Try to reproduce #1844 with json, not succeeding yet.

src/test/java/com/fasterxml/jackson/databind/deser/merge/MapMerge1844Test.java

@@ -0,0 +1,69 @@
+package com.fasterxml.jackson.databind.deser.merge;
+
+import java.util.*;
+
+import com.fasterxml.jackson.annotation.JsonProperty;
+import com.fasterxml.jackson.databind.BaseMapTest;
+import com.fasterxml.jackson.databind.ObjectMapper;
+
+// for [databind#1844]
+public class MapMerge1844Test extends BaseMapTest
+{
+    static class TestMap1844 {
+        public Map<String, Integer> getMapStringInteger() {
+            return mapStringInteger;
+        }
+
+        @JsonProperty("key1")
+        public void setMapStringInteger(Map<String, Integer> mapStringInteger) {
+            this.mapStringInteger = mapStringInteger;
+        }
+
+        public Map<Integer, Integer> getMapIntegerInteger() {
+            return mapIntegerInteger;
+        }
+
+        @JsonProperty("key2")
+        public void setMapIntegerInteger(Map<Integer, Integer> mapIntegerInteger) {
+            this.mapIntegerInteger = mapIntegerInteger;
+        }
+
+        private Map<String, Integer> mapStringInteger = new HashMap<>();
+
+        private Map<Integer, Integer> mapIntegerInteger = new HashMap<>();
+    }
+
+    // for [databind#1844]
+    public void testMap1844() throws Exception
+    {
+        final ObjectMapper mapper = newObjectMapper();
+        mapper.setDefaultMergeable(true);
+
+        TestMap1844 testMap = new TestMap1844();
+        TestMap1844 testMap1 = new TestMap1844();
+
+        final String f1 = aposToQuotes(
+"{ 'key1' : {\n"
++"  '1': 1, '2': 2, '3': 3\n"
++"}, 'key2': {\n"
++"  '1': 1, '2': 2, '3': 3\n"
++"} }"
+);
+        final String f2 = aposToQuotes(
+"{ 'key1' : {\n"
++"  '1': 2, '2': 3, '3': 5\n"
++"}, 'key2': {\n"
++"  '1': 2, '2': 3, '3': 5\n"
++"} }"
+);
+
+        testMap = mapper.readerForUpdating(testMap).readValue(f1);
+        testMap1 = mapper.readerForUpdating(testMap1).readValue(f1);
+        testMap = mapper.readerForUpdating(testMap).readValue(f2);
+        testMap1 = mapper.readerForUpdating(testMap1).readValue(f2);
+//        System.out.println(testMap.getMapIntegerInteger().get(1));
+//        System.out.println(testMap.getMapStringInteger().get("1"));
+        assertEquals(testMap.getMapIntegerInteger().get(1), testMap1.getMapIntegerInteger().get(1));
+        assertEquals(testMap.getMapStringInteger().get("1"), testMap1.getMapStringInteger().get("1"));
+    }
+}