Compiler crashes with NPE during type instantiation of a variable

[nerush]

Instead of reporting compilation error only, compiler crashes with NPE for the following source:

object Main {
    abstract class Factory[T <: Int] {
      def size: T
      def create: Array[T] = Array.ofDim(size)
    }
}

Actual error:

-- Error: Main.scala:5:44 ------------------------------------------------------
5 |    def create: Array[T] = Array.ofDim(size)
  |                                            ^
  |                                  No ClassTag available for Factory.this.T

exception occurred while typechecking Main.scala

exception occurred while compiling Main.scala
Exception in thread "main" java.lang.NullPointerException
	at dotty.tools.dotc.core.Types$TypeVar.instantiateWith(Types.scala:3338)
	at dotty.tools.dotc.core.Types$TypeVar.instantiate(Types.scala:3352)
	at dotty.tools.dotc.typer.Inferencing$.op$5$$anonfun$1(Inferencing.scala:264)
	at scala.collection.mutable.ResizableArray.foreach(ResizableArray.scala:59)
	at scala.collection.mutable.ResizableArray.foreach$(ResizableArray.scala:52)
	at scala.collection.mutable.ArrayBuffer.foreach(ArrayBuffer.scala:48)
	at dotty.tools.dotc.typer.Inferencing$.op$4(Inferencing.scala:265)
	at dotty.tools.dotc.typer.Inferencing$.interpolate$1(Inferencing.scala:221)
	at dotty.tools.dotc.typer.Inferencing$.interpolateUndetVars(Inferencing.scala:267)
	at dotty.tools.dotc.typer.Typer.op$2(Typer.scala:1859)
	at dotty.tools.dotc.typer.Typer.op$1(Typer.scala:1858)
	at dotty.tools.dotc.typer.Typer.adapt(Typer.scala:1857)
	at dotty.tools.dotc.typer.Typer.op$7(Typer.scala:1696)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1692)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:1718)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:1741)
	at dotty.tools.dotc.typer.Typer.op$14(Typer.scala:1409)
	at dotty.tools.dotc.typer.Typer.typedClassDef(Typer.scala:1339)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:1623)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1678)
	at dotty.tools.dotc.typer.Typer.op$7(Typer.scala:1696)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1692)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:1718)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:1741)
	at dotty.tools.dotc.typer.Typer.op$14(Typer.scala:1409)
	at dotty.tools.dotc.typer.Typer.typedClassDef(Typer.scala:1339)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:1623)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1678)
	at dotty.tools.dotc.typer.Typer.op$7(Typer.scala:1696)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1692)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:1718)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:1741)
	at dotty.tools.dotc.typer.Typer.op$11(Typer.scala:1522)
	at dotty.tools.dotc.typer.Typer.typedPackageDef(Typer.scala:1509)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:1662)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1679)
	at dotty.tools.dotc.typer.Typer.op$7(Typer.scala:1696)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1692)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:1753)
	at dotty.tools.dotc.typer.FrontEnd.typeCheck$$anonfun$1(FrontEnd.scala:64)
	at scala.compat.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
	at dotty.tools.dotc.typer.FrontEnd.monitor(FrontEnd.scala:32)
	at dotty.tools.dotc.typer.FrontEnd.typeCheck(FrontEnd.scala:68)
	at dotty.tools.dotc.typer.FrontEnd.runOn$$anonfun$3(FrontEnd.scala:93)
	at scala.compat.java8.JProcedure1.apply(JProcedure1.java:18)
	at scala.compat.java8.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:389)
	at dotty.tools.dotc.typer.FrontEnd.runOn(FrontEnd.scala:93)
	at dotty.tools.dotc.Run.op$1(Run.scala:124)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$2$$anonfun$1(Run.scala:122)
	at scala.compat.java8.JProcedure1.apply(JProcedure1.java:18)
	at scala.compat.java8.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.IndexedSeqOptimized.foreach(IndexedSeqOptimized.scala:32)
	at scala.collection.IndexedSeqOptimized.foreach$(IndexedSeqOptimized.scala:29)
	at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:191)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:135)
	at scala.compat.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
	at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:86)
	at dotty.tools.dotc.Run.compileUnits(Run.scala:137)
	at dotty.tools.dotc.Run.compileSources(Run.scala:95)
	at dotty.tools.dotc.Run.compile(Run.scala:79)
	at dotty.tools.dotc.Driver.doCompile(Driver.scala:26)
	at dotty.tools.dotc.Driver.process(Driver.scala:124)
	at dotty.tools.dotc.Driver.process(Driver.scala:93)
	at dotty.tools.dotc.Driver.process(Driver.scala:105)
	at dotty.tools.dotc.Driver.main(Driver.scala:132)
	at dotty.tools.dotc.Main.main(Main.scala)

Compiler crashes in Types class because instantiateWith method of TypeVar class does not take into account that owningState field may be null when permanent instance type of the variable is instantiated.

I have this bug already fixed in the branch with additional minor improvements like removed unused imports and fixed typo.

Shall I create pull request ?

[allanrenucci]

There is only one place where owningState is set to null Types:3311:

private[core] def inst_=(tp: Type) = {
  myInst = tp
  if (tp.exists) owningState = null // no longer needed; null out to avoid a memory leak
}

The comment says that owningState should no longer be used at this point. So I am not sure that guarding against null value is the right fix.

Feel free to open a PR, if you think you have a proper fix.

[odersky]

Fixed by #3726