Session without IO states yields uncompilable code

[senier]

package Test is

   type T1 is mod 2 ** 8;
   type T2 is
      message
         F : T1;
      end message;
   generic
   session S with
      Initial => Start,
      Final   => Done
   is
   begin
      state Start is
         M : T2;
      begin
         M := T2'(F => 42);
      transition
         goto Done
      exception
         goto Done
      end Start;

      state Done is null state;
   end S;
end Test;

$ gprbuild 
using project file /opt/GNAT-Community-2021/share/gpr/_default.gpr
Compile
   [Ada]          rflx-test-s.adb
rflx-test-s.adb:83:22: (style) space not allowed
rflx-test-s.adb:83:22: error: missing operand
gprbuild: *** compilation phase failed

   function In_IO_State return Boolean is 
     (P_Next_State in );