bug#28107: 26.0.50; Byte compilation shows an unused var warning for an used variable

[Tino Calancha <tino.calancha@gmail.com>]

Write a file '/tmp/test.el' with following content:
--8<-----------------------------cut here---------------start------------->8---
;; -*- lexical-binding: t; -*-

;; Compiles OK
(defun test ()
  (let ((lst (list "foo" 1)))
    (when (cdr lst)
      (equal (cdr lst) 1))))

;; Compiles OK
(defun getval (x) (cdr x))
(defun test3 ()
  (let ((alist (list (cons "foo" 1) (cons "bar" 2))))
    (dolist (x alist)
      (when (getval x)
        (equal (getval x) (alist-get (car x) alist))))))

;; Warning: value returned from (cdr x) is unused
(defun test2 ()
  (let ((alist (list (cons "foo" 1) (cons "bar" 2))))
    (dolist (x alist)
    (when (cdr x)
      (equal (cdr x) (alist-get (car x) alist))))))

--8<-----------------------------cut here---------------end--------------->8---

emacs -Q
M-x byte-compile-file /tmp/test.el RET
I got a warning from test2 func:
Warning: value returned from (cdr x) is unused

But the value is used as the `when' condition, and as `equal'
1st argument.


In GNU Emacs 26.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.11)
 of 2017-08-16
Repository revision: 3305dec5387021791eb09a93df5ab784b2297dc8