Fix assertion failure in type inference (#13352)

crates/ide/src/highlight_related.rs

@@ -1373,6 +1373,22 @@ fn main() {
     ().func$0();
      //^^^^
 }
+"#,
+        );
+    }
+
+    #[test]
+    fn test_assoc_type_highlighting() {
+        check(
+            r#"
+trait Trait {
+    type Output;
+      // ^^^^^^
+}
+impl Trait for () {
+    type Output$0 = ();
+      // ^^^^^^
+}
 "#,
         );
     }