c++: Add support for -std=c++23

Derived from the changes that added C++2a support in 2017.
r8-3237-g026a79f70cf33f836ea5275eda72d4870a3041e5

No C++23 features are added here.
Use of -std=c++23 sets __cplusplus to 202100L.

$ g++ -std=c++23 -dM -E -x c++ - < /dev/null | grep cplusplus
 #define __cplusplus 202100L

gcc/
	* doc/cpp.texi (__cplusplus): Document value for -std=c++23
	or -std=gnu++23.
	* doc/invoke.texi: Document -std=c++23 and -std=gnu++23.
	* dwarf2out.c (highest_c_language): Recognise C++20 and C++23.
	(gen_compile_unit_die): Recognise C++23.

gcc/c-family/
	* c-common.h (cxx_dialect): Add cxx23 as a dialect.
	* c.opt: Add options for -std=c++23, std=c++2b, -std=gnu++23
	and -std=gnu++2b
	* c-opts.c (set_std_cxx23): New.
	(c_common_handle_option): Set options when -std=c++23 is enabled.
	(c_common_post_options): Adjust comments.
	(set_std_cxx20): Likewise.

gcc/testsuite/
	* lib/target-supports.exp (check_effective_target_c++2a):
	Check for C++2a or C++23.
	(check_effective_target_c++20_down): New.
	(check_effective_target_c++23_only): New.
	(check_effective_target_c++23): New.
	* g++.dg/cpp23/cplusplus.C: New.

libcpp/
	* include/cpplib.h (c_lang): Add CXX23 and GNUCXX23.
	* init.c (lang_defaults): Add rows for CXX23 and GNUCXX23.
	(cpp_init_builtins): Set __cplusplus to 202100L for C++23.

8 files changed, 100 insertions, 9 deletions

diff --git a/gcc/c-family/c-common.h b/gcc/c-family/c-common.h
index a65c78f..f30b6c6 100644
--- a/gcc/c-family/c-common.h
+++ b/gcc/c-family/c-common.h
@@ -738,7 +738,9 @@ enum cxx_dialect {
   /* C++17 */
   cxx17,
   /* C++20 */
-  cxx20
+  cxx20,
+  /* C++23 */
+  cxx23
 };
 
 /* The C++ dialect being used. C++98 is the default.  */
diff --git a/gcc/c-family/c-opts.c b/gcc/c-family/c-opts.c
index 3cdf41b..bd15b9c 100644
--- a/gcc/c-family/c-opts.c
+++ b/gcc/c-family/c-opts.c
@@ -113,6 +113,7 @@ static void set_std_cxx11 (int);
 static void set_std_cxx14 (int);
 static void set_std_cxx17 (int);
 static void set_std_cxx20 (int);
+static void set_std_cxx23 (int);
 static void set_std_c89 (int, int);
 static void set_std_c99 (int);
 static void set_std_c11 (int);
@@ -649,6 +650,12 @@ c_common_handle_option (size_t scode, const char *arg, HOST_WIDE_INT value,
 	set_std_cxx20 (code == OPT_std_c__20 /* ISO */);
       break;
 
+    case OPT_std_c__23:
+    case OPT_std_gnu__23:
+      if (!preprocessing_asm_p)
+	set_std_cxx23 (code == OPT_std_c__23 /* ISO */);
+      break;
+
     case OPT_std_c90:
     case OPT_std_iso9899_199409:
       if (!preprocessing_asm_p)
@@ -1019,7 +1026,7 @@ c_common_post_options (const char **pfilename)
 	warn_narrowing = 1;
 
       /* Unless -f{,no-}ext-numeric-literals has been used explicitly,
-	 for -std=c++{11,14,17,2a} default to -fno-ext-numeric-literals.  */
+	 for -std=c++{11,14,17,20,23} default to -fno-ext-numeric-literals.  */
       if (flag_iso && !global_options_set.x_flag_ext_numeric_literals)
 	cpp_opts->ext_numeric_literals = 0;
     }
@@ -1763,7 +1770,7 @@ set_std_cxx20 (int iso)
   flag_no_gnu_keywords = iso;
   flag_no_nonansi_builtin = iso;
   flag_iso = iso;
-  /* C++17 includes the C11 standard library.  */
+  /* C++20 includes the C11 standard library.  */
   flag_isoc94 = 1;
   flag_isoc99 = 1;
   flag_isoc11 = 1;
@@ -1773,6 +1780,24 @@ set_std_cxx20 (int iso)
   lang_hooks.name = "GNU C++20";
 }
 
+/* Set the C++ 2023 standard (without GNU extensions if ISO).  */
+static void
+set_std_cxx23 (int iso)
+{
+  cpp_set_lang (parse_in, iso ? CLK_CXX23: CLK_GNUCXX23);
+  flag_no_gnu_keywords = iso;
+  flag_no_nonansi_builtin = iso;
+  flag_iso = iso;
+  /* C++23 includes the C11 standard library.  */
+  flag_isoc94 = 1;
+  flag_isoc99 = 1;
+  flag_isoc11 = 1;
+  /* C++23 includes coroutines.  */
+  flag_coroutines = true;
+  cxx_dialect = cxx23;
+  lang_hooks.name = "GNU C++23";
+}
+
 /* Args to -d specify what to dump.  Silently ignore
    unrecognized options; they may be aimed at toplev.c.  */
 static void
diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index 1766364..b729be7f 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -2214,6 +2214,14 @@ std=c++20
 C++ ObjC++
 Conform to the ISO 2020 C++ draft standard (experimental and incomplete support).
 
+std=c++2b
+C++ ObjC++ Alias(std=c++23)
+Conform to the ISO 2020 C++ draft standard (experimental and incomplete support).
+
+std=c++23
+C++ ObjC++ Undocumented
+Conform to the ISO 2023 C++ draft standard (experimental and incomplete support).
+
 std=c11
 C ObjC
 Conform to the ISO 2011 C standard.
@@ -2285,13 +2293,21 @@ C++ ObjC++
 Conform to the ISO 2017 C++ standard with GNU extensions.
 
 std=gnu++2a
-C++ ObjC++ Alias(std=gnu++20)
+C++ ObjC++ Alias(std=gnu++20) Undocumented
 Conform to the ISO 2020 C++ draft standard with GNU extensions (experimental and incomplete support).
 
 std=gnu++20
 C++ ObjC++
 Conform to the ISO 2020 C++ draft standard with GNU extensions (experimental and incomplete support).
 
+std=gnu++2b
+C++ ObjC++ Alias(std=gnu++23)
+Conform to the ISO 2023 C++ draft standard with GNU extensions (experimental and incomplete support).
+
+std=gnu++23
+C++ ObjC++ Undocumented
+Conform to the ISO 2023 C++ draft standard with GNU extensions (experimental and incomplete support).
+
 std=gnu11
 C ObjC
 Conform to the ISO 2011 C standard with GNU extensions.
diff --git a/gcc/doc/cpp.texi b/gcc/doc/cpp.texi
index 25f2625..2c109bb 100644
--- a/gcc/doc/cpp.texi
+++ b/gcc/doc/cpp.texi
@@ -1907,9 +1907,10 @@ selected, the value of the macro is
 @code{201103L} for the 2011 C++ standard,
 @code{201402L} for the 2014 C++ standard,
 @code{201703L} for the 2017 C++ standard,
-or an unspecified value strictly larger than @code{201703L} for the
-experimental languages enabled by @option{-std=c++2a} and
-@option{-std=gnu++2a}.
+@code{202002L} for the 2020 C++ standard,
+or an unspecified value strictly larger than @code{202002L} for the
+experimental languages enabled by @option{-std=c++23} and
+@option{-std=gnu++23}.
 
 @item __OBJC__
 This macro is defined, with value 1, when the Objective-C compiler is in
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 34218a7..d9e49c4 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -2430,6 +2430,18 @@ GNU dialect of @option{-std=c++20}.
 Support is experimental, and could change in incompatible ways in
 future releases.
 The name @samp{gnu++2a} is deprecated.
+
+@item c++2b
+@itemx c++23
+The next revision of the ISO C++ standard, planned for
+2023.  Support is highly experimental, and will almost certainly
+change in incompatible ways in future releases.
+
+@item gnu++2b
+@itemx gnu++23
+GNU dialect of @option{-std=c++2b}.  Support is highly experimental,
+and will almost certainly change in incompatible ways in future
+releases.
 @end table
 
 @item -fgnu89-inline
diff --git a/gcc/dwarf2out.c b/gcc/dwarf2out.c
index 2ec3449..0a61d14 100644
--- a/gcc/dwarf2out.c
+++ b/gcc/dwarf2out.c
@@ -24492,6 +24492,10 @@ static char *producer_string;
 static const char *
 highest_c_language (const char *lang1, const char *lang2)
 {
+  if (strcmp ("GNU C++23", lang1) == 0 || strcmp ("GNU C++23", lang2) == 0)
+    return "GNU C++23";
+  if (strcmp ("GNU C++20", lang1) == 0 || strcmp ("GNU C++20", lang2) == 0)
+    return "GNU C++20";
   if (strcmp ("GNU C++17", lang1) == 0 || strcmp ("GNU C++17", lang2) == 0)
     return "GNU C++17";
   if (strcmp ("GNU C++14", lang1) == 0 || strcmp ("GNU C++14", lang2) == 0)
@@ -24597,7 +24601,8 @@ gen_compile_unit_die (const char *filename)
 	  else if (strcmp (language_string, "GNU C++14") == 0)
 	    language = DW_LANG_C_plus_plus_14;
 	  else if (strcmp (language_string, "GNU C++17") == 0
-		   || strcmp (language_string, "GNU C++20") == 0)
+		   || strcmp (language_string, "GNU C++20") == 0
+		   || strcmp (language_string, "GNU C++23") == 0)
 	    /* For now.  */
 	    language = DW_LANG_C_plus_plus_14;
 	}
diff --git a/gcc/testsuite/g++.dg/cpp23/cplusplus.C b/gcc/testsuite/g++.dg/cpp23/cplusplus.C
new file mode 100644
index 0000000..29a941b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/cplusplus.C
@@ -0,0 +1,4 @@
+// { dg-do compile }
+// { dg-options "-std=c++23" }
+
+static_assert(__cplusplus > 202002L);
diff --git a/gcc/testsuite/lib/target-supports.exp b/gcc/testsuite/lib/target-supports.exp
index 0d351c8..af46c77 100644
--- a/gcc/testsuite/lib/target-supports.exp
+++ b/gcc/testsuite/lib/target-supports.exp
@@ -9503,7 +9503,10 @@ proc check_effective_target_c++2a_only { } {
     return 0
 }
 proc check_effective_target_c++2a { } {
-    return [check_effective_target_c++2a_only]
+    if [check_effective_target_c++2a_only] {
+	return 1
+    }
+    return [check_effective_target_c++23]
 }
 
 proc check_effective_target_c++20_only { } {
@@ -9513,6 +9516,29 @@ proc check_effective_target_c++20_only { } {
 proc check_effective_target_c++20 { } {
     return [check_effective_target_c++2a]
 }
+proc check_effective_target_c++20_down { } {
+    if ![check_effective_target_c++] {
+	return 0
+    }
+    return [expr ![check_effective_target_c++23] ]
+}
+
+proc check_effective_target_c++23_only { } {
+    global cxx_default
+    if ![check_effective_target_c++] {
+	return 0
+    }
+    if [check-flags { { } { } { -std=c++23 -std=gnu++23 -std=c++2b -std=gnu++2b } }] {
+	return 1
+    }
+    if { $cxx_default == "c++23" && [check-flags { { } { } { } { -std=* } }] } {
+	return 1
+    }
+    return 0
+}
+proc check_effective_target_c++23 { } {
+    return [check_effective_target_c++23_only]
+}
 
 # Check for C++ Concepts support, i.e. -fconcepts flag.
 proc check_effective_target_concepts { } {