Add a regression test

Author: oli-obk

tests/ui/type-alias-impl-trait/defining-scope-self.rs

@@ -0,0 +1,21 @@
+#![feature(type_alias_impl_trait)]
+
+// edition: 2021
+// check-pass
+
+type MyIter = impl Iterator<Item = i32>;
+
+struct Foo {
+    iter: MyIter,
+}
+
+// ensure that `self` is seen as `Foo` and thus
+// counts as a defining use for `MyIter` due to the
+// struct field.
+impl Foo {
+    fn set_iter(&mut self) {
+        self.iter = [1, 2, 3].into_iter();
+    }
+}
+
+fn main() {}