Add test checking that Index<T: ?Sized> works

src/test/ui/unsized-locals/unsized-index.rs

@@ -0,0 +1,23 @@
+// compile-pass
+
+// `std::ops::Index` has an `: ?Sized` bound on the `Idx` type param. This is
+// an accidental left-over from the times when it `Index` was by-reference.
+// Tightening the bound now could be a breaking change. Although no crater
+// regression were observed (https://github.com/rust-lang/rust/pull/59527),
+// let's be conservative and just add a test for this.
+#![feature(unsized_locals)]
+
+use std::ops;
+
+pub struct A;
+
+impl ops::Index<str> for A {
+    type Output = ();
+    fn index(&self, _: str) -> &Self::Output { panic!() }
+}
+
+impl ops::IndexMut<str> for A {
+    fn index_mut(&mut self, _: str) -> &mut Self::Output { panic!() }
+}
+
+fn main() {}