[C2x] Support -std=c23 and -std=gnu23

C2x was finalized at the June 2023 WG14 meeting. The DIS is out for
balloting and the comment period for that closes later this year/early
next year. While that does leave an opportunity for more changes to the
standard during the DIS ballot resolution process, only editorial
changes are anticipated and as a result, the C committee considers C2x
to be final. The committee took a straw poll on what we'd prefer the
informal name of the standard be, and we decided it should be called
C23 regardless of what year ISO publishes it.

However, because the final publication is not out, this patch does not
add the language standard alias for the -std=iso9899:<year> spelling of
the standard mode; that will be added once ISO finally publishes the
document and the year chosen will match the publication date.

This also changes the value of __STDC_VERSION__ from the placeholder
value 202000L to the final value 202311L.

Subsequent patches will start renaming things from c2x to c23, cleaning
up documentation, etc.

Differential Revision: https://reviews.llvm.org/D157606

clang-tools-extra/clangd/index/StdLib.cpp

@@ -60,7 +60,7 @@ LangStandard::Kind standardFromOpts(const LangOptions &LO) {
     return LangStandard::lang_cxx98;
   }
   if (LO.C2x)
-    return LangStandard::lang_c2x;
+    return LangStandard::lang_c23;
   // C17 has no new features, so treat {C11,C17} as C17.
   if (LO.C11)
     return LangStandard::lang_c17;

clang/docs/ReleaseNotes.rst

@@ -98,6 +98,10 @@ C Language Changes
 
 C2x Feature Support
 ^^^^^^^^^^^^^^^^^^^
+- Clang now accepts ``-std=c23`` and ``-std=gnu23`` as language standard modes,
+  and the ``__STDC_VERSION__`` macro now expands to ``202311L`` instead of its
+  previous placeholder value. Clang continues to accept ``-std=c2x`` and
+  ``-std=gnu2x`` as aliases for C23 and GNU C23, respectively.
 
 Non-comprehensive list of changes in this release
 -------------------------------------------------

clang/include/clang/Basic/LangStandards.def

@@ -87,13 +87,17 @@ LANGSTANDARD(gnu17, "gnu17",
              LineComment | C99 | C11 | C17 | Digraphs | GNUMode | HexFloat)
 LANGSTANDARD_ALIAS(gnu17, "gnu18")
 
-// C2x modes
-LANGSTANDARD(c2x, "c2x",
-             C, "Working Draft for ISO C2x",
+// C23 modes
+LANGSTANDARD(c23, "c23",
+             C, "Working Draft for ISO C23",
              LineComment | C99 | C11 | C17 | C2x | Digraphs | HexFloat)
-LANGSTANDARD(gnu2x, "gnu2x",
-             C, "Working Draft for ISO C2x with GNU extensions",
+LANGSTANDARD_ALIAS_DEPR(c23, "c2x")
+LANGSTANDARD(gnu23, "gnu23",
+             C, "Working Draft for ISO C23 with GNU extensions",
              LineComment | C99 | C11 | C17 | C2x | 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).
 
 // C++ modes
 LANGSTANDARD(cxx98, "c++98",

clang/lib/Frontend/InitPreprocessor.cpp

@@ -438,9 +438,8 @@ static void InitializeStandardPredefinedMacros(const TargetInfo &TI,
   //      value is, are implementation-defined.
   // (Removed in C++20.)
   if (!LangOpts.CPlusPlus) {
-    // FIXME: Use correct value for C23.
     if (LangOpts.C2x)
-      Builder.defineMacro("__STDC_VERSION__", "202000L");
+      Builder.defineMacro("__STDC_VERSION__", "202311L");
     else if (LangOpts.C17)
       Builder.defineMacro("__STDC_VERSION__", "201710L");
     else if (LangOpts.C11)

clang/lib/Headers/limits.h

@@ -67,9 +67,7 @@
 #define CHAR_BIT  __CHAR_BIT__
 
 /* C2x 5.2.4.2.1 */
-/* FIXME: This is using the placeholder dates Clang produces for these macros
-   in C2x mode; switch to the correct values once they've been published. */
-#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202000L
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
 #define BOOL_WIDTH   __BOOL_WIDTH__
 #define CHAR_WIDTH   CHAR_BIT
 #define SCHAR_WIDTH  CHAR_BIT

clang/lib/Headers/stdalign.h

@@ -10,10 +10,8 @@
 #ifndef __STDALIGN_H
 #define __STDALIGN_H
 
-/* FIXME: This is using the placeholder dates Clang produces for these macros
-   in C2x mode; switch to the correct values once they've been published. */
 #if defined(__cplusplus) ||                                                    \
-    (defined(__STDC_VERSION__) && __STDC_VERSION__ < 202000L)
+    (defined(__STDC_VERSION__) && __STDC_VERSION__ < 202311L)
 #ifndef __cplusplus
 #define alignas _Alignas
 #define alignof _Alignof

clang/lib/Headers/stdarg.h

@@ -23,13 +23,11 @@ typedef __builtin_va_list va_list;
 #define _VA_LIST
 #endif
 
-/* FIXME: This is using the placeholder dates Clang produces for these macros
-   in C2x mode; switch to the correct values once they've been published. */
-#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202000L
-/* C2x does not require the second parameter for va_start. */
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
+/* C23 does not require the second parameter for va_start. */
 #define va_start(ap, ...) __builtin_va_start(ap, 0)
 #else
-/* Versions before C2x do require the second parameter. */
+/* Versions before C23 do require the second parameter. */
 #define va_start(ap, param) __builtin_va_start(ap, param)
 #endif
 #define va_end(ap)          __builtin_va_end(ap)

clang/lib/Headers/stdatomic.h

@@ -45,16 +45,14 @@ extern "C" {
 #define ATOMIC_POINTER_LOCK_FREE    __CLANG_ATOMIC_POINTER_LOCK_FREE
 
 /* 7.17.2 Initialization */
-/* FIXME: This is using the placeholder dates Clang produces for these macros
-   in C2x mode; switch to the correct values once they've been published. */
-#if (defined(__STDC_VERSION__) && __STDC_VERSION__ < 202000L) ||               \
+#if (defined(__STDC_VERSION__) && __STDC_VERSION__ < 202311L) ||               \
     defined(__cplusplus)
-/* ATOMIC_VAR_INIT was removed in C2x, but still remains in C++23. */
+/* ATOMIC_VAR_INIT was removed in C23, but still remains in C++23. */
 #define ATOMIC_VAR_INIT(value) (value)
 #endif
 
 #if ((defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201710L &&              \
-      __STDC_VERSION__ < 202000L) ||                                           \
+      __STDC_VERSION__ < 202311L) ||                                           \
      (defined(__cplusplus) && __cplusplus >= 202002L)) &&                      \
     !defined(_CLANG_DISABLE_CRT_DEPRECATION_WARNINGS)
 /* ATOMIC_VAR_INIT was deprecated in C17 and C++20. */

clang/lib/Headers/stddef.h

@@ -97,14 +97,12 @@ using ::std::nullptr_t;
 #undef __need_NULL
 #endif /* defined(__need_NULL) */
 
-/* FIXME: This is using the placeholder dates Clang produces for these macros
-   in C2x mode; switch to the correct values once they've been published. */
-#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202000L
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
 typedef typeof(nullptr) nullptr_t;
-#endif /* defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202000L */
+#endif /* defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L */
 
 #if defined(__need_STDDEF_H_misc) && defined(__STDC_VERSION__) &&              \
-    __STDC_VERSION__ >= 202000L
+    __STDC_VERSION__ >= 202311L
 #define unreachable() __builtin_unreachable()
 #endif /* defined(__need_STDDEF_H_misc) && >= C23 */
 

clang/lib/Headers/stdint.h

@@ -499,9 +499,8 @@ typedef __UINTMAX_TYPE__ uintmax_t;
 # define INT64_MAX           INT64_C( 9223372036854775807)
 # define INT64_MIN         (-INT64_C( 9223372036854775807)-1)
 # define UINT64_MAX         UINT64_C(18446744073709551615)
-/* FIXME: This is using the placeholder dates Clang produces for these macros
-   in C2x mode; switch to the correct values once they've been published. */
-#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202000L
+
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
 # define UINT64_WIDTH         64
 # define INT64_WIDTH          UINT64_WIDTH
 
@@ -545,9 +544,7 @@ typedef __UINTMAX_TYPE__ uintmax_t;
 # define INT_FAST64_MAX    __INT_LEAST64_MAX
 # define UINT_FAST64_MAX  __UINT_LEAST64_MAX
 
-/* FIXME: This is using the placeholder dates Clang produces for these macros
-   in C2x mode; switch to the correct values once they've been published. */
-#if defined(__STDC_VERSION__) &&  __STDC_VERSION__ >= 202000L
+#if defined(__STDC_VERSION__) &&  __STDC_VERSION__ >= 202311L
 # define UINT_LEAST64_WIDTH __UINT_LEAST64_WIDTH
 # define INT_LEAST64_WIDTH  UINT_LEAST64_WIDTH
 # define UINT_FAST64_WIDTH  __UINT_LEAST64_WIDTH
@@ -586,9 +583,7 @@ typedef __UINTMAX_TYPE__ uintmax_t;
 # undef __UINT_LEAST8_MAX
 # define __UINT_LEAST8_MAX  UINT56_MAX
 
-/* FIXME: This is using the placeholder dates Clang produces for these macros
-   in C2x mode; switch to the correct values once they've been published. */
-#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202000L
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
 # define UINT56_WIDTH         56
 # define INT56_WIDTH          UINT56_WIDTH
 # define UINT_LEAST56_WIDTH   UINT56_WIDTH
@@ -635,9 +630,7 @@ typedef __UINTMAX_TYPE__ uintmax_t;
 # undef __UINT_LEAST8_MAX
 # define __UINT_LEAST8_MAX  UINT48_MAX
 
-/* FIXME: This is using the placeholder dates Clang produces for these macros
-   in C2x mode; switch to the correct values once they've been published. */
-#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202000L
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
 #define UINT48_WIDTH         48
 #define INT48_WIDTH          UINT48_WIDTH
 #define UINT_LEAST48_WIDTH   UINT48_WIDTH
@@ -684,9 +677,7 @@ typedef __UINTMAX_TYPE__ uintmax_t;
 # undef __UINT_LEAST8_MAX
 # define __UINT_LEAST8_MAX  UINT40_MAX
 
-/* FIXME: This is using the placeholder dates Clang produces for these macros
-   in C2x mode; switch to the correct values once they've been published. */
-#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202000L
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
 # define UINT40_WIDTH         40
 # define INT40_WIDTH          UINT40_WIDTH
 # define UINT_LEAST40_WIDTH   UINT40_WIDTH
@@ -727,9 +718,7 @@ typedef __UINTMAX_TYPE__ uintmax_t;
 # undef __UINT_LEAST8_MAX
 # define __UINT_LEAST8_MAX  UINT32_MAX
 
-/* FIXME: This is using the placeholder dates Clang produces for these macros
-   in C2x mode; switch to the correct values once they've been published. */
-#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202000L
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
 # define UINT32_WIDTH         32
 # define INT32_WIDTH          UINT32_WIDTH
 # undef __UINT_LEAST32_WIDTH
@@ -749,9 +738,7 @@ typedef __UINTMAX_TYPE__ uintmax_t;
 # define INT_FAST32_MAX    __INT_LEAST32_MAX
 # define UINT_FAST32_MAX  __UINT_LEAST32_MAX
 
-/* FIXME: This is using the placeholder dates Clang produces for these macros
-   in C2x mode; switch to the correct values once they've been published. */
-#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202000L
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
 # define UINT_LEAST32_WIDTH __UINT_LEAST32_WIDTH
 # define INT_LEAST32_WIDTH  UINT_LEAST32_WIDTH
 # define UINT_FAST32_WIDTH  __UINT_LEAST32_WIDTH
@@ -784,9 +771,7 @@ typedef __UINTMAX_TYPE__ uintmax_t;
 # undef __UINT_LEAST8_MAX
 # define __UINT_LEAST8_MAX  UINT24_MAX
 
-/* FIXME: This is using the placeholder dates Clang produces for these macros
-   in C2x mode; switch to the correct values once they've been published. */
-#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202000L
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
 # define UINT24_WIDTH         24
 # define INT24_WIDTH          UINT24_WIDTH
 # define UINT_LEAST24_WIDTH   UINT24_WIDTH
@@ -819,9 +804,7 @@ typedef __UINTMAX_TYPE__ uintmax_t;
 # undef __UINT_LEAST8_MAX
 # define __UINT_LEAST8_MAX  UINT16_MAX
 
-/* FIXME: This is using the placeholder dates Clang produces for these macros
-   in C2x mode; switch to the correct values once they've been published. */
-#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202000L
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
 # define UINT16_WIDTH         16
 # define INT16_WIDTH          UINT16_WIDTH
 # undef __UINT_LEAST16_WIDTH
@@ -839,9 +822,7 @@ typedef __UINTMAX_TYPE__ uintmax_t;
 # define INT_FAST16_MAX    __INT_LEAST16_MAX
 # define UINT_FAST16_MAX  __UINT_LEAST16_MAX
 
-/* FIXME: This is using the placeholder dates Clang produces for these macros
-   in C2x mode; switch to the correct values once they've been published. */
-#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202000L
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
 # define UINT_LEAST16_WIDTH __UINT_LEAST16_WIDTH
 # define INT_LEAST16_WIDTH  UINT_LEAST16_WIDTH
 # define UINT_FAST16_WIDTH  __UINT_LEAST16_WIDTH
@@ -862,9 +843,7 @@ typedef __UINTMAX_TYPE__ uintmax_t;
 # undef __UINT_LEAST8_MAX
 # define __UINT_LEAST8_MAX  UINT8_MAX
 
-/* FIXME: This is using the placeholder dates Clang produces for these macros
-   in C2x mode; switch to the correct values once they've been published. */
-#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202000L
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
 # define UINT8_WIDTH         8
 # define INT8_WIDTH          UINT8_WIDTH
 # undef __UINT_LEAST8_WIDTH
@@ -880,9 +859,7 @@ typedef __UINTMAX_TYPE__ uintmax_t;
 # define INT_FAST8_MAX    __INT_LEAST8_MAX
 # define UINT_FAST8_MAX  __UINT_LEAST8_MAX
 
-/* FIXME: This is using the placeholder dates Clang produces for these macros
-   in C2x mode; switch to the correct values once they've been published. */
-#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202000L
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
 # define UINT_LEAST8_WIDTH __UINT_LEAST8_WIDTH
 # define INT_LEAST8_WIDTH  UINT_LEAST8_WIDTH
 # define UINT_FAST8_WIDTH  __UINT_LEAST8_WIDTH
@@ -908,9 +885,7 @@ typedef __UINTMAX_TYPE__ uintmax_t;
 #define    SIZE_MAX      __SIZE_MAX__
 
 /* C2x 7.20.2.4 Width of integer types capable of holding object pointers. */
-/* FIXME: This is using the placeholder dates Clang produces for these macros
-   in C2x mode; switch to the correct values once they've been published. */
-#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202000L
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
 /* NB: The C standard requires that these be the same value, but the compiler
    exposes separate internal width macros. */
 #define INTPTR_WIDTH  __INTPTR_WIDTH__
@@ -929,9 +904,7 @@ typedef __UINTMAX_TYPE__ uintmax_t;
 #define UINTMAX_MAX  __UINTMAX_MAX__
 
 /* C2x 7.20.2.5 Width of greatest-width integer types. */
-/* FIXME: This is using the placeholder dates Clang produces for these macros
-   in C2x mode; switch to the correct values once they've been published. */
-#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202000L
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
 /* NB: The C standard requires that these be the same value, but the compiler
    exposes separate internal width macros. */
 #define INTMAX_WIDTH __INTMAX_WIDTH__
@@ -965,9 +938,7 @@ typedef __UINTMAX_TYPE__ uintmax_t;
 #define UINTMAX_C(v) __int_c(v, __UINTMAX_C_SUFFIX__)
 
 /* C2x 7.20.3.x Width of other integer types. */
-/* FIXME: This is using the placeholder dates Clang produces for these macros
-   in C2x mode; switch to the correct values once they've been published. */
-#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202000L
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
 #define PTRDIFF_WIDTH    __PTRDIFF_WIDTH__
 #define SIG_ATOMIC_WIDTH __SIG_ATOMIC_WIDTH__
 #define SIZE_WIDTH       __SIZE_WIDTH__

clang/test/C/C11/n1330.c

@@ -58,9 +58,7 @@ void test(void) {
   _Static_assert(1.0f, "this should not compile"); // expected-warning {{expression is not an integer constant expression; folding it to a constant is a GNU extension}}
 }
 
-// FIXME: This is using the placeholder date Clang produces for the macro in
-// C2x mode; switch to the correct value once it's been published.
-#if __STDC_VERSION__ < 202000L
+#if __STDC_VERSION__ < 202311L
 // The use of a _Static_assert in a K&R C function definition is prohibited per
 // 6.9.1p6 requiring each declaration to have a declarator (which a static
 // assertion does not have) and only declare identifiers from the identifier

clang/test/C/drs/dr0xx.c

@@ -234,7 +234,7 @@ void dr031(int i) {
  */
 int dr032 = (1, 2); /* expected-warning {{left operand of comma operator has no effect}} */
 
-#if __STDC_VERSION__ < 202000L
+#if __STDC_VERSION__ < 202311L
 /* WG14 DR035: partial
  * Questions about definition of functions without a prototype
  */
@@ -251,7 +251,7 @@ void dr035_2(c) /* expected-warning {{a function definition without a prototype
    */
   int test = q; /* expected-error {{use of undeclared identifier 'q'}} */
 }
-#endif /* __STDC_VERSION__ < 202000L */
+#endif /* __STDC_VERSION__ < 202311L */
 
 /* WG14 DR038: yes
  * Questions about argument substitution during macro expansion
@@ -339,7 +339,7 @@ void dr050(void) {
   (void)NULL; /* expected-error {{use of undeclared identifier 'NULL'}} */
 }
 
-#if __STDC_VERSION__ < 202000L
+#if __STDC_VERSION__ < 202311L
 /* WG14 DR053: yes
  * Accessing a pointer to a function with a prototype through a pointer to
  * pointer to function without a prototype
@@ -356,7 +356,7 @@ void dr053(void) {
   fpp = &fp1;
   (**fpp)(3); /* expected-warning {{passing arguments to a function without a prototype is deprecated in all versions of C and is not supported in C2x}} */
 }
-#endif /* __STDC_VERSION__ < 202000L */
+#endif /* __STDC_VERSION__ < 202311L */
 
 /* WG14 DR064: yes
  * Null pointer constants
@@ -385,7 +385,7 @@ void dr068(void) {
 #endif
 }
 
-#if __STDC_VERSION__ < 202000L
+#if __STDC_VERSION__ < 202311L
 /* WG14: DR070: yes
  * Interchangeability of function arguments
  *
@@ -406,7 +406,7 @@ void dr070_2(void) {
   dr070_1(6);
   dr070_1(6U); /* Pedantically UB */
 }
-#endif /* __STDC_VERSION__ < 202000L */
+#endif /* __STDC_VERSION__ < 202311L */
 
 /* WG14 DR071: yes
  * Enumerated types

clang/test/C/drs/dr2xx.c

@@ -231,7 +231,7 @@ void dr251(void) {
   struct dr251_fred *ptr; /* expected-error {{use of 'dr251_fred' with tag type that does not match previous declaration}} */
 }
 
-#if __STDC_VERSION__ < 202000L
+#if __STDC_VERSION__ < 202311L
 /* WG14 DR252: yes
  * Incomplete argument types when calling non-prototyped functions
  */
@@ -250,7 +250,7 @@ void dr252(void) {
                                     expected-warning {{passing arguments to 'dr252_no_proto' without a prototype is deprecated in all versions of C and is not supported in C2x}}
                                   */
 }
-#endif /* __STDC_VERSION__ < 202000L */
+#endif /* __STDC_VERSION__ < 202311L */
 
 /* WG14 DR258: yes
  * Ordering of "defined" and macro replacement