detect valid extension main functions (#1227)

Fixes #KT-18083

idea/src/org/jetbrains/kotlin/idea/inspections/UnusedReceiverParameterInspection.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2016 JetBrains s.r.o.
+ * 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.
@@ -24,6 +24,7 @@ import com.intellij.psi.PsiElementVisitor
 import org.jetbrains.kotlin.descriptors.CallableDescriptor
 import org.jetbrains.kotlin.descriptors.ReceiverParameterDescriptor
 import org.jetbrains.kotlin.idea.KotlinBundle
+import org.jetbrains.kotlin.idea.MainFunctionDetector
 import org.jetbrains.kotlin.idea.caches.resolve.analyze
 import org.jetbrains.kotlin.idea.core.isOverridable
 import org.jetbrains.kotlin.idea.refactoring.changeSignature.KotlinChangeSignatureConfiguration
@@ -54,6 +55,8 @@ class UnusedReceiverParameterInspection : AbstractKotlinInspection() {
 
                 val callable = callableDeclaration.descriptor
 
+                if (callable != null && MainFunctionDetector.isMain(callable)) return
+
                 var used = false
                 callableDeclaration.acceptChildren(object : KtVisitorVoid() {
                     override fun visitKtElement(element: KtElement) {