v. 11.1.0.0
Reduce[Arg[a] + Arg[b] == Arg[a*b], {a, b}]
Edited to add steps to reproduce: Start Mathematica from scratch, enter that input, and within a minute Mathematica beeps, and when I ask it why the beep (under the Help menu), it says the Kernel has quit. That's what I mean by "crash". For me it's normally reproducible, though on one trial it ran for a few minutes without beeping (until I gave up and cancelled.)
