[#11600] Vector.from(ArraySeq) copies elems rather than reusing

unsafeArray

There were issues with reusing the ArraySeq unsafeArray which arose in
subsequent calls to Vector#updated which would then clone underlying
arrays, and insert into them the new updated element. However the
underlying array.clone() would return a new array with the same
componentType as the passed varargs, rather than an Array[AnyRef] which
is required in the general case.

src/library/scala/collection/immutable/Vector.scala

@@ -33,8 +33,20 @@ object Vector extends StrictOptimizedSeqFactory[Vector] {
       case as: ArraySeq[E] if as.length <= 32 && as.unsafeArray.isInstanceOf[Array[AnyRef]] =>
         if (as.isEmpty) NIL
         else {
-          val v = new Vector(0, as.length, 0)
-          v.display0 = as.unsafeArray.asInstanceOf[Array[AnyRef]]
+          val unsafeArray = as.unsafeArray
+          val len = unsafeArray.length
+          val v = new Vector(0, len, 0)
+          val display0 = new Array[Any](len)
+          if (unsafeArray.isInstanceOf[Array[AnyRef]]) {
+            System.arraycopy(unsafeArray, 0, display0, 0, len)
+          } else {
+            var i = 0
+            while (i < len) {
+              display0(i) = unsafeArray(i)
+              i += 1
+            }
+          }
+          v.display0 = display0.asInstanceOf[Array[AnyRef]]
           v.depth = 1
           releaseFence()
           v
@@ -45,17 +57,11 @@ object Vector extends StrictOptimizedSeqFactory[Vector] {
 
         if (knownSize == 0) empty[E]
         else if (knownSize > 0 && knownSize <= 32) {
-          val display0 = new Array[AnyRef](knownSize)
-
-          var i = 0
-          val iterator = it.iterator
-          while (iterator.hasNext) {
-            display0(i) = iterator.next().asInstanceOf[AnyRef]
-            i += 1
-          }
+          val display0 = new Array[Any](knownSize)
+          it.iterator.copyToArray(display0)
           val v = new Vector[E](0, knownSize, 0)
           v.depth = 1
-          v.display0 = display0
+          v.display0 = display0.asInstanceOf[Array[AnyRef]]
           releaseFence()
           v
         } else {

test/junit/scala/collection/immutable/VectorTest.scala

@@ -219,4 +219,48 @@ class VectorTest {
       assertArrayEquals(s"<${v2.length}>.slice($j, $k)", v2.toArray.slice(j, k), v2.iterator.slice(j, k).toArray)
     }
   }
+
+  @Test
+  def i11600(): Unit = {
+    locally {
+      abstract class Base
+      class Derived1 extends Base
+      class Derived2 extends Base
+      val d1 = new Derived1
+      val d2 = new Derived2
+
+      locally {
+        val arraySeq = ArraySeq(d1)
+        val vector = Vector(arraySeq: _*)
+        assertEquals(arraySeq, ArraySeq(d1)) // ensure arraySeq is not mutated
+        assertEquals(vector.updated(0, d2), Vector(d2))
+      }
+
+      locally {
+        val list = List(d1)
+        val vector = Vector.from(list)
+        assertEquals(list, vector)
+        assertEquals(List(d2), vector.updated(0, d2))
+      }
+    }
+
+    locally {
+      // ensure boxing logic works:
+      val arraySeq = ArraySeq(1,2,3,4,5)
+      val vector = Vector(arraySeq: _*)
+
+      assertEquals(1 to 5, vector)
+      assertEquals(vector.updated(0, 20), Vector(20,2,3,4,5))
+      assertEquals(vector.updated(0, ""), Vector("",2,3,4,5))
+      assertEquals(1 to 5, arraySeq) // ensure arraySeq is not mutated
+    }
+    locally {
+      // ensure boxing logic works:
+      val arr = Array(1)
+      val vector = Vector.from(arr)
+      assertEquals(arr.toList, vector)
+      assertEquals(List(20), vector.updated(0, 20))
+      assertEquals(List(""), vector.updated(0, ""))
+    }
+  }
 }