https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92735
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> --- For the case reported here, Clang and EDG do reject it, but I'm not yet convinced GCC is wrong to accept it. The implicit instantiation of is_Foo<Bad> causes: "the implicit instantiation of the declarations, but not of the definitions, of the non-deleted class member functions, member classes, scoped member enumerations, static data members, member templates, and friends;" and: "in particular, the initialization (and any associated side effects) of a static data member does not occur unless the static data member is itself used in a way that requires the definition of the static data member to exist." Nothing requires the instantiation of is_Foo<Bad>::hello.
