add test for ice #121472

Fixes #121472

tests/ui/type-alias-impl-trait/underef-index-out-of-bounds-121472.rs

@@ -0,0 +1,16 @@
+// test for ICE #121472 index out of bounds un_derefer.rs
+#![feature(type_alias_impl_trait)]
+
+trait T {}
+
+type Alias<'a> = impl T;
+
+struct S;
+impl<'a> T for &'a S {}
+
+fn with_positive(fun: impl Fn(Alias<'_>)) {}
+
+fn main() {
+    with_positive(|&n| ());
+    //~^ ERROR mismatched types
+}

tests/ui/type-alias-impl-trait/underef-index-out-of-bounds-121472.stderr

@@ -0,0 +1,23 @@
+error[E0308]: mismatched types
+  --> $DIR/underef-index-out-of-bounds-121472.rs:14:20
+   |
+LL | type Alias<'a> = impl T;
+   |                  ------ the expected opaque type
+...
+LL |     with_positive(|&n| ());
+   |                    ^^
+   |                    |
+   |                    expected opaque type, found `&_`
+   |                    expected due to this
+   |
+   = note: expected opaque type `Alias<'_>`
+                found reference `&_`
+help: consider removing `&` from the pattern
+   |
+LL -     with_positive(|&n| ());
+LL +     with_positive(|n| ());
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.