Error message mentions type errors and non-existent type parameter on trait with an associated type

[ftxqxd]

#![feature(associated_types)]

trait Foo {
    type T;
    fn foo(&self) -> <Self as Foo>::T;
}

struct Bar;

impl Foo for Bar {
    type T = int;
    fn foo(&self) -> int { 1 }
}

fn main() {
    let x = box Bar as Box<Foo>;
    x.foo();
}

This fails to compile with:

assoc_typ_dyn.rs:16:28: 16:31 error: unsupported associated type binding
assoc_typ_dyn.rs:16     let x = box Bar as Box<Foo>;
                                               ^~~
assoc_typ_dyn.rs:16:13: 16:32 error: type mismatch: the type `Bar` implements the trait `Foo<int>`, but the trait `Foo<[type error]>` is required (expected type error, found int)
assoc_typ_dyn.rs:16     let x = box Bar as Box<Foo>;
                                ^~~~~~~~~~~~~~~~~~~
assoc_typ_dyn.rs:16:13: 16:32 note: the trait `Foo` must be implemented for the cast to the object type `Foo<[type error]>`
assoc_typ_dyn.rs:16     let x = box Bar as Box<Foo>;
                                ^~~~~~~~~~~~~~~~~~~

Not only does this mention type errors when it probably shouldn’t, but it also seems to be mistaking the associated type for a type parameter. Changing the cast to as Box<Foo<int>> as it seems to suggest doesn’t work, and the syntax from the RFC (Box<Foo<T = int>>) doesn’t seem to be implemented yet.

[ftxqxd]

The error message for this makes sense now, and the Box<Foo<T = i32>> syntax works now. I think this can be closed.