Proposed resolution:

This wording is relative to N4727.

1. Edit [depr.strstreambuf.virtuals] as indicated:

   pos_type seekpos(pos_type sp, ios_base::openmode which
                    = ios_base::in | ios_base::out) override;
   

   -17- Effects: […]

   -18- For a sequence to be positioned, if its next pointer is a null pointer, the positioning operation fails. Otherwise, the function determines newoff from static_cast<off_type>(sp).offset():

   […]

[ 2018-06-18 after reflector discussion ]

Priority set to 4

strstreambuf refers to a nonexistent member function of fpos in the specification of the member function seekpos, [depr.strstreambuf.virtuals]/18 (emphasize mine):

For a sequence to be positioned, if its next pointer is a null pointer, the positioning operation fails. Otherwise, the function determines newoff from sp.offset():

The intent is clearly to get the corresponding streamoff from the fpos, as p19 says "the resultant offset newoff (of type off_type)". The mechanism to make that conversion is a normal explicit conversion, as indicated in the last row of the table in [fpos.operations].