Patternmatch Unreachable Warning

[CyrilFMoser]

Compiler version

3.4.1

Minimized code

sealed trait T_A[A, B]
sealed trait T_B[C]
case class CC_D[A, C]() extends T_A[A, C]
case class CC_E() extends T_B
case class CC_G[A, C](c: C) extends T_A[A, C]
val v_a: T_A[Boolean, T_B[Boolean]] = CC_G(null)
val v_b = v_a match {
  case CC_D()  => 0
  case CC_G(_) => 1
}

Output

The value of v_b is 1

Expectation

The compiler gives an unreachable warning for case CC_G(_) , so it should not be able to reach that case.

[Gedochao]

The unreachable case indeed is reached when this similar repro is run:

sealed trait T_A[A, B]
sealed trait T_B[C]
case class CC_D[A, C]() extends T_A[A, C]
case class CC_E() extends T_B
case class CC_G[A, C](c: C) extends T_A[A, C]
@main def main = {
  val v_a: T_A[Boolean, T_B[Boolean]] = CC_G(null)
  val v_b = v_a match {
    case CC_D()  => 0
    case CC_G(_) => 1
  }
  println(s"the unreachable case v_b: $v_b")
}

scala-cli run repro.scala               
# Compiling project (Scala 3.4.2, JVM (17))
# [warn] ./repro.scala:10:10
# [warn] Unreachable case
# [warn]     case CC_G(_) => 1
# [warn]          ^^^^^^^
# Compiled project (Scala 3.4.2, JVM (17))
# the unreachable case v_b: 1

[Gedochao]

Very similar issues which were reported together with this, potentially duplicates:

• Patternmatch Unreachable Warning #20122
• Patternmatch Unreachable Warning #20121