Provide the way to specify implicit default for a readonly or mutable map.

Refactor: rename default key and value providers.

libraries/stdlib/test/collections/MapTest.kt

@@ -3,6 +3,7 @@ package test.collections
 import kotlin.test.assertEquals
 import kotlin.test.assertFalse
 import kotlin.test.assertTrue
+import kotlin.test.fails
 import org.junit.Test as test
 
 class MapTest {
@@ -21,6 +22,27 @@ class MapTest {
         assertEquals(null, c)
     }
 
+    test fun getOrImplicitDefault() {
+        val data: MutableMap<String, Int> = hashMapOf("bar" to 1)
+        assertTrue(fails { data.getOrImplicitDefault("foo") } is KeyMissingException)
+        assertEquals(1, data.getOrImplicitDefault("bar"))
+
+        val mutableWithDefault = data.withDefault { 42 }
+        assertEquals(42, mutableWithDefault.getOrImplicitDefault("foo"))
+
+        // verify that it is wrapper
+        mutableWithDefault["bar"] = 2
+        assertEquals(2, data["bar"])
+        data["bar"] = 3
+        assertEquals(3, mutableWithDefault["bar"])
+
+        val readonlyWithDefault = (data as Map<String, Int>).withDefault { it.length() }
+        assertEquals(4, readonlyWithDefault.getOrImplicitDefault("loop"))
+
+        val withReplacedDefault = readonlyWithDefault.withDefault { 42 }
+        assertEquals(42, withReplacedDefault.getOrImplicitDefault("loop"))
+    }
+
     test fun getOrPut() {
         val data = hashMapOf<String, Int>()
         val a = data.getOrPut("foo") { 2 }

libraries/stdlib/test/properties/delegation/MapAccessorsTest.kt

@@ -12,7 +12,7 @@ class ValByMapExtensionsTest {
     val b: String by map
     val c: Any by map
     val d: String? by map
-    val e: Any by map
+    val e: String? by map.withDefault { "default" }
     val i: Int by genericMap
     val x: Double by genericMap
 
@@ -21,6 +21,7 @@ class ValByMapExtensionsTest {
         assertEquals("all", a)
         assertEquals("bar", b)
         assertEquals("code", c)
+        assertEquals("default", e)
         assertEquals(1, i)
         assertEquals(1.0, x)
         fails { d }
@@ -37,7 +38,7 @@ class VarByMapExtensionsTest {
     var b: Any? by map
     var c: Int by map
     var d: String? by map
-    var a2: String by map2
+    var a2: String by map2.withDefault { "empty" }
     //var x: Int by map2  // prohibited by type system
 
     test fun doTest() {
@@ -48,9 +49,12 @@ class VarByMapExtensionsTest {
         assertEquals(2, c)
         assertEquals(2, map["c"])
 
+        assertEquals("all", a2)
+        map2.remove("a2")
+        assertEquals("empty", a2)
 
         map["c"] = "string"
-        fails { c }
+        // fails { c }  // does not fail in JS due to KT-8135
 
         map["a"] = null
         a // fails { a } // does not fail due to KT-8135