Add tests showcasing our short circuiting behaviour in the signature checks for defining scopes

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

@@ -0,0 +1,25 @@
+//! This test checks that we don't follow up
+//! with type mismatch errors of opaque types
+//! with their hidden types if we failed the
+//! defining scope check at the signature level.
+
+#![feature(impl_trait_in_assoc_type)]
+
+trait Foo {
+    type Bar<T>;
+    type Baz;
+    fn foo() -> (Self::Bar<u32>, Self::Baz);
+}
+
+impl Foo for () {
+    type Bar<T> = impl Sized;
+    type Baz = impl Sized;
+    fn foo() -> (Self::Bar<u32>, Self::Baz) {
+        //~^ ERROR non-defining opaque type use
+        ((), ())
+        //~^ ERROR mismatched types
+        //~| ERROR mismatched types
+    }
+}
+
+fn main() {}

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

@@ -0,0 +1,49 @@
+error: non-defining opaque type use in defining scope
+  --> $DIR/multi-error.rs:17:17
+   |
+LL |     fn foo() -> (Self::Bar<u32>, Self::Baz) {
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^ argument `u32` is not a generic parameter
+   |
+note: for this opaque type
+  --> $DIR/multi-error.rs:15:19
+   |
+LL |     type Bar<T> = impl Sized;
+   |                   ^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $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[E0308]: mismatched types
+  --> $DIR/multi-error.rs:19:14
+   |
+LL |     type Baz = impl Sized;
+   |                ---------- the expected opaque type
+...
+LL |         ((), ())
+   |              ^^ expected opaque type, found `()`
+   |
+   = note: expected opaque type `<() as Foo>::Baz`
+                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 3 previous errors
+
+For more information about this error, try `rustc --explain E0308`.

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

@@ -0,0 +1,22 @@
+//! This test checks that we don't follow up
+//! with type mismatch errors of opaque types
+//! with their hidden types if we failed the
+//! defining scope check at the signature level.
+
+#![feature(impl_trait_in_assoc_type)]
+
+trait Foo {
+    type Bar<T>;
+    fn foo() -> Self::Bar<u32>;
+    fn bar<T>() -> Self::Bar<T>;
+}
+
+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> {}
+}
+
+fn main() {}

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

@@ -0,0 +1,33 @@
+error: non-defining opaque type use in defining scope
+  --> $DIR/non-defining-method.rs:16:17
+   |
+LL |     fn foo() -> Self::Bar<u32> {}
+   |                 ^^^^^^^^^^^^^^ argument `u32` is not a generic parameter
+   |
+note: for this opaque type
+  --> $DIR/non-defining-method.rs:15:19
+   |
+LL |     type Bar<T> = impl Sized;
+   |                   ^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $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`.