BUG #18730: Inequality comparison operators and SMALLINT negative immediate value

The following bug has been logged on the website:

Bug reference:      18730
Logged by:          Nat Makarevitch
Email address:      nat@makarevitch.org
PostgreSQL version: 17.2
Operating system:   Linux
Description:

Hi!

create table test_smallint(smint smallint);

insert into test_smallint values (-1);

select * from test_smallint where smint<>-2;
 smint 
═══════
    -1
(1 row)

BEWARE: there is a space between '=' and '-':
select * from test_smallint where smint!= -2;
 smint 
═══════
    -1
(1 row)

So far, so good.

BEWARE: there isn't any space between '=' and '-':
select * from test_smallint where smint!=-2;
ERROR:  operator does not exist: smallint !=- integer
LINE 1: select * from test_smallint where smint!=-2;
                                               ^
HINT:  No operator matches the given name and argument types. You might need
to add explicit type casts.

Isn't the operator "!=" theoritically "an alias, which is converted to <> at
a very early stage of parsing" (as per
https://www.postgresql.org/docs/17/functions-comparison.html )?

AFAIK there is no "=-" alias/macro, and I can't see any reason to implicitly
cast "-2" to INTEGER while "2" type stays inferred.

Thank you!