Add regression test

tests/ui/type-alias-impl-trait/hidden_behind_struct_field3.rs

@@ -0,0 +1,28 @@
+//! This test demonstrates a bug where we accidentally
+//! detected opaque types in struct fields, but only if nested
+//! in projections of another opaque type.
+//@ check-pass
+
+#![feature(impl_trait_in_assoc_type)]
+
+struct Bar;
+
+trait Trait: Sized {
+    type Assoc2;
+    type Assoc;
+    fn foo() -> Self::Assoc;
+}
+
+impl Trait for Bar {
+    type Assoc2 = impl std::fmt::Debug;
+    type Assoc = impl Iterator<Item = Foo>;
+    fn foo() -> Self::Assoc {
+        vec![Foo { field: () }].into_iter()
+    }
+}
+
+struct Foo {
+    field: <Bar as Trait>::Assoc2,
+}
+
+fn main() {}