Why this expression is not constant expression

I have the following code:

struct S {
    const bool x = true;
    constexpr const bool& f() { return x; }
} s{};

int main() {
  static_assert(s.f()); // error: s is not declared as 'constexpr';
}

Note that, this is a dupe for the question How to make this pointer constant expression because the the expression E = s.f() does not evaluates lvalue-to-rvalue conversion ([expr.const]/(5.8)).

Also, this question is not a dupe for the question How to get std array size in a constant expression because the expression E = s.f() does evaluates this in a constexpr function S::f that's evaluated as part of the expression s.f() ([expr.const]/(5.1)).

Now my question is, which bullet in [expr.const]/5 (if any) is applied to the above example?