Compiler incorrectly reasons about types of mutable data sometimes

[Denis Budyak]

Steps to reproduce:

(defparameter *g* (cons 1 2))
(defun g () (setf (car *g*) 'a))

(defun f (x)
  (when (typep x '(cons fixnum))
    (g)
    (if (typep x '(cons fixnum))
      "Still cons fixnum!"
      "Not cons fixnum")))

(print (f *g*))
;; EOF
Put this in a file, compile and load. It would print
"Still cons fixnum" while x is actually (not cons fixnum) at the moment of printing.

sbcl 1.4.0
Linux deb8 3.16.0-4-amd64 #1 SMP Debian 3.16.39-1+deb8u1 (2017-02-22) x86_64 GNU/Linux
features were not changed:
(:64-BIT :64-BIT-REGISTERS :ALIEN-CALLBACKS :ANSI-CL :ASH-RIGHT-VOPS
 :C-STACK-IS-CONTROL-STACK :COMMON-LISP :COMPACT-INSTANCE-HEADER
 :COMPARE-AND-SWAP-VOPS :COMPLEX-FLOAT-VOPS :CYCLE-COUNTER :ELF :FLOAT-EQL-VOPS
 :FP-AND-PC-STANDARD-SAVE :GENCGC :IEEE-FLOATING-POINT :IMMOBILE-CODE
 :IMMOBILE-SPACE :INLINE-CONSTANTS :INTEGER-EQL-VOP :LARGEFILE :LINKAGE-TABLE
 :LINUX :LITTLE-ENDIAN :MEMORY-BARRIER-VOPS :MULTIPLY-HIGH-VOPS
 :OS-PROVIDES-BLKSIZE-T :OS-PROVIDES-DLADDR :OS-PROVIDES-DLOPEN
 :OS-PROVIDES-GETPROTOBY-R :OS-PROVIDES-POLL :OS-PROVIDES-PUTWC
 :OS-PROVIDES-SUSECONDS-T :PACKAGE-LOCAL-NICKNAMES :PRECISE-ARG-COUNT-ERROR
 :RAW-INSTANCE-INIT-VOPS :RAW-SIGNED-WORD :RELOCATABLE-HEAP :SB-DOC :SB-EVAL
 :SB-FUTEX :SB-LDB :SB-PACKAGE-LOCKS :SB-SIMD-PACK :SB-SOURCE-LOCATIONS
 :SB-THREAD :SB-TRACEROOT :SB-UNICODE :SBCL :STACK-ALLOCATABLE-CLOSURES
 :STACK-ALLOCATABLE-FIXED-OBJECTS :STACK-ALLOCATABLE-LISTS
 :STACK-ALLOCATABLE-VECTORS :STACK-GROWS-DOWNWARD-NOT-UPWARD :SYMBOL-INFO-VOPS
 :UNBIND-N-VOP :UNDEFINED-FUN-RESTARTS :UNIX :UNWIND-TO-FRAME-AND-CALL-VOP
 :X86-64)

[Denis Budyak (budden73)]

Draft of a fix is here:
https://github.com/budden/sbclt/commit/e0529216934e824769b85b3bd34866099c371a75

[Stas Boukarev (stassats)]

There's already https://bugs.launchpad.net/sbcl/+bug/1732737
And the patch just removes any type information, whether the lvar can be modified or not.