Provide protected toArray implementation in AbstractCollection.

Relates to #KT-13898

libraries/stdlib/common/src/kotlin/CollectionsH.kt

@@ -158,4 +158,8 @@ header operator fun <K, V> MutableMap<K, V>.set(key: K, value: V): Unit
 
 // from Grouping.kt
 public header fun <T, K> Grouping<T, K>.eachCount(): Map<K, Int>
-// public header inline fun <T, K> Grouping<T, K>.eachSumOf(valueSelector: (T) -> Int): Map<K, Int>
+// public header inline fun <T, K> Grouping<T, K>.eachSumOf(valueSelector: (T) -> Int): Map<K, Int>
+
+internal header fun copyToArrayImpl(collection: Collection<*>): Array<Any?>
+
+internal header fun <T> copyToArrayImpl(collection: Collection<*>, array: Array<T>): Array<T>

libraries/stdlib/src/kotlin/collections/AbstractCollection.kt

@@ -31,7 +31,14 @@ public abstract class AbstractCollection<out E> protected constructor() : Collec
         if (it === this) "(this Collection)" else it.toString()
     }
 
+    /**
+     * Returns new array of type `Array<Any?>` with the elements of this collection.
+     */
+    protected open fun toArray(): Array<Any?> = copyToArrayImpl(this)
 
+    /**
+     * Fills the provided [array] or creates new array of the same type
+     * and fills it with the elements of this collection.
+     */
+    protected open fun <T> toArray(array: Array<T>): Array<T> = copyToArrayImpl(this, array)
 }
-
-

libraries/stdlib/src/kotlin/collections/Collections.kt

@@ -182,6 +182,16 @@ internal fun <T> List<T>.optimizeReadOnlyList() = when (size) {
     else -> this
 }
 
+@JvmVersion
+@kotlin.internal.InlineOnly
+internal inline fun copyToArrayImpl(collection: Collection<*>): Array<Any?> =
+        kotlin.jvm.internal.CollectionToArray.toArray(collection)
+
+@JvmVersion
+@kotlin.internal.InlineOnly
+internal inline fun <T> copyToArrayImpl(collection: Collection<*>, array: Array<T>): Array<T> =
+        kotlin.jvm.internal.CollectionToArray.toArray(collection, array)
+
 // copies typed varargs array to array of objects
 @JvmVersion
 private fun <T> Array<out T>.copyToArrayOfAny(isVarargs: Boolean): Array<Any?> =

libraries/stdlib/test/collections/CollectionTest.kt

@@ -905,4 +905,30 @@ class CollectionTest {
         assertTrue(listOf(1) is RandomAccess, "Default singleton list is RandomAccess")
         assertTrue(emptyList<Int>() is RandomAccess, "Empty list is RandomAccess")
     }
+
+    @Test fun abstractCollectionToArray() {
+        class TestCollection<out E>(val data: Collection<E>) : AbstractCollection<E>() {
+            val invocations = mutableListOf<String>()
+            override val size get() = data.size
+            override fun iterator() = data.iterator()
+
+            override fun toArray(): Array<Any?> {
+                invocations += "toArray1"
+                return data.toTypedArray()
+            }
+            public override fun <T> toArray(array: Array<T>): Array<T> {
+                invocations += "toArray2"
+                return super.toArray(array)
+            }
+        }
+        val data = listOf("abc", "def")
+        val coll = TestCollection(data)
+
+        val arr1 = coll.toTypedArray()
+        assertEquals(data, arr1.asList())
+        assertTrue("toArray1" in coll.invocations || "toArray2" in coll.invocations)
+
+        val arr2: Array<String> = coll.toArray(Array(coll.size + 1) { "" })
+        assertEquals(data + listOf(null), arr2.asList())
+    }
 }