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6]
+        var base = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))
+        }
+        base.append(contentsOf: [7, 8, 9])
+        var data = base[4..<9]
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBufferPointer { data.append($0.baseAddress!, count: $0.count) }
+        XCTAssertEqual(data, Data(bytes: [4, 5, 6, 7, 8, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_slice_mutableBacking_appendData() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6]
+        var base = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))
+        }
+        base.append(contentsOf: [7, 8, 9])
+        var data = base[4..<9]
+        data.append(Data(bytes: [0xFF, 0xFF]))
+        XCTAssertEqual(data, Data(bytes: [4, 5, 6, 7, 8, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_slice_mutableBacking_appendBuffer() {
+        var base = Data(referencing: NSData(bytes: "hello world", length: 11))
+        base.append(contentsOf: [1, 2, 3, 4, 5, 6])
+        var data = base[4..<9]
+        let bytes: [UInt8] = [1, 2, 3]
+        bytes.withUnsafeBufferPointer { data.append($0) }
+        XCTAssertEqual(data, Data(bytes: [0x6f, 0x20, 0x77, 0x6f, 0x72, 0x1, 0x2, 0x3]))
+    }
+    
+    func test_validateMutation_slice_mutableBacking_appendSequence() {
+        var base = Data(referencing: NSData(bytes: "hello world", length: 11))
+        base.append(contentsOf: [1, 2, 3, 4, 5, 6])
+        var data = base[4..<9]
+        let seq = repeatElement(UInt8(1), count: 3)
+        data.append(contentsOf: seq)
+        XCTAssertEqual(data, Data(bytes: [0x6f, 0x20, 0x77, 0x6f, 0x72, 0x1, 0x1, 0x1]))
+    }
+    
+    func test_validateMutation_slice_mutableBacking_appendContentsOf() {
+        var base = Data(referencing: NSData(bytes: "hello world", length: 11))
+        base.append(contentsOf: [1, 2, 3, 4, 5, 6])
+        var data = base[4..<9]
+        let bytes: [UInt8] = [1, 2, 3]
+        data.append(contentsOf: bytes)
+        XCTAssertEqual(data, Data(bytes: [0x6f, 0x20, 0x77, 0x6f, 0x72, 0x1, 0x2, 0x3]))
+    }
+    
+    func test_validateMutation_slice_mutableBacking_resetBytes() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6]
+        var base = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))
+        }
+        base.append(contentsOf: [7, 8, 9])
+        var data = base[4..<9]
+        data.resetBytes(in: 5..<8)
+        XCTAssertEqual(data, Data(bytes: [4, 0, 0, 0, 8]))
+    }
+    
+    func test_validateMutation_slice_mutableBacking_replaceSubrange() {
+        var base = Data(referencing: NSData(bytes: "hello world", length: 11))
+        base.append(contentsOf: [1, 2, 3, 4, 5, 6])
+        var data = base[4..<9]
+        let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        data.replaceSubrange(range, with: Data(bytes: [0xFF, 0xFF]))
+        XCTAssertEqual(data, Data(bytes: [0x6f, 0xFF, 0xFF, 0x72]))
+    }
+    
+    func test_validateMutation_slice_mutableBacking_replaceSubrangeCountableRange() {
+        var base = Data(referencing: NSData(bytes: "hello world", length: 11))
+        base.append(contentsOf: [1, 2, 3, 4, 5, 6])
+        var data = base[4..<9]
+        let range: CountableRange<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        data.replaceSubrange(range, with: Data(bytes: [0xFF, 0xFF]))
+        XCTAssertEqual(data, Data(bytes: [0x6f, 0xFF, 0xFF, 0x72]))
+    }
+    
+    func test_validateMutation_slice_mutableBacking_replaceSubrangeWithBuffer() {
+        var base = Data(referencing: NSData(bytes: "hello world", length: 11))
+        base.append(contentsOf: [1, 2, 3, 4, 5, 6])
+        var data = base[4..<9]
+        let replacement: [UInt8] = [0xFF, 0xFF]
+        let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        replacement.withUnsafeBufferPointer { (buffer: UnsafeBufferPointer<UInt8>) in
+            data.replaceSubrange(range, with: buffer)
+        }
+        XCTAssertEqual(data, Data(bytes: [0x6f, 0xFF, 0xFF, 0x72]))
+    }
+    
+    func test_validateMutation_slice_mutableBacking_replaceSubrangeWithCollection() {
+        var base = Data(referencing: NSData(bytes: "hello world", length: 11))
+        base.append(contentsOf: [1, 2, 3, 4, 5, 6])
+        var data = base[4..<9]
+        let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        let replacement: [UInt8] = [0xFF, 0xFF]
+        data.replaceSubrange(range, with:replacement)
+        XCTAssertEqual(data, Data(bytes: [0x6f, 0xFF, 0xFF, 0x72]))
+    }
+    
+    func test_validateMutation_slice_mutableBacking_replaceSubrangeWithBytes() {
+        var base = Data(referencing: NSData(bytes: "hello world", length: 11))
+        base.append(contentsOf: [1, 2, 3, 4, 5, 6])
+        var data = base[4..<9]
+        let replacement: [UInt8] = [0xFF, 0xFF]
+        let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        replacement.withUnsafeBytes {
+            data.replaceSubrange(range, with: $0.baseAddress!, count: 2)
+        }
+        XCTAssertEqual(data, Data(bytes: [0x6f, 0xFF, 0xFF, 0x72]))
+    }
+    
+    func test_validateMutation_cow_mutableBacking_withUnsafeMutableBytes() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        data.append(contentsOf: [1, 2, 3, 4, 5, 6])
+        holdReference(data) {
+            data.withUnsafeMutableBytes { (ptr: UnsafeMutablePointer<UInt8>) in
+                ptr.advanced(by: 5).pointee = 0xFF
+            }
+            XCTAssertEqual(data[data.startIndex.advanced(by: 5)], 0xFF)
+        }
+    }
+    
+    func test_validateMutation_cow_mutableBacking_appendBytes() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        data.append(contentsOf: [1, 2, 3, 4, 5, 6])
+        holdReference(data) {
+            data.append("hello", count: 5)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 16)], 6)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 17)], 0x68)
+        }
+    }
+    
+    func test_validateMutation_cow_mutableBacking_appendData() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        data.append(contentsOf: [1, 2, 3, 4, 5, 6])
+        holdReference(data) {
+            data.append("hello", count: 5)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 16)], 6)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 17)], 0x68)
+        }
+    }
+    
+    func test_validateMutation_cow_mutableBacking_appendBuffer() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        data.append(contentsOf: [1, 2, 3, 4, 5, 6])
+        holdReference(data) {
+            let other = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+            data.append(other)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 16)], 6)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 17)], 0)
+        }
+    }
+    
+    func test_validateMutation_cow_mutableBacking_appendSequence() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        data.append(contentsOf: [1, 2, 3, 4, 5, 6])
+        holdReference(data) {
+            let seq = repeatElement(UInt8(1), count: 10)
+            data.append(contentsOf: seq)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 16)], 6)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 17)], 1)
+        }
+    }
+    
+    func test_validateMutation_cow_mutableBacking_appendContentsOf() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        data.append(contentsOf: [1, 2, 3, 4, 5, 6])
+        holdReference(data) {
+            let bytes: [UInt8] = [1, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+            data.append(contentsOf: bytes)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 16)], 6)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 17)], 1)
+        }
+    }
+    
+    func test_validateMutation_cow_mutableBacking_resetBytes() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        data.append(contentsOf: [1, 2, 3, 4, 5, 6])
+        holdReference(data) {
+            data.resetBytes(in: 5..<8)
+            XCTAssertEqual(data, Data(bytes: [0x68, 0x65, 0x6c, 0x6c, 0x6f, 0x00, 0x00, 0x00, 0x72, 0x6c, 0x64, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06]))
+        }
+    }
+    
+    func test_validateMutation_cow_mutableBacking_replaceSubrange() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        data.append(contentsOf: [1, 2, 3, 4, 5, 6])
+        holdReference(data) {
+            let range: Range<Data.Index> = data.startIndex.advanced(by: 4)..<data.startIndex.advanced(by: 9)
+            let replacement = Data(bytes: [0xFF, 0xFF])
+            data.replaceSubrange(range, with: replacement)
+            XCTAssertEqual(data, Data(bytes: [0x68, 0x65, 0x6c, 0x6c, 0xff, 0xff, 0x6c, 0x64, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06]))
+        }
+    }
+    
+    func test_validateMutation_cow_mutableBacking_replaceSubrangeCountableRange() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        data.append(contentsOf: [1, 2, 3, 4, 5, 6])
+        holdReference(data) {
+            let range: CountableRange<Data.Index> = data.startIndex.advanced(by: 4)..<data.startIndex.advanced(by: 9)
+            let replacement = Data(bytes: [0xFF, 0xFF])
+            data.replaceSubrange(range, with: replacement)
+            XCTAssertEqual(data, Data(bytes: [0x68, 0x65, 0x6c, 0x6c, 0xff, 0xff, 0x6c, 0x64, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06]))
+        }
+    }
+    
+    func test_validateMutation_cow_mutableBacking_replaceSubrangeWithBuffer() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        data.append(contentsOf: [1, 2, 3, 4, 5, 6])
+        holdReference(data) {
+            let range: Range<Data.Index> = data.startIndex.advanced(by: 4)..<data.startIndex.advanced(by: 9)
+            let replacement: [UInt8] = [0xFF, 0xFF]
+            replacement.withUnsafeBufferPointer { (buffer: UnsafeBufferPointer<UInt8>) in
+                data.replaceSubrange(range, with: buffer)
+            }
+            XCTAssertEqual(data, Data(bytes: [0x68, 0x65, 0x6c, 0x6c, 0xff, 0xff, 0x6c, 0x64, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06]))
+        }
+    }
+    
+    func test_validateMutation_cow_mutableBacking_replaceSubrangeWithCollection() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        data.append(contentsOf: [1, 2, 3, 4, 5, 6])
+        holdReference(data) {
+            let replacement: [UInt8] = [0xFF, 0xFF]
+            let range: Range<Data.Index> = data.startIndex.advanced(by: 4)..<data.startIndex.advanced(by: 9)
+            data.replaceSubrange(range, with: replacement)
+            XCTAssertEqual(data, Data(bytes: [0x68, 0x65, 0x6c, 0x6c, 0xff, 0xff, 0x6c, 0x64, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06]))
+        }
+    }
+    
+    func test_validateMutation_cow_mutableBacking_replaceSubrangeWithBytes() {
+        var data = Data(referencing: NSData(bytes: "hello world", length: 11))
+        data.append(contentsOf: [1, 2, 3, 4, 5, 6])
+        holdReference(data) {
+            let replacement: [UInt8] = [0xFF, 0xFF]
+            let range: Range<Data.Index> = data.startIndex.advanced(by: 4)..<data.startIndex.advanced(by: 9)
+            replacement.withUnsafeBytes {
+                data.replaceSubrange(range, with: $0.baseAddress!, count: 2)
+            }
+            XCTAssertEqual(data, Data(bytes: [0x68, 0x65, 0x6c, 0x6c, 0xff, 0xff, 0x6c, 0x64, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_mutableBacking_withUnsafeMutableBytes() {
+        var base = Data(referencing: NSData(bytes: "hello world", length: 11))
+        base.append(contentsOf: [1, 2, 3, 4, 5, 6])
+        var data = base[4..<9]
+        holdReference(data) {
+            data.withUnsafeMutableBytes { (ptr: UnsafeMutablePointer<UInt8>) in
+                ptr.advanced(by: 1).pointee = 0xFF
+            }
+            XCTAssertEqual(data[data.startIndex.advanced(by: 1)], 0xFF)
+        }
+    }
+    
+    func test_validateMutation_slice_cow_mutableBacking_appendBytes() {
+        let bytes: [UInt8] = [0, 1, 2]
+        var base = bytes.withUnsafeBytes { (ptr) in
+            return Data(referencing: NSData(bytes: ptr.baseAddress!, length: ptr.count))
+        }
+        base.append(contentsOf: [3, 4, 5])
+        var data = base[1..<4]
+        holdReference(data) {
+            let bytesToAppend: [UInt8] = [6, 7, 8]
+            bytesToAppend.withUnsafeBytes { (ptr) in
+                data.append(ptr.baseAddress!.assumingMemoryBound(to: UInt8.self), count: ptr.count)
+            }
+            XCTAssertEqual(data, Data(bytes: [1, 2, 3, 6, 7, 8]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_mutableBacking_appendData() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6]
+        var base = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))
+        }
+        base.append(contentsOf: [7, 8, 9])
+        var data = base[4..<9]
+        holdReference(data) {
+            data.append(Data(bytes: [0xFF, 0xFF]))
+            XCTAssertEqual(data, Data(bytes: [4, 5, 6, 7, 8, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_mutableBacking_appendBuffer() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6]
+        var base = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))
+        }
+        base.append(contentsOf: [7, 8, 9])
+        var data = base[4..<9]
+        holdReference(data) {
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            bytes.withUnsafeBufferPointer{ data.append($0) }
+            XCTAssertEqual(data, Data(bytes: [4, 5, 6, 7, 8, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_mutableBacking_appendSequence() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6]
+        var base = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))
+        }
+        base.append(contentsOf: [7, 8, 9])
+        var data = base[4..<9]
+        holdReference(data) {
+            let bytes = repeatElement(UInt8(0xFF), count: 2)
+            data.append(contentsOf: bytes)
+            XCTAssertEqual(data, Data(bytes: [4, 5, 6, 7, 8, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_mutableBacking_appendContentsOf() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6]
+        var base = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))
+        }
+        base.append(contentsOf: [7, 8, 9])
+        var data = base[4..<9]
+        holdReference(data) {
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            data.append(contentsOf: bytes)
+            XCTAssertEqual(data, Data(bytes: [4, 5, 6, 7, 8, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_mutableBacking_resetBytes() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6]
+        var base = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))
+        }
+        base.append(contentsOf: [7, 8, 9])
+        var data = base[4..<9]
+        holdReference(data) {
+            data.resetBytes(in: 5..<8)
+            XCTAssertEqual(data, Data(bytes: [4, 0, 0, 0, 8]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_mutableBacking_replaceSubrange() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6]
+        var base = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))
+        }
+        base.append(contentsOf: [7, 8, 9])
+        var data = base[4..<9]
+        holdReference(data) {
+            let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            data.replaceSubrange(range, with: Data(bytes: [0xFF, 0xFF]))
+            XCTAssertEqual(data, Data(bytes: [4, 0xFF, 0xFF, 8]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_mutableBacking_replaceSubrangeCountableRange() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6]
+        var base = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))
+        }
+        base.append(contentsOf: [7, 8, 9])
+        var data = base[4..<9]
+        holdReference(data) {
+            let range: CountableRange<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            data.replaceSubrange(range, with: Data(bytes: [0xFF, 0xFF]))
+            XCTAssertEqual(data, Data(bytes: [4, 0xFF, 0xFF, 8]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_mutableBacking_replaceSubrangeWithBuffer() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6]
+        var base = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))
+        }
+        base.append(contentsOf: [7, 8, 9])
+        var data = base[4..<9]
+        holdReference(data) {
+            let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            bytes.withUnsafeBufferPointer { data.replaceSubrange(range, with: $0) }
+            XCTAssertEqual(data, Data(bytes: [4, 0xFF, 0xFF, 8]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_mutableBacking_replaceSubrangeWithCollection() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6]
+        var base = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))
+        }
+        base.append(contentsOf: [7, 8, 9])
+        var data = base[4..<9]
+        holdReference(data) {
+            let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            data.replaceSubrange(range, with: bytes)
+            XCTAssertEqual(data, Data(bytes: [4, 0xFF, 0xFF, 8]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_mutableBacking_replaceSubrangeWithBytes() {
+        let baseBytes: [UInt8] = [0, 1, 2, 3, 4, 5, 6]
+        var base = baseBytes.withUnsafeBufferPointer {
+            return Data(referencing: NSData(bytes: $0.baseAddress!, length: $0.count))
+        }
+        base.append(contentsOf: [7, 8, 9])
+        var data = base[4..<9]
+        holdReference(data) {
+            let range: Range<Data.Index> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            bytes.withUnsafeBytes { data.replaceSubrange(range, with: $0.baseAddress!, count: 2) }
+            XCTAssertEqual(data, Data(bytes: [4, 0xFF, 0xFF, 8]))
+        }
+    }
+    
+    func test_validateMutation_customBacking_withUnsafeMutableBytes() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))
+        data.withUnsafeMutableBytes { (ptr: UnsafeMutablePointer<UInt8>) in
+            ptr.advanced(by: 5).pointee = 0xFF
+        }
+        XCTAssertEqual(data, Data(bytes: [1, 1, 1, 1, 1, 0xFF, 1, 1, 1, 1]))
+    }
+    
+#if false // this requires factory patterns
+    func test_validateMutation_customBacking_appendBytes() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBufferPointer { data.append($0.baseAddress!, count: $0.count) }
+        XCTAssertEqual(data, Data(bytes: [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_customBacking_appendData() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))
+        data.append(Data(bytes: [0xFF, 0xFF]))
+        XCTAssertEqual(data, Data(bytes: [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_customBacking_appendBuffer() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBufferPointer { (buffer) in
+            data.append(buffer)
+        }
+        XCTAssertEqual(data, Data(bytes: [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0xFF, 0xFF]))
+        
+    }
+    
+    func test_validateMutation_customBacking_appendSequence() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))
+        data.append(contentsOf: repeatElement(UInt8(0xFF), count: 2))
+        XCTAssertEqual(data, Data(bytes: [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_customBacking_appendContentsOf() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))
+        data.append(contentsOf: [0xFF, 0xFF])
+        XCTAssertEqual(data, Data(bytes: [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_customBacking_resetBytes() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))
+        data.resetBytes(in: 5..<8)
+        XCTAssertEqual(data, Data(bytes: [1, 1, 1, 1, 1, 0, 0, 0, 1, 1]))
+    }
+    
+    func test_validateMutation_customBacking_replaceSubrange() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))
+        let range: Range<Int> = 1..<4
+        data.replaceSubrange(range, with: Data(bytes: [0xFF, 0xFF]))
+        XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 1, 1, 1, 1, 1, 1]))
+    }
+    
+    func test_validateMutation_customBacking_replaceSubrangeCountableRange() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))
+        let range: CountableRange<Int> = 1..<4
+        data.replaceSubrange(range, with: Data(bytes: [0xFF, 0xFF]))
+        XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 1, 1, 1, 1, 1, 1]))
+    }
+    
+    func test_validateMutation_customBacking_replaceSubrangeWithBuffer() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        let range: Range<Int> = 1..<4
+        bytes.withUnsafeBufferPointer { (buffer) in
+            data.replaceSubrange(range, with: buffer)
+        }
+        XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 1, 1, 1, 1, 1, 1]))
+    }
+    
+    func test_validateMutation_customBacking_replaceSubrangeWithCollection() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))
+        let range: Range<Int> = 1..<4
+        data.replaceSubrange(range, with: [0xFF, 0xFF])
+        XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 1, 1, 1, 1, 1, 1]))
+    }
+    
+    func test_validateMutation_customBacking_replaceSubrangeWithBytes() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        let range: Range<Int> = 1..<5
+        bytes.withUnsafeBufferPointer { (buffer) in
+            data.replaceSubrange(range, with: buffer.baseAddress!, count: buffer.count)
+        }
+        XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 1, 1, 1, 1, 1]))
+    }
+    
+    func test_validateMutation_slice_customBacking_withUnsafeMutableBytes() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))[4..<9]
+        data.withUnsafeMutableBytes { (ptr: UnsafeMutablePointer<UInt8>) in
+            ptr.advanced(by: 1).pointee = 0xFF
+        }
+        XCTAssertEqual(data[data.startIndex.advanced(by: 1)], 0xFF)
+    }
+    
+    func test_validateMutation_slice_customBacking_appendBytes() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))[4..<9]
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBytes { ptr in
+            data.append(ptr.baseAddress!.assumingMemoryBound(to: UInt8.self), count: ptr.count)
+        }
+        XCTAssertEqual(data, Data(bytes: [1, 1, 1, 1, 1, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_slice_customBacking_appendData() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))[4..<9]
+        data.append(Data(bytes: [0xFF, 0xFF]))
+        XCTAssertEqual(data, Data(bytes: [1, 1, 1, 1, 1, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_slice_customBacking_appendBuffer() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))[4..<9]
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBufferPointer { (buffer) in
+            data.append(buffer)
+        }
+        XCTAssertEqual(data, Data(bytes: [1, 1, 1, 1, 1, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_slice_customBacking_appendSequence() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))[4..<9]
+        let seq = repeatElement(UInt8(0xFF), count: 2)
+        data.append(contentsOf: seq)
+        XCTAssertEqual(data, Data(bytes: [1, 1, 1, 1, 1, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_slice_customBacking_appendContentsOf() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))[4..<9]
+        data.append(contentsOf: [0xFF, 0xFF])
+        XCTAssertEqual(data, Data(bytes: [1, 1, 1, 1, 1, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_slice_customBacking_resetBytes() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))[4..<9]
+        data.resetBytes(in: 5..<8)
+        XCTAssertEqual(data, Data(bytes: [1, 0, 0, 0, 1]))
+    }
+    
+    func test_validateMutation_slice_customBacking_replaceSubrange() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))[4..<9]
+        let range: Range<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        data.replaceSubrange(range, with: Data(bytes: [0xFF, 0xFF]))
+        XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 1]))
+    }
+    
+    func test_validateMutation_slice_customBacking_replaceSubrangeCountableRange() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))[4..<9]
+        let range: CountableRange<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        data.replaceSubrange(range, with: Data(bytes: [0xFF, 0xFF]))
+        XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 1]))
+    }
+    
+    func test_validateMutation_slice_customBacking_replaceSubrangeWithBuffer() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))[4..<9]
+        let range: Range<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBufferPointer { (buffer) in
+            data.replaceSubrange(range, with: buffer)
+        }
+        XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 1]))
+    }
+    
+    func test_validateMutation_slice_customBacking_replaceSubrangeWithCollection() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))[4..<9]
+        let range: Range<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        data.replaceSubrange(range, with: [0xFF, 0xFF])
+        XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 1]))
+    }
+    
+    func test_validateMutation_slice_customBacking_replaceSubrangeWithBytes() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))[4..<9]
+        let range: Range<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBytes { buffer in
+            data.replaceSubrange(range, with: buffer.baseAddress!, count: 2)
+        }
+        XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 1]))
+    }
+    
+    func test_validateMutation_cow_customBacking_withUnsafeMutableBytes() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))
+        holdReference(data) {
+            data.withUnsafeMutableBytes { (ptr: UnsafeMutablePointer<UInt8>) in
+                ptr.advanced(by: 5).pointee = 0xFF
+            }
+            XCTAssertEqual(data[data.startIndex.advanced(by: 5)], 0xFF)
+        }
+    }
+    
+    func test_validateMutation_cow_customBacking_appendBytes() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))
+        holdReference(data) {
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            bytes.withUnsafeBufferPointer { (buffer) in
+                data.append(buffer.baseAddress!, count: buffer.count)
+            }
+            XCTAssertEqual(data, Data(bytes: [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_cow_customBacking_appendData() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))
+        holdReference(data) {
+            data.append(Data(bytes: [0xFF, 0xFF]))
+            XCTAssertEqual(data, Data(bytes: [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_cow_customBacking_appendBuffer() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))
+        holdReference(data) {
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            bytes.withUnsafeBufferPointer { data.append($0) }
+            XCTAssertEqual(data, Data(bytes: [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_cow_customBacking_appendSequence() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))
+        holdReference(data) {
+            data.append(contentsOf: repeatElement(UInt8(0xFF), count: 2))
+            XCTAssertEqual(data, Data(bytes: [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_cow_customBacking_appendContentsOf() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))
+        holdReference(data) {
+            data.append(contentsOf: [0xFF, 0xFF])
+            XCTAssertEqual(data, Data(bytes: [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_cow_customBacking_resetBytes() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))
+        holdReference(data) {
+            data.resetBytes(in: 5..<8)
+            XCTAssertEqual(data, Data(bytes: [1, 1, 1, 1, 1, 0, 0, 0, 1, 1]))
+        }
+    }
+    
+    func test_validateMutation_cow_customBacking_replaceSubrange() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))
+        holdReference(data) {
+            let range: Range<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            data.replaceSubrange(range, with: Data(bytes: [0xFF, 0xFF]))
+            XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 1]))
+        }
+    }
+    
+    func test_validateMutation_cow_customBacking_replaceSubrangeCountableRange() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))
+        holdReference(data) {
+            let range: CountableRange<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            data.replaceSubrange(range, with: Data(bytes: [0xFF, 0xFF]))
+            XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 1]))
+        }
+    }
+    
+    func test_validateMutation_cow_customBacking_replaceSubrangeWithBuffer() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))
+        holdReference(data) {
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            let range: Range<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            bytes.withUnsafeBufferPointer { data.replaceSubrange(range, with: $0) }
+            XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 1]))
+        }
+    }
+    
+    func test_validateMutation_cow_customBacking_replaceSubrangeWithCollection() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))
+        holdReference(data) {
+            let range: Range<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            data.replaceSubrange(range, with: [0xFF, 0xFF])
+            XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 1]))
+        }
+    }
+    
+    func test_validateMutation_cow_customBacking_replaceSubrangeWithBytes() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))
+        holdReference(data) {
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            let range: Range<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            bytes.withUnsafeBytes {
+                data.replaceSubrange(range, with: $0.baseAddress!, count: $0.count)
+            }
+            XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 1]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_customBacking_withUnsafeMutableBytes() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))[4..<9]
+        holdReference(data) {
+            data.withUnsafeMutableBytes { (ptr: UnsafeMutablePointer<UInt8>) in
+                ptr.advanced(by: 1).pointee = 0xFF
+            }
+            XCTAssertEqual(data[data.startIndex.advanced(by: 1)], 0xFF)
+        }
+    }
+    
+    func test_validateMutation_slice_cow_customBacking_appendBytes() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))[4..<9]
+        holdReference(data) {
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            bytes.withUnsafeBufferPointer { (buffer) in
+                data.append(buffer.baseAddress!, count: buffer.count)
+            }
+            XCTAssertEqual(data, Data(bytes: [1, 1, 1, 1, 1, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_customBacking_appendData() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))[4..<9]
+        holdReference(data) {
+            data.append(Data(bytes: [0xFF, 0xFF]))
+            XCTAssertEqual(data, Data(bytes: [1, 1, 1, 1, 1, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_customBacking_appendBuffer() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))[4..<9]
+        holdReference(data) {
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            bytes.withUnsafeBufferPointer { data.append($0) }
+            XCTAssertEqual(data, Data(bytes: [1, 1, 1, 1, 1, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_customBacking_appendSequence() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))[4..<9]
+        holdReference(data) {
+            data.append(contentsOf: repeatElement(UInt8(0xFF), count: 2))
+            XCTAssertEqual(data, Data(bytes: [1, 1, 1, 1, 1, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_customBacking_appendContentsOf() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))[4..<9]
+        holdReference(data) {
+            data.append(contentsOf: [0xFF, 0xFF])
+            XCTAssertEqual(data, Data(bytes: [1, 1, 1, 1, 1, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_customBacking_resetBytes() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))[4..<9]
+        holdReference(data) {
+            data.resetBytes(in: 5..<8)
+            XCTAssertEqual(data, Data(bytes: [1, 0, 0, 0, 1]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_customBacking_replaceSubrange() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))[4..<9]
+        holdReference(data) {
+            let range: Range<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            data.replaceSubrange(range, with: Data(bytes: [0xFF, 0xFF]))
+            XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 1]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_customBacking_replaceSubrangeCountableRange() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))[4..<9]
+        holdReference(data) {
+            let range: CountableRange<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            data.replaceSubrange(range, with: Data(bytes: [0xFF, 0xFF]))
+            XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 1]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_customBacking_replaceSubrangeWithBuffer() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))[4..<9]
+        holdReference(data) {
+            let range: Range<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            bytes.withUnsafeBufferPointer { data.replaceSubrange(range, with: $0) }
+            XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 1]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_customBacking_replaceSubrangeWithCollection() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))[4..<9]
+        holdReference(data) {
+            let range: Range<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            data.replaceSubrange(range, with: [0xFF, 0xFF])
+            XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 1]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_customBacking_replaceSubrangeWithBytes() {
+        var data = Data(referencing: AllOnesImmutableData(length: 10))[4..<9]
+        holdReference(data) {
+            let range: Range<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            bytes.withUnsafeBytes {
+                data.replaceSubrange(range, with: $0.baseAddress!, count: $0.count)
+            }
+            XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 1]))
+        }
+    }
+    
+    func test_validateMutation_customMutableBacking_withUnsafeMutableBytes() {
+        var data = Data(referencing: AllOnesData(length: 1))
+        data.count = 10
+        data.withUnsafeMutableBytes { (ptr: UnsafeMutablePointer<UInt8>) in
+            ptr.advanced(by: 5).pointee = 0xFF
+        }
+        XCTAssertEqual(data[data.startIndex.advanced(by: 5)], 0xFF)
+    }
+    
+    func test_validateMutation_customMutableBacking_appendBytes() {
+        var data = Data(referencing: AllOnesData(length: 1))
+        data.count = 10
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBufferPointer { data.append($0.baseAddress!, count: $0.count) }
+        XCTAssertEqual(data, Data(bytes: [1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_customMutableBacking_appendData() {
+        var data = Data(referencing: AllOnesData(length: 1))
+        data.count = 10
+        data.append(Data(bytes: [0xFF, 0xFF]))
+        XCTAssertEqual(data, Data(bytes: [1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_customMutableBacking_appendBuffer() {
+        var data = Data(referencing: AllOnesData(length: 1))
+        data.count = 10
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBufferPointer { data.append($0) }
+        XCTAssertEqual(data, Data(bytes: [1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_customMutableBacking_appendSequence() {
+        var data = Data(referencing: AllOnesData(length: 1))
+        data.count = 10
+        data.append(contentsOf: repeatElement(UInt8(0xFF), count: 2))
+        XCTAssertEqual(data, Data(bytes: [1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_customMutableBacking_appendContentsOf() {
+        var data = Data(referencing: AllOnesData(length: 1))
+        data.count = 10
+        data.append(contentsOf: [0xFF, 0xFF])
+        XCTAssertEqual(data, Data(bytes: [1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_customMutableBacking_resetBytes() {
+        var data = Data(referencing: AllOnesData(length: 1))
+        data.count = 10
+        data.resetBytes(in: 5..<8)
+        XCTAssertEqual(data.count, 10)
+        XCTAssertEqual(data[data.startIndex.advanced(by: 0)], 1)
+        XCTAssertEqual(data[data.startIndex.advanced(by: 5)], 0)
+        XCTAssertEqual(data[data.startIndex.advanced(by: 6)], 0)
+        XCTAssertEqual(data[data.startIndex.advanced(by: 7)], 0)
+    }
+    
+    func test_validateMutation_customMutableBacking_replaceSubrange() {
+        var data = Data(referencing: AllOnesData(length: 1))
+        data.count = 10
+        let range: Range<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        data.replaceSubrange(range, with: Data(bytes: [0xFF, 0xFF]))
+        XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 0]))
+    }
+    
+    func test_validateMutation_customMutableBacking_replaceSubrangeCountableRange() {
+        var data = Data(referencing: AllOnesData(length: 1))
+        data.count = 10
+        let range: CountableRange<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        data.replaceSubrange(range, with: Data(bytes: [0xFF, 0xFF]))
+        XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 0]))
+    }
+    
+    func test_validateMutation_customMutableBacking_replaceSubrangeWithBuffer() {
+        var data = Data(referencing: AllOnesData(length: 1))
+        data.count = 10
+        let range: Range<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBufferPointer { data.replaceSubrange(range, with: $0) }
+        XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 0]))
+    }
+    
+    func test_validateMutation_customMutableBacking_replaceSubrangeWithCollection() {
+        var data = Data(referencing: AllOnesData(length: 1))
+        data.count = 10
+        let range: Range<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        data.replaceSubrange(range, with: [0xFF, 0xFF])
+        XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 0]))
+    }
+    
+    func test_validateMutation_customMutableBacking_replaceSubrangeWithBytes() {
+        var data = Data(referencing: AllOnesData(length: 1))
+        data.count = 10
+        let range: Range<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBufferPointer { data.replaceSubrange(range, with: $0.baseAddress!, count: $0.count) }
+        XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 0]))
+    }
+    
+    func test_validateMutation_slice_customMutableBacking_withUnsafeMutableBytes() {
+        var base = Data(referencing: AllOnesData(length: 1))
+        base.count = 10
+        var data = base[4..<9]
+        data.withUnsafeMutableBytes { (ptr: UnsafeMutablePointer<UInt8>) in
+            ptr.advanced(by: 1).pointee = 0xFF
+        }
+        XCTAssertEqual(data[data.startIndex.advanced(by: 1)], 0xFF)
+    }
+    
+    func test_validateMutation_slice_customMutableBacking_appendBytes() {
+        var base = Data(referencing: AllOnesData(length: 1))
+        base.count = 10
+        var data = base[4..<9]
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBufferPointer { data.append($0.baseAddress!, count: $0.count) }
+        XCTAssertEqual(data, Data(bytes: [0, 0, 0, 0, 0, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_slice_customMutableBacking_appendData() {
+        var base = Data(referencing: AllOnesData(length: 1))
+        base.count = 10
+        var data = base[4..<9]
+        data.append(Data(bytes: [0xFF, 0xFF]))
+        XCTAssertEqual(data, Data(bytes: [0, 0, 0, 0, 0, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_slice_customMutableBacking_appendBuffer() {
+        var base = Data(referencing: AllOnesData(length: 1))
+        base.count = 10
+        var data = base[4..<9]
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBufferPointer { data.append($0) }
+        XCTAssertEqual(data, Data(bytes: [0, 0, 0, 0, 0, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_slice_customMutableBacking_appendSequence() {
+        var base = Data(referencing: AllOnesData(length: 1))
+        base.count = 10
+        var data = base[4..<9]
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBufferPointer { data.append($0) }
+        XCTAssertEqual(data, Data(bytes: [0, 0, 0, 0, 0, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_slice_customMutableBacking_appendContentsOf() {
+        var base = Data(referencing: AllOnesData(length: 1))
+        base.count = 10
+        var data = base[4..<9]
+        data.append(contentsOf: [0xFF, 0xFF])
+        XCTAssertEqual(data, Data(bytes: [0, 0, 0, 0, 0, 0xFF, 0xFF]))
+    }
+    
+    func test_validateMutation_slice_customMutableBacking_resetBytes() {
+        var base = Data(referencing: AllOnesData(length: 1))
+        base.count = 10
+        var data = base[4..<9]
+        data.resetBytes(in: 5..<8)
+        
+        XCTAssertEqual(data[data.startIndex.advanced(by: 1)], 0)
+        XCTAssertEqual(data[data.startIndex.advanced(by: 2)], 0)
+        XCTAssertEqual(data[data.startIndex.advanced(by: 3)], 0)
+    }
+    
+    func test_validateMutation_slice_customMutableBacking_replaceSubrange() {
+        var base = Data(referencing: AllOnesData(length: 1))
+        base.count = 10
+        var data = base[4..<9]
+        let range: Range<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        data.replaceSubrange(range, with: Data(bytes: [0xFF, 0xFF]))
+        XCTAssertEqual(data, Data(bytes: [0, 0xFF, 0xFF, 0]))
+    }
+    
+    func test_validateMutation_slice_customMutableBacking_replaceSubrangeCountableRange() {
+        var base = Data(referencing: AllOnesData(length: 1))
+        base.count = 10
+        var data = base[4..<9]
+        let range: CountableRange<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        data.replaceSubrange(range, with: Data(bytes: [0xFF, 0xFF]))
+        XCTAssertEqual(data, Data(bytes: [0, 0xFF, 0xFF, 0]))
+    }
+    
+    func test_validateMutation_slice_customMutableBacking_replaceSubrangeWithBuffer() {
+        var base = Data(referencing: AllOnesData(length: 1))
+        base.count = 10
+        var data = base[4..<9]
+        let range: Range<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBufferPointer { data.replaceSubrange(range, with: $0) }
+        XCTAssertEqual(data, Data(bytes: [0, 0xFF, 0xFF, 0]))
+    }
+    
+    func test_validateMutation_slice_customMutableBacking_replaceSubrangeWithCollection() {
+        var base = Data(referencing: AllOnesData(length: 1))
+        base.count = 10
+        var data = base[4..<9]
+        let range: Range<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        data.replaceSubrange(range, with: [0xFF, 0xFF])
+        XCTAssertEqual(data, Data(bytes: [0, 0xFF, 0xFF, 0]))
+    }
+    
+    func test_validateMutation_slice_customMutableBacking_replaceSubrangeWithBytes() {
+        var base = Data(referencing: AllOnesData(length: 1))
+        base.count = 10
+        var data = base[4..<9]
+        let range: Range<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+        let bytes: [UInt8] = [0xFF, 0xFF]
+        bytes.withUnsafeBufferPointer { data.replaceSubrange(range, with: $0.baseAddress!, count: $0.count) }
+        XCTAssertEqual(data, Data(bytes: [0, 0xFF, 0xFF, 0]))
+    }
+    
+    func test_validateMutation_cow_customMutableBacking_withUnsafeMutableBytes() {
+        var data = Data(referencing: AllOnesData(length: 1))
+        data.count = 10
+        holdReference(data) {
+            data.withUnsafeMutableBytes { (ptr: UnsafeMutablePointer<UInt8>) in
+                ptr.advanced(by: 5).pointee = 0xFF
+            }
+            XCTAssertEqual(data[data.startIndex.advanced(by: 5)], 0xFF)
+        }
+    }
+    
+    func test_validateMutation_cow_customMutableBacking_appendBytes() {
+        var data = Data(referencing: AllOnesData(length: 1))
+        data.count = 10
+        holdReference(data) {
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            bytes.withUnsafeBufferPointer { data.append($0.baseAddress!, count: $0.count) }
+            XCTAssertEqual(data, Data(bytes: [1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_cow_customMutableBacking_appendData() {
+        var data = Data(referencing: AllOnesData(length: 1))
+        data.count = 10
+        holdReference(data) {
+            data.append(Data(bytes: [0xFF, 0xFF]))
+            XCTAssertEqual(data, Data(bytes: [1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_cow_customMutableBacking_appendBuffer() {
+        var data = Data(referencing: AllOnesData(length: 1))
+        data.count = 10
+        holdReference(data) {
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            bytes.withUnsafeBufferPointer { data.append($0) }
+            XCTAssertEqual(data, Data(bytes: [1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_cow_customMutableBacking_appendSequence() {
+        var data = Data(referencing: AllOnesData(length: 1))
+        data.count = 10
+        holdReference(data) {
+            data.append(contentsOf: repeatElement(UInt8(0xFF), count: 2))
+            XCTAssertEqual(data, Data(bytes: [1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_cow_customMutableBacking_appendContentsOf() {
+        var data = Data(referencing: AllOnesData(length: 1))
+        data.count = 10
+        holdReference(data) {
+            data.append(contentsOf: [0xFF, 0xFF])
+            XCTAssertEqual(data, Data(bytes: [1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_cow_customMutableBacking_resetBytes() {
+        var data = Data(referencing: AllOnesData(length: 10))
+        holdReference(data) {
+            data.resetBytes(in: 5..<8)
+            XCTAssertEqual(data.count, 10)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 0)], 1)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 5)], 0)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 6)], 0)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 7)], 0)
+        }
+    }
+    
+    func test_validateMutation_cow_customMutableBacking_replaceSubrange() {
+        var data = Data(referencing: AllOnesData(length: 1))
+        data.count = 10
+        holdReference(data) {
+            let range: Range<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            data.replaceSubrange(range, with: Data(bytes: [0xFF, 0xFF]))
+            XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 0])) 
+        }
+    }
+    
+    func test_validateMutation_cow_customMutableBacking_replaceSubrangeCountableRange() {
+        var data = Data(referencing: AllOnesData(length: 1))
+        data.count = 10
+        holdReference(data) {
+            let range: CountableRange<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            data.replaceSubrange(range, with: Data(bytes: [0xFF, 0xFF]))
+            XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 0])) 
+        }
+    }
+    
+    func test_validateMutation_cow_customMutableBacking_replaceSubrangeWithBuffer() {
+        var data = Data(referencing: AllOnesData(length: 1))
+        data.count = 10
+        holdReference(data) {
+            let range: Range<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            bytes.withUnsafeBufferPointer { data.replaceSubrange(range, with: $0) }
+            XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 0])) 
+        }
+    }
+    
+    func test_validateMutation_cow_customMutableBacking_replaceSubrangeWithCollection() {
+        var data = Data(referencing: AllOnesData(length: 1))
+        data.count = 10
+        holdReference(data) {
+            let range: Range<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            data.replaceSubrange(range, with: [0xFF, 0xFF])
+            XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 0])) 
+        }
+    }
+    
+    func test_validateMutation_cow_customMutableBacking_replaceSubrangeWithBytes() {
+        var data = Data(referencing: AllOnesData(length: 1))
+        data.count = 10
+        holdReference(data) {
+            let range: Range<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            bytes.withUnsafeBufferPointer { data.replaceSubrange(range, with: $0.baseAddress!, count: $0.count) }
+            XCTAssertEqual(data, Data(bytes: [1, 0xFF, 0xFF, 0])) 
+        }
+    }
+    
+    func test_validateMutation_slice_cow_customMutableBacking_withUnsafeMutableBytes() {
+        var base = Data(referencing: AllOnesData(length: 1))
+        base.count = 10
+        var data = base[4..<9]
+        holdReference(data) {
+            data.withUnsafeMutableBytes { (ptr: UnsafeMutablePointer<UInt8>) in
+                ptr.advanced(by: 1).pointee = 0xFF
+            }
+            XCTAssertEqual(data[data.startIndex.advanced(by: 1)], 0xFF)
+        }
+    }
+    
+    func test_validateMutation_slice_cow_customMutableBacking_appendBytes() {
+        var base = Data(referencing: AllOnesData(length: 1))
+        base.count = 10
+        var data = base[4..<9]
+        holdReference(data) {
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            bytes.withUnsafeBufferPointer { data.append($0.baseAddress!, count: $0.count) }
+            XCTAssertEqual(data, Data(bytes: [0, 0, 0, 0, 0, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_customMutableBacking_appendData() {
+        var base = Data(referencing: AllOnesData(length: 1))
+        base.count = 10
+        var data = base[4..<9]
+        holdReference(data) {
+            data.append(Data(bytes: [0xFF, 0xFF]))
+            XCTAssertEqual(data, Data(bytes: [0, 0, 0, 0, 0, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_customMutableBacking_appendBuffer() {
+        var base = Data(referencing: AllOnesData(length: 1))
+        base.count = 10
+        var data = base[4..<9]
+        holdReference(data) {
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            bytes.withUnsafeBufferPointer { data.append($0) }
+            XCTAssertEqual(data, Data(bytes: [0, 0, 0, 0, 0, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_customMutableBacking_appendSequence() {
+        var base = Data(referencing: AllOnesData(length: 1))
+        base.count = 10
+        var data = base[4..<9]
+        holdReference(data) {
+            data.append(contentsOf: repeatElement(UInt8(0xFF), count: 2))
+            XCTAssertEqual(data, Data(bytes: [0, 0, 0, 0, 0, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_customMutableBacking_appendContentsOf() {
+        var base = Data(referencing: AllOnesData(length: 1))
+        base.count = 10
+        var data = base[4..<9]
+        holdReference(data) {
+            data.append(contentsOf: [0xFF, 0xFF])
+            XCTAssertEqual(data, Data(bytes: [0, 0, 0, 0, 0, 0xFF, 0xFF]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_customMutableBacking_resetBytes() {
+        var base = Data(referencing: AllOnesData(length: 1))
+        base.count = 10
+        var data = base[4..<9]
+        holdReference(data) {
+            data.resetBytes(in: 5..<8)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 1)], 0)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 2)], 0)
+            XCTAssertEqual(data[data.startIndex.advanced(by: 3)], 0)
+        }
+    }
+    
+    func test_validateMutation_slice_cow_customMutableBacking_replaceSubrange() {
+        var base = Data(referencing: AllOnesData(length: 1))
+        base.count = 10
+        var data = base[4..<9]
+        holdReference(data) {
+            let range: Range<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            data.replaceSubrange(range, with: Data(bytes: [0xFF, 0xFF]))
+            XCTAssertEqual(data, Data(bytes: [0, 0xFF, 0xFF, 0]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_customMutableBacking_replaceSubrangeCountableRange() {
+        var base = Data(referencing: AllOnesData(length: 1))
+        base.count = 10
+        var data = base[4..<9]
+        holdReference(data) {
+            let range: CountableRange<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            data.replaceSubrange(range, with: Data(bytes: [0xFF, 0xFF]))
+            XCTAssertEqual(data, Data(bytes: [0, 0xFF, 0xFF, 0]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_customMutableBacking_replaceSubrangeWithBuffer() {
+        var base = Data(referencing: AllOnesData(length: 1))
+        base.count = 10
+        var data = base[4..<9]
+        holdReference(data) {
+            let range: Range<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            bytes.withUnsafeBufferPointer { data.replaceSubrange(range, with: $0) }
+            XCTAssertEqual(data, Data(bytes: [0, 0xFF, 0xFF, 0]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_customMutableBacking_replaceSubrangeWithCollection() {
+        var base = Data(referencing: AllOnesData(length: 1))
+        base.count = 10
+        var data = base[4..<9]
+        holdReference(data) {
+            let range: Range<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            data.replaceSubrange(range, with: [0xFF, 0xFF])
+            XCTAssertEqual(data, Data(bytes: [0, 0xFF, 0xFF, 0]))
+        }
+    }
+    
+    func test_validateMutation_slice_cow_customMutableBacking_replaceSubrangeWithBytes() {
+        var base = Data(referencing: AllOnesData(length: 1))
+        base.count = 10
+        var data = base[4..<9]
+        holdReference(data) {
+            let range: Range<Int> = data.startIndex.advanced(by: 1)..<data.endIndex.advanced(by: -1)
+            let bytes: [UInt8] = [0xFF, 0xFF]
+            bytes.withUnsafeBufferPointer { data.replaceSubrange(range, with: $0.baseAddress!, count: $0.count) }
+            XCTAssertEqual(data, Data(bytes: [0, 0xFF, 0xFF, 0]))
+        }
+    }
+#endif
+    
+    func test_sliceHash() {
+        let base1 = Data(bytes: [0, 0xFF, 0xFF, 0])
+        let base2 = Data(bytes: [0, 0xFF, 0xFF, 0])
+        let base3 = Data(bytes: [0xFF, 0xFF, 0xFF, 0])
+        let sliceEmulation = Data(bytes: [0xFF, 0xFF])
+        XCTAssertEqual(base1.hashValue, base2.hashValue)
+        let slice1 = base1[base1.startIndex.advanced(by: 1)..<base1.endIndex.advanced(by: -1)]
+        let slice2 = base2[base2.startIndex.advanced(by: 1)..<base2.endIndex.advanced(by: -1)]
+        let slice3 = base3[base3.startIndex.advanced(by: 1)..<base3.endIndex.advanced(by: -1)]
+        XCTAssertEqual(slice1.hashValue, sliceEmulation.hashValue)
+        XCTAssertEqual(slice1.hashValue, slice2.hashValue)
+        XCTAssertEqual(slice2.hashValue, slice3.hashValue)
+    }
+
+    func test_slice_resize_growth() {
+        var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])[4..<9]
+        data.resetBytes(in: data.endIndex.advanced(by: -1)..<data.endIndex.advanced(by: 1))
+        XCTAssertEqual(data, Data(bytes: [4, 5, 6, 7, 0, 0]))
+    }
+    
+    /*
+    func test_sliceEnumeration() {
+        var base = DispatchData.empty
+        let bytes: [UInt8] = [0, 1, 2, 3, 4]
+        base.append(bytes.withUnsafeBytes { DispatchData(bytes: $0) })
+        base.append(bytes.withUnsafeBytes { DispatchData(bytes: $0) })
+        base.append(bytes.withUnsafeBytes { DispatchData(bytes: $0) })
+        let data = ((base as AnyObject) as! Data)[3..<11]
+        var regionRanges: [Range<Int>] = []
+        var regionData: [Data] = []
+        data.enumerateBytes { (buffer, index, _) in
+            regionData.append(Data(bytes: buffer.baseAddress!, count: buffer.count))
+            regionRanges.append(index..<index + buffer.count)
+        }
+        XCTAssertEqual(regionRanges.count, 3)
+        XCTAssertEqual(Range<Data.Index>(3..<5), regionRanges[0])
+        XCTAssertEqual(Range<Data.Index>(5..<10), regionRanges[1])
+        XCTAssertEqual(Range<Data.Index>(10..<11), regionRanges[2])
+        XCTAssertEqual(Data(bytes: [3, 4]), regionData[0]) //fails
+        XCTAssertEqual(Data(bytes: [0, 1, 2, 3, 4]), regionData[1]) //passes
+        XCTAssertEqual(Data(bytes: [0]), regionData[2]) //fails
+    }
+ */
 }
 
diff --git a/TestFoundation/TestNSDictionary.swift b/TestFoundation/TestNSDictionary.swift
index 096f0a6..fa09cb0 100644
--- a/TestFoundation/TestNSDictionary.swift
+++ b/TestFoundation/TestNSDictionary.swift
@@ -211,7 +211,7 @@
             let d1: NSDictionary = ["Hello":["world":"again"]]
             let isWritten = d1.write(toFile: testFilePath!, atomically: true)
             if(isWritten) {
-                let dict = NSMutableDictionary.init(contentsOfFile: testFilePath!)
+                let dict = NSDictionary(contentsOfFile: testFilePath!)
                 XCTAssert(dict == d1)
             } else {
                 XCTFail("Write to file failed")
diff --git a/TestFoundation/TestNSKeyedArchiver.swift b/TestFoundation/TestNSKeyedArchiver.swift
index 1a03419..4ccc05e 100644
--- a/TestFoundation/TestNSKeyedArchiver.swift
+++ b/TestFoundation/TestNSKeyedArchiver.swift
@@ -106,6 +106,7 @@
             ("test_archive_user_class", test_archive_user_class),
             ("test_archive_uuid_bvref", test_archive_uuid_byref),
             ("test_archive_uuid_byvalue", test_archive_uuid_byvalue),
+            ("test_archive_unhashable", test_archive_unhashable),
         ]
     }
 
@@ -312,4 +313,30 @@
         let uuid = UUID()
         return test_archive(uuid, classes: [NSUUID.self])
     }
+
+    func test_archive_unhashable() {
+        let data = """
+            {
+              "args": {},
+              "headers": {
+                "Accept": "*/*",
+                "Accept-Encoding": "deflate, gzip",
+                "Accept-Language": "en",
+                "Connection": "close",
+                "Host": "httpbin.org",
+                "User-Agent": "TestFoundation (unknown version) curl/7.54.0"
+              },
+              "origin": "0.0.0.0",
+              "url": "https://httpbin.org/get"
+            }
+            """.data(using: .utf8)!
+        do {
+            let json = try JSONSerialization.jsonObject(with: data)
+            _ = NSKeyedArchiver.archivedData(withRootObject: json)
+            XCTAssert(true, "NSKeyedArchiver.archivedData handles unhashable")
+        }
+        catch {
+            XCTFail("test_archive_unhashable, de-serialization error \(error)")
+        }
+    }
 }
diff --git a/TestFoundation/TestNSNumber.swift b/TestFoundation/TestNSNumber.swift
index b34d45c..9d2ac9c 100644
--- a/TestFoundation/TestNSNumber.swift
+++ b/TestFoundation/TestNSNumber.swift
@@ -1091,8 +1091,8 @@
 
     func test_Equals() {
         // Booleans: false only equals 0, true only equals 1
-        XCTAssertTrue(NSNumber(value: true) == NSNumber(value: true))
-        XCTAssertTrue(NSNumber(value: true) == NSNumber(value: 1))
+        XCTAssertTrue(NSNumber(value: true) == NSNumber(value: Bool(true)))
+        XCTAssertTrue(NSNumber(value: true) == NSNumber(value: Int(1)))
         XCTAssertTrue(NSNumber(value: true) == NSNumber(value: Float(1)))
         XCTAssertTrue(NSNumber(value: true) == NSNumber(value: Double(1)))
         XCTAssertTrue(NSNumber(value: true) == NSNumber(value: Int8(1)))
@@ -1102,8 +1102,8 @@
         XCTAssertTrue(NSNumber(value: true) != NSNumber(value: Double(1234.56)))
         XCTAssertTrue(NSNumber(value: true) != NSNumber(value: 2))
         XCTAssertTrue(NSNumber(value: true) != NSNumber(value: Int.max))
-        XCTAssertTrue(NSNumber(value: false) == NSNumber(value: false))
-        XCTAssertTrue(NSNumber(value: false) == NSNumber(value: 0))
+        XCTAssertTrue(NSNumber(value: false) == NSNumber(value: Bool(false)))
+        XCTAssertTrue(NSNumber(value: false) == NSNumber(value: Int(0)))
         XCTAssertTrue(NSNumber(value: false) == NSNumber(value: Float(0)))
         XCTAssertTrue(NSNumber(value: false) == NSNumber(value: Double(0)))
         XCTAssertTrue(NSNumber(value: false) == NSNumber(value: Int8(0)))
diff --git a/TestFoundation/TestNSTextCheckingResult.swift b/TestFoundation/TestNSTextCheckingResult.swift
index f008fc2..181898f 100644
--- a/TestFoundation/TestNSTextCheckingResult.swift
+++ b/TestFoundation/TestNSTextCheckingResult.swift
@@ -34,17 +34,17 @@
            let searchRange = NSMakeRange(0,7)
            let match: NSTextCheckingResult =  regex.firstMatch(in: searchString, options: searchOptions, range: searchRange)!
            //Positive offset
-           var result = match.resultByAdjustingRangesWithOffset(1)
+           var result = match.adjustingRanges(offset: 1)
            XCTAssertEqual(result.range(at: 0).location, 6)
            XCTAssertEqual(result.range(at: 1).location, NSNotFound)
            XCTAssertEqual(result.range(at: 2).location, 6)
            //Negative offset
-           result = match.resultByAdjustingRangesWithOffset(-2)
+           result = match.adjustingRanges(offset: -2)
            XCTAssertEqual(result.range(at: 0).location, 3)
            XCTAssertEqual(result.range(at: 1).location, NSNotFound)
            XCTAssertEqual(result.range(at: 2).location, 3)
            //ZeroOffset
-           result = match.resultByAdjustingRangesWithOffset(0)
+           result = match.adjustingRanges(offset: 0)
            XCTAssertEqual(result.range(at: 0).location, 5)
            XCTAssertEqual(result.range(at: 1).location, NSNotFound)
            XCTAssertEqual(result.range(at: 2).location, 5)
diff --git a/TestFoundation/TestOperationQueue.swift b/TestFoundation/TestOperationQueue.swift
index 02f95c4..1dd8c50 100644
--- a/TestFoundation/TestOperationQueue.swift
+++ b/TestFoundation/TestOperationQueue.swift
@@ -28,6 +28,7 @@
             ("test_MainQueueGetter", test_MainQueueGetter),
             ("test_CurrentQueueOnMainQueue", test_CurrentQueueOnMainQueue),
             ("test_CurrentQueueOnBackgroundQueue", test_CurrentQueueOnBackgroundQueue),
+            ("test_CurrentQueueOnBackgroundQueueWithSelfCancel", test_CurrentQueueOnBackgroundQueueWithSelfCancel),
             ("test_CurrentQueueWithCustomUnderlyingQueue", test_CurrentQueueWithCustomUnderlyingQueue),
             ("test_CurrentQueueWithUnderlyingQueueResetToNil", test_CurrentQueueWithUnderlyingQueueResetToNil),
         ]
@@ -135,6 +136,20 @@
         waitForExpectations(timeout: 1)
     }
     
+    func test_CurrentQueueOnBackgroundQueueWithSelfCancel() {
+        let operationQueue = OperationQueue()
+        operationQueue.maxConcurrentOperationCount = 1
+        let expectation = self.expectation(description: "Background execution")
+        operationQueue.addOperation {
+            XCTAssertEqual(operationQueue, OperationQueue.current)
+            expectation.fulfill()
+            // Canceling operation X from inside operation X should not cause the app to a crash
+            operationQueue.cancelAllOperations()
+        }
+        
+        waitForExpectations(timeout: 1)
+    }
+
     func test_CurrentQueueWithCustomUnderlyingQueue() {
         let expectation = self.expectation(description: "Background execution")
         
diff --git a/TestFoundation/TestProcess.swift b/TestFoundation/TestProcess.swift
index 54775b5..d19340b 100644
--- a/TestFoundation/TestProcess.swift
+++ b/TestFoundation/TestProcess.swift
@@ -28,13 +28,11 @@
                    ("test_pipe_stdout", test_pipe_stdout),
                    ("test_pipe_stderr", test_pipe_stderr),
                    ("test_current_working_directory", test_current_working_directory),
-                   // disabled for now
-                   // ("test_pipe_stdout_and_stderr_same_pipe", test_pipe_stdout_and_stderr_same_pipe),
+                   ("test_pipe_stdout_and_stderr_same_pipe", test_pipe_stdout_and_stderr_same_pipe),
                    ("test_file_stdout", test_file_stdout),
-                   // disabled for now
-                   // ("test_passthrough_environment", test_passthrough_environment),
-                   // ("test_no_environment", test_no_environment),
-                   // ("test_custom_environment", test_custom_environment),
+                   ("test_passthrough_environment", test_passthrough_environment),
+                   ("test_no_environment", test_no_environment),
+                   ("test_custom_environment", test_custom_environment),
         ]
     }
     
@@ -183,7 +181,7 @@
             XCTFail("Could not read stdout")
             return
         }
-        // testing the return value of an external process is does not port well, and may change.
+        // testing the return value of an external process does not port well, and may change.
         // XCTAssertEqual(string, "/bin/cat: invalid_file_name: No such file or directory\n")
     }
 
@@ -197,6 +195,9 @@
         process.standardOutput = pipe
         process.standardError = pipe
 
+        // Clear the environment to stop the malloc debug flags used in Xcode debug being
+        // set in the subprocess.
+        process.environment = [:]
         process.launch()
         process.waitUntilExit()
         XCTAssertEqual(process.terminationStatus, 1)
@@ -206,7 +207,12 @@
             XCTFail("Could not read stdout")
             return
         }
-        XCTAssertEqual(string, "/bin/cat: invalid_file_name: No such file or directory\n")
+
+        // Remove the leading '/bin/' since on macOS '/bin/cat' just outputs 'cat:'
+        let searchStr = "/bin/"
+        let errMsg = string.replacingOccurrences(of: searchStr, with: "", options: [.literal, .anchored],
+                                              range: searchStr.startIndex..<searchStr.endIndex)
+        XCTAssertEqual(errMsg, "cat: invalid_file_name: No such file or directory\n")
     }
 
     func test_file_stdout() {
@@ -234,7 +240,7 @@
     
     func test_passthrough_environment() {
         do {
-            let output = try runTask(["/usr/bin/env"], environment: nil)
+            let (output, _) = try runTask(["/usr/bin/env"], environment: nil)
             let env = try parseEnv(output)
             XCTAssertGreaterThan(env.count, 0)
         } catch let error {
@@ -244,7 +250,7 @@
 
     func test_no_environment() {
         do {
-            let output = try runTask(["/usr/bin/env"], environment: [:])
+            let (output, _) = try runTask(["/usr/bin/env"], environment: [:])
             let env = try parseEnv(output)
             XCTAssertEqual(env.count, 0)
         } catch let error {
@@ -255,7 +261,7 @@
     func test_custom_environment() {
         do {
             let input = ["HELLO": "WORLD", "HOME": "CUPERTINO"]
-            let output = try runTask(["/usr/bin/env"], environment: input)
+            let (output, _) = try runTask(["/usr/bin/env"], environment: input)
             let env = try parseEnv(output)
             XCTAssertEqual(env, input)
         } catch let error {
@@ -267,9 +273,9 @@
         do {
             let previousWorkingDirectory = FileManager.default.currentDirectoryPath
 
-            // `bash` will not be found if the current working directory is not set correctly.
-            let _ = try runTask(["bash", "-c", "exit 0"], currentDirectoryPath: "/bin")
-
+            // Darwin Foundation requires the full path to the executable (.launchPath)
+            let (output, _) = try runTask(["/bin/bash", "-c", "pwd"], currentDirectoryPath: "/bin")
+            XCTAssertEqual(output.trimmingCharacters(in: .newlines), "/bin")
             XCTAssertEqual(previousWorkingDirectory, FileManager.default.currentDirectoryPath)
         } catch let error {
             XCTFail("Test failed: \(error)")
@@ -297,21 +303,26 @@
     case InvalidEnvironmentVariable(String)
 }
 
-private func runTask(_ arguments: [String], environment: [String: String]? = nil, currentDirectoryPath: String? = nil) throws -> String {
+private func runTask(_ arguments: [String], environment: [String: String]? = nil, currentDirectoryPath: String? = nil) throws -> (String, String) {
     let process = Process()
 
     var arguments = arguments
     process.launchPath = arguments.removeFirst()
     process.arguments = arguments
-    process.environment = environment
+    // Darwin Foundation doesnt allow .environment to be set to nil although the documentation
+    // says it is an optional. https://developer.apple.com/documentation/foundation/process/1409412-environment
+    if let e = environment {
+        process.environment = e
+    }
 
     if let directoryPath = currentDirectoryPath {
         process.currentDirectoryPath = directoryPath
     }
 
-    let pipe = Pipe()
-    process.standardOutput = pipe
-    process.standardError = pipe
+    let stdoutPipe = Pipe()
+    let stderrPipe = Pipe()
+    process.standardOutput = stdoutPipe
+    process.standardError = stderrPipe
     process.launch()
     process.waitUntilExit()
 
@@ -319,12 +330,17 @@
         throw Error.TerminationStatus(process.terminationStatus)
     }
 
-    let data = pipe.fileHandleForReading.availableData
-    guard let output = String(data: data, encoding: .utf8) else {
-        throw Error.UnicodeDecodingError(data)
+    let stdoutData = stdoutPipe.fileHandleForReading.availableData
+    guard let stdout = String(data: stdoutData, encoding: .utf8) else {
+        throw Error.UnicodeDecodingError(stdoutData)
     }
 
-    return output
+    let stderrData = stderrPipe.fileHandleForReading.availableData
+    guard let stderr = String(data: stderrData, encoding: .utf8) else {
+        throw Error.UnicodeDecodingError(stderrData)
+    }
+
+    return (stdout, stderr)
 }
 
 private func parseEnv(_ env: String) throws -> [String: String] {
diff --git a/TestFoundation/TestThread.swift b/TestFoundation/TestThread.swift
index 28478a4..6098b02 100644
--- a/TestFoundation/TestThread.swift
+++ b/TestFoundation/TestThread.swift
@@ -41,14 +41,15 @@
     
     func test_threadStart() {
         let condition = NSCondition()
+        condition.lock()
+
         let thread = Thread() {
             condition.lock()
             condition.broadcast()
             condition.unlock()
         }
         thread.start()
-        
-        condition.lock()
+
         let ok = condition.wait(until: Date(timeIntervalSinceNow: 10))
         condition.unlock()
         XCTAssertTrue(ok, "NSCondition wait timed out")
@@ -95,6 +96,8 @@
         XCTAssertTrue(c.isEqual(t))
 
         let condition = NSCondition()
+        condition.lock()
+
         let thread = Thread() {
             condition.lock()
             XCTAssertFalse(Thread.isMainThread)
@@ -104,7 +107,6 @@
         }
         thread.start()
 
-        condition.lock()
         let ok = condition.wait(until: Date(timeIntervalSinceNow: 10))
         condition.unlock()
         XCTAssertTrue(ok, "NSCondition wait timed out")
diff --git a/TestFoundation/TestUserDefaults.swift b/TestFoundation/TestUserDefaults.swift
index de01e04..86a2e97 100644
--- a/TestFoundation/TestUserDefaults.swift
+++ b/TestFoundation/TestUserDefaults.swift
@@ -18,9 +18,26 @@
 class TestUserDefaults : XCTestCase {
 	static var allTests : [(String, (TestUserDefaults) -> () throws -> ())] {
 		return [
-			// __kCFXMLPropertyListDomainCallBacks is causing a failure
-			// ("test_createUserDefaults", test_createUserDefaults ),
-			// ("test_getRegisteredDefaultItem", test_getRegisteredDefaultItem ),
+			("test_createUserDefaults", test_createUserDefaults ),
+			("test_getRegisteredDefaultItem", test_getRegisteredDefaultItem ),
+			("test_getRegisteredDefaultItem_NSString", test_getRegisteredDefaultItem_NSString ),
+			("test_getRegisteredDefaultItem_String", test_getRegisteredDefaultItem_String ),
+			("test_getRegisteredDefaultItem_NSURL", test_getRegisteredDefaultItem_NSURL ),
+			("test_getRegisteredDefaultItem_URL", test_getRegisteredDefaultItem_URL ),
+			("test_getRegisteredDefaultItem_NSData", test_getRegisteredDefaultItem_NSData ),
+			("test_getRegisteredDefaultItem_Data)", test_getRegisteredDefaultItem_Data ),
+			("test_getRegisteredDefaultItem_BoolFromString", test_getRegisteredDefaultItem_BoolFromString ),
+			("test_getRegisteredDefaultItem_IntFromString", test_getRegisteredDefaultItem_IntFromString ),
+			("test_getRegisteredDefaultItem_DoubleFromString", test_getRegisteredDefaultItem_DoubleFromString ),
+			("test_setValue_NSString", test_setValue_NSString ),
+			("test_setValue_String", test_setValue_String ),
+			("test_setValue_NSURL", test_setValue_NSURL ),
+			("test_setValue_URL", test_setValue_URL ),
+			("test_setValue_NSData", test_setValue_NSData ),
+			("test_setValue_Data", test_setValue_Data ),
+			("test_setValue_BoolFromString", test_setValue_BoolFromString ),
+			("test_setValue_IntFromString", test_setValue_IntFromString ),
+			("test_setValue_DoubleFromString", test_setValue_DoubleFromString ),
 		]
 	}
 
@@ -40,4 +57,197 @@
 		
 		XCTAssertEqual(defaults.integer(forKey: "key1"), 5)
 	}
+	
+	func test_getRegisteredDefaultItem_NSString() {
+		let defaults = UserDefaults.standard
+		
+		// Register a NSString value. UserDefaults.string(forKey:) is supposed to return the NSString as a String
+		defaults.register(defaults: ["key1": "hello" as NSString])
+		
+		//make sure we don't have anything in the saved plist.
+		defaults.removeObject(forKey: "key1")
+		
+		XCTAssertEqual(defaults.string(forKey: "key1"), "hello")
+	}
+
+	func test_getRegisteredDefaultItem_String() {
+		let defaults = UserDefaults.standard
+		
+		// Register a String value. UserDefaults.string(forKey:) is supposed to return the String
+		defaults.register(defaults: ["key1": "hello"])
+		
+		//make sure we don't have anything in the saved plist.
+		defaults.removeObject(forKey: "key1")
+		
+		XCTAssertEqual(defaults.string(forKey: "key1"), "hello")
+	}
+
+	func test_getRegisteredDefaultItem_NSURL() {
+		let defaults = UserDefaults.standard
+		
+		// Register an NSURL value. UserDefaults.url(forKey:) is supposed to return the URL
+		defaults.register(defaults: ["key1": NSURL(fileURLWithPath: "/hello/world")])
+		
+		//make sure we don't have anything in the saved plist.
+		defaults.removeObject(forKey: "key1")
+		
+		XCTAssertEqual(defaults.url(forKey: "key1"), URL(fileURLWithPath: "/hello/world"))
+	}
+
+	func test_getRegisteredDefaultItem_URL() {
+		let defaults = UserDefaults.standard
+		
+		// Register an URL value. UserDefaults.url(forKey:) is supposed to return the URL
+		defaults.register(defaults: ["key1": URL(fileURLWithPath: "/hello/world")])
+		
+		//make sure we don't have anything in the saved plist.
+		defaults.removeObject(forKey: "key1")
+		
+		XCTAssertEqual(defaults.url(forKey: "key1"), URL(fileURLWithPath: "/hello/world"))
+	}
+
+	func test_getRegisteredDefaultItem_NSData() {
+		let defaults = UserDefaults.standard
+		let bytes = [0, 1, 2, 3, 4] as [UInt8]
+		
+		// Register an NSData value. UserDefaults.data(forKey:) is supposed to return the Data
+		defaults.register(defaults: ["key1": NSData(bytes: bytes, length: bytes.count)])
+		
+		//make sure we don't have anything in the saved plist.
+		defaults.removeObject(forKey: "key1")
+		
+		XCTAssertEqual(defaults.data(forKey: "key1"), Data(bytes: bytes))
+	}
+	
+	func test_getRegisteredDefaultItem_Data() {
+		let defaults = UserDefaults.standard
+		let bytes = [0, 1, 2, 3, 4] as [UInt8]
+		
+		// Register a Data value. UserDefaults.data(forKey:) is supposed to return the Data
+		defaults.register(defaults: ["key1": Data(bytes: bytes)])
+		
+		//make sure we don't have anything in the saved plist.
+		defaults.removeObject(forKey: "key1")
+		
+		XCTAssertEqual(defaults.data(forKey: "key1"), Data(bytes: bytes))
+	}
+
+	func test_getRegisteredDefaultItem_BoolFromString() {
+		let defaults = UserDefaults.standard
+		
+		// Register a boolean default value as a string. UserDefaults.bool(forKey:) is supposed to return the parsed Bool value
+		defaults.register(defaults: ["key1": "YES"])
+		
+		//make sure we don't have anything in the saved plist.
+		defaults.removeObject(forKey: "key1")
+		
+		XCTAssertEqual(defaults.bool(forKey: "key1"), true)
+	}
+	
+	func test_getRegisteredDefaultItem_IntFromString() {
+		let defaults = UserDefaults.standard
+		
+		// Register an int default value as a string. UserDefaults.integer(forKey:) is supposed to return the parsed Int value
+		defaults.register(defaults: ["key1": "1234"])
+		
+		//make sure we don't have anything in the saved plist.
+		defaults.removeObject(forKey: "key1")
+		
+		XCTAssertEqual(defaults.integer(forKey: "key1"), 1234)
+	}
+	
+	func test_getRegisteredDefaultItem_DoubleFromString() {
+		let defaults = UserDefaults.standard
+		
+		// Register a double default value as a string. UserDefaults.double(forKey:) is supposed to return the parsed Double value
+		defaults.register(defaults: ["key1": "12.34"])
+		
+		//make sure we don't have anything in the saved plist.
+		defaults.removeObject(forKey: "key1")
+		
+		XCTAssertEqual(defaults.double(forKey: "key1"), 12.34)
+	}
+	
+	func test_setValue_NSString() {
+		let defaults = UserDefaults.standard
+		
+		// Set a NSString value. UserDefaults.string(forKey:) is supposed to return the NSString as a String
+		defaults.set("hello" as NSString, forKey: "key1")
+		
+		XCTAssertEqual(defaults.string(forKey: "key1"), "hello")
+	}
+	
+	func test_setValue_String() {
+		let defaults = UserDefaults.standard
+		
+		// Register a String value. UserDefaults.string(forKey:) is supposed to return the String
+		defaults.set("hello", forKey: "key1")
+		
+		XCTAssertEqual(defaults.string(forKey: "key1"), "hello")
+	}
+
+	func test_setValue_NSURL() {
+		let defaults = UserDefaults.standard
+		
+		// Set a NSURL value. UserDefaults.url(forKey:) is supposed to return the NSURL as a URL
+		defaults.set(NSURL(fileURLWithPath: "/hello/world"), forKey: "key1")
+		
+		XCTAssertEqual(defaults.url(forKey: "key1"), URL(fileURLWithPath: "/hello/world"))
+	}
+
+	func test_setValue_URL() {
+		let defaults = UserDefaults.standard
+		
+		// Set a URL value. UserDefaults.url(forKey:) is supposed to return the URL
+		defaults.set(URL(fileURLWithPath: "/hello/world"), forKey: "key1")
+		
+		XCTAssertEqual(defaults.url(forKey: "key1"), URL(fileURLWithPath: "/hello/world"))
+	}
+
+	func test_setValue_NSData() {
+		let defaults = UserDefaults.standard
+		let bytes = [0, 1, 2, 3, 4] as [UInt8]
+		
+		// Set a NSData value. UserDefaults.data(forKey:) is supposed to return the Data
+		defaults.set(NSData(bytes: bytes, length: bytes.count), forKey: "key1")
+		
+		XCTAssertEqual(defaults.data(forKey: "key1"), Data(bytes: bytes))
+	}
+	
+	func test_setValue_Data() {
+		let defaults = UserDefaults.standard
+		let bytes = [0, 1, 2, 3, 4] as [UInt8]
+		
+		// Set a Data value. UserDefaults.data(forKey:) is supposed to return the Data
+		defaults.set(Data(bytes: bytes), forKey: "key1")
+		
+		XCTAssertEqual(defaults.data(forKey: "key1"), Data(bytes: bytes))
+	}
+
+	func test_setValue_BoolFromString() {
+		let defaults = UserDefaults.standard
+		
+		// Register a boolean default value as a string. UserDefaults.bool(forKey:) is supposed to return the parsed Bool value
+		defaults.set("YES", forKey: "key1")
+		
+		XCTAssertEqual(defaults.bool(forKey: "key1"), true)
+	}
+	
+	func test_setValue_IntFromString() {
+		let defaults = UserDefaults.standard
+		
+		// Register an int default value as a string. UserDefaults.integer(forKey:) is supposed to return the parsed Int value
+		defaults.set("1234", forKey: "key1")
+		
+		XCTAssertEqual(defaults.integer(forKey: "key1"), 1234)
+	}
+	
+	func test_setValue_DoubleFromString() {
+		let defaults = UserDefaults.standard
+		
+		// Register a double default value as a string. UserDefaults.double(forKey:) is supposed to return the parsed Double value
+		defaults.set("12.34", forKey: "key1")
+		
+		XCTAssertEqual(defaults.double(forKey: "key1"), 12.34)
+	}
 }
diff --git a/TestFoundation/TestUtils.swift b/TestFoundation/TestUtils.swift
index b587b36..860e129 100644
--- a/TestFoundation/TestUtils.swift
+++ b/TestFoundation/TestUtils.swift
@@ -41,7 +41,7 @@
                     print(err)
                     return false
                 }
-            } else if !isDir {
+            } else if !isDir.boolValue {
                 return false
             }
             
diff --git a/TestFoundation/XDGTestHelper.swift b/TestFoundation/XDGTestHelper.swift
index bdf58a0..9692199 100644
--- a/TestFoundation/XDGTestHelper.swift
+++ b/TestFoundation/XDGTestHelper.swift
@@ -35,7 +35,7 @@
         storage.setCookie(simpleCookie!)
         let fm = FileManager.default
         let destPath = xdg_data_home! + "/xdgTestHelper/.cookies.shared"
-        var isDir = false
+        var isDir: ObjCBool = false
         let exists = fm.fileExists(atPath: destPath, isDirectory: &isDir) 
         if (!exists) {
             print("Expected cookie path: ", destPath)
diff --git a/build.py b/build.py
index 00d8588..efe0ce1 100755
--- a/build.py
+++ b/build.py
@@ -282,6 +282,7 @@
 	'CoreFoundation/PlugIn.subproj/CFPlugIn_PlugIn.c',
 	'CoreFoundation/Preferences.subproj/CFApplicationPreferences.c',
 	'CoreFoundation/Preferences.subproj/CFPreferences.c',
+    'CoreFoundation/Preferences.subproj/CFXMLPreferencesDomain.c',
 	# 'CoreFoundation/RunLoop.subproj/CFMachPort.c',
 	# 'CoreFoundation/RunLoop.subproj/CFMessagePort.c',
 	'CoreFoundation/RunLoop.subproj/CFRunLoop.c',