Are you mad that you would do such a thing?
The “dass du meinst” clause seems superfluous, yet there is a feeling of it being used to throw the weight of the accusation (being crazy) on to the intention (meinen) rather than the act itself. Comments?
Very good observation!
The difference between
“Bist du verrückt, so etwas machen zu wollen?”
“Bist du verrückt, dass du meinst, so etwas machen zu wollen?”
is that in the second case, you believe that the person is mislead about his or her own intention.
Are you mad, that you fancy to want to do such a thing?