op_ref false positive

[euclio]

With the following code: https://play.rust-lang.org/?gist=1aa5789d7082019e91bac7b093304ad5&version=stable

Clippy suggests removing the reference, but without the reference the Rust compiler complains that type annotations are needed.

I would expect the lint to not fire in this case.

[oli-obk]

That is so weird. I think this is rather a rustc bug... I'll investigate

[arielb1]

That's a trait-system issue. Operator overloading does succeed in this case. Without the autoref, you get these trait bounds:

u16: Add<$0>
u8: Into<$0>

Because the compiler only considers each trait bound alone, it can't see that the only solution is $0 = u16.

If you add a reference, instead yoou get

u16: Add<&'a $0>
u8: Into<$0>

And on the first bound, only the impl Add<&u16> for u16 impl matches, which allows a solution.