bug#20241: 25.0.50; `setq' with only one argument

[Glenn Morris]

Alan Mackenzie wrote:

>> setq' should not be called with only one argument.  It "works", with the
>> effect of assigning a value of nil, but it is poor form.  Typically this
>> is a sign of a typo.
>
> Fixed in emacs-25.  The interpreter now signals a wrong-number-of-args
> error, and the byte compiler issues a warning.

What on earth is this incompatible change doing in a feature-frozen
release branch?

Also, the combination of a compiler _warning_ and a run-time _error_
makes no sense.