https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117684
--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It was fixed by r267253
re PR c++/87934 (struct with NSDMI of enum makes initialization a
non-constant expression)
PR c++/87934
* constexpr.c (cxx_eval_constant_expression) <case CONSTRUCTOR>: Do
re-process TREE_CONSTANT CONSTRUCTORs if they aren't reduced
constant
expressions.
That's supposed to be fixed in GCC 8.3 as well as 9.1
