"Introduce 'when' subject" intention: suggest for "this" expression

#KT-12567 Fixed

idea/src/org/jetbrains/kotlin/idea/intentions/branchedTransformations/branchedTransformationUtils.kt

@@ -59,15 +59,14 @@ fun KtWhenExpression.getSubjectToIntroduce(): KtExpression?  {
         for (condition in conditions) {
             if (condition !is KtWhenConditionWithExpression) return null
 
-            val candidate = condition.expression?.getWhenConditionSubjectCandidate() as? KtNameReferenceExpression ?: return null
+            val candidate = condition.expression?.getWhenConditionSubjectCandidate() ?: return null
+            if (candidate !is KtNameReferenceExpression && candidate !is KtThisExpression) return null
 
             if (lastCandidate == null) {
                 lastCandidate = candidate
-            }
+            } else if (!lastCandidate.matches(candidate)) {
-            else if (!lastCandidate.matches(candidate)) {
                 return null
             }
-
         }
     }
 

idea/testData/inspectionsLocal/branched/introduceWhenSubject/this.kt

@@ -0,0 +1,7 @@
+fun Number.test() {
+    <caret>when {
+        this is Int -> {}
+        this is Long -> {}
+        this is Short -> {}
+    }
+}

idea/testData/inspectionsLocal/branched/introduceWhenSubject/this.kt.after

@@ -0,0 +1,7 @@
+fun Number.test() {
+    <caret>when (this) {
+        is Int -> {}
+        is Long -> {}
+        is Short -> {}
+    }
+}

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

@@ -460,6 +460,11 @@ public class LocalInspectionTestGenerated extends AbstractLocalInspectionTest {
                 runTest("idea/testData/inspectionsLocal/branched/introduceWhenSubject/lineBreaksAndComments.kt");
             }
 
+            @TestMetadata("this.kt")
+            public void testThis() throws Exception {
+                runTest("idea/testData/inspectionsLocal/branched/introduceWhenSubject/this.kt");
+            }
+
             @TestMetadata("whenWithEqualityTests.kt")
             public void testWhenWithEqualityTests() throws Exception {
                 runTest("idea/testData/inspectionsLocal/branched/introduceWhenSubject/whenWithEqualityTests.kt");