Proposed resolution:

In [filebuf.virtuals] paragraph 9, change all uses of int in the description of setbuf() to streamsize.

In [filebuf] the function setbuf() gets an object of type streamsize as second argument. However, in [filebuf.virtuals] paragraph 9 the second argument is of type int.

As far as I can see this is not really a contradiction because everything is consistent if streamsize is typedef to be int. However, this is almost certainly not what was intended. The same thing happened to basic_istream::ignore(), as described in issue 172.