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

Compiler has not sufficient evidence that S#T can be used as argument for test in concrete instance.

Consider this hypotecical example for weakened scala compiler

trait A2 extends A{
  type T <: AnyRef
}

class A3 extends A2{
  override type T = Integer

  def test(t: Integer): Unit = println(t * 2)
}

So B[A2] should accept instance of A3 along with anything that is <: AnyRef while A3 needs exactly Integer for its own test implementation

You can catch concrete type in the definition of B, to make sure what type will be used

case class B[S <: A, ST](a: S {type T = ST}, t: ST) {
  def test() = a.test(t) 
}