default argument desugaring does not properly deal with patterns

[scabug]

[extempore says: this is an intersection of implicit + default + unapply. If we let it get past the NPE (which is from a Block not having a symbol) we see what has happened:

"foo" match {
  case {
    val x$$1: java.lang.String = <unapply-selector>;
    val x$$2: Option[String] = TestingObject.this.Matcher.unapply$$default$$2(x$$1);
    TestingObject.this.Matcher.unapply(x$$1)(x$$2)
  } <unapply> ((x @ _)) => scala.this.Predef.println(x)
};

There is nothing in place to deal with symbol-free Blocks popping up inside patterns.

• original report follows --

Here's the class being compiled.
It compiles fine if an implicit val is defined in main().

object TestingObject {

    "foo" match {
      case Matcher(result) => println(result)
    }

  object Matcher{
    def unapply(s: String)(implicit secondParam: Option[String] = None) = Some("Got: " + s + " and " + secondParam)
  }
}

Here's the error the compiler throws:

Exception in thread "main" java.lang.NullPointerException
	at scala.tools.nsc.typechecker.Unapplies$$class.unapplyTypeList(Unapplies.scala:30)
	at scala.tools.nsc.Global$$analyzer$$.unapplyTypeList(Global.scala:291)
	at scala.tools.nsc.typechecker.Typers$$Typer.doTypedApply(Typers.scala:2509)
	at scala.tools.nsc.typechecker.Typers$$Typer.doTypedApply(Typers.scala:2424)
	at scala.tools.nsc.typechecker.Typers$$Typer.typedApply$$1(Typers.scala:3285)
	at scala.tools.nsc.typechecker.Typers$$Typer.typed1(Typers.scala:3919)
	at scala.tools.nsc.typechecker.Typers$$Typer.typed(Typers.scala:4073)
	at scala.tools.nsc.typechecker.Typers$$Typer.typedPattern(Typers.scala:4161)
	at scala.tools.nsc.typechecker.Typers$$Typer.typedCase(Typers.scala:1904)
	at scala.tools.nsc.typechecker.Typers$$Typer$$$$anonfun$$typedCases$$1.apply(Typers.scala:1927)
	...

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-3353?orig=1
Reporter: Graham Lea (grlea)

[scabug]

@dubochet said:
Removed IDEA-specific bits of the stack trace. Assigned to Martin as this is a typer error.

[scabug]

@paulp said:
Reassigning to rytz with new description to get it back in the hopper.

[scabug]

@lrytz said:
OK, I see.. I'm not sure what to do.

Using default arguments in patterns is currently not supported, the fact that it's not disallowed for this extractor is rather a missed case, maybe we should disallow it for now.

There's a whole space to explore on the interaction of named/default arguments and pattern matching. If we start doing this, we will end up with this kind of blocks during pattern matching (and btw, blocks don't need to have symbols).

[scabug]

@hubertp said:
Just as a side note. Bug #4425 was closed because it no longer crashes, but the thing is it currently gives a wrong error message and is related to whatever is decided for this bug. So once we fix this bug, test for #4425 needs to be fixed as well.

[scabug]

@magarciaEPFL said:

Still crashing:

Welcome to Scala version 2.10.0-20120502-222338-24be606216 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_29).
Type in expressions to have them evaluated.
Type :help for more information.

scala> object TestingObject {
     |
     |     "foo" match {
     |       case Matcher(result) => println(result)
     |     }
     |
     |   object Matcher{
     |     def unapply(s: String)(implicit secondParam: Option[String] = None) = Some("Got: " + s + " and " + secondParam)
     |   }
     | }
error:
     while compiling:  <console>
       current phase:  typer
     library version:  version 2.10.0-20120502-222338-24be606216
    compiler version:  version 2.10.0-20120502-222338-24be606216
  reconstructed args:

uncaught exception during compilation: java.lang.NullPointerException
java.lang.NullPointerException
        at scala.tools.nsc.typechecker.Unapplies$class.unapplyTypeList(Unapplies.scala:28)
        at scala.tools.nsc.interpreter.ReplGlobal$$anon$1.unapplyTypeList(ReplGlobal.scala:24)
        at scala.tools.nsc.typechecker.Typers$Typer.doTypedApply(Typers.scala:3046)
        at scala.tools.nsc.typechecker.Typers$Typer.doTypedApply(Typers.scala:2994)
        at scala.tools.nsc.typechecker.Typers$Typer.typedApply$1(Typers.scala:4111)
        at scala.tools.nsc.typechecker.Typers$Typer.typed1(Typers.scala:4819)
        at scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:4992)
        at scala.tools.nsc.interpreter.ReplGlobal$$anon$1$$anon$2.typed(ReplGlobal.scala:29)
        at scala.tools.nsc.typechecker.Typers$Typer$$anonfun$typedPattern$1.apply(Typers.scala:5106)
        at scala.tools.nsc.typechecker.Typers$Typer$$anonfun$typedPattern$1.apply(Typers.scala:5106)
        at scala.tools.nsc.typechecker.Contexts$Context.withImplicitsDisabledAllowEnrichment(Contexts.scala:211)
        at scala.tools.nsc.typechecker.Typers$Typer.typedPattern(Typers.scala:5106)
        at scala.tools.nsc.typechecker.Typers$Typer.typedCase(Typers.scala:2156)
        at scala.tools.nsc.typechecker.Typers$Typer$$anonfun$typedCases$1.apply(Typers.scala:2206)
        at scala.tools.nsc.typechecker.Typers$Typer$$anonfun$typedCases$1.apply(Typers.scala:2205)
        at scala.collection.immutable.List.loop$1(List.scala:163)
        at scala.collection.immutable.List.mapConserve(List.scala:179)
        at scala.tools.nsc.typechecker.Typers$Typer.typedCases(Typers.scala:2205)
        at scala.tools.nsc.typechecker.Typers$Typer.typedMatch(Typers.scala:2220)

[scabug]

@adriaanm said:
I can fix this in the new pattern matcher, but first names-defaults has to support leaving the dummy in place for an unapply call with defaults
A reference to argDummy (see below) should not be extracted to a new local variable.

val argDummy  = context.owner.newValue(nme.SELECTOR_DUMMY, fun.pos, SYNTHETIC) setInfo pt

[scabug]

@adriaanm said (edited on Jan 29, 2013 1:01:14 AM UTC):

{
        val x$1: String = <unapply-selector>;
        val x$2: Option[String] = TestingObject.this.Matcher.unapply$default$2(x$1);
        TestingObject.this.Matcher.unapply(x$1)(x$2)
}

should be

{
        val x$1: Option[String] = TestingObject.this.Matcher.unapply$default$1();
        TestingObject.this.Matcher.unapply(<unapply-selector>)(x$1)
}

[scabug]

@adriaanm said:
in the mean time, an implementation restriction would be a good enough fix for 2.10.1-RC1

[scabug]

@lrytz said:
scala/scala#2043