Should be getting a constraint error in this code

From: Keith Boruff (kboruff_at_optonline.net)
Date: 07/24/04 

Date: Sat, 24 Jul 2004 02:32:44 GMT

Hey all,

Forgive me for posting such a trivial problem here but I couldn't seem
to find an answer in any of the FAQs I know of nor DejaNews.

Though I'm not a novice programmer, I am a novice Ada programmer.

In my studies this evening, I wrote the following trivial program to
force a constraint error:

with Ada.Text_Io; use Ada.Text_Io;

procedure Constraint_Tests is

   type Exam_Mark is range 0 .. 100;

   English : Exam_Mark := 72;
   Math : Exam_Mark := 68;
   Computing : Exam_Mark := 76;
   Average : Exam_Mark;

begin

  Put("Average exam mark is ");

  -- I expect a constraint exception in this statement
  -- because English + Math + Computing > 100
  -- NOTE: On my machine, Exam_Mark'Base'Last = 127
  Average := (English + Math + Computing) / 3;
  Put_line(Exam_Mark'Image(Average));

end Constraint_Tests;

Ok, the problem I'm having is that I'm expecting a constraint exception
to occur because (English + Math + Computing) = 216 and the largest
value of its base is 127 (on my machine).

However, I'm not getting this at all. I get no constraint errors
whatsoever and what's worse, Average = 243. First of all, why didn't I
get a constraint error here (since Average is of type Exam_Mark)?
Secondly, where in the heck did 243 come from?

Now, if I omit the statement in the code and instead write:
Average := English + Math + Computing;

I get a constraint error as expected. When I use the '/' operator
though, all bets are off.

Is there something I'm not seeing here? Is there something about the /
operator I should know about?

Or... could this possibly be a bug in the compiler itself (using GNAT)?

I'm embarrassed to be posting such a trivial program but it really
bothers me that the behavior is not what I expect.

If anyone has any suggestions, I'd appreciate some info on the matter...
even if it's to tell me to RTFM (... as long as you provide a link).

Thanks,
Keith Boruff