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Clang |release| |ReleaseNotesTitle|
.. contents::
:depth: 2
Written by the `LLVM Team <>`_
.. only:: PreRelease
.. warning::
These are in-progress notes for the upcoming Clang |version| release.
Release notes for previous releases can be found on
`the Releases Page <>`_.
This document contains the release notes for the Clang C/C++/Objective-C
frontend, part of the LLVM Compiler Infrastructure, release |release|. Here we
describe the status of Clang in some detail, including major
improvements from the previous release and new feature work. For the
general LLVM release notes, see `the LLVM
documentation <>`_. For the libc++ release notes,
see `this page <>`_. All LLVM releases
may be downloaded from the `LLVM releases web site <>`_.
For more information about Clang or LLVM, including information about the
latest release, please see the `Clang Web Site <>`_ or the
`LLVM Web Site <>`_.
Potentially Breaking Changes
These changes are ones which we think may surprise users when upgrading to
Clang |release| because of the opportunity they pose for disruption to existing
code bases.
- Clang's default C++/ObjC++ standard is now ``gnu++17`` instead of ``gnu++14``.
This means Clang will by default accept code using features from C++17 and
conforming GNU extensions. Projects incompatible with C++17 can add
``-std=gnu++14`` to their build settings to restore the previous behaviour.
- The ``-fexperimental-new-pass-manager`` and ``-fno-legacy-pass-manager``
flags have been removed. These flags have been silently ignored since Clang 15.
- Clang's resource directory path previously included the full clang version.
It now includes only the major version. The new resource directory is
``$prefix/lib/clang/$CLANG_MAJOR_VERSION`` and can be queried using
``clang -print-resource-dir``, just like before.
- The ``-Wimplicit-function-declaration`` and ``-Wimplicit-int`` warnings
now default to an error in C99, C11, and C17. As of C2x,
support for implicit function declarations and implicit int has been removed,
and the warning options will have no effect. Specifying ``-Wimplicit-int`` in
C89 mode will now issue warnings instead of being a noop.
**NOTE**: We recommend that projects using configure scripts verify that the
results do not change before/after setting
``-Werror=implicit-function-declarations`` or ``-Wimplicit-int`` to avoid
incompatibility with Clang 16.
- ``-Wincompatible-function-pointer-types`` now defaults to an error in all C
language modes. It may be downgraded to a warning with
``-Wno-error=incompatible-function-pointer-types`` or disabled entirely with
**NOTE:** We recommend that projects using configure scripts verify that the
results do not change before/after setting
``-Werror=incompatible-function-pointer-types`` to avoid incompatibility with
Clang 16.
.. code-block:: c
void func(const int *i);
void other(void) {
void (*fp)(int *) = func; // Previously a warning, now a downgradable error.
- To match GCC, ``__ppc64__`` is no longer defined on PowerPC64 targets. Use
``__powerpc64__`` instead.
- ``-p`` is rejected for all targets which are not AIX or OpenBSD. ``-p`` led
to an ``-Wunused-command-line-argument`` warning in previous releases.
C/C++ Language Potentially Breaking Changes
- Clang now disallows types whose sizes aren't a multiple of their alignments
to be used as the element type of arrays.
.. code-block:: c
typedef char int8_a16 __attribute__((aligned(16)));
int8_a16 array[4]; // Now diagnosed as the element size not being a multiple of the array alignment.
- When compiling for Windows in MSVC compatibility mode (for example by using
clang-cl), the compiler will now propagate dllimport/export declspecs in
explicit specializations of class template member functions (`#54717
.. code-block:: c++
template <typename> struct __declspec(dllexport) S {
void f();
template<> void S<int>::f() {} // clang-cl will now dllexport this.
This matches what MSVC does, so it improves compatibility, but it can also
cause errors for code which clang-cl would previously accept, for example:
.. code-block:: c++
template <typename> struct __declspec(dllexport) S {
void f();
template<> void S<int>::f() = delete; // Error: cannot delete dllexport function.
.. code-block:: c++
template <typename> struct __declspec(dllimport) S {
void f();
template<> void S<int>::f() {}; // Error: cannot define dllimport function.
These errors also match MSVC's behavior.
- Clang now diagnoses indirection of ``void *`` in C++ mode as a warning which
defaults to an error. This is compatible with ISO C++, GCC, ICC, and MSVC. This
is also now a SFINAE error so constraint checking and SFINAE checking can be
compatible with other compilers. It is expected that this will be upgraded to
an error-only diagnostic in the next Clang release.
.. code-block:: c++
void func(void *p) {
*p; // Now diagnosed as a warning-as-error.
- Clang now diagnoses use of a bit-field as an instruction operand in Microsoft
style inline asm blocks as an error. Previously, a bit-field operand yielded
the address of the allocation unit the bit-field was stored within; reads or
writes therefore had the potential to read or write nearby bit-fields.
(`#57791 <>`_)
.. code-block:: c++
typedef struct S {
unsigned bf:1;
} S;
void f(S s) {
__asm {
mov eax, // Now diagnosed as an error.
mov, eax // Now diagnosed as an error.
- Clang now diagnoses if structs/unions with the same name are different in
different used modules. Behavior in C and Objective-C language modes now is
the same as in C++.
C++ Specific Potentially Breaking Changes
- Clang now prohibits non-inline externally-visible definitions in C++
standard header units as per ``[module.import/6]``. In Clang 15,
these definitions were allowed. Note that such definitions are ODR
violations if the header is included more than once.
- Clang will now correctly diagnose as ill-formed a constant expression where an
enum without a fixed underlying type is set to a value outside the range of
the enumeration's values.
.. code-block:: c++
enum E { Zero, One, Two, Three, Four };
constexpr E Val1 = (E)3; // Ok
constexpr E Val2 = (E)7; // Ok
constexpr E Val3 = (E)8; // Now diagnosed as out of the range [0, 7]
constexpr E Val4 = (E)-1; // Now diagnosed as out of the range [0, 7]
Due to the extended period of time this bug was present in major C++
implementations (including Clang), this error has the ability to be
downgraded into a warning (via: ``-Wno-error=enum-constexpr-conversion``) to
provide a transition period for users. This diagnostic is expected to turn
into an error-only diagnostic in the next Clang release.
(`#50055 <>`_)
- As a side effect of implementing DR692/DR1395/DR1432, Clang now rejects some
overloaded function templates as ambiguous when one of the candidates has a
trailing parameter pack.
.. code-block:: c++
template <typename T> void g(T, T = T());
template <typename T, typename... U> void g(T, U...);
void h() {
// This is rejected due to ambiguity between the pack and the
// default argument. Only parameters with arguments are considered during
// partial ordering of function templates.
ABI Changes in This Version
- GCC doesn't pack non-POD members in packed structs unless the packed
attribute is also specified on the member. Clang historically did perform
such packing. Clang now matches the gcc behavior
(except on Darwin, PS4 and AIX).
You can switch back to the old ABI behavior with the flag:
- GCC allows POD types to have defaulted special members. Clang historically
classified such types as non-POD (for the purposes of Itanium ABI). Clang now
matches the gcc behavior (except on Darwin, PS4, AIX and z/OS). You can switch
back to the old ABI behavior with the flag: ``-fclang-abi-compat=15.0``.
What's New in Clang |release|?
Some of the major new features and improvements to Clang are listed
here. Generic improvements to Clang as a whole or to its underlying
infrastructure are described first, followed by language-specific
sections with improvements to Clang's support for those languages.
C++ Language Changes
C++20 Feature Support
Newly implemented papers:
- Implemented `P0848: Conditionally Trivial Special Member Functions <>`_.
Note: The handling of deleted functions is not yet compliant, as Clang
does not implement `DR1496 <>`_
and `DR1734 <>`_.
- Implemented `P1091R3 <>`_ and `P1381R1 <>`_ to
support capturing structured bindings in lambdas (`#52720 <>`_,
`#54300 <>`_,
`#54301 <>`_,
`#49430 <>`_).
- Implemented `P2468R2: The Equality Operator You Are Looking For <>`_.
- Implemented `P2113R0: Proposed resolution for 2019 comment CA 112 <>`_
([temp.func.order]p6.2.1 is not implemented, matching GCC).
- Implemented `P0634R3: Down with typename! <>`_,
which removes the requirement for the ``typename`` keyword in certain contexts.
- Implemented `P0857R0: Fixing small-ish functionality gaps in constraints <>`_,
which specifies constrained lambdas and constrained template *template-parameter*\s.
- Implemented `P0960R3 <>`_
and `P1975R0 <>`_,
which allow parenthesized aggregate-initialization.
Notable Bug Fixes to C++20 Features:
- Clang now correctly delays the instantiation of function constraints until
the time of checking, which should now allow the libstdc++ ranges implementation
to work for at least trivial examples.
(`#44178 <>`_)
- Consider explicitly defaulted constexpr/consteval special member function
template instantiation to be constexpr/consteval even though a call to such
a function cannot appear in a constant expression.
(C++14 [dcl.constexpr]p6 (CWG DR647/CWG DR1358))
- Correctly defer dependent immediate function invocations until template instantiation.
(`#55601 <>`_)
- Correctly set expression evaluation context as 'immediate function context' in
consteval functions.
(`#51182 <>`_)
- Skip rebuilding lambda expressions in arguments of immediate invocations.
(`#56183 <>`_,
`#51695 <>`_,
`#50455 <>`_,
`#54872 <>`_,
`#54587 <>`_)
- Clang implements DR2621, correcting a defect in ``using enum`` handling. The
name is found via ordinary lookup so typedefs are found.
- Implemented CWG2635 as a Defect Report, which prohibits structured bindings from being constrained.
- When using header modules, inclusion of a private header and violations of
the `use-declaration rules
<>`_ are now
diagnosed even when the includer is a textual header. This change can be
temporarily reversed with ``-Xclang
-fno-modules-validate-textual-header-includes``, but this flag will be
removed in a future Clang release.
C++2b Feature Support
- Implemented `P2324R2: Support label at end of compound statement <>`_.
- Implemented `P1169R4: static operator() <>`_ and `P2589R1: static operator[] <>`_.
- Implemented `P2513R3: char8_t Compatibility and Portability Fix <>`_.
This change was applied to C++20 as a Defect Report.
- Implemented `P2647R1: Permitting static constexpr variables in constexpr functions <>`_.
- Implemented `CWG2640: Allow more characters in an n-char sequence <>`_.
Resolutions to C++ Defect Reports
- Implemented `DR692 <>`_, `DR1395 <>`_,
and `DR1432 <>`_. The fix for DR1432 is speculative since the
issue is still open and has no proposed resolution at this time. A speculative fix
for DR1432 is needed to prevent regressions that would otherwise occur due to DR692.
- Implemented `DR2358 <>`_, which allows init captures in lambdas in default arguments.
- Implemented `DR2654 <>`_ which undeprecates
all compound assignments operations on volatile qualified variables.
- Implemented `DR2631 <>`_. Invalid ``consteval`` calls in default arguments and default
member initializers are diagnosed when and if the default is used.
This changes the value of ``std::source_location::current()``
used in default parameters calls compared to previous versions of Clang.
(`#56379 <>`_)
C Language Changes
- Adjusted ``-Wformat`` warnings according to `WG14 N2562 <>`_.
Clang will now consider default argument promotions in ``printf``, and remove
unnecessary warnings. Especially ``int`` argument with specifier ``%hhd`` and
C2x Feature Support
- Implemented `WG14 N2662 <>`_,
so the ``[[maybe_unused]]`` attribute may be applied to a label to silence an
``-Wunused-label`` warning.
- Implemented `WG14 N2508 <>`_,
so labels can placed everywhere inside a compound statement.
- Implemented `WG14 N2927 <>`_,
the Not-so-magic ``typeof`` operator. Also implemented
`WG14 N2930 <>`_,
renaming ``remove_quals``, so the ``typeof_unqual`` operator is also
supported. Both of these operators are supported only in C2x mode. The
``typeof`` operator specifies the type of the given parenthesized expression
operand or type name, including all qualifiers. The ``typeof_unqual``
operator is similar to ``typeof`` except that all qualifiers are removed,
including atomic type qualification and type attributes which behave like a
qualifier, such as an address space attribute.
.. code-block:: c
__attribute__((address_space(1))) const _Atomic int Val;
typeof(Val) OtherVal; // type is '__attribute__((address_space(1))) const _Atomic int'
typeof_unqual(Val) OtherValUnqual; // type is 'int'
- Implemented `WG14 N3042 <>`_,
Introduce the nullptr constant. This introduces a new type ``nullptr_t``,
declared in ``<stddef.h>`` which represents the type of the null pointer named
constant, ``nullptr``. This constant is implicitly convertible to any pointer
type and represents a type-safe null value.
Note, there are some known incompatibilities with this same feature in C++.
The following examples were discovered during implementation and are subject
to change depending on how national body comments are resolved by WG14 (C
status is based on standard requirements, not necessarily implementation
.. code-block:: c
nullptr_t null_val;
(nullptr_t)nullptr; // Rejected in C, accepted in C++, Clang accepts
(void)(1 ? nullptr : 0); // Rejected in C, accepted in C++, Clang rejects
(void)(1 ? null_val : 0); // Rejected in C, accepted in C++, Clang rejects
bool b1 = nullptr; // Accepted in C, rejected in C++, Clang rejects
b1 = null_val; // Accepted in C, rejected in C++, Clang rejects
null_val = 0; // Rejected in C, accepted in C++, Clang rejects
void func(nullptr_t);
func(0); // Rejected in C, accepted in C++, Clang rejects
- Implemented `WG14 N2975 <>`_,
Relax requirements for va_start. In C2x mode, functions can now be declared
fully variadic and the ``va_start`` macro no longer requires passing a second
argument (though it accepts a second argument for backwards compatibility).
If a second argument is passed, it is neither expanded nor evaluated in C2x
.. code-block:: c
void func(...) { // Invalid in C17 and earlier, valid in C2x and later.
va_list list;
va_start(list); // Invalid in C17 and earlier, valid in C2x and later.
- Diagnose type definitions in the ``type`` argument of ``__builtin_offsetof``
as a conforming C extension according to
`WG14 N2350 <>`_.
Also documents the builtin appropriately. Note, a type definition in C++
continues to be rejected.
OpenCL Kernel Language Changes
- Improved diagnostics for C++ templates used in kernel arguments.
- Removed redundant pragma for the ``arm-integer-dot-product`` extension.
- Improved support of enqueued block for AMDGPU.
- Added ``nounwind`` attribute implicitly to all functions.
- Improved builtin functions support:
* Allow disabling default header-based feature/extension support by passing ``-D__undef_<feature>``.
* Fixed conditional definition of the depth image and ``read_write image3d`` builtins.
Non-comprehensive list of changes in this release
- It's now possible to set the crash diagnostics directory through
the environment variable ``CLANG_CRASH_DIAGNOSTICS_DIR``.
The ``-fcrash-diagnostics-dir`` flag takes precedence.
- Unicode support has been updated to support Unicode 15.0.
New unicode codepoints are supported as appropriate in diagnostics,
C and C++ identifiers, and escape sequences.
- In identifiers, Clang allows a restricted set of additional mathematical symbols
as an extension. These symbols correspond to a proposed Unicode
`Mathematical notation profile for default identifiers
(`#54732 <>`_)
- Clang now supports loading multiple configuration files. The files from
default configuration paths are loaded first, unless ``--no-default-config``
option is used. All files explicitly specified using ``--config=`` option
are loaded afterwards.
- When loading default configuration files, clang now unconditionally uses
the real target triple (respecting options such as ``--target=`` and ``-m32``)
rather than the executable prefix. The respective configuration files are
also loaded when clang is called via an executable without a prefix (e.g.
plain ``clang``).
- Default configuration paths were partially changed. Clang now attempts to load
``<triple>-<driver>.cfg`` first, and falls back to loading both
``<driver>.cfg`` and ``<triple>.cfg`` if the former is not found. `Triple`
is the target triple and `driver` first tries the canonical name
for the driver (respecting ``--driver-mode=``), and then the name found
in the executable.
- If the environment variable ``SOURCE_DATE_EPOCH`` is set, it specifies a UNIX
timestamp to be used in replacement of the current date and time in
the ``__DATE__``, ``__TIME__``, and ``__TIMESTAMP__`` macros. See
- Clang now supports ``__has_constexpr_builtin`` function-like macro that
evaluates to 1 if the builtin is supported and can be constant evaluated.
It can be used to writing conditionally constexpr code that uses builtins.
- The time profiler (using ``-ftime-trace`` option) now traces various constant
evaluation events.
- Clang can now generate a PCH when using ``-fdelayed-template-parsing`` for
code with templates containing loop hint pragmas, OpenMP pragmas, and
``#pragma unused``.
New Compiler Flags
- Implemented ``-fcoro-aligned-allocation`` flag. This flag implements
Option 2 of P2014R0 aligned allocation of coroutine frames
(`P2014R0 <>`_).
With this flag, the coroutines will try to lookup aligned allocation
function all the time. The compiler will emit an error if it fails to
find aligned allocation function. So if the user code implemented self
defined allocation function for coroutines, the existing code will be
broken. A little divergence with P2014R0 is that clang will lookup
``::operator new(size_­t, std::aligned_val_t, nothrow_­t)`` if there is
``get_­return_­object_­on_­allocation_­failure``. We feel this is more consistent
with the intention.
- Added ``--no-default-config`` to disable automatically loading configuration
files using default paths.
- Added the new level, ``3``, to the ``-fstrict-flex-arrays=`` flag. The new
level is the strict, standards-conforming mode for flexible array members. It
recognizes only incomplete arrays as flexible array members (which is how the
feature is defined by the C standard).
.. code-block:: c
struct foo {
int a;
int b[]; // Flexible array member.
struct bar {
int a;
int b[0]; // NOT a flexible array member.
- Added ``-fmodule-output`` to enable the one-phase compilation model for
standard C++ modules. See
`Standard C++ Modules <>`_
for more information.
- Added ``-Rpass-analysis=stack-frame-layout`` which will emit new diagnostic
information about the layout of stack frames through the remarks
infrastructure. Since it uses remarks the diagnostic information is available
both on the CLI, and in a machine readable format.
Deprecated Compiler Flags
- ``-enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang``
has been deprecated. The flag will be removed in Clang 18.
``-ftrivial-auto-var-init=zero`` is now available unconditionally, to be
compatible with GCC.
- ``-fcoroutines-ts`` has been deprecated. The flag will be removed in Clang 17.
Please use ``-std=c++20`` or higher to use standard C++ coroutines instead.
- ``-fmodules-ts`` has been deprecated. The flag will be removed in Clang 17.
Please use ``-std=c++20`` or higher to use standard C++ modules instead.
Modified Compiler Flags
- Clang now permits specifying ``--config=`` multiple times, to load multiple
configuration files.
- The option ``-rtlib=platform`` can now be used for all targets to override
a different option value (either one set earlier on the command line, or a
built-in hardcoded default). (Previously the MSVC and Darwin targets didn't
allow this parameter combination.)
Removed Compiler Flags
- Clang now no longer supports ``-cc1 -fconcepts-ts``. This flag has been deprecated
and encouraged use of ``-std=c++20`` since Clang 10, so we're now removing it.
Attribute Changes in Clang
- Added support for ``__attribute__((guard(nocf)))`` and C++-style
``[[clang::guard(nocf)]]``, which is equivalent to ``__declspec(guard(nocf))``
when using the MSVC environment. This is to support enabling Windows Control
Flow Guard checks with the ability to disable them for specific functions when
using the MinGW environment. This attribute is only available for Windows
- Introduced a new function attribute ``__attribute__((nouwtable))`` to suppress
LLVM IR ``uwtable`` function attribute.
- Updated the value returned by ``__has_c_attribute(nodiscard)`` to ``202003L``
based on the final date specified by the C2x committee draft. We already
supported the ability to specify a message in the attribute, so there were no
changes to the attribute behavior.
- Updated the value returned by ``__has_c_attribute(fallthrough)`` to ``201910L``
based on the final date specified by the C2x committee draft. We previously
used ``201904L`` (the date the proposal was seen by the committee) by mistake.
There were no other changes to the attribute behavior.
- Introduced a new record declaration attribute ``__attribute__((enforce_read_only_placement))``
to support analysis of instances of a given type focused on read-only program
memory placement. It emits a warning if something in the code provably prevents
an instance from a read-only memory placement.
- Introduced new attribute ``__attribute__((target_version("cpu_features")))``
and expanded the functionality of the existing attribute
``__attribute__((target_clones("cpu_features1","cpu_features2",...)))`` to
support Function Multi Versioning on AArch64 target. It detects at runtime
which function versions are supported by CPU and calls the one with highest
priority. Refer to `clang attributes
<>`_ for
more details.
Improvements to Clang's diagnostics
- Disabled a FIX-IT suggested for a case of bad conversion in system headers.
- Clang will now check compile-time determinable string literals as format strings.
(`#55805 <>`_)
- ``-Wformat`` now recognizes ``%b`` for the ``printf``/``scanf`` family of
functions and ``%B`` for the ``printf`` family of functions.
(`#56885 <>`_)
- Introduced ``-Wsingle-bit-bitfield-constant-conversion``, grouped under
``-Wbitfield-constant-conversion``, which diagnoses implicit truncation when
``1`` is assigned to a 1-bit signed integer bitfield. To reduce
potential false positives, this diagnostic will not diagnose use of the
``true`` macro (from ``<stdbool.h>``) in C language mode despite the macro
being defined to expand to ``1``. (`#53253 <>`_)
- Clang will now print more information about failed static assertions. In
particular, simple static assertion expressions are evaluated to their
compile-time value and printed out if the assertion fails.
- Diagnostics about uninitialized ``constexpr`` variables have been improved
to mention the missing constant initializer.
- Correctly diagnose a future keyword if it exists as a keyword in the higher
language version and specifies in which version it will be a keyword. This
supports both C and C++.
- ``no_sanitize("...")`` on a global variable for known but not relevant
sanitizers is now just a warning. It now says that this will be ignored
instead of incorrectly saying no_sanitize only applies to functions and
- No longer mention ``reinterpet_cast`` in the invalid constant expression
diagnostic note when in C mode.
- Clang will now give a more suitale diagnostic for declaration of block
scope identifiers that have external/internal linkage that has an initializer.
(`#57478 <>`_)
- A new analysis pass will helps preserve sugar when combining deductions, in an
order agnostic way. This will be in effect when deducing template arguments,
when deducing function return type from multiple return statements, for the
conditional operator, and for most binary operations. Type sugar is combined
in a way that strips the sugar which is different between terms, and preserves
those which are common.
- Correctly diagnose the use of an integer literal without a suffix whose
underlying type is ``long long`` or ``unsigned long long`` as an extension in
C89 mode . Clang previously only diagnosed if the literal had an explicit
``LL`` suffix.
- Clang now correctly diagnoses index that refers past the last possible element
of FAM-like arrays.
- Clang now correctly emits a warning when dereferencing a void pointer in C mode.
(`#53631 <>`_)
- Clang will now diagnose an overload set where a candidate has a constraint that
refers to an expression with a previous error as nothing viable, so that it
doesn't generate strange cascading errors, particularly in cases where a
subsuming constraint fails, which would result in a less-specific overload to
be selected.
- Add a fix-it hint for the ``-Wdefaulted-function-deleted`` warning to
explicitly delete the function.
- Fixed an accidental duplicate diagnostic involving the declaration of a
function definition without a prototype which is preceded by a static
declaration of the function with a prototype.
(`#58181 <>`_)
- Copy-elided initialization of lock scopes is now handled differently in
``-Wthread-safety-analysis``: annotations on the move constructor are no
longer taken into account, in favor of annotations on the function returning
the lock scope by value. This could result in new warnings if code depended
on the previous undocumented behavior. As a side effect of this change,
constructor calls outside of initializer expressions are no longer ignored,
which can result in new warnings (or make existing warnings disappear).
- The wording of diagnostics regarding arithmetic on fixed-sized arrays and
pointers is improved to include the type of the array and whether it's cast
to another type. This should improve comprehension for why an index is
- Clang now correctly points to the problematic parameter for the ``-Wnonnull``
warning. (`#58273 <>`_)
- Introduced ``-Wcast-function-type-strict`` and
``-Wincompatible-function-pointer-types-strict`` to warn about function type
mismatches in casts and assignments that may result in runtime indirect call
`Control-Flow Integrity (CFI)
<>`_ failures. The
``-Wcast-function-type-strict`` diagnostic is grouped under
``-Wcast-function-type`` as it identifies a more strict set of potentially
problematic function type casts.
- Clang will now disambiguate NTTP types when printing diagnostic that contain NTTP types.
(`#57562 <>`_)
- Better error recovery for pack expansion of expressions.
(`#58673 <>`_)
- Better diagnostics when the user has missed ``auto`` in a declaration.
(`#49129 <>`_)
- Clang now diagnoses use of invalid or reserved module names in a module
export declaration. Both are diagnosed as an error, but the diagnostic is
suppressed for use of reserved names in a system header.
- ``-Winteger-overflow`` will diagnose overflow in more cases.
(`#58944 <>`_)
- Clang has an internal limit of 2GB of preprocessed source code per
compilation, including source reachable through imported AST files such as
PCH or modules. When Clang hits this limit, it now produces notes mentioning
which header and source files are consuming large amounts of this space.
``#pragma clang __debug sloc_usage`` can also be used to request this report.
- Clang no longer permits the keyword 'bool' in a concept declaration as a
concepts-ts compatibility extension.
- Clang now diagnoses overflow undefined behavior in a constant expression while
evaluating a compound assignment with remainder as operand.
- Add ``-Wreturn-local-addr``, a GCC alias for ``-Wreturn-stack-address``.
- Clang now suppresses ``-Wlogical-op-parentheses`` on ``(x && a || b)`` and ``(a || b && x)``
only when ``x`` is a string literal.
- Clang will now reject the GNU extension address of label in coroutines explicitly.
(`#56436 <>`_)
- Clang now automatically adds ``[[clang::lifetimebound]]`` to the parameters of
``std::move, std::forward`` et al, this enables Clang to diagnose more cases
where the returned reference outlives the object.
- Fix clang not properly diagnosing the failing subexpression when chained
binary operators are used in a ``static_assert`` expression.
- ``-Wtautological-compare`` missed warnings for tautological comparisons
involving a negative integer literal.
(`#42918 <>`_)
- ``-Wcomma`` is no longer emitted for void returning functions.
(`#57151 <>`_)
Bug Fixes in This Version
Bug Fixes to Compiler Builtins
- ``stdatomic.h`` will use the internal declarations when targeting pre-C++-23
on Windows platforms as the MSVC support requires newer C++ standard.
- Correct ``_Static_assert`` to accept the same set of extended integer
constant expressions as is accpted in other contexts that accept them.
(`#57687 <>`_)
- Fix crashes caused when builtin C++ language extension type traits were
instantiated by templates with an unexpected number of arguments.
(`#57008 <>`_)
- Fix ``__builtin_assume_aligned`` crash when the 1st arg is array type.
(`#57169 <>`_)
- Fixes to builtin template emulation of regular templates.
(`#42102 <>`_,
`#51928 <>`_)
- Fix incorrect handling of inline builtins with asm labels.
- The builtin type trait ``__is_aggregate`` now returns ``true`` for arrays of incomplete
types in accordance with the suggested fix for `LWG3823 <>`_
Bug Fixes to Attribute Support
- Fixes an accepts-invalid bug in C when using a ``_Noreturn`` function
specifier on something other than a function declaration.
(`#56800 <>`_)
- No longer assert/miscompile when trying to make a vectorized ``_BitInt`` type
using the ``ext_vector_type`` attribute (the ``vector_size`` attribute was
already properly diagnosing this case).
- Invalid destructor names are no longer accepted on template classes.
(`#56772 <>`_)
- Improve compile-times with large dynamic array allocations with trivial
(`#56774 <>`_)
- Fix a crash when a ``btf_type_tag`` attribute is applied to the pointee of
a function pointer.
- In C mode, when ``e1`` has ``__attribute__((noreturn))`` but ``e2`` doesn't,
``(c ? e1 : e2)`` is no longer considered noreturn, fixing a miscompilation.
(`#59792 <>`_)
- GNU attributes being applied prior to standard attributes would be handled
improperly, which was corrected to match the behaviour exhibited by GCC.
(`#58229 <>`_)
- Fix issue using __attribute__((format)) on non-variadic functions that expect
more than one formatted argument.
- Clang will now no longer treat a C 'overloadable' function without a prototype as
a variadic function with the attribute. This should make further diagnostics more
Bug Fixes to C++ Support
- Address the thread identification problems in coroutines.
(`#47177 <>`_,
`#47179 <>`_)
- Reject non-type template arguments formed by casting a non-zero integer
to a pointer in pre-C++17 modes, instead of treating them as null
- Fix template arguments of pointer and reference not taking the type as
part of their identity.
(`#47136 <>`_)
- Fix handling of unexpanded packs in template argument expressions.
(`#58679 <>`_)
- Fix bug with ``using enum`` that could lead to enumerators being treated as if
they were part of an overload set.
(`#58067 <>`_,
`#59014 <>`_,
`#54746 <>`_)
- Fix bug where constant evaluation treated a pointer to member that points to
a weak member as never being null. Such comparisons are now treated as
- Fix issue that the standard C++ modules importer will call global
constructor/destructor for the global variables in the importing modules.
(`#59765 <>`_)
- Reject in-class defaulting of previously declared comparison operators.
(`#51227 <>`_)
- Do not hide templated base members introduced via using-decl in derived class
(useful specially for constrained members). (`#50886 <>`_)
Concepts Specific Fixes:
- Class member variables are now in scope when parsing a ``requires`` clause.
(`#55216 <>`_)
- Required parameter pack to be provided at the end of the concept parameter list.
(`#48182 <>`_)
- Correctly handle access-checks in requires expression. (`#53364 <>`_,
`#53334 <>`_)
- Fixed an issue with concept requirement evaluation, where we incorrectly allowed implicit
conversions to bool for a requirement. (`#54524 <>`_)
- Fix a crash when emitting a concept-related diagnostic.
(`#57415 <>`_)
- Fix a crash when trying to form a recovery expression on a call inside a
constraint, which re-evaluated the same constraint.
(`#53213 <>`_,
`#45736 <>`_)
- Fix an issue when performing constraints partial ordering on non-template
functions. (`#56154 <>`_)
- Fix a number of recursively-instantiated constraint issues, which would possibly
result in a stack overflow.
(`#44304 <>`_,
`#50891 <>`_)
- Finished implementing C++ DR2565, which results in a requirement becoming
not satisfied in the event of an instantiation failures in a requires expression's
parameter list. We previously handled this correctly in a constraint evaluation
context, but not in a requires clause evaluated as a boolean.
Consteval Specific Fixes:
- Fix a crash when generating code coverage information for an
``if consteval`` statement.
(`#57377 <>`_)
- Fixed a crash-on-valid with consteval evaluation of a list-initialized
constructor for a temporary object.
(`#55871 <>`_)
- Fixes an assert crash caused by looking up missing vtable information on ``consteval``
virtual functions. (`#55065 <>`_)
Bug Fixes to AST Handling
- A SubstTemplateTypeParmType can now represent the pack index for a
substitution from an expanded pack.
(`#56099 <>`_)
- The template arguments of a variable template being accessed as a
member will now be represented in the AST.
Miscellaneous Bug Fixes
- Clang configuration files are now read through the virtual file system
rather than the physical one, if these are different.
- Fix an issue where -frewrite-includes generated line control directives with
incorrect line numbers in some cases when a header file used an end of line
character sequence that differed from the primary source file.
(`#59736 <>`_)
- Clang 14 predeclared some builtin POSIX library functions in ``gnu2x`` mode,
and Clang 15 accidentally stopped predeclaring those functions in that
language mode. Clang 16 now predeclares those functions again.
(`#56607 <>`_)
- Fix the bug of inserting the ``ZeroInitializationFixit`` before the template
argument list of ``VarTemplateSpecializationDecl``.
- Fix the bug where Clang emits constrained float intrinsics when specifying
``-ffp-model=strict -ffp-model=fast``.
Miscellaneous Clang Crashes Fixed
- Fix a crash when evaluating a multi-dimensional array's array filler
expression is element-dependent.
(`#50601 <>`_)
- Fix an assert that triggers a crash during template name lookup when a type is
incomplete but was not also a TagType.
(`#57387 <>`_)
- Fix a crash when attempting to default a virtual constexpr non-special member
function in a derived class.
(`#57431 <>`_)
- Fix ``-Wpre-c++17-compat`` crashing Clang when compiling C++20 code which
contains deduced template specializations.
(`#57369 <>`_,
`#57643 <>`_,
`#57793 <>`_)
- Fix a crash upon stray coloncolon token in C2x mode.
- Fix assert that triggers a crash during some types of list initialization that
generate a CXXTemporaryObjectExpr instead of a InitListExpr.
(`#58302 <>`_,
`#58753 <>`_,
`#59100 <>`_)
- Fix a crash where we attempt to define a deleted destructor.
(`#57516 <>`_)
- Fix an issue that triggers a crash if we instantiate a hidden friend functions.
(`#54457 <>`_)
- Fix an issue that makes Clang crash on lambda template parameters.
(`#57960 <>`_)
- Fixed a crash in C++20 mode in Clang and Clangd when compile source
with compilation errors.
(`#53628 <>`_)
- Fix sanity check when value initializing an empty union so that it takes into
account anonymous structs which is a GNU extension.
(`#58800 <>`_)
Target Specific Changes
X86 Support
- Support ``-mindirect-branch-cs-prefix`` for call and jmp to indirect thunk.
- Fix 32-bit ``__fastcall`` and ``__vectorcall`` ABI mismatch with MSVC.
- Add ISA of ``AMX-FP16`` which support ``_tile_dpfp16ps``.
- Switch ``AVX512-BF16`` intrinsics types from ``short`` to ``__bf16``.
- Add support for ``PREFETCHI`` instructions.
- Support ISA of ``CMPCCXADD``.
* Support intrinsic of ``_cmpccxadd_epi32``.
* Support intrinsic of ``_cmpccxadd_epi64``.
- Add support for ``RAO-INT`` instructions.
* Support intrinsic of ``_aadd_i32/64``
* Support intrinsic of ``_aand_i32/64``
* Support intrinsic of ``_aor_i32/64``
* Support intrinsic of ``_axor_i32/64``
- Support ISA of ``AVX-IFMA``.
* Support intrinsic of ``_mm(256)_madd52hi_avx_epu64``.
* Support intrinsic of ``_mm(256)_madd52lo_avx_epu64``.
- Support ISA of ``AVX-VNNI-INT8``.
* Support intrinsic of ``_mm(256)_dpbssd(s)_epi32``.
* Support intrinsic of ``_mm(256)_dpbsud(s)_epi32``.
* Support intrinsic of ``_mm(256)_dpbuud(s)_epi32``.
- Support ISA of ``AVX-NE-CONVERT``.
* Support intrinsic of ``_mm(256)_bcstnebf16_ps``.
* Support intrinsic of ``_mm(256)_bcstnesh_ps``.
* Support intrinsic of ``_mm(256)_cvtneebf16_ps``.
* Support intrinsic of ``_mm(256)_cvtneeph_ps``.
* Support intrinsic of ``_mm(256)_cvtneobf16_ps``.
* Support intrinsic of ``_mm(256)_cvtneoph_ps``.
* Support intrinsic of ``_mm(256)_cvtneps_avx_pbh``.
- ``-march=raptorlake``, ``-march=meteorlake`` and ``-march=emeraldrapids`` are now supported.
- ``-march=sierraforest``, ``-march=graniterapids`` and ``-march=grandridge`` are now supported.
- Lift _BitInt() supported max width from 128 to 8388608.
- Support intrinsics of ``_mm(256)_reduce_(add|mul|or|and)_epi8/16``.
- Support intrinsics of ``_mm(256)_reduce_(max|min)_ep[i|u]8/16``.
Arm and AArch64 Support
- The target(..) function attributes for AArch64 now accept:
* ``"arch=<arch>"`` strings, that specify the architecture for a function as per the ``-march`` option.
* ``"cpu=<cpu>"`` strings, that specify the cpu for a function as per the ``-mcpu`` option.
* ``"tune=<cpu>"`` strings, that specify the tune cpu for a function as per ``-mtune``.
* ``"+<feature>"``, ``"+no<feature>"`` enables/disables the specific feature, for compatibility with GCC target attributes.
* ``"<feature>"``, ``"no-<feature>"`` enabled/disables the specific feature, for backward compatibility with previous releases.
- ``-march`` values for targeting armv2, armv2A, armv3 and armv3M have been removed.
Their presence gave the impression that Clang can correctly generate code for
them, which it cannot.
- Support has been added for the following processors (-mcpu identifiers in parenthesis):
* Arm Cortex-A715 (cortex-a715).
* Arm Cortex-X3 (cortex-x3).
* Arm Neoverse V2 (neoverse-v2)
- Strict floating point has been enabled for AArch64, which means that
``-ftrapping-math``, ``-frounding-math``, ``-ffp-model=strict``, and
``-ffp-exception-behaviour=<arg>`` are now accepted.
Windows Support
- For the MinGW driver, added the options ``-mguard=none``, ``-mguard=cf`` and
``-mguard=cf-nochecks`` (equivalent to ``/guard:cf-``, ``/guard:cf`` and
``/guard:cf,nochecks`` in clang-cl) for enabling Control Flow Guard checks
and generation of address-taken function table.
- Switched from SHA1 to BLAKE3 for PDB type hashing / ``-gcodeview-ghash``
- Fixed code generation with emulated TLS, when the emulated TLS is enabled
by default (with downstream patches; no upstream configurations default
to this configuration, but some mingw downstreams change the default
in this way).
- Improved detection of MinGW cross sysroots for finding sysroots provided
by Linux distributions such as Fedora. Also improved such setups by
avoiding to include ``/usr/include`` among the include paths when cross
compiling with a cross sysroot based in ``/usr``.
- Fix incorrect alignment attribute on the this parameter of certain
non-complete destructors when using the Microsoft ABI.
`Issue 60465 <>`_.
LoongArch Support
- Clang now supports LoongArch. Along with the backend, clang is able to build a
large corpus of Linux applications. Test-suite 100% pass.
- Support basic option ``-march=`` which is used to select the target
architecture, i.e. the basic set of ISA modules to be enabled. Possible values
are ``loongarch64`` and ``la464``.
- Support basic option ``-mabi=`` which is used to select the base ABI type.
Possible values are ``lp64d``, ``lp64f``, ``lp64s``, ``ilp32d``, ``ilp32f``
and ``ilp32s``.
- Support extended options: ``-msoft-float``, ``-msingle-float``, ``-mdouble-float`` and ``mfpu=``.
See `LoongArch toolchain conventions <>`_.
RISC-V Support
- ``sifive-7-rv32`` and ``sifive-7-rv64`` are no longer supported for ``-mcpu``.
Use ``sifive-e76``, ``sifive-s76``, or ``sifive-u74`` instead.
- Native detections via ``-mcpu=native`` and ``-mtune=native`` are supported.
- Fix interaction of ``-mcpu`` and ``-march``, RISC-V backend will take the
architecture extension union of ``-mcpu`` and ``-march`` before, and now will
take architecture extensions from ``-march`` if both are given.
- An ABI mismatch between GCC and Clang that related to the
sign/zero-extension of integer scalars was fixed.
- An overall simplification of the RISC-V Vector intrinsics are done. The
simplification is based on
`riscv-non-isa/rvv-intrinsic-doc#186 <>`_.
- Intrinsics of `vcompress` and `vmerge` have been adjusted to have interfaces
be aligned among `vvm`, `vxm` intrinsics. The adjustment is base on
`riscv-non-isa/rvv-intrinsic-doc#185 < non-isa/rvv-intrinsic-doc/pull/185>`_.
- All RISC-V Vector intrinsics now share a `__riscv_` prefix, based on the
naming convention defined by
`riscv-non-isa/riscv-c-api-doc#31 <>`_.
- Note that the RISC-V Vector C intrinsics are still under development. The RVV
C Intrinsic Task Group is working towards a ratified v1.0.
- The rvv-intrinsic-doc provides `compatible headers <>`_ for transition from the previous implemented version to the current (v0.11) version.
CUDA/HIP Language Changes in Clang
- Allow the use of ``__noinline__`` as a keyword (instead of ``__attribute__((noinline))``)
in lambda declarations.
CUDA Support
- Clang now supports CUDA SDK up to 11.8
- Added support for targeting sm_{87,89,90} GPUs.
AIX Support
* When using ``-shared``, the clang driver now invokes llvm-nm to create an
export list if the user doesn't specify one via linker flag or pass an
alternative export control option.
* Driver work done for ``-pg`` to link with the right paths and files.
- Improved support for `-bcdtors:mbr` and `-bcdtors:csect` linker flags
when linking with -fprofile-generate.
- Enabled LTO support. Requires AIX 7.2 TL5 SP3 or newer, or AIX 7.3. LTO
support is implemented with the `` plugin. To specify a
different plugin, use the linker option `-bplugin:<path to plugin>`.
To pass options to the plugin, use the linker option `-bplugin_opt:<option>`.
- ``-mcpu`` option's values are checked against a list of known CPUs. An error
is reported if the specified CPU model is not found.
WebAssembly Support
- The -mcpu=generic configuration now enables sign-ext and mutable-globals. These
proposals are standardized and available in all major engines.
DWARF Support in Clang
Previously when emitting DWARFv4 and tuning for GDB, Clang would use DWARF v2's
``DW_AT_bit_offset`` and ``DW_AT_data_member_location``. Clang now uses DWARF v4's
``DW_AT_data_bit_offset`` regardless of tuning.
Support for ``DW_AT_data_bit_offset`` was added in GDB 8.0. For earlier versions,
you can use the ``-gdwarf-3`` option to emit compatible DWARF.
Floating Point Support in Clang
- The driver option ``-menable-unsafe-fp-math`` has been removed. To enable
unsafe floating-point optimizations use ``-funsafe-math-optimizations`` or
``-ffast-math`` instead.
- Add ``__builtin_elementwise_sin`` and ``__builtin_elementwise_cos`` builtins for floating point types only.
AST Matchers
- Add ``isInAnoymousNamespace`` matcher to match declarations in an anonymous namespace.
- Add ``RemoveSemicolon`` option for removing ``;`` after a non-empty function definition.
- Add ``RequiresExpressionIndentation`` option for configuring the alignment of requires-expressions.
The default value of this option is ``OuterScope``, which differs in behavior from clang-format 15.
To match the default behavior of clang-format 15, use the ``Keyword`` value.
- Add ``IntegerLiteralSeparator`` option for fixing integer literal separators
in C++, C#, Java, and JavaScript.
- Add ``BreakAfterAttributes`` option for breaking after a group of C++11
attributes before a function declaration/definition name.
- Add ``InsertNewlineAtEOF`` option for inserting a newline at EOF if missing.
- Add ``LineEnding`` option to deprecate ``DeriveLineEnding`` and ``UseCRLF``.
- Introduced the new function ``clang_getUnqualifiedType``, which mimics
the behavior of ``QualType::getUnqualifiedType`` for ``CXType``.
- Introduced the new function ``clang_getNonReferenceType``, which mimics
the behavior of ``QualType::getNonReferenceType`` for ``CXType``.
- Introduced the new function ``clang_CXXMethod_isDeleted``, which queries
whether the method is declared ``= delete``.
- Introduced the new function ``clang_CXXMethod_isCopyAssignmentOperator``,
which identifies whether a method cursor is a copy-assignment
- Introduced the new function ``clang_CXXMethod_isMoveAssignmentOperator``,
which identifies whether a method cursor is a move-assignment
- ``clang_Cursor_getNumTemplateArguments``, ``clang_Cursor_getTemplateArgumentKind``,
``clang_Cursor_getTemplateArgumentType``, ``clang_Cursor_getTemplateArgumentValue`` and
``clang_Cursor_getTemplateArgumentUnsignedValue`` now work on struct, class,
and partial template specialization cursors in addition to function cursors.
Static Analyzer
- Removed the deprecated ``-analyzer-store`` and
``-analyzer-opt-analyze-nested-blocks`` analyzer flags.
``scanbuild`` was also updated accordingly.
Passing these flags will result in a hard error.
- Deprecate the ``consider-single-element-arrays-as-flexible-array-members``
analyzer-config option.
This option will be still accepted, but a warning will be displayed.
This option will be rejected, thus turned into a hard error starting with
``clang-17``. Use ``-fstrict-flex-array=<N>`` instead if necessary.
- Trailing array objects of structs with single elements will be considered
as flexible-array-members. Use ``-fstrict-flex-array=<N>`` to define
what should be considered as flexible-array-member if needed.
.. _release-notes-sanitizers:
- ``-fsanitize-memory-param-retval`` is turned on by default. With
``-fsanitize=memory``, passing uninitialized variables to functions and
returning uninitialized variables from functions is more aggressively
reported. ``-fno-sanitize-memory-param-retval`` restores the previous
Additional Information
A wide variety of additional information is available on the `Clang web
page <>`_. The web page contains versions of the
API documentation which are up-to-date with the Git version of
the source code. You can access versions of these documents specific to
this release by going into the "``clang/docs/``" directory in the Clang
If you have any questions or comments about Clang, please feel free to
contact us on the `Discourse forums (Clang Frontend category)