bug#69739: 30.0.50; `type-of` is not precise enough

[Stefan Monnier]

>> >> Still, any change as suggested here would be an incompatible change, so
>> >> there's risk it'll break some code out there (`type-of` is not used very
>> >> often, but it *is* used).  Another option is to introduce a new function
>> >> which does the same as `type-of` but with changes like the ones above.
>> >> (we could even decide to give it a `cl-generic-` prefix to discourage
>> >> its use elsewhere so we can be more free to change its return value in
>> >> the future).
>> > I'd prefer a non-breaking change, if possible and reasonable.
>> 
>> Suggestion for a good name for that new function?
>
> cl-generic-type-of is one.
>
>> Should it make `type-of` obsolete?
>
> I don't have an opinion on this.  Depends on how much type-of is used,
> I guess.  Maybe we should wait for the next major release and decide
> whether to deprecate then?

In my review of uses of `type-of`, I found:

- The most common seemed to be when generating error messages like
  (error "Arg should be an int rather than a %s" (type-of arg))
- The next most common was misuses like (eq (type-of FOO) 'BAR)
  which should be using `BAR-p` instead (or `cl-typep`).
- The next most common was when `type-of` is applied to a struct
  (i.e. it returns the same as (aref FOO 0)).  Since this was new
  functionality in Emacs-26, I'm surprised at how widespread it is.
- There were a few that are combined with case/pcase and would probably
  be better served by `cl-typecase`.

I think that marking it obsolete wouldn't be an option if the new
function is named something like `cl-generic-type-of`: it would have to
be a more "neutral" name.

The reason why I'd like to make it obsolete (if it can't be changed) is
to try and avoid having two slightly-different functions where most
users don't actually care about the difference.


        Stefan