IArray.toArray: Deprecate broken method

The cast in the original method is unsound. Any attempt at fixing
this would break source-compatibility, so this commit only deprecates
it, to be revisited for 3.1.

Author: smarter

library/src/scala/IArray.scala

@@ -249,9 +249,11 @@ object IArray:
   extension [T](arr: IArray[T]) def takeWhile(p: T => Boolean): IArray[T] =
     genericArrayOps(arr).takeWhile(p)
 
-  /** Returns a mutable copy of this immutable array. */
-  extension [T](arr: IArray[T]) def toArray: Array[T] =
-    arr.clone.asInstanceOf[Array[T]]
+  extension [T](arr: IArray[T])
+    /** Returns a mutable copy of this immutable array. */
+    @deprecated("This method implementation is incorrect and calling it can crash your program, please use `IArray.genericWrapArray(myIArray).toArray` instead.", "3.0.1")
+    def toArray: Array[T] =
+      arr.clone.asInstanceOf[Array[T]]
 
   extension [T](arr: IArray[T])
     def ++[U >: T: ClassTag](suffix: IArray[U]): IArray[U] = genericArrayOps(arr) ++ suffix.toSeq

tests/neg-custom-args/deprecation/i12597.scala

@@ -0,0 +1,5 @@
+@main def Test =
+  val a: IArray[Int] = IArray(2)
+  val b: IArray[Any] = a
+  val c = b.toArray // error: deprecated
+  c(0) = ""

tests/run/i12597.scala

@@ -0,0 +1,5 @@
+@main def Test =
+  val a: IArray[Int] = IArray(2)
+  val b: IArray[Any] = a
+  val c = IArray.genericWrapArray(b).toArray
+  c(0) = ""