8358801: javac produces class that does not pass verifier.

src/jdk.compiler/share/classes/com/sun/tools/javac/jvm/Gen.java

@@ -756,13 +756,25 @@ public CondItem genCond(JCTree _tree, boolean markBranches) {
             }
             CondItem result = genCond(tree.expr, markBranches);
             code.endScopes(limit);
+            //make sure variables defined in the let expression are not included
+            //in the defined variables for jumps that go outside of this let
+            //expression:
+            undefineVariablesInChain(result.falseJumps, limit);
+            undefineVariablesInChain(result.trueJumps, limit);
             return result;
         } else {
             CondItem result = genExpr(_tree, syms.booleanType).mkCond();
             if (markBranches) result.tree = _tree;
             return result;
         }
     }
+        //where:
+        private void undefineVariablesInChain(Chain toClear, int limit) {
+            while (toClear != null) {
+                toClear.state.defined.excludeFrom(limit);
+                toClear = toClear.next;
+            }
+        }
 
     public Code getCode() {
         return code;

test/langtools/tools/javac/patterns/T8358801.java

@@ -0,0 +1,132 @@
+/*
+ * 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 8358801
+ * @summary Verify variables introduced by let expressions are correctly undefined
+ * @library /tools/lib
+ */
+
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.util.Arrays;
+
+import javax.tools.JavaFileObject;
+import javax.tools.ToolProvider;
+
+import com.sun.source.util.JavacTask;
+import java.lang.classfile.ClassFile;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Objects;
+import javax.tools.JavaCompiler;
+
+import toolbox.JavaTask;
+import toolbox.Task;
+import toolbox.TestRunner;
+import toolbox.ToolBox;
+
+public class T8358801 extends TestRunner {
+    private ToolBox tb;
+
+    public static void main(String... args) throws Exception {
+        new T8358801().runTests();
+    }
+
+    T8358801() {
+        super(System.err);
+        tb = new ToolBox();
+    }
+
+    public void runTests() throws Exception {
+        runTests(m -> new Object[] { Paths.get(m.getName()) });
+    }
+
+    @Test
+    public void testPatternsInJava(Path base) throws Exception {
+        Path classes = base.resolve("classes");
+
+        List<JavaFileObject> files = new ArrayList<>();
+        files.add(new ToolBox.JavaSource(
+            """
+            public class Main {
+                private boolean test(String s, int i) {
+                    if (s.subSequence(0, 1) instanceof Runnable r) {
+                        return true;
+                    }
+
+                    switch (i) {
+                        case 0:
+                            String clashing1 = null;
+                            String clashing2 = null;
+                            String clashing3 = null;
+                            String clashing4 = null;
+                            return true;
+                        default:
+                            System.out.println("correct");
+                            return true;
+                    }
+                }
+
+                public static void main(String[] args) {
+                    new Main().test("hello", 1);
+                }
+            }
+            """
+        ));
+
+        if (Files.exists(classes)) {
+            tb.cleanDirectory(classes);
+        } else {
+            Files.createDirectories(classes);
+        }
+
+        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
+        Iterable<String> options = Arrays.asList("-d", classes.toString());
+        JavacTask task = (JavacTask) compiler.getTask(null, null, null, options, null, files);
+
+        task.generate();
+
+        List<VerifyError> errors = ClassFile.of().verify(classes.resolve("Main.class"));
+
+        if (!errors.isEmpty()) {
+            throw new AssertionError("verify errors found: " + errors);
+        }
+
+        List<String> log =
+            new JavaTask(tb).classpath(classes.toString())
+                            .className("Main")
+                            .run()
+                            .writeAll()
+                            .getOutputLines(Task.OutputKind.STDOUT);
+        List<String> expected = List.of("correct");
+
+        if (!Objects.equals(log, expected)) {
+            throw new AssertionError("Incorrect result, expected: " + expected +
+                                     ", got: " + log);
+        }
+    }
+
+}