Why std::declval<int>() = std::declval<int>() is invalid?

Question

I'm trying to use detector I wrote which according to std::experiment::is_detected_v to check whether a type is assignable. However std::declval<int>() = std::declval<int>() is invalid and only std::declval<int&>() = std::declval<int>() is valid.

Why does this happen?

Answer 1

Look at the signature of std::declval:

template<class T>
typename std::add_rvalue_reference<T>::type declval() noexcept;

std::declval<int>() has type int&&, an rvalue(xvalue). You can't assign to an rvalue int.