ClassCastException with structural types, value classes

[scabug]

A structural type with a value class parameter throws a ClassCastException at runtime.

object D {
  def main(args: Array[String]) {
    val a = new { def y[T](x: X[T]) = x.i }
    val x = new X(3)
    val t = a.y(x)
    println(t)
  }
}
class X[T](val i: T) extends AnyVal

java.lang.ClassCastException: X cannot be cast to java.lang.Integer
	at scala.runtime.BoxesRunTime.unboxToInt(Unknown Source)
	at D$.main(D.scala:5)
	at D.main(D.scala)
{code}

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-6336?orig=1
Reporter: @harrah
Affected Versions: 2.10.0-M7, 2.10.0
Other Milestones: 2.10.0

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@jsuereth said:
Again, bumping to you, just pass back if it's not a blocker/critical.

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@retronym said (edited on Sep 9, 2012 8:15:49 PM UTC):
(Comments originally lodged, accidentally, on #6337)

A play-by-play in the REPL:

scala> class X[T](val i: T) extends AnyVal
defined class X

scala> object a { def y[T](x: X[T]) = x.i }
defined module a

scala> val t = a.y(new X(0))
t: Int = 0

scala> val t = (a: {def y[T](x: X[T]): T}).y(new X(0))
warning: there were 1 feature warnings; re-run with -feature for details
java.lang.ClassCastException: X cannot be cast to java.lang.Integer

A very similar problem: methods with erased value classes as parameters can't be invoked with reflection.

scala> class X[T](val i: T) extends AnyVal
defined class X

scala> object a { def y[T](x: X[T]) = x.i }
defined module a

scala> val cm = reflect.runtime.currentMirror
cm: reflect.runtime.universe.Mirror = JavaMirror with scala.tools.nsc.interpreter.IMain$TranslatingClassLoader@4bcc946b of type class scala.tools.nsc.interpreter.IMain$TranslatingClassLoader with classpath [(memory)] and parent being scala.tools.nsc.util.ScalaClassLoader$URLClassLoader@642423ad of type class scala.tools.nsc.util.ScalaClassLoader$URLClassLoader with classpath [file:/Library/Java/JavaVirtualMachines/1.6.0_27-b07-395.jdk/Contents/Classes/classes.jar,file:/Library/Java/JavaVirtualMachines/1.6.0_27-b07-395.jdk/Contents/Classes/ui.jar,file:/Library/Java/JavaVirtualMachines/1.6.0_27-b07-395.jdk/Contents/Classes/jsse.jar,file:/Library/Java/JavaVirtualMachines/1.6.0_27-b07-395.jdk/Contents/Classes/jce.jar,file:/Library/Java/JavaVirtualMachines/1.6.0_27-b07-395.jdk/Contents/Class...
scala> val moduleA = cm.reflect(a)
moduleA: reflect.runtime.universe.InstanceMirror = instance mirror for a$@6ff64801

scala> val methodY = moduleA.symbol.typeSignature.declarations.last.asMethod
methodY: reflect.runtime.universe.MethodSymbol = method y

scala> moduleA.reflectMethod(methodY)
res10: reflect.runtime.universe.MethodMirror = method mirror for a.y[T](x: X[T]): T (bound to a$@6ff64801)

scala> res10(new X(0))
java.lang.NoClassDefFoundError: no Java class corresponding to ErasedValueType(X[T]) found
	at scala.reflect.runtime.JavaMirrors$JavaMirror.typeToJavaClass(JavaMirrors.scala:1160)

Currently, method signatures that accept value classes are compiled to accept the unboxed type:

scala> class X(val i: Int) extends AnyVal
defined class X

scala> class C1 { def foo(x: X) = x.i * 2 }
defined class C1

scala> :javap C1
Compiled from "<console>"
public class C1 extends java.lang.Object{
    public int foo(int);
    public C1();
}

The compiler unboxes the X at the call site (and eliminates allocation of a new X, where possible). But for reflective callers, this never happens.

It seems to me that both problems would be solved if we left the signatures unmolested, and created a shadow method that accepts the unboxed value:

public class C1 extends java.lang.Object{
    public int foo(X)
    public int foo$unboxed(int);
    public C1();
}

The first method would delegate to the second, and the compiler would endeavour to call the second directly where possible. This would parallel the way that a specialized method can still be called from a generic call site.

It would also avoid double definition errors like:

scala> class C3 { def foo(x: X) = x.i * 2; def foo(i: Int) = i * 2 }
<console>:11: error: double definition:
method foo:(i: Int)Int and
method foo:(x: X)Int at line 11
have same type after erasure: (i: Int)Int
       class C3 { def foo(x: X) = x.i * 2; def foo(i: Int) = i * 2 }
                                               ^

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@retronym said:
Bringing this to the attention of [scala-sips] https://groups.google.com/d/topic/scala-sips/KSm99KoiYNA/discussion

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@retronym said:
Another example from #6347.

scala> class ValueClass(val n: Int) extends AnyVal
defined class ValueClass
scala> def createStructural = new { | def withValueClass(valueClass: ValueClass) = () | }
createStructural: Object{def withValueClass(valueClass: ValueClass): Unit}
scala> createStructural.withValueClass(new ValueClass(1))

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@jsuereth said:
That ones seems a bit intense.

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@retronym said:
It's really just a different flavour of this error, it just fails sooner.

Martin's short-term suggestion on [scala-sips] is to disallow value classes in structural types. The followup might be to a) add boxed bridge methods outlined above; or b) make structural type dispatch / reflection aware of the necessary boxing/unboxing.

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@jsuereth said:
This is now disallowed. Marking as resolved. Should I open a feature request to make this work in the future?

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@harrah said:
Probably needs to be disallowed in the return type as well:

object D {
  def main(args: Array[String]) {
    val a = new { def y[T](x: T): X[T] = new X(x) }
    val t = a.y(3).i + 1
    println(t)
  }
}

class X[T](val i: T) extends AnyVal

java.lang.ClassCastException: java.lang.Integer cannot be cast to X
   at D$.main(D.scala:4)
   at D.main(D.scala)

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@retronym said (edited on Sep 17, 2012 9:48:55 PM UTC):
One more to test (I'll check it tonight if noone beats me to it)

scala> class X[T](val i: T) extends AnyVal
defined class X

scala> (new { def y[T, XT <: X[T]](xt: XT) = xt.i }).y[Int, X[Int]](new X(0))
java.lang.VerifyError: (class: , method: <init> signature: ()V) Expecting to find object/array on stack

UPDATE:

This example work fine. The error I was seeing was from an older milestone, M5.

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@jsuereth said:
Look at the comments, we still have a missing case issue.

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@odersky said:
New pull request: scala/scala#1342