Unhelpful error message for specialization-related type projection failures.

[rphmeier]

Playground: http://is.gd/QMg2a8

[rphmeier]

Maybe related to this comment: https://github.com/rust-lang/rust/blob/ffca6c3e/src/librustc/traits/project.rs#L71 and #32205?

edit: definitely related to the comment: http://is.gd/Qjh6tB

[rphmeier]

seems that this is by design? if so, there should be an error indicative of it.

[estebank]

Current output:

error[E0271]: type mismatch resolving `<(i32, f32) as Sameness>::Same == False`
  --> src/main.rs:49:5
   |
40 | fn not_same<A, B>() where (A, B): NotSame {}
   |                                   ------- required by this bound in `not_same`
...
49 |     not_same::<i32, f32>();
   |     ^^^^^^^^^^^^^^^^^^^^ expected associated type, found struct `False`
   |
   = note: expected associated type `<(i32, f32) as Sameness>::Same`
                       found struct `False`
   = help: consider constraining the associated type `<(i32, f32) as Sameness>::Same` to `False` or calling a method that returns `<(i32, f32) as Sameness>::Same`
   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
   = note: required because of the requirements on the impl of `NotSame` for `(i32, f32)`