failed AVER: (NULL SB-C::CURRENT) (during compilation of QL package MISC-EXTENSIONS)

[Paul F. Dietz]

Simplified input showing the bug:

(defun f ()
  `(let ()
     . ,(progn
          (let () (setq x y))
          (nreverse z))))

==>

failed AVER: (NULL SB-C::CURRENT)
This is probably a bug in SBCL itself. (Alternatively, SBCL
might have been corrupted by bad user code, e.g. by an undefined
Lisp operation like (FMAKUNBOUND 'COMPILE), or by stray pointers
from alien code or from unsafe Lisp code; or there might be a
bug in the OS or hardware that SBCL is running on.) If it seems
to be a bug in SBCL itself, the maintainers would like to know
about it. Bug reports are welcome on the SBCL mailing lists,
which you can find at <http://sbcl.sourceforge.net/>.
   [Condition of type SB-INT:BUG]

Restarts:
 0: [RETRY] Retry SLIME REPL evaluation request.
 1: [*ABORT] Return to SLIME's top level.
 2: [REMOVE-FD-HANDLER] Remove #<SB-IMPL::HANDLER INPUT on descriptor 4: #<FUNCTION (LABELS SWANK/SBCL::RUN :IN SWANK/BACKEND:ADD-FD-HANDLER) {100A84CFBB}>>
 3: [ABORT] Exit debugger, returning to top level.

Backtrace:
  0: #x52AC8997 (SB-C::FIND-ORIGINAL-SOURCE (SB-C::ORIGINAL-SOURCE-START 5 2 3 1 2 ...))
  1: #x52B88791 (SB-C::FIND-ERROR-CONTEXT (X) NIL #<FUNCTION IDENTITY>)
  2: #x52B8958F (SB-C::NOTE-UNDEFINED-REFERENCE X :VARIABLE)
  3: #x530EFACC (SB-C::IR1-CONVERT-SETQ #<SB-C::CTRAN 1 {1010AA5A73}> #<SB-C::CTRAN 2 {1010AA5C53}> NIL (SETQ X Y))
  4: #x52A281C5 (SB-C::IR1-CONVERT #<SB-C::CTRAN 1 {1010AA5A73}> #<SB-C::CTRAN 2 {1010AA5C53}> NIL (SETQ X Y))
  5: #x52A25A8C (SB-C::IR1-CONVERT-PROGN-BODY #<SB-C::CTRAN 1 {1010AA5A73}> #<SB-C::CTRAN 2 {1010AA5C53}> NIL ((SETQ X Y)))
  6: #x52A87ADC (SB-C::%PROCESSING-DECLS NIL NIL NIL #<SB-C::CTRAN 2 {1010AA5C53}> NIL NIL #<FUNCTION (LAMBDA (SB-C::NEXT SB-C::RESULT #:LEXENV0) :IN SB-C::IR1-TRANSLATE-LOCALLY) {1010AA5C9B}>)
 [...]

x86-64, "2.2.11.27-18b1e1d31"