>> I'm fine with it. I can see having 'f' and 's' both mean cast functions, but 's' means safe, but the extra boolean works too and we'll be fine with either method.
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> I can work on this part if you don't have time.

Do you mean change pg_cast.casterrorsafe from boolean to char?

No, I meant implementing the syntax for being able to declare a custom CAST function as safe (or not). Basically adding the [SAFE] to

CREATE CAST (source_type AS target_type)
    WITH [SAFE] FUNCTION function_name [ (argument_type [, ...]) ]

I'm not tied to this syntax choice, but this one seemed the most obvious and least invasive.

But this brings up an interesting point: if a cast is declared as WITHOUT FUNCTION aka COERCION_METHOD_BINARY, then the cast can never fail, and we should probably check for that because a cast that cannot fail can ignore the DEFAULT clause altogether and fall back to being an ordinary CAST().

Currently pg_cast.casterrorsafe works just fine.
if the cast function is not applicable, the castfunc would be InvalidOid.
also the cast function is either error safe or not, I don't see a
usage case for the third value.

Agreed, it's fine. A committer may want a char with 's'/'u' values to keep all the options char types, but even if they do that's a very minor change.