Fix for polymorphic custom map key serializer

src/main/java/com/fasterxml/jackson/databind/ObjectWriter.java

@@ -871,7 +871,7 @@ public void writeValue(JsonGenerator gen, Object value)
                 _serializerProvider(_config).serializeValue(gen, value, _prefetch.rootType,
                         _prefetch.valueSerializer);
             } else if (_prefetch.typeSerializer != null) {
-                _serializerProvider(_config).serializePolymorphic(gen, value, _prefetch.typeSerializer);
+                _serializerProvider(_config).serializePolymorphic(gen, value, _prefetch.rootType, _prefetch.typeSerializer);
             } else {
                 _serializerProvider(_config).serializeValue(gen, value);
             }
@@ -1078,7 +1078,7 @@ protected final void _configAndWriteValue(JsonGenerator gen, Object value) throw
                 _serializerProvider(_config).serializeValue(gen, value, _prefetch.rootType,
                         _prefetch.valueSerializer);
             } else if (_prefetch.typeSerializer != null) {
-                _serializerProvider(_config).serializePolymorphic(gen, value, _prefetch.typeSerializer);
+                _serializerProvider(_config).serializePolymorphic(gen, value, _prefetch.rootType, _prefetch.typeSerializer);
             } else {
                 _serializerProvider(_config).serializeValue(gen, value);
             }
@@ -1113,7 +1113,7 @@ private final void _writeCloseable(JsonGenerator gen, Object value, Serializatio
                 _serializerProvider(cfg).serializeValue(gen, value, _prefetch.rootType,
                         _prefetch.valueSerializer);
             } else if (_prefetch.typeSerializer != null) {
-                _serializerProvider(cfg).serializePolymorphic(gen, value, _prefetch.typeSerializer);
+                _serializerProvider(cfg).serializePolymorphic(gen, value, _prefetch.rootType, _prefetch.typeSerializer);
             } else {
                 _serializerProvider(cfg).serializeValue(gen, value);
             }
@@ -1157,7 +1157,7 @@ private final void _writeCloseableValue(JsonGenerator gen, Object value, Seriali
                 _serializerProvider(cfg).serializeValue(gen, value, _prefetch.rootType,
                         _prefetch.valueSerializer);
             } else if (_prefetch.typeSerializer != null) {
-                _serializerProvider(cfg).serializePolymorphic(gen, value, _prefetch.typeSerializer);
+                _serializerProvider(cfg).serializePolymorphic(gen, value, _prefetch.rootType, _prefetch.typeSerializer);
             } else {
                 _serializerProvider(cfg).serializeValue(gen, value);
             }

src/main/java/com/fasterxml/jackson/databind/ser/DefaultSerializerProvider.java

@@ -269,15 +269,28 @@ public void serializeValue(JsonGenerator gen, Object value, JavaType rootType, J
      *
      * @since 2.5
      */
-    public void serializePolymorphic(JsonGenerator gen, Object value, TypeSerializer typeSer)
+    public void serializePolymorphic(JsonGenerator gen, Object value, JavaType rootType, TypeSerializer typeSer)
             throws IOException
     {
         if (value == null) {
             _serializeNull(gen);
             return;
         }
+
+        // Let's ensure types are compatible at this point
+        if ((rootType != null) && !rootType.getRawClass().isAssignableFrom(value.getClass())) {
+            _reportIncompatibleRootType(value, rootType);
+        }
+
+        JsonSerializer<Object> ser = null;
+
         final Class<?> type = value.getClass();
-        JsonSerializer<Object> ser = findValueSerializer(type, null);
+        if (rootType.isMapLikeType()) {
+            // Ensure custom key serializers are used for map types when using polymorphic serialization.
+            ser = findTypedValueSerializer(rootType, true, null);
+        } else {
+            ser = findValueSerializer(type, null);
+        }
 
         final boolean wrap;
         String rootName = _config.getRootName();

src/test/java/com/fasterxml/jackson/databind/seq/PolyMapWriterTest.java

@@ -0,0 +1,60 @@
+package com.fasterxml.jackson.databind.seq;
+
+import java.io.IOException;
+import java.util.HashMap;
+import java.util.Map;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+import com.fasterxml.jackson.annotation.JsonAutoDetect.Visibility;
+import com.fasterxml.jackson.annotation.PropertyAccessor;
+import com.fasterxml.jackson.core.JsonGenerator;
+import com.fasterxml.jackson.core.JsonProcessingException;
+import com.fasterxml.jackson.databind.JsonSerializer;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.ObjectWriter;
+import com.fasterxml.jackson.databind.SerializerProvider;
+import com.fasterxml.jackson.databind.module.SimpleModule;
+import com.fasterxml.jackson.databind.type.MapType;
+
+public class PolyMapWriterTest {
+
+    static class CustomKey {
+        String a;
+        int b;
+    }
+
+    public class CustomKeySerializer extends JsonSerializer<CustomKey> {
+        @Override
+        public void serialize(CustomKey key, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException, JsonProcessingException {
+            jsonGenerator.writeFieldName(key.a + "," + key.b);
+        }
+    }
+
+    @Test
+    public void testPolyCustomKeySerializer() throws Exception {
+        ObjectMapper mapper = new ObjectMapper();
+        mapper.setVisibility(PropertyAccessor.ALL, Visibility.NONE);
+        mapper.setVisibility(PropertyAccessor.FIELD, Visibility.ANY);
+        mapper.enableDefaultTyping(ObjectMapper.DefaultTyping.NON_FINAL);
+
+        SimpleModule module = new SimpleModule("keySerializerModule");
+        module.addKeySerializer(CustomKey.class, new CustomKeySerializer());
+        mapper.registerModule(module);
+
+        Map<CustomKey, String> map = new HashMap<CustomKey, String>();
+        CustomKey key = new CustomKey();
+        key.a = "foo";
+        key.b = 1;
+        map.put(key, "bar");
+
+        final MapType type = mapper.getTypeFactory().constructMapType(
+                Map.class, CustomKey.class, String.class);
+        final ObjectWriter writer = mapper.writerFor(type);
+        String json = writer.writeValueAsString(map);
+
+        Assert.assertEquals("[\"java.util.HashMap\",{\"foo,1\":\"bar\"}]", json);
+    }
+
+}