[Bug c++/106362] New: -Wc++20-compat should not warn with __extension__

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106362

            Bug ID: 106362
           Summary: -Wc++20-compat should not warn with __extension__
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mpolacek at gcc dot gnu.org
  Target Milestone: ---

As noted here:
<https://sourceware.org/pipermail/libc-alpha/2022-July/140825.html>, the
following should probably not generate a warning:

__extension__ int char8_t;

$ ./cc1plus -quiet g.C -Wc++20-compat
g.C:1:19: warning: identifier ‘char8_t’ is a keyword in C++20 [-Wc++20-compat]
    1 | __extension__ int char8_t;
      |                   ^~~~~~~