bug#62020: Lisp reader: dotted pair notation not working when initial elements are omitted

[Mattias Engdegård]

Eli, thank you for bringing this to our attention.

>> one should be able to evaluate e.g.:
>>    (. 1)
>> to:
>>    1

Federico, it would be very easy to change the reader into behaving that way and 
I'll do that if required, but before I or anyone else change code or docs, and 
above all much more important and interesting would be to hear exactly why it 
matters to you and how you were affected by this corner of the reader semantics.

Could be it that you saw the manual passage, decided to try it out -- which is 
good, we want more people to do that -- and observed that Emacs and the manual 
didn't agree on that point?

As far as I can tell while researching ahead of the previous changed, the 
documented (old) reader semantics was merely emergent behaviour of an 
under-constrained implementation, never a purposeful design for user 
convenience.

No other Lisp (Common Lisp, Scheme etc) implementation known to me provides 
such a reader 'feature', and no evidence of any use of it was found at the 
time. This is why your report is of such interest: did it actually break 
existing code, and if so, how exactly?