getClass on Unit not erased properly

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You shouldn't be able to cast your way to a VerifyError.

object Test {
  final val UNIT: AnyRef with Unit = ().asInstanceOf[AnyRef with Unit]
  
  def main(args: Array[String]): Unit = {
    println(UNIT.getClass)
  }
}

% scala Test
java.lang.VerifyError: (class: Test$, method: main signature: ([Ljava/lang/String;)V) Incompatible object argument for function call
	at Test.main(a.scala)

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-5568?orig=1
Reporter: @paulp
Assignee: @JamesIry
See #6326, #6108

[scabug]

@magarciaEPFL said:
The optimizer can't optimize code that doesn't typecheck, the JVM can't run code that doesn't typecheck, for example the ICode below

(the CALL_METHOD scala.Unit.getClass expects an Object on the stack but finds nothing (the Predef module instance is there to be consumed by the forthcoming Predef.println invocation)

  def main(args: Array[String] (ARRAY[REF(class String)])): Unit {
  locals: value args
  startBlock: 1
  blocks: [1]
  
  1: 
    5	LOAD_MODULE object Predef
    5	CALL_METHOD scala.Unit.getClass (dynamic)
    5	CALL_METHOD scala.Predef.println (dynamic)
    5	RETURN(UNIT)
    
  }

How does that code find its way till GenICode? Like so:

After superaccessors:

[[syntax trees at end of superaccessors]]// Scala source: bt4.scala
package <empty> {
  object Test extends scala.AnyRef {
    def <init>(): Test.type = {
      Test.super.<init>();
      ()
    };
    final private[this] val UNIT: Unit = ().asInstanceOf[Unit];
    final <stable> <accessor> def UNIT: Unit = Test.this.UNIT;
    def main(args: Array[String]): Unit = scala.this.Predef.println(Test.this.UNIT.getClass())
  }
}

After erasure:

[[syntax trees at end of erasure]]// Scala source: bt4.scala
package <empty> {
  object Test extends Object {
    def <init>(): ... = {
      Test.super.<init>();
      ()
    };
    final private[this] val UNIT: Object = scala.runtime.BoxedUnit.UNIT.$asInstanceOf[Object]();
    final <stable> <accessor> def UNIT(): Object = Test.this.UNIT;
    def main(args: Array[String]): Unit = scala.this.Predef.println(().getClass())
  }
}

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@magarciaEPFL said (edited on May 4, 2012 4:47:12 PM UTC):
In transform.Erasure, the Apply tree Test.this.UNIT().getClass() is matched by

  case Apply(fn @ Select(qual, _), Nil) if interceptedMethods(fn.symbol) =>

but then doesn't get into this branch

      // Rewrite 5.getClass to ScalaRunTime.anyValClass(5)
      else if (isPrimitiveValueClass(qual.tpe.typeSymbol))
        global.typer.typed(
          gen.mkRuntimeCall(
            nme.anyValClass,
            List(qual, typer.resolveErasureTag(tree.pos, qual.tpe.widen, true))))

because the qualifier's tpe reads Test.<refinement>.type

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@som-snytt said (edited on Oct 4, 2012 6:11:39 AM UTC):
I stumbled upon this without trying to do any evil. It looks similar.

The error is: java.lang.VerifyError: (class: fracked/Test$$anonfun$fraction$1, method: applyOrElse signature: (ILscala/Function1;)Ljava/lang/Object;) Expecting to find object/array on stack

package fracked

object Test {
  //def fraction: PartialFunction[Int, Int] = { case d if d != 0 => 42 / d }
  def fraction: PartialFunction[Int, Int] = { case d if d != 0 => println(d.getClass); 42 / d }
  //def fraction: PartialFunction[Int, Int] = { case d: Int if d != 0 => println(d.getClass); 42 / d } // ok
  //def fraction: PartialFunction[Int, Int] = { case d if d != 0 => println("hello"); 42 / d }
  def main(args: Array[String]) {
    println(fraction(42))
  }
}

[scabug]

@JamesIry said:
Can reproduce also with (5.asInstanceOf[AnyRef with Int]).getClass

[scabug]

@JamesIry said:
Two branches to consider. The first is the one Miguel pointed to

} else if (isPrimitiveValueClass(qual.tpe.typeSymbol)) {
   // Rewrite 5.getClass to ScalaRunTime.anyValClass(5)
   global.typer.typed(gen.mkRuntimeCall(nme.anyValClass, List(qual, typer.resolveClassTag(tree.pos, qual.tpe.widen))))
}

If we go into that one (by hacking in a bit of force) the code that gets generated is very strange and brittle

3:	getstatic	#24; //Field scala/runtime/ScalaRunTime$.MODULE$:Lscala/runtime/ScalaRunTime$;
   6:	getstatic	#30; //Field scala/runtime/BoxedUnit.UNIT:Lscala/runtime/BoxedUnit;
   9:	getstatic	#35; //Field scala/reflect/ClassTag$.MODULE$:Lscala/reflect/ClassTag$;
   12:	ldc	#4; //class java/lang/Object
   14:	invokevirtual	#39; //Method scala/reflect/ClassTag$.apply:(Ljava/lang/Class;)Lscala/reflect/ClassTag;
   17:	invokevirtual	#43; //Method scala/runtime/ScalaRunTime$.anyValClass:(Ljava/lang/Object;Lscala/reflect/ClassTag;)Ljava/lang/Class;

The result is "java.lang.Object"

If instead we go into the next branch

} else if (fn.symbol == AnyVal_getClass) {
  tree setSymbol Object_getClass

Then what we get is totally reasonable

   3:	getstatic	#25; //Field scala/runtime/BoxedUnit.UNIT:Lscala/runtime/BoxedUnit;
   6:	invokevirtual	#29; //Method java/lang/Object.getClass:()Ljava/lang/Class;

And the result is class scala.runtime.BoxedUnit. Which is nice because that's exactly the same answer as when we do [Unit with AnyRef] instad of [AnyRef with Unit].

What we're currently doing, though, is falling all the way to the default branch and that's getting us into trouble

} else {
  tree
}

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@JamesIry said:
scala/scala#1904