Add regression test for supertrait associated type normalization through dyn

tests/ui/associated-types/normalize-supertrait-projection-in-dyn-61083.rs

@@ -0,0 +1,28 @@
+//! Regression test for <https://github.com/rust-lang/rust/issues/61083>.
+//!
+//! An associated type projection in a supertrait bound (`Bar<T>: Foo<T::Item>`)
+//! failed to normalize when the `Bar` bound was reached through a trait object,
+//! so passing the object to a function expecting `Foo<u32>` was rejected.
+
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+//@ check-pass
+
+trait Foo<T> {}
+
+trait Bar<T: Iterator>: Foo<T::Item> {}
+
+fn a(_x: &(impl Foo<u32> + ?Sized)) {}
+
+// The `dyn` form is the one that used to fail to normalize `T::Item` to `u32`.
+fn b(y: &dyn Bar<std::vec::IntoIter<u32>>) {
+    a(y)
+}
+
+// The equivalent `impl Trait` form always compiled; keep it so both paths stay pinned.
+fn c(y: &(impl Bar<std::vec::IntoIter<u32>> + ?Sized)) {
+    a(y)
+}
+
+fn main() {}