Additional test demonstrating check for full trait ref

tests/ui/type-alias-impl-trait/not-matching-trait-refs-isnt-defining.rs

@@ -0,0 +1,33 @@
+#![feature(impl_trait_in_assoc_type)]
+
+trait Foo<T> {
+    type Assoc;
+
+    fn test() -> u32;
+}
+
+struct DefinesOpaque;
+impl Foo<DefinesOpaque> for () {
+    type Assoc = impl Sized;
+
+    // This test's return type is `u32`, *not* the opaque that is defined above.
+    // Previously we were only checking that the self type of the assoc matched,
+    // but this doesn't account for other impls with different trait substs.
+    fn test() -> <() as Foo<NoOpaques>>::Assoc {
+        let _: <Self as Foo<DefinesOpaque>>::Assoc = "";
+        //~^ ERROR mismatched types
+
+        1
+    }
+}
+
+struct NoOpaques;
+impl Foo<NoOpaques> for () {
+    type Assoc = u32;
+
+    fn test() -> u32 {
+        1
+    }
+}
+
+fn main() {}

tests/ui/type-alias-impl-trait/not-matching-trait-refs-isnt-defining.stderr

@@ -0,0 +1,22 @@
+error[E0308]: mismatched types
+  --> $DIR/not-matching-trait-refs-isnt-defining.rs:17:54
+   |
+LL |     type Assoc = impl Sized;
+   |                  ---------- the expected opaque type
+...
+LL |         let _: <Self as Foo<DefinesOpaque>>::Assoc = "";
+   |                -----------------------------------   ^^ expected opaque type, found `&str`
+   |                |
+   |                expected due to this
+   |
+   = note: expected opaque type `<() as Foo<DefinesOpaque>>::Assoc`
+                found reference `&'static str`
+note: this item must have the opaque type in its signature in order to be able to register hidden types
+  --> $DIR/not-matching-trait-refs-isnt-defining.rs:16:5
+   |
+LL |     fn test() -> <() as Foo<NoOpaques>>::Assoc {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.