http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34996
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-09-29 12:25:00 UTC --- Derived1::X must be defined in every translation unit that needs it. An explicit instantiation is *required* to not instantiate members of base classes: [temp.explicit]/8 An explicit instantiation that names a class template specialization is also an explicit instantiation of the same kind (declaration or definition) of each of its members (not including members inherited from base classes) ...
