Send + Sync and Sync + Send are distinct valid types

[arielb1]

STR

#![feature(specialization)]

trait Teq<U:?Sized> {
    fn types_eq() -> &'static str;
}

impl<U:?Sized> Teq<U> for U {
    fn types_eq() -> &'static str { "types are equal" }
}

impl<U:?Sized, V:?Sized> Teq<U> for V {
    default fn types_eq() -> &'static str { "types differ" }
}

fn main() {
    println!("{}", <Send+Sync as Teq<Sync+Send>>::types_eq());
}

Expected Result

This should not compile, or should print "types are equal"

Actual Result

This prints "types differ".

[arielb1]

I think this should be fixed to not compiling, right?

[durka]

Why should they be different?

[arielb1]

They should be the same type (rather, this code should not compile) The bug is that they are different.

[durka]

I still don't understand, sorry... what compile error do you expect? If they were the same type, it would print "types are equal"? Are you saying that neither of them should be accepted as a type?

[arielb1]

@durka

I am going to write a PR that will make neither of them be accepted as a type.

[durka]

Okay... maybe in the PR you can explain why, as I keep asking? FWIW, Any + Send + Sync and Any + Sync + Send are equal using your code above.

[arielb1]

The problem type is Sync + Send without any additional trait. Any + Sync + Send is a perfectly good type, and will not be removed.

[durka]

Is it because they are both marker traits? Or both auto traits?

[cyplo]

Hello ! Any changes on this one ?

[RalfJung]

The two types differing it at least consistent with this being rejected due to a type mismatch:

fn foo(x: Box<Send+Sync>) -> Box<Sync+Send> { x }