Request parameters are rejected when nested inside expressions that SQLPage explicitly emulates for sqlpage.* function arguments. This affects all SQLPage functions, not only send_mail.

Minimal side-effect-free reproducer:

select sqlpage.url_encode(
    json_object('recipient', :recipient)
);

Posting recipient=a@example.com produces:

Unsupported sqlpage function argument:
sqlpage.url_encode(JSON_OBJECT('recipient', CAST(?1 AS TEXT)))

"CAST(?1 AS TEXT)" is an sql expression, which cannot be passed as a nested sqlpage function argument.

Expected: :recipient remains a request-backed parameter while json_object is evaluated by SQLPage, and the resulting JSON is passed to sqlpage.url_encode.

Scope verified from the extraction pipeline and targeted experiments:

• Affects :post, $post_or_get, and query-only request parameters.
• Affects parameters nested in the supported json_object/json_array variants, concat, coalesce, ||, and compositions of those expressions.
• Affects any sqlpage.* function and both SELECT and SET pre-query calls.
• Applies to every database dialect: generated placeholders are always wrapped in CAST(...); only placeholder syntax and cast type vary.
• Direct arguments such as sqlpage.url_encode(:recipient) work.
• Literals and nested SQLPage functions work.
• Arbitrary database expressions remain intentionally unsupported and are not part of this bug.
• Assigning the database expression first is a workaround:

set payload = json_object('recipient', :recipient);
select sqlpage.url_encode($payload);

Root cause: ParameterExtractor rewrites nested request parameters before converting the enclosing SQLPage function. make_placeholder_for_index wraps generated placeholders in a dialect-specific CAST. The nested argument converter supports raw placeholders and the explicitly emulated expressions, but has no Expr::Cast handling, so it mistakes SQLPage's own generated cast for an unsupported database expression.

Existing tests cover direct request arguments and intentional rejection of arbitrary SQL expressions, but not a request parameter nested in an emulated expression.

I searched open and closed issues for nested SQLPage arguments, json_object, generated casts, and function arguments. #708 concerns PostgreSQL KEY VALUE JSON syntax and is unrelated.