bug#12677: 24.2.50; `nreverse' complains that ("foo") is not `listp'

[Stefan Monnier]

> Debugger entered--Lisp error: (wrong-type-argument listp ("c:/drews-lisp-20=
> /CONTRIB/"))

My guess is that the code ended up doing something morally equivalent
to (nreverse '(1 2 . 3)).  Try it and you'll see a similarly confusing
error message.
The reason is that by the time you get to the error, the list has
already been "nreversed" by side-effect, so the arg (which still points
to the cons cell which used to be the first element of the line) now
still points to the same cons-cell, but that cons-cell's cdr has been
set to nil because it was to become the last element of the
returned list.

I've installed a change in trunk which makes the above error include the
problematic non-nil tail, rather than the first element who looks
confusingly normal.


        Stefan