#![feature(specialization)]

use std::fmt::Debug;

fn foo(v :impl Sized) -> impl Sized {
    #[derive(Debug)]
    struct T<I>(I);
	T(v)
}

fn main() {
	trait MaybeDebug {
		fn maybe_debug(&self);
	}
	impl<T> MaybeDebug for T {
		default fn maybe_debug(&self) {
		    println!("Can't debug, sorry :(");
		}
	}
	impl<T: Debug> MaybeDebug for T {
		fn maybe_debug(&self) {
			println!("{:?}", self);
		}
	}
	foo(42).maybe_debug();
	struct NoDebug;
	foo(NoDebug).maybe_debug();
}