Introduce KProperty{1,2}.getExtensionDelegate

These two functions are supposed to fix an inconvenience in the design of
KProperty{1,2}.getDelegate for extension properties, where you have to pass an
instance of the extension receiver which is going to be completely ignored

 #KT-8384 Fixed

core/reflection.jvm/src/kotlin/reflect/full/KProperties.kt

@@ -0,0 +1,48 @@
+/*
+ * Copyright 2010-2017 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+@file:JvmName("KProperties")
+package kotlin.reflect.full
+
+import kotlin.reflect.KProperty1
+import kotlin.reflect.KProperty2
+import kotlin.reflect.jvm.internal.KPropertyImpl
+
+/**
+ * Returns the instance of a delegated **extension property**, or `null` if this property is not delegated.
+ * Throws an exception if this is not an extension property.
+ *
+ * @see [KProperty1.getDelegate]
+ */
+@SinceKotlin("1.1")
+fun KProperty1<*, *>.getExtensionDelegate(): Any? {
+    @Suppress("UNCHECKED_CAST")
+    return (this as KProperty1<Any?, *>).getDelegate(KPropertyImpl.EXTENSION_PROPERTY_DELEGATE)
+}
+
+/**
+ * Returns the instance of a delegated **member extension property**, or `null` if this property is not delegated.
+ * Throws an exception if this is not an extension property.
+ *
+ * @param receiver the instance of the class used to retrieve the value of the property delegate.
+ *
+ * @see [KProperty2.getDelegate]
+ */
+@SinceKotlin("1.1")
+fun <D> KProperty2<D, *, *>.getExtensionDelegate(receiver: D): Any? {
+    @Suppress("UNCHECKED_CAST")
+    return (this as KProperty2<D, Any?, *>).getDelegate(receiver, KPropertyImpl.EXTENSION_PROPERTY_DELEGATE)
+}

core/reflection.jvm/src/kotlin/reflect/jvm/internal/KPropertyImpl.kt

@@ -87,6 +87,12 @@ internal abstract class KPropertyImpl<out R> private constructor(
 
     protected fun getDelegate(field: Field?, receiver: Any?): Any? =
             try {
+                if (receiver === EXTENSION_PROPERTY_DELEGATE) {
+                    if (descriptor.extensionReceiverParameter == null) {
+                        throw RuntimeException("'$this' is not an extension property and thus getExtensionDelegate() " +
+                                               "is not going to work, use getDelegate() instead")
+                    }
+                }
                 field?.get(receiver)
             }
             catch (e: IllegalAccessException) {
@@ -164,6 +170,10 @@ internal abstract class KPropertyImpl<out R> private constructor(
             computeCallerForAccessor(isGetter = false)
         }
     }
+
+    companion object {
+        val EXTENSION_PROPERTY_DELEGATE = Any()
+    }
 }