src/test/ui/extern/extern-types-trait-impl.rs

// run-pass
#![allow(dead_code)]
// Test that traits can be implemented for extern types.

#![feature(extern_types)]

extern {
    type A;
}

trait Foo {
    fn foo(&self) { }
}

impl Foo for A {
    fn foo(&self) { }
}

fn assert_foo<T: ?Sized + Foo>() { }

fn use_foo<T: ?Sized + Foo>(x: &dyn Foo) {
    x.foo();
}

fn main() {
    assert_foo::<A>();
}