Kaartic Sivaraam [off-list ref] writes:

Wait, I missed a contradiction here.
..
Documentation/SubmittingPatches says:

quoted

 - Some clever tricks, like using the !! operator with arithmetic
   constructs, can be extremely confusing to others.

What does "with arithmetic constructs" mean?  Would it refer to
things like

	!!i != !!(j + 3)

that unnecessarily obfuscates what is going on?

The primary reason why !!ptr is good in the code that this patch
touches is because what is doubly negated is a pointer, not an
integer or other things.  The called function does *not* limit its
input to 0 or 1 (it wants 0 for false and everything else for true),
so we wouldn't be doing !!i if what we are passing is already an
integer.  But we cannot just pass a pointer to such a parameter
without getting the compiler upset.