Class can_invoke

include/function2/function2.hpp:262–262

/// SFINAE here.
template <typename T, typename Args, typename = void>
struct can_invoke : std::false_type {};
template <typename T, typename... Args>
struct can_invoke<T, identity<Args...>,
                  decltype((void)std::declval<T>()(std::declval<Args>()...))>