plpgsql doesn't check a number of expressions and number of target variables correctly

Hello

I found a plpgsql bug:

create or replace function fx()
returns void as $$
declare a int; b int;
begin
  select 10,20 into a;
end;
$$ language plpgsql;

this is bug, or minimally potential source of strange behave.

Regards

Pavel Stehule