findOrCreateModuleVar picks the wrong symbol

[scabug]

I was looking today at refchecks in order to learn how modules are transformed. In so doing I spotted a bug.

scalac wrongly tells there's an error in the following program (the program is OK, in fact):

class D { 
  object p // (program point 1) 
}  

class C { 
  def m: D = { 
    if("abc".length == 0) {
      object p       // (program point 2)
    }
    null 
  } 
}

Summary: eliminateModuleDefs(tree: Tree) wants to expand the ModuleDef at "program point 2". As part of that, it invokes

  def findOrCreateModuleVar() = localTyper.typedPos(tree.pos) {
    lazy val createModuleVar = gen.mkModuleVarDef(sym)
    sym.owner.info.decl(nme.moduleVarName(sym.name.toTermName)) match {
      // In case we are dealing with local symbol then we already have
      // to correct error with forward reference
      case NoSymbol => createModuleVar
      case vsym     => ValDef(vsym)
    }
  }

with tree bound to that ModuleDef. Normal behavior would be to enter the case NoSymbol, however it finds a vsym (surprisingly, for the lowering of another module, that in "program point 1").

The assumptions under which findOrCreateModuleVar are written appear to be: (a) for a module owned by class, sym.owner.info is a ClassInfo and thus decl() on it looks up where intended. (b) for a module owned by something else, that same lookup will be performed on an EmptyScope.

Assumption (b) proves wrong when sym.owner.info returns a NullaryMethodType. In that case, NullaryMethodType.decls delegates to resultType.decls. In our case, that resultType ("D") happens to have as member an object with the same name, but not the one we're looking for.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-5012?orig=1 Reporter: @magarciaEPFL Affected Versions: 2.9.2

[scabug]

Commit Message Bot (anonymous) said: (extempore in r25757) Fix bug in ModuleDef elimination.

Fixes corner case diagnosed by miguel. Closes #5012, no review.