Can't express "T : Foo<X1> implies T : Foo<X2>"

[vadimcn]

[Previously reported on rust-dev]

I'm trying to say "Any type that implements Foo<X1> also implements Foo<X2>":

trait Foo<T> {}
struct X1;
struct X2;

impl<T: Foo<X1>> Foo<X2> for T {}

fn main() {}

but this does not compile:

test.rs:5:1: 5:34 error: expected Foo<X2>, but found Foo<X1> (expected struct X2 but found struct X1)
test.rs:5 impl<T: Foo<X1>> Foo<X2> for T {}

This is supposedly related to Rust not distinguishing between different instantiations of generic traits, though you'd never guess that from the error message...

[vadimcn]

cc #5527

[steveklabnik]

This now compiles successfully.