From cd6a50432f5874966aa8eddf8786cfcb71d903e6 Mon Sep 17 00:00:00 2001
From: Liam Miller-Cushon <cushon@google.com>
Date: Mon, 25 Mar 2024 10:40:08 -0700
Subject: [PATCH] Handle exhaustive switches on sealed types in MissingDefault

PiperOrigin-RevId: 618889679
---
 .../bugpatterns/MissingDefault.java           | 23 +++++++++++++++++++
 .../bugpatterns/MissingDefaultTest.java       | 21 +++++++++++++++++
 2 files changed, 44 insertions(+)

diff --git a/core/src/main/java/com/google/errorprone/bugpatterns/MissingDefault.java b/core/src/main/java/com/google/errorprone/bugpatterns/MissingDefault.java
index 07fe35f2f59..5ea53624666 100644
--- a/core/src/main/java/com/google/errorprone/bugpatterns/MissingDefault.java
+++ b/core/src/main/java/com/google/errorprone/bugpatterns/MissingDefault.java
@@ -33,6 +33,8 @@
 import com.sun.source.tree.StatementTree;
 import com.sun.source.tree.SwitchTree;
 import com.sun.tools.javac.code.Type;
+import com.sun.tools.javac.tree.JCTree;
+import java.lang.reflect.Field;
 import java.util.List;
 import java.util.Optional;
 import javax.lang.model.element.ElementKind;
@@ -55,6 +57,9 @@ public Description matchSwitch(SwitchTree tree, VisitorState state) {
     }
     Optional<? extends CaseTree> maybeDefault = getSwitchDefault(tree);
     if (!maybeDefault.isPresent()) {
+      if (isExhaustive(tree)) {
+        return NO_MATCH;
+      }
       Description.Builder description = buildDescription(tree);
       if (!tree.getCases().isEmpty()) {
         // Inserting the default after the last case is easier than finding the closing brace
@@ -95,4 +100,22 @@ public Description matchSwitch(SwitchTree tree, VisitorState state) {
         .addFix(SuggestedFix.postfixWith(defaultCase, " // fall out"))
         .build();
   }
+
+  private static final Field IS_EXHAUSTIVE = getIsExhaustive();
+
+  private static Field getIsExhaustive() {
+    try {
+      return JCTree.JCSwitch.class.getField("isExhaustive");
+    } catch (NoSuchFieldException e) {
+      return null;
+    }
+  }
+
+  private static boolean isExhaustive(SwitchTree tree) {
+    try {
+      return IS_EXHAUSTIVE != null && IS_EXHAUSTIVE.getBoolean(tree);
+    } catch (IllegalAccessException e) {
+      throw new LinkageError(e.getMessage(), e);
+    }
+  }
 }
diff --git a/core/src/test/java/com/google/errorprone/bugpatterns/MissingDefaultTest.java b/core/src/test/java/com/google/errorprone/bugpatterns/MissingDefaultTest.java
index 9799b78fe4f..977467e0d17 100644
--- a/core/src/test/java/com/google/errorprone/bugpatterns/MissingDefaultTest.java
+++ b/core/src/test/java/com/google/errorprone/bugpatterns/MissingDefaultTest.java
@@ -282,4 +282,25 @@ public void arrowComment() {
             "}")
         .doTest();
   }
+
+  @Test
+  public void exhaustiveExpressionSwitch() {
+    assumeTrue(RuntimeVersion.isAtLeast21());
+    compilationHelper
+        .addSourceLines(
+            "Test.java",
+            "class Test {",
+            "  sealed interface I permits A, B {}",
+            "  abstract static class S {}",
+            "  static final class A extends S implements I {}",
+            "  static final class B extends S implements I {}",
+            "  void f(I i) {",
+            "    switch (i) {",
+            "      case A a -> System.err.println(a);",
+            "      case B b -> System.err.println(b);",
+            "    };",
+            "  }",
+            "}")
+        .doTest();
+  }
 }