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

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.