8372635: Lambdas do not copy over SYNTHETIC flag for local variables

Reviewed-by: vromero, liach

src/jdk.compiler/share/classes/com/sun/tools/javac/comp/LambdaToMethod.java

@@ -1152,7 +1152,7 @@ public class LambdaToMethod extends TreeTranslator {
                     propagateAnnos = false;
                     break;
                 case LOCAL_VAR:
-                    ret = new VarSymbol(sym.flags() & FINAL, sym.name, sym.type, translatedSym);
+                    ret = new VarSymbol(sym.flags(), sym.name, sym.type, translatedSym);
                     ret.pos = sym.pos;
                     // If sym.data == ElementKind.EXCEPTION_PARAMETER,
                     // set ret.data = ElementKind.EXCEPTION_PARAMETER too.
@@ -1164,7 +1164,8 @@ public class LambdaToMethod extends TreeTranslator {
                     }
                     break;
                 case PARAM:
-                    ret = new VarSymbol((sym.flags() & FINAL) | PARAMETER, sym.name, types.erasure(sym.type), translatedSym);
+                    Assert.check((sym.flags() & PARAMETER) != 0);
+                    ret = new VarSymbol(sym.flags(), sym.name, types.erasure(sym.type), translatedSym);
                     ret.pos = sym.pos;
                     break;
                 default:

test/langtools/tools/javac/patterns/SyntheticVariables.java

@@ -0,0 +1,138 @@
+/*
+ * Copyright (c) 2025, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @bug 8372635
+ * @enablePreview
+ * @summary Verify temporary variables created for pattern matching are
+ *          marked as synthetic.
+ * @compile -g SyntheticVariables.java
+ * @run main SyntheticVariables
+ */
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.lang.classfile.Attributes;
+import java.lang.classfile.ClassFile;
+import java.lang.classfile.ClassModel;
+import java.lang.classfile.MethodModel;
+import java.lang.classfile.attribute.CodeAttribute;
+import java.lang.classfile.attribute.LocalVariableTableAttribute;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+public class SyntheticVariables {
+
+    public static void main(String[] args) throws IOException {
+        try (InputStream in = Test.class.getClassLoader().getResource(Test.class.getName().replace('.', '/') + ".class").openStream()) {
+            ClassModel model = ClassFile.of().parse(in.readAllBytes());
+            Map<String, MethodModel> name2Method = model.methods().stream().collect(Collectors.toMap(m -> m.methodName().stringValue(), m -> m));
+            assertEquals(Set.of("str", "b", "b2", "this", "l", "o"), localVars(name2Method.get("testInMethod")));
+            assertEquals(Set.of("str", "b", "b2", "l", "o"), localVars(name2Method.get("lambda$testInLambda$0")));
+        }
+    }
+
+    private static Set<String> localVars(MethodModel method) {
+        CodeAttribute code = method.findAttribute(Attributes.code()).orElseThrow();
+        LocalVariableTableAttribute lvt = code.findAttribute(Attributes.localVariableTable()).orElseThrow();
+        return lvt.localVariables()
+                  .stream()
+                  .map(info -> info.name().stringValue())
+                  .collect(Collectors.toSet());
+    }
+
+    private static void assertEquals(Set<String> expected, Set<String> actual) {
+        if (!Objects.equals(expected, actual)) {
+            throw new AssertionError("Unexpected value, expected: " + expected +
+                                     ", got: " + actual);
+        }
+    }
+
+    public record Test(Object o) {
+        private void testInMethod() {
+            Object o = create();
+
+            //synthetic variable for the instanceof's expression
+            boolean b = o.toString() instanceof String str && str.isEmpty();
+
+            System.err.println(b);
+
+            //synthetic variable for the switch's selector and index:
+            switch (create()) {
+                //synthetic variable for the nested component values,
+                //and for the synthetic catch over the getters
+                case Test(Test(String str)) when str.isEmpty() -> System.err.println(1);
+                case Test(Test(String str)) -> System.err.println(2);
+                default -> System.err.println(2);
+            }
+
+            List<Integer> l = List.of(0);
+
+            //synthetic variable for case where the static and dynamic types
+            //don't match for primitive patterns:
+            boolean b2 = l.get(0) instanceof int;
+
+            System.err.println(b2);
+        }
+
+        private void testInLambda() {
+            Object o = create();
+            Runnable r = () -> {
+
+                //synthetic variable for the instanceof's expression
+                boolean b = o.toString() instanceof String str && str.isEmpty();
+
+                System.err.println(b);
+
+                //synthetic variable for the switch's selector and index:
+                switch (create()) {
+                    //synthetic variable for the nested component values,
+                    //and for the synthetic catch over the getters
+                    case Test(Test(String str)) when str.isEmpty() -> System.err.println(1);
+                    case Test(Test(String str)) -> System.err.println(2);
+                    default -> System.err.println(2);
+                }
+
+                List<Integer> l = List.of(0);
+
+                //synthetic variable for case where the static and dynamic types
+                //don't match for primitive patterns:
+                boolean b2 = l.get(0) instanceof int;
+
+                System.err.println(b2);
+            };
+        }
+        private static String val = "v";
+        public static Test create() {
+            try {
+                return new Test(new Test(val));
+            } finally {
+                val += val;
+            }
+        }
+    }
+}