Delta message aggregate with assignment from 'Opaque generates invalid code

[jklmnn]

When using a delta message aggregate to assign a message field from a 'Opaque expression:

Option.Data := Option_Data'Opaque;

I get the following error:

rflx-test-session.adb:127:94: error: invalid operand types for operator "<"
rflx-test-session.adb:127:94: error: left operand has type "Bit_Length" defined at rflx-rflx_generic_types.ads:35, instance at rflx-rflx_types.ads:14
rflx-test-session.adb:127:94: error: right operand has type "Length" defined at rflx-universal.ads:74

Full reproducing spec:

with Universal; 

package Test is 

   type Result is (M_Valid, M_Invalid) with Size => 2;

   type Option_Data is
      message
         Length : Universal::Length;                                                                                  
         Data : Opaque
            with Size => Length * 8;
      end message;

   generic
      Channel : Channel with Readable, Writable; -- §S-P-C-RW
      -- §S-P-F-R-S
      with function Get_Option_Data
         (Data : Opaque)
      return Option_Data;
   session Session with
      Initial => Start,
      Final => Terminated
   is
      Message : Universal::Message; -- §S-D-V-T-M, §S-D-V-E-N
      Option : Universal::Option;
   begin
      state Start is
      begin
         Channel'Read (Message); -- §S-S-A-RD-V
      transition
         goto Process
            if Message'Valid -- §S-S-T-VAT, §S-E-AT-V-V
               and Message.Message_Type = Universal::MT_Data -- §S-S-T-S, §S-E-S-V, §S-S-T-L
               and Message.Length = 3 -- §S-S-T-S, §S-E-S-V, §S-S-T-L
         goto Terminated -- §S-S-T-N
      end Start;

      state Process is
         Option_Data : Option_Data;
      begin
         -- §S-S-A-A-CL, §S-E-CL-V, §S-E-CL-S
         Option_Data := Get_Option_Data (Message.Data);
         Option'Reset;
         Option.Option_Type := Universal::OT_Data;
         Option.Length := Option_Data.Length;
         Option.Data := Option_Data'Opaque;
      transition
         goto Reply -- §S-S-T-N
      exception
         goto Terminated -- §S-S-E
      end Process;

      state Reply is
      begin
         Channel'Write (Option); -- §S-S-A-WR-V
      transition
         goto Terminated -- §S-S-T-N
      end Reply;

      state Terminated is null state; -- §S-S-N
   end Session;

end Test;