[stdlib] Use merge sort for comparables too

backend.native/tests/build.gradle

@@ -1468,8 +1468,12 @@ task sort1(type: RunKonanTest) {
     source = "runtime/collections/sort1.kt"
 }
 
-task sortWith(type: RunKonanTest) {
+// TODO: Enable devirtualization and make the test not standalone
+// when KT-26315 is fixed.
+task sortWith(type: RunStandaloneKonanTest) {
     source = "runtime/collections/SortWith.kt"
+    flags = ['-tr', '--disable', 'devirtualization']
+    arguments = ['--ktest_logger=SILENT']
 }
 
 task if_else(type: RunKonanTest) {

backend.native/tests/runtime/collections/SortWith.kt

@@ -24,8 +24,18 @@ fun Array<Int>.assertSorted(cmp: Comparator<Int>, message: String = "") {
     }
 }
 
+fun Array<MyComparable>.assertSorted(message: String = "") {
+    for (i in 1 until size) {
+        assertTrue(this[i - 1] <= this[i], message)
+    }
+}
+
 data class ComparatorInfo(val name: String, val comparator: Comparator<Int>, val isCorrect: Boolean)
 
+class MyComparable (val value: Int, val comparator: Comparator<Int>): Comparable<MyComparable> {
+    override fun compareTo(other: MyComparable): Int = comparator.compare(value, other.value)
+}
+
 // Assert that the array is sorted in terms of a comparator only for correct/partially correct cases
 val comparators = listOf<ComparatorInfo>(
     ComparatorInfo("Correct increasing", correctIncreasing ,  true),
@@ -68,8 +78,8 @@ val arrays = listOf<Array<Int>>(
 @Test fun runTest() {
     arrays.forEach { array ->
         comparators.forEach {
+            // Test with custom comparator
             val arrayUnderTest = array.copyOf()
-
             arrayUnderTest.sortWith(it.comparator)
             if (it.isCorrect) {
                 arrayUnderTest.assertSorted(it.comparator, """
@@ -78,6 +88,20 @@ val arrays = listOf<Array<Int>>(
                     Array after sorting: ${arrayUnderTest.joinToString()}
                 """.trimIndent())
             }
+
+            // Test of a custom comparable
+            val comparableArrayUnderTest = Array(array.size) { i ->
+                MyComparable(array[i], it.comparator)
+            }
+            comparableArrayUnderTest.sort()
+
+            if (it.isCorrect) {
+                comparableArrayUnderTest.assertSorted("""
+                    Assert sorted failed for Comparable: "${it.name}"
+                    Array: ${array.joinToString()}
+                    Array after sorting: ${comparableArrayUnderTest.joinToString()}
+                """.trimIndent())
+            }
         }
     }
 }