--- /dev/null
+#![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();
+}
--- /dev/null
+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`.