https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89237
--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> --- ICC also rejects it with the same message as GCC even. MSVC rejects it with: <source>(9): error C2794: 'type': is not a member of any direct or indirect base class of 'enable_if<false,void>' <source>(9): warning C4305: 'specialization': truncation from 'size_t' to 'bool' If I change "sizeof(T)" to "sizeof(T)!=0" as what I thought the conversion would be, then clang rejects it with the same error as GCC. So I suspect it is rejecting the specialization due to the conversion from size_t to bool which I thought I had saw a bug about that.
