Class template inheriting from incomplete class

Question

This code fails to compile because of the inheritance from an incomplete type (https://godbolt.org/z/G35wj9):

template<typename>
class Incomplete;

class T : public Incomplete<T> {};

template<typename>
class Incomplete {};

int main()
{
  [[maybe_unused]] T x;
}

I believed that this rule also applied to class templates. However, this code compiles fine (https://godbolt.org/z/cU6GNt):

template<typename>
class Incomplete;

template<int d>
class T : public Incomplete<T<d>> {};

template<typename>
class Incomplete {};

int main()
{
  [[maybe_unused]] T<1> x;
}

When class templates are involved, is the base class only required to be complete at the point of instantiation?

Answer 1

When class templates are involved, is the base class only required to be complete at the point of instantiation?

If it's a dependent base then yes. By that virtue a compiler has no idea what Incomplete<T<d>> is at the point of template definition. After all, for some values of d we can have ourselves a specialization of Incomplete<T<d>> that is entirely different from the primary template declaration.

template<>
class Incomplete<T<0>> {};

This is not a circular dependency. Simply naming the specialization T<0> does not cause it to be instantiated. It's just a type name. But it does mean the compiler has no recourse but wait until it can check the base class is valid.

On the other hand, if the base class is not a dependent type, then using it as base would be ill-formed.