test for reachable private impl

tests/ui/reachable/auxiliary/foreign-priv-aux.rs

@@ -0,0 +1,21 @@
+trait PrivTrait {
+    fn priv_fn(&self);
+}
+
+pub struct ImplPrivTrait;
+
+impl PrivTrait for ImplPrivTrait {
+    fn priv_fn(&self) {}
+}
+
+pub struct Wrapper<T>(T);
+
+pub trait PubTrait {
+    fn pub_fn(&self);
+}
+
+impl<T: PrivTrait> PubTrait for Wrapper<T> {
+    fn pub_fn(&self) {
+        self.0.priv_fn()
+    }
+}

tests/ui/reachable/foreign-priv.rs

@@ -0,0 +1,12 @@
+// aux-build:foreign-priv-aux.rs
+// build-pass
+
+#![crate_type = "lib"]
+
+extern crate foreign_priv_aux;
+
+use foreign_priv_aux::{ImplPrivTrait, PubTrait, Wrapper};
+
+pub fn foo(x: Wrapper<ImplPrivTrait>) {
+    x.pub_fn();
+}