Explicit this inspection: fix false positive with subclass and extension

#KT-27550 Fixed

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

@@ -30,6 +30,7 @@ import org.jetbrains.kotlin.psi.*
 import org.jetbrains.kotlin.psi.psiUtil.getChildOfType
 import org.jetbrains.kotlin.resolve.BindingContext
 import org.jetbrains.kotlin.synthetic.SyntheticJavaPropertyDescriptor
+import org.jetbrains.kotlin.types.typeUtil.isSubtypeOf
 
 class ExplicitThisInspection : AbstractKotlinInspection() {
 
@@ -55,6 +56,7 @@ class ExplicitThisInspection : AbstractKotlinInspection() {
             val scope = expression.getResolutionScope(context) ?: return
 
             val referenceExpression = reference as? KtNameReferenceExpression ?: reference.getChildOfType() ?: return
+            val receiverType = context[BindingContext.EXPRESSION_TYPE_INFO, thisExpression]?.type ?: return
 
             //we avoid overload-related problems by enforcing that there is only one candidate
             val name = referenceExpression.getReferencedNameAsName()
@@ -62,10 +64,11 @@ class ExplicitThisInspection : AbstractKotlinInspection() {
             if (referenceExpression.getCallableDescriptor() is SyntheticJavaPropertyDescriptor) {
                 if (candidates.map { it.containingDeclaration }.distinct().size != 1) return
             } else {
-                if (candidates.size != 1) return
+                val candidate = candidates.singleOrNull() ?: return
+                val extensionType = candidate.extensionReceiverParameter?.type
+                if (extensionType != null && extensionType != receiverType && receiverType.isSubtypeOf(extensionType)) return
             }
 
-            val receiverType = context[BindingContext.EXPRESSION_TYPE_INFO, thisExpression]?.type ?: return
             val expressionFactory = scope.getFactoryForImplicitReceiverWithSubtypeOf(receiverType) ?: return
 
             val label = thisExpression.getLabelName() ?: ""
@@ -85,9 +88,7 @@ class ExplicitThisInspection : AbstractKotlinInspection() {
 
         override fun applyFix(project: Project, descriptor: ProblemDescriptor) {
             val thisExpression = descriptor.psiElement as? KtThisExpression ?: return
-            val parent = thisExpression.parent
-
-            when (parent) {
+            when (val parent = thisExpression.parent) {
                 is KtDotQualifiedExpression -> parent.replace(parent.selectorExpression ?: return)
                 is KtCallableReferenceExpression -> thisExpression.delete()
             }

idea/testData/inspectionsLocal/explicitThis/differentReceiverInstanceExtension2.kt

@@ -0,0 +1,12 @@
+// WITH_RUNTIME
+// PROBLEM: none
+
+open class Foo
+
+class SubFoo : Foo()
+
+val Foo.bar: String get() = ""
+
+fun Foo.func() = Foo().apply {
+    <caret>this@func.bar
+}

idea/testData/inspectionsLocal/explicitThis/differentReceiverInstanceExtension3.kt

@@ -0,0 +1,12 @@
+// WITH_RUNTIME
+// PROBLEM: none
+
+open class Foo
+
+class SubFoo : Foo()
+
+val Foo.bar: String get() = ""
+
+fun SubFoo.func() = Foo().apply {
+    <caret>this@func.bar
+}

idea/testData/inspectionsLocal/explicitThis/differentReceiverInstanceExtension4.kt

@@ -0,0 +1,11 @@
+// WITH_RUNTIME
+
+open class Foo
+
+class SubFoo : Foo()
+
+val SubFoo.bar: String get() = ""
+
+fun SubFoo.func() = Foo().apply {
+    <caret>this@func.bar
+}

idea/testData/inspectionsLocal/explicitThis/differentReceiverInstanceExtension4.kt.after

@@ -0,0 +1,11 @@
+// WITH_RUNTIME
+
+open class Foo
+
+class SubFoo : Foo()
+
+val SubFoo.bar: String get() = ""
+
+fun SubFoo.func() = Foo().apply {
+    bar
+}

idea/tests/org/jetbrains/kotlin/idea/inspections/LocalInspectionTestGenerated.java

@@ -3758,6 +3758,21 @@ public class LocalInspectionTestGenerated extends AbstractLocalInspectionTest {
             runTest("idea/testData/inspectionsLocal/explicitThis/differentReceiverInstanceExtension.kt");
         }
 
+        @TestMetadata("differentReceiverInstanceExtension2.kt")
+        public void testDifferentReceiverInstanceExtension2() throws Exception {
+            runTest("idea/testData/inspectionsLocal/explicitThis/differentReceiverInstanceExtension2.kt");
+        }
+
+        @TestMetadata("differentReceiverInstanceExtension3.kt")
+        public void testDifferentReceiverInstanceExtension3() throws Exception {
+            runTest("idea/testData/inspectionsLocal/explicitThis/differentReceiverInstanceExtension3.kt");
+        }
+
+        @TestMetadata("differentReceiverInstanceExtension4.kt")
+        public void testDifferentReceiverInstanceExtension4() throws Exception {
+            runTest("idea/testData/inspectionsLocal/explicitThis/differentReceiverInstanceExtension4.kt");
+        }
+
         @TestMetadata("differentReceiverType.kt")
         public void testDifferentReceiverType() throws Exception {
             runTest("idea/testData/inspectionsLocal/explicitThis/differentReceiverType.kt");