c: Add -std=c2y, -std=gnu2y, -Wc23-c2y-compat, C2Y _Generic with type operand

The first new C2Y feature, _Generic where the controlling operand is a
type name rather than an expression (as defined in N3260), was voted
into C2Y today.  (In particular, this form of _Generic allows
distinguishing qualified and unqualified versions of a type.)  This
feature also includes allowing the generic associations to specify
incomplete and function types.

Add this feature to GCC, along with the -std=c2y, -std=gnu2y and
-Wc23-c2y-compat options to control when and how it is diagnosed.  As
usual, the feature is allowed by default in older standards modes,
subject to diagnosis with -pedantic, -pedantic-errors or
-Wc23-c2y-compat.

Bootstrapped with no regressions on x86_64-pc-linux-gnu.

gcc/
	* doc/cpp.texi (__STDC_VERSION__): Document C2Y handling.
	* doc/invoke.texi (-Wc23-c2y-compat, -std=c2y, -std=gnu2y):
	Document options.
	(-std=gnu23): Update documentation.
	* doc/standards.texi (C Language): Document C2Y.  Update C23
	description.
	* config/rl78/rl78.cc (rl78_option_override): Handle "GNU C2Y"
	language name.
	* dwarf2out.cc (highest_c_language, gen_compile_unit_die):
	Likewise.

gcc/c-family/
	* c-common.cc (flag_isoc2y): New.
	(flag_isoc99, flag_isoc11, flag_isoc23): Update comments.
	* c-common.h (flag_isoc2y): New.
	(clk_c, flag_isoc23): Update comments.
	* c-opts.cc (set_std_c2y): New.
	(c_common_handle_option): Handle OPT_std_c2y and OPT_std_gnu2y.
	(set_std_c89, set_std_c99, set_std_c11, set_std_c17, set_std_c23):
	Set flag_isoc2y.
	(set_std_c23): Update comment.
	* c.opt (Wc23-c2y-compat, std=c2y, std=gnu2y): New.
	* c.opt.urls: Regenerate.

gcc/c/
	* c-errors.cc (pedwarn_c23): New.
	* c-parser.cc (disable_extension_diagnostics)
	(restore_extension_diagnostics): Save and restore
	warn_c23_c2y_compat.
	(c_parser_generic_selection): Handle type name as controlling
	operand.  Allow incomplete and function types subject to
	pedwarn_c23 calls.
	* c-tree.h (pedwarn_c23): New.

gcc/testsuite/
	* gcc.dg/c23-generic-1.c, gcc.dg/c23-generic-2.c,
	gcc.dg/c23-generic-3.c, gcc.dg/c23-generic-4.c,
	gcc.dg/c2y-generic-1.c, gcc.dg/c2y-generic-2.c,
	gcc.dg/c2y-generic-3.c, gcc.dg/gnu2y-generic-1.c: New tests.
	* gcc.dg/c23-tag-6.c: Use -pedantic-errors.

libcpp/
	* include/cpplib.h (CLK_GNUC2Y, CLK_STDC2Y): New.
	* init.cc (lang_defaults): Add GNUC2Y and STDC2Y entries.
	(cpp_init_builtins): Define __STDC_VERSION__ to 202500L for GNUC2Y
	and STDC2Y.

2 files changed, 7 insertions, 1 deletions

diff --git a/libcpp/include/cpplib.h b/libcpp/include/cpplib.h
index c62374d..d76817c 100644
--- a/libcpp/include/cpplib.h
+++ b/libcpp/include/cpplib.h
@@ -172,8 +172,9 @@ enum cpp_ttype
 
 /* C language kind, used when calling cpp_create_reader.  */
 enum c_lang {CLK_GNUC89 = 0, CLK_GNUC99, CLK_GNUC11, CLK_GNUC17, CLK_GNUC23,
+	     CLK_GNUC2Y,
 	     CLK_STDC89, CLK_STDC94, CLK_STDC99, CLK_STDC11, CLK_STDC17,
-	     CLK_STDC23,
+	     CLK_STDC23, CLK_STDC2Y,
 	     CLK_GNUCXX, CLK_CXX98, CLK_GNUCXX11, CLK_CXX11,
 	     CLK_GNUCXX14, CLK_CXX14, CLK_GNUCXX17, CLK_CXX17,
 	     CLK_GNUCXX20, CLK_CXX20, CLK_GNUCXX23, CLK_CXX23,
diff --git a/libcpp/init.cc b/libcpp/init.cc
index c457fa6..9ae06a9 100644
--- a/libcpp/init.cc
+++ b/libcpp/init.cc
@@ -109,12 +109,14 @@ static const struct lang_flags lang_defaults[] =
   /* GNUC11   */  { 1,  0,  1,  1,  1,  0,    0,  1,   1,   1,   0,    0,     0,     0,   0,      1,   1,     0,   0,   0,      0,      0,    0 },
   /* GNUC17   */  { 1,  0,  1,  1,  1,  0,    0,  1,   1,   1,   0,    0,     0,     0,   0,      1,   1,     0,   0,   0,      0,      0,    0 },
   /* GNUC23   */  { 1,  0,  1,  1,  1,  1,    0,  1,   1,   1,   0,    1,     1,     0,   1,      1,   1,     1,   0,   1,      1,      0,    1 },
+  /* GNUC2Y   */  { 1,  0,  1,  1,  1,  1,    0,  1,   1,   1,   0,    1,     1,     0,   1,      1,   1,     1,   0,   1,      1,      0,    1 },
   /* STDC89   */  { 0,  0,  0,  0,  0,  0,    1,  0,   0,   0,   0,    0,     0,     1,   0,      0,   0,     0,   0,   0,      0,      0,    0 },
   /* STDC94   */  { 0,  0,  0,  0,  0,  0,    1,  1,   0,   0,   0,    0,     0,     1,   0,      0,   0,     0,   0,   0,      0,      0,    0 },
   /* STDC99   */  { 1,  0,  1,  1,  0,  0,    1,  1,   0,   0,   0,    0,     0,     1,   0,      0,   0,     0,   0,   0,      0,      0,    0 },
   /* STDC11   */  { 1,  0,  1,  1,  1,  0,    1,  1,   1,   0,   0,    0,     0,     1,   0,      0,   0,     0,   0,   0,      0,      0,    0 },
   /* STDC17   */  { 1,  0,  1,  1,  1,  0,    1,  1,   1,   0,   0,    0,     0,     1,   0,      0,   0,     0,   0,   0,      0,      0,    0 },
   /* STDC23   */  { 1,  0,  1,  1,  1,  1,    1,  1,   1,   0,   0,    1,     1,     0,   1,      1,   1,     1,   0,   1,      1,      0,    1 },
+  /* STDC2Y   */  { 1,  0,  1,  1,  1,  1,    1,  1,   1,   0,   0,    1,     1,     0,   1,      1,   1,     1,   0,   1,      1,      0,    1 },
   /* GNUCXX   */  { 0,  1,  1,  1,  0,  1,    0,  1,   0,   0,   0,    0,     0,     0,   0,      1,   1,     0,   0,   0,      0,      0,    1 },
   /* CXX98    */  { 0,  1,  0,  1,  0,  1,    1,  1,   0,   0,   0,    0,     0,     1,   0,      0,   1,     0,   0,   0,      0,      0,    1 },
   /* GNUCXX11 */  { 1,  1,  1,  1,  1,  1,    0,  1,   1,   1,   1,    0,     0,     0,   0,      1,   1,     0,   0,   0,      0,      0,    1 },
@@ -595,6 +597,9 @@ cpp_init_builtins (cpp_reader *pfile, int hosted)
   else if (CPP_OPTION (pfile, lang) == CLK_STDC23
 	   || CPP_OPTION (pfile, lang) == CLK_GNUC23)
     _cpp_define_builtin (pfile, "__STDC_VERSION__ 202311L");
+  else if (CPP_OPTION (pfile, lang) == CLK_STDC2Y
+	   || CPP_OPTION (pfile, lang) == CLK_GNUC2Y)
+    _cpp_define_builtin (pfile, "__STDC_VERSION__ 202500L");
   else if (CPP_OPTION (pfile, lang) == CLK_STDC17
 	   || CPP_OPTION (pfile, lang) == CLK_GNUC17)
     _cpp_define_builtin (pfile, "__STDC_VERSION__ 201710L");