Proposed resolution:

Change [streambuf.virt.get] paragraph 1, showmanyc()returns clause, by replacing the word "supplied" with the words "extracted from the stream".

The description of the meaning of the result of showmanyc() seems to be rather strange: It uses calls to underflow(). Using underflow() is strange because this function only reads the current character but does not extract it, uflow() would extract the current character. This should be fixed to use sbumpc() instead.