GROOVY-11889: STC: instanceof check rejects UnionTypeClassNode members as incompatible types

Author: paulk-asert

src/main/java/org/codehaus/groovy/transform/stc/StaticTypeCheckingVisitor.java

@@ -982,7 +982,16 @@ && isAssignment(enclosingBinaryExpression.getOperation().getType())) {
             } else if (op == KEYWORD_INSTANCEOF) {
                 rType = rightExpression.getType();
                 if (lType instanceof WideningCategories.LowestUpperBoundClassNode) lType = lType.getUnresolvedSuperClass();
-                if (!lType.isInterface() && !rType.isInterface() && !(lType.isDerivedFrom(rType) || rType.isDerivedFrom(lType))) {
+                if (lType instanceof UnionTypeClassNode union) {
+                    boolean compatible = rType.isInterface();
+                    for (int i = 0; !compatible && i < union.getDelegates().length; i += 1) {
+                        ClassNode delegate = union.getDelegates()[i];
+                        compatible = delegate.isInterface() || delegate.isDerivedFrom(rType) || rType.isDerivedFrom(delegate);
+                    }
+                    if (!compatible) {
+                        addError("Incompatible instanceof types: " + prettyPrintTypeName(lType) + " and " + prettyPrintTypeName(rType), expression);
+                    }
+                } else if (!lType.isInterface() && !rType.isInterface() && !(lType.isDerivedFrom(rType) || rType.isDerivedFrom(lType))) {
                     addError("Incompatible instanceof types: " + prettyPrintTypeName(lType) + " and " + prettyPrintTypeName(rType), expression);
                 }
                 pushInstanceOfTypeInfo(leftExpression, rightExpression);

src/test/groovy/groovy/transform/stc/TypeInferenceSTCTest.groovy

@@ -537,8 +537,8 @@ class TypeInferenceSTCTest extends StaticTypeCheckingTestCase {
         '''
     }
 
-    // GROOVY-7971: negated || instanceof — re-check instanceof in else branch
-    @Test @org.junit.jupiter.api.Disabled('requires instanceof compatibility fix for UnionTypeClassNode')
+    // GROOVY-11889: negated || instanceof — re-check instanceof in else branch
+    @Test
     void testInstanceOf25() {
         assertScript '''
             void test(Object x) {
@@ -569,6 +569,52 @@ class TypeInferenceSTCTest extends StaticTypeCheckingTestCase {
         '''
     }
 
+    // GROOVY-11889: instanceof interface check on union type
+    @Test
+    void testInstanceOf27() {
+        assertScript '''
+            void test(Object x) {
+                if (x instanceof String || x instanceof Integer) {
+                    if (x instanceof Serializable) {
+                        assert true
+                    }
+                }
+            }
+            test('hello')
+            test(42)
+        '''
+    }
+
+    // GROOVY-11889: union containing interface delegate
+    @Test
+    void testInstanceOf28() {
+        assertScript '''
+            void test(Object x) {
+                if (x instanceof Serializable || x instanceof String) {
+                    if (x instanceof String) {
+                        assert true
+                    }
+                }
+            }
+            test('hello')
+        '''
+    }
+
+    // GROOVY-11889: incompatible instanceof on union type should produce error
+    @Test
+    void testInstanceOf29() {
+        shouldFailWithMessages '''
+            void test(Object x) {
+                if (x instanceof String || x instanceof Integer) {
+                    if (x instanceof Long) {
+                        // unreachable - neither String nor Integer is related to Long
+                    }
+                }
+            }
+        ''',
+        'Incompatible instanceof types'
+    }
+
     // GROOVY-5226
     @Test
     void testNestedInstanceOf1() {