basic_streambuf::seekoff what should be returned when ios_base::in | ios_base::out is specified?

27.6.3.4.2 Buffer management and positioning

pos_type seekoff(off_type off, ios_base::seekdir way,
    ios_base::openmode which = ios_base::in | ios_base::out);

• Effects: Alters the stream positions within one or more of the controlled sequences in a way that is defined separately for each class derived from basic_streambuf in this Clause (27.8.2.4, 27.9.1.5).
• Default behavior: Returns pos_type(off_type(-1)).

So far, so good. The basic_streambuf derivation I'm using can alter its position separately for ios_base::in and/or ios_base::out. But, what do I need to return when both are specified?

If you specify ios_base::in or ios_base::out, we would return new stream position of the specific sequence.