This issue is a continuation of the disussion with @seberg in PR #3501:

In my opinion, having __array_priority__ logic in umath is just broken. It should be ripped out. A tell-tale sign of this is that the logic is activated only for two-argument ufuncs with one output. What's so special about them? Why should they behave differently from three-argument ufuncs? Also, as @seberg pointed out, why should a ufunc ever return NotImplemented?

See here:

• https://github.com/numpy/numpy/blob/master/numpy/core/src/umath/ufunc_object.c#L2362
• https://github.com/numpy/numpy/blob/master/numpy/core/src/umath/ufunc_object.c#L1971