docs/ReleaseNotes.rst

===========================================
Clang |release| |ReleaseNotesTitle|
===========================================

.. contents::
   :local:
   :depth: 2

Written by the `LLVM Team <https://llvm.org/>`_

.. 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 <https://llvm.org/releases/>`_.

Introduction
============

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 <https://llvm.org/docs/ReleaseNotes.html>`_. For the libc++ release notes,
see `this page <https://libcxx.llvm.org/ReleaseNotes.html>`_. All LLVM releases
may be downloaded from the `LLVM releases web site <https://llvm.org/releases/>`_.

For more information about Clang or LLVM, including information about the
latest release, please see the `Clang Web Site <https://clang.llvm.org>`_ or the
`LLVM Web Site <https://llvm.org>`_.

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.


C/C++ Language Potentially Breaking Changes
-------------------------------------------
- Indirect edges of asm goto statements under certain circumstances may now be
  split. In previous releases of clang, that means for the following code the
  two inputs may have compared equal in the inline assembly.  This is no longer
  guaranteed (and necessary to support outputs along indirect edges, which is
  now supported as of this release). This change is more consistent with the
  behavior of GCC.

  .. code-block:: c

    foo: asm goto ("# %0 %1"::"i"(&&foo)::foo);

C++ Specific Potentially Breaking Changes
-----------------------------------------
- Clang won't search for coroutine_traits in std::experimental namespace any more.
  Clang will only search for std::coroutine_traits for coroutines then.

ABI Changes in This Version
---------------------------
- ``__is_trivial`` has changed for a small category of classes with constrained default constructors (`#60697 <https://github.com/llvm/llvm-project/issues/60697>`_).
  *FIXME: Remove this note if we've backported this change to the Clang 16 branch.*

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
--------------------
- Improved ``-O0`` code generation for calls to ``std::forward_like``. Similarly to
  ``std::move, std::forward`` et al. it is now treated as a compiler builtin and implemented
  directly rather than instantiating the definition from the standard library.
- Implemented `CWG2518 <https://wg21.link/CWG2518>`_ which allows ``static_assert(false)``
  to not be ill-formed when its condition is evaluated in the context of a template definition.

C++20 Feature Support
^^^^^^^^^^^^^^^^^^^^^
- Support for out-of-line definitions of constrained templates has been improved.
  This partially fixes `#49620 <https://github.com/llvm/llvm-project/issues/49620>`_.
- Lambda templates with a requires clause directly after the template parameters now parse
  correctly if the requires clause consists of a variable with a dependent type.
  (`#61278 <https://github.com/llvm/llvm-project/issues/61278>`_)
- Announced C++20 Coroutines is fully supported on all targets except Windows, which
  still has some stability and ABI issues.

C++2b Feature Support
^^^^^^^^^^^^^^^^^^^^^

- Implemented `P2036R3: Change scope of lambda trailing-return-type <https://wg21.link/P2036R3>`_
  and `P2579R0 Mitigation strategies for P2036 <https://wg21.link/P2579R0>`_.
  These proposals modify how variables captured in lambdas can appear in trailing return type
  expressions and how their types are deduced therein, in all C++ language versions.

Resolutions to C++ Defect Reports
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

C Language Changes
------------------
- Support for outputs from asm goto statements along indirect edges has been
  added. (`#53562 <https://github.com/llvm/llvm-project/issues/53562>`_)

C2x Feature Support
^^^^^^^^^^^^^^^^^^^
- Implemented the ``unreachable`` macro in freestanding ``<stddef.h>`` for
  `WG14 N2826 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2826.pdf>`_

- Removed the ``ATOMIC_VAR_INIT`` macro in C2x and later standards modes, which
  implements `WG14 N2886 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2886.htm>`_

- Implemented `WG14 N2934 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2934.pdf>`_
  which introduces the ``bool``, ``static_assert``, ``alignas``, ``alignof``,
  and ``thread_local`` keywords in C2x.

Non-comprehensive list of changes in this release
-------------------------------------------------
- Clang now saves the address of ABI-indirect function parameters on the stack,
  improving the debug information available in programs compiled without
  optimizations.
- Clang now supports ``__builtin_nondeterministic_value`` that returns a
  nondeterministic value of the same type as the provided argument.
- Clang now supports ``__builtin_FILE_NAME()`` which returns the same
  information as the ``__FILE_NAME__`` macro (the presumed file name
  from the invocation point, with no path components included).

New Compiler Flags
------------------

Deprecated Compiler Flags
-------------------------

Modified Compiler Flags
-----------------------

Removed Compiler Flags
-------------------------
- The deprecated flag `-fmodules-ts` is removed. Please use ``-std=c++20``
  or higher to use standard C++ modules instead.
- The deprecated flag `-fcoroutines-ts` is removed. Please use ``-std=c++20``
  or higher to use standard C++ coroutines instead.
- The CodeGen flag `-lower-global-dtors-via-cxa-atexit` which affects how global
  destructors are lowered for MachO is removed without replacement. The default
  of `-lower-global-dtors-via-cxa-atexit=true` is now the only supported way.

Attribute Changes in Clang
--------------------------
- Introduced a new function attribute ``__attribute__((unsafe_buffer_usage))``
  to be worn by functions containing buffer operations that could cause out of
  bounds memory accesses. It emits warnings at call sites to such functions when
  the flag ``-Wunsafe-buffer-usage`` is enabled.
- ``__declspec`` attributes can now be used together with the using keyword. Before
  the attributes on ``__declspec`` was ignored, while now it will be forwarded to the
  point where the alias is used.
- Introduced a new ``USR`` (unified symbol resolution) clause inside of the
  existing ``__attribute__((external_source_symbol))`` attribute. Clang's indexer
  uses the optional USR value when indexing Clang's AST. This value is expected
  to be generated by an external compiler when generating C++ bindings during
  the compilation of the foreign language sources (e.g. Swift).
- The ``__has_attribute``, ``__has_c_attribute`` and ``__has_cpp_attribute``
  preprocessor operators now return 1 also for attributes defined by plugins.

Improvements to Clang's diagnostics
-----------------------------------
- We now generate a diagnostic for signed integer overflow due to unary minus
  in a non-constant expression context.
  (`#31643 <https://github.com/llvm/llvm-project/issues/31643>`_)
- Clang now warns by default for C++20 and later about deprecated capture of
  ``this`` with a capture default of ``=``. This warning can be disabled with
  ``-Wno-deprecated-this-capture``.
- Clang had failed to emit some ``-Wundefined-internal`` for members of a local
  class if that class was first introduced with a forward declaration.
- Diagnostic notes and fix-its are now generated for ``ifunc``/``alias`` attributes
  which point to functions whose names are mangled.
- Diagnostics relating to macros on the command line of a preprocessed assembly
  file or precompiled header are now reported as coming from the file
  ``<command line>`` instead of ``<built-in>``.
- Clang constexpr evaluator now provides a more concise diagnostic when calling
  function pointer that is known to be null.
- Clang now avoids duplicate warnings on unreachable ``[[fallthrough]];`` statements
  previously issued from ``-Wunreachable-code`` and ``-Wunreachable-code-fallthrough``
  by prioritizing ``-Wunreachable-code-fallthrough``.
- Clang now correctly diagnoses statement attributes ``[[clang::always_inine]]`` and
  ``[[clang::noinline]]`` when used on a statement with dependent call expressions.

Bug Fixes in This Version
-------------------------

- Fix crash when diagnosing incorrect usage of ``_Nullable`` involving alias
  templates.
  (`#60344 <https://github.com/llvm/llvm-project/issues/60344>`_)
- Fix confusing warning message when ``/clang:-x`` is passed in ``clang-cl``
  driver mode and emit an error which suggests using ``/TC`` or ``/TP``
  ``clang-cl`` options instead.
  (`#59307 <https://github.com/llvm/llvm-project/issues/59307>`_)
- Fix assert that fails when the expression causing the this pointer to be
  captured by a block is part of a constexpr if statement's branch and
  instantiation of the enclosing method causes the branch to be discarded.
- Fix __VA_OPT__ implementation so that it treats the concatenation of a
  non-placemaker token and placemaker token as a non-placemaker token.
  (`#60268 <https://github.com/llvm/llvm-project/issues/60268>`_)
- Fix crash when taking the address of a consteval lambda call operator.
  (`#57682 <https://github.com/llvm/llvm-project/issues/57682>`_)
- Clang now support export declarations in the language linkage.
  (`#60405 <https://github.com/llvm/llvm-project/issues/60405>`_)
- Fix aggregate initialization inside lambda constexpr.
  (`#60936 <https://github.com/llvm/llvm-project/issues/60936>`_)
- No longer issue a false positive diagnostic about a catch handler that cannot
  be reached despite being reachable. This fixes
  `#61177 <https://github.com/llvm/llvm-project/issues/61177>`_ in anticipation
  of `CWG2699 <https://wg21.link/CWG2699>_` being accepted by WG21.
- Fix crash when parsing fold expression containing a delayed typo correction.
  (`#61326 <https://github.com/llvm/llvm-project/issues/61326>`_)
- Fix crash when dealing with some member accesses outside of class or member
  function context.
  (`#37792 <https://github.com/llvm/llvm-project/issues/37792>`_) and
  (`#48405 <https://github.com/llvm/llvm-project/issues/48405>`_)
- Fix crash when using ``[[clang::always_inline]]`` or ``[[clang::noinline]]``
  statement attributes on a call to a template function in the body of a
  template function.
- Fix coroutines issue where ``get_return_object()`` result was always eargerly
  converted to the return type. Eager initialization (allowing RVO) is now only
  perfomed when these types match, otherwise deferred initialization is used,
  enabling short-circuiting coroutines use cases. This fixes
  (`#56532 <https://github.com/llvm/llvm-project/issues/56532>`_) in
  antecipation of `CWG2563 <https://cplusplus.github.io/CWG/issues/2563.html>_`.

Bug Fixes to Compiler Builtins
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Bug Fixes to Attribute Support
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Bug Fixes to C++ Support
^^^^^^^^^^^^^^^^^^^^^^^^

- Fix crash on invalid code when looking up a destructor in a templated class
  inside a namespace.
  (`#59446 <https://github.com/llvm/llvm-project/issues/59446>`_)
- Fix crash when evaluating consteval constructor of derived class whose base
  has more than one field.
  (`#60166 <https://github.com/llvm/llvm-project/issues/60166>`_)
- Fix an issue about ``decltype`` in the members of class templates derived from
  templates with related parameters.
  (`#58674 <https://github.com/llvm/llvm-project/issues/58674>`_)
- Fix incorrect deletion of the default constructor of unions in some
  cases. (`#48416 <https://github.com/llvm/llvm-project/issues/48416>`_)
- No longer issue a pre-C++2b compatibility warning in ``-pedantic`` mode
  regading overloaded `operator[]` with more than one parmeter or for static
  lambdas. (`#61582 <https://github.com/llvm/llvm-project/issues/61582>`_)

Bug Fixes to AST Handling
^^^^^^^^^^^^^^^^^^^^^^^^^

Miscellaneous Bug Fixes
^^^^^^^^^^^^^^^^^^^^^^^

Miscellaneous Clang Crashes Fixed
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Target Specific Changes
-----------------------

AMDGPU Support
^^^^^^^^^^^^^^

- Linking for AMDGPU now uses ``--no-undefined`` by default. This causes
  undefined symbols in the created module to be a linker error. To prevent this,
  pass ``-Wl,--undefined`` if compiling directly, or ``-Xoffload-linker
  --undefined`` if using an offloading language.

X86 Support
^^^^^^^^^^^

Arm and AArch64 Support
^^^^^^^^^^^^^^^^^^^^^^^

- The hard-float ABI is now available in Armv8.1-M configurations that
  have integer MVE instructions (and therefore have FP registers) but
  no scalar or vector floating point computation. Previously, trying
  to select the hard-float ABI on such a target (via
  ``-mfloat-abi=hard`` or a triple ending in ``hf``) would silently
  use the soft-float ABI instead.

- Clang builtin ``__arithmetic_fence`` and the command line option ``-fprotect-parens``
  are now enabled for AArch64.

Windows Support
^^^^^^^^^^^^^^^

LoongArch Support
^^^^^^^^^^^^^^^^^

- Patchable function entry (``-fpatchable-function-entry``) is now supported
  on LoongArch.

RISC-V Support
^^^^^^^^^^^^^^
- Added ``-mrvv-vector-bits=`` option to give an upper and lower bound on vector
  length. Valid values are powers of 2 between 64 and 65536. A value of 32
  should eventually be supported. We also accept "zvl" to use the Zvl*b
  extension from ``-march`` or ``-mcpu`` to the be the upper and lower bound.

CUDA/HIP Language Changes
^^^^^^^^^^^^^^^^^^^^^^^^^

CUDA Support
^^^^^^^^^^^^

AIX Support
^^^^^^^^^^^

WebAssembly Support
^^^^^^^^^^^^^^^^^^^

AVR Support
^^^^^^^^^^^
- The definition of ``USHRT_MAX`` in the freestanding ``<limits.h>`` no longer
  overflows on AVR (where ``sizeof(int) == sizeof(unsigned short)``).  The type
  of ``USHRT_MAX`` is now ``unsigned int`` instead of ``int``, as required by
  the C standard.

DWARF Support in Clang
----------------------

Floating Point Support in Clang
-------------------------------
- Add ``__builtin_elementwise_log`` builtin for floating point types only.
- Add ``__builtin_elementwise_log10`` builtin for floating point types only.
- Add ``__builtin_elementwise_log2`` builtin for floating point types only.
- Add ``__builtin_elementwise_exp`` builtin for floating point types only.
- Add ``__builtin_elementwise_exp2`` builtin for floating point types only.
- Add ``__builtin_set_flt_rounds`` builtin for X86, x86_64, Arm and AArch64 only.

AST Matchers
------------

- Add ``coroutineBodyStmt`` matcher.

- The ``hasBody`` matcher now matches coroutine body nodes in
  ``CoroutineBodyStmts``.

clang-format
------------

- Add ``NextLineOnly`` style to option ``PackConstructorInitializers``.
  Compared to ``NextLine`` style, ``NextLineOnly`` style will not try to
  put the initializers on the current line first, instead, it will try to
  put the initializers on the next line only.

libclang
--------

- Introduced the new function ``clang_CXXMethod_isExplicit``,
  which identifies whether a constructor or conversion function cursor
  was marked with the explicit identifier.

- Introduced the new ``CXIndex`` constructor function
  ``clang_createIndexWithOptions``, which allows storing precompiled preambles
  in memory or overriding the precompiled preamble storage path.

- Deprecated two functions ``clang_CXIndex_setGlobalOptions`` and
  ``clang_CXIndex_setInvocationEmissionPathOption`` in favor of the new
  function ``clang_createIndexWithOptions`` in order to improve thread safety.

- Added check in ``clang_getFieldDeclBitWidth`` for whether a bit-field
  has an evaluable bit width. Fixes undefined behavior when called on a
  bit-field whose width depends on a template paramter.

- ``clang_parseTranslationUnit`` and ``clang_parseTranslationUnit2`` have been
  changed to automatically locate the Clang installation directory relative to
  the location of the libclang binary and use it for system headers installed
  alongside the Clang installation. It is no longer necessary to manually
  locate such system headers or use the ``clang_parseTranslationUnit2FullArgv``
  function for this purpose if libclang has been installed in the default
  location.
 
Static Analyzer
---------------
- Fix incorrect alignment attribute on the this parameter of certain
  non-complete destructors when using the Microsoft ABI.
  (`#60465 <https://github.com/llvm/llvm-project/issues/60465>`_)

.. _release-notes-sanitizers:

Sanitizers
----------


Additional Information
======================

A wide variety of additional information is available on the `Clang web
page <https://clang.llvm.org/>`_. 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
tree.

If you have any questions or comments about Clang, please feel free to
contact us on the `Discourse forums (Clang Frontend category)
<https://discourse.llvm.org/c/clang/6>`_.