Variance trick with path-dependent types

Here's another one for implicits and path dependent types. I don't understand why I need to be so verbose here: (Note -- I found the answer, see below)

trait B
trait C[X]
trait A { def call[B1 <: B](implicit b: B1): C[B1] }
trait D extends B {
  def set(c: C[this.type])开发者_JAVA技巧: Unit
}

first try:

def test1(a: A)(implicit d: D: Unit =
  d.set(a.call) // found C[D] -- required C[d.type]

second try:

def test2(a: A)(implicit d: D): Unit =
  d.set(a.call[d.type]) // could not find implicit value for parameter b: d.type

third try:

def test3(a: A)(implicit d: D): Unit =
  d.set(a.call[d.type](d))  // works. why so much clutter!?

---

the Scala 2.9 REPL helps us (thanks, whoever added this useful message!). Here for the test1:

 found   : C[D]
 required: C[d.type]
Note: D >: d.type, but trait C is invariant in type X.
You may wish to define X as -X instead. (SLS 4.5)
              d.set( a.call ) // found C[D] -- required C[d.type]
                       ^

Thus: changing trait C[ X ] to trait C[ -X ] makes test1 work as expected.

---

Are you sure you want this.type? Scala's "this type" is not the same as what is more commonly known as "MyType". See this discussion on this.type, as well as the discussion linked by the MyType question.