Decision on "must define before use" for opaque types

Nominating for T-lang to decide whether the following rule should hold for opaque types:

If the body of an item that may define the hidden type of some opaque does define that hidden type, it must do so syntactically before using the opaque type in a non-defining way.

This restriction is called "must define before use."

Consider:

use core::convert::identity;

struct I;
struct IShow;
impl I { fn show(&self) -> IShow { IShow } }

struct OnIShow;
trait OnI { fn show(&self) -> OnIShow { OnIShow } }
impl OnI for I {}

fn test(n: bool) -> impl OnI {
    let true = n else { loop {} };
    let x = test(!n); //~ NOTE this is the opaque type
    let _: OnIShow = x.show(); //~ NOTE this is a non-defining use
    //~^ ERROR if the body registers a hidden type for the opaque, it
    //         must do so *before* using it opaquely
    let _: IShow = identity::<I>(x).show();
    //~^ NOTE this registers a hidden type for the opaque, but does so
    //        too late
    loop {}
}

fn main() {}

Because the code above works today, this would be a breaking change for RPIT.

Start with the issue's RPIT example and the stated “must define before use” rule for opaque types. Determine whether the T-lang decision has been made and, if so, identify the compiler entry points and tests for the affected behavior; done requires an agreed language decision and corresponding validation.