CompileTimeConstantExpressionMatcher: Handle switch expressions

This change updates CompileTimeConstantExpressionMatcher to support switch
expressions, ensuring that all branches are constant expressions.

PiperOrigin-RevId: 789743986

Author: graememorgan

annotations/src/main/java/com/google/errorprone/annotations/CompileTimeConstant.java

@@ -41,6 +41,8 @@
  *   <li>the expression is a {@link String}, and formed from the concatenation of symbols which meet
  *       these conditions, or
  *   <li>the expression is a ternary condition, where both branches satisfy these conditions, or
+ *   <li>the expression is a switch expression, where every case is either a constant expression or
+ *       throws, or
  *   <li>the expression is an immutable collection with all values known to satisfy these conditions
  *       (for example, {@code ImmutableSet.of("a", "b", "c")}).
  * </ol>

check_api/src/main/java/com/google/errorprone/matchers/CompileTimeConstantExpressionMatcher.java

@@ -30,11 +30,14 @@
 import com.google.errorprone.annotations.CompileTimeConstant;
 import com.google.errorprone.suppliers.Supplier;
 import com.sun.source.tree.BinaryTree;
+import com.sun.source.tree.CaseTree.CaseKind;
 import com.sun.source.tree.ConditionalExpressionTree;
 import com.sun.source.tree.ExpressionTree;
 import com.sun.source.tree.IdentifierTree;
 import com.sun.source.tree.MethodInvocationTree;
 import com.sun.source.tree.ParenthesizedTree;
+import com.sun.source.tree.SwitchExpressionTree;
+import com.sun.source.tree.ThrowTree;
 import com.sun.source.tree.Tree;
 import com.sun.source.tree.Tree.Kind;
 import com.sun.source.tree.TypeCastTree;
@@ -102,6 +105,17 @@ public Boolean visitConditionalExpression(ConditionalExpressionTree tree, Void u
                   && tree.getFalseExpression().accept(this, null);
             }
 
+            @Override
+            public Boolean visitSwitchExpression(SwitchExpressionTree tree, Void unused) {
+              // This is intentionally quite restrictive: only accept bare values and throws.
+              return tree.getCases().stream()
+                  .allMatch(
+                      c ->
+                          c.getCaseKind().equals(CaseKind.RULE)
+                              && (c.getBody().accept(this, null)
+                                  || c.getBody() instanceof ThrowTree));
+            }
+
             @Override
             public Boolean visitMethodInvocation(MethodInvocationTree tree, Void unused) {
               return IMMUTABLE_FACTORY.matches(tree, state)

core/src/test/java/com/google/errorprone/bugpatterns/CompileTimeConstantCheckerTest.java

@@ -887,4 +887,68 @@ public abstract class CompileTimeConstantTestCase {
             """)
         .doTest();
   }
+
+  @Test
+  public void switchExpression_allBranchesConstant() {
+    compilationHelper
+        .addSourceLines(
+            "Test.java",
+            """
+            import com.google.errorprone.annotations.CompileTimeConstant;
+
+            public class Test {
+              @CompileTimeConstant
+              final String s =
+                  switch (1) {
+                    case 1 -> "a";
+                    case 2 -> "b";
+                    default -> "c";
+                  };
+            }
+            """)
+        .doTest();
+  }
+
+  @Test
+  public void switchExpression_notAllBranchesConstant() {
+    compilationHelper
+        .addSourceLines(
+            "Test.java",
+            """
+            import com.google.errorprone.annotations.CompileTimeConstant;
+
+            public class Test {
+              @CompileTimeConstant
+              final String s =
+                  // BUG: Diagnostic contains:
+                  switch (1) {
+                    case 1 -> "a";
+                    case 2 -> toString();
+                    default -> "c";
+                  };
+            }
+            """)
+        .doTest();
+  }
+
+  @Test
+  public void switchExpression_onlyConsiderReturningBranches() {
+    compilationHelper
+        .addSourceLines(
+            "Test.java",
+            """
+            import com.google.errorprone.annotations.CompileTimeConstant;
+
+            public class Test {
+              @CompileTimeConstant
+              final String s =
+                  switch (1) {
+                    case 1 -> "a";
+                    case 2 -> "b";
+                    default -> throw new RuntimeException();
+                  };
+            }
+            """)
+        .doTest();
+  }
 }