Additional notes (January, 2023):

The following example might be related:

  #include <type_traits>

  struct Bar {
    struct Baz {
      int a = 0;
    };
    static_assert(std::is_default_constructible_v<Baz>);
  };

Consider an example like:

  struct A {
    struct B {
      auto foo() { return 0; }
    };
    decltype(B().foo()) x;
  };

There does not appear to be a prohibition of cases like this, where the type of a member depends on the definition of a member function.

(See also issues 1360, 1397, and 2335.)