[Bug libstdc++/57691] freestanding libstdc++ has compile error

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57691

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I've opened PR 57699 -- it would be nice not to have to write (void) in our
headers.