Reading values of static const data members without definitions: what governs these rules?

Consider the following program:

struct Empty    {          };
struct NonEmpty { int x{}; };

struct S {
    static const Empty     e;   // declaration
    static const NonEmpty  n;   // declaration
    static const int       a;   // declaration
    static const int       b{}; // declaration and initializer
};

Empty    ge;  // declaration & definition
NonEmpty gn;  // declaration & definition
int      ga;  // declaration & definition

int main() {
    auto le1 = S::e; // OK
    auto ln1 = S::n; // #1 error: undefined reference
    auto la1 = S::a; // #2 error: undefined reference
    auto lb1 = S::b; // OK

    auto le2 = ::ge;  // OK
    auto ln2 = ::gn;  // OK
    auto la2 = ::ga;  // OK
}

None of the static data members of S are defined, but of the two integral type static data members S::a and S::b the latter specifies a brace-or-equal-initializer at its in-class declaration, as it may, as per [class.static.data]/4. S::e and S::n may not specify brace-or-equal-initializer:s at their in-class declaration.

All these static data members of S have, like their global namespace-scope counterpart variables ge, gn and ga, static storage duration. As per [basic.start.static]/2 they all thus undergo zero initialization as part of static initialization.

The program above is, however, rejected by GCC and Clang (various language versions, various compiler versions) at #1 and #2, when trying to copy-initialize local variables from the S::n and S::a static data members.

I'm assuming the compilers are correct, but I cannot find the normative section which supports this rejection, or maybe rather, why the program accept the case with the empty class static data member S::e. That the case of S::b is accepted "makes sense", but again I can't sort this out normatively (odr-use, conv.lval, basic.life, ... ?).

Question

• What normative text explains why the program above rejects reading the value of S::e and S::a as ill-formed, whilst it accepts read the values of S::e (empty) and S::b (initialized)? Say, in N4868.