Add test to verify #3251 is fixed by 2.16

pom.xml

@@ -99,6 +99,11 @@
       <artifactId>junit-jupiter</artifactId>
       <scope>test</scope>
     </dependency>
+    <dependency> <!-- added in 2.16 -->
+      <groupId>org.assertj</groupId>
+      <artifactId>assertj-core</artifactId>
+      <scope>test</scope>
+    </dependency>
     <dependency>
       <groupId>org.powermock</groupId>
       <artifactId>powermock-core</artifactId>

release-notes/CREDITS-2.x

@@ -1657,6 +1657,11 @@ Antti Lampinen (arlampin@github)
    and is marked `Access.WRITE_ONLY`
   (2.15.1)
 
+Kevin Baes (BaesKevin@github)
+ * Reported #3251: Generic class with generic field of runtime type `Double` is deserialized
+   as `BigDecimal` when used with `@JsonTypeInfo` and `JsonTypeInfo.As.EXISTING_PROPERTY`
+ (2.16.0)
+
 David Schlosnagle (schlosna@github)
  * Contributed #4008: Optimize `ObjectNode` findValue(s) and findParent(s) fast paths
   (2.16.0)

release-notes/VERSION-2.x

@@ -10,6 +10,9 @@ Project: jackson-databind
  (contributed by Joo-Hyuk K)
 #2787: Mix-ins do not work for `Enum`s
  (fix contributed by Joo-Hyuk K)
+#3251: Generic class with generic field of runtime type `Double` is deserialized
+  as `BigDecimal` when used with `@JsonTypeInfo` and `JsonTypeInfo.As.EXISTING_PROPERTY`
+ (reported by Kevin B)
 #3647: `@JsonIgnoreProperties` not working with `@JsonValue`
  (reported by @ThatSneakyRaccoon)
  (fix contributed by Joo-Hyuk K)

src/test/java/com/fasterxml/jackson/databind/jsontype/ExistingPropertyTest.java

@@ -11,6 +11,8 @@
 import com.fasterxml.jackson.databind.BaseMapTest;
 import com.fasterxml.jackson.databind.ObjectMapper;
 
+import static org.assertj.core.api.Assertions.assertThat;
+
 public class ExistingPropertyTest extends BaseMapTest
 {
     /**
@@ -199,6 +201,45 @@ public interface Base2785  {
     static class Impl2785 implements Base2785 {
     }
 
+    // [databind#3251]: Double vs BigDecimal
+    @JsonTypeInfo(
+        use = JsonTypeInfo.Id.NAME,
+        property = "type_alias"
+    )
+    static class GenericWrapperWithNew3251<T> {
+        private final T value;
+
+        @JsonCreator
+        public GenericWrapperWithNew3251(@JsonProperty("value") T value) {
+            this.value = value;
+        }
+
+        public T getValue() {
+            return value;
+        }
+    }
+
+    @JsonTypeInfo(
+        use = JsonTypeInfo.Id.NAME,
+        include = JsonTypeInfo.As.EXISTING_PROPERTY,
+        property = "fieldType",
+        visible = true,
+        defaultImpl = GenericWrapperWithExisting3251.class
+    )
+    static class GenericWrapperWithExisting3251<T> {
+        public String fieldType;
+        private final T value;
+
+        @JsonCreator
+        public GenericWrapperWithExisting3251(@JsonProperty("value") T value) {
+            this.value = value;
+        }
+
+        public T getValue() {
+            return value;
+        }
+    }
+
     // [databind#3271]
     @JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.EXISTING_PROPERTY,
             visible = true, property = "type", defaultImpl = DefaultShape3271.class)
@@ -490,4 +531,30 @@ public void testDeserializationNull() throws Exception {
         Shape3271 deserShape = MAPPER.readValue("{\"type\":null}", Shape3271.class);
         assertNull(deserShape.getType()); // error: "expected null, but was:<null>"
     }
+
+    // [databind#3251]: Double vs BigDecimal
+    public void test3251WithNewProperty() throws Exception
+    {
+        GenericWrapperWithNew3251<?> wrapper = new GenericWrapperWithNew3251<>(123.5);
+
+        String json = MAPPER.writeValueAsString(wrapper);
+        GenericWrapperWithNew3251<?> actualWrapper = MAPPER.readValue(json, GenericWrapperWithNew3251.class);
+
+        assertThat(actualWrapper).satisfies(it -> assertThat(it.getValue()).isEqualTo(123.5));
+        assertThat(actualWrapper.getValue()).isInstanceOf(Double.class);
+        assertThat(json).contains("\"value\":123.5");
+    }
+
+    public void test3251WithExistingProperty() throws Exception
+    {
+        GenericWrapperWithExisting3251<?> wrapper = new GenericWrapperWithExisting3251<>(123.5);
+
+        String json = MAPPER.writeValueAsString(wrapper);
+        GenericWrapperWithExisting3251<?> actualWrapper = MAPPER.readValue(json, GenericWrapperWithExisting3251.class);
+
+        assertThat(actualWrapper).satisfies(it -> assertThat(it.getValue()).isEqualTo(123.5));
+        assertThat(actualWrapper.getValue()).isInstanceOf(Double.class);
+        assertThat(json).contains("\"value\":123.5");
+    }
+
 }