BUG #2490: '||' and type casting for user defined types

The following bug has been logged online:

Bug reference:      2490
Logged by:          Nikolay Samokhvalov
Email address:      pg-bugs@samokhvalov.com
PostgreSQL version: CVS
Operating system:   fedora core 5
Description:        '||' and type casting for user defined types
Details:

Assume that we are creating foolish type 'aaa', which works like
varchar(3),
or is simply 'string, which length is not more than 3'.

In/out functions for this type:

Datum
aaa_in(PG_FUNCTION_ARGS)
{
    char                 *s = PG_GETARG_CSTRING(0);
    int                 len = strlen(s);
    VarChar                *result;

    if (len > 3)
        ereport(ERROR,
            (errcode(ERRCODE_EXTERNAL_ROUTINE_EXCEPTION),
             errmsg("too much chars: length=\"%d\"",
                    len)));

    result = (VarChar *) palloc(len + VARHDRSZ);
    VARATT_SIZEP(result) = len + VARHDRSZ;
    memcpy(VARDATA(result), s, len);

    PG_RETURN_VARCHAR_P(result);
}

Datum
aaa_out(PG_FUNCTION_ARGS)
{
    VarChar    *s = PG_GETARG_VARCHAR_P(0);
    char    *result;
    int32    len;

    /* copy and add null term */
    len = VARSIZE(s) - VARHDRSZ;
    result = palloc(len + 1);
    memcpy(result, VARDATA(s), len);
    result[len] = '\0';

    PG_RETURN_CSTRING(result);
}


SQL code:

CREATE FUNCTION aaa_in(cstring)
    RETURNS aaa
    AS 'MODULE_PATHNAME'
    LANGUAGE C RETURNS NULL ON NULL INPUT;

CREATE FUNCTION aaa_out(aaa)
    RETURNS cstring
    AS 'MODULE_PATHNAME'
    LANGUAGE C RETURNS NULL ON NULL INPUT;

CREATE TYPE aaa (
    INTERNALLENGTH = -1,
    INPUT = aaa_in,
    OUTPUT = aaa_out,
    STORAGE = extended
);

CREATE CAST (aaa AS text) WITHOUT FUNCTION
    AS IMPLICIT;
CREATE CAST (text AS aaa) WITHOUT FUNCTION
    AS IMPLICIT;

Well, let's do some tests. After applying sql code in the database 'trash':

trash=# select 'asd'::aaa;
 aaa
-----
 asd
(1 row)

trash=# select 'asdf'::aaa;
ERROR:  too much chars: length="4"
trash=# select ('as' || 'df')::aaa;
 aaa
------
 asdf
(1 row)

In the last case, function aaa_in() wasn't invoked at all and we obtained
the sting of type aaa with length > 3...