[1/1,v2] c++/106423: Fix pragma suppression of -Wc++20-compat diagnostics.

  Gcc's '#pragma GCC diagnostic' directives are processed in "early mode"
(see handle_pragma_diagnostic_early) for the C++ frontend and, as such,
require that the target diagnostic option be enabled for the preprocessor
(see c_option_is_from_cpp_diagnostics).  This change modifies the
-Wc++20-compat option definition to register it as a preprocessor option
so that its associated diagnostics can be suppressed.  The changes also
implicitly disable the option in C++20 and later modes.  These changes
are consistent with the definition of the -Wc++11-compat option.

This support is motivated by the need to suppress the following diagnostic
otherwise issued in C++17 and earlier modes due to the char8_t typedef
present in the uchar.h header file in glibc 2.36.
  warning: identifier ‘char8_t’ is a keyword in C++20 [-Wc++20-compat]

Tests are added to validate suppression of both -Wc++11-compat and
-Wc++20-compat related diagnostics (fixes were only needed for the C++20
case).

Fixes https://gcc.gnu.org/PR106423.

gcc/c-family/ChangeLog:
	* c-opts.cc (c_common_post_options): Disable -Wc++20-compat diagnostics
	in C++20 and later.
	* c.opt (Wc++20-compat): Enable hooks for the preprocessor.

gcc/testsuite/ChangeLog:
	* g++.dg/cpp0x/keywords2.C: New test.
	* g++.dg/cpp2a/keywords2.C: New test.

libcpp/ChangeLog:
	* include/cpplib.h (cpp_warning_reason): Add CPP_W_CXX20_COMPAT.
	* init.cc (cpp_create_reader): Add cpp_warn_cxx20_compat.
---
 gcc/c-family/c-opts.cc                 |  7 +++++++
 gcc/c-family/c.opt                     |  2 +-
 gcc/cp/parser.cc                       |  5 ++++-
 gcc/testsuite/g++.dg/cpp0x/keywords2.C | 16 ++++++++++++++++
 gcc/testsuite/g++.dg/cpp2a/keywords2.C | 13 +++++++++++++
 libcpp/include/cpplib.h                |  4 ++++
 libcpp/init.cc                         |  1 +
 7 files changed, 46 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/keywords2.C
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/keywords2.C
  

Are there any further concerns with this patch? If not, I extend my 
gratitude to anyone so kind as to commit this for me as I don't have 
commit access.

I just noticed that I neglected to add a ChangeLog entry for the comment 
addition to gcc/cp/parser.cc. Noted inline below. I can re-send the 
patch with that update if desired.

Tom.

On 8/1/22 2:49 PM, Tom Honermann wrote:
> Gcc's '#pragma GCC diagnostic' directives are processed in "early mode"
> (see handle_pragma_diagnostic_early) for the C++ frontend and, as such,
> require that the target diagnostic option be enabled for the preprocessor
> (see c_option_is_from_cpp_diagnostics).  This change modifies the
> -Wc++20-compat option definition to register it as a preprocessor option
> so that its associated diagnostics can be suppressed.  The changes also
> implicitly disable the option in C++20 and later modes.  These changes
> are consistent with the definition of the -Wc++11-compat option.
>
> This support is motivated by the need to suppress the following diagnostic
> otherwise issued in C++17 and earlier modes due to the char8_t typedef
> present in the uchar.h header file in glibc 2.36.
>    warning: identifier ‘char8_t’ is a keyword in C++20 [-Wc++20-compat]
>
> Tests are added to validate suppression of both -Wc++11-compat and
> -Wc++20-compat related diagnostics (fixes were only needed for the C++20
> case).
>
> Fixeshttps://gcc.gnu.org/PR106423.
>
> gcc/c-family/ChangeLog:
> 	* c-opts.cc (c_common_post_options): Disable -Wc++20-compat diagnostics
> 	in C++20 and later.
> 	* c.opt (Wc++20-compat): Enable hooks for the preprocessor.

gcc/cp/ChangeLog:
     * parser.cc (cp_lexer_saving_tokens): Add comment regarding 
diagnostic requirements.

>
> gcc/testsuite/ChangeLog:
> 	* g++.dg/cpp0x/keywords2.C: New test.
> 	* g++.dg/cpp2a/keywords2.C: New test.
>
> libcpp/ChangeLog:
> 	* include/cpplib.h (cpp_warning_reason): Add CPP_W_CXX20_COMPAT.
> 	* init.cc (cpp_create_reader): Add cpp_warn_cxx20_compat.
> ---
>   gcc/c-family/c-opts.cc                 |  7 +++++++
>   gcc/c-family/c.opt                     |  2 +-
>   gcc/cp/parser.cc                       |  5 ++++-
>   gcc/testsuite/g++.dg/cpp0x/keywords2.C | 16 ++++++++++++++++
>   gcc/testsuite/g++.dg/cpp2a/keywords2.C | 13 +++++++++++++
>   libcpp/include/cpplib.h                |  4 ++++
>   libcpp/init.cc                         |  1 +
>   7 files changed, 46 insertions(+), 2 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/keywords2.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/keywords2.C
>
> diff --git a/gcc/c-family/c-opts.cc b/gcc/c-family/c-opts.cc
> index b9f01a65ed7..1ea37ba9742 100644
> --- a/gcc/c-family/c-opts.cc
> +++ b/gcc/c-family/c-opts.cc
> @@ -1046,6 +1046,13 @@ c_common_post_options (const char **pfilename)
>     else if (warn_narrowing == -1)
>       warn_narrowing = 0;
>   
> +  if (cxx_dialect >= cxx20)
> +    {
> +      /* Don't warn about C++20 compatibility changes in C++20 or later.  */
> +      warn_cxx20_compat = 0;
> +      cpp_opts->cpp_warn_cxx20_compat = 0;
> +    }
> +
>     /* C++17 has stricter evaluation order requirements; let's use some of them
>        for earlier C++ as well, so chaining works as expected.  */
>     if (c_dialect_cxx ()
> diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
> index 44e1a60ce24..dfdebd596ef 100644
> --- a/gcc/c-family/c.opt
> +++ b/gcc/c-family/c.opt
> @@ -455,7 +455,7 @@ Wc++2a-compat
>   C++ ObjC++ Warning Alias(Wc++20-compat) Undocumented
>   
>   Wc++20-compat
> -C++ ObjC++ Var(warn_cxx20_compat) Warning LangEnabledBy(C++ ObjC++,Wall)
> +C++ ObjC++ Var(warn_cxx20_compat) Warning LangEnabledBy(C++ ObjC++,Wall) Init(0) CPP(cpp_warn_cxx20_compat) CppReason(CPP_W_CXX20_COMPAT)
>   Warn about C++ constructs whose meaning differs between ISO C++ 2017 and ISO C++ 2020.
>   
>   Wc++11-extensions
> diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
> index 4f67441eeb1..c3584446827 100644
> --- a/gcc/cp/parser.cc
> +++ b/gcc/cp/parser.cc
> @@ -924,7 +924,10 @@ cp_lexer_saving_tokens (const cp_lexer* lexer)
>   /* Store the next token from the preprocessor in *TOKEN.  Return true
>      if we reach EOF.  If LEXER is NULL, assume we are handling an
>      initial #pragma pch_preprocess, and thus want the lexer to return
> -   processed strings.  */
> +   processed strings.
> +
> +   Diagnostics issued from this function must have their controlling option (if
> +   any) in c.opt annotated as a libcpp option via the CppReason property.  */
>   
>   static void
>   cp_lexer_get_preprocessor_token (unsigned flags, cp_token *token)
> diff --git a/gcc/testsuite/g++.dg/cpp0x/keywords2.C b/gcc/testsuite/g++.dg/cpp0x/keywords2.C
> new file mode 100644
> index 00000000000..d67d01e31ed
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/keywords2.C
> @@ -0,0 +1,16 @@
> +// { dg-do compile { target c++98_only } }
> +// { dg-options "-Wc++11-compat" }
> +
> +// Validate suppression of -Wc++11-compat diagnostics.
> +#pragma GCC diagnostic ignored "-Wc++11-compat"
> +int alignof;
> +int alignas;
> +int constexpr;
> +int decltype;
> +int noexcept;
> +int nullptr;
> +int static_assert;
> +int thread_local;
> +int _Alignas;
> +int _Alignof;
> +int _Thread_local;
> diff --git a/gcc/testsuite/g++.dg/cpp2a/keywords2.C b/gcc/testsuite/g++.dg/cpp2a/keywords2.C
> new file mode 100644
> index 00000000000..8714a7b26b7
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/keywords2.C
> @@ -0,0 +1,13 @@
> +// { dg-do compile { target c++17_down } }
> +// { dg-options "-Wc++20-compat" }
> +
> +// Validate suppression of -Wc++20-compat diagnostics.
> +#pragma GCC diagnostic ignored "-Wc++20-compat"
> +int constinit;
> +int consteval;
> +int requires;
> +int concept;
> +int co_await;
> +int co_yield;
> +int co_return;
> +int char8_t;
> diff --git a/libcpp/include/cpplib.h b/libcpp/include/cpplib.h
> index 3eba6f74b57..9d90c18e4f2 100644
> --- a/libcpp/include/cpplib.h
> +++ b/libcpp/include/cpplib.h
> @@ -547,6 +547,9 @@ struct cpp_options
>     /* True if warn about differences between C++98 and C++11.  */
>     bool cpp_warn_cxx11_compat;
>   
> +  /* True if warn about differences between C++17 and C++20.  */
> +  bool cpp_warn_cxx20_compat;
> +
>     /* Nonzero if bidirectional control characters checking is on.  See enum
>        cpp_bidirectional_level.  */
>     unsigned char cpp_warn_bidirectional;
> @@ -655,6 +658,7 @@ enum cpp_warning_reason {
>     CPP_W_C90_C99_COMPAT,
>     CPP_W_C11_C2X_COMPAT,
>     CPP_W_CXX11_COMPAT,
> +  CPP_W_CXX20_COMPAT,
>     CPP_W_EXPANSION_TO_DEFINED,
>     CPP_W_BIDIRECTIONAL
>   };
> diff --git a/libcpp/init.cc b/libcpp/init.cc
> index f4ab83d2145..cca3c1dc1e7 100644
> --- a/libcpp/init.cc
> +++ b/libcpp/init.cc
> @@ -202,6 +202,7 @@ cpp_create_reader (enum c_lang lang, cpp_hash_table *table,
>     CPP_OPTION (pfile, cpp_warn_c90_c99_compat) = -1;
>     CPP_OPTION (pfile, cpp_warn_c11_c2x_compat) = -1;
>     CPP_OPTION (pfile, cpp_warn_cxx11_compat) = 0;
> +  CPP_OPTION (pfile, cpp_warn_cxx20_compat) = 0;
>     CPP_OPTION (pfile, cpp_warn_deprecated) = 1;
>     CPP_OPTION (pfile, cpp_warn_long_long) = 0;
>     CPP_OPTION (pfile, dollars_in_ident) = 1;
  

If there are no further concerns, could a C++ or libcpp maintainer 
please commit this for me?

Thank you!

Tom.

On 8/4/22 12:42 PM, Tom Honermann via Gcc-patches wrote:
> Are there any further concerns with this patch? If not, I extend my 
> gratitude to anyone so kind as to commit this for me as I don't have 
> commit access.
>
> I just noticed that I neglected to add a ChangeLog entry for the 
> comment addition to gcc/cp/parser.cc. Noted inline below. I can 
> re-send the patch with that update if desired.
>
> Tom.
>
> On 8/1/22 2:49 PM, Tom Honermann wrote:
>> Gcc's '#pragma GCC diagnostic' directives are processed in "early mode"
>> (see handle_pragma_diagnostic_early) for the C++ frontend and, as such,
>> require that the target diagnostic option be enabled for the 
>> preprocessor
>> (see c_option_is_from_cpp_diagnostics).  This change modifies the
>> -Wc++20-compat option definition to register it as a preprocessor option
>> so that its associated diagnostics can be suppressed.  The changes also
>> implicitly disable the option in C++20 and later modes.  These changes
>> are consistent with the definition of the -Wc++11-compat option.
>>
>> This support is motivated by the need to suppress the following 
>> diagnostic
>> otherwise issued in C++17 and earlier modes due to the char8_t typedef
>> present in the uchar.h header file in glibc 2.36.
>>    warning: identifier ‘char8_t’ is a keyword in C++20 [-Wc++20-compat]
>>
>> Tests are added to validate suppression of both -Wc++11-compat and
>> -Wc++20-compat related diagnostics (fixes were only needed for the C++20
>> case).
>>
>> Fixeshttps://gcc.gnu.org/PR106423.
>>
>> gcc/c-family/ChangeLog:
>>     * c-opts.cc (c_common_post_options): Disable -Wc++20-compat 
>> diagnostics
>>     in C++20 and later.
>>     * c.opt (Wc++20-compat): Enable hooks for the preprocessor.
>
> gcc/cp/ChangeLog:
>     * parser.cc (cp_lexer_saving_tokens): Add comment regarding 
> diagnostic requirements.
>
>>
>> gcc/testsuite/ChangeLog:
>>     * g++.dg/cpp0x/keywords2.C: New test.
>>     * g++.dg/cpp2a/keywords2.C: New test.
>>
>> libcpp/ChangeLog:
>>     * include/cpplib.h (cpp_warning_reason): Add CPP_W_CXX20_COMPAT.
>>     * init.cc (cpp_create_reader): Add cpp_warn_cxx20_compat.
>> ---
>>   gcc/c-family/c-opts.cc                 |  7 +++++++
>>   gcc/c-family/c.opt                     |  2 +-
>>   gcc/cp/parser.cc                       |  5 ++++-
>>   gcc/testsuite/g++.dg/cpp0x/keywords2.C | 16 ++++++++++++++++
>>   gcc/testsuite/g++.dg/cpp2a/keywords2.C | 13 +++++++++++++
>>   libcpp/include/cpplib.h                |  4 ++++
>>   libcpp/init.cc                         |  1 +
>>   7 files changed, 46 insertions(+), 2 deletions(-)
>>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/keywords2.C
>>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/keywords2.C
>>
>> diff --git a/gcc/c-family/c-opts.cc b/gcc/c-family/c-opts.cc
>> index b9f01a65ed7..1ea37ba9742 100644
>> --- a/gcc/c-family/c-opts.cc
>> +++ b/gcc/c-family/c-opts.cc
>> @@ -1046,6 +1046,13 @@ c_common_post_options (const char **pfilename)
>>     else if (warn_narrowing == -1)
>>       warn_narrowing = 0;
>>   +  if (cxx_dialect >= cxx20)
>> +    {
>> +      /* Don't warn about C++20 compatibility changes in C++20 or 
>> later.  */
>> +      warn_cxx20_compat = 0;
>> +      cpp_opts->cpp_warn_cxx20_compat = 0;
>> +    }
>> +
>>     /* C++17 has stricter evaluation order requirements; let's use 
>> some of them
>>        for earlier C++ as well, so chaining works as expected. */
>>     if (c_dialect_cxx ()
>> diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
>> index 44e1a60ce24..dfdebd596ef 100644
>> --- a/gcc/c-family/c.opt
>> +++ b/gcc/c-family/c.opt
>> @@ -455,7 +455,7 @@ Wc++2a-compat
>>   C++ ObjC++ Warning Alias(Wc++20-compat) Undocumented
>>     Wc++20-compat
>> -C++ ObjC++ Var(warn_cxx20_compat) Warning LangEnabledBy(C++ 
>> ObjC++,Wall)
>> +C++ ObjC++ Var(warn_cxx20_compat) Warning LangEnabledBy(C++ 
>> ObjC++,Wall) Init(0) CPP(cpp_warn_cxx20_compat) 
>> CppReason(CPP_W_CXX20_COMPAT)
>>   Warn about C++ constructs whose meaning differs between ISO C++ 
>> 2017 and ISO C++ 2020.
>>     Wc++11-extensions
>> diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
>> index 4f67441eeb1..c3584446827 100644
>> --- a/gcc/cp/parser.cc
>> +++ b/gcc/cp/parser.cc
>> @@ -924,7 +924,10 @@ cp_lexer_saving_tokens (const cp_lexer* lexer)
>>   /* Store the next token from the preprocessor in *TOKEN. Return true
>>      if we reach EOF.  If LEXER is NULL, assume we are handling an
>>      initial #pragma pch_preprocess, and thus want the lexer to return
>> -   processed strings.  */
>> +   processed strings.
>> +
>> +   Diagnostics issued from this function must have their controlling 
>> option (if
>> +   any) in c.opt annotated as a libcpp option via the CppReason 
>> property.  */
>>     static void
>>   cp_lexer_get_preprocessor_token (unsigned flags, cp_token *token)
>> diff --git a/gcc/testsuite/g++.dg/cpp0x/keywords2.C 
>> b/gcc/testsuite/g++.dg/cpp0x/keywords2.C
>> new file mode 100644
>> index 00000000000..d67d01e31ed
>> --- /dev/null
>> +++ b/gcc/testsuite/g++.dg/cpp0x/keywords2.C
>> @@ -0,0 +1,16 @@
>> +// { dg-do compile { target c++98_only } }
>> +// { dg-options "-Wc++11-compat" }
>> +
>> +// Validate suppression of -Wc++11-compat diagnostics.
>> +#pragma GCC diagnostic ignored "-Wc++11-compat"
>> +int alignof;
>> +int alignas;
>> +int constexpr;
>> +int decltype;
>> +int noexcept;
>> +int nullptr;
>> +int static_assert;
>> +int thread_local;
>> +int _Alignas;
>> +int _Alignof;
>> +int _Thread_local;
>> diff --git a/gcc/testsuite/g++.dg/cpp2a/keywords2.C 
>> b/gcc/testsuite/g++.dg/cpp2a/keywords2.C
>> new file mode 100644
>> index 00000000000..8714a7b26b7
>> --- /dev/null
>> +++ b/gcc/testsuite/g++.dg/cpp2a/keywords2.C
>> @@ -0,0 +1,13 @@
>> +// { dg-do compile { target c++17_down } }
>> +// { dg-options "-Wc++20-compat" }
>> +
>> +// Validate suppression of -Wc++20-compat diagnostics.
>> +#pragma GCC diagnostic ignored "-Wc++20-compat"
>> +int constinit;
>> +int consteval;
>> +int requires;
>> +int concept;
>> +int co_await;
>> +int co_yield;
>> +int co_return;
>> +int char8_t;
>> diff --git a/libcpp/include/cpplib.h b/libcpp/include/cpplib.h
>> index 3eba6f74b57..9d90c18e4f2 100644
>> --- a/libcpp/include/cpplib.h
>> +++ b/libcpp/include/cpplib.h
>> @@ -547,6 +547,9 @@ struct cpp_options
>>     /* True if warn about differences between C++98 and C++11. */
>>     bool cpp_warn_cxx11_compat;
>>   +  /* True if warn about differences between C++17 and C++20. */
>> +  bool cpp_warn_cxx20_compat;
>> +
>>     /* Nonzero if bidirectional control characters checking is on.  
>> See enum
>>        cpp_bidirectional_level.  */
>>     unsigned char cpp_warn_bidirectional;
>> @@ -655,6 +658,7 @@ enum cpp_warning_reason {
>>     CPP_W_C90_C99_COMPAT,
>>     CPP_W_C11_C2X_COMPAT,
>>     CPP_W_CXX11_COMPAT,
>> +  CPP_W_CXX20_COMPAT,
>>     CPP_W_EXPANSION_TO_DEFINED,
>>     CPP_W_BIDIRECTIONAL
>>   };
>> diff --git a/libcpp/init.cc b/libcpp/init.cc
>> index f4ab83d2145..cca3c1dc1e7 100644
>> --- a/libcpp/init.cc
>> +++ b/libcpp/init.cc
>> @@ -202,6 +202,7 @@ cpp_create_reader (enum c_lang lang, 
>> cpp_hash_table *table,
>>     CPP_OPTION (pfile, cpp_warn_c90_c99_compat) = -1;
>>     CPP_OPTION (pfile, cpp_warn_c11_c2x_compat) = -1;
>>     CPP_OPTION (pfile, cpp_warn_cxx11_compat) = 0;
>> +  CPP_OPTION (pfile, cpp_warn_cxx20_compat) = 0;
>>     CPP_OPTION (pfile, cpp_warn_deprecated) = 1;
>>     CPP_OPTION (pfile, cpp_warn_long_long) = 0;
>>     CPP_OPTION (pfile, dollars_in_ident) = 1;
  

Sorry for the delay, I'm travelling with limited internet. I will commit it
early next week if no one beats me to it.

On Thu, Aug 11, 2022, 2:45 PM Tom Honermann via Gcc-patches <
gcc-patches@gcc.gnu.org> wrote:

> If there are no further concerns, could a C++ or libcpp maintainer
> please commit this for me?
>
> Thank you!
>
> Tom.
>
> On 8/4/22 12:42 PM, Tom Honermann via Gcc-patches wrote:
> > Are there any further concerns with this patch? If not, I extend my
> > gratitude to anyone so kind as to commit this for me as I don't have
> > commit access.
> >
> > I just noticed that I neglected to add a ChangeLog entry for the
> > comment addition to gcc/cp/parser.cc. Noted inline below. I can
> > re-send the patch with that update if desired.
> >
> > Tom.
> >
> > On 8/1/22 2:49 PM, Tom Honermann wrote:
> >> Gcc's '#pragma GCC diagnostic' directives are processed in "early mode"
> >> (see handle_pragma_diagnostic_early) for the C++ frontend and, as such,
> >> require that the target diagnostic option be enabled for the
> >> preprocessor
> >> (see c_option_is_from_cpp_diagnostics).  This change modifies the
> >> -Wc++20-compat option definition to register it as a preprocessor option
> >> so that its associated diagnostics can be suppressed.  The changes also
> >> implicitly disable the option in C++20 and later modes.  These changes
> >> are consistent with the definition of the -Wc++11-compat option.
> >>
> >> This support is motivated by the need to suppress the following
> >> diagnostic
> >> otherwise issued in C++17 and earlier modes due to the char8_t typedef
> >> present in the uchar.h header file in glibc 2.36.
> >>    warning: identifier ‘char8_t’ is a keyword in C++20 [-Wc++20-compat]
> >>
> >> Tests are added to validate suppression of both -Wc++11-compat and
> >> -Wc++20-compat related diagnostics (fixes were only needed for the C++20
> >> case).
> >>
> >> Fixeshttps://gcc.gnu.org/PR106423.
> >>
> >> gcc/c-family/ChangeLog:
> >>     * c-opts.cc (c_common_post_options): Disable -Wc++20-compat
> >> diagnostics
> >>     in C++20 and later.
> >>     * c.opt (Wc++20-compat): Enable hooks for the preprocessor.
> >
> > gcc/cp/ChangeLog:
> >     * parser.cc (cp_lexer_saving_tokens): Add comment regarding
> > diagnostic requirements.
> >
> >>
> >> gcc/testsuite/ChangeLog:
> >>     * g++.dg/cpp0x/keywords2.C: New test.
> >>     * g++.dg/cpp2a/keywords2.C: New test.
> >>
> >> libcpp/ChangeLog:
> >>     * include/cpplib.h (cpp_warning_reason): Add CPP_W_CXX20_COMPAT.
> >>     * init.cc (cpp_create_reader): Add cpp_warn_cxx20_compat.
> >> ---
> >>   gcc/c-family/c-opts.cc                 |  7 +++++++
> >>   gcc/c-family/c.opt                     |  2 +-
> >>   gcc/cp/parser.cc                       |  5 ++++-
> >>   gcc/testsuite/g++.dg/cpp0x/keywords2.C | 16 ++++++++++++++++
> >>   gcc/testsuite/g++.dg/cpp2a/keywords2.C | 13 +++++++++++++
> >>   libcpp/include/cpplib.h                |  4 ++++
> >>   libcpp/init.cc                         |  1 +
> >>   7 files changed, 46 insertions(+), 2 deletions(-)
> >>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/keywords2.C
> >>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/keywords2.C
> >>
> >> diff --git a/gcc/c-family/c-opts.cc b/gcc/c-family/c-opts.cc
> >> index b9f01a65ed7..1ea37ba9742 100644
> >> --- a/gcc/c-family/c-opts.cc
> >> +++ b/gcc/c-family/c-opts.cc
> >> @@ -1046,6 +1046,13 @@ c_common_post_options (const char **pfilename)
> >>     else if (warn_narrowing == -1)
> >>       warn_narrowing = 0;
> >>   +  if (cxx_dialect >= cxx20)
> >> +    {
> >> +      /* Don't warn about C++20 compatibility changes in C++20 or
> >> later.  */
> >> +      warn_cxx20_compat = 0;
> >> +      cpp_opts->cpp_warn_cxx20_compat = 0;
> >> +    }
> >> +
> >>     /* C++17 has stricter evaluation order requirements; let's use
> >> some of them
> >>        for earlier C++ as well, so chaining works as expected. */
> >>     if (c_dialect_cxx ()
> >> diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
> >> index 44e1a60ce24..dfdebd596ef 100644
> >> --- a/gcc/c-family/c.opt
> >> +++ b/gcc/c-family/c.opt
> >> @@ -455,7 +455,7 @@ Wc++2a-compat
> >>   C++ ObjC++ Warning Alias(Wc++20-compat) Undocumented
> >>     Wc++20-compat
> >> -C++ ObjC++ Var(warn_cxx20_compat) Warning LangEnabledBy(C++
> >> ObjC++,Wall)
> >> +C++ ObjC++ Var(warn_cxx20_compat) Warning LangEnabledBy(C++
> >> ObjC++,Wall) Init(0) CPP(cpp_warn_cxx20_compat)
> >> CppReason(CPP_W_CXX20_COMPAT)
> >>   Warn about C++ constructs whose meaning differs between ISO C++
> >> 2017 and ISO C++ 2020.
> >>     Wc++11-extensions
> >> diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
> >> index 4f67441eeb1..c3584446827 100644
> >> --- a/gcc/cp/parser.cc
> >> +++ b/gcc/cp/parser.cc
> >> @@ -924,7 +924,10 @@ cp_lexer_saving_tokens (const cp_lexer* lexer)
> >>   /* Store the next token from the preprocessor in *TOKEN. Return true
> >>      if we reach EOF.  If LEXER is NULL, assume we are handling an
> >>      initial #pragma pch_preprocess, and thus want the lexer to return
> >> -   processed strings.  */
> >> +   processed strings.
> >> +
> >> +   Diagnostics issued from this function must have their controlling
> >> option (if
> >> +   any) in c.opt annotated as a libcpp option via the CppReason
> >> property.  */
> >>     static void
> >>   cp_lexer_get_preprocessor_token (unsigned flags, cp_token *token)
> >> diff --git a/gcc/testsuite/g++.dg/cpp0x/keywords2.C
> >> b/gcc/testsuite/g++.dg/cpp0x/keywords2.C
> >> new file mode 100644
> >> index 00000000000..d67d01e31ed
> >> --- /dev/null
> >> +++ b/gcc/testsuite/g++.dg/cpp0x/keywords2.C
> >> @@ -0,0 +1,16 @@
> >> +// { dg-do compile { target c++98_only } }
> >> +// { dg-options "-Wc++11-compat" }
> >> +
> >> +// Validate suppression of -Wc++11-compat diagnostics.
> >> +#pragma GCC diagnostic ignored "-Wc++11-compat"
> >> +int alignof;
> >> +int alignas;
> >> +int constexpr;
> >> +int decltype;
> >> +int noexcept;
> >> +int nullptr;
> >> +int static_assert;
> >> +int thread_local;
> >> +int _Alignas;
> >> +int _Alignof;
> >> +int _Thread_local;
> >> diff --git a/gcc/testsuite/g++.dg/cpp2a/keywords2.C
> >> b/gcc/testsuite/g++.dg/cpp2a/keywords2.C
> >> new file mode 100644
> >> index 00000000000..8714a7b26b7
> >> --- /dev/null
> >> +++ b/gcc/testsuite/g++.dg/cpp2a/keywords2.C
> >> @@ -0,0 +1,13 @@
> >> +// { dg-do compile { target c++17_down } }
> >> +// { dg-options "-Wc++20-compat" }
> >> +
> >> +// Validate suppression of -Wc++20-compat diagnostics.
> >> +#pragma GCC diagnostic ignored "-Wc++20-compat"
> >> +int constinit;
> >> +int consteval;
> >> +int requires;
> >> +int concept;
> >> +int co_await;
> >> +int co_yield;
> >> +int co_return;
> >> +int char8_t;
> >> diff --git a/libcpp/include/cpplib.h b/libcpp/include/cpplib.h
> >> index 3eba6f74b57..9d90c18e4f2 100644
> >> --- a/libcpp/include/cpplib.h
> >> +++ b/libcpp/include/cpplib.h
> >> @@ -547,6 +547,9 @@ struct cpp_options
> >>     /* True if warn about differences between C++98 and C++11. */
> >>     bool cpp_warn_cxx11_compat;
> >>   +  /* True if warn about differences between C++17 and C++20. */
> >> +  bool cpp_warn_cxx20_compat;
> >> +
> >>     /* Nonzero if bidirectional control characters checking is on.
> >> See enum
> >>        cpp_bidirectional_level.  */
> >>     unsigned char cpp_warn_bidirectional;
> >> @@ -655,6 +658,7 @@ enum cpp_warning_reason {
> >>     CPP_W_C90_C99_COMPAT,
> >>     CPP_W_C11_C2X_COMPAT,
> >>     CPP_W_CXX11_COMPAT,
> >> +  CPP_W_CXX20_COMPAT,
> >>     CPP_W_EXPANSION_TO_DEFINED,
> >>     CPP_W_BIDIRECTIONAL
> >>   };
> >> diff --git a/libcpp/init.cc b/libcpp/init.cc
> >> index f4ab83d2145..cca3c1dc1e7 100644
> >> --- a/libcpp/init.cc
> >> +++ b/libcpp/init.cc
> >> @@ -202,6 +202,7 @@ cpp_create_reader (enum c_lang lang,
> >> cpp_hash_table *table,
> >>     CPP_OPTION (pfile, cpp_warn_c90_c99_compat) = -1;
> >>     CPP_OPTION (pfile, cpp_warn_c11_c2x_compat) = -1;
> >>     CPP_OPTION (pfile, cpp_warn_cxx11_compat) = 0;
> >> +  CPP_OPTION (pfile, cpp_warn_cxx20_compat) = 0;
> >>     CPP_OPTION (pfile, cpp_warn_deprecated) = 1;
> >>     CPP_OPTION (pfile, cpp_warn_long_long) = 0;
> >>     CPP_OPTION (pfile, dollars_in_ident) = 1;
>
>