Do not allow to delegate readonly property to a map with incompatible value type.

Infer type of delegated readonly property from the map value type.

libraries/stdlib/src/kotlin/properties/MapAccessors.kt

@@ -11,7 +11,7 @@ package kotlin.properties
  *
  * @throws NoSuchElementException when the map doesn't contain value for the property name and doesn't provide an implicit default (see [withDefault]).
  */
-public fun <V> Map<in String, *>.getValue(thisRef: Any?, property: PropertyMetadata): V = getOrImplicitDefault(property.name) as V
+public fun <V, V1: V> Map<in String, @Exact V>.getValue(thisRef: Any?, property: PropertyMetadata): V1 = getOrImplicitDefault(property.name) as V1
 
 /**
  * Returns the value of the property for the given object from this mutable map.

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

@@ -9,11 +9,13 @@ class ValByMapExtensionsTest {
     val map: Map<String, String> = hashMapOf("a" to "all", "b" to "bar", "c" to "code")
     val genericMap = mapOf<String, Any?>("i" to 1, "x" to 1.0)
 
-    val a: String by map
+    val a by map
     val b: String by map
     val c: Any by map
     val d: String? by map
-    val e: String? by map.withDefault { "default" }
+    val e: String by map.withDefault { "default" }
+    val f: String? by map.withDefault { null }
+    // val n: Int by map // prohibited by type system
     val i: Int by genericMap
     val x: Double by genericMap
 
@@ -23,6 +25,7 @@ class ValByMapExtensionsTest {
         assertEquals("bar", b)
         assertEquals("code", c)
         assertEquals("default", e)
+        assertEquals(null, f)
         assertEquals(1, i)
         assertEquals(1.0, x)
         assertTrue(assertFails { d } is NoSuchElementException)