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

hi.
more preparation work has been committed.

1. SQL/JSON patch [1] added keyword ERROR
2. CoerceViaIo, CoerceToDomain can be evaluated error safe. see commit [2].
3. ExprState added ErrorSaveContext point, so before calling ExecInitExprRec
set valid ErrorSaveContext for ExprState->escontext we should evaluate
expression error softly.
see commit [2] also.

I only found oracle implement, [3].
Based on my reading of [4], it seems CAST(EXPRESSION AS TYPE DEFAULT
def_expr ON ERROR)
is not included in SQL:2023.

anyway, just share my POC based on the previous patch in this thread.
it will work for domain over composite, composite over domain.
example:
CREATE DOMAIN d_char3_not_null as char(3) NOT NULL;
CREATE TYPE comp_domain_with_typmod AS (a d_char3_not_null, b int);
SELECT CAST('(,42)' AS comp_domain_with_typmod DEFAULT NULL ON ERROR);
--return NULL

[1]: https://git.postgresql.org/cgit/postgresql.git/diff/src/backend/parser/gram.y?id=6185c9737cf48c9540782d88f12bd2912d6ca1cc
[2]: https://git.postgresql.org/cgit/postgresql.git/commit/?id=aaaf9449ec6be62cb0d30ed3588dc384f56274bf
[3] https://docs.oracle.com/en/database/oracle/oracle-database/23/sqlrf/CAST.html
[4] https://peter.eisentraut.org/blog/2023/04/04/sql-2023-is-finished-here-is-whats-new