Re: CONSTRAINT_ERROR - why?

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Alfred Hilscher <Alfred.Hilscher@t-online.de> writes:

> Dennis Lee Bieber schrieb:
> > 
> > From the user guide:
> > 
> > -gnato
> >    Enables overflow checking for integer operations. This causes GNAT
> > to generate slower and larger executable programs by adding code to
> > check for overflow (resulting in raising Constraint_Error as required
> > by standard Ada semantics). These overflow checks correspond to
> > situations in which the true value of the result of an operation may be
> 
> "... the result of an operation ..." The result of the operation "/"
> _does_ fit into a BYTE.

The Constraint_Error comes from the implicit conversion from universal
integer to BYTE, which happens to the *operands* of "/".  It's got
nothing to do with the "/" itself.

It's a Range_Check, by the way, not an Overflow_Check.  I don't know if
the -gnato switch understands that.

The relevant RM paragraph is 4.6(28).  It is a mistake, in my opinion
(although I didn't realize it at the time).  All the other operations on
modular types wrap around -- clearly type conversion should work the
same way.  (Randy will say that *none* of the operations should wrap
around, and I could go along with that, too.  But have one "gotcha"
operation work differently from all the rest is poor language design.)

> > outside the base range of the result type. The following example shows
> > the distinction:

- Bob