BUG #15272: creating an index function terminate with error

The following bug has been logged on the website:

Bug reference: 15272
Logged by: karim loumour
Email address: loumour(at)gmail(dot)com
PostgreSQL version: 10.4
Operating system: RHEL 7
Description:

I created the function bellow :

CREATE OR REPLACE FUNCTION f_unaccent(text) RETURNS text AS $$
SELECT lower(unaccent('unaccent', $1))
$$ LANGUAGE sql IMMUTABLE;

I can call this function correctly :

select upper(f_unaccent(nom)) from geoname limit 10;
upper
----------------------
CANAL LLISA
TORRENT LLIMOIS
BARRANC DEL LLEMPO
CASA LLECSIA
BORDA DEL LLECSIA
PONT DEL LLARG
FONTANAL LLARG
CASA LLARG
CANAL DEL LLARG
TORRENT DE LA LLAMPA
(10 rows)

But, when i need to create an index using that function, i have the
following error:

dref=# create index geo_test on geoname(upper(f_unaccent(nom))) ;
ERROR: function unaccent(unknown, text) does not exist
LINE 2: SELECT lower(unaccent('unaccent', $1))
^
HINT: No function matches the given name and argument types. You might need
to add explicit type casts.
QUERY:
SELECT lower(unaccent('unaccent', $1))

CONTEXT: SQL function "f_unaccent" during inlining