https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127615
--- Comment #1 from Joseph S. Myers <jsm28 at gcc dot gnu.org> --- It's nothing to do with C++; it's more an extension that long predates constexpr (empirically, before constant expression semantics were tightened up around GCC 4.5, people were relying in just about every context on being able to use things that GCC could fold to a constant, and so all the diagnostics for things that weren't constant expressions as defined by the standard generally became pedwarns-if-pedantic).
