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

[Tom Honermann <tom@honermann.net>]

On 7/27/22 7:09 PM, Joseph Myers wrote:
> On Sun, 24 Jul 2022, Tom Honermann via Gcc-patches 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
> There are lots of C++ warning options, all of which should support pragma
> suppression regardless of whether they are relevant to the preprocessor or
> not.  Do they all need this kind of handling, or is it only -Wc++20-compat
> that has some kind of problem?

I had only checked -Wc++20-compat when working on the patch.

I did some spot checking now and confirmed that suppression works as 
expected for C++ for at least the following warnings:
   -Wuninitialized
   -Warray-compare
   -Wbool-compare
   -Wtautological-compare
   -Wterminate

I don't know the diagnostic framework well. As best I can tell, this 
issue is specific to the -Wc++20-compat option and when the particular 
diagnostic is issued (e.g., during lexing as opposed to during parsing). 
The following call chains appear to be relevant.
   cp_lexer_new_main -> cp_lexer_handle_early_pragma -> 
c_invoke_early_pragma_handler
   cp_parser_* -> cp_parser_pragma -> c_invoke_pragma_handler
   (where * might be "declaration", "toplevel_declaration", 
"class_head", "objc_interstitial_code", ...)

The -Wc++20-compat enabled warning regarding new keywords in C++20 is 
issued from cp_lexer_get_preprocessor_token.

Tom.