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.