fn_anyelement

postgres=# create or replace function a (b anyelement) returns anyelement as $$ select b $$ language sql;
CREATE FUNCTION
postgres=# select pg_typeof(a(1::bigint));
 pg_typeof
-----------
 bigint
(1 row)

postgres=# select pg_typeof(a(1::text));
 pg_typeof
-----------
 text
(1 row)