The while-while loop

One of the first constructs learned in c is the while loop, with the do-while loop following shortly thereafter.  Today, while inspecting some code (whose location I will keep anonymous to protect the guilty), I ran into the wicked uncle of these innocent statements:

The while-while loop

I kid you not.  At first I thought, well that's not even legal c; how is this even getting through the compiler?  So I isolated it to ensure I wasn't tricked by some conditional compilation, and sure enough, GCC is perfectly fine with it:

So I played with it a bit, to see which condition is actually the one controlling the loop:

Let the first condition run to 20:

It appears to ignore the second condition here.

Let the second condition run to 20:

Time limit exceeded???

Aha!

Then it occured to me what is happening.  These are two entirely independent while loops.  Written more clearly, they could look like this:

And then the behaviour in the examples then becomes obvious.

For now I can think of no reasonable reason to format code to look like this naughty while-while loop, and yet the language happily lets you shoot yourself in the foot with it.  Beware!