Why scala does not unify this type lambda with underlying type?

Question

trait A {
  type T
  def test(t: T): Unit
}

case class B[S <: A](a: S, t : S#T) {
  def test() = a.test(t) // Error: type mismatch;
    // found   : B.this.t.typ         


        

Answer 1

I could come up with encodings (removed the type parameters for simplification):

scala> :paste
// Entering paste mode (ctrl-D to finish)

def test0(a: A)(t : a.T) = a.test(t) 

abstract class B{
  val a: A
  val t: a.T
  def test = a.test(t)
}

// Exiting paste mode, now interpreting.

test0: (a: A)(t: a.T)Unit
defined class B

This on the other hand didn't work with case classes arguments (nor classes' for that matter).

One of the reasons your encoding wouldn't work:

scala> def test1(a: A)(t : A#T) = a.test(t) 
:12: error: type mismatch;
 found   : t.type (with underlying type A#T)
 required: a.T
       def test1(a: A)(t : A#T) = a.test(t)

The important part is required: a.T (versus A#T). The test method in A doesn't take any T, it takes T this.T, or in other words, the T belonging to one particular instance of A.