Non-specific message: "not a valid result type of an unapply method of an extractor"

[dsbos]

(Some) errors with extractor unapply methods yield only a general "not a valid result type of an unapply method of an extractor" message, which doesn't really help the user identify and fix whatever the actual error is.

Compiler version

3.2.2

Minimized example

object ExtractorsMessages {

  {  // Works fine:
    case class TypicalCaseClass(x: String)

    object Extractor:
      def unapply(x: String): TypicalCaseClass = ???

    (null: AnyRef) match
      case Extractor(out1) =>
  }

  {
    case class NoArgCaseClass()

    object Extractor:
      def unapply(x: String): NoArgCaseClass = ???

    (null: AnyRef) match
      // *** On following line, compiler reports just "NoArgCaseClass is not a
      // valid result type of an unapply method of an extractor" (neither
      // identifying the specific condition(s) that failed to be met (in this
      // case, having at least one product element (right?)) nor stating the
      // conditions that need to be met (even it can't/doesn't identify which
      // part failed to be met)):
      case Extractor() =>
  }

  { // Attempt at creating non-case class with "manual" additions to make it
    // sufficient to use with an extractor:

    class AttemptedPlainClass(val x: String) extends Product
    object AttemptedPlainClass {
      def apply(x: String): AttemptedPlainClass = ???
    }

    object Extractor:
      def unapply(x: String): AttemptedPlainClass = ???

    (null: AnyRef) match
      // *** Again, compiler just says "... is not a valid result type of an
      // unapply method of an extractor."  (In this case, I don't yet know why
      // it's not valid (presumbly something case classes have that I forgot to
      // provide for my non-case class), since the compiler didn't say anything
      // more and since
      // https://docs.scala-lang.org/scala3/reference/changed-features/pattern-matching.html#product-match
      // seems quite unclear):

      case Extractor(out1) =>
  }

}

Output

At case Extractor(out1) => lines annotated with comments above, compiler errors say:
NoArgCaseClass is not a valid result type of an unapply method of an extractor.
and
AttemptedPlainClass is not a valid result type of an unapply method of an extractor.

Expectation

(If not a valid expectation, then at least a hope/wish/eventual improvement:) That the compiler would be more specific about why the results types are not valid.

(FYI, I was exploring Scala 3 extractors, trying to follow the documentation at https://docs.scala-lang.org/scala3/reference/changed-features/pattern-matching.html, which seems a little unclear.)

[nicolasstucki]

The error states

-- [E108] Declaration Error: i17077.scala:26:20 -----------------------------
26 |      case Extractor() =>
   |           ^^^^^^^^^^^
   | NoArgCaseClass is not a valid result type of an unapply method of an extractor.
   |
   | longer explanation available when compiling with `-explain`

If we run it with -explain we get

-- [E108] Declaration Error: i 17077.scala:26:20 -----------------------------
26 |      case Extractor() =>
   |           ^^^^^^^^^^^
   | NoArgCaseClass is not a valid result type of an unapply method of an extractor.
   |----------------------------------------------------------------------------
   | Explanation (enabled by `-explain`)
   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
   |
   | To be used as an extractor, an unapply method has to return a type that either:
   |  - has members isEmpty: Boolean and get: S (usually an Option[S])
   |  - is a Boolean
   |  - is a Product (like a Tuple2[T1, T2])
   |
   | class A(val i: Int)
   |
   | object B {
   |   def unapply(a: A): Option[Int] = Some(a.i)
   | }
   |
   | object C {
   |   def unapply(a: A): Boolean = a.i == 2
   | }
   |
   | object D {
   |   def unapply(a: A): (Int, Int) = (a.i, a.i)
   | }
   |
   | object Test {
   |   def test(a: A) = a match {
   |     case B(1) => 1
   |     case a @ C() => 2
   |     case D(3, 3) => 3
   |   }
   | }
   |       
    ----------------------------------------------------------------------------

[nicolasstucki]

I noticed that scala-cli does not show the -explain part of the message. Also not sure how to pass that flag to it.

[nicolasstucki]

Note that in the -explain:

• We do not explain why a case class without arguments does not work. Are those supposed to work?
• We do not explain that products require the _1, ..._N members

[nicolasstucki]

The short error message should state which of the conditions was missing if the result class does match some of the constraints. For example, if we have a Product but miss the _1 methods, we could include this detail in the message.

[nicolasstucki]

The empty case class is probably not a valid result because we expect a case object in those situations.

[prolativ]

@nicolasstucki regarding -explain & scala-cli: VirtusLab/scala-cli#1285
TLDR: This is something that couldn't be fixed directly in scala-cli so it was fixed in the compiler instead (fix available in nightly). As a workaround working with any compiler version one needs to use --server=false