Why Implementation for is_copy_assignable doesn't work?

Question

This is my attempt at an implementation for is_copy_assignable:

template<typename, typename = void>
struct IsCopyAssignable : std::false_type
{};

template<typename T>
struct IsCopyAssignable<T, decltype(std::add_lvalue_reference<T>::type = std::add_lvalue_reference<const T>::type, void())> : std::true_type
{};

It was a failure.

Here's the test cases:

int main()
{
    struct Structure {};
    std::cout << "IsCopyAssignable=\n";
    std::cout << IsCopyAssignable<int>::value << '\n'; // should be true
    std::cout << IsCopyAssignable<int&>::value << '\n'; // should be true
    std::cout << IsCopyAssignable<const int&>::value << '\n'; // should be false
    std::cout << IsCopyAssignable<const double&>::value << '\n'; // should be false
    std::cout << IsCopyAssignable<class Structure>::value << '\n'; // should be true
    std::cout << '\n';
}

They all print false.

(I realized then that declval in combination with the handy void_t - and decltype of course - can be used for things like these instead.) But I still do not understand exactly why this one doesn't work. I think we want to test whether const T& can be assigned to T& (as the copy assignment operator does). Well, why then?

Answer 1

Your decltype(std::add_lvalue_reference<T>::type = std::add_lvalue_reference<const T>::type, void()) is ill formed for every T as std::add_lvalue_reference<T>::type is actually not a value, but a type.

std::declval might help:

You want to check instead expression std::declval<T&>() = std::declval<const T&>() is valid.

So

template<typename T>
struct IsCopyAssignable<T,
                       decltype(std::declval<T&>() = std::declval<const T&>(), void())>
       : std::true_type
{};