std::function with noexcept in C++17

std::function‘s definition hasn’t changed in the current working draft:

template<class T>
class function; // not defined

template<class R, class... ArgTypes>
class function<R(ArgTypes...)> {
    /* ... */
};

Since void() noexcept doesn’t match the partial specialization, std::function<void() noexcept> is an incomplete type. Both Clang and GCC trunk diagnose this accordingly.