gistfile1.txt

object Wat {
  trait TypeSum[T, U] {
    type SumType

    def add(t: T, u: U): SumType
  }

  implicit def unitLeft[T]: TypeSum[Unit, T] = new TypeSum[Unit, T] {
    type SumType = T
    def add(u: Unit, t: T) = t
  }

  implicit def unitRight[T]: TypeSum[T, Unit] = new TypeSum[T, Unit] {
    type SumType = T
    def add(t: T, u: Unit) = t
  }

  implicit def anySum[T, X]: TypeSum[T, X] = new TypeSum[T, X] {
    type SumType = (T, X)

    def add(t: T, x: X) = (t, x)
  }

  def combine[A, B](a: A, b: B)(implicit ta: TypeSum[A, B]) = ta.add(a,b)

  assert(combine(combine((), 3), ()) == 3) // Not a problem

  val t = combine(combine((), 3), ()) // Doesn't compile like a boss. 
}

% scalac -Xexperimental bip.scala 
% scala -e 'println(Wat.t)'
3