Scala 2.11 Type Variance Changes

Question

In Scala 2.10.4 this compiles:

trait Foo[-U,T]{
  type Contra = U
}

but in 2.11.0 the same fails with:

contravariant type U occurs in invariant position in type U of type Contra trait Foo[-U,T] {type Contra = U}

Is there a workaround available? Trying to port over a Scala library to 2.11 and the contravariant type is required in order to get a boatload of implicit defs picked up by the compiler (i.e. making U invariant doesn't seem to be an option).

Thanks

Answer 1

I can't imagine there being a work around available. The reason I say this is all about supporting path-dependent types:

 def foo[T <: Foo[A,B]](that: T): that.Contra

which places the Contra type in the wrong position. You can not return a contravariant type as a result of an operation. Perhaps the search and validation of these requires so much work that the compiler authors decided this small corner case created too much of a burden or it is a compiler bug that you've uncovered.

By the way, this is just wild speculation on my part. I'd have to read the compiler code to figure out which way is which.