• Subject: Re: Integers-related trap in Lua 5.3
• From: Sean Conner <sean@...>
• Date: Wed, 18 May 2016 15:00:12 -0400

```It was thus said that the Great Roberto Ierusalimschy once stated:
> > It was thus said that the Great Roberto Ierusalimschy once stated:
> > >
> > > One problem with all proposed solutions is math.mininteger. The
> > > lexer reads numbers without signal. (Several languages do the
> > > same, e.g. C).  This means that -9223372036854775808 is read as
> > > -(9223372036854775808) (a unary minus applied over a positive
> > > constant). But 9223372036854775808 is not a valid integer, so it would
> > > result in a float (or nil, or inf, or error). Whatever the choice, the
> > > final value of -9223372036854775808 would not be an integer, despite
> > > it being a valid integer.
> >
> >   I haven't looked at the code, but couldn't the lexer read the minus sign
> > and set a flag. Then read in the number as an unsigned quantity and as long
> > as it's not a float (no decimal point, no 'e' or 'E', etc.) then check
> > against LLONG_MIN and LLONG_MAX? 9223372036854775808 *is* representable as
> > an unsigned quantity.
>
> The lexer cannot distinguish between a "-9223372036854775808" in
> "x = -9223372036854775808" and in "x = x-9223372036854775808". It
> handles both the same way, so the only way is a minus and then
> a constant. (As I said, this is a common thing in programming
> languages; C and Java, for instance, have this same rule.) There
> are work arounds, but they are work arounds... Java, for instance,
> has an axplicit provision for that case:

There are two cases here:

x = x-9223372036854775808
x = x--9223372036854775808

In the first case, the '-' is the operator and you are trying to subtract a
positive integer that exceeds the bounds for a positive integer (and then
"something happens" but that "something happens" is beyond the scope for
now).  In the second case, you have a '-' as an operator, and a '-' that is
part of the number (a unary operator).  So you end up with subtracting a
negative number, which *can* be represented as an integer.

Or does the lexer not handle this like this at all?

-spc

```