RFC 1214 warnings with recursive associated traits

[Popog]

use std::ops::{Index};

trait Trait: Index<usize>
where <Self as Index<usize>>::Output: Sized {
    type Associated: Trait;
}

Incorrectly generates E0277 warning.

<anon>:3:1: 7:2 warning: the trait `core::marker::Sized` is not implemented for the type `<<<Self as Trait>::Associated as Trait>::Associated as core::ops::Index<usize>>::Output` [E0277]
<anon>:3 trait Trait: Index<usize>
<anon>:4 where <Self as Index<usize>>::Output: Sized,
<anon>:5 <Self::Associated as Index<usize>>::Output: Sized {
<anon>:6     type Associated: Trait;
<anon>:7 }

[brson]

Currently generates an error

rustc 1.17.0 (56124baa9 2017-04-24)

error[E0277]: the trait bound `<<Self as Trait>::Associated as std::ops::Index<usize>>::Output: std::marker::Sized` is not satisfied
 --> <anon>:3:1
  |
3 |   trait Trait: Index<usize>
  |  _^ starting here...
4 | | where <Self as Index<usize>>::Output: Sized {
5 | |     type Associated: Trait;
6 | | }
  | |_^ ...ending here: the trait `std::marker::Sized` is not implemented for `<<Self as Trait>::Associated as std::ops::Index<usize>>::Output`
  |
  = help: consider adding a `where <<Self as Trait>::Associated as std::ops::Index<usize>>::Output: std::marker::Sized` bound
  = note: required by `Trait`

error: aborting due to previous error

[nikomatsakis]

I think this is effectively a dup -- or at least a variant of -- #20671. The problem, I believe, is that the trait checker doesn't "elaboate" the requirement that type Assocated: Trait into the requirement that Output will be sized as well.