[Bug c++/86533] [9.0 regression] Compile error on valid code: error: no matching function for call to 'std::allocator::allocator(const _Tp_alloc_type&)' { return _Map_alloc_type(_M_get_Tp_allocator());

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86533

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
My best guess is that you've messed up your GCC installation, because
_GLIBCXX20_CONSTEXPR should be defined in <bits/c++config.h> like so:

#ifndef _GLIBCXX20_CONSTEXPR
# if __cplusplus > 201703L
#  define _GLIBCXX20_CONSTEXPR constexpr
# else
#  define _GLIBCXX20_CONSTEXPR
# endif
#endif

If you have a headers using that macro then you should have the definition of
that macro too.

Without preprocessed source there's no way to tell.