Associated types should be permitted in impl headers

[carllerche]

Code

#![feature(associated_types)]

pub trait Foo {
    type Bar: Sized;

    fn invoke(&self, bar: <Self as Foo>::Bar);
}

pub struct Wrap<F, B> {
    foo: F,
    bar: B,
}

impl<F: Foo> Wrap<F, <F as Foo>::Bar> {
    // Does not compile  ^^
}

pub fn main() {
}

Output

$ rustc at.rs
at.rs:14:22: 14:37 error: this associated type is not allowed in this context
at.rs:14 impl<F: Foo> Wrap<F, <F as Foo>::Bar> {
                              ^~~~~~~~~~~~~~~
error: aborting due to previous error

[nikomatsakis]

While I really want this to work, I think it will be relatively low priority. Once there is an explicit syntax for naming associated types (#18432), it can be worked around relatively easily, and to do this right is quite challenging (resolving <T as Foo>::Bar requires trait resolution; but here we have to do it as part of the impl, so there is a cyclic dependency). Given the existence of a workaround, I'm personally not inclined to hack something up but rather just to wait and fix the general issue.

[nikomatsakis]

These are now permitted but don't, I think, work correctly most of the time.

[nikomatsakis]

PR #20757 more or less fixes this.

[steveklabnik]

That PR was merged, and this does compile today.