bug#24876: 26.0.50; alist copied by copy-tree shares structure with original

[Gemini Lasswell]

copy-tree will recursively copy structures inside of vectors if you pass
it a non-nil vecp argument. But it fails to recursively copy structures
inside of vectors in alists constructed with dotted pairs. To reproduce,
enter the following code into *scratch* and M-x eval-buffer RET:

(defun my-copy-tree-bug ()
  (let* ((vec-ab (vector 'a 'b))
         (vec-abcd (vector vec-ab (vector 'c 'd)))
         (vec-alist (list (list 'key vec-abcd)))
         (vec-alist-copy (copy-tree vec-alist t))
         (vec-dotted-alist (list (cons 'key vec-abcd)))
         (vec-dotted-alist-copy (copy-tree vec-dotted-alist t)))
    (message "Before:")
    (message " Original: %S \tCopy: %S" vec-alist vec-alist-copy)
    (message " Original: %S \tCopy: %S" vec-dotted-alist vec-dotted-alist-copy)

    (aset vec-ab 0 'foo)
    (message "After change to original:")
    (message " Original: %S \tCopy: %S" vec-alist vec-alist-copy)
    (message " Original: %S \tCopy: %S" vec-dotted-alist 
vec-dotted-alist-copy)))

(my-copy-tree-bug)

Result:
The copy is independent of the original of the alist constructed
with a list but not independent of the original of the alist constructed
with a dotted pair.



In GNU Emacs 26.0.50.23 (x86_64-apple-darwin15.6.0, NS appkit-1404.47 Version 
10.11.6 (Build 15G1004))
 of 2016-11-03 built on rainbow.local
Repository revision: 1a2e210db7d85d8cf5f7205a0e4acc49f2ac6e67
Windowing system distributor 'Apple', version 10.3.1404
Recent messages:
 Original: ((key . [(foo b) (c d)]))    Copy: ((key . [(foo b) (c d)]))
" Original: ((key . [(foo b) (c d)]))   Copy: ((key . [(foo b) (c d)]))"
my-copy-tree-bug
Before:
 Original: ((key [[a b] [c d]]))        Copy: ((key [[a b] [c d]]))
 Original: ((key . [[a b] [c d]]))      Copy: ((key . [[a b] [c d]]))
After change to original:
 Original: ((key [[foo b] [c d]]))      Copy: ((key [[a b] [c d]]))
 Original: ((key . [[foo b] [c d]]))    Copy: ((key . [[foo b] [c d]]))
" Original: ((key . [[foo b] [c d]]))   Copy: ((key . [[foo b] [c d]]))"

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