test: serialization of subtype collections without properties

Co-authored-by-AI: IBM Bob (1.126.0+bob2.2.0)

src/test/java/org/eclipse/yasson/customization/polymorphism/SubtypeCollectionTest.java

diff --git a/src/test/java/org/eclipse/yasson/customization/polymorphism/SubtypeCollectionTest.java b/src/test/java/org/eclipse/yasson/customization/polymorphism/SubtypeCollectionTest.java
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+/*
+ * Copyright (c) 2026 Contributors to the Eclipse Foundation
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License v. 2.0 which is available at
+ * http://www.eclipse.org/legal/epl-2.0,
+ * or the Eclipse Distribution License v. 1.0 which is available at
+ * http://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * SPDX-License-Identifier: EPL-2.0 OR BSD-3-Clause
+ */
+
+package org.eclipse.yasson.customization.polymorphism;
+
+import java.util.List;
+import java.util.Set;
+
+import jakarta.json.bind.annotation.JsonbCreator;
+import jakarta.json.bind.annotation.JsonbSubtype;
+import jakarta.json.bind.annotation.JsonbTypeInfo;
+import org.junit.jupiter.api.Test;
+
+import static org.hamcrest.CoreMatchers.containsString;
+import static org.hamcrest.CoreMatchers.is;
+import static org.hamcrest.MatcherAssert.assertThat;
+
+import static org.eclipse.yasson.Jsonbs.defaultJsonb;
+
+/**
+ * Regression tests for https://github.com/eclipse-ee4j/yasson/issues/775
+ *
+ * The root cause was a stale key left in the serialization context after the
+ * polymorphism discriminator lambda wrote its key+value pair directly to the
+ * generator (bypassing KeyWriter and its context.setKey(null) cleanup).
+ *
+ * The stale key caused the outer KeyWriter — wrapping the per-element object
+ * serializer chain — to call generator.writeKey() while the generator was in
+ * IN_ARRAY state when processing any element after the first.
+ *
+ * Three conditions are required to trigger the bug:
+ *   1. A type annotated with @JsonbTypeInfo (adds the raw discriminator lambda).
+ *   2. The subtype has NO other serializable properties (so no subsequent
+ *      KeyWriter-wrapped property serializer clears the stale key).
+ *   3. More than one instance appears consecutively in a collection or array
+ *      (so the outer KeyWriter fires again on the second element).
+ */
+public class SubtypeCollectionTest {
+
+    // =========================================================================
+    // Scenario 1 – sealed interface + record subtypes (original report, #775)
+    // =========================================================================
+
+    @JsonbTypeInfo(key = "type", value = {
+            @JsonbSubtype(alias = "optionA", type = OptionA.class),
+            @JsonbSubtype(alias = "optionB", type = OptionB.class),
+    })
+    public sealed interface TestJsonbInterface permits OptionA, OptionB {
+    }
+
+    public record OptionA() implements TestJsonbInterface {
+        @JsonbCreator
+        public OptionA {
+        }
+    }
+
+    public record OptionB() implements TestJsonbInterface {
+        @JsonbCreator
+        public OptionB {
+        }
+    }
+
+    public record TestJsonbSetRecord(Set<TestJsonbInterface> interfaceSet) {
+        @JsonbCreator
+        public TestJsonbSetRecord {
+        }
+    }
+
+    public record TestJsonbListRecord(List<TestJsonbInterface> interfaceList) {
+        @JsonbCreator
+        public TestJsonbListRecord {
+        }
+    }
+
+    @Test
+    public void testSingleElementSetSerialization() {
+        String json = defaultJsonb.toJson(new TestJsonbSetRecord(Set.of(new OptionA())));
+        assertThat(json, containsString("\"type\":\"optionA\""));
+    }
+
+    /** Reproduces issue #775. */
+    @Test
+    public void testMultipleElementSetSerialization() {
+        String json = defaultJsonb.toJson(new TestJsonbSetRecord(Set.of(new OptionA(), new OptionB())));
+        assertThat(json, containsString("\"type\":\"optionA\""));
+        assertThat(json, containsString("\"type\":\"optionB\""));
+    }
+
+    @Test
+    public void testMultipleElementListSerialization() {
+        String json = defaultJsonb.toJson(new TestJsonbListRecord(List.of(new OptionA(), new OptionB(), new OptionA())));
+        assertThat(json, is("{\"interfaceList\":[{\"type\":\"optionA\"},{\"type\":\"optionB\"},{\"type\":\"optionA\"}]}"));
+    }
+
+    // =========================================================================
+    // Scenario 2 – plain classes, interface with @JsonbTypeInfo
+    // =========================================================================
+
+    @JsonbTypeInfo(key = "kind", value = {
+            @JsonbSubtype(alias = "alpha", type = Alpha.class),
+            @JsonbSubtype(alias = "beta",  type = Beta.class),
+    })
+    public interface PlainInterface {
+    }
+
+    /** Plain class – no fields, no record. */
+    public static class Alpha implements PlainInterface {
+    }
+
+    /** Plain class – no fields, no record. */
+    public static class Beta implements PlainInterface {
+    }
+
+    public static class PlainWrapper {
+        public List<PlainInterface> items;
+
+        public PlainWrapper(List<PlainInterface> items) {
+            this.items = items;
+        }
+    }
+
+    @Test
+    public void testMultipleElementListPlainClassSerialization() {
+        String json = defaultJsonb.toJson(new PlainWrapper(List.of(new Alpha(), new Beta(), new Alpha())));
+        assertThat(json, is("{\"items\":[{\"kind\":\"alpha\"},{\"kind\":\"beta\"},{\"kind\":\"alpha\"}]}"));
+    }
+
+    // =========================================================================
+    // Scenario 3 – array instead of collection
+    // =========================================================================
+
+    @Test
+    public void testMultipleElementArraySerialization() {
+        PlainInterface[] arr = { new Alpha(), new Beta(), new Alpha() };
+        String json = defaultJsonb.toJson(arr);
+        assertThat(json, is("[{\"kind\":\"alpha\"},{\"kind\":\"beta\"},{\"kind\":\"alpha\"}]"));
+    }
+
+    // =========================================================================
+    // Scenario 4 – repeated same subtype in a collection
+    // =========================================================================
+
+    @Test
+    public void testRepeatedSameSubtypeInListSerialization() {
+        String json = defaultJsonb.toJson(new PlainWrapper(List.of(new Alpha(), new Alpha())));
+        assertThat(json, is("{\"items\":[{\"kind\":\"alpha\"},{\"kind\":\"alpha\"}]}"));
+    }
+}