fix: create JMESPATH flattenIfPossible functions for Lists (#1169)

runtime/runtime-core/common/src/aws/smithy/kotlin/runtime/util/JMESPath.kt

@@ -66,10 +66,20 @@ public fun Any?.type(): String = when (this) {
     else -> throw Exception("Undetected type for: $this")
 }
 
+// Collection `flattenIfPossible` functions
 @InternalApi
 @JvmName("noOpUnnestedCollection")
 public inline fun <reified T> Collection<T>.flattenIfPossible(): Collection<T> = this
 
 @InternalApi
 @JvmName("flattenNestedCollection")
 public inline fun <reified T> Collection<Collection<T>>.flattenIfPossible(): Collection<T> = flatten()
+
+// List `flattenIfPossible` functions
+@InternalApi
+@JvmName("noOpUnnestedCollection")
+public inline fun <reified T> List<T>.flattenIfPossible(): List<T> = this
+
+@InternalApi
+@JvmName("flattenNestedCollection")
+public inline fun <reified T> List<List<T>>.flattenIfPossible(): List<T> = flatten()

runtime/runtime-core/common/test/aws/smithy/kotlin/runtime/util/JmesPathTest.kt

@@ -0,0 +1,49 @@
+package aws.smithy.kotlin.runtime.util
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertTrue
+
+class JmesPathTest {
+    @Test
+    fun flattenNestedLists() {
+        val nestedList = listOf(
+            listOf(1, 2, 3),
+            listOf(4, 5),
+            listOf(6),
+        )
+        val flattenedList = nestedList.flattenIfPossible()
+        assertEquals(listOf(1, 2, 3, 4, 5, 6), flattenedList)
+    }
+
+    @Test
+    fun flattenEmptyNestedLists() {
+        val nestedList = listOf(
+            listOf<Int>(),
+            listOf(),
+            listOf(),
+        )
+        val flattenedList = nestedList.flattenIfPossible()
+        assertTrue(flattenedList.isEmpty())
+    }
+
+    @Test
+    fun flattenNestedEmptyAndNonEmptyNestedLists() {
+        val nestedList = listOf(
+            listOf(1, 2),
+            listOf(),
+            listOf(3, 4, 5),
+        )
+        val flattenedList = nestedList.flattenIfPossible()
+        assertEquals(listOf(1, 2, 3, 4, 5), flattenedList)
+    }
+
+    @Test
+    fun flattenList() {
+        val nestedList = listOf(
+            listOf(1, 2, 3),
+        )
+        val flattenedList = nestedList.flattenIfPossible()
+        assertEquals(listOf(1, 2, 3), flattenedList)
+    }
+}