[C23] Update user-facing docs for C23

This changes some public references of C2x to be C23, corrects standard
citations to use the final paragraph numbers, and adds some information
about differences between C17 and C23 modes.

clang/docs/LanguageExtensions.rst

@@ -612,7 +612,7 @@ to perform additional operations on certain scalar and vector types.
 
 Let ``T`` be one of the following types:
 
-* an integer type (as in C2x 6.2.5p19), but excluding enumerated types and _Bool
+* an integer type (as in C23 6.2.5p22), but excluding enumerated types and ``bool``
 * the standard floating types float or double
 * a half-precision floating point type, if one is supported on the target
 * a vector type.
@@ -1462,8 +1462,8 @@ Conditional ``explicit``               __cpp_conditional_explicit       C++20
 ``static operator()``                  __cpp_static_call_operator       C++23         C++03
 -------------------------------------- -------------------------------- ------------- -------------
 Designated initializers (N494)                                          C99           C89
-Array & element qualification (N2607)                                   C2x           C89
-Attributes (N2335)                                                      C2x           C89
+Array & element qualification (N2607)                                   C23           C89
+Attributes (N2335)                                                      C23           C89
 ====================================== ================================ ============= =============
 
 Type Trait Primitives
@@ -3669,7 +3669,7 @@ A predefined typedef for the target-specific ``va_list`` type.
 
 A builtin function for the target-specific ``va_start`` function-like macro.
 The ``parameter-name`` argument is the name of the parameter preceding the
-ellipsis (``...``) in the function signature. Alternatively, in C2x mode or
+ellipsis (``...``) in the function signature. Alternatively, in C23 mode or
 later, it may be the integer literal ``0`` if there is no parameter preceding
 the ellipsis. This function initializes the given ``__builtin_va_list`` object.
 It is undefined behavior to call this function on an already initialized

clang/docs/MatrixTypes.rst

@@ -33,7 +33,7 @@ program is ill-formed.
 Currently, the element type of a matrix is only permitted to be one of the
 following types:
 
-* an integer type (as in C2x 6.2.5p19), but excluding enumerated types and ``_Bool``
+* an integer type (as in C23 6.2.5p22), but excluding enumerated types and ``bool``
 * the standard floating types ``float`` or ``double``
 * a half-precision floating point type, if one is supported on the target
 
@@ -68,7 +68,7 @@ rows and columns are the same and the value's elements can be converted to the
 element type of the result type. The result is a matrix where each element is
 the converted corresponding element.
 
-A value of any real type (as in C2x 6.2.5p17) can be converted to a matrix type
+A value of any real type (as in C23 6.2.5p14) can be converted to a matrix type
 if it can be converted to the element type of the matrix. The result is a
 matrix where all elements are the converted original value.
 

clang/docs/ReleaseNotes.rst

@@ -96,7 +96,7 @@ C Language Changes
   constant expressions.  This change is more consistent with the behavior of
   GCC.
 
-C2x Feature Support
+C23 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

clang/docs/UsersManual.rst

@@ -3172,7 +3172,7 @@ Differences between various standard modes
 
 clang supports the -std option, which changes what language mode clang uses.
 The supported modes for C are c89, gnu89, c94, c99, gnu99, c11, gnu11, c17,
-gnu17, c2x, gnu2x, and various aliases for those modes. If no -std option is
+gnu17, c23, gnu23, and various aliases for those modes. If no -std option is
 specified, clang defaults to gnu17 mode. Many C99 and C11 features are
 supported in earlier modes as a conforming extension, with a warning. Use
 ``-pedantic-errors`` to request an error if a feature from a later standard
@@ -3224,6 +3224,19 @@ Differences between ``*11`` and ``*17`` modes:
 
 -  ``__STDC_VERSION__`` is defined to ``201710L`` rather than ``201112L``.
 
+Differences between ``*17`` and ``*23`` modes:
+
+- ``__STDC_VERSION__`` is defined to ``202311L`` rather than ``201710L``.
+- ``nullptr`` and ``nullptr_t`` are supported, only in ``*23`` mode.
+- ``ATOMIC_VAR_INIT`` is removed from ``*23`` mode.
+- ``bool``, ``true``, ``false``, ``alignas``, ``alignof``, ``static_assert``,
+  and ``thread_local` are now first-class keywords, only in ``*23`` mode.
+- ``typeof`` and ``typeof_unqual`` are supported, only ``*23`` mode.
+- Bit-precise integers (``_BitInt(N)``) are supported by default in ``*23``
+  mode, and as an extension in ``*17`` and earlier modes.
+- ``[[]]`` attributes are supported by default in ``*23`` mode, and as an
+  extension in ``*17`` and earlier modes.
+
 GCC extensions not implemented yet
 ----------------------------------