No warning is triggered for non-exhaustive pattern matching on case class field

[kevin-lee]

Compiler version

3.0.0-RC3 from print scalaVersion on sbt console.

Minimized code

sealed trait Status
object Status {
  case object Active   extends Status
  case object Inactive extends Status
}

final case class Foo(status: Status)
def bar(user: Foo): Unit = user match {
  case Foo(Status.Active) =>
    println("active")
    // no compile-time warning for missing Status.Inactive case
}

scalacOptions has

List(
  "-unchecked",
  "-deprecation",
  "-feature",
  "-Xfatal-warnings"
)

NOTE:

• Replacing the Status above with the enum like below doesn't make any difference.

  enum Status {
    case Active
    case Inactive
  }

• final case class (i.e. final case class Foo(status: Status)) makes no difference.

Output

It compiles without any warning or error even with -Xfatal-warnings, and it results in the following runtime error.

[error] (run-main-1) scala.MatchError: Foo(Inactive) (of class kevinlee.Example1$Foo)
[error] scala.MatchError: Foo(Inactive) (of class kevinlee.Example1$Foo)
...

Expectation

The compile-time warning like

match may not be exhaustive.
It would fail on the following input: Foo(Inactive)
    def bar(user: Foo): Unit = user match {

(or the compile-time error with "-Xfatal-warnings" compiler option)

[liufengyun]

Exhaustivity check is not triggered since Foo is not sealed.

Changing final case class Foo to sealed case class Foo will get the warning:

sealed trait Status
object Status {
  case object Active   extends Status
  case object Inactive extends Status
}

sealed case class Foo(status: Status)
def bar(user: Foo): Unit = user match {
  case Foo(Status.Active) =>
    println("active")
    // no compile-time warning for missing Status.Inactive case
}

-- [E029] Pattern Match Exhaustivity Warning: examples/i12337.scala:8:27 -------
8 |def bar(user: Foo): Unit = user match {
  |                           ^^^^
  |                           match may not be exhaustive.
  |
  |                           It would fail on pattern case: Foo(Inactive)

[kevin-lee]

@liufengyun Is it a changed behavior in Scala 3? The exactly the same code as mine in Scala 2 triggers non-exhaustive warning.

[liufengyun]

@liufengyun Is it a changed behavior in Scala 3? The exactly the same code as mine in Scala 2 triggers non-exhaustive warning.

I don't think it's changed intentionally. It's just not well specified whether we should perform more checks in the absence of sealed. Given that it's supported in Scala 2, we could make Scala 3 to align with Scala 2 behavior.

[kevin-lee]

@liufengyun Here is the same code in Scala 2.13.5. As you can see, the non-exhaustive warning is triggered.
https://scastie.scala-lang.org/AXKSl4K5S26cPjTE8b1kdw
https://scastie.scala-lang.org/KoU8DeZvTuKMlTfHFmHYeg (updated with non-final case class)

[som-snytt]

final is also not required to see the warning in Scala 2. All that is required is that it walk like a duck.

[kevin-lee]

@som-snytt Thanks. Yeah I forgot to mention that it triggers the warning in Scala 2 without final.
I probably shouldn't have put final there as it may cause confusion. I'll fix the example code. Thanks @som-snytt.