Add test for issue-70877

Author: JohnTitor

src/test/ui/impl-trait/issues/issue-70877.rs

@@ -0,0 +1,38 @@
+#![feature(type_alias_impl_trait)]
+#![feature(impl_trait_in_bindings)]
+#![allow(incomplete_features)]
+
+type FooArg<'a> = &'a dyn ToString;
+type FooRet = impl std::fmt::Debug;
+
+type FooItem = Box<dyn Fn(FooArg) -> FooRet>;
+type Foo = impl Iterator<Item = FooItem>; //~ ERROR: type mismatch
+
+#[repr(C)]
+struct Bar(u8);
+
+impl Iterator for Bar {
+    type Item = FooItem;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        Some(Box::new(quux))
+    }
+}
+
+fn quux(st: FooArg) -> FooRet {
+    Some(st.to_string())
+}
+
+fn ham() -> Foo {
+    Bar(1)
+}
+
+fn oof() -> impl std::fmt::Debug {
+    let mut bar = ham();
+    let func = bar.next().unwrap();
+    return func(&"oof");
+}
+
+fn main() {
+    let _ = oof();
+}

src/test/ui/impl-trait/issues/issue-70877.stderr

@@ -0,0 +1,15 @@
+error[E0271]: type mismatch resolving `<Bar as Iterator>::Item == Box<(dyn for<'r> Fn(&'r (dyn ToString + 'r)) -> Option<String> + 'static)>`
+  --> $DIR/issue-70877.rs:9:12
+   |
+LL | type FooRet = impl std::fmt::Debug;
+   |               -------------------- the expected opaque type
+...
+LL | type Foo = impl Iterator<Item = FooItem>;
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected opaque type, found enum `Option`
+   |
+   = note: expected struct `Box<(dyn for<'r> Fn(&'r (dyn ToString + 'r)) -> impl Debug + 'static)>`
+              found struct `Box<(dyn for<'r> Fn(&'r (dyn ToString + 'r)) -> Option<String> + 'static)>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0271`.