BUG #19472: CAST(-32768::SMALLINT AS REAL) fails with "SMALLINT out of range" but -32768 is valid SMALLINT value

The following bug has been logged on the website:

Bug reference: 19472
Logged by: Pisces Mar.
Email address: vectorplanck(at)gmail(dot)com
PostgreSQL version: 17.6
Operating system: x86_64-windows
Description:

Issue Description:
When casting -32768 to SMALLINT and then to REAL using the :: operator,
PostgreSQL throws an error "smallint out of range", even though -32768 is a
valid value within the SMALLINT range (-32768 to +32767).

Steps to Reproduce:
1. Execute: SELECT CAST(-32768 :: SMALLINT AS REAL);
Result: ERROR: SMALLINT out of range

2. Execute: SELECT CAST((-32768) :: SMALLINT AS REAL);
Result: Successfully returns -32768

3. Execute: SELECT CAST(CAST(-32768 AS SMALLINT) AS REAL);
Result: Successfully returns -32768

Root Cause:
The expression -32768 :: SMALLINT is being parsed as -(32768 :: SMALLINT)
instead of (-32768) :: SMALLINT. Since 32768 exceeds the maximum positive
value for SMALLINT (32767), it causes an out-of-range error.

Expected Behavior:
-32768 should be recognized as a valid SMALLINT value (the minimum value for
smallint is -32768), and the cast to REAL should succeed.

Workaround:
Use parentheses: SELECT CAST((-32768) :: SMALLINT AS REAL);
Or use standard CAST syntax: SELECT CAST(-32768 AS SMALLINT);

Environment:
PostgreSQL Version: 17.6
Operating System: x86_64-windows