regression test for issue 80949.

src/test/ui/mir/issue-80949.rs

@@ -0,0 +1,34 @@
+// build-pass
+
+trait Trait { type Item; }
+
+impl<'a, X> Trait for &'a Vec<X> {
+    type Item = &'a X;
+}
+
+impl<X> Trait for Box<dyn Trait<Item = X>> {
+    type Item = X;
+}
+
+fn make_dyn_trait(_: &()) -> Box<dyn Trait<Item = &()>> {
+    todo!()
+}
+
+fn diff<'a, M, N, S>(_: N, _: S)
+where
+    M: 'a,
+    N: Trait<Item = &'a M>,
+    S: Trait<Item = &'a M>,
+{
+    todo!()
+}
+
+fn may_panic<X>(_: X) { }
+
+fn main() {
+    let dyn_trait = make_dyn_trait(&());
+    let storage = vec![()];
+    let _x = may_panic(());
+    let storage_ref = &storage;
+    diff(dyn_trait, storage_ref);
+}