[Bug libstdc++/46821] swapping empty classes

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46821

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> 2010-12-06 14:04:58 UTC ---
I think Marc's right, tuple::swap is specified to call swap for each element,
and that is definitely something users can detect (even if there's rarely a
good reason to do so for empty classes)

Your change looks safe and correct to me