Spurious 'conflicting implementations' error when specializing a type with a compilation error

[Aaron1011]

The following code:

#![feature(specialization)]

struct BadStruct {
    err: MissingType
}

trait MyTrait<T> {
    fn foo();
}

impl<T, D> MyTrait<T> for D {
    default fn foo() {}
}

impl<T> MyTrait<T> for BadStruct {
    fn foo() {}
}

gives the following errors:

error[E0412]: cannot find type `MissingType` in this scope
 --> src/lib.rs:4:10
  |
4 |     err: MissingType
  |          ^^^^^^^^^^^ not found in this scope

error[E0119]: conflicting implementations of trait `MyTrait<_>` for type `BadStruct`:
  --> src/lib.rs:15:1
   |
11 | impl<T, D> MyTrait<T> for D {
   | --------------------------- first implementation here
...
15 | impl<T> MyTrait<T> for BadStruct {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `BadStruct`

error: aborting due to 2 previous errors

For some reason, the fact that the definition of BadStruct has an error (MissingType is not defining) causes a 'conflicting implementations' error to be emitted when BadStruct has a specialized impl. If MissingType is changed to a type which actually exists (e.g. ()), the 'conflicting implementations' error disappears.

I found this when working on rustc - a missing use statement caused 30 spurious specialization-related errors to be emiited, which all disappeared when I added the missing import.

[JohnTitor]

Current output:

error[E0412]: cannot find type `MissingType` in this scope
 --> src/main.rs:5:10
  |
5 |     err: MissingType
  |          ^^^^^^^^^^^ not found in this scope

error: aborting due to previous error

The E0119 is no longer shown, marking as E-needs-test.

[gurry]

This issue regressed due to PR #123491. @Aaron1011 please reopen it.