Eli Zaretskii <eliz@gnu.org> writes:

>> Cc: contovob@tcd.ie, emacs-devel@gnu.org
>> From: Paul Eggert <eggert@cs.ucla.edu>
>> Date: Sat, 7 Jul 2018 01:41:38 -0700
>> 
>> Eli Zaretskii wrote:
>> > the documentation still doesn't say that 'length' signals
>> > an error for anything that is not a sequence, which is what
>> > proper-list-p is relying on.
>> 
>> The proposed implementation of proper-list-p does not rely on such a signal, 
>
> That's not my reading of the implementation:
>
>    (and (listp object) (ignore-errors (length object)))
>                         ^^^^^^^^^^^^^

The call to 'length' is wrapped in 'ignore-errors' in order to catch the
errors signalled in 'Flength' by 'FOR_EACH_TAIL' (in case of
circularity) and 'CHECK_LIST_END' (in case of dottedness).  'length'
shouldn't signal a 'wrong-type-argument' for a non-sequence argument
because it is only called on objects which satisfy 'listp':

    (and (listp object) (ignore-errors (length object)))
         ^^^^^^^^^^^^^^

Having said that, would something like the following documentation
change be welcome, for completeness?