https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91371
--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> --- I haven't fully tested this idea yet, but ... There are two kinds of function type we need to detect: referenceable functions, and abominable functions. A referenceable function type is a non-class type that can be converted from T& to T* (by function-to-pointer conversion). An abominable function type is a non-referenceable type that is not cv void. So: template<typename T, typename = void> struct is_referenceable : false_type { }; template<typename T> struct is_referenceable<T, __void_t<T&>> : true_type { }; template<typename T, bool = __is_class(T) || __is_union(T), typename = void> struct is_referenceable_function : false_type { }; template<typename T> struct is_referenceable_function<T, false, __void_t<decltype(static_cast<T*>(std::declval<T&>()))>> : true_type { }; template<typename T> struct is_abominable_function : __not_<__or_<is_referenceable<T>, is_void<T>>>::type { }; template<typename T> struct is_function : __or_<is_referenceable_function<T>, is_abominable_function<T>> { }; The current definition of __is_referenceable depends on is_object which depends on is_function, but I think we can simplify it as shown above, and reverse the dependency.