bug#31718: 26.1; Strange behavior of `cond'

[Ikumi Keita]

The special form `cond' does not return expected value under a specific
condition.

[How to reproduce]
1. Save the following file as /tmp/test.el
---- test.el ---------------------------------------------------------
(defun xyz (arg)
  "dummy"
;    (cond ((eq arg nil) ; OK
;    (cond ((eq arg 'abc) ; OK
;    (cond ((eq arg 'def) ; OK
    (cond ((eq arg 'default) ; NG
;    (cond ((eq arg 'default1) ; OK
           "A")
          (t
           "B")))

(byte-compile 'xyz)

(insert (xyz t)) ; should "B", but "A" in actual
----------------------------------------------------------------------
2. emacs-26.1 -Q -l /tmp/test.el
3. Expected result:
   "B" is inserted in *scratch*
   Actual result:
   "A" is inserted in *scratch*

[Some notes related to this issue]
a. As the comments written in the above code, only the symbol `default'
   triggers this behavior.
b. Emacs 25.3 doesn't have this problem.
c. If `xyz' is not byte compiled, `cond' works as expected.
d. It seems that the following entry in NEWS is related:
** Certain cond/pcase/cl-case forms are now compiled using a faster jump
table implementation.  This uses a new bytecode op 'switch', which
isn't compatible with previous Emacs versions.  This functionality can
be disabled by setting 'byte-compile-cond-use-jump-table' to nil.
   If I replace the line `(byte-compile 'xyz)' with
(let ((byte-compile-cond-use-jump-table nil))
  (byte-compile 'xyz))
   in the above example, `cond' works as expected.


In GNU Emacs 26.1 (build 1, x86_64-unknown-freebsd11.1, GTK+ Version 3.22.29)
 of 2018-05-30 built on freebsd.vmware
Windowing system distributor 'The X.Org Foundation', version 11.0.11804000
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