Possibly ill-formed pattern match involving method name crashes the compiler with NPE

[maemre]

I accidentally created a path-dependent pattern match clause involving a method and it crashed the compiler. I minimized it to the code snippet below. I tried it with Scala 2.12.3 and OpenJDK 1.8.0_131 on Ubuntu 16.04.

object Test {
  def Foo(a: Int): Char = ???

  object Bar

  def crash[A](): Boolean = Bar match {
    case Foo.Bar ⇒ true
    case _ ⇒ false
  }
}

This is the resulting stack trace:

$ scalac parser.scala
error: java.lang.NullPointerException
	at scala.tools.nsc.ast.TreeInfo$ValueClass$.$anonfun$valueUnbox$1(TreeInfo.scala:45)
	at scala.tools.nsc.ast.TreeInfo$ValueClass$.valueUnbox(TreeInfo.scala:45)
	at scala.tools.nsc.ast.TreeInfo$ValueClass$Unbox$.unapply(TreeInfo.scala:50)
	at scala.tools.nsc.ast.TreeInfo$ValueClass$BoxAndUnbox$.unapply(TreeInfo.scala:64)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:37)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:22)
	at scala.reflect.internal.Trees.itransform(Trees.scala:1379)
	at scala.reflect.internal.Trees.itransform$(Trees.scala:1347)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.transform(TypingTransformers.scala:44)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:35)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:22)
	at scala.reflect.internal.Trees.itransform(Trees.scala:1432)
	at scala.reflect.internal.Trees.itransform$(Trees.scala:1347)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.transform(TypingTransformers.scala:44)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:35)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:22)
	at scala.reflect.api.Trees$Transformer.$anonfun$transformStats$1(Trees.scala:2589)
	at scala.reflect.api.Trees$Transformer.transformStats(Trees.scala:2587)
	at scala.reflect.internal.Trees.itransform(Trees.scala:1377)
	at scala.reflect.internal.Trees.itransform$(Trees.scala:1347)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.transform(TypingTransformers.scala:44)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:35)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:22)
	at scala.reflect.internal.Trees.$anonfun$itransform$2(Trees.scala:1374)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.atOwner(TypingTransformers.scala:30)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.atOwner(TypingTransformers.scala:25)
	at scala.reflect.internal.Trees.itransform(Trees.scala:1372)
	at scala.reflect.internal.Trees.itransform$(Trees.scala:1347)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.transform(TypingTransformers.scala:44)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:35)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:22)
	at scala.reflect.api.Trees$Transformer.$anonfun$transformStats$1(Trees.scala:2589)
	at scala.reflect.api.Trees$Transformer.transformStats(Trees.scala:2587)
	at scala.reflect.internal.Trees.itransform(Trees.scala:1415)
	at scala.reflect.internal.Trees.itransform$(Trees.scala:1347)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.super$transform(TypingTransformers.scala:40)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.$anonfun$transform$1(TypingTransformers.scala:40)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.atOwner(TypingTransformers.scala:30)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.transform(TypingTransformers.scala:25)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:35)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:22)
	at scala.reflect.api.Trees$Transformer.transformTemplate(Trees.scala:2563)
	at scala.reflect.internal.Trees.$anonfun$itransform$4(Trees.scala:1419)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.atOwner(TypingTransformers.scala:30)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.atOwner(TypingTransformers.scala:25)
	at scala.reflect.internal.Trees.itransform(Trees.scala:1418)
	at scala.reflect.internal.Trees.itransform$(Trees.scala:1347)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.transform(TypingTransformers.scala:44)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:35)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:22)
	at scala.reflect.api.Trees$Transformer.$anonfun$transformStats$1(Trees.scala:2589)
	at scala.reflect.api.Trees$Transformer.transformStats(Trees.scala:2587)
	at scala.reflect.internal.Trees.$anonfun$itransform$7(Trees.scala:1437)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.atOwner(TypingTransformers.scala:30)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.atOwner(TypingTransformers.scala:25)
	at scala.reflect.internal.Trees.itransform(Trees.scala:1437)
	at scala.reflect.internal.Trees.itransform$(Trees.scala:1347)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.super$transform(TypingTransformers.scala:40)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.$anonfun$transform$2(TypingTransformers.scala:42)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.atOwner(TypingTransformers.scala:30)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.transform(TypingTransformers.scala:25)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:35)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:22)
	at scala.tools.nsc.ast.Trees$Transformer.transformUnit(Trees.scala:140)
	at scala.tools.nsc.transform.Transform$Phase.apply(Transform.scala:30)
	at scala.tools.nsc.Global$GlobalPhase.$anonfun$applyPhase$1(Global.scala:426)
	at scala.tools.nsc.Global$GlobalPhase.applyPhase(Global.scala:419)
	at scala.tools.nsc.Global$GlobalPhase.$anonfun$run$1(Global.scala:390)
	at scala.tools.nsc.Global$GlobalPhase.$anonfun$run$1$adapted(Global.scala:390)
	at scala.collection.Iterator.foreach(Iterator.scala:929)
	at scala.collection.Iterator.foreach$(Iterator.scala:929)
	at scala.collection.AbstractIterator.foreach(Iterator.scala:1417)
	at scala.tools.nsc.Global$GlobalPhase.run(Global.scala:390)
	at scala.tools.nsc.Global$Run.compileUnitsInternal(Global.scala:1431)
	at scala.tools.nsc.Global$Run.compileUnits(Global.scala:1416)
	at scala.tools.nsc.Global$Run.compileSources(Global.scala:1412)
	at scala.tools.nsc.Global$Run.compile(Global.scala:1515)
	at scala.tools.nsc.Driver.doCompile(Driver.scala:35)
	at scala.tools.nsc.MainClass.doCompile(Main.scala:24)
	at scala.tools.nsc.Driver.process(Driver.scala:55)
	at scala.tools.nsc.Driver.main(Driver.scala:68)
	at scala.tools.nsc.Main.main(Main.scala)
Exception in thread "main" java.lang.NullPointerException
	at scala.tools.nsc.ast.TreeInfo$ValueClass$.$anonfun$valueUnbox$1(TreeInfo.scala:45)
	at scala.tools.nsc.ast.TreeInfo$ValueClass$.valueUnbox(TreeInfo.scala:45)
	at scala.tools.nsc.ast.TreeInfo$ValueClass$Unbox$.unapply(TreeInfo.scala:50)
	at scala.tools.nsc.ast.TreeInfo$ValueClass$BoxAndUnbox$.unapply(TreeInfo.scala:64)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:37)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:22)
	at scala.reflect.internal.Trees.itransform(Trees.scala:1379)
	at scala.reflect.internal.Trees.itransform$(Trees.scala:1347)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.transform(TypingTransformers.scala:44)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:35)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:22)
	at scala.reflect.internal.Trees.itransform(Trees.scala:1432)
	at scala.reflect.internal.Trees.itransform$(Trees.scala:1347)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.transform(TypingTransformers.scala:44)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:35)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:22)
	at scala.reflect.api.Trees$Transformer.$anonfun$transformStats$1(Trees.scala:2589)
	at scala.reflect.api.Trees$Transformer.transformStats(Trees.scala:2587)
	at scala.reflect.internal.Trees.itransform(Trees.scala:1377)
	at scala.reflect.internal.Trees.itransform$(Trees.scala:1347)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.transform(TypingTransformers.scala:44)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:35)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:22)
	at scala.reflect.internal.Trees.$anonfun$itransform$2(Trees.scala:1374)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.atOwner(TypingTransformers.scala:30)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.atOwner(TypingTransformers.scala:25)
	at scala.reflect.internal.Trees.itransform(Trees.scala:1372)
	at scala.reflect.internal.Trees.itransform$(Trees.scala:1347)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.transform(TypingTransformers.scala:44)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:35)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:22)
	at scala.reflect.api.Trees$Transformer.$anonfun$transformStats$1(Trees.scala:2589)
	at scala.reflect.api.Trees$Transformer.transformStats(Trees.scala:2587)
	at scala.reflect.internal.Trees.itransform(Trees.scala:1415)
	at scala.reflect.internal.Trees.itransform$(Trees.scala:1347)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.super$transform(TypingTransformers.scala:40)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.$anonfun$transform$1(TypingTransformers.scala:40)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.atOwner(TypingTransformers.scala:30)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.transform(TypingTransformers.scala:25)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:35)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:22)
	at scala.reflect.api.Trees$Transformer.transformTemplate(Trees.scala:2563)
	at scala.reflect.internal.Trees.$anonfun$itransform$4(Trees.scala:1419)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.atOwner(TypingTransformers.scala:30)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.atOwner(TypingTransformers.scala:25)
	at scala.reflect.internal.Trees.itransform(Trees.scala:1418)
	at scala.reflect.internal.Trees.itransform$(Trees.scala:1347)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.transform(TypingTransformers.scala:44)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:35)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:22)
	at scala.reflect.api.Trees$Transformer.$anonfun$transformStats$1(Trees.scala:2589)
	at scala.reflect.api.Trees$Transformer.transformStats(Trees.scala:2587)
	at scala.reflect.internal.Trees.$anonfun$itransform$7(Trees.scala:1437)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.atOwner(TypingTransformers.scala:30)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.atOwner(TypingTransformers.scala:25)
	at scala.reflect.internal.Trees.itransform(Trees.scala:1437)
	at scala.reflect.internal.Trees.itransform$(Trees.scala:1347)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.super$transform(TypingTransformers.scala:40)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.$anonfun$transform$2(TypingTransformers.scala:42)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.atOwner(TypingTransformers.scala:30)
	at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.transform(TypingTransformers.scala:25)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:35)
	at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:22)
	at scala.tools.nsc.ast.Trees$Transformer.transformUnit(Trees.scala:140)
	at scala.tools.nsc.transform.Transform$Phase.apply(Transform.scala:30)
	at scala.tools.nsc.Global$GlobalPhase.$anonfun$applyPhase$1(Global.scala:426)
	at scala.tools.nsc.Global$GlobalPhase.applyPhase(Global.scala:419)
	at scala.tools.nsc.Global$GlobalPhase.$anonfun$run$1(Global.scala:390)
	at scala.tools.nsc.Global$GlobalPhase.$anonfun$run$1$adapted(Global.scala:390)
	at scala.collection.Iterator.foreach(Iterator.scala:929)
	at scala.collection.Iterator.foreach$(Iterator.scala:929)
	at scala.collection.AbstractIterator.foreach(Iterator.scala:1417)
	at scala.tools.nsc.Global$GlobalPhase.run(Global.scala:390)
	at scala.tools.nsc.Global$Run.compileUnitsInternal(Global.scala:1431)
	at scala.tools.nsc.Global$Run.compileUnits(Global.scala:1416)
	at scala.tools.nsc.Global$Run.compileSources(Global.scala:1412)
	at scala.tools.nsc.Global$Run.compile(Global.scala:1515)
	at scala.tools.nsc.Driver.doCompile(Driver.scala:35)
	at scala.tools.nsc.MainClass.doCompile(Main.scala:24)
	at scala.tools.nsc.Driver.process(Driver.scala:55)
	at scala.tools.nsc.Driver.main(Driver.scala:68)
	at scala.tools.nsc.Main.main(Main.scala)

[hrhino]

Even smaller:

scala> def Foo(i: Int) = i
Foo: (i: Int)Int

scala> val Foo.Crash = ???
java.lang.NullPointerException
	at scala.tools.nsc.ast.TreeInfo$ValueClass$.$anonfun$valueUnbox$1(TreeInfo.scala:45)
	at scala.tools.nsc.ast.TreeInfo$ValueClass$.valueUnbox(TreeInfo.scala:45)

We are failing to adapt Foo's type (i: Int)Char to a non-method type, duly marking it as error-typed, and then, in a magnificent gesture of clemency, allow the tree to pass unpunished all the way down to posterasure, where it becomes yet another statistic in the shockingly high erroneous tree recidivism rate.

EDIT:

scala> type Q = Foo.type
<console>:12: error: erroneous or inaccessible type
       type Q = Foo.type

I forget... is that better or worse than a compiler crash?

[adriaanm]

Thanks for the report, @maemre, and the fix, @hrhino! Pretty amazing that that loophole's been open-ish

[som-snytt]

Both tests added to the other PR.

@hrhino came so close to the shortest crasher, that it's a shame to mark this as

Duplicates #10731

[lrytz]

Fixed in scala/scala#6273