You're missing my point: that is possible in an indexscan, but *not* in a bitmap indexscan, because the index AM APIs are totally different in the two cases. In a bitmap scan, nothing more than a TID bitmap is ever returned out to anyplace that could execute arbitrary expressions.
I had thought it must already be able to execute arbitrary expressions, due to the ability to already support user-defined btree ops (and ops of non-btree types in the case of other index types).
No. An index AM is only expected to be able to evaluate clauses of the form <indexed_column> <indexable_operator> <constant>, and the key restriction there is that the operator is one that the AM has volunteered to support. Well, actually, it's the opclass more than the AM that determines this, but anyway it's not just some random operator; more than likely, the AM and/or opclass has got special logic about the operator.
Isn't that pretty much exactly the point made by Jeff and Simon, that index AM is currently only allowed to handle the indexable operators, i.e. operators that it can explicitly optimize (e.g. use to walk the btree and such), and completely ignores the other operators despite having all the columns in the index. Which means we'll have to do the heap fetch, which usually means a significant performance hit.
This also ties into Robert's point about evaluation of operators against index entries for dead or invisible rows. Indexable operators are much less likely than others to have unexpected side-effects.
I certainly understand there are cases that require care - like the leakproof thing pointed out by Robert for example. I don't immediately see why evaluation against dead rows would be a problem.
Apart from other problems discussed, I think it could also lead to
a performance penality for the cases when the qual condition is
costly as evaluating such a qual against lot of dead rows would be a
time consuming operation. I am not sure, but I think some of the
other well know databases might not have any such problems as
they store visibility info inside index due to which they don't need to
