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[clang] Alias iso9899:2024 to C23, update documentation #138459

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May 5, 2025
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[clang] Alias iso9899:2024 to C23, update documentation #138459

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9 changes: 9 additions & 0 deletions clang/docs/CommandGuide/clang.rst
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Expand Up @@ -146,6 +146,15 @@ Language Selection and Mode Options

ISO C 2017 with GNU extensions

| ``c23``
| ``iso9899:2024``

ISO C 2023

| ``gnu23``

ISO C 2023 with GNU extensions

The default C language standard is ``gnu17``, except on PS4, where it is
``gnu99``.

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1 change: 1 addition & 0 deletions clang/docs/ReleaseNotes.rst
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Expand Up @@ -241,6 +241,7 @@ C2y Feature Support

C23 Feature Support
^^^^^^^^^^^^^^^^^^^
- Clang now accepts ``-std=iso9899:2024`` as an alias for C23.
- Added ``__builtin_c23_va_start()`` for compatibility with GCC and to enable
better diagnostic behavior for the ``va_start()`` macro in C23 and later.
This also updates the definition of ``va_start()`` in ``<stdarg.h>`` to use
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7 changes: 3 additions & 4 deletions clang/include/clang/Basic/LangStandards.def
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Expand Up @@ -89,15 +89,14 @@ LANGSTANDARD_ALIAS(gnu17, "gnu18")

// C23 modes
LANGSTANDARD(c23, "c23",
C, "Working Draft for ISO C23",
C, "ISO C 2023",
LineComment | C99 | C11 | C17 | C23 | Digraphs | HexFloat)
LANGSTANDARD_ALIAS(c23, "iso9899:2024")
LANGSTANDARD_ALIAS_DEPR(c23, "c2x")
LANGSTANDARD(gnu23, "gnu23",
C, "Working Draft for ISO C23 with GNU extensions",
C, "ISO C 2023 with GNU extensions",
LineComment | C99 | C11 | C17 | C23 | Digraphs | GNUMode | HexFloat)
LANGSTANDARD_ALIAS_DEPR(gnu23, "gnu2x")
// FIXME: Add the alias for iso9899:202* once we know the year ISO publishes
// the document (expected to be 2024).

// C2y modes
LANGSTANDARD(c2y, "c2y",
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5 changes: 2 additions & 3 deletions clang/test/Driver/unknown-std.c
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Expand Up @@ -15,11 +15,10 @@
// CHECK-NEXT: note: use 'gnu11' for 'ISO C 2011 with GNU extensions' standard
// CHECK-NEXT: note: use 'c17', 'iso9899:2017', 'c18', or 'iso9899:2018' for 'ISO C 2017' standard
// CHECK-NEXT: note: use 'gnu17' or 'gnu18' for 'ISO C 2017 with GNU extensions' standard
// CHECK-NEXT: note: use 'c23' for 'Working Draft for ISO C23' standard
// CHECK-NEXT: note: use 'gnu23' for 'Working Draft for ISO C23 with GNU extensions' standard
// CHECK-NEXT: note: use 'c23' or 'iso9899:2024' for 'ISO C 2023' standard
// CHECK-NEXT: note: use 'gnu23' for 'ISO C 2023 with GNU extensions' standard
// CHECK-NEXT: note: use 'c2y' for 'Working Draft for ISO C2y' standard
// CHECK-NEXT: note: use 'gnu2y' for 'Working Draft for ISO C2y with GNU extensions' standard

// Make sure that no other output is present.
// CHECK-NOT: {{^.+$}}