@Kitonum 

Okay now you are just making me feel dumb xD Thanks a bunch man

Thanks guys. Now for this situation the Domain thing turns out great. However how would deal with this sort of limitation?

f := unapply(piecewise(0 <= x and x <= 2 and x-2 <= y and y <= x, y^2+x, undefined), x, y)

graf := plot3d(f(x, y), x = 0 .. 2, y = -2 .. 2, orientation = [220, 75])