bug#65051: internal_equal manipulates symbols with position without checking symbols-with-pos-enabled.

[Dmitry Gutov <dmitry@gutov.dev>]

Hi again, Alan,

On 11/08/2023 15:05, Alan Mackenzie wrote:

>>> I think it was possibly a design error to have text
>>> properties conceptually as a part of a string/buffer rather than
>>> something associated with it, like an overlay.  The fact that equal
>>> ignores these properties supports this view.
> 
>> We needed a reference to access the properties from. Overlays are
>> different because they attach to a buffer. There is nothing else to
>> attach to when you have a string value.
> 
> This is arbitrary; overlays _could_ have been made attachable to
> strings, in which case text properties need not have been.  That would
> have prevented all the heart searching when considering equal with
> strings.

Then we would have some "metadata" that's part of the value, and some 
that is not part of the value. How would we look those up, though? 
Through a global registry?

equal-including-properties is useful enough, by the way. In the tests, 
at least.

>> Which seems very similar to the situation with symbols, I think.
> 
> There are practical differences.  Having symbols with position simply
> handled as their bare symbols would slow down Emacs quite a lot.  That's
> why we have symbols-with-pos-enabled.  But you know that.

Does the current impl of 'equal' create worse performance as well? That 
would be a good argument to change it.

 >  Currently,
 > the working of s-w-p-enabled is inconsistent, and should be fixed, which
 > is what this bug is about.

Inconsistent with what? If we're talking about the relation between 
EQUAL and EQ, objects that are EQ have to be EQUAL, but those that are 
EQUAL don't have to be EQ.

Anyway, I'd like to offer a question from a different perspective: 
should two symbols-with-positions where the positions are different but 
the symbol is the same, be equal between each other? If yes (which is my 
reading of fns.c:2755), then it makes sense for them to be equal-able to 
symbols without positions as well.