From https://groups.google.com/forum/#!topic/sage-support/L75JgocZc3I verbatim (but the syntax for Maxima should be very similar here):
a,b=var('a b'); assume(4*b^2-4*a^2<0); assume((b-a)*(b+a)<0); integrate(1/(a-b*sin(x)),x,-oo,oo)
It complains and asks whether (b-a)*(b+a) is negative or positive.
Downstream at http://trac.sagemath.org/ticket/17249