Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

>
>
> I didn't implement the [ FORMAT <cast template> ] part for now.
> please check the attached regress test and tests expected result.
>

Question about this:

+/*
+ * Push steps to evaluate a SafeTypeCastExpr and its various subsidiary
expressions.
+ * We already handle CoerceViaIO, CoerceToDomain, and ArrayCoerceExpr error
+ * softly. However, FuncExpr (e.g., int84) cannot be made error-safe.
+ * In such cases, we wrap the source expression and target type
information into
+ * a CoerceViaIO node instead.
+ */

I'm not sure we _can_ just fall back to the CoerceViaIO if there is a
defined cast from TypeA -> TypeB. I seem to recall there was some reason we
couldn't do that, possibly to do with how it handled rounding, but I have
no clear memory of it.

Aside from that, I like what you've done with making SafeTypeCastExpr be
its own node type and not saddling regular typecasts with the overhead.