Full name of submitter (unless configured in github; will be published with the issue): Hubert Tong

Reference (section label): expr.cast

Link to reflector thread (if any): N/A

Issue description:
In https://eel.is/c++draft/expr.cast#4, we say:

If a conversion can be interpreted in more than one way as a static_cast followed by a const_cast, the conversion is ill-formed.

In the following, we see that the choice of static_cast can differ between casting to an lvalue reference type or an rvalue reference type.

struct A {};
struct B : A {};
const B b;
void f() {
  (A &&)b; // ill-formed?
  const_cast<A &&>(static_cast<const A &>(b));
  const_cast<A &&>(static_cast<const volatile A &&>(b));
}

Suggested resolution:
If a conversion can be interpreted in more than one way as a static_cast, not arbitrarily adding cv-qualification and to an rvalue reference type if and only if converting to an rvalue reference type, followed by a const_cast, the conversion is ill-formed.