Partial substitution of trait associated types results in compiltion failure

[alexbool]

Consider a following playground:

#![crate_type = "lib"]

trait A<X> {
}

trait B {
    type X;
    type A: A<Self::X>;
}

fn oops<T: B<X=String>>(_: &T) {
}

It fails to compile with

   Compiling playground v0.0.1 (file:///playground)
error[E0277]: the trait bound `<T as B>::A: A<std::string::String>` is not satisfied
  --> src/lib.rs:11:1
   |
11 | / fn oops<T: B<X=String>>(_: &T) {
12 | | }
   | |_^ the trait `A<std::string::String>` is not implemented for `<T as B>::A`
   |
   = help: consider adding a `where <T as B>::A: A<std::string::String>` bound
   = note: required by `B`

error: aborting due to previous error

error: Could not compile `playground`.

To learn more, run the command again with --verbose.

[alexbool]

cc @nikomatsakis (or I don't know exactly who might help with this)

[nikomatsakis]

This is a normalization failure. I think the best way to fix this is moving towards lazy normalization (a move that is very much in progress).

[steveklabnik]

Triage: no change directly, this still fails to compile. I believe the lazy normalization work is ongoing.