BUG #16510: Count Distinct with non distinct column in combination with string constants throws error

The following bug has been logged on the website:

Bug reference: 16510
Logged by: Aceonline
Email address: aceonline(at)gmx(dot)de
PostgreSQL version: 11.7
Operating system: Unix (bitnami/postgresql:11.7.0 image)
Description:

Minimal Reproduction:
CREATE TABLE public.testtable (
column1 int4 NOT NULL,
column2 varchar NULL
);

insert into public.testtable values(1, 'test1');
insert into public.testtable values(2, 'test1');

SELECT
COUNT( DISTINCT (testtable.column2, 'blub') )
FROM
public.testtable;