Pushed. I moved the check into DefineIndex, as that's where user-facing
complaints about indexes generally ought to be.
If you're planning on back-patching this, please don't. :) It'll literally ruin my life.
I've got an extension that's actually a custom Access Method, and for reasons that are probably too boring to go into here, it requires that the first column in the index be a function that takes the ctid. Ie, something akin to:
CREATE INDEX idx ON table (my_func('table', ctid), other_func(table));
The AM implementation itself doesn't actually use the result of my_func(), but that construct is necessary so I can detect certain queries that look like:
SELECT FROM table WHERE my_func('table', ctid) ==> 'index condition'
I don't mind that you're changing this for 9.6... 9.6 is going to change so much other stuff around custom AMs that I'll deal with it when the time comes, but back-patching this into 9.3/4/5 would make life very difficult.
Thanks for listening!
eric