[Bug libstdc++/89464] shared_ptr_base.h: error: '__tag' was not declared in this scope (gcc-8.3.0 regression?)

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89464

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
alignas is a keyword in C++, it's not supposed to be a macro at all.

GCC 8.3.0 now uses alignas in the standard library headers, which is OK because
it's a C++ feature supported by the compiler. Stop defining a macro with the
name of a keyword, that's undefined.