[Invalid accepted] E0568 can be bypassed by `where Self`

[codehz]

I tried this code:

#![feature(auto_traits)]

auto trait CanCopy where Self: Copy {}

I expected to see this happen: got E0568 because the unstable book say supertraits for auto trait are unsound

Instead, this happened: Compiler accepted those code

Playground link:
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=57126ad5d71bd5bbaa913416b70fae67

Meta

rustc --version --verbose:

1.53.0-nightly

(2021-04-09 dae9d6ac3e9a8fb389cf)
(rust playground)

nightly feature tracking issue: #13231

[nbdd0121]

A slight tweak with the example in #29859, this is indeed unsound.

#![feature(auto_traits)]

auto trait Magic where Self: Copy {}
impl<T: Magic> Magic for T {}

fn copy<T: Magic>(x: T) -> (T, T) { (x, x) }

#[derive(Debug)]
struct NoClone;

fn main() {
    let (a, b) = copy(NoClone);
    println!("{:?} {:?}", a, b);
}

This compiles while it shouldn't.

@rustbot label: +requires-nightly

[rustbot]

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[apiraino]

Assigning priority as discussed as part of the Prioritization Working Group procedure and removing I-prioritize.

edit: upgrading to p-high because by looking at the code samples, the unsoundness looks easy to trigger

@rustbot label -I-prioritize +P-high