Unreachable case involving path dependent types

[bishabosha]

Compiler version

3.0.0

Minimized code

object Examples {

  case class Leaf1() extends i.Root
  case class Leaf2() extends i.Branch

  val i = new Inner()

  class Inner {

    sealed trait Root
    sealed trait Branch extends Root

    // simulate ordinal method of a Mirror.SumOf generated at this call site
    def myOrdinal(r: Root): Int = r match {
      case _: Examples.Leaf1 => 0
      case _: Inner.this.Branch => 1
    }
  }

}

Output

-- [E030] Match case Unreachable Warning: example3.scala:16:11 -----------------
16 |      case _: Inner.this.Branch => 1
   |           ^^^^^^^^^^^^^^^^^^^^
   |           Unreachable case
1 warning found

Expectation

in Scala 2.13.6 we get the type error

cmd0.sc:14: pattern type is incompatible with expected type;
 found   : ammonite.$sess.cmd0.Examples.Leaf1
 required: Inner.this.Root
      case _: Examples.Leaf1 => 0
                       ^
Compilation Failed

[bishabosha]

the unreachable case warning seems confusing because it only appears when the Leaf1 case is above it, which is unrelated. It also does not seem to be related to the children of Branch - for example, this alternative method is accepted:

def myOrdinal(r: Root): Int = r match {
  case _: Examples.Leaf1 => 0
  case _: Examples.Leaf2 => 1
}

[liufengyun]

the unreachable case warning seems confusing because it only appears when the Leaf1 case is above it, which is unrelated

The problem is related to how we select qualified children of a sealed type. We need to tweak the subtyping so that Leaf2 is a qualified child of Inner.this.Branch despite that Leaf2 is not a subtype of Inner.this.Branch. I'll propose a fix.

[liufengyun]

Fixed as part of #12549.