Add test for rust-lang#70121

Author: JohnTitor

src/test/ui/type-alias-impl-trait/issue-70121.rs

@@ -0,0 +1,23 @@
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+
+pub type Successors<'a> = impl Iterator<Item = &'a ()>;
+
+pub fn f<'a>() -> Successors<'a> {
+    None.into_iter()
+}
+
+pub trait Tr {
+    type Item;
+}
+
+impl<'a> Tr for &'a () {
+    type Item = Successors<'a>;
+}
+
+pub fn kazusa<'a>() -> <&'a () as Tr>::Item {
+    None.into_iter()
+}
+
+fn main() {}