C++14 `result_of` in VS 2015 Update 1

type_traits.hpp

#include <type_traits>

namespace std {

  namespace detail {

    // A dummy tag type that indicates substitution failure. It should be hidden
    // as best as possible, in this case it's just under the `detail` namespace.
    struct fail;

    // Perform the necessary expression SFINAE in a supported context.
    // Note that it leverages `std::invoke` which is carefully written by STL to
    // avoid the limitations around the partial expression SFINAE support.

    template <typename, typename...>
    fail result_of_type(...);

    template <typename F, typename... Args>
    auto result_of_type(int)
        -> decltype(std::invoke(std::declval<F>(), std::declval<Args>()...));

    // `decltype(result_of_type<F, Args...>(0))` is either `fail` in the case of
    // substitution failure, or the return type of `invoke(f, args...)`.
    // `result_of_impl` provides member typedef `type = R` if `R != fail`.

    template <typename R>
    using result_of_impl = std::enable_if<!std::is_same<R, fail>::value, R>;

    template <typename>
    struct result_of;

    template <typename F, typename... Args>
    struct result_of<F(Args...)>
        : result_of_impl<decltype(result_of_type<F, Args...>(0))> {};

  }  // namespace detail

  using detail::result_of;

}  // namespace std