Review fixes: special case of toMap for singleton map.

#KT-9108

js/js.libraries/src/core/kotlin.kt

@@ -92,7 +92,9 @@ internal fun <T> arrayPlusCollection(array: dynamic, collection: Collection<T>):
 }
 
 // no singleton map implementation in js, return map as is
-internal inline fun <K, V> Map<K, V>.toSingletonMap(): Map<K, V> = this
+internal inline fun <K, V> Map<K, V>.toSingletonMapOrSelf(): Map<K, V> = this
+
+internal inline fun <K, V> Map<out K, V>.toSingletonMap(): Map<K, V> = this.toMutableMap()
 
 internal inline fun <T> Array<out T>.copyToArrayOfAny(isVarargs: Boolean): Array<out Any?> =
         if (isVarargs)

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

@@ -445,6 +445,7 @@ public fun <K, V, M : MutableMap<in K, in V>> Sequence<Pair<K, V>>.toMap(destina
  */
 public fun <K, V> Map<out K, V>.toMap(): Map<K, V> = when (size) {
     0 -> emptyMap()
+    1 -> toSingletonMap()
     else -> toMutableMap()
 }
 
@@ -551,11 +552,16 @@ public inline operator fun <K, V> MutableMap<in K, in V>.plusAssign(map: Map<K,
 // do not expose for now @kotlin.internal.InlineExposed
 internal fun <K, V> Map<K, V>.optimizeReadOnlyMap() = when (size) {
     0 -> emptyMap()
-    1 -> toSingletonMap()
+    1 -> toSingletonMapOrSelf()
     else -> this
 }
 
-// creates a singleton copy of map, if it there is specialization available in target platform
+// creates a singleton copy of map, if there is specialization available in target platform, otherwise returns itself
 @kotlin.jvm.JvmVersion
-internal fun <K, V> Map<K, V>.toSingletonMap(): Map<K, V>
-        = with (entries.iterator().next()) { Collections.singletonMap(key, value) }
+@kotlin.internal.InlineOnly
+internal inline fun <K, V> Map<K, V>.toSingletonMapOrSelf(): Map<K, V> = toSingletonMap()
+
+// creates a singleton copy of map
+@kotlin.jvm.JvmVersion
+internal fun <K, V> Map<out K, V>.toSingletonMap(): Map<K, V>
+    = with (entries.iterator().next()) { Collections.singletonMap(key, value) }

libraries/stdlib/test/collections/MapTest.kt

@@ -170,6 +170,8 @@ class MapTest {
         assertEquals(expected, pairs.asIterable().toMap())
         assertEquals(expected, pairs.asSequence().toMap())
         assertEquals(expected, expected.toMap())
+        assertEquals(mapOf("a" to 1), expected.filterKeys { it == "a" }.toMap())
+        assertEquals(emptyMap(), expected.filter { false }.toMap())
 
         val mutableMap = expected.toMutableMap()
         assertEquals(expected, mutableMap)
@@ -192,6 +194,9 @@ class MapTest {
 
         val mutableMap2 = mutableMap.toMap(mutableMapOf())
         assertEquals(expected, mutableMap2)
+
+        val mutableMap3 = mutableMap.toMap(hashMapOf<CharSequence, Any>())
+        assertEquals<Map<*, *>>(expected, mutableMap3)
     }
 
     @test fun createWithSelector() {