Order of type parameter declarations matters

[catamorphism]

trait A {
  fn a<X, Y>(c: X);
}

struct B {
 x: int;
}

impl B: A {
  fn a<Y, X>(c: X) {}
}

This fails with:

/Users/TimChevalier/rust/src/test/run-pass/issue-2611-2.rs:15:2: 15:21 error: method `a` has an incompatible type: expected type parameter but found type parameter
/Users/TimChevalier/rust/src/test/run-pass/issue-2611-2.rs:15   fn a<Y, X>(c: X) {}
                                                                ^~~~~~~~~~~~~~~~~~~

I think it should pass, since simply reordering the ty param bindings doesn't change meaning. It would be kind of a pain to fix this right now since ty params are still positional.

[catamorphism]

(Separately, the "expected type parameter but found type parameter" in the error message is gratuitous.)

[nikomatsakis]

Indeed the order does matter. There are many subtle cases to consider and allowing arbitrary re-ordering is quite a complication. I'd be inclined to mark this as a Won't Fix, myself.

[catamorphism]

Ok, fair enough!