Generic trait impl conflicts with all other impls

[sfackler]

~ > cat test.rs
trait Foo {}

trait Bar {}

impl<T: Bar> Foo for T {}

impl Foo for int {}
~ > rustc --lib test.rs
test.rs:7:0: 7:19 error: conflicting implementations for a trait
test.rs:7 impl Foo for int {}
          ^~~~~~~~~~~~~~~~~~~
test.rs:5:0: 5:25 note: note conflicting implementation here
test.rs:5 impl<T: Bar> Foo for T {}
          ^~~~~~~~~~~~~~~~~~~~~~~~~
test.rs:5:0: 5:25 error: conflicting implementations for a trait
test.rs:5 impl<T: Bar> Foo for T {}
          ^~~~~~~~~~~~~~~~~~~~~~~~~
test.rs:7:0: 7:19 note: note conflicting implementation here
test.rs:7 impl Foo for int {}
          ^~~~~~~~~~~~~~~~~~~
error: aborting due to 2 previous errors

I think this is related to #3429 but is far more general.

[pnkfelix]

subbug of #5527. I briefly tried to find whether this is a duplicate of another bug (besides #3429, which I think it is a dupe of, but this seems to be a simpler illustration of the issue so it may be worth keeping separately so that future bug seekers will identify it as for what they are looking).

[japaric]

This works after the multidispatch PR (#17669) landed and should be closed.

[alexcrichton]

Thanks @japaric!