~ > 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.