Clang 16.0.0git (In-Progress) Release Notes

Written by the LLVM Team

Warning

These are in-progress notes for the upcoming Clang 16 release. Release notes for previous releases can be found on the Download Page.

Introduction

This document contains the release notes for the Clang C/C++/Objective-C frontend, part of the LLVM Compiler Infrastructure, release 16.0.0git. 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. 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.

Note that if you are reading this file from a Git checkout or the main Clang web page, this document applies to the next release, not the current one. To see the release notes for a specific release, please see the releases page.

Potentially Breaking Changes

These changes are ones which we think may surprise users when upgrading to Clang 16.0.0git because of the opportunity they pose for disruption to existing code bases.

  • 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 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. Fixes Issue 50055.

  • -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 -Wno-implicit-function-pointer-types.

    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.

    void func(const int *i);
void other(void) {
  void (*fp)(int *) = func; // Previously a warning, now a downgradable error.
}

  • Clang now disallows types whose sizes aren’t a multiple of their alignments to be used as the element type of arrays.

    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.

What’s New in Clang 16.0.0git?

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.

Bug Fixes

  • Correct _Static_assert to accept the same set of extended integer constant expressions as is accpted in other contexts that accept them. This fixes Issue 57687.
  • Fixes an accepts-invalid bug in C when using a _Noreturn function specifier on something other than a function declaration. This fixes Issue 56800.
  • Fix #56772 - invalid destructor names were incorrectly accepted on template classes.
  • Improve compile-times with large dynamic array allocations with trivial constructors. This fixes Issue 56774.
  • 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).
  • Fix clang not properly diagnosing the failing subexpression when chained binary operators are used in a static_assert expression.
  • Fix a crash when evaluating a multi-dimensional array’s array filler expression is element-dependent. This fixes Issue 50601.
  • Fixed a crash-on-valid with consteval evaluation of a list-initialized constructor for a temporary object. This fixes Issue 55871.
  • Fix #57008 - Builtin C++ language extension type traits instantiated by a template with unexpected number of arguments cause an assertion fault.
  • Fix multi-level pack expansion of undeclared function parameters. This fixes Issue 56094.
  • Fix #57151. -Wcomma is emitted for void returning functions.
  • -Wtautological-compare missed warnings for tautological comparisons involving a negative integer literal. This fixes Issue 42918.
  • Fix a crash when generating code coverage information for an if consteval statement. This fixes Issue 57377.
  • Fix assert that triggers a crash during template name lookup when a type was incomplete but was not also a TagType. This fixes Issue 57387.
  • Fix a crash when emitting a concept-related diagnostic. This fixes Issue 57415.
  • Fix a crash when attempting to default a virtual constexpr non-special member function in a derived class. This fixes Issue 57431
  • Fix a crash where we attempt to define a deleted destructor. This fixes Issue 57516
  • Fix __builtin_assume_aligned crash when the 1st arg is array type. This fixes Issue 57169
  • Clang configuration files are now read through the virtual file system rather than the physical one, if these are different.
  • 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 clear.
  • Fixes to builtin template emulation of regular templates. Issue 42102 Issue 51928
  • A SubstTemplateTypeParmType can now represent the pack index for a substitution from an expanded pack. Issue 56099
  • Fix -Wpre-c++17-compat crashing Clang when compiling C++20 code which contains deduced template specializations. This Fixes Issue 57369 Issue 57643 Issue 57793
  • Respect constructor constraints during class template argument deduction (CTAD). This is the suggested resolution to CWG DR2628. Issue 57646 Issue 43829
  • Fixed a crash in C++20 mode in Clang and Clangd when compile source with compilation errors. Issue 53628
  • The template arguments of a variable template being accessed as a member will now be represented in the AST.
  • Fix incorrect handling of inline builtins with asm labels.
  • 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.

Improvements to Clang’s diagnostics

  • Clang will now check compile-time determinable string literals as format strings. Fixes Issue 55805:.
  • -Wformat now recognizes %b for the printf/scanf family of functions and %B for the printf family of functions. Fixes Issue 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. This fixes Issue 53253. 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.
  • 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 varaibles have been improved to mention the missing constant initializer.
  • Correctly diagnose a future keyword if it exist as a keyword in the higher language version and specifies in which version it will be a keyword. This supports both c and c++ language.
  • When diagnosing multi-level pack expansions of mismatched lengths, Clang will now, in most cases, be able to point to the relevant outer parameter.
  • 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 methods.
  • 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. Fixes Issue 57478:.
  • New analysis pass will now help 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 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 diagnoses a warning when defercencing a void pointer in C mode. This fixes Issue 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.

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.
  • 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.
  • 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.
  • 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.

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.

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.

Modified Compiler Flags

  • Clang now permits specifying --config= multiple times, to load multiple configuration files.

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 targets.
  • 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.

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.

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.

C Language Changes in Clang

  • 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 %hd.

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.

    __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'

C++ Language Changes in Clang

  • Implemented DR692, DR1395 and DR1432. Use the -fclang-abi-compat=15 option to get the old partial ordering behavior regarding packs. Note that the fix for DR1432 is speculative that there is no wording or even resolution for this issue. A speculative fix for DR1432 is needed because it fixes regressions caused by DR692.
  • 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.

C++20 Feature Support

  • Support capturing structured bindings in lambdas (P1091R3 and P1381R1). This fixes issues Issue 52720, Issue 54300, Issue 54301, and Issue 49430.
  • 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. This fixes Issue 55601.
  • Implemented “Conditionally Trivial Special Member Functions” (P0848). Note: The handling of deleted functions is not yet compliant, as Clang does not implement DR1496 and DR1734.
  • Class member variables are now in scope when parsing a requires clause. Fixes Issue 55216.
  • Correctly set expression evaluation context as ‘immediate function context’ in consteval functions. This fixes Issue 51182.
  • Fixes an assert crash caused by looking up missing vtable information on consteval virtual functions. Fixes Issue 55065.
  • Skip rebuilding lambda expressions in arguments of immediate invocations. This fixes Issue 56183, Issue 51695, Issue 50455, Issue 54872, Issue 54587.
  • 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. This fixes Issue 44178.
  • Clang implements DR2621, correcting a defect in using enum handling. The name is found via ordinary lookup so typedefs are found.
  • Implemented P0634r3, which removes the requirement for the typename keyword in certain contexts.

C++2b Feature Support

RISC-V Support in Clang

  • sifive-7-rv32 and sifive-7-rv64 are no longer supported for -mcpu. Use sifive-e76, sifive-s76, or sifive-u74 instead.

X86 Support in Clang

  • Support -mindirect-branch-cs-prefix for call and jmp to indirect thunk.
  • Fix 32-bit __fastcall and __vectorcall ABI mismatch with MSVC.

Arm and AArch64 Support in Clang

  • 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.
  • Add driver and tuning support for Neoverse V2 via the flag -mcpu=neoverse-v2. Native detection is also supported via -mcpu=native.

libclang

  • 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.
  • 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.

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 behavior.

New Issues Found

  • …

Python Binding Changes

The following methods have been added:

  • …

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) <https://discourse.llvm.org/c/clang/6>`_.