The trait object Foo+Copy implements the trait Copy

[arielb1]

select considers the trait object type Foo+Copy to implement the trait Copy, and therefore its supertraits Clone and Sized. This is totally bogus, as the trait object type is not Sized.

For example:

use std::{fmt, mem};

fn size_of_copy<T: Copy+?Sized>() -> usize { mem::size_of::<T>() }

fn main() {
    println!("{}", size_of_copy::<fmt::Debug+Copy>());
}

This obviously should not compile, but it does (and prints 16, because of the size_of unsized hack).

[arielb1]

cc @nikomatsakis

[nikomatsakis]

@arielb1 indeed.

[nikomatsakis]

triage: P-high

[arielb1]

I am working on a fix.

[nikomatsakis]

@arielb1 saw your ping on IRC; it seems like we simply should consider Copy to be object-unsafe. Ideally with a warning period.

[nikomatsakis]

(I've always thought that unsized types would be move-only.)