Scala: Lower type bound A without B

Question

I want to achieve this:

implicit def aConvertable(obj: A): B = new B(obj)
implicit def aConvertableWithoutB[T <: A without B](obj: T): C = new C(obj)

What is the correct way to say: "A without B"?

Answer 1

I'm assuming that B is a subtype of A. The trick is to introduce an implicit "AllowedConversion" trait that works for all A's, and then two implicit instances for B that have the same type, so that they're ambiguous for B:

object Foo {

  trait A
  class B(a: A) extends A

  class C(a: A) 


  class AllowedConversion[T]

  implicit def allowedForAs[T<:A] = new AllowedConversion[T]
  implicit def contradictoryForBs[T <: B] = new AllowedConversion[T]
  implicit def contradictoryForBs2[T <: B] = new AllowedConversion[T]


  implicit def aConvertable(obj: A): B = new B(obj)
  implicit def aConvertableWithoutB[T <: A](obj: T)(implicit allowed: AllowedConversion[T]): C = new C(obj)

  // error!
  // new B(new A{}):C

  // ok!
  new A{}:C

}