Invalid substructure coalescing in COMPILE-FILE?

[Jan Moringen]

For a file bla.lisp with content

  (defun x ()
    (rest '(a (&rest x . #1=(&rest y)) #1# a)))

  (defun y ()
    (rest '(b (&rest x . #1=(&rest y)) #1# b)))

  (defun z ()
    (rest '(c (&rest x . #1=(&rest y)) #1# c)))

  (let ((x (x)) (y (y)) (z (z)))
    (format t "~3S ~S [~X] ~S [~X]~@
               ~3S ~S [~X] ~S [~X]~@
               ~3S ~S [~X] ~S [~X]~%"
            (eq (cddr (car x)) (cadr x)) (car x) (sb-kernel:get-lisp-obj-address (car x)) (cadr x) (sb-kernel:get-lisp-obj-address (cadr x))
            (eq (cddr (car y)) (cadr y)) (car y) (sb-kernel:get-lisp-obj-address (car y)) (cadr y) (sb-kernel:get-lisp-obj-address (cadr y))
            (eq (cddr (car z)) (cadr z)) (car z) (sb-kernel:get-lisp-obj-address (car z)) (cadr z) (sb-kernel:get-lisp-obj-address (cadr z))))

when I execute (I tried two newer SBCL versions as well, with the same result)

  > uname -a
  Linux al 6.1.0-9-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.1.27-1 (2023-05-08) x86_64 GNU/Linux
  > sbcl --version
  SBCL 2.2.9.debian
  > sbcl --noinform --no-userinit --eval '(print *features*)' --quit
  (:X86-64 :GENCGC :64-BIT :ANSI-CL :COMMON-LISP :ELF :IEEE-FLOATING-POINT :LINUX
   :LITTLE-ENDIAN :PACKAGE-LOCAL-NICKNAMES :SB-CORE-COMPRESSION :SB-LDB
   :SB-PACKAGE-LOCKS :SB-THREAD :SB-UNICODE :SBCL :UNIX)
  > sbcl --noinform --no-userinit --eval '(progn (compile-file "/tmp/bla.lisp") (load "/tmp/bla.fasl"))' --quit
  ; compiling file "/tmp/bla.lisp" (written 26 JUN 2023 08:43:19 PM):

  ; wrote /tmp/bla.fasl
  ; compilation finished in 0:00:00.008
  T (&REST X &REST Y) [1001565447] (&REST Y) [1001565427]
  NIL (&REST X &REST Y) [1001565447] (&REST Y) [10015654A7]
  NIL (&REST X &REST Y) [1001565447] (&REST Y) [10015654A7]

I expect all three EQ calls to return true. Instead, only the first call returns true. It is also surprising that the return values are different between the three calls.

I believe this behavior violates "The file compiler is required to preserve eqlness of substructures within a file. Preserving eqlness means that subobjects that are the same in the source code must be the same in the corresponding compiled code." in https://novaspec.org/cl/3_2_Compilation#sec_3_2_4_4.