[fpc-pascal] How to let compiler print waring for the dropped returned-vale

[ZHANG Dao-yuan]

E.g.

   function f0: boolean;
   begin
     f0:= false;
   end;
begin
   f0;
end;

the returned value f0 is dropped.
Compile it and the compiler won't print any warning.

Any modifier or cmd option can make the compiler print warning for this 
case?
(my compiler is 2.4.5 x86_64 Linux)