java: don't report duplicate switch branches when they fall through (IDEA-223905)

GitOrigin-RevId: 9017b2cb574e41d4acb8b11f7408472823c910e3

java/java-impl/src/com/intellij/codeInspection/DuplicateBranchesInSwitchInspection.java

@@ -1,4 +1,4 @@
-// Copyright 2000-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license that can be found in the LICENSE file.
+// Copyright 2000-2021 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license that can be found in the LICENSE file.
 package com.intellij.codeInspection;
 
 import com.intellij.codeInspection.util.InspectionMessage;
@@ -223,7 +223,7 @@ public class DuplicateBranchesInSwitchInspection extends LocalInspectionTool {
       return previousBranch; // the code without a label is not allowed in 'switch', just ignore it
     }
     Branch branch = new Branch(labels, statementList, hasImplicitBreak, comments.fetchTexts());
-    if (previousBranch == null || !previousBranch.canFallThrough()) {
+    if (!branch.canFallThrough() && (previousBranch == null || !previousBranch.canFallThrough())) {
       int hash = branch.hash();
       List<Branch> branches = branchesByHash.get(hash);
       if (branches == null) branchesByHash.put(hash, branches = new ArrayList<>());

java/java-tests/testData/inspection/duplicateBranchesInSwitch/DuplicateFallThrough.java

@@ -0,0 +1,19 @@
+public class DuplicateFallThrough {
+
+  public static void test(int x) {
+    switch (x) {
+      case 0:
+        System.out.println("a");
+      case 1:
+        System.out.println("b");
+        break;
+      case 2:
+        System.out.println("a");
+      case 3:
+        System.out.println("c");
+        break;
+      default:
+        System.out.println("d");
+    }
+  }
+}

java/java-tests/testSrc/com/intellij/java/codeInspection/DuplicateBranchesInSwitchTest.kt

@@ -1,4 +1,4 @@
-// Copyright 2000-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license that can be found in the LICENSE file.
+// Copyright 2000-2021 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license that can be found in the LICENSE file.
 package com.intellij.java.codeInspection
 
 import com.intellij.JavaTestUtil
@@ -25,6 +25,7 @@ class DuplicateBranchesInSwitchTest : LightJavaCodeInsightFixtureTestCase() {
   fun testFallThrough() = doTest()
   fun testFallThroughBefore() = doTest()
   fun testAllFallThrough() = doTest()
+  fun testDuplicateFallThrough() = doTest()
   fun testNoLastBreak() = doTest()
   fun testFallThroughToBreak() = doTest()
   fun testThreeDuplicates() = doTest()