Eric Niebler wrote:
template < class , class = void > struct result_of_aux {};
template < class F, class ...Args> struct result_of_aux<F(Args...), decltype(std::declval<F>()(std::declval<Args>()...), void ())> { typedef decltype(std::declval<F>()(std::declval<Args>()...)) type; };
What is void()? That's not any expression I recognize. It's a function type.
5.2.3 [expr.type.conv] p2 The expression T(), where T is a simple-type-specifier or typename-specifier for a non-array complete object type or the (possibly cv -qualified) void type, creates a prvalue of the specified type, which is value-initialized ( 8.5; no initialization is done for the void() case).
Anyway, keep in mind that std::declval<F>()(...) could have an incomplete type, so you can't really use it in an expression. You should use it in decltype,
All the use of `std::declval<F>()(...)` is in decltype and so I don't see any problem in the code. Do you mean struct result_of_aux< F(Args...) , decltype(std::declval<F>()(std::declval<Args>()...), void ()) > can be a hard error? Regards, Michel