Case classes with additional custom unapply produce pattern match warning

[jbwheatley]

reproduction steps

using Scala 2.13.4, I have a case class with an additional custom unapply method.

final case class Foo(value: String)

object Foo {
  def unapply(str: String): Option[Foo] = Some(Foo(str))

  def extract(id: Foo): String =
    id match {
      case Foo(a) => a
    }
}

problem

Gives a warning:

match may not be exhaustive.
[warn] It would fail on the following input: Foo(_)
[warn]     id match {
[warn]     ^

Adding a further unapply method as suggested here #12232 did make the warning go away:

def unapply(arg: Foo): Some[String] = Some(arg.value)

However, the discussion on that issue seemed to point to not using case classes as the root of the problem. It seems like a bit of a gotcha that adding another unapply method breaks the out-of-the-box pattern matching for case classes.

[joroKr21]

If I'm not mistaken the custom unapply is not additional but completely replaces the generated one.

[jbwheatley]

The generated unapply still works: https://scastie.scala-lang.org/u7WW2ykKTKu9KUukQUaZWg

Edit: And I've been happily using this pattern on scala 2.13.3 without the warning, and without adding

def unapply(arg: Foo): Some[String] = Some(arg.value)

[joroKr21]

Oh sorry I didn't notice the signature in your original example is swapped:

  def unapply(str: String): Option[Foo] = Some(Foo(str))

[dwijnand]

Sadly it looks like making that extract-and-convert unapply irrefutable doesn't fix this:

def unapply(str: String): Some[Foo] = Some(Foo(str))

[jbwheatley]

@dwijnand Yeah I had also tried that 😕