Patternmatch Unreachable Warning

[CyrilFMoser]

Compiler version

3.4.1

Minimized code

sealed trait T_A[A, B]
type X = T_A[
  Byte,
  Byte
]
case class CC_B[A](
    a: A
) extends T_A[A, X]
val v_a: T_A[X, X] = CC_B(null)

val v_b = v_a match {
  case CC_B(_) => 0
  case _       => 1
}

Output

The value of v_b is 0

Expectation

The compiler gives an unreachable warning for case CC_B(_), 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_A[A, B]
type X = T_A[
  Byte,
  Byte
]
case class CC_B[A](
    a: A
) extends T_A[A, X]
val v_a: T_A[X, X] = CC_B(null)

val v_b = v_a match {
  case CC_B(_) => 0
  case _       => 1
}
println(s"unreachable case v_b: $v_b")

scala-cli run repro-script.sc
Compiling project (Scala 3.4.2, JVM (17))
[warn] ./repro-script.sc:12:8
[warn] Unreachable case
[warn]   case CC_B(_) => 0
[warn]        ^^^^^^^
Compiled project (Scala 3.4.2, JVM (17))
unreachable case v_b: 0

[Gedochao]

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

• Patternmatch Unreachable Warning #20122
• Patternmatch Unreachable Warning #20123