Can't provide non-overlapping trait impls based on assoc type trait bound

[cristicbz]

use std::iter::IntoIterator;

trait Foo {
    fn foo(&self);
}

trait Bar {
    fn bar(&self);
}

impl<I> Foo for I
        where I: IntoIterator,
              I::Item: Bar {
    fn foo(&self) {
    }
}

impl Foo for Vec<u8> {
    fn foo(&self) {
    }
}

fn main() {}

fails with

<anon>:11:1: 16:2 error: conflicting implementations for trait `Foo` [E0119]
<anon>:11 impl<I> Foo for I
<anon>:12         where I: IntoIterator,
<anon>:13               I::Item: Bar {
<anon>:14     fn foo(&self) {
<anon>:15     }
<anon>:16 }
<anon>:18:1: 21:2 note: note conflicting implementation here
<anon>:18 impl Foo for Vec<u8> {
<anon>:19     fn foo(&self) {
<anon>:20     }
<anon>:21 }

See playpen.

Since u8 does not implement Bar, the impl-s are actually non-overlapping.

Possibly related to #20400.

[kdeeee]

still occur this error inrustc 1.11.0 (9b21dcd6a 2016-08-15)
error message is conflicting implementations of trait 'Foo' for type 'std::vec::Vec<u8>'

[dhardy]

I've hit this bug too:

https://github.com/dhardy/rand/blob/master/src/distributions/default.rs#L41

[dtolnay]

Let's continue to track this under #20400 -- the resolution will surely be the same and it's better to keep the relevant discussion in one place.