CRTP std::is_default_constructible not working as expected

Question

template <class T>
class Base {
     static_assert(!std::is_default_constructible<T>::value,
                   "T must not be default constructible");
};

struct X1 : Base<X1> {};
struct X2 : Base<X2> {
   X2() = default;
};
struct X3 : Base<X3> {
   X3() {};
};
struct X4 : Base<X4> {
   X4() : Base{} {};
};

struct Y1 {};

int main() {
    // all compile. They shouldn't
    X1 x1; X2 x2; X3 x3; X4 x4; 
    // all compile. They shouldn't:
    Base<X1> bx1; Base<X2> bx2; Base<X3> bx3; Base<X4> bx4;  

    Base<Y1> by1; // static assert fires. This is the expected behavior
}

The static_assert at class level doesn't fire for any of the X classes. But kicks in for Y (which doesn't derive Base)

It works correctly if the static_assert is moved inside the constructor of Base

What is the reason that is_default_constructible<T> is always false at class level if T derives from Base?

Ideone

Answer 1

When Base<X1> is instantiated in the inheritance list of X1, X1 is an incomplete type. This means that X1 is not default-constructible when the class-scope static_assert is checked.

The constructor for Base is only instantiated upon use, at which point X1 is now a complete type and is default-constructible. This is why the static_assert fires when inside the constructor, but not at class-scope.