-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.