> Copying const qualified storage to non-const qualified storage is the
> only safe and truly portable way you can discard const.

This depends on what you mean by "safe and truly portable" and "discard
const".

I have no problem with using deconst() for something like iov_base when
used with writev(), because writev _won't_ modify the pointed-to
storage.  (If the interface were designed to be used const-poisoned,
writev would use a different struct from readv.)  How is doing this
unsafe or nonportable?  Or does it not count as discarding const?

> Since a "volatile" and "const" make no real sense togethr in a cast

...huh?  volatile const makes perfect sense: the object must be read
exactly as the source implies, but the source promises it will not
write to it through that path.  A device register is a typical example.
A pointer to a value modified by a signal handler might be another (the
handler, obviously, would have to use a non-const reference to the
storage to do the modification.)

>         char *magic = (volatile char *) MAGIC;

Discarding volatile is just as verboten as discarding const.

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