Delegate properties to property references

#KT-8658

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

@@ -0,0 +1,106 @@
+/*
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+@file:Suppress("PackageDirectoryMismatch")
+package kotlin
+
+import kotlin.reflect.*
+
+/**
+ * An extension operator that allows delegating a read-only property of type [V]
+ * to a property reference to a property of type [V] or its subtype.
+ *
+ * @receiver A property reference to a read-only or mutable property of type [V] or its subtype.
+ * The reference is without a receiver, i.e. it either references a top-level property or
+ * has the receiver bound to it.
+ *
+ * Example:
+ *
+ * ```
+ * class Login(val username: String)
+ * val defaultLogin = Login("Admin")
+ * val defaultUsername by defaultLogin::username
+ * // equivalent to
+ * val defaultUserName get() = defaultLogin.username
+ * ```
+ */
+@SinceKotlin("1.4")
+@kotlin.internal.InlineOnly
+public inline operator fun <V> KProperty0<V>.getValue(thisRef: Any?, property: KProperty<*>): V {
+    return get()
+}
+
+/**
+ * An extension operator that allows delegating a mutable property of type [V]
+ * to a property reference to a mutable property of the same type [V].
+ *
+ * @receiver A property reference to a mutable property of type [V].
+ * The reference is without a receiver, i.e. it either references a top-level property or
+ * has the receiver bound to it.
+ *
+ * Example:
+ *
+ * ```
+ * class Login(val username: String, var incorrectAttemptCounter: Int = 0)
+ * val defaultLogin = Login("Admin")
+ * var defaultLoginAttempts by defaultLogin::incorrectAttemptCounter
+ * // equivalent to
+ * var defaultLoginAttempts: Int
+ *     get() = defaultLogin.incorrectAttemptCounter
+ *     set(value) { defaultLogin.incorrectAttemptCounter = value }
+ * ```
+ */
+@SinceKotlin("1.4")
+@kotlin.internal.InlineOnly
+public inline operator fun <V> KMutableProperty0<V>.setValue(thisRef: Any?, property: KProperty<*>, value: V) {
+    set(value)
+}
+
+
+/**
+ * An extension operator that allows delegating a read-only member or extension property of type [V]
+ * to a property reference to a member or extension property of type [V] or its subtype.
+ *
+ * @receiver A property reference to a read-only or mutable property of type [V] or its subtype.
+ * The reference has an unbound receiver of type [T].
+ *
+ * Example:
+ *
+ * ```
+ * class Login(val username: String)
+ * val Login.user by Login::username
+ * // equivalent to
+ * val Login.user get() = this.username
+ * ```
+ */
+@SinceKotlin("1.4")
+@kotlin.internal.InlineOnly
+public inline operator fun <T, V> KProperty1<T, V>.getValue(thisRef: T, property: KProperty<*>): V {
+    return get(thisRef)
+}
+
+/**
+ * An extension operator that allows delegating a mutable member or extension property of type [V]
+ * to a property reference to a member or extension mutable property of the same type [V].
+ *
+ * @receiver A property reference to a read-only or mutable property of type [V] or its subtype.
+ * The reference has an unbound receiver of type [T].
+ *
+ * Example:
+ *
+ * ```
+ * class Login(val username: String, var incorrectAttemptCounter: Int)
+ * var Login.attempts by Login::incorrectAttemptCounter
+ * // equivalent to
+ * var Login.attempts: Int
+ *     get() = this.incorrectAttemptCounter
+ *     set(value) { this.incorrectAttemptCounter = value }
+ * ```
+ */
+@SinceKotlin("1.4")
+@kotlin.internal.InlineOnly
+public inline operator fun <T, V> KMutableProperty1<T, V>.setValue(thisRef: T, property: KProperty<*>, value: V) {
+    set(thisRef, value)
+}