Provide flatMapIndexed operation

- similar to flatMap, but transform function takes index and element

#KT-36894

libraries/stdlib/common/src/generated/_Strings.kt

@@ -785,6 +785,39 @@ public inline fun <R> CharSequence.flatMap(transform: (Char) -> Iterable<R>): Li
     return flatMapTo(ArrayList<R>(), transform)
 }
 
+/**
+ * Returns a single list of all elements yielded from results of [transform] function being invoked on each character
+ * and its index in the original char sequence.
+ * 
+ * @sample samples.collections.Collections.Transformations.flatMapIndexed
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <R> CharSequence.flatMapIndexed(transform: (index: Int, Char) -> Iterable<R>): List<R> {
+    return flatMapIndexedTo(ArrayList<R>(), transform)
+}
+
+/**
+ * Appends all elements yielded from results of [transform] function being invoked on each character
+ * and its index in the original char sequence, to the given [destination].
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <R, C : MutableCollection<in R>> CharSequence.flatMapIndexedTo(destination: C, transform: (index: Int, Char) -> Iterable<R>): C {
+    var index = 0
+    for (element in this) {
+        val list = transform(index++, element)
+        destination.addAll(list)
+    }
+    return destination
+}
+
 /**
  * Appends all elements yielded from results of [transform] function being invoked on each character of original char sequence, to the given [destination].
  */