Proposed resolution:

Change 17.3.2.1.2 [lib.bitmask.types] operator~ from:

   bitmask operator~ ( bitmask X )
     { return static_cast< bitmask>(static_cast<int_type>(~ X)); }

to:

   bitmask operator~ ( bitmask X )
     { return static_cast< bitmask>(~static_cast<int_type>(X)); }