E0119 errors are unhelpful when doing things involving types from another crate

[glandium]

I was trying to do something like this:

use std::io;

enum Foo<T> {
    Bar(T),
    Qux,
}

impl<T: PartialEq> PartialEq for Foo<T> {
    fn eq(&self, other: &Self) -> bool {
        match (self, other) {
            (&Foo::Bar(ref a), &Foo::Bar(ref b)) => a == b,
            (&Foo::Qux, &Foo::Qux) => true,
            (_, _) => false,
        }
    }
}

impl PartialEq for Foo<io::Error> {
    fn eq(&self, other: &Self) -> bool {
        unimplemented!()
    }
}

And got the following error as a result:

error: conflicting implementations of trait `std::cmp::PartialEq` for type `Foo<std::io::Error>`: [--explain E0119]
  --> <anon>:18:1
   |>
18 |> impl PartialEq for Foo<io::Error> {
   |> ^
note: conflicting implementation is here:
 --> <anon>:8:1
  |>
8 |> impl<T: PartialEq> PartialEq for Foo<T> {
  |> ^

error: aborting due to previous error

In and of itself, there is no straightforward explanation as to why this is happening, since io::Error doesn't implement PartialEq.

[gnzlbg]

I ran into this and it took me a while to figure out what is wrong with my code:

trait A {}
trait B {}

trait Foo<T> { } 

impl<T, U> Foo<T> for U: A { ... }
impl<T, U> Foo<T> for U: B { ... }

The only information that the error gives is that there are two conflicting implementations.

I would have understood the problem faster if the error message would tell me why are these two implementations conflicting.

[qnighy]

Seems to be a duplicate of #23980.

[Mark-Simulacrum]

Closing as a duplicate of #23980. (Thanks @qnighy!)