Patternmatch Unreachable Warning

[CyrilFMoser]

Compiler version

3.4.1

Minimized code

sealed trait T_B[C, D]
case class CC_A()
case class CC_B[A, C](a: A) extends T_B[C, CC_A]
case class CC_C[C, D](a: T_B[C, D])
    extends T_B[Int, CC_A]
case class CC_E(a: CC_C[Char, Byte])
val v_a: T_B[Int, CC_A] = CC_B(CC_E(CC_C(null)))
val v_b = v_a match {
  case CC_B(CC_E(CC_C(_))) => 0
  case _              => 1
}

Output

The value of v_b is 0

Expectation

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

[Gedochao]

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

// repro-script.sc
sealed trait T_B[C, D]
case class CC_A()
case class CC_B[A, C](a: A) extends T_B[C, CC_A]
case class CC_C[C, D](a: T_B[C, D])
    extends T_B[Int, CC_A]
case class CC_E(a: CC_C[Char, Byte])
val v_a: T_B[Int, CC_A] = CC_B(CC_E(CC_C(null)))
val v_b = v_a match {
  case CC_B(CC_E(CC_C(_))) => 0
  case _              => 1
}
println(s"unreachable case v_b: $v_b")

scala-cli run repro-script.sc
[warn] ./repro-script.sc:9:8
[warn] Unreachable case
[warn]   case CC_B(CC_E(CC_C(_))) => 0
[warn]        ^^^^^^^^^^^^^^^^^^^
unreachable case v_b: 0

[Gedochao]

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

• Patternmatch Unreachable Warning #20123
• Patternmatch Unreachable Warning #20121