Re: Division by zero

[Keith Thompson <kst-u@mib.org>]

Jeffrey Carter <spam@spam.com> writes:
[...]
> I suppose a compiler could tell if "/" had been redefined for a type,
> and produce warnings only in the case where it had not, but I don't
> know of any compilers that do.

I do:

% gcc --version
gcc (GCC) 4.0.0
Copyright (C) 2005 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

% cat -n foo.adb
     1  procedure Foo is
     2      X : Integer := 1;
     3      Y : Integer := 1;
     4  begin
     5      declare
     6          function "/"(Left, Right: Integer) return Integer is
     7          begin
     8              return Right;
     9          end "/";
    10      begin
    11          X := X / 0;
    12          X := 1 / 0;
    13      end;
    14      Y := Y / 0;
    15      Y := 1 / 0;
    16  end Foo;
% gcc -c foo.adb
foo.adb:14:14: warning: division by zero
foo.adb:14:14: warning: "Constraint_Error" will be raised at run time
foo.adb:15:12: division by zero
foo.adb:15:12: static expression raises "Constraint_Error"

I'd be surprised if this weren't the case.  By the time the compiler
is deciding whether to issue a warning, it knows that the "/" being
invoked is a user-defined operator rather than a predefined operator
(and probably doesn't care that its name is "/").

If a compiler issued a warning on a call to a user-defined "/"
function with a second parameter of 0, I'd consider it a bug (unless
it was a result of an analysis of the actual function).

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
San Diego Supercomputer Center             <*>  <http://users.sdsc.edu/~kst>
We must do something.  This is something.  Therefore, we must do this.