Expanding a pattern match leads to "head of empty list"

[b-studios]

When trying to compile this syntactically correct, but nonsensical program the compiler crashes:

object Bug {
  val x = 42
  x match {case {42}.toString => 42}
}

Beginning of the trace:

exception while transforming Bug.x match
  {
    case 42.toString() => 42
  } of class class dotty.tools.dotc.ast.Trees$Match # 79
exception while transforming () extends Object() {
  val x: Int = 42
  Bug.x match
    {
      case 42.toString() => 42
    }
} of class class dotty.tools.dotc.ast.Trees$Template # 81
exception while transforming @scala.annotation.internal.SourceFile("examples/bug.scala") final module class
  Bug$
() extends Object() {
  val x: Int = 42
  Bug.x match
    {
      case 42.toString() => 42
    }
} of class class dotty.tools.dotc.ast.Trees$TypeDef # 82
exception while transforming package <empty> {
  final lazy module val Bug: Bug$ = new Bug$()
  @scala.annotation.internal.SourceFile("examples/bug.scala") final module class

  Bug$() extends Object() {
    val x: Int = 42
    Bug.x match
      {
        case 42.toString() => 42
      }
  }
} of class class dotty.tools.dotc.ast.Trees$PackageDef # 83

exception occurred while compiling examples/bug.scala
Exception in thread "main" java.util.NoSuchElementException: head of empty list
	at scala.collection.immutable.Nil$.head(List.scala:420)
	at scala.collection.immutable.Nil$.head(List.scala:417)
	at dotty.tools.dotc.transform.PatternMatcher$Translator$ScalacPatternExpanders$ScalacPatternExpander$class.applyMethodTypes(PatternMatcher.scala:1863)
	at dotty.tools.dotc.transform.PatternMatcher$Translator$ScalacPatternExpanders$alignPatterns$.applyMethodTypes(PatternMatcher.scala:1904)
	at dotty.tools.dotc.transform.PatternMatcher$Translator$ScalacPatternExpanders$alignPatterns$.apply(PatternMatcher.scala:1955)
	at dotty.tools.dotc.transform.PatternMatcher$Translator$ScalacPatternExpanders$alignPatterns$.apply(PatternMatcher.scala:1978)
	at dotty.tools.dotc.transform.PatternMatcher$Translator$MatchTranslator$ExtractorCall$.apply(PatternMatcher.scala:1412)
	at dotty.tools.dotc.transform.PatternMatcher$Translator$MatchTranslator$BoundTree.extractor$lzycompute(PatternMatcher.scala:1127)
	at dotty.tools.dotc.transform.PatternMatcher$Translator$MatchTranslator$BoundTree.extractor(PatternMatcher.scala:1127)
	at dotty.tools.dotc.transform.PatternMatcher$Translator$MatchTranslator$BoundTree.paramType$1(PatternMatcher.scala:1185)
	at dotty.tools.dotc.transform.PatternMatcher$Translator$MatchTranslator$BoundTree.extractorStep(PatternMatcher.scala:1201)
	at dotty.tools.dotc.transform.PatternMatcher$Translator$MatchTranslator$BoundTree.nextStep(PatternMatcher.scala:1222)
	at dotty.tools.dotc.transform.PatternMatcher$Translator$MatchTranslator$BoundTree.translate(PatternMatcher.scala:1231)
	at dotty.tools.dotc.transform.PatternMatcher$Translator$MatchTranslator$class.translatePattern(PatternMatcher.scala:1338)
	at dotty.tools.dotc.transform.PatternMatcher$Translator$OptimizingMatchTranslator.translatePattern(PatternMatcher.scala:72)
	at dotty.tools.dotc.transform.PatternMatcher$Translator$MatchTranslator$class.translateCase(PatternMatcher.scala:1335)
	at dotty.tools.dotc.transform.PatternMatcher$Translator$OptimizingMatchTranslator.translateCase(PatternMatcher.scala:72)
	at dotty.tools.dotc.transform.PatternMatcher$Translator$MatchTranslator$$anonfun$29.apply(PatternMatcher.scala:1299)
	at dotty.tools.dotc.transform.PatternMatcher$Translator$MatchTranslator$$anonfun$29.apply(PatternMatcher.scala:1299)
	at scala.collection.immutable.List.map(List.scala:273)

[liufengyun]

This is related to #1774 , blocks inside patterns is not well supported in Dotty. In Scalac, the code above is syntactically invalid, but the pattern matcher can handle blocks that are synthesized.

[b-studios]

I guess the problem for this particular issue is that the grammar allows path selections and blocks at the same time. Is there any reason why you would want to select a member of a block? At least the examples in #1774 don't do that.

I could imagine an example like:

  x match { 
    case {
      object Even {
        def unapply(n: Int): Boolean = n % 2 == 0
      }
      object foo {
        val even = Even
      }
      foo
    }.even => 42 
  }

But here Dotty tells me:

-- [E008] Member Not Found Error: examples/bug.scala ---------------------------
 9 |    case {
10 |      object Even {
11 |        def unapply(n: Int): Boolean = n % 2 == 0
12 |      }
13 |      object foo {
14 |        val even = Even
15 |      }
16 |      foo
   |         ^
   |      value `even` is not a member of Object - did you mean `Object.eq`?

If we want to forbid selection of blocks, I guess we need to change the grammar to

 SimplePattern1    ::=  Path
                     |  `{' Block `}'
-                    |  SimplePattern1 `.' id

Other selections like Foo.Bar.Baz are already supported by Path.

[nicolasstucki]

The current exception is

exception while typing 42.toString.unapply of class class dotty.tools.dotc.ast.Trees$Select # 89
exception while typing 42.toString() of class class dotty.tools.dotc.ast.Trees$Apply # 78
exception while typing Bug.x match
  {
    case 42.toString() => 42
  } of class class dotty.tools.dotc.ast.Trees$Match # 81
exception while typing
  final module
 class Bug() extends Object() {
  val x: Int = 42
  Bug.x match
    {
      case 42.toString() => 42
    }
} of class class dotty.tools.dotc.ast.Trees$TypeDef # 84
exception while typing package <empty> {
  final lazy module val Bug: Bug = new Bug()
  
    final module
   class Bug() extends Object() {
    val x: Int = 42
    Bug.x match
      {
        case 42.toString() => 42
      }
  }
} of class class dotty.tools.dotc.ast.Trees$PackageDef # 85