Re: Precedence of standard comparison operators

Attached is an improved patch that includes optional warnings for
constructs that changed parsing. It's not quite 100% but I think it's
about 90% correct; the difference in size between this and the previous
patch should be a pretty fair indication of what it's going to cost us
to have a warning capability.

What's missing from this version is that it can't tell the difference
between LIKE/ILIKE/SIMILAR TO and the underlying operators, that is,
it sees "a LIKE b" as "a ~~ b" because that's what the grammar emits.
However, those inputs have different operator-precedence behavior.

Likewise, we can't tell the difference between
xmlexpr IS NOT DOCUMENT
NOT (xmlexpr IS DOCUMENT)
because the grammar converts the former into the latter --- and again,
those two things have different precedence behavior.

It wouldn't take very much additional code to fix these things by changing
what the grammar emits; but I'm running out of energy for today. In any
case, I thought I should put this up and see if this general approach is
going to satisfy people's concerns about making such a change.

regards, tom lane