[v2] c++: Add support for -std=c++2b

[PATCH v2] c++: Add support for -std=c++2b

Derived from the changes that added C++2a support in 2017.
https://gcc.gnu.org/g:026a79f70cf33f836ea5275eda72d4870a3041e5

No C++2b features are added here.
Use of -std=c++2b sets __cplusplus to 202100L.

$ g++ -std=c++2b -dM -E -x c++ - < /dev/null | grep cplusplus
#define __cplusplus 202100L

Changes since v1 (8th Jan 2021):
* As suggested by Jonathan Wakely:
  __cplusplus set to 202100L rather than 202101L.  Use of a non-existent date
  helps indicate this is not a true standard, yet is a value greater
than 202002L.
* As suggested by Jakub Jelinek:
  Fixed typos and formatting.
  Added C++23 support to dwarf2out.c, including missing C++20 support
in highest_c_language.
* Regarding suggestion by Marek Polacek to refer to C++23 rather than C++2b.
  Left the option as -std=c++2b for now.  It may be premature to assume the next
  version of the standard will be named C++23.  Use of c++2b also reinforces
  the experimental nature of GCC's C++23 implementation.


gcc/

    Add support for -std=c++2b
    * doc/cpp.texi (__cplusplus): Document value for -std=c++2b
    or -std=gnu++2b.
    * doc/invoke.texi: Document -std=c++2b and -std=gnu++2b.

gcc/c-family

    Add support for -std=c++2b
    * c-common.h (cxx_dialect): Add cxx2b as a dialect.
    * c.opt: Add options for -std=c++2b and -std=gnu++2b.
    * c-opts.c (set_std_cxx2b): New.
    (c_common_handle_option): Set options when -std=c++2b is enabled.
    (c_common_post_options): Adjust comments.
    (set_std_cxx20): Likewise.
    * dwarf2out.c (highest_c_language): Recognise C++20 and C++23.
    (gen_compile_unit_die): Recognise C++23.

gcc/testsuite

    Add support for -std=c++2b
    * lib/target-supports.exp (check_effective_target_c++2a_only):
    rename to check_effective_target_c++20_only.
    (check_effective_target_c++2a): rename to check_effective_target_c++20.
    (check_effective_target_c++20): Return 1
    if check_effective_target_c++20_only or
    if check_effective_target_c++2b.
    (check_effective_target_c++20_down): New.
    (check_effective_target_c++2a_only): New.
    (check_effective_target_c++2a): New.
    * g++.dg/cpp2b/cplusplus.C: New.

libcpp
    Add support for -std=c++2b
    * include/cpplib.h (c_lang): Add CXX2B and GNUCXX2B.
    * init.c (lang_defaults): Add rows for CXX2B and GNUCXX2B.
    (cpp_init_builtins): Set __cplusplus to 202100L for C++2b.
---
 gcc/c-family/c-common.h                |    4 ++-
 gcc/c-family/c-opts.c                  |   29 ++++++++++++++++++++++--
 gcc/c-family/c.opt                     |    8 ++++++
 gcc/doc/cpp.texi                       |    7 +++--
 gcc/doc/invoke.texi                    |   10 ++++++++
 gcc/dwarf2out.c                        |    7 +++++
 gcc/testsuite/g++.dg/cpp2b/cplusplus.C |    4 +++
 gcc/testsuite/lib/target-supports.exp  |   39 +++++++++++++++++++++++----------
 libcpp/include/cpplib.h                |    3 +-
 libcpp/init.c                          |    7 +++++
 10 files changed, 98 insertions(+), 20 deletions(-)

+  /* CXX2B    */  { 1,  1,  1,  1,  1,  1,  1,   1,   1,   1,    1,
  1,     0,   1,      1,   1,     0 },
   /* ASM      */  { 0,  0,  1,  0,  0,  0,  0,   0,   0,   0,    0,
  0,     0,   0,      0,   0,     0 }
 };

@@ -540,7 +542,10 @@ cpp_init_builtins (cpp_reader *pfile, int hosted)

   if (CPP_OPTION (pfile, cplusplus))
     {
-      if (CPP_OPTION (pfile, lang) == CLK_CXX20
+      if (CPP_OPTION (pfile, lang) == CLK_CXX2B
+      || CPP_OPTION (pfile, lang) == CLK_GNUCXX2B)
+    _cpp_define_builtin (pfile, "__cplusplus 202100L");
+      else if (CPP_OPTION (pfile, lang) == CLK_CXX20
       || CPP_OPTION (pfile, lang) == CLK_GNUCXX20)
     _cpp_define_builtin (pfile, "__cplusplus 202002L");
       else if (CPP_OPTION (pfile, lang) == CLK_CXX17

On 1/10/21 7:28 PM, Paul Fee via Gcc-patches wrote:
> [PATCH v2] c++: Add support for -std=c++2b

Thanks!

This patch was corrupted by word wrap, so it won't apply; if you can't 
suppress word wrap in your mail client, please send the patch as an 
attachment instead.

Also remember to use git gcc-verify before sending the patch.

> Derived from the changes that added C++2a support in 2017.
> https://gcc.gnu.org/g:026a79f70cf33f836ea5275eda72d4870a3041e5
> 
> No C++2b features are added here.
> Use of -std=c++2b sets __cplusplus to 202100L.
> 
> $ g++ -std=c++2b -dM -E -x c++ - < /dev/null | grep cplusplus
> #define __cplusplus 202100L
> 
> Changes since v1 (8th Jan 2021):
> * As suggested by Jonathan Wakely:
>    __cplusplus set to 202100L rather than 202101L.  Use of a non-existent date
>    helps indicate this is not a true standard, yet is a value greater
> than 202002L.
> * As suggested by Jakub Jelinek:
>    Fixed typos and formatting.
>    Added C++23 support to dwarf2out.c, including missing C++20 support
> in highest_c_language.

> * Regarding suggestion by Marek Polacek to refer to C++23 rather than C++2b.
>    Left the option as -std=c++2b for now.  It may be premature to assume the next
>    version of the standard will be named C++23.  Use of c++2b also reinforces
>    the experimental nature of GCC's C++23 implementation.

Hmm, I don't think it's that premature; the C++ committee has been very 
serious about time-based releases every three years.  I think it makes 
sense for the advertised flag to be c++2b, but let's also go ahead and 
add the c++23 flags as hidden, and use cxx23 internally.

> gcc/
> 
>      Add support for -std=c++2b
>      * doc/cpp.texi (__cplusplus): Document value for -std=c++2b
>      or -std=gnu++2b.
>      * doc/invoke.texi: Document -std=c++2b and -std=gnu++2b.
> 
> gcc/c-family
> 
>      Add support for -std=c++2b
>      * c-common.h (cxx_dialect): Add cxx2b as a dialect.
>      * c.opt: Add options for -std=c++2b and -std=gnu++2b.
>      * c-opts.c (set_std_cxx2b): New.
>      (c_common_handle_option): Set options when -std=c++2b is enabled.
>      (c_common_post_options): Adjust comments.
>      (set_std_cxx20): Likewise.
>      * dwarf2out.c (highest_c_language): Recognise C++20 and C++23.
>      (gen_compile_unit_die): Recognise C++23.

dwarf2out.c isn't in c-family.

> gcc/testsuite
> 
>      Add support for -std=c++2b
>      * lib/target-supports.exp (check_effective_target_c++2a_only):
>      rename to check_effective_target_c++20_only.
>      (check_effective_target_c++2a): rename to check_effective_target_c++20.
>      (check_effective_target_c++20): Return 1
>      if check_effective_target_c++20_only or
>      if check_effective_target_c++2b.
>      (check_effective_target_c++20_down): New.
>      (check_effective_target_c++2a_only): New.
>      (check_effective_target_c++2a): New.
>      * g++.dg/cpp2b/cplusplus.C: New.
> 
> libcpp
>      Add support for -std=c++2b
>      * include/cpplib.h (c_lang): Add CXX2B and GNUCXX2B.
>      * init.c (lang_defaults): Add rows for CXX2B and GNUCXX2B.
>      (cpp_init_builtins): Set __cplusplus to 202100L for C++2b.
> ---
>   gcc/c-family/c-common.h                |    4 ++-
>   gcc/c-family/c-opts.c                  |   29 ++++++++++++++++++++++--
>   gcc/c-family/c.opt                     |    8 ++++++
>   gcc/doc/cpp.texi                       |    7 +++--
>   gcc/doc/invoke.texi                    |   10 ++++++++
>   gcc/dwarf2out.c                        |    7 +++++
>   gcc/testsuite/g++.dg/cpp2b/cplusplus.C |    4 +++
>   gcc/testsuite/lib/target-supports.exp  |   39 +++++++++++++++++++++++----------
>   libcpp/include/cpplib.h                |    3 +-
>   libcpp/init.c                          |    7 +++++
>   10 files changed, 98 insertions(+), 20 deletions(-)
> 
> diff --git a/gcc/c-family/c-common.h b/gcc/c-family/c-common.h
> index a65c78f7240..f562cdebf4c 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++2b (C++23?) */
> +  cxx2b
>   };
> 
>   /* 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 3cdf41bc6e2..15f120d475d 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_cxx2b (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__2b:
> +    case OPT_std_gnu__2b:
> +      if (!preprocessing_asm_p)
> +    set_std_cxx2b (code == OPT_std_c__2b /* 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++{11,14,17,20,2b} 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++ 202b standard (without GNU extensions if ISO).  */
> +static void
> +set_std_cxx2b (int iso)
> +{
> +  cpp_set_lang (parse_in, iso ? CLK_CXX2B: CLK_GNUCXX2B);
> +  flag_no_gnu_keywords = iso;
> +  flag_no_nonansi_builtin = iso;
> +  flag_iso = iso;
> +  /* C++2b includes the C11 standard library.  */
> +  flag_isoc94 = 1;
> +  flag_isoc99 = 1;
> +  flag_isoc11 = 1;
> +  /* C++2b includes coroutines.  */
> +  flag_coroutines = true;
> +  cxx_dialect = cxx2b;
> +  lang_hooks.name = "GNU C++20"; /* Pretend C++20 until standardization.  */
> +}
> +
>   /* 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 1766364806e..273772a0cd5 100644
> --- a/gcc/c-family/c.opt
> +++ b/gcc/c-family/c.opt
> @@ -2214,6 +2214,10 @@ std=c++20
>   C++ ObjC++
>   Conform to the ISO 2020 C++ draft standard (experimental and
> incomplete support).
> 
> +std=c++2b
> +C++ ObjC++
> +Conform to the ISO 2023(?) C++ draft standard (experimental and
> incomplete support).
> +
>   std=c11
>   C ObjC
>   Conform to the ISO 2011 C standard.
> @@ -2292,6 +2296,10 @@ 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++
> +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 25f2625d8bd..f801024affd 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++2b} and
> +@option{-std=gnu++2b}.
> 
>   @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 226b8ade430..11a6e9a23c3 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -2430,6 +2430,16 @@ 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
> +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
> +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 8b6890a5097..97df6604373 100644
> --- a/gcc/dwarf2out.c
> +++ b/gcc/dwarf2out.c
> @@ -24445,6 +24445,10 @@ 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)
> @@ -24550,7 +24554,8 @@ 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/cpp2b/cplusplus.C
> b/gcc/testsuite/g++.dg/cpp2b/cplusplus.C
> new file mode 100644
> index 00000000000..615d9f081de
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2b/cplusplus.C
> @@ -0,0 +1,4 @@
> +// { dg-do compile }
> +// { dg-options "-std=c++2b" }
> +
> +static_assert(__cplusplus > 202002L);
> diff --git a/gcc/testsuite/lib/target-supports.exp
> b/gcc/testsuite/lib/target-supports.exp
> index 5cf0f4218a6..31be4de12a5 100644
> --- a/gcc/testsuite/lib/target-supports.exp
> +++ b/gcc/testsuite/lib/target-supports.exp
> @@ -9434,7 +9434,6 @@ proc check_effective_target_c++14_only { } {
>       }
>       return 0
>   }
> -
>   proc check_effective_target_c++14 { } {
>       if [check_effective_target_c++14_only] {
>       return 1
> @@ -9475,7 +9474,6 @@ proc check_effective_target_c++17_only { } {
>       }
>       return 0
>   }
> -
>   proc check_effective_target_c++17 { } {
>       if [check_effective_target_c++17_only] {
>       return 1
> @@ -9489,12 +9487,12 @@ proc check_effective_target_c++17_down { } {
>       return [expr ![check_effective_target_c++2a] ]
>   }
> 
> -proc check_effective_target_c++2a_only { } {
> +proc check_effective_target_c++20_only { } {
>       global cxx_default
>       if ![check_effective_target_c++] {
>       return 0
>       }
> -    if [check-flags { { } { } { -std=c++2a -std=gnu++2a -std=c++20
> -std=gnu++20 } }] {
> +    if [check-flags { { } { } { -std=c++20 -std=gnu++20 -std=c++2a
> -std=gnu++2a } }] {
>       return 1
>       }
>       if { $cxx_default == "c++20" && [check-flags { { } { } { } {
> -std=* } }] } {
> @@ -9502,18 +9500,37 @@ proc check_effective_target_c++2a_only { } {
>       }
>       return 0
>   }
> -proc check_effective_target_c++2a { } {
> -    return [check_effective_target_c++2a_only]
> +proc check_effective_target_c++20 { } {
> +    if [check_effective_target_c++20_only] {
> +    return 1
> +    }
> +    return [check_effective_target_c++2b]
>   }
> -
> -proc check_effective_target_c++20_only { } {
> -    return [check_effective_target_c++2a_only]
> +proc check_effective_target_c++20_down { } {
> +    if ![check_effective_target_c++] {
> +    return 0
> +    }
> +    return [expr ![check_effective_target_c++20] ]

This should be !c++2b.

>   }
> 
> -proc check_effective_target_c++20 { } {
> -    return [check_effective_target_c++2a]
> +proc check_effective_target_c++2b_only { } {
> +    global cxx_default
> +    if ![check_effective_target_c++] {
> +    return 0
> +    }
> +    if [check-flags { { } { } { -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++2b { } {
> +    return [check_effective_target_c++2b_only]
>   }
> 
> +
>   # Check for C++ Concepts support, i.e. -fconcepts flag.
>   proc check_effective_target_concepts { } {
>       if [check_effective_target_c++2a] {
> diff --git a/libcpp/include/cpplib.h b/libcpp/include/cpplib.h
> index bcc50ba45f6..c213fbac0b1 100644
> --- a/libcpp/include/cpplib.h
> +++ b/libcpp/include/cpplib.h
> @@ -173,7 +173,8 @@ enum c_lang {CLK_GNUC89 = 0, CLK_GNUC99,
> CLK_GNUC11, CLK_GNUC17, CLK_GNUC2X,
>            CLK_STDC2X,
>            CLK_GNUCXX, CLK_CXX98, CLK_GNUCXX11, CLK_CXX11,
>            CLK_GNUCXX14, CLK_CXX14, CLK_GNUCXX17, CLK_CXX17,
> -         CLK_GNUCXX20, CLK_CXX20, CLK_ASM};
> +         CLK_GNUCXX20, CLK_CXX20, CLK_GNUCXX2B, CLK_CXX2B,
> +         CLK_ASM};
> 
>   /* Payload of a NUMBER, STRING, CHAR or COMMENT token.  */
>   struct GTY(()) cpp_string {
> diff --git a/libcpp/init.c b/libcpp/init.c
> index a5103c4d454..80543f90a51 100644
> --- a/libcpp/init.c
> +++ b/libcpp/init.c
> @@ -119,6 +119,8 @@ static const struct lang_flags lang_defaults[] =
>     /* CXX17    */  { 1,  1,  1,  1,  1,  1,  1,   1,   1,   1,    1,
>    1,     0,   1,      0,   1,     0 },
>     /* GNUCXX20 */  { 1,  1,  1,  1,  1,  0,  1,   1,   1,   1,    1,
>    1,     0,   1,      1,   1,     0 },
>     /* CXX20    */  { 1,  1,  1,  1,  1,  1,  1,   1,   1,   1,    1,
>    1,     0,   1,      1,   1,     0 },
> +  /* GNUCXX2B */  { 1,  1,  1,  1,  1,  0,  1,   1,   1,   1,    1,
>    1,     0,   1,      1,   1,     0 },
> +  /* CXX2B    */  { 1,  1,  1,  1,  1,  1,  1,   1,   1,   1,    1,
>    1,     0,   1,      1,   1,     0 },
>     /* ASM      */  { 0,  0,  1,  0,  0,  0,  0,   0,   0,   0,    0,
>    0,     0,   0,      0,   0,     0 }
>   };
> 
> @@ -540,7 +542,10 @@ cpp_init_builtins (cpp_reader *pfile, int hosted)
> 
>     if (CPP_OPTION (pfile, cplusplus))
>       {
> -      if (CPP_OPTION (pfile, lang) == CLK_CXX20
> +      if (CPP_OPTION (pfile, lang) == CLK_CXX2B
> +      || CPP_OPTION (pfile, lang) == CLK_GNUCXX2B)
> +    _cpp_define_builtin (pfile, "__cplusplus 202100L");

Let's add a comment here.

> +      else if (CPP_OPTION (pfile, lang) == CLK_CXX20
>         || CPP_OPTION (pfile, lang) == CLK_GNUCXX20)
>       _cpp_define_builtin (pfile, "__cplusplus 202002L");
>         else if (CPP_OPTION (pfile, lang) == CLK_CXX17
> 

Jason

On Tue, Jan 19, 2021 at 11:28 PM Jason Merrill <jason@redhat.com> wrote:
>
> On 1/10/21 7:28 PM, Paul Fee via Gcc-patches wrote:
> > [PATCH v2] c++: Add support for -std=c++2b
>
> Thanks!
>
> This patch was corrupted by word wrap, so it won't apply; if you can't
> suppress word wrap in your mail client, please send the patch as an
> attachment instead.
Seems that "Plain text mode" in gmail isn't plain enough.  I'll use an
attachment for patch v3.
>
> Also remember to use git gcc-verify before sending the patch.
Thanks for the tip.  It helps me learn the GCC patch posting process.
>
> > Derived from the changes that added C++2a support in 2017.
> > https://gcc.gnu.org/g:026a79f70cf33f836ea5275eda72d4870a3041e5
> >
> > No C++2b features are added here.
> > Use of -std=c++2b sets __cplusplus to 202100L.
> >
> > $ g++ -std=c++2b -dM -E -x c++ - < /dev/null | grep cplusplus
> > #define __cplusplus 202100L
> >
> > Changes since v1 (8th Jan 2021):
> > * As suggested by Jonathan Wakely:
> >    __cplusplus set to 202100L rather than 202101L.  Use of a non-existent date
> >    helps indicate this is not a true standard, yet is a value greater
> > than 202002L.
> > * As suggested by Jakub Jelinek:
> >    Fixed typos and formatting.
> >    Added C++23 support to dwarf2out.c, including missing C++20 support
> > in highest_c_language.
>
> > * Regarding suggestion by Marek Polacek to refer to C++23 rather than C++2b.
> >    Left the option as -std=c++2b for now.  It may be premature to assume the next
> >    version of the standard will be named C++23.  Use of c++2b also reinforces
> >    the experimental nature of GCC's C++23 implementation.
>
> Hmm, I don't think it's that premature; the C++ committee has been very
> serious about time-based releases every three years.  I think it makes
> sense for the advertised flag to be c++2b, but let's also go ahead and
> add the c++23 flags as hidden, and use cxx23 internally.
Ok, I'll change from only exposing -std=c++2b to the end goal of
-std=c++23.  You pointed out below a flaw in target-supports.exp.
Checking "make check-c++", I found that I'd also broken many tests
while adjusting c++2a to be c++20.  I'll take the 2a->20 change out of
this patch.  It can be in a separate patch.  Seeing the cost of
changing from "2a" to "20", I'll post up patch v3 with support for
-std=c++23 so that a "2b->23" change isn't needed in a few years.
GCC's C++23 will still be experimental, despite the flag be
-std=c++23.
>
> > gcc/
> >
> >      Add support for -std=c++2b
> >      * doc/cpp.texi (__cplusplus): Document value for -std=c++2b
> >      or -std=gnu++2b.
> >      * doc/invoke.texi: Document -std=c++2b and -std=gnu++2b.
> >
> > gcc/c-family
> >
> >      Add support for -std=c++2b
> >      * c-common.h (cxx_dialect): Add cxx2b as a dialect.
> >      * c.opt: Add options for -std=c++2b and -std=gnu++2b.
> >      * c-opts.c (set_std_cxx2b): New.
> >      (c_common_handle_option): Set options when -std=c++2b is enabled.
> >      (c_common_post_options): Adjust comments.
> >      (set_std_cxx20): Likewise.
> >      * dwarf2out.c (highest_c_language): Recognise C++20 and C++23.
> >      (gen_compile_unit_die): Recognise C++23.
>
> dwarf2out.c isn't in c-family.
>
> > gcc/testsuite
> >
> >      Add support for -std=c++2b
> >      * lib/target-supports.exp (check_effective_target_c++2a_only):
> >      rename to check_effective_target_c++20_only.
> >      (check_effective_target_c++2a): rename to check_effective_target_c++20.
> >      (check_effective_target_c++20): Return 1
> >      if check_effective_target_c++20_only or
> >      if check_effective_target_c++2b.
> >      (check_effective_target_c++20_down): New.
> >      (check_effective_target_c++2a_only): New.
> >      (check_effective_target_c++2a): New.
> >      * g++.dg/cpp2b/cplusplus.C: New.
> >
> > libcpp
> >      Add support for -std=c++2b
> >      * include/cpplib.h (c_lang): Add CXX2B and GNUCXX2B.
> >      * init.c (lang_defaults): Add rows for CXX2B and GNUCXX2B.
> >      (cpp_init_builtins): Set __cplusplus to 202100L for C++2b.
> > ---
> >   gcc/c-family/c-common.h                |    4 ++-
> >   gcc/c-family/c-opts.c                  |   29 ++++++++++++++++++++++--
> >   gcc/c-family/c.opt                     |    8 ++++++
> >   gcc/doc/cpp.texi                       |    7 +++--
> >   gcc/doc/invoke.texi                    |   10 ++++++++
> >   gcc/dwarf2out.c                        |    7 +++++
> >   gcc/testsuite/g++.dg/cpp2b/cplusplus.C |    4 +++
> >   gcc/testsuite/lib/target-supports.exp  |   39 +++++++++++++++++++++++----------
> >   libcpp/include/cpplib.h                |    3 +-
> >   libcpp/init.c                          |    7 +++++
> >   10 files changed, 98 insertions(+), 20 deletions(-)
> >
> > diff --git a/gcc/c-family/c-common.h b/gcc/c-family/c-common.h
> > index a65c78f7240..f562cdebf4c 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++2b (C++23?) */
> > +  cxx2b
> >   };
> >
> >   /* 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 3cdf41bc6e2..15f120d475d 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_cxx2b (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__2b:
> > +    case OPT_std_gnu__2b:
> > +      if (!preprocessing_asm_p)
> > +    set_std_cxx2b (code == OPT_std_c__2b /* 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++{11,14,17,20,2b} 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++ 202b standard (without GNU extensions if ISO).  */
> > +static void
> > +set_std_cxx2b (int iso)
> > +{
> > +  cpp_set_lang (parse_in, iso ? CLK_CXX2B: CLK_GNUCXX2B);
> > +  flag_no_gnu_keywords = iso;
> > +  flag_no_nonansi_builtin = iso;
> > +  flag_iso = iso;
> > +  /* C++2b includes the C11 standard library.  */
> > +  flag_isoc94 = 1;
> > +  flag_isoc99 = 1;
> > +  flag_isoc11 = 1;
> > +  /* C++2b includes coroutines.  */
> > +  flag_coroutines = true;
> > +  cxx_dialect = cxx2b;
> > +  lang_hooks.name = "GNU C++20"; /* Pretend C++20 until standardization.  */
> > +}
> > +
> >   /* 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 1766364806e..273772a0cd5 100644
> > --- a/gcc/c-family/c.opt
> > +++ b/gcc/c-family/c.opt
> > @@ -2214,6 +2214,10 @@ std=c++20
> >   C++ ObjC++
> >   Conform to the ISO 2020 C++ draft standard (experimental and
> > incomplete support).
> >
> > +std=c++2b
> > +C++ ObjC++
> > +Conform to the ISO 2023(?) C++ draft standard (experimental and
> > incomplete support).
> > +
> >   std=c11
> >   C ObjC
> >   Conform to the ISO 2011 C standard.
> > @@ -2292,6 +2296,10 @@ 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++
> > +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 25f2625d8bd..f801024affd 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++2b} and
> > +@option{-std=gnu++2b}.
> >
> >   @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 226b8ade430..11a6e9a23c3 100644
> > --- a/gcc/doc/invoke.texi
> > +++ b/gcc/doc/invoke.texi
> > @@ -2430,6 +2430,16 @@ 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
> > +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
> > +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 8b6890a5097..97df6604373 100644
> > --- a/gcc/dwarf2out.c
> > +++ b/gcc/dwarf2out.c
> > @@ -24445,6 +24445,10 @@ 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)
> > @@ -24550,7 +24554,8 @@ 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/cpp2b/cplusplus.C
> > b/gcc/testsuite/g++.dg/cpp2b/cplusplus.C
> > new file mode 100644
> > index 00000000000..615d9f081de
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp2b/cplusplus.C
> > @@ -0,0 +1,4 @@
> > +// { dg-do compile }
> > +// { dg-options "-std=c++2b" }
> > +
> > +static_assert(__cplusplus > 202002L);
> > diff --git a/gcc/testsuite/lib/target-supports.exp
> > b/gcc/testsuite/lib/target-supports.exp
> > index 5cf0f4218a6..31be4de12a5 100644
> > --- a/gcc/testsuite/lib/target-supports.exp
> > +++ b/gcc/testsuite/lib/target-supports.exp
> > @@ -9434,7 +9434,6 @@ proc check_effective_target_c++14_only { } {
> >       }
> >       return 0
> >   }
> > -
> >   proc check_effective_target_c++14 { } {
> >       if [check_effective_target_c++14_only] {
> >       return 1
> > @@ -9475,7 +9474,6 @@ proc check_effective_target_c++17_only { } {
> >       }
> >       return 0
> >   }
> > -
> >   proc check_effective_target_c++17 { } {
> >       if [check_effective_target_c++17_only] {
> >       return 1
> > @@ -9489,12 +9487,12 @@ proc check_effective_target_c++17_down { } {
> >       return [expr ![check_effective_target_c++2a] ]
> >   }
> >
> > -proc check_effective_target_c++2a_only { } {
> > +proc check_effective_target_c++20_only { } {
> >       global cxx_default
> >       if ![check_effective_target_c++] {
> >       return 0
> >       }
> > -    if [check-flags { { } { } { -std=c++2a -std=gnu++2a -std=c++20
> > -std=gnu++20 } }] {
> > +    if [check-flags { { } { } { -std=c++20 -std=gnu++20 -std=c++2a
> > -std=gnu++2a } }] {
> >       return 1
> >       }
> >       if { $cxx_default == "c++20" && [check-flags { { } { } { } {
> > -std=* } }] } {
> > @@ -9502,18 +9500,37 @@ proc check_effective_target_c++2a_only { } {
> >       }
> >       return 0
> >   }
> > -proc check_effective_target_c++2a { } {
> > -    return [check_effective_target_c++2a_only]
> > +proc check_effective_target_c++20 { } {
> > +    if [check_effective_target_c++20_only] {
> > +    return 1
> > +    }
> > +    return [check_effective_target_c++2b]
> >   }
> > -
> > -proc check_effective_target_c++20_only { } {
> > -    return [check_effective_target_c++2a_only]
> > +proc check_effective_target_c++20_down { } {
> > +    if ![check_effective_target_c++] {
> > +    return 0
> > +    }
> > +    return [expr ![check_effective_target_c++20] ]
>
> This should be !c++2b.
Indeed, thanks for highlighting my mistake.
>
> >   }
> >
> > -proc check_effective_target_c++20 { } {
> > -    return [check_effective_target_c++2a]
> > +proc check_effective_target_c++2b_only { } {
> > +    global cxx_default
> > +    if ![check_effective_target_c++] {
> > +    return 0
> > +    }
> > +    if [check-flags { { } { } { -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++2b { } {
> > +    return [check_effective_target_c++2b_only]
> >   }
> >
> > +
> >   # Check for C++ Concepts support, i.e. -fconcepts flag.
> >   proc check_effective_target_concepts { } {
> >       if [check_effective_target_c++2a] {
> > diff --git a/libcpp/include/cpplib.h b/libcpp/include/cpplib.h
> > index bcc50ba45f6..c213fbac0b1 100644
> > --- a/libcpp/include/cpplib.h
> > +++ b/libcpp/include/cpplib.h
> > @@ -173,7 +173,8 @@ enum c_lang {CLK_GNUC89 = 0, CLK_GNUC99,
> > CLK_GNUC11, CLK_GNUC17, CLK_GNUC2X,
> >            CLK_STDC2X,
> >            CLK_GNUCXX, CLK_CXX98, CLK_GNUCXX11, CLK_CXX11,
> >            CLK_GNUCXX14, CLK_CXX14, CLK_GNUCXX17, CLK_CXX17,
> > -         CLK_GNUCXX20, CLK_CXX20, CLK_ASM};
> > +         CLK_GNUCXX20, CLK_CXX20, CLK_GNUCXX2B, CLK_CXX2B,
> > +         CLK_ASM};
> >
> >   /* Payload of a NUMBER, STRING, CHAR or COMMENT token.  */
> >   struct GTY(()) cpp_string {
> > diff --git a/libcpp/init.c b/libcpp/init.c
> > index a5103c4d454..80543f90a51 100644
> > --- a/libcpp/init.c
> > +++ b/libcpp/init.c
> > @@ -119,6 +119,8 @@ static const struct lang_flags lang_defaults[] =
> >     /* CXX17    */  { 1,  1,  1,  1,  1,  1,  1,   1,   1,   1,    1,
> >    1,     0,   1,      0,   1,     0 },
> >     /* GNUCXX20 */  { 1,  1,  1,  1,  1,  0,  1,   1,   1,   1,    1,
> >    1,     0,   1,      1,   1,     0 },
> >     /* CXX20    */  { 1,  1,  1,  1,  1,  1,  1,   1,   1,   1,    1,
> >    1,     0,   1,      1,   1,     0 },
> > +  /* GNUCXX2B */  { 1,  1,  1,  1,  1,  0,  1,   1,   1,   1,    1,
> >    1,     0,   1,      1,   1,     0 },
> > +  /* CXX2B    */  { 1,  1,  1,  1,  1,  1,  1,   1,   1,   1,    1,
> >    1,     0,   1,      1,   1,     0 },
> >     /* ASM      */  { 0,  0,  1,  0,  0,  0,  0,   0,   0,   0,    0,
> >    0,     0,   0,      0,   0,     0 }
> >   };
> >
> > @@ -540,7 +542,10 @@ cpp_init_builtins (cpp_reader *pfile, int hosted)
> >
> >     if (CPP_OPTION (pfile, cplusplus))
> >       {
> > -      if (CPP_OPTION (pfile, lang) == CLK_CXX20
> > +      if (CPP_OPTION (pfile, lang) == CLK_CXX2B
> > +      || CPP_OPTION (pfile, lang) == CLK_GNUCXX2B)
> > +    _cpp_define_builtin (pfile, "__cplusplus 202100L");
>
> Let's add a comment here.
>
> > +      else if (CPP_OPTION (pfile, lang) == CLK_CXX20
> >         || CPP_OPTION (pfile, lang) == CLK_GNUCXX20)
> >       _cpp_define_builtin (pfile, "__cplusplus 202002L");
> >         else if (CPP_OPTION (pfile, lang) == CLK_CXX17
> >
>
> Jason
>