RE: [External] : Undefined behavior for the function ‘last’ or an error?

[Drew Adams]

> The built-in Emacs Lisp function ‘last’ accepts
> a list as its argument:
> 
> (last '(a b c)) => (c)
> 
> It also (surprisingly) accepts a non-list as its argument:
> 
> (last 'a) => a
> (last 3.14) => 3.14
> (last "string of chars") => "string of chars"

Congratulations!

This regression was introduced in Emacs 24.5.
Prior to that it would raise an error.

In 24.5 `safe-length' was substituted for
`length', and that results in what you see.

(`C-h f safe-length'.)

IMO this should be fixed.  Someone(TM) will
maybe say it's intentional (but then it
should be doc'd).

`safe-length' is good, in avoiding pbs with
circular lists etc.  But either some test
for a list (or a sequence) should be used
or this nontraditional behavior for `last'
should be doc'd.  (`last' should of course
work with dotted lists.)

`last' is one of the oldest Lisp functions.
We really shouldn't be fiddling with its
behavior in unexpected ways.