Clang 20.0.0git (In-Progress) Release Notes

Written by the LLVM Team

Warning

These are in-progress notes for the upcoming Clang 20 release. Release notes for previous releases can be found on the Releases Page.

Introduction

This document contains the release notes for the Clang C/C++/Objective-C frontend, part of the LLVM Compiler Infrastructure, release 20.0.0git. 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 20.0.0git because of the opportunity they pose for disruption to existing code bases.

  • The le32 and le64 targets have been removed.

  • The clang-rename tool has been removed.

C/C++ Language Potentially Breaking Changes

C++ Specific Potentially Breaking Changes

  • The type trait builtin __is_nullptr has been removed, since it has very few users and can be written as __is_same(__remove_cv(T), decltype(nullptr)), which GCC supports as well.

  • 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.

    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 ill-formed, out of the range [0, 7]
constexpr E Val4 = (E)-1; // Now ill-formed, out of the range [0, 7]

    Since Clang 16, it has been possible to suppress the diagnostic via -Wno-enum-constexpr-conversion, to allow for a transition period for users. Now, in Clang 20, it is no longer possible to suppress the diagnostic.

  • Extraneous template headers are now ill-formed by default. This error can be disable with -Wno-error=extraneous-template-head.

    template <> // error: extraneous template head
template <typename T>
void f();

ABI Changes in This Version

  • Fixed Microsoft name mangling of placeholder, auto and decltype(auto), return types for MSVC 1920+. This change resolves incompatibilities with code compiled by MSVC 1920+ but will introduce incompatibilities with code compiled by earlier versions of Clang unless such code is built with the compiler option -fms-compatibility-version=19.14 to imitate the MSVC 1914 mangling behavior.

AST Dumping Potentially Breaking Changes

Clang Frontend Potentially Breaking Changes

Clang Python Bindings Potentially Breaking Changes

  • Parts of the interface returning string results will now return the empty string "" when no result is available, instead of None.

  • Calling a property on the CompletionChunk or CompletionString class statically now leads to an error, instead of returning a CachedProperty object that is used internally. Properties are only available on instances.

  • For a single-line SourceRange and a SourceLocation in the same line, but after the end of the SourceRange, SourceRange.__contains__ used to incorrectly return True. (#22617), (#52827)

What’s New in Clang 20.0.0git?

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

  • Allow single element access of GCC vector/ext_vector_type object to be constant expression. Supports the V.xyzw syntax and other tidbits as seen in OpenCL. Selecting multiple elements is left as a future work.

  • Implement CWG1815. Support lifetime extension of temporary created by aggregate initialization using a default member initializer.

  • Accept C++26 user-defined static_assert messages in C++11 as an extension.

  • Add __builtin_elementwise_popcount builtin for integer types only.

C++2c Feature Support

C++23 Feature Support

  • Removed the restriction to literal types in constexpr functions in C++23 mode.

C++20 Feature Support

Resolutions to C++ Defect Reports

  • Allow calling initializer list constructors from initializer lists with a single element of the same type instead of always copying. (CWG2137: List-initialization from object of same type <https://cplusplus.github.io/CWG/issues/2137.html>)

  • Speculative resolution for CWG2311 implemented so that the implementation of CWG2137 doesn’t remove previous cases where guaranteed copy elision was done. Given a prvalue e of class type T, T{e} will try to resolve an initializer list constructor and will use it if successful. Otherwise, if there is no initializer list constructor, the copy will be elided as if it was T(e). (CWG2311: Missed case for guaranteed copy elision <https://cplusplus.github.io/CWG/issues/2311.html>)

  • Casts from a bit-field to an integral type is now not considered narrowing if the width of the bit-field means that all potential values are in the range of the target type, even if the type of the bit-field is larger. (CWG2627: Bit-fields and narrowing conversions)

  • nullptr is now promoted to void* when passed to a C-style variadic function. (CWG722: Can nullptr be passed to an ellipsis?)

  • Allow void{} as a prvalue of type void. (CWG2351: void{}).

  • Clang now has improved resolution to CWG2398, allowing class templates to have default arguments deduced when partial ordering.

  • Clang now allows comparing unequal object pointers that have been cast to void * in constant expressions. These comparisons always worked in non-constant expressions. (CWG2749: Treatment of “pointer to void” for relational comparisons).

  • Reject explicit object parameters with type void (this void). (CWG2915: Explicit object parameters of type void).

C Language Changes

C2y Feature Support

C23 Feature Support

  • Clang now supports N3029 Improved Normal Enumerations.

Non-comprehensive list of changes in this release

  • The floating point comparison builtins (__builtin_isgreater, __builtin_isgreaterequal, __builtin_isless, etc.) and __builtin_signbit can now be used in constant expressions.

New Compiler Flags

  • The -fc++-static-destructors={all,thread-local,none} flag was added to control which C++ variables have static destructors registered: all (the default) does so for all variables, thread-local only for thread-local variables, and none (which corresponds to the existing -fno-c++-static-destructors flag) skips all static destructors registration.

Deprecated Compiler Flags

  • -fheinous-gnu-extensions is deprecated; it is now equivalent to specifying -Wno-error=invalid-gnu-asm-cast and may be removed in the future.

Modified Compiler Flags

  • The -ffp-model option has been updated to enable a more limited set of optimizations when the fast argument is used and to accept a new argument, aggressive. The behavior of -ffp-model=aggressive is equivalent to the previous behavior of -ffp-model=fast. The updated -ffp-model=fast behavior no longer assumes finite math only and uses the promoted algorithm for complex division when possible rather than the less basic (limited range) algorithm.

Removed Compiler Flags

  • The compiler flag -Wenum-constexpr-conversion (and the Wno-, Wno-error- derivatives) is now removed, since it’s no longer possible to suppress the diagnostic (see above). Users can expect an unknown warning diagnostic if it’s still in use.

Attribute Changes in Clang

  • Clang now disallows more than one __attribute__((ownership_returns(class, idx))) with different class names attached to one function.

  • Introduced a new format attribute __attribute__((format(syslog, 1, 2))) from OpenBSD.

  • The hybrid_patchable attribute is now supported on ARM64EC targets. It can be used to specify that a function requires an additional x86-64 thunk, which may be patched at runtime.

  • [[clang::lifetimebound]] is now explicitly disallowed on explicit object member functions where they were previously silently ignored.

  • Clang now automatically adds [[clang::lifetimebound]] to the parameters of std::span, std::string_view constructors, this enables Clang to capture more cases where the returned reference outlives the object. (#100567)

  • Clang now correctly diagnoses the use of btf_type_tag in C++ and ignores it; this attribute is a C-only attribute, and caused crashes with template instantiation by accidentally allowing it in C++ in some circumstances. (#106864)

  • Introduced a new attribute [[clang::coro_await_elidable]] on coroutine return types to express elideability at call sites where the coroutine is invoked under a safe elide context.

  • Introduced a new attribute [[clang::coro_await_elidable_argument]] on function parameters to propagate safe elide context to arguments if such function is also under a safe elide context.

Improvements to Clang’s diagnostics

  • Some template related diagnostics have been improved.

    void foo() { template <typename> int i; } // error: templates can only be declared in namespace or class scope

struct S {
 template <typename> int i; // error: non-static data member 'i' cannot be declared as a template
};

  • Clang now has improved diagnostics for functions with explicit ‘this’ parameters. Fixes #97878

  • Clang now diagnoses dangling references to fields of temporary objects. Fixes #81589.

  • Clang now diagnoses undefined behavior in constant expressions more consistently. This includes invalid shifts, and signed overflow in arithmetic.

  • -Wdangling-assignment-gsl is enabled by default.

  • Clang now always preserves the template arguments as written used to specialize template type aliases.

  • Clang now diagnoses the use of main in an extern context as invalid according to [basic.start.main] p3. Fixes #101512.

  • Clang now diagnoses when the result of a [[nodiscard]] function is discarded after being cast in C. Fixes #104391.

  • Don’t emit duplicated dangling diagnostics. (#93386).

  • Improved diagnostic when trying to befriend a concept. (#45182).

  • Added the -Winvalid-gnu-asm-cast diagnostic group to control warnings about use of “noop” casts for lvalues (a GNU extension). This diagnostic is a warning which defaults to being an error, is enabled by default, and is also controlled by the now-deprecated -fheinous-gnu-extensions flag.

  • Added the -Wdecls-in-multiple-modules option to assist users to identify multiple declarations in different modules, which is the major reason of the slow compilation speed with modules. This warning is disabled by default and it needs to be explicitly enabled or by -Weverything.

  • Improved diagnostic when trying to overload a function in an extern "C" context. (#80235)

  • Clang now respects lifetimebound attribute for the assignment operator parameter. (#106372).

  • The lifetimebound and GSL analysis in clang are coherent, allowing clang to detect more use-after-free bugs. (#100549).

  • Clang now diagnoses dangling cases where a gsl-pointer is constructed from a gsl-owner object inside a container (#100384).

  • Clang now warns for u8 character literals used in C23 with -Wpre-c23-compat instead of -Wpre-c++17-compat.

  • Clang now diagnose when importing module implementation partition units in module interface units.

  • Don’t emit bogus dangling diagnostics when [[gsl::Owner]] and [[clang::lifetimebound]] are used together (#108272).

Improvements to Clang’s time-trace

Improvements to Coverage Mapping

Bug Fixes in This Version

  • Fixed the definition of ATOMIC_FLAG_INIT in <stdatomic.h> so it can be used in C++.

  • Fixed a failed assertion when checking required literal types in C context. (#101304).

  • Fixed a crash when trying to transform a dependent address space type. Fixes #101685.

  • Fixed a crash when diagnosing format strings and encountering an empty delimited escape sequence (e.g., "\o{}"). #102218

Bug Fixes to Compiler Builtins

  • Fix crash when atomic builtins are called with pointer to zero-size struct (#90330)

  • Clang now allows pointee types of atomic builtin arguments to be complete template types that was not instantiated elsewhere.

  • __noop can now be used in a constant expression. (#102064)

Bug Fixes to Attribute Support

Bug Fixes to C++ Support

  • Fixed a crash when an expression with a dependent __typeof__ type is used as the operand of a unary operator. (#97646)

  • Fixed incorrect pack expansion of init-capture references in requires expresssions.

  • Fixed a failed assertion when checking invalid delete operator declaration. (#96191)

  • Fix a crash when checking destructor reference with an invalid initializer. (#97230)

  • Clang now correctly parses potentially declarative nested-name-specifiers in pointer-to-member declarators.

  • Fix a crash when checking the initialzier of an object that was initialized with a string literal. (#82167)

  • Fix a crash when matching template template parameters with templates which have parameters of different class type. (#101394)

  • Clang now correctly recognizes the correct context for parameter substitutions in concepts, so it doesn’t incorrectly complain of missing module imports in those situations. (#60336)

  • Fix init-capture packs having a size of one before being instantiated. (#63677)

  • Clang now preserves the unexpanded flag in a lambda transform used for pack expansion. (#56852), (#85667), (#99877).

  • Fixed a bug when diagnosing ambiguous explicit specializations of constrained member functions.

  • Fixed an assertion failure when selecting a function from an overload set that includes a specialization of a conversion function template.

  • Correctly diagnose attempts to use a concept name in its own definition; A concept name is introduced to its scope sooner to match the C++ standard. (#55875)

  • Properly reject defaulted relational operators with invalid types for explicit object parameters, e.g., bool operator==(this int, const Foo&) (#100329), and rvalue reference parameters.

  • Properly reject defaulted copy/move assignment operators that have a non-reference explicit object parameter.

  • Clang now properly handles the order of attributes in extern blocks. (#101990).

  • Fixed an assertion failure by preventing null explicit object arguments from being deduced. (#102025).

  • Correctly check constraints of explicit instantiations of member functions. (#46029)

  • When performing partial ordering of function templates, clang now checks that the deduction was consistent. Fixes (#18291).

  • Fixed an assertion failure about a constraint of a friend function template references to a value with greater template depth than the friend function template. (#98258)

  • Clang now rebuilds the template parameters of out-of-line declarations and specializations in the context of the current instantiation in all cases.

  • Fix evaluation of the index of dependent pack indexing expressions/types specifiers (#105900)

  • Correctly handle subexpressions of an immediate invocation in the presence of implicit casts. (#105558)

  • Clang now correctly handles direct-list-initialization of a structured bindings from an array. (#31813)

  • Mangle placeholders for deduced types as a template-prefix, such that mangling of template template parameters uses the correct production. (#106182)

  • Fixed an assertion failure when converting vectors to int/float with invalid expressions. (#105486)

  • Template parameter names are considered in the name lookup of out-of-line class template specialization right before its declaration context. (#64082)

  • Fixed a constraint comparison bug for friend declarations. (#78101)

  • Fix handling of _ as the name of a lambda’s init capture variable. (#107024)

  • Fix an issue with dependent source location expressions (#106428), (#81155), (#80210), (#85373)

  • Fixed a bug in the substitution of empty pack indexing types. (#105903)

  • Clang no longer tries to capture non-odr used default arguments of template parameters of generic lambdas (#107048)

  • Fixed a bug where defaulted comparison operators would remove const from base classes. (#102588)

  • Fix a crash when using source_location in the trailing return type of a lambda expression. (#67134)

  • A follow-up fix was added for (#61460), as the previous fix was not entirely correct. (#86361)

  • Fixed a crash in the typo correction of an invalid CTAD guide. (#107887)

  • Fixed a crash when clang tries to subtitute parameter pack while retaining the parameter pack. (#63819), (#107560)

  • Fix a crash when a static assert declaration has an invalid close location. (#108687)

  • Avoided a redundant friend declaration instantiation under a certain consteval context. (#107175)

  • Fixed an assertion failure in debug mode, and potential crashes in release mode, when diagnosing a failed cast caused indirectly by a failed implicit conversion to the type of the constructor parameter.

Bug Fixes to AST Handling

  • Fixed a crash that occurred when dividing by zero in complex integer division. (#55390).

Miscellaneous Bug Fixes

Miscellaneous Clang Crashes Fixed

  • Fixed a crash in C due to incorrect lookup that members in nested anonymous struct/union can be found as ordinary identifiers in struct/union definition. (#31295)

  • Fixed a crash caused by long chains of sizeof and other similar operators that can be followed by a non-parenthesized expression. (#45061)

  • Fixed an crash when compiling #pragma STDC FP_CONTRACT DEFAULT with -ffp-contract=fast-honor-pragmas. (#104830)

  • Fixed a crash when function has more than 65536 parameters. Now a diagnostic is emitted. (#35741)

  • Fixed -ast-dump crashes on codes involving concept with -ast-dump-decl-types. (#94928)

OpenACC Specific Changes

Target Specific Changes

AMDGPU Support

X86 Support

  • The MMX vector intrinsic functions from *mmintrin.h which operate on __m64 vectors, such as _mm_add_pi8, have been reimplemented to use the SSE2 instruction-set and XMM registers unconditionally. These intrinsics are therefore no longer supported if MMX is enabled without SSE2 – either from targeting CPUs from the Pentium-MMX through the Pentium 3, or explicitly via passing arguments such as -mmmx -mno-sse2. MMX assembly code remains supported without requiring SSE2, including inside inline-assembly.

  • The compiler builtins such as __builtin_ia32_paddb which formerly implemented the above MMX intrinsic functions have been removed. Any uses of these removed functions should migrate to the functions defined by the *mmintrin.h headers. A mapping can be found in the file clang/www/builtins.py.

  • Support ISA of AVX10.2. * Supported MINMAX intrinsics of *_(mask(z)))_minmax(ne)_p[s|d|h|bh] and *_(mask(z)))_minmax_s[s|d|h].

Arm and AArch64 Support

Android Support

Windows Support

  • Clang no longer allows references inside a union when emulating MSVC 1900+ even if fms-extensions is enabled. Starting with VS2015, MSVC 1900, this Microsoft extension is no longer allowed and always results in an error. Clang now follows the MSVC behavior in this scenario. When -fms-compatibility-version=18.00 or prior is set on the command line this Microsoft extension is still allowed as VS2013 and prior allow it.

LoongArch Support

RISC-V Support

  • The option -mcmodel=large for the large code model is supported.

CUDA/HIP Language Changes

CUDA Support

AIX Support

NetBSD Support

WebAssembly Support

AVR Support

DWARF Support in Clang

Floating Point Support in Clang

Fixed Point Support in Clang

AST Matchers

  • Fixed an issue with the hasName and hasAnyName matcher when matching inline namespaces with an enclosing namespace of the same name.

  • Fixed an ordering issue with the hasOperands matcher occurring when setting a binding in the first matcher and using it in the second matcher.

  • Fixed a crash when traverse lambda expr with invalid captures. (#106444)

clang-format

  • Adds BreakBinaryOperations option.

libclang

  • Add clang_isBeforeInTranslationUnit. Given two source locations, it determines whether the first one comes strictly before the second in the source code.

Static Analyzer

New features

  • MallocChecker now checks for ownership_returns(class, idx) and ownership_takes(class, idx) attributes with class names different from “malloc”. Clang static analyzer now reports an error if class of allocation and deallocation function mismatches. Documentation.

Crash and bug fixes

Improvements

  • Improved the handling of the ownership_returns attribute. Now, Clang reports an error if the attribute is attached to a function that returns a non-pointer value. Fixes (#99501)

Moved checkers

  • The checker alpha.security.MallocOverflow was deleted because it was badly implemented and its agressive logic produced too many false positives. To detect too large arguments passed to malloc, consider using the checker alpha.taint.TaintedAlloc.

Sanitizers

  • Introduced Realtime Sanitizer, activated by using the -fsanitize=realtime flag. This sanitizer detects unsafe system library calls, such as memory allocations and mutex locks. If any such function is called during invocation of a function marked with the [[clang::nonblocking]] attribute, an error is printed to the console and the process exits non-zero.

  • Added the -fsanitize-undefined-ignore-overflow-pattern flag which can be used to disable specific overflow-dependent code patterns. The supported patterns are: add-signed-overflow-test, add-unsigned-overflow-test, negated-unsigned-const, and unsigned-post-decr-while. The sanitizer instrumentation can be toggled off for all available patterns by specifying all. Conversely, you may disable all exclusions with none which is the default.

    /// specified with ``-fsanitize-undefined-ignore-overflow-pattern=add-unsigned-overflow-test``
int common_overflow_check_pattern(unsigned base, unsigned offset) {
  if (base + offset < base) { /* ... */ } // The pattern of `a + b < a`, and other re-orderings, won't be instrumented
}

/// specified with ``-fsanitize-undefined-ignore-overflow-pattern=add-signed-overflow-test``
int common_overflow_check_pattern_signed(signed int base, signed int offset) {
  if (base + offset < base) { /* ... */ } // The pattern of `a + b < a`, and other re-orderings, won't be instrumented
}

/// specified with ``-fsanitize-undefined-ignore-overflow-pattern=negated-unsigned-const``
void negation_overflow() {
  unsigned long foo = -1UL; // No longer causes a negation overflow warning
  unsigned long bar = -2UL; // and so on...
}

/// specified with ``-fsanitize-undefined-ignore-overflow-pattern=unsigned-post-decr-while``
void while_post_decrement() {
  unsigned char count = 16;
  while (count--) { /* ... */ } // No longer causes unsigned-integer-overflow sanitizer to trip
}

    Many existing projects have a large amount of these code patterns present. This new flag should allow those projects to enable integer sanitizers with less noise.

Python Binding Changes

  • Fixed an issue that led to crashes when calling Type.get_exception_specification_kind.

OpenMP Support

  • Added support for ‘omp assume’ directive.

Improvements

  • Improve the handling of mapping array-section for struct containing nested structs with user defined mappers

  • num_teams and thead_limit now accept multiple expressions when it is used along in target teams ompx_bare construct. This allows the target region to be launched with multi-dim grid on GPUs.

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 tree.

If you have any questions or comments about Clang, please feel free to contact us on the Discourse forums (Clang Frontend category).