Simplify WrappedArray for boxed primitives

[lrytz]

In 2.12, a mutable.ArraySeq wraps an Array[AnyRef], which means primitives are boxed in the underlying array.

WrappedArray by default uses unboxed arrays for primitives, but it can also wrap an array of boxed primitives.

scala> import scala.collection.mutable.WrappedArray
import scala.collection.mutable.WrappedArray

scala> val a = WrappedArray.make(Array(1))
a: scala.collection.mutable.WrappedArray[Int] = WrappedArray(1)

scala> a.array.getClass
res0: Class[_ <: Array[Int]] = class [I

scala> val a = WrappedArray.make(Array(1): Array[Any]).asInstanceOf[WrappedArray[Int]]
a: scala.collection.mutable.WrappedArray[Int] = WrappedArray(1)

scala> a.array.getClass
res1: Class[_ <: Array[Int]] = class [Ljava.lang.Object;

Can / should we make this easier to do?

Also, can we avoid this problem here:

scala> a.array
java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [I

Once things are settled, update the docs at https://github.com/scala/collection-strawman/wiki/FAQ#new-deprecations

[lrytz]

For creating a WrappedArray[Int] with an underlying array of boxed primitives, there is also

scala> WrappedArray.untagged(1,2,3)
res5: scala.collection.mutable.WrappedArray[Int] = WrappedArray(1, 2, 3)

scala> WrappedArray.untagged(1,2,3).array.getClass
res7: Class[_ <: Array[Int]] = class [Ljava.lang.Object;

[lrytz]

A related issue:

scala> WrappedArray.untagged(1).elemTag.newArray(1)
java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [I

[szeiger]

If we want WrappedArray to allow boxed and unboxed representations we need to drop the element types in elemTag and array, at least in WrappedArray. They are probably safe to keep in the specialized subclasses.

This one per se is more of a puzzler than a bug:

scala> val a = WrappedArray.make(Array(1): Array[Any]).asInstanceOf[WrappedArray[Int]]
a: scala.collection.mutable.WrappedArray[Int] = WrappedArray(1)

Array(1): Array[Any] creates an Array[Object] (not an Array[Int] cast to Array[Any]). If you cast the WrappedArray to WrappedArray[Int] you end up with an Array[Object] pretending to be an Array[Int]. But there are legitimate ways (without a cast) to end up with the same boxed representation (as in your later comment).