Re: integer overflow handling for most-negative-fixnum

[Paul Eggert <eggert@cs.ucla.edu>]

On 07/18/2018 07:39 PM, Andy Moreton wrote:
>> +  (should-error (read (format "#x%x" most-negative-fixnum))
>> +                :type 'overflow-error)
>> +  (should-error (read (format "#o%o" most-negative-fixnum))
>> +                :type 'overflow-error)
> However, these tests (and the overflow behaviour) seem completely wrong
> to me. The reported error is for an out of range fixnum, when what was
> tested was an explicitly valid integer value.

Here, the code is executing something like (read "#x2000000000000000"), 
which is indeed out of range for a fixnum, since fixnums are signed and 
go up only to #x1fffffffffffffff (on 64-bit hosts).

> Why are non-base10 numbers treated as signed ?

Emacs fixnums are signed; there is no 'unsigned' type in Emacs Lisp. 
Although we could of course add such a type, it'd be better to expend 
our limited development resources on adding bignums. The 'unsigned' type 
in C has been a glitch magnet.

It might perhaps be useful to add an Emacs Lisp syntax for negative 
hexadecimal numbers, e.g., -#x10 would be equivalent to -16.

> Why does the error message ignore the base of the input value ? It
> should give a more accurate description of the problematic input.

Yes, it should. I'll see if I can pry loose some time to look into that.