Possible resolution:

Change in 11.4.4 [special] paragraph 6 as follows:

An eligible special member function is a special member function for which:

• the function is not deleted,
• the associated constraints (13.5 [temp.constr]), if any, are satisfied, and
• no special member function of the same kind and at most as explict (9.2.3 [dcl.fct.spec]) whose associated constraints, if any, are satisfied is more constrained (13.5.5 [temp.constr.order]).,

where a function F1 is at most as explicit as a function F2 if

• both are explicit or neither is or
• F1 is not explicit and F2 is explicit.

Consider:

  template <class T>
  struct X {
    X() = default;
    X(const X&) = default;
  private:
    explicit X(const X&) requires (T::value) = default;  // #1
  };

  struct S { static constexpr bool value = true; };

  int main() {
    const X<S> x;
    X<S> x2 = x;     // #2
  }

According to 11.4.4 [special] paragraph 6, the public copy constructor is not eligible, because #1 is more constrained. This causes it to be deleted (9.6.2 [dcl.fct.def.default] paragraph 5). Yet, the public copy constructor is selected by overload resolution when used at #2.