Compilation fails due to range integer with single value

[treiher]

package Test is

    type B is range 1 .. 1 with Size => 1;

    type D is
       message
          A : B
             then C
                with Length => A * 8;
          C : Opaque;
       end message;

    type E is null message;

    for D use (C => E);

end Test;

rflx-test.ads:34:11: warning: can never be greater than, could replace by "=" [-gnatwc]

   compilation of rflx-test.ads failed

 33    function Valid (Val : RFLX.Test.B_Base) return Boolean is
 34      (Val >= 1);