Constructor instantiation in a world of guaranteed copy elision

Question

Consider this example:

template <typename T>
using type = typename T::type;

template <typename T>
struct A
{
    A(type<T>);
};

A<int> f();
A<int> g() { return f(); }

Neither gcc nor clang compile this code due to int not having a nested type typedef. But why is that constructor being instantiated at all? f() is a prvalue of the same type as the return of g(), there shouldn't even be a move there. What is causing us to instantiate the bad constructor?

Answer 1

The constructor is a bit of a red herring. The same would happen if it was any other member function.

template <typename T>
struct A
{
    void foo(type<T>); // Same error
};

This is on account of [temp.inst]/2

The implicit instantiation of a class template specialization causes the implicit instantiation of the declarations, but not of the definitions, default arguments, or noexcept-specifiers of the class member functions, [...]

The declaration is instantiated, so type<T> has to be well-formed.