Add regression test

src/test/ui/type-alias-impl-trait/auxiliary/collect_hidden_types.rs

@@ -0,0 +1,21 @@
+#![feature(type_alias_impl_trait)]
+
+// edition:2018
+
+use std::future::Future;
+
+pub trait Service<Request> {
+    type Future: Future<Output = ()>;
+    fn call(&mut self, req: Request) -> Self::Future;
+}
+
+// NOTE: the pub(crate) here is critical
+pub(crate) fn new() -> () {}
+
+pub struct A;
+impl Service<()> for A {
+    type Future = impl Future<Output = ()>;
+    fn call(&mut self, _: ()) -> Self::Future {
+        async { new() }
+    }
+}

src/test/ui/type-alias-impl-trait/collect_hidden_types.rs

@@ -0,0 +1,22 @@
+// aux-build:collect_hidden_types.rs
+use collect_hidden_types::Service;
+use std::future::Future;
+use std::pin::Pin;
+use std::task::Context;
+
+// build-pass
+
+// edition:2018
+
+extern crate collect_hidden_types;
+
+fn broken(mut a: collect_hidden_types::A, cx: &mut Context<'_>) {
+    let mut fut = a.call(());
+    let _ = unsafe { Pin::new_unchecked(&mut fut) }.poll(cx);
+}
+
+pub async fn meeb(cx: &mut Context<'_>) {
+    broken(collect_hidden_types::A, cx);
+}
+
+fn main() {}