Why Scala doesn't deduce trait type parameters?

trait foo[F] {
  def test: F
}

class ah extends foo[(Int,Int) => Int] {
  def test = (i: Int,j: Int) => i+j
}

So the question is, why Scala known to be so smart about types cannot just deduce type (Int,Int) => Int from the type of test asking me instead to specify it? Or It is still possible? Or maybe this is backed by some thoughts that I don't have in mind.