Re: Weird behavior with modular type

[Georg Bauhaus <rm.dash-bauhaus@futureapps.de>]

On 07.02.12 19:02, Adam Beneschan wrote:
> On Feb 7, 9:24 am, Yannick Duch�ne (Hibou57)
> <yannick_duch...@yahoo.fr> wrote:
>>
>> Now, the following, with added "Natural'Pos (...)", did not even compile:
>>
>>     with Ada.Text_IO;
>>     procedure Bug is
>>
>>        type Modular_Type is mod 256;
>>        subtype Index_Type is Natural range 1 .. 3;
>>
>>        E : Modular_Type := 255;
>>        I : Index_Type := 3;
>>        B : Boolean;
>>
>>     begin
>>        B := E < Natural'Pos (10 ** I);
>>        Ada.Text_IO.Put_Line (Boolean'Image (B));
>>     end Bug;
>>
>> That's counter intuitive. I would expect the first to behave as the
>> second, with either a failure to compile or range error at run-time.
> 
> By the way, I don't see any reason this shouldn't compile, although
> offhand I think it should raise a Constraint_Error at runtime (since
> 1024 would be the right argument of a function "<" (Left, Right : E)
> return Boolean and 1024 can't be converted to E without a
> Constraint_Error).  When I compile on GNAT (4.5.2), it says "static
> expression fails Constraint_Check".  This looks like a GNAT bug,
> though, because there's no static expression.  Apparently it thinks
> it's static because it knows the value of I must still be 3 at that
> point, but that doesn't make it a static expression according to the
> language rules, so the program shouldn't be rejected.

This spit for the assignment to B seems to confirm:

      | Node #44 N_Identifier "false" (Node_Id=1727) (analyzed)
      |  Rewritten: original node = N_Op_Lt "Olt" (Node_Id=42471)
      | ...
      |  Chars = "false" (Name_Id=300001085)
      |  Is_Static_Expression = True
      |  Etype = N_Defining_Identifier "boolean" (Entity_Id=12s)
      |  Entity = N_Defining_Identifier "false" (Entity_Id=107s)

-gnatdg gives

   b := false;