Unsupported cast when casting to a one field struct using the parenthesis syntax

[maingoh]

Describe the bug

The following raise an error

SELECT (foo.my_id)::struct<id string> as foo FROM ...

However using the row() function it works:

SELECT row(foo.my_id)::struct<id string> as foo FROM ...

Error message/log

Unsupported function: cast(character varying) -> struct<id character varying>;

### To Reproduce

_No response_

### Expected behavior

The `()` syntax for creating struct should work with one field. When I add a second one it works. I currently explicitely use `row()` as a workaround.

### How did you deploy RisingWave?

K8s operator

### The version of RisingWave

v1.8.2

### Additional context

_No response_

[xiangjinwu]

The ROW keyword is actually optional as long as you have more than one field in the expression

https://www.postgresql.org/docs/current/rowtypes.html#ROWTYPES-CONSTRUCTING

This is actually PostgreSQL behavior, which is essential to differentiate it from parentheses used for precedence: (int_x + int_y) * int_z.

If you know Python, it is also the case that (int_x) is an integer while (int_x,) with an awkward trailing comma is a tuple containing an integer field. The trailing comma syntax is not adopted by PostgreSQL, so does RisingWave.