Message7353

The problem here is that PyInstance#__add__ does not follow the contract of PyObject#__add__ in that it must return null when a user defined add impl returns NotImplemented

Currently PyInstance#instance___add__ defers to invoke_ex to do the work. However there's an invoke_ex_richcmp method that is doing the right thing (returning null for NotImplemented). Either that method should be used instead (and more appropriately renamed) or maybe invoke_ex should just adopt that logic.

Note that this problem affects likely every binary op in PyInstance, not just add (which is one reason why invoke_ex might have to adopt the logic).

So this fix could use a few more tests other than add. You *might* be able to recycle some of test_descr_jy.BinopCombinationsTestCase for this (but there's no need to go overboard)