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)

Yes, of course. What do you expect? If you don't use the return value for any purpose it is dropped.

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

Yes, there is a compiler switch that lets the compiler show an error when not using function return values. But I don't have it at hand now. Just look into the documentation.

Thanks, I've found it. {$x-} or {$ExtendedSyntax off}