Dotty: Wrong type constructor parameter inference of Nothing when using contra-variant implicit parameter.

[Sciss]

Minimized code

trait Measure[-C, M]

implicit object PathMeasure extends Measure[Long, (Int, Long)]

object FingerTree {
  def empty[V, A](implicit m: Measure[A, V]): FingerTree[V, A] = ???
}
trait FingerTree[V, +A] {
  def :+[A1 >: A](b: A1)(implicit m: Measure[A1, V]): FingerTree[V, A1]
}

trait Test {
  def readPathComponent(): Long

  def read(sz: Int): Any = {
    var tree  = FingerTree.empty(PathMeasure)  // problem origin
    var i = 0
    while (i < sz) {
      tree = tree :+ readPathComponent()   // problem shows here
      i += 1
    }
    ???
  }
}

Output

In the line 'problem shows here':

Found:    Long
Required: Nothing

Thus var tree has type FingerTree[?, Nothing] instead of FingerTree[(Int, Long), Long]

Expectation

Should not infer Nothing (no problem in Scala 2.13)

[griggt]

Minimized further, without implicits:

trait Foo[+A] {
  def foo[A1 >: A](a: A1): Foo[A1]
}

trait Bar[-B]

trait X {
  def bla[A](m: Bar[A]): Foo[A]

  def baz(x: Bar[Int]): Unit = {
    var t = bla(x)
    t = t.foo(0)
  }
}

[info] Compiling 1 Scala source to /src/dotty-issues/i9842/target/scala-0.27/classes ...
[error] -- [E007] Type Mismatch Error: /src/dotty-issues/i9842/src/main/scala/Main.scala:12:14 
[error] 12 |    t = t.foo(0)
[error]    |              ^
[error]    |              Found:    (0 : Int)
[error]    |              Required: Nothing

Compilation succeeds with Scala 2.13.3.

[odersky]

These are as intended. Scala 2 has a built-in "fear of Nothing": That is, it will never infer Nothing and will instead infer the upper bound. That leads to other problems when Nothing is the right type to infer and is an irregular exception. Scala 3 does not special-case Nothing in this way anymore. In

var t = bla(x)

The best type to infer for t is indeed Foo[Nothing]. If you want a wider type, you have to annotate the var.