bug#37943: assoc-default is wrongly flagged as side-effect-free.

[Alan Mackenzie]

Hello, Emacs.

Emacs 27.

In the source for assoc-default (in subr.el) appears

    (declare (side-effect-free t))

.  This is not true when the TEST argument to assoc-default itself
changes the match-data.

I suggest removing this declaration from the function rather than
amending the code with a save-match-data.  There are calls to
assoc-default in set-auto-mode where the caller relies on the match data
set by the function when TEST is string-match.  :-(

-- 
Alan Mackenzie (Nuremberg, Germany).