Proposed resolution:

Change the sentence from:

For a sequence to be positioned, if its next pointer (either gptr() or pptr()) is a null pointer, the positioning operation fails.

to:

For a sequence to be positioned, if its next pointer (either gptr() or pptr()) is a null pointer and the new offset newoff is nonzero, the positioning operation fails.

[ Sydney: Agreed that this is an annoying problem: seeking to zero should be legal. Bill will provide wording. ]

  pos_type basic_stringbuf::seekoff(off_type, ios_base::seekdir,
                                    ios_base::openmode);

is obliged to fail if nothing has been inserted into the stream. This is unnecessary and undesirable. It should be permissible to seek to an effective offset of zero.