#![feature(specialization)]

use std::fmt::Debug;

trait CanDoFoo {
	fn do_foo(&mut self);
}

fn foo() -> impl Debug {
    #[derive(Debug)]
    struct T;
    // Whether this impl is commented out or not decides whether hi is printed.
    // This documents impl Trait leakage of a property of the returned type
    /*
	impl CanDoFoo for T {
		fn do_foo(&mut self) {
			println!("hi");
		}
	}
	*/
	T
}

fn main() {
	trait MaybeDoFoo {
		fn maybe_do_foo(&mut self);
	}
	impl<T> MaybeDoFoo for T {
		default fn maybe_do_foo(&mut self) { }
	}
	impl<T: CanDoFoo> MaybeDoFoo for T {
		fn maybe_do_foo(&mut self) {
			self.do_foo();
		}
	}
	foo().maybe_do_foo();
}