bug#25166: 26.0.50; It shouldn't be possible to set the function cell of nil and t

[Philipp <p.stephani2@gmail.com>]

You can set the function cell of nil and t using `fset' and friends.
But you can't call the `nil' function using (nil) (it does work with
(t)).  I think that attempting to set the function cell of nil and t is
almost always a bug -- probably the programmer wanted to set a real
symbol, but some of the constants got passed.  I propose to signal an
error (e.g. `setting-constant') whenever the function cell of nil and t
is modified; maybe the same should happen for keywords.


In GNU Emacs 26.0.50.5 (x86_64-unknown-linux-gnu, GTK+ Version 3.10.8)
 of 2016-12-10 built on localhost
Repository revision: dc62ea38e36b5de47893a60a76d66b7a79bf6437
Windowing system distributor 'The X.Org Foundation', version 11.0.11702000
System Description:	Ubuntu 14.04 LTS

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Memory information:
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