'assoc' is not side-effect-free; constprop its pure subset

Since a supplied test function can do anything, assoc is not
side-effect-free (bug#44018).  However, with only two arguments it is
pure and should be optimised accordingly.

* lisp/emacs-lisp/byte-opt.el (side-effect-free-fns): Remove 'assoc'.
(byte-optimize-assoc): Constant-propagate through 2-arg assoc calls.
* test/lisp/emacs-lisp/bytecomp-tests.el
(byte-opt-testsuite-arith-data): Add test cases.

lisp/emacs-lisp/byte-opt.el

@@ -830,11 +830,13 @@
 (defun byte-optimize-assoc (form)
   ;; Replace 2-argument `assoc' with `assq', `rassoc' with `rassq',
   ;; if the first arg is a symbol.
-  (if (and (= (length form) 3)
-           (byte-optimize--constant-symbol-p (nth 1 form)))
-      (cons (if (eq (car form) 'assoc) 'assq 'rassq)
-            (cdr form))
-    form))
+  (cond
+   ((/= (length form) 3)
+    form)
+   ((byte-optimize--constant-symbol-p (nth 1 form))
+    (cons (if (eq (car form) 'assoc) 'assq 'rassq)
+          (cdr form)))
+   (t (byte-optimize-constant-args form))))
 
 (defun byte-optimize-memq (form)
   ;; (memq foo '(bar)) => (and (eq foo 'bar) '(bar))
@@ -1144,7 +1146,7 @@
 ;; I wonder if I missed any :-\)
 (let ((side-effect-free-fns
        '(% * + - / /= 1+ 1- < <= = > >= abs acos append aref ash asin atan
-	 assoc assq
+	 assq
 	 boundp buffer-file-name buffer-local-variables buffer-modified-p
 	 buffer-substring byte-code-function-p
 	 capitalize car-less-than-car car cdr ceiling char-after char-before

test/lisp/emacs-lisp/bytecomp-tests.el

@@ -365,7 +365,12 @@
             '(((a b)) a b (c) (d)))
     (mapcar (lambda (x) (cond ((memq '(a b) x) 1)
                               ((equal x '(c)) 2)))
-            '(((a b)) a b (c) (d))))
+            '(((a b)) a b (c) (d)))
+
+    (assoc 'b '((a 1) (b 2) (c 3)))
+    (assoc "b" '(("a" 1) ("b" 2) ("c" 3)))
+    (let ((x '((a 1) (b 2) (c 3)))) (assoc 'c x))
+    (assoc 'a '((a 1) (b 2) (c 3)) (lambda (u v) (not (equal u v)))))
   "List of expression for test.
 Each element will be executed by interpreter and with
 bytecompiled code, and their results compared.")