A confusing expression?

[bearophile]

mwarning:
> Turning it into a syntax error sounds to be the right way to do to me.
> From a mathematical syntax pov, it's undefined behavior.

Thank you for your answer. You may be right, but this a hard sell :-)
I'd like to know what others think about it. Eventually if the conditions are fit I can restart this thread in the main D newsgroup...

Bye,
bearophile