warning: type alias `Foo` is more private than the item `foo`
  --> $DIR/privacy.rs:6:1
   |
LL | pub fn foo() -> Foo {
   | ^^^^^^^^^^^^^^^^^^^ function `foo` is reachable at visibility `pub`
   |
note: but type alias `Foo` is only usable at visibility `pub(crate)`
  --> $DIR/privacy.rs:5:1
   |
LL | type Foo = (impl Sized, u8);
   | ^^^^^^^^
   = note: `#[warn(private_interfaces)]` on by default

warning: 1 warning emitted