Relax generic variance in Array.flatten

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

@@ -30,7 +30,7 @@ public inline fun <reified T> Array<out T>?.orEmpty(): Array<out T> = this ?: ar
 /**
  * Returns a single list of all elements from all arrays in the given array.
  */
-public fun <T> Array<Array<out T>>.flatten(): List<T> {
+public fun <T> Array<out Array<out T>>.flatten(): List<T> {
     val result = ArrayList<T>(sumBy { it.size })
     for (element in this) {
         result.addAll(element)

libraries/stdlib/test/collections/ArraysTest.kt

@@ -956,6 +956,19 @@ class ArraysTest {
         assertEquals(listOf(2), arrayOf("a", null, "test").mapIndexedNotNull { index, it -> it?.run { if (index != 0) length / index else null  } })
     }
 
+    @test fun flattenArray() {
+        val arr1: Array<Array<Int>> = arrayOf(arrayOf(1, 2, 3), arrayOf(4, 5, 6))
+        val arr2: Array<out Array<Int>> = arr1
+        val arr3: Array<out Array<out Int>> = arr1
+        val arr4: Array<Array<out Int>> = arr1 as Array<Array<out Int>>
+
+        val expected = listOf(1, 2, 3, 4, 5, 6)
+        assertEquals(expected, arr1.flatten())
+        assertEquals(expected, arr2.flatten())
+        assertEquals(expected, arr3.flatten())
+        assertEquals(expected, arr4.flatten())
+    }
+
     @test fun asListPrimitives() {
         // Array of primitives
         val arr = intArrayOf(1, 2, 3, 4, 2, 5)