Given an IDL type definition like:

  mytype:
  bson_serialization_type: any
  cpp_type: MyType
  deserializer: "mongo::parseMyType"
  serializer: "::mongo::serializeMyType"

Serialization will call `mongo::serializeMyType(const MyType& val, StringData fieldName, BSONObjBuilder* builder)`, however when the serialization type is narrowed, such as the following, the "::" prefix denoting a free function is ignored, and the type is serialized as `myType.serializeMyType(StringData fieldName, BSONObjBuilder* builder) const`.

  mytype:
  bson_serialization_type: [int, long, decimal, double]
  cpp_type: MyType
  deserializer: "mongo::parseMyType"
  serializer: "::mongo::serializeMyType"

The behavior current exhibited by the `any` case at top is correct, while the more narrow type below is not.