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.
