std::unexpected constructor constraint

Question

I am reading the cpp 23 standard and I stumbled upon std::unexpected.

Section expected.un.cons defines

template<class Err = E>
constexpr explicit unexpected(Err&& e);

with the following costraint (among others)

is_same_v<remove_cvref_t<Err>, unexpected> is false; 

When would this expression be true given that unexpected is a class template?

Answer 1

unexpected is the injected-class-name in the scope of the class template. It refers to the current specialization of the template. It does not refer to the template itself as the same name would outside the class scope.

For example if you write

template<typename T>
struct X {
    static X x;
};

then X<T>::x is of type X<T>. You don't have to repeat the template argument list in the declaration of x. If none is given, then it resolves to the injected-class-name which refers to the current specialization of the template with the same template argument list.

The constraint is there so that the constructor can't be chosen over the copy constructor in overload resolution, which may otherwise happen for non-const lvalue arguments.