bug#30408: Checking for loss of information on integer conversion

[Paul Eggert]

Eli Zaretskii wrote:

> Emacs Lisp is not used to write software that controls
> aircraft and spaceships

Actually, I maintain Emacs Lisp code that controls timestamps used in aircraft 
and spaceships. I'm not saying that Emacs itself runs the aircraft and 
spaceships, but it definitely is used to develop software and data used there. 
As luck would have it, I'm currently engaged in an email thread about time 
transfer between Earth and Mars (yes, this is really a thing and people are 
trying to do it with millisecond precision) that is related to a project where I 
regularly use Emacs Lisp. See the thread containing this message:

https://mm.icann.org/pipermail/tz/2018-February/026257.html

> More generally, why signaling an error by default in this case is a
> good idea? ...  That would
> be similar to behavior of equivalent constructs in C programs

Sure, and C compilers typically issue diagnostics for situations similar to 
what's in Bug#30408. For example, for this C program:

int a = 18446744073709553664;

GCC issues a diagnostic, whereas for the similar Emacs Lisp program:

(setq b 18446744073709553664)

Emacs silently substitutes a number that is off by 2048. It's the latter 
behavior that causes the sort of problem seen in Bug#30408.

When people write a floating-point number they naturally expect it to have some 
fuzz. But when they write an integer they expect it to be represented exactly, 
and not to be rounded.  Emacs already reports an overflow error for the 
following code that attempts to use the same mathematical value:

(setq c #x10000000000000800)

so it's not like it would be a huge change to do something similar for decimal 
integers.

When Emacs was originally developed, its integers were typically 28 bits (not 
counting sign) and floating-point numbers could typically represent integers 
exactly up to 53 bits (not counting sign), so the old Emacs behavior was 
somewhat defensible: although it didn't do bignums, at least it could represent 
integers nearly twice as wide as fixnums. However, nowadays Emacs integers 
typically have more precision than floating point numbers, and the old Emacs 
behavior is more likely to lead to counterintuitive results such as those 
described in Bug#30408.

On thinking about it in the light of your comments, I suppose it's confusing 
that the proposal used a new signal 'inexact', whereas it should just signal 
overflow. After all, that's what string_to_number already does for out-of-range 
hexadecimal integers. That issue is easily fixed. Revised patch attached.

0001-Avoid-losing-info-when-converting-integers.patch
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