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Scala pattern match is not exhaustive on nested case classes #9630

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scabug opened this issue Jan 25, 2016 · 7 comments
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Scala pattern match is not exhaustive on nested case classes #9630

scabug opened this issue Jan 25, 2016 · 7 comments
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@retronym
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patmat
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2.12.0-M4

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@scabug
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scabug commented Jan 25, 2016

Hi,

I've got a case class hierarchy to encode some request and processing errors:

sealed trait OpError
sealed trait RequestErrorType
sealed trait ProcessingErrorType

final case class InvalidEndpoint(reason: String) extends RequestErrorType
final case class InvalidParameters(reason: String) extends RequestErrorType

final case class InvalidFormat(response: String) extends ProcessingErrorType
final case class EntityNotFound(id: Long) extends ProcessingErrorType

final case class RequestError(errorType: RequestErrorType) extends OpError
final case class ProcessingError(errorType: ProcessingErrorType) extends OpError

If I write a simple match across all patterns:

def printMatches(error: OpError): Unit = error match {
  case RequestError(InvalidEndpoint(reason)) => //print something
  case RequestError(InvalidParameters(reason)) => //print something
  case ProcessingError(InvalidFormat(format)) => //print something
  case ProcessingError(EntityNotFound(entityId)) => //print something
}

the compiler gives me a warning about missing match:

match may not be exhaustive.
 It would fail on the following input: ProcessingError(_)
 def printMatches(error: OpError): Unit = error match {

ProcessingError takes in a ProcessingErrorType with only two extensions: InvalidFormat and EntityNotFound, both which are accounted for in the pattern match.
@scabug
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scabug commented Jan 25, 2016

Imported From: https://issues.scala-lang.org/browse/SI-9630?orig=1
Reporter: sanjiv sahayam (sanjsmailbox-at-gmail.com)

@scabug
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scabug commented Jan 25, 2016

sanjiv sahayam (sanjsmailbox-at-gmail.com) said:
Could be linked to scala/scala#2737 as per @Retronyms suggestion.

@scabug
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scabug commented Jan 25, 2016

@retronym said (edited on Jan 25, 2016 9:26:50 AM UTC):
A workaround appears to be to chose different names for the field of RequestError and ProcessingError.

Here's a slightly minimized version:

sealed trait Base
final case class Base_1(sameName: Some[Any]) extends Base
final case class Base_2(sameName: Nested) extends Base

sealed trait Nested
final case class Nested_1(x: Any) extends Nested
final case class Nested_2(y: Any) extends Nested

class Test {
  def test(b: Base): Unit = b match {
    case Base_1(Some(_)) =>
    case Base_2(Nested_1(_)) =>
    case Base_2(Nested_2(_)) =>
  }
}

@scabug
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scabug commented Jan 25, 2016

@retronym said:
Here's a potential fix: https://github.com/scala/scala/compare/2.12.x...retronym:ticket/9630?expand=1

I'll discuss this bug with @adriaanm to see if that's the right way forward.

@scabug
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scabug commented Jan 25, 2016

sanjiv sahayam (sanjsmailbox-at-gmail.com) said:
Thanks for looking into this Jason. I can verify that the workaround works! :)

It does seem weird that the name of a parameter would lead to this bug.

@scabug
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scabug commented Jan 25, 2016

@retronym said:
It seems that that strangeness is the root cause for a few different bugs (including some mentioned by [~japgolly] on your mailing list thread.

See the additional test cases in https://github.com/scala/scala/compare/2.12.x...retronym:ticket/9630?expand=1 for more details.

Thanks for your report, which triggered me to look at these again with fresh eyes.

@scabug
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scabug commented Jan 27, 2016

@retronym said:
scala/scala#4919

@scabug scabug closed this as completed Jan 29, 2016
@scabug scabug added the patmat label Apr 7, 2017
@scabug scabug added this to the 2.12.0-M4 milestone Apr 7, 2017
@didii didii mentioned this issue Jun 16, 2017
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