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Empty WrappedArray.ofRef.array throws ClassCastException #9941

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scabug opened this issue Sep 29, 2016 · 4 comments
Open

Empty WrappedArray.ofRef.array throws ClassCastException #9941

scabug opened this issue Sep 29, 2016 · 4 comments
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library:collections
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@scabug
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scabug commented Sep 29, 2016

.array on an empty WrappedArray$ofRef throws a ClassCastException.

Here is a simple example where calling .array on an Array hits it via an implicit conversion from Array to WrappedArray$ofRef:

scala> case class A(n: Int)
defined class A

scala> val arr: Array[A] = Array()
arr: Array[A] = Array()

scala> arr.array
java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [LA;
  ... 32 elided

Here's the above implicit unrolled:

scala> val wrapped: scala.collection.mutable.WrappedArray[A] = arr
wrapped: scala.collection.mutable.WrappedArray[A] = WrappedArray()

scala> wrapped.array
java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [LA;
  ... 32 elided

Other, primitive WrappedArray subclasses seem to not have the issue:

scala> val ints: Array[Int] = Array()
ints: Array[Int] = Array()

scala> ints.array
res6: Array[Int] = Array()

I haven't fully pieced together exactly where/why the exception occurs, but it seems unexpected/buggy for Array to have an always-available (via an implicit) .array method that will throw such an inscrutable exception on empty Arrays.
@scabug
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scabug commented Sep 29, 2016

Imported From: https://issues.scala-lang.org/browse/SI-9941?orig=1
Reporter: Ryan Williams (rdub)
Affected Versions: 2.11.8, 2.12.0-RC1

@scabug scabug added the quickfix label Apr 7, 2017
@scabug scabug added this to the Backlog milestone Apr 7, 2017
@selrach selrach mentioned this issue Jun 14, 2017
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@eugengarkusha
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Additional example(just hit this problem on my project):

the following code

object ArrayIssue {
  def main(args: Array[String]): Unit ={
    // inferred type of k is Array[_ <: String]
    val k = if (true )Array.empty else Array.empty[String]
    println(k)
  }
}

throws
java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.String;

the main method decompiles to:

    {
        String k[] = (String[])Array$.MODULE$.empty(ClassTag$.MODULE$.Nothing());
        Predef$.MODULE$.println(k);
    }

reproducible on 2.13: https://scastie.scala-lang.org/wvg8VYr2QNSa11r8WmIDFQ

@SethTisue
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not fixed in Scala 3 (as of 3.3.2-RC1-bin-20230613-d83aa49-NIGHTLY)

@som-snytt
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The improvement

Welcome to Scala 2.13.11 (OpenJDK 64-Bit Server VM, Java 20.0.1).
Type in expressions for evaluation. Or try :help.

scala> def f(b: Boolean) = if (b) Array.empty else Array("hello, world")
def f(b: Boolean): Array[_ <: String]

scala> f(true)
java.lang.ClassCastException: class [Ljava.lang.Object; cannot be cast to class [Ljava.lang.String; ([Ljava.lang.Object; and [Ljava.lang.String; are in module java.base of loader 'bootstrap')
  at f(<console>:1)
  ... 30 elided

but

Welcome to Scala 3.3.0 (20.0.1, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.

scala> List(Array(42), Array("hello, world"))
val res0: List[Array[? >: Int & String <: Int | String]] = List(Array(42), Array(hello, world))

scala> def f(b: Boolean) = if (b) Array(42) else Array("hello, world")
def f(b: Boolean): Array[? >: Int & String <: Int | String]

scala> f(true)
val res5: Array[? >: Int & String <: Int | String] = Array(42)

scala> f(true).array
val res6: Array[?] = Array(42)

scala> val xs: Array[String] = Array.empty
val xs: Array[String] = Array()

scala> xs.array
java.lang.ClassCastException: class [Ljava.lang.Object; cannot be cast to class [Ljava.lang.String; ([Ljava.lang.Object; and [Ljava.lang.String; are in module java.base of loader 'bootstrap')
  ... 33 elided

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