This is needed because :: casting binds more tightly than minus, so for
example -1::integer is not the same as (-1)::integer, and there are cases
where the difference is important.  In particular this caused a failure
in SELECT DISTINCT ... ORDER BY ... where expressions that should have
matched were seen as different by the parser; but I suspect that there
could be other cases where failure to parenthesize leads to subtler
semantic differences in reloaded rules.  Per report from Alexandr Popov.

require SELECT privilege as well, since you normally need to read existing
column values within such commands.  This behavior is according to spec,
but we'd never documented it before.  Per gripe from Volkan Yazici.