There exists a conversion from streampos to streamoff, so that the expression can be interpreted as =09(streamoff)pos - 1 or =09pos.operator-(1) In both cases conversions are used (from streampos to streamoff, and from int to long). The ambiguouity can be resolved be writing =09pos - 1L
Probably the best way of writing it would then be "pos - (streamoff)1", yielding a streampos. I didn't know that streamoff existed, and its existence does explain the problem I was having. Thank you very much!
Regards, Bart