Another approach for sorting nulls.

libraries/stdlib/src/kotlin/util/Ordering.kt

@@ -68,13 +68,19 @@ public inline fun <T> compareValuesBy(a: T, b: T, selector: (T) -> Comparable<*>
     return compareValues(selector(a), selector(b))
 }
 
+public inline fun <T, K> compareValuesBy(a: T, b: T, comparator: Comparator<K>, selector: (T) -> K): Int {
+    return comparator.compare(selector(a), selector(b))
+}
+
 // Not so useful without type inference for receiver of expression
 // compareValuesWith(v1, v2, compareBy { it.prop1 } thenByDescending { it.prop2 })
-///**
-// * Compares two values using the specified [comparator].
-// */
-//@suppress("NOTHING_TO_INLINE")
-//public inline fun <T> compareValuesWith(a: T, b: T, comparator: Comparator<T>): Int = comparator.compare(a, b)
+/**
+ * Compares two values using the specified [comparator].
+ */
+@suppress("NOTHING_TO_INLINE")
+public inline fun <T> compareValuesWith(a: T, b: T, comparator: Comparator<T>): Int = comparator.compare(a, b)
+
+
 
 /**
  * Compares two nullable [Comparable] values. Null is considered less than any value.
@@ -114,6 +120,12 @@ inline public fun <T> compareBy(inlineOptions(InlineOption.ONLY_LOCAL_RETURN) co
     }
 }
 
+public fun <T, K> compareBy(comparator: Comparator<K>, selector: (T) -> K): Comparator<T> {
+    return object : Comparator<T> {
+        public override fun compare(a: T, b: T): Int = compareValuesBy(a, b, comparator, selector)
+    }
+}
+
 /**
  * Creates a descending comparator using the function to transform value to a [Comparable] instance for comparison.
  */
@@ -172,31 +184,31 @@ inline public fun <T> Comparator<T>.thenComparator(inlineOptions(InlineOption.ON
 
 // Not so useful without type inference for receiver of expression
 /**
- * Extends the given comparator of non-nullable values to a comparator of nullable values
+ * Extends the given [comparator] of non-nullable values to a comparator of nullable values
  * considering null value less than any other value.
  */
-public fun <T: Any> Comparator<T>.nullsFirst(): Comparator<T?> {
+public fun <T: Any> nullsFirst(comparator: Comparator<T>): Comparator<T?> {
     return object: Comparator<T?> {
         override fun compare(a: T?, b: T?): Int {
             if (a === b) return 0
             if (a == null) return -1
             if (b == null) return 1
-            return this@nullsFirst.compare(a, b)
+            return comparator.compare(a, b)
         }
     }
 }
 
 /**
- * Extends the given comparator of non-nullable values to a comparator of nullable values
+ * Extends the given [comparator] of non-nullable values to a comparator of nullable values
  * considering null value greater than any other value.
  */
-public fun <T: Any> Comparator<T>.nullsLast(): Comparator<T?> {
+public fun <T: Any> nullsLast(comparator: Comparator<T>): Comparator<T?> {
     return object: Comparator<T?> {
         override fun compare(a: T?, b: T?): Int {
             if (a === b) return 0
             if (a == null) return 1
             if (b == null) return -1
-            return this@nullsLast.compare(a, b)
+            return comparator.compare(a, b)
         }
     }
 }

libraries/stdlib/test/OrderingTest.kt

@@ -39,7 +39,7 @@ class OrderingTest {
         val v2: Item? = null
         val diff = compareValuesBy(v1, v2) { it?.rating }
         assertTrue(diff > 0)
-        val diff2 = compareBy<Item> { it.rating }.thenBy { it.name }.nullsLast().compare(v1, v2)
+        val diff2 = nullsLast(compareBy<Item> { it.rating }.thenBy { it.name }).compare(v1, v2)
         assertTrue(diff2 < 0)
     }
 

libraries/stdlib/test/collections/ArraysTest.kt

@@ -827,9 +827,9 @@ class ArraysTest {
     test fun sortedNullableBy() {
         fun String.nullIfEmpty() = if (isEmpty()) null else this
         arrayOf(null, "").let {
-            expect(listOf(null, "")) { it.sortedWith(compareBy<String> { it }.nullsFirst()) }
-            expect(listOf("", null)) { it.sortedWith(compareByDescending<String> { it }.nullsLast()) }
-            expect(listOf("", null)) { it.sortedWith(compareByDescending<String> { it.nullIfEmpty() }.nullsLast() ) }
+            expect(listOf(null, "")) { it.sortedWith(nullsFirst(compareBy { it })) }
+            expect(listOf("", null)) { it.sortedWith(nullsLast(compareByDescending { it })) }
+            expect(listOf("", null)) { it.sortedWith(nullsLast(compareByDescending { it.nullIfEmpty() })) }
         }
     }
 

libraries/stdlib/test/collections/CollectionTest.kt

@@ -572,13 +572,19 @@ class CollectionTest {
 
     test fun sortedNullableBy() {
         fun String.nullIfEmpty() = if (isEmpty()) null else this
-        listOf(null, "").let {
-//            expect(listOf(null, "")) { it.sortedBy { it.orEmpty()}}
-//            expect(listOf("", null)) { it.sortedByDescending { it.orEmpty() }}
-//            expect(listOf("", null)) { it.sortedByDescending { it?.nullIfEmpty() }}
-            expect(listOf(null, "")) { it.sortedWith(compareBy<String> { it }.nullsFirst()) }
-            expect(listOf("", null)) { it.sortedWith(compareByDescending<String> { it }.nullsLast()) }
-            expect(listOf("", null)) { it.sortedWith(compareByDescending<String> { it.nullIfEmpty() }.nullsLast() ) }
+        listOf(null, "", "a").let {
+            expect(listOf(null, "", "a")) { it.sortedWith(nullsFirst(compareBy { it })) }
+            expect(listOf("a", "", null)) { it.sortedWith(nullsLast(compareByDescending { it })) }
+            expect(listOf(null, "a", "")) { it.sortedWith(nullsFirst(compareByDescending { it.nullIfEmpty() })) }
+        }
+    }
+
+    test fun sortedByNullable() {
+        fun String.nonEmptyLength() = if (isEmpty()) null else length()
+        listOf("", "sort", "abc").let {
+            assertEquals(listOf("", "abc", "sort"), it.sortedBy { it.nonEmptyLength() })
+            assertEquals(listOf("sort", "abc", ""), it.sortedByDescending { it.nonEmptyLength() })
+            assertEquals(listOf("abc", "sort", ""), it.sortedWith(compareBy(nullsLast(compareBy<Int> {it})) { it.nonEmptyLength()}))
         }
     }