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Pattern-matcher spuriously spouts "unreachable code" #9411

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scabug opened this issue Jul 22, 2015 · 3 comments
Closed

Pattern-matcher spuriously spouts "unreachable code" #9411

scabug opened this issue Jul 22, 2015 · 3 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 Jul 22, 2015

In following snippet:

object OhNoes {

  sealed trait F
  sealed abstract class FA extends F
  sealed abstract class FB extends F

  case object FA1 extends FA
  case object FB1 extends FB
  case object FB2 extends FB

  sealed trait G
  case object G1 extends G
  case object G2 extends G

  sealed trait H
  case class H1(a: FB, b: G) extends H
  case class H2(a: F)        extends H

  val demo: H => Unit = {
    case H1(FB1, G1) =>
    case H1(FB2, G2) =>
    case H2(_: FB) =>
    case H2(_: FA) =>
    case H1(FB1, G2) =>
    case H1(FB2, G1) =>
  }
}

the naughty little compiler lies and reports:

[warn] /tmp/ah/demo.scala:23: unreachable code
[warn]     case H2(_: FA) =>

Very interestingly, H1 and H2 both having a parameter with the same name seems to be pertinent. If you rename H2's "a" to "b" you get a different (incorrect) warning:

[warn] It would fail on the following input: H2(_)
[warn]   val demo: H => Unit = {

Renaming H2's "a" to "x" the warnings disappear.

Finally, rearranging the order of cases produces different, incorrect warnings, likely related to #9399 and #9398. Example:

  val demo2: H => Unit = {
    case H2(_: FA) =>
    case H2(_: FB) =>
    case H1(FB1, G1) =>
    case H1(FB2, G1) =>
    case H1(FB1, G2) =>
    case H1(FB2, G2) =>
  }

produces

[warn] It would fail on the following inputs: H1(_, G1), H1(_, G2)
[warn]   val demo2: H => Unit = {
@scabug
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scabug commented Jul 22, 2015

Imported From: https://issues.scala-lang.org/browse/SI-9411?orig=1
Reporter: David Barri (japgolly)
Affected Versions: 2.11.7
See #9398, #9399

@scabug
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scabug commented Jan 25, 2016

@retronym said:
This one is also fixed in https://github.com/scala/scala/compare/2.12.x...retronym:ticket/9630?expand=1

@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
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