c++: Add support for -std={c,gnu}++2{c,6}

It seems prudent to add C++26 now that the first C++26 papers have been
approved.  I followed commit r11-6920 as well as r8-3237.

Since C++23 is essentially finished and its __cplusplus value has
settled to 202302L, I've updated cpp_init_builtins and marked
-std=c++2b Undocumented and made -std=c++23 no longer Undocumented.

As for __cplusplus, I've chosen 202400L:

  $ xg++ -std=c++26 -dM -E -x c++ - < /dev/null | grep cplusplus
  #define __cplusplus 202400L

I've verified the patch with a simple test, exercising the new
directives.  Don't forget to update your GXX_TESTSUITE_STDS!

This patch does not add -Wc++26-extensions.

gcc/c-family/ChangeLog:

	* c-common.h (cxx_dialect): Add cxx26 as a dialect.
	* c-opts.cc (set_std_cxx26): New.
	(c_common_handle_option): Set options when -std={c,gnu}++2{c,6} is
	enabled.
	(c_common_post_options): Adjust comments.
	* c.opt: Add options for -std=c++26, std=c++2c, -std=gnu++26,
	and -std=gnu++2c.
	(std=c++2b): Mark as Undocumented.
	(std=c++23): No longer Undocumented.

gcc/ChangeLog:

	* doc/cpp.texi (__cplusplus): Document value for -std=c++26 and
	-std=gnu++26.  Document that for C++23, its value is 202302L.
	* doc/invoke.texi: Document -std=c++26 and -std=gnu++26.
	* dwarf2out.cc (highest_c_language): Handle GNU C++26.
	(gen_compile_unit_die): Likewise.

libcpp/ChangeLog:

	* include/cpplib.h (c_lang): Add CXX26 and GNUCXX26.
	* init.cc (lang_defaults): Add rows for CXX26 and GNUCXX26.
	(cpp_init_builtins): Set __cplusplus to 202400L for C++26.
	Set __cplusplus to 202302L for C++23.

gcc/testsuite/ChangeLog:

	* lib/target-supports.exp (check_effective_target_c++23): Return
	1 also if check_effective_target_c++26.
	(check_effective_target_c++23_down): New.
	(check_effective_target_c++26_only): New.
	(check_effective_target_c++26): New.
	* g++.dg/cpp23/cplusplus.C: Adjust expected value.
	* g++.dg/cpp26/cplusplus.C: New test.

1 files changed, 9 insertions, 4 deletions

diff --git a/libcpp/init.cc b/libcpp/init.cc
index c508f06..693feaa 100644
--- a/libcpp/init.cc
+++ b/libcpp/init.cc
@@ -127,6 +127,8 @@ static const struct lang_flags lang_defaults[] =
   /* CXX20    */  { 1,  1,  1,  1,  1,  1,    1,  1,   1,   1,   1,    1,     1,     0,   1,      1,   1,     0,   0,   0,      0,      0,    1 },
   /* GNUCXX23 */  { 1,  1,  1,  1,  1,  1,    0,  1,   1,   1,   1,    1,     1,     0,   1,      1,   1,     0,   1,   1,      1,      1,    1 },
   /* CXX23    */  { 1,  1,  1,  1,  1,  1,    1,  1,   1,   1,   1,    1,     1,     0,   1,      1,   1,     0,   1,   1,      1,      1,    1 },
+  /* GNUCXX26 */  { 1,  1,  1,  1,  1,  1,    0,  1,   1,   1,   1,    1,     1,     0,   1,      1,   1,     0,   1,   1,      1,      1,    1 },
+  /* CXX26    */  { 1,  1,  1,  1,  1,  1,    1,  1,   1,   1,   1,    1,     1,     0,   1,      1,   1,     0,   1,   1,      1,      1,    1 },
   /* ASM      */  { 0,  0,  1,  0,  0,  0,    0,  0,   0,   0,   0,    0,     0,     0,   0,      0,   0,     0,   0,   0,      0,      0,    0 }
 };
 
@@ -561,11 +563,14 @@ cpp_init_builtins (cpp_reader *pfile, int hosted)
 
   if (CPP_OPTION (pfile, cplusplus))
     {
-      /* C++23 is not yet a standard.  For now, use an invalid
-       * year/month, 202100L, which is larger than 202002L.  */
-      if (CPP_OPTION (pfile, lang) == CLK_CXX23
+      /* C++26 is not yet a standard.  For now, use an invalid
+	 year/month, 202400L, which is larger than 202302L.  */
+      if (CPP_OPTION (pfile, lang) == CLK_CXX26
+	  || CPP_OPTION (pfile, lang) == CLK_GNUCXX26)
+	_cpp_define_builtin (pfile, "__cplusplus 202400L");
+      else if (CPP_OPTION (pfile, lang) == CLK_CXX23
 	  || CPP_OPTION (pfile, lang) == CLK_GNUCXX23)
-	_cpp_define_builtin (pfile, "__cplusplus 202100L");
+	_cpp_define_builtin (pfile, "__cplusplus 202302L");
       else if (CPP_OPTION (pfile, lang) == CLK_CXX20
 	  || CPP_OPTION (pfile, lang) == CLK_GNUCXX20)
 	_cpp_define_builtin (pfile, "__cplusplus 202002L");