BUG #14193: cann't modify composite array's element in plpgsql?

The following bug has been logged on the website:

Bug reference: 14193
Logged by: Zhou Digoal
Email address: digoal(at)126(dot)com
PostgreSQL version: 9.5.3
Operating system: CentOS 6.x x64
Description:

postgres=# do language plpgsql $$
declare
tmp_t t[];
begin
tmp_t[1] := '(1,"test","2016-01-01")'::t;
raise notice '%', tmp_t;
tmp_t[1].info := 'new';
raise notice '%', tmp_t;
end;
$$;
ERROR: syntax error at or near "."
LINE 7: tmp_t[1].info := 'new';
^

^

Cann't grant value to tmp_t[1].info but can use it to caculate?
postgres=# do language plpgsql $$
declare
tmp_t t[];
tmp t;
begin
tmp_t[1] := '(1,"test","2016-01-01")'::t;
raise notice '%', tmp_t;
tmp := tmp_t[1];
tmp.info = 'new' || tmp_t[1].info;
tmp_t[1] := tmp;
raise notice '%', tmp_t;
end;
$$;
NOTICE: {"(1,test,\"2016-01-01 00:00:00\")"}
NOTICE: {"(1,newtest,\"2016-01-01 00:00:00\")"}
DO