c: Add __typeof_unqual__ and __typeof_unqual support

As I mentioned in my stdckdint.h mail, I think having __ prefixed
keywords for the typeof_unqual keyword which can be used in earlier
language modes can be useful, not all code can be switched to C23
right away.

The following patch implements that.  It keeps the non-C23 behavior
for it for the _Noreturn functions to stay compatible with how
__typeof__ behaves.

I think we don't need it for C++, in C++ we have standard
traits to remove qualifiers etc.

2023-08-11  Jakub Jelinek  <jakub@redhat.com>

gcc/
	* doc/extend.texi (Typeof): Document typeof_unqual
	and __typeof_unqual__.
gcc/c-family/
	* c-common.cc (c_common_reswords): Add __typeof_unqual
	and __typeof_unqual__ spellings of typeof_unqual.
gcc/c/
	* c-parser.cc (c_parser_typeof_specifier): Handle
	__typeof_unqual and __typeof_unqual__ as !is_std.
gcc/testsuite/
	* gcc.dg/c11-typeof-2.c: New test.
	* gcc.dg/c11-typeof-3.c: New test.
	* gcc.dg/gnu11-typeof-3.c: New test.
	* gcc.dg/gnu11-typeof-4.c: New test.

1 files changed, 7 insertions, 0 deletions

diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 89c5b4e..73a9972 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -843,6 +843,13 @@ Thus, @code{array (pointer (char), 4)} is the type of arrays of 4
 pointers to @code{char}.
 @end itemize
 
+The ISO C2X operator @code{typeof_unqual} is available in ISO C2X mode
+and its result is the non-atomic unqualified version of what @code{typeof}
+operator returns.  Alternate spelling @code{__typeof_unqual__} is
+available in all C modes and provides non-atomic unqualified version of
+what @code{__typeof__} operator returns.
+@xref{Alternate Keywords}.
+
 @cindex @code{__auto_type} in GNU C
 In GNU C, but not GNU C++, you may also declare the type of a variable
 as @code{__auto_type}.  In that case, the declaration must declare