8353565: Javac throws "inconsistent stack types at join point" exception

Reviewed-by: vromero, liach, mcimadamore

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

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1999, 2024, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1999, 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
@@ -1807,11 +1807,7 @@ public class Code {
             for (int i=0; i<stacksize; ) {
                 Type t = stack[i];
                 Type tother = other.stack[i];
-                Type result =
+                Type result = commonSuperClass(t, tother);
-                    t==tother ? t :
-                    types.isSubtype(t, tother) ? tother :
-                    types.isSubtype(tother, t) ? t :
-                    error();
                 int w = width(result);
                 stack[i] = result;
                 if (w == 2) Assert.checkNull(stack[i+1]);
@@ -1820,8 +1816,23 @@ public class Code {
             return this;
         }
 
-        Type error() {
+        private Type commonSuperClass(Type t1, Type t2) {
-            throw new AssertionError("inconsistent stack types at join point");
+            if (t1 == t2) {
+                return t1;
+            } else if (types.isSubtype(t1, t2)) {
+                return t2;
+            } else if (types.isSubtype(t2, t1)) {
+                return t1;
+            } else {
+                Type lub = types.lub(t1, t2);
+
+                if (lub.hasTag(BOT)) {
+                    throw Assert.error("Cannot find a common super class of: " +
+                                       t1 + " and " + t2);
+                }
+
+                return types.erasure(lub);
+            }
         }
 
         void dump() {

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

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1999, 2024, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1999, 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
@@ -1853,7 +1853,6 @@ public class Gen extends JCTree.Visitor {
                 if (switchResult != null)
                     switchResult.load();
 
-                code.state.forceStackTop(tree.target.type);
                 targetEnv.info.addExit(code.branch(goto_));
                 code.markDead();
             }
@@ -1969,7 +1968,6 @@ public class Gen extends JCTree.Visitor {
             int startpc = genCrt ? code.curCP() : 0;
             code.statBegin(tree.truepart.pos);
             genExpr(tree.truepart, pt).load();
-            code.state.forceStackTop(tree.type);
             if (genCrt) code.crt.put(tree.truepart, CRT_FLOW_TARGET,
                                      startpc, code.curCP());
             thenExit = code.branch(goto_);
@@ -1979,7 +1977,6 @@ public class Gen extends JCTree.Visitor {
             int startpc = genCrt ? code.curCP() : 0;
             code.statBegin(tree.falsepart.pos);
             genExpr(tree.falsepart, pt).load();
-            code.state.forceStackTop(tree.type);
             if (genCrt) code.crt.put(tree.falsepart, CRT_FLOW_TARGET,
                                      startpc, code.curCP());
         }

test/langtools/tools/javac/switchexpr/ExpressionSwitchEmbedding.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018, 2020, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2018, 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
@@ -23,7 +23,7 @@
 
 /*
  * @test
- * @bug 8214031 8214114 8236546
+ * @bug 8214031 8214114 8236546 8353565
  * @summary Verify switch expressions embedded in various statements work properly.
  * @compile ExpressionSwitchEmbedding.java
  * @run main ExpressionSwitchEmbedding
@@ -32,6 +32,7 @@
 public class ExpressionSwitchEmbedding {
     public static void main(String... args) {
         new ExpressionSwitchEmbedding().run();
+        new ExpressionSwitchEmbedding().runStackMapMergingTest();
     }
 
     private void run() {
@@ -330,6 +331,110 @@ public class ExpressionSwitchEmbedding {
         }
     }
 
+    private void runStackMapMergingTest() {
+        //JDK-8353565: verify that two types neither of which is a subtype of the other
+        //can be merged while computing StackMaps.
+        if (!(computeTypeAtMergePoint1(E.A, E.A) instanceof Impl1a)) {
+            throw new AssertionError("Unexpected result");
+        }
+        if (runMethodForInterfaceTypeAtMergePoint1(E.A, E.A) != 1) {
+            throw new AssertionError("Unexpected result");
+        }
+        if (runMethodForInterfaceTypeAtMergePoint2(E.A, E.A) != 2) {
+            throw new AssertionError("Unexpected result");
+        }
+    }
+
+    private Root computeTypeAtMergePoint1(E e1, E e2) {
+        return (Root) switch (e1) {
+            case A -> switch (e2) {
+                case A -> new Impl1a();
+                case B -> new Impl1b();
+                case C -> new Impl1c();
+            };
+            case B -> switch (e2) {
+                case A -> new Impl2a();
+                case B -> new Impl2b();
+                case C -> new Impl2c();
+            };
+            case C -> switch (e2) {
+                case A -> new Impl3a();
+                case B -> new Impl3b();
+                case C -> new Impl3c();
+            };
+        };
+    }
+
+    private int runMethodForInterfaceTypeAtMergePoint1(E e1, E e2) {
+        return (switch (e1) {
+            case A -> switch (e2) {
+                case A -> new C1();
+                case B -> new C1();
+                case C -> new C1();
+            };
+            case B -> switch (e2) {
+                case A -> new C2();
+                case B -> new C2();
+                case C -> new C2();
+            };
+            case C -> switch (e2) {
+                case A -> new C3();
+                case B -> new C3();
+                case C -> new C3();
+            };
+        }).test1();
+    }
+
+    private int runMethodForInterfaceTypeAtMergePoint2(E e1, E e2) {
+        return (switch (e1) {
+            case A -> switch (e2) {
+                case A -> new C1();
+                case B -> new C1();
+                case C -> new C1();
+            };
+            case B -> switch (e2) {
+                case A -> new C2();
+                case B -> new C2();
+                case C -> new C2();
+            };
+            case C -> switch (e2) {
+                case A -> new C3();
+                case B -> new C3();
+                case C -> new C3();
+            };
+        }).test2();
+    }
+
+    private static class Root {}
+    private static class Base1 extends Root {}
+    private static class Impl1a extends Base1 {}
+    private static class Impl1b extends Base1 {}
+    private static class Impl1c extends Base1 {}
+    private static class Base2 extends Root {}
+    private static class Impl2a extends Base2 {}
+    private static class Impl2b extends Base2 {}
+    private static class Impl2c extends Base2 {}
+    private static class Base3 extends Root {}
+    private static class Impl3a extends Base3 {}
+    private static class Impl3b extends Base3 {}
+    private static class Impl3c extends Base3 {}
+
+    private static interface RootInterface1 {
+        public default int test1() {
+            return 1;
+        }
+    }
+    private static interface RootInterface2 {
+        public default int test2() {
+            return 2;
+        }
+    }
+    private static class C1 implements RootInterface1, RootInterface2 {}
+    private static class C2 implements RootInterface1, RootInterface2 {}
+    private static class C3 implements RootInterface1, RootInterface2 {}
+
+    enum E {A, B, C;}
+
     private void throwException() {
         throw new RuntimeException();
     }