Stack overflow error on compile

[gzoller]

Issue description

Something I changed in my code now causes a StackOverflowError on compile.
Dotty: 0.23.0-bin-20200220-228e593-NIGHTLY
Code: https://github.com/gzoller/dotty_reflection/tree/broken

Should be noted that I'm not using any macro/metaprogramming in the code (that I'm aware of)

I'm using a custom build of scalameta munit, but you don't need test code to reproduce this issue--you can safely comment out this dependency in build.sbt.

I've been trying to narrow down what I changed to cause it to do this, but thus far haven't found a culprit. Will continue narrowing and post any learnings on this bug report.

Steps

1. Download repo
2. sbt compile

Crash output (click arrow to expand)

[error] ## Exception when compiling 14 sources to /Users/wmy965/git/dotty_reflection/target/scala-0.23/classes
[error] java.lang.StackOverflowError
[error] java.base/java.util.WeakHashMap.getTable(WeakHashMap.java:350)
[error] java.base/java.util.WeakHashMap.put(WeakHashMap.java:451)
[error] dotty.tools.dotc.core.SymDenotations$InheritedCacheImpl.addDependent(SymDenotations.scala:2436)
[error] dotty.tools.dotc.core.SymDenotations$BaseDataImpl.apply(SymDenotations.scala:2526)
[error] dotty.tools.dotc.core.SymDenotations$ClassDenotation.baseData(SymDenotations.scala:1673)
[error] dotty.tools.dotc.core.SymDenotations$ClassDenotation.baseClassSet(SymDenotations.scala:1684)
[error] dotty.tools.dotc.core.SymDenotations$ClassDenotation.derivesFrom(SymDenotations.scala:1709)
[error] dotty.tools.dotc.core.TypeComparer.provablyDisjoint(TypeComparer.scala:2261)
[error] dotty.tools.dotc.core.TypeComparer.provablyDisjoint(TypeComparer.scala:2333)
[error] dotty.tools.dotc.core.TypeComparer.provablyDisjoint$$anonfun$1(TypeComparer.scala:2274)
[error] scala.collection.immutable.List.forall(List.scala:358)
[error] dotty.tools.dotc.core.TypeComparer.provablyDisjoint(TypeComparer.scala:2274)
[error] dotty.tools.dotc.core.TypeComparer.provablyDisjoint(TypeComparer.scala:2333)
[error] dotty.tools.dotc.core.TypeComparer.provablyDisjoint$$anonfun$1(TypeComparer.scala:2274)
[error] scala.collection.immutable.List.forall(List.scala:358)
(repeats)

[gzoller]

Timing is everything--just found the change that crashes the compiler.

1. In UnionInfo.scala, in object UnionKind, comment out case t: UnionContainer if t.hasUnion => true

Then it compiles fine.

[gzoller]

More learnings... code for context:

object UnionKind:
  def unapply(f: FieldInfo): Boolean = 
    f.fieldType match {
      case _: StaticUnionInfo => true
      case t: AliasInfo if t.isUnion =>  true
      case t: UnionContainer if t.hasUnion => true  // this line here causes crash
      case _ => false
    }

I discovered that if I move the problematic UnionContainer line to the first position, it compiles fine:

object UnionKind:
  def unapply(f: FieldInfo): Boolean = 
    f.fieldType match {
      case t: UnionContainer if t.hasUnion => true  // same line moved here works
      case _: StaticUnionInfo => true
      case t: AliasInfo if t.isUnion =>  true
      case _ => false
    }

[odersky]

It would be helpful to minimize this further. The fact that the stack overflow happens in provablyDisjoint means it's related to pattern matching exhaustivity checking.

Also, did you try to increase the JVM stack size with -Xss? Just to make sure it's a true overflow and not just a very deep recursion.

[gzoller]

I bumped -Xss to 5M and can confirm I still got the overflow error.

[liufengyun]

@gzoller I tried to minimize from your repo, it's almost done. However, I couldn't find the definition for UnionContainer anywhere. Could you please provide its definition?

trait FieldInfo {
  val fieldType: ALL_TYPE
}

opaque type TypeSymbol = String

trait ConcreteType {
  val name: String
  val typeParameters: List[TypeSymbol]
}

type ALL_TYPE = ConcreteType | TypeSymbol

case class StaticUnionInfo(name: String, typeParameters: List[TypeSymbol]) extends ConcreteType
case class AliasInfo(name: String, typeParameters: List[TypeSymbol]) extends ConcreteType

object UnionKind {
  def unapply(f: FieldInfo): Boolean =
    f.fieldType match {
      case _: StaticUnionInfo => true
      case t: AliasInfo if t.isUnion =>  true
      case t: UnionContainer if t.hasUnion => true  // this line here causes crash
      case _ => false
    }
}

[gzoller]

It's in ReflectedThing.scala:

trait UnionContainer:
  val hasUnion: Boolean