コンパイル時に結果をチェックできる引き算

dtmodoki.ads

package dtmodoki is

   -- minus functions ensuring Left >= Right (Left - Right >= 0)
   generic
      X : Natural;
      Y : Natural;
   package Natural_Minus is

      subtype Result is Natural range X - Y .. X - Y;

      function F return Result is (X - Y);

   end Natural_Minus;

   package Calc_5_Minus_3 is new Natural_Minus (5, 3); -- OK

   package Calc_3_Minus_5 is new Natural_Minus (3, 5); -- warned

--dtmodoki.ads:19:04: warning: in instantiation at line 9
--dtmodoki.ads:19:04: warning: static value out of range of type "Standard.Natural"
--dtmodoki.ads:19:04: warning: "Constraint_Error" will be raised at run time

end dtmodoki;