[ssp]

Tom Duff himself writes about it here:

http://doc.cat-v.org/bell_labs/duffs_device

In particular,

    The device is legal dpANS C. I cannot quote chapter and verse, but
    Larry Rosler, who was chairman of the language subcommittee (I think),
    has assured me that X3J11 considered it carefully and decided that
    it was legal. Somewhere I have a note from dmr certifying that all the
    compilers that he believes in accept it. Of course, the device is also
    legal C++, since Bjarne uses it in his book.