Add regression test for ICE

Author: JonathanBrouwer

tests/incremental/explicit_supertraits_containing_assoc_item.rs

@@ -0,0 +1,19 @@
+//@ revisions: cpass1 cpass2
+//@ edition: 2024
+// regression test for https://github.com/rust-lang/rust/issues/158093
+
+trait Super {
+    type Assoc;
+}
+
+trait Sub: Super {}
+
+impl<T: ?Sized> Super for T {
+    type Assoc = i32;
+}
+
+fn illegal(x: &dyn Sub<Assoc = impl Sized>) -> &dyn Super<Assoc = impl Sized> {
+    x
+}
+
+fn main() {}