--- 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;"
"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.