privacy: Fix private-in-public check for existential types

src/test/ui/privacy/private-in-public-existential.rs

@@ -0,0 +1,15 @@
+// compile-pass
+
+#![feature(existential_type)]
+#![deny(private_in_public)]
+
+pub existential type Pub: Default;
+
+#[derive(Default)]
+struct Priv;
+
+fn check() -> Pub {
+    Priv
+}
+
+fn main() {}