Minor improvement to basetype compatibility checking (wrt #1861)

Author: cowtowncoder

src/main/java/com/fasterxml/jackson/databind/jsontype/impl/StdTypeResolverBuilder.java

@@ -8,6 +8,7 @@
 import com.fasterxml.jackson.databind.annotation.NoClass;
 import com.fasterxml.jackson.databind.cfg.MapperConfig;
 import com.fasterxml.jackson.databind.jsontype.*;
+import com.fasterxml.jackson.databind.util.ClassUtil;
 
 /**
  * Default {@link TypeResolverBuilder} implementation.
@@ -135,8 +136,21 @@ public TypeDeserializer buildTypeDeserializer(DeserializationConfig config,
                      || (_defaultImpl == NoClass.class)) {
                 defaultImpl = config.getTypeFactory().constructType(_defaultImpl);
             } else {
-                defaultImpl = config.getTypeFactory()
-                    .constructSpecializedType(baseType, _defaultImpl);
+                if (baseType.hasRawClass(_defaultImpl)) { // common enough to check
+                    defaultImpl = baseType;
+                } else if (baseType.isTypeOrSuperTypeOf(_defaultImpl)) {
+                    // most common case with proper base type...
+                    defaultImpl = config.getTypeFactory()
+                            .constructSpecializedType(baseType, _defaultImpl);
+                } else {
+                    // 05-Apr-2018, tatu: [databind#1861] Not sure what would be the best way
+                    //    to handle, but for 2.9, let's consider case of "sibling" defaultImpl...
+                    // ... Ugh. Not declared to throw `JsonMappingException`, so...
+                    throw new IllegalArgumentException(
+                            String.format("Invalid \"defaultImpl\" (%s): not a subtype of basetype (%s)",
+                                    ClassUtil.nameOf(_defaultImpl), ClassUtil.nameOf(baseType.getRawClass()))
+                            );
+                }
             }
         }