Re: ALTER TYPE 3: add facility to identify further no-work cases

On Thu, Jan 27, 2011 at 09:02:26AM -0500, Robert Haas wrote:
> OK. I was thinking that instead moving this into
> eval_const_expressions(), we just make the logic in
> find_coercion_pathway() call the "exemptor" function (or whatever we
> call it) right around here:
> 
>                         switch (castForm->castmethod)
>                         {
>                                 case COERCION_METHOD_FUNCTION:
>                                result = COERCION_PATH_FUNC;
>                                         *funcid = castForm->castfunc;
>                                         break;
>                                 case COERCION_METHOD_INOUT:
>                                         result = COERCION_PATH_COERCEVIAIO;
>                                         break;
>                                 case COERCION_METHOD_BINARY:
>                                         result = COERCION_PATH_RELABELTYPE;
>                                         break;
>                                 default:
>                                         elog(ERROR, "unrecognized
> castmethod: %d",
>                                                  (int) castForm->castmethod);
>                                         break;
>                         }
> 
> If it's COERCION_METHOD_FUNCTION, then instead of unconditionally
> setting the result to COERCION_PATH_FUNC, we inquire - based on the
> typemods - whether it's OK to downgrade to a
> COERCION_PATH_RELABELTYPE.  The only fly in the ointment is that
> find_coercion_pathway() doesn't current get the typemods.  Not sure
> how ugly that would be to fix.

Basically, this is a stylistic variation of the approach from my original at3
patch.  I believe I looked at that particular positioning and decided that the
extra arguments and effects on non-parse_coerce.c callers were undesirable.
Debatable as any style issue, though.  Note that you need to do the same thing
in find_typmod_coercion_function().