diff --git a/lib/Serialization/ASTReader.cpp b/lib/Serialization/ASTReader.cpp
index b4946ab..c5edca7 100644
--- a/lib/Serialization/ASTReader.cpp
+++ b/lib/Serialization/ASTReader.cpp
@@ -4050,16 +4050,18 @@
     // If this module was validated as a system module, and it is now a
     // non-system module, emit a warning about ignoring differences in
     // diagnostic options.
+    //
+    // In any case, if it has been validated in this process somehow, we're
+    // stuck with it (and it should be good enough!).
     bool IsSystem;
     if (BufferMgr->isValidatedByAncestor(F.FileName, IsSystem)) {
       if (IsSystem) {
         Module *M = getTopImportImplicitModule(ModuleMgr, PP);
         assert(M && "missing module");
-        if (!M->IsSystem) {
+        if (!M->IsSystem)
           Diag(diag::warn_module_system_bit_conflict) << F.FileName;
-          return Success;
-        }
       }
+      return Success;
     }
   }
 
diff --git a/lib/Serialization/ModuleManager.cpp b/lib/Serialization/ModuleManager.cpp
index cefacc1..d3c1a5d 100644
--- a/lib/Serialization/ModuleManager.cpp
+++ b/lib/Serialization/ModuleManager.cpp
@@ -93,13 +93,7 @@
     New->FileName = FileName.str();
     New->File = Entry;
     New->ImportLoc = ImportLoc;
-    Chain.push_back(New);
-    if (!New->isModule())
-      PCHChain.push_back(New);
-    if (!ImportedBy)
-      Roots.push_back(New);
     NewModule = true;
-    ModuleEntry = New;
 
     New->InputFilesValidationTimestamp = 0;
     if (New->Kind == MK_ImplicitModule) {
@@ -146,6 +140,34 @@
 
     // Initialize the stream.
     PCHContainerRdr.ExtractPCH(New->Buffer->getMemBufferRef(), New->StreamFile);
+
+    if (ExpectedSignature != ASTFileSignature({{0}})) {
+      // Check the signature before hooking it up, so we can still delete it
+      // safely.
+      New->Signature = ReadSignature(New->StreamFile);
+      if (New->Signature != ExpectedSignature) {
+        ErrorStr = New->Signature != ASTFileSignature({{0}})
+                       ? "signature mismatch"
+                       : "could not read module signature";
+
+        bool IsSystem;
+        if (!FileMgr.getPCMCache()->isValidatedByAncestor(New->FileName,
+                                                          IsSystem)) {
+          FileMgr.invalidateCache(New->File);
+          FileMgr.getPCMCache()->removeFromConsistentBuffer(New->FileName);
+        }
+        delete New;
+        return OutOfDate;
+      }
+    }
+
+    // We're keeping this module.  Hook it up.
+    Chain.push_back(New);
+    if (!New->isModule())
+      PCHChain.push_back(New);
+    if (!ImportedBy)
+      Roots.push_back(New);
+    ModuleEntry = New;
   } else if (ExpectedSignature != ASTFileSignature({{0}})) {
     // Only check the signature here if the module has already been loaded.  New
     // modules may be invalidated and rebuilt, and the logic for that is in
diff --git a/test/Modules/Inputs/warning-mismatch/Mismatch.h b/test/Modules/Inputs/warning-mismatch/Mismatch.h
new file mode 100644
index 0000000..a07b0ee
--- /dev/null
+++ b/test/Modules/Inputs/warning-mismatch/Mismatch.h
@@ -0,0 +1 @@
+struct Mismatch { int i; };
diff --git a/test/Modules/Inputs/warning-mismatch/System.h b/test/Modules/Inputs/warning-mismatch/System.h
new file mode 100644
index 0000000..8e69e70
--- /dev/null
+++ b/test/Modules/Inputs/warning-mismatch/System.h
@@ -0,0 +1,2 @@
+#import "Mismatch.h"
+struct System { int i; };
diff --git a/test/Modules/Inputs/warning-mismatch/module.modulemap b/test/Modules/Inputs/warning-mismatch/module.modulemap
new file mode 100644
index 0000000..c22cde4
--- /dev/null
+++ b/test/Modules/Inputs/warning-mismatch/module.modulemap
@@ -0,0 +1,7 @@
+module System [system] {
+  header "System.h"
+}
+
+module Mismatch {
+  header "Mismatch.h"
+}
diff --git a/test/Modules/warning-mismatch.m b/test/Modules/warning-mismatch.m
new file mode 100644
index 0000000..7c83664
--- /dev/null
+++ b/test/Modules/warning-mismatch.m
@@ -0,0 +1,9 @@
+// RUN: rm -rf %t.cache
+// RUN: echo "@import Mismatch;" >%t.m
+// RUN: %clang_cc1 -Wno-system-headers -fdisable-module-hash -fmodules-cache-path=%t.cache -fmodules -fimplicit-module-maps -I %S/Inputs/warning-mismatch %t.m -fsyntax-only -Rmodule-build
+// RUN: echo -----------------------------------------------
+// RUN: %clang_cc1 -Wsystem-headers -fdisable-module-hash -fmodules-cache-path=%t.cache -fmodules -fimplicit-module-maps -I %S/Inputs/warning-mismatch %s -fsyntax-only -Rmodule-build
+
+// This testcase triggers a warning flag mismatch in an already validated header.
+@import Mismatch;
+@import System;