Treat opaque types failing the signature defining scope check as defining, as we already errored and can hide subsequent errors this way.

compiler/rustc_ty_utils/src/opaque_types.rs

@@ -72,14 +72,15 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for OpaqueTypeCollector<'tcx> {
                 if !self.seen.insert(alias_ty.def_id.expect_local()) {
                     return ControlFlow::Continue(());
                 }
+
+                self.opaques.push(alias_ty.def_id.expect_local());
+
                 match self.tcx.uses_unique_generic_params(alias_ty.substs, CheckRegions::Bound) {
                     Ok(()) => {
                         // FIXME: implement higher kinded lifetime bounds on nested opaque types. They are not
                         // supported at all, so this is sound to do, but once we want to support them, you'll
                         // start seeing the error below.
 
-                        self.opaques.push(alias_ty.def_id.expect_local());
-
                         // Collect opaque types nested within the associated type bounds of this opaque type.
                         for (pred, span) in self
                             .tcx

tests/ui/generic-associated-types/issue-88595.rs

@@ -19,5 +19,4 @@ impl<'a> A<'a> for C {
     type B<'b> = impl Clone;
 
     fn a(&'a self) -> Self::B<'a> {} //~ ERROR: non-defining opaque type use in defining scope
-    //~^ ERROR: mismatched types
 }

tests/ui/generic-associated-types/issue-88595.stderr

@@ -10,25 +10,5 @@ note: for this opaque type
 LL |     type B<'b> = impl Clone;
    |                  ^^^^^^^^^^
 
-error[E0308]: mismatched types
+error: aborting due to previous error
-  --> $DIR/issue-88595.rs:21:23
-   |
-LL |     type B<'b> = impl Clone;
-   |                  ---------- the expected opaque type
-LL |
-LL |     fn a(&'a self) -> Self::B<'a> {}
-   |        -              ^^^^^^^^^^^ expected opaque type, found `()`
-   |        |
-   |        implicitly returns `()` as its body has no tail or `return` expression
-   |
-   = note: expected opaque type `<C as A<'a>>::B<'a>`
-                found unit type `()`
-note: this item must have the opaque type in its signature in order to be able to register hidden types
-  --> $DIR/issue-88595.rs:21:8
-   |
-LL |     fn a(&'a self) -> Self::B<'a> {}
-   |        ^
 
-error: aborting due to 2 previous errors
-
-For more information about this error, try `rustc --explain E0308`.

tests/ui/type-alias-impl-trait/multi-error.rs

@@ -17,7 +17,6 @@ impl Foo for () {
     fn foo() -> (Self::Bar<u32>, Self::Baz) {
         //~^ ERROR non-defining opaque type use
         ((), ())
-        //~^ ERROR mismatched types
     }
 }
 

tests/ui/type-alias-impl-trait/multi-error.stderr

@@ -10,23 +10,5 @@ note: for this opaque type
 LL |     type Bar<T> = impl Sized;
    |                   ^^^^^^^^^^
 
-error[E0308]: mismatched types
+error: aborting due to previous error
-  --> $DIR/multi-error.rs:19:10
-   |
-LL |     type Bar<T> = impl Sized;
-   |                   ---------- the expected opaque type
-...
-LL |         ((), ())
-   |          ^^ expected opaque type, found `()`
-   |
-   = note: expected opaque type `<() as Foo>::Bar<u32>`
-                found unit type `()`
-note: this item must have the opaque type in its signature in order to be able to register hidden types
-  --> $DIR/multi-error.rs:17:8
-   |
-LL |     fn foo() -> (Self::Bar<u32>, Self::Baz) {
-   |        ^^^
 
-error: aborting due to 2 previous errors
-
-For more information about this error, try `rustc --explain E0308`.

tests/ui/type-alias-impl-trait/non-defining-method.rs

@@ -15,7 +15,6 @@ impl Foo for () {
     type Bar<T> = impl Sized;
     fn foo() -> Self::Bar<u32> {}
     //~^ ERROR non-defining opaque type use
-    //~| ERROR mismatched types
     fn bar<T>() -> Self::Bar<T> {}
 }
 

tests/ui/type-alias-impl-trait/non-defining-method.stderr

@@ -10,24 +10,5 @@ note: for this opaque type
 LL |     type Bar<T> = impl Sized;
    |                   ^^^^^^^^^^
 
-error[E0308]: mismatched types
+error: aborting due to previous error
-  --> $DIR/non-defining-method.rs:16:17
-   |
-LL |     type Bar<T> = impl Sized;
-   |                   ---------- the expected opaque type
-LL |     fn foo() -> Self::Bar<u32> {}
-   |        ---      ^^^^^^^^^^^^^^ expected opaque type, found `()`
-   |        |
-   |        implicitly returns `()` as its body has no tail or `return` expression
-   |
-   = note: expected opaque type `<() as Foo>::Bar<u32>`
-                found unit type `()`
-note: this item must have the opaque type in its signature in order to be able to register hidden types
-  --> $DIR/non-defining-method.rs:16:8
-   |
-LL |     fn foo() -> Self::Bar<u32> {}
-   |        ^^^
 
-error: aborting due to 2 previous errors
-
-For more information about this error, try `rustc --explain E0308`.