ClassCastException with structural types, value classes #6336

scabug · 2012-09-07T15:16:07Z

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}

The text was updated successfully, but these errors were encountered:

scabug · 2012-09-07T15:16:07Z

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

scabug · 2012-09-07T16:16:20Z

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

scabug · 2012-09-09T13:32:05Z

@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 }
                                               ^

scabug · 2012-09-09T21:32:57Z

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

scabug · 2012-09-10T13:47:40Z

@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))

scabug · 2012-09-10T14:21:38Z

@jsuereth said:
That ones seems a bit intense.

scabug · 2012-09-10T14:42:47Z

@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.

scabug · 2012-09-17T11:57:18Z

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

scabug · 2012-09-17T14:42:26Z

@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)

scabug · 2012-09-17T16:12:37Z

@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.

scabug · 2012-09-17T16:55:24Z

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

scabug · 2012-09-18T15:29:52Z

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

scabug closed this as completed Sep 20, 2012

scabug added critical backend structural types valueclass labels Apr 7, 2017

scabug added this to the 2.10.0-M7 milestone Apr 7, 2017

scabug assigned odersky Apr 7, 2017

This was referenced Apr 7, 2017

generic signatures must account for value class erasure rules #6344

Closed

Using value classes as method parameters in structurally defined classes is broken #6347

Closed

Reflection: cannot invoke a method that takes a value class parameter #6411

Closed

steinybot mentioned this issue Feb 29, 2024

ClassCastException when attempting to use structural refinement that returns a value class scala/scala3#19833

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ClassCastException with structural types, value classes #6336

ClassCastException with structural types, value classes #6336

scabug commented Sep 7, 2012 •

edited by hrhino

scabug commented Sep 7, 2012

scabug commented Sep 7, 2012

scabug commented Sep 9, 2012

scabug commented Sep 9, 2012

scabug commented Sep 10, 2012

scabug commented Sep 10, 2012

scabug commented Sep 10, 2012

scabug commented Sep 17, 2012

scabug commented Sep 17, 2012 •

edited by retronym

scabug commented Sep 17, 2012

scabug commented Sep 17, 2012

scabug commented Sep 18, 2012

ClassCastException with structural types, value classes #6336

ClassCastException with structural types, value classes #6336

Comments

scabug commented Sep 7, 2012 • edited by hrhino

scabug commented Sep 7, 2012

scabug commented Sep 7, 2012

scabug commented Sep 9, 2012

scabug commented Sep 9, 2012

scabug commented Sep 10, 2012

scabug commented Sep 10, 2012

scabug commented Sep 10, 2012

scabug commented Sep 17, 2012

scabug commented Sep 17, 2012 • edited by retronym

scabug commented Sep 17, 2012

scabug commented Sep 17, 2012

scabug commented Sep 18, 2012

scabug commented Sep 7, 2012 •

edited by hrhino

scabug commented Sep 17, 2012 •

edited by retronym