[Bug c++/107543] New: c-family/c-cppbuiltin.cc: Undefine __GNUC_STDC_INLINE__ in C++ mode?

["i at maskray dot me" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107543

            Bug ID: 107543
           Summary: c-family/c-cppbuiltin.cc: Undefine
                    __GNUC_STDC_INLINE__ in C++ mode?
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: i at maskray dot me
  Target Milestone: ---

-fgnu89-inline / C99 / C++  inline modes are all different. For C++, perhaps
neither 
__GNUC_STDC_INLINE__ nor __GNUC_GNUC_INLINE__ should be defined.

% g++ -dM -E -xc++ /dev/null | grep __GNUC_STDC_INLINE__
#define __GNUC_STDC_INLINE__ 1


gcc/doc/cpp.texi does not talk about C++

@item __GNUC_STDC_INLINE__
GCC defines this macro if functions declared @code{inline} will be
handled according to the ISO C99 or later standards.  Object files will contain
externally visible definitions of all functions declared @code{extern
inline}.  They will not contain definitions of any functions declared
@code{inline} without @code{extern}.

If this macro is defined, GCC supports the @code{gnu_inline} function
attribute as a way to always get the gnu90 behavior.


Side note: there is "warning: command-line option ‘-fgnu89-inline’ is valid for
C/ObjC but not for C++" but  __attribute__((gnu_inline)) is usable in C++ mode
and has strange codegen behaviors.