Why can't a class static constexpr value depend on a class static constexpr function?

Question

Anyone know if there is something wrong with the following or if it is a bug?

#include <utility>

struct X {
    template <typename...Ts>
    static constexpr std::size_t fn() {
        return 1;
    }
    static constexpr std::size_t value = fn();
};

Demo

Looks like msvc 19.22 seems to be ok with it, but clang and gcc don't like it.

If I move the fn() template external to the struct, it works fine. clang says:

error: constexpr variable 'value' must be initialized by a constant expression

and gcc says:

static constexpr std::size_t X::fn() [with Ts = {}; std::size_t = long unsigned int]' used before its definition

Also, can't define later:

struct X {
    template <typename...Ts>
    static constexpr std::size_t fn() {
        return 1;
    }
    static constexpr std::size_t value;
};
constexpr std::size_t X::value = X::fn();

Demo

However, it can be specified as a constexpr function:

struct X {
    template <typename...Ts>
    static constexpr std::size_t fn() {
        return 1;
    }
    static constexpr std::size_t value() {
        return fn();
    }
};

Demo

This appears to be caused by the class not being complete at use. If so, it would seem to be a deficiency since static member vars/fns don't depend on the class.

EDIT

Thanks @Rakete1111. Looking at the duplicate you provided, I got this quote from the standard (emphasis mine):

9.4.2 [class.static.data] p3: ... A static data member of literal type can be declared in the class definition with the constexpr specifier; if so, its declaration shall specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression.

As can is not the same as must, this further indicates a deficiency as my second example should have then worked.