Re: Allow ON CONFLICT DO UPDATE to return EXCLUDED values

I’ve looked through this patch. As far as I can tell, everything looks good, working and well commented.
The only nitpick I could find is a mispelling "EXLCUDED" → "EXCLUDED" in src/test/regress/expected/returning.out:464.

A maybe bigger question, is it nice that EXCLUDED is null when no conflict occurred? I can see the logic, but I think ergonomics wise it’d be nicer to have the proposed values in EXCLUDED, no matter what happened later. Then one can check EXCLUDED.value = NEW.value to see if one’s changes were added, for example.