BUG #14275: cursor's variable in pgsql doesn't respect scope

The following bug has been logged on the website:

Bug reference: 14275
Logged by: Oleg Klimovich
Email address: klimych(at)tut(dot)by
PostgreSQL version: 9.5.3
Operating system: Windows 7, Windows 8, Ubuntu 14.04
Description:

DO $$
DECLARE
cur cursor for select 1; -- (1)
BEGIN
open cur; -- (2)

DECLARE
cur cursor for select 2; -- (3)
BEGIN
open cur; -- (4)
close cur; -- (5)
END;

close cur; -- (6)
end $$;

Executing of the code gives error "cursor "cur" already in use". Evedently,
PG in statement (4) refers to the variable, defined in statement (1). (and I
expect it should be variable, defined in statement (3)).
Futhermore, same error exists even across different functions:

create function func1() returns void as $$
declare
cur cursor for select 1;
BEGIN
open cur;
close cur;
end
$$
LANGUAGE 'plpgsql';

create function func2() returns void as $$
declare
cur cursor for select 1;
BEGIN
open cur;

PERFORM func1();

close cur;
end
$$
LANGUAGE 'plpgsql';

select func2();

So, cursor's variable is kind of global. I just hope it's a bug and not
"feature" (at least I haven't found mention of such behaviour in
documentation)