Must destructor be just available (public) or fully valid for default initialized class members?

Please consider a struct A having a field u of type U<R> with a default initializer. The destructor ~U<R> is only declared:

template<typename T>
struct U {
    ~U();
};

struct R;

struct A {
    U<R> u = U<R>{};
};

All compilers accept this code, demo: https://gcc.godbolt.org/z/oqMjTovMo

But if we define the destructor ~U<R> as follows:

template<typename T>
struct U {
    ~U() { static_assert( sizeof(T) > 0 ); }
};

then the current compilers diverge. MSVC keeps accepting the program, while GCC/Clang print the error

error: invalid application of 'sizeof' to an incomplete type 'R'

demo: https://gcc.godbolt.org/z/713TzPd6v

Obviously, the compiler must verify destructor availability of default initialing class members in case an exception occurs during construction. But does the standard require that the compiler just check the availability of the destructor (as MSVC does), or the compiler should verify its body as well? MSVC behavior looks more convenient here since it permits forward declaration of R by the moment of struct A definition.

P. S. This questing has not only purely theoretical interest. If one replaces U here with std::unique_ptr then it explains why class fields of type std::unique_ptr<R> are accepted by MSVC and rejected by GCC/Clang for incomplete classes R.