typmod is always -1

Hello

I am playing with custom typmod. I did simple wrapper over varchar type.

PG_FUNCTION_INFO_V1(mvarcharin);

Datum
mvarcharin(PG_FUNCTION_ARGS)
{
elog(NOTICE, ">>>>>>>>%d", PG_GETARG_INT32(2));
return DirectFunctionCall3(varchar,
DirectFunctionCall3(varcharin,

PG_GETARG_DATUM(0),

PG_GETARG_DATUM(1),

Int32GetDatum(-1)),
PG_GETARG_DATUM(2),
/* original typmod */
BoolGetDatum(true));
/* explit casting, quite truncate */
}

CREATE TYPE mvarchar (
INPUT = mvarcharin,
OUTPUT = mvarcharout,
LIKE = pg_catalog.varchar,
typmod_in = pg_catalog.varchartypmodin,
typmod_out = pg_catalog.varchartypmodout
);

I have a problem - every call of mvarcharin is with typmod = -1.

postgres=# create table x(a mvarchar(3));
CREATE TABLE
Time: 29,930 ms
postgres=# \d x
Table "public.x"
┌────────┬─────────────┬───────────┐
│ Column │ Type │ Modifiers │
├────────┴─────────────┴───────────┤
│ a │ mvarchar(3) │ │
└──────────────────────────────────┘

postgres=# INSERT INTO x values('abcdef');
NOTICE: >>>>>>>>-1
INSERT 0 1
Time: 2,244 ms
postgres=#

can somebody navigate me?

regards
Pavel Stehule