`implied_bounds_entailment` diagnostic when associated type not in scope

[DaniPopes]

Code

trait Identity {
    type Identity;
}
impl<T> Identity for T {
    type Identity = T;
}

trait Trait {
    type Assoc: Identity;
    fn tokenize(&self) -> <Self::Assoc as Identity>::Identity;
}

impl Trait for () {
    type Assoc = DoesNotExist;
    fn tokenize(&self) -> <Self::Assoc as Identity>::Identity {
        unimplemented!()
    }
}

Current output

error[E0412]: cannot find type `DoesNotExist` in this scope
  --> issue.rs:14:18
   |
14 |     type Assoc = DoesNotExist;
   |                  ^^^^^^^^^^^^ not found in this scope

error: impl method assumes more implied bounds than the corresponding trait method
  --> issue.rs:15:5
   |
15 |     fn tokenize(&self) -> <Self::Assoc as Identity>::Identity {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #105572 <https://github.com/rust-lang/rust/issues/105572>
   = note: `#[deny(implied_bounds_entailment)]` on by default

Desired output

error[E0412]: cannot find type `DoesNotExist` in this scope
  --> issue.rs:14:18
   |
14 |     type Assoc = DoesNotExist;
   |                  ^^^^^^^^^^^^ not found in this scope

Rationale and extra context

When implementing the trait, if the associated type is set to something that is not in scope / doesn't exist, the compiler emits a implied_bounds_entailment diagnostic. I don't think this should be emitted because there is no way to calculate a type's trait bounds when it doesn't even exist.

Might be related to #108544, but I decided to file a separate issue because this does not involve any lifetimes.

Other cases

No response

Anything else?

Tested with stable 1.70.0, beta 1.71.0-beta.1 and nightly 1.72.0-nightly (d59363a 2023-06-01)