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Failing guard in pattern match causes incorrect behavior of later matches #4482

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scabug opened this issue Apr 14, 2011 · 5 comments
Closed

Failing guard in pattern match causes incorrect behavior of later matches #4482

scabug opened this issue Apr 14, 2011 · 5 comments
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@scabug scabug commented Apr 14, 2011

=== What steps will reproduce the problem? ===
Have two traits, one of which has an abstract method. Write a pattern-match expression, as below, where you first match trait A (with a pattern guard), then trait B, then trait A again. If you pass in an object that extends both A and B, and does NOT pass the boolean guard, the second match is skipped and the third match is selected. If you remove the first match (with the pattern guard) the behavior is as expected.

Basically, the presence of one failing match is causing the following match to fail, which it shouldn't.

Welcome to Scala version 2.9.0.RC1 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_24).
Type in expressions to have them evaluated.
Type :help for more information.

scala> {
     |   trait Foo  {
     |     def i: Int
     |   }
     |   trait Bar
     | 
     |   case class Spam(i: Int) extends Foo with Bar
     | 
     |   def matchParent(p:Any) = p match {
     |     case f:Foo if f.i == 1 => 1
     |     case _:Bar => 2
     |     case _:Foo => 3
     |   }
     | 
     |   matchParent(Spam(3))
     | }
res0: Int = 3

scala> {
     |   trait Foo  {
     |     def i: Int
     |   }
     |   trait Bar
     | 
     |   case class Spam(i: Int) extends Foo with Bar
     | 
     |   def matchParent(p:Any) = p match {
     |     case _:Bar => 2
     |     case _:Foo => 3
     |   }
     | 
     |   matchParent(Spam(3))
     | }
res1: Int = 2

=== What is the expected behavior? ===
Both of the above expressions should resolve to 2.

=== What do you see instead? ===
The first expression resolves to 3, skipping the second match.

=== What versions of the following are you using? ===

  • Scala: 2.9.0.RC1 (also occurs in 2.8.1)
  • Java: 1.6.0_24
  • Operating system: OS X 10.6.7
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@scabug scabug commented Apr 14, 2011

Imported From: https://issues.scala-lang.org/browse/SI-4482?orig=1
Reporter: Stephen Judkins (stephenjudkins)
Attachments:

  • bug4482.patch (created on Apr 14, 2011 8:15:19 PM UTC, 876 bytes)
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@scabug scabug commented Apr 14, 2011

Stephen Judkins (stephenjudkins) said:
Patch adding failing test

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@scabug scabug commented Apr 14, 2011

Stephen Judkins (stephenjudkins) said:
The problem seems to be introduced in the "explicitouter" compiler stage:

    def matchParent(p: Any): Int = {
      <synthetic> def gd2(x$$1: com.wtf.Main.Foo): Boolean = x$$1.i().==(1);
      {
        <synthetic> val temp5: Any = p;
        if (temp5.isInstanceOf[com.wtf.Main.Foo]())
          {
            if (gd2(temp5.asInstanceOf[com.wtf.Main.Foo]()))
              {
                1
              }
            else
              {
                3
              }
          }
        else
          if (temp5.isInstanceOf[com.wtf.Main.Bar]())
            {
              2
            }
          else
            throw new MatchError(temp5)
      }
    };

It appears the generated code branches on whether it's an instance of "Foo" first, only branching based on the guard after it's determined it will match one of the "Foo" cases.

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@scabug scabug commented Apr 18, 2011

@magarciaEPFL said:
Yes, matchParent(Spam(3)) returns 3 when defined as

  def matchParent(p:Any) = p match {
    case f:Foo if f.i == 1 => 1
    case _:Bar => 2
    case _:Foo => 3
  }

However when compiled with -Ypmat-naive it returns 2.

The other option that comes to mind (-Yno-squeeze) makes compiled code return 3.

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@scabug scabug commented May 11, 2012

@soc said:
Fixed in scala/scala@4c04213

@scabug scabug closed this May 11, 2012
@scabug scabug added the patmat label Apr 7, 2017
@scabug scabug added this to the 2.10.0 milestone Apr 7, 2017
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