basic_types.idl currently exports safeInt/safeInt64 types for numbers, and the latter two can accept either BSON int or BSON long values during parse, but when serializing, they explicitly store a single given type (int for safeInt or int64_t for safeInt64).

Some commands such as count want to serialize as a BSON int when the value can fit into int32_t, or a BSON long when the value is larger.  A standard IDL type which uses the BSONObjBuilder::appendNumber overload to select the encoded type based on value would simplify TypedCommand reply structures.