Bind to second the entity of (template parameter) TYPE that is bound to the second reference of other. Note that, unlike the primary template, these constructors must be declared and defaulted as the user-declared copy-assignment operator would inhibit the declaration of the move constructor, which in turn would mean that the implicitly declared constructors for the derived pair class would be just the copy constructor taking a const reference, even when the second element is move-only. Declaring the move constructor deletes the copy constructor unless it too is declared and defaulted.