Detecting inadvertent use of integer division

[Jason House]

Don Wrote:

> Walter Bright wrote:
> > Don wrote:
> >> Consider this notorious piece of code:
> >>
> >> assert(x>1);
> >> double y = 1 / x;
> >>
> >> This calculates y as the reciprocal of x, if x is a floating-point 
> >> number. But if x is an integer, an integer division is performed 
> >> instead of a floating-point one, and y will be 0.
> >>
> >> It's a very common newbie trap, but I find it still catches me 
> >> occasionally, especially when dividing two variables or compile-time 
> >> constants.
> >>
> >> In the opPow thread there were a couple of mentions of inadvertent 
> >> integer division, and how Python is removing this error by making / 
> >> always mean floating-point division, and introducing a new operator 
> >> for integer division.
> >>
> >> We could largely eliminate this type of bug without doing anything so 
> >> drastic. Most of the problem just comes from C's cavalier attitude to 
> >> implicit casting. All we'd need to do is tighten the implicit 
> >> conversion rules for int->float, in the same way that the int->uint 
> >> rules have been tightened:
> >>
> >> "If an integer expression has an inexact result (ie, involves an 
> >> inexact integer divison), that expression cannot be implicitly cast to 
> >> a floating-point type."
> > 
> > But the compiler cannot reliably tell if it will produce an inexact result.
> > 
> > 
> >> (This means that double y = int_val / 1;  is OK, and also:
> >>  double z = 90/3; would be OK. An alternative rule would be:
> >> "If an integer expression involves integer divison, that expression 
> >> cannot be implicitly cast to a floating-point type").
> > 
> > This is kinda complicated if one has, say:
> > 
> >     double z = x/y + 3;
> 
> Integer expressions remain inexact until there's a cast.
> 
> (It's very simple to implement, you just use the integer range code, 
> adding an 'inexact' flag. Division sets the flag, casts clear the flag, 
> everything else just propagates it if a unary operation, or ORs the two 
> flags if a binary operation).

What about function calls?
double z = abs(x/y);

Regardless, your proposal is a simple incremental improvement, and I'd love to see it in D.

Also, one more thought: should similar rigor be used for implicit float -> double conversions?