CL-USER> (handler-bind ((style-warning #'break))
           (eval '(defgeneric frob (bar)
                   (:generic-function-class filtered-functions:filtered-function)
                   (:method (bar)))))

;;;;;;;;;;;;;

The value
  #<SB-INT:SIMPLE-STYLE-WARNING "The variable ~S is defined but never used." {100E1771B3}>

is not of type
  (OR (VECTOR CHARACTER) (VECTOR NIL) FUNCTION BASE-STRING)
when binding SB-KERNEL::DATUM
   [Condition of type TYPE-ERROR]

Restarts:
 0: [MUFFLE-WARNING] Skip warning.
 1: [RETRY] Retry SLIME REPL evaluation request.
 2: [*ABORT] Return to SLIME's top level.
 3: [ABORT] abort thread (#<THREAD "repl-thread" RUNNING {100BF41DE3}>)

Backtrace:
  0: (BREAK #<SB-INT:SIMPLE-STYLE-WARNING "The variable ~S is defined but never used." {100E1771B3}>) [more]
      Locals:
        SB-DEBUG::ARG-0 = #<SB-INT:SIMPLE-STYLE-WARNING "The variable ~S is defined but never used." {100E1771B3}>
        SB-DEBUG::MORE = NIL
  1: (SB-KERNEL::%SIGNAL #<SB-INT:SIMPLE-STYLE-WARNING "The variable ~S is defined but never used." {100E1771B3}>)
      Locals:
        CONDITION = #<SB-INT:SIMPLE-STYLE-WARNING "The variable ~S is defined but never used." {100E1771B3}>
        HANDLER-CLUSTERS = (((#<SB-KERNEL::CLASSOID-CELL SB-IMPL::EVAL-ERROR> . #<CLOSURE # {7F213AC6533B}>)) ((#<SB-KERNEL::CLASSOID-CELL SB-C:COMPILER-ERROR> . #<FUNCTION # {52225A3B}>)) ..)
  2: (SB-C::COMPILER-STYLE-WARNING-HANDLER #<SB-INT:SIMPLE-STYLE-WARNING "The variable ~S is defined but never used." {100E1771B3}>)
      Locals:
        CONDITION = #<SB-INT:SIMPLE-STYLE-WARNING "The variable ~S is defined but never used." {100E1771B3}>
  3: (SB-KERNEL::%SIGNAL #<SB-INT:SIMPLE-STYLE-WARNING "The variable ~S is defined but never used." {100E1771B3}>)
      Locals:
        CONDITION = #<SB-INT:SIMPLE-STYLE-WARNING "The variable ~S is defined but never used." {100E1771B3}>
        HANDLER-CLUSTERS = (((#<SB-KERNEL::CLASSOID-CELL COMPILER-NOTE> . #<FUNCTION MUFFLE-WARNING>)) ((#<SB-KERNEL::CLASSOID-CELL SB-IMPL::EVAL-ERROR> . #<CLOSURE # {7F213AC64C4B}>)) ..)
  4: ((FLET SB-KERNEL::%WARN :IN "SYS:SRC;CODE;WARM-ERROR.LISP") "The variable ~S is defined but never used." #<SB-KERNEL::CONDITION-CLASSOID STYLE-WARNING> SB-INT:SIMPLE-STYLE-WARNING BAR)
      Locals:
        CONDITION = #<SB-INT:SIMPLE-STYLE-WARNING "The variable ~S is defined but never used." {100E1771B3}>
        SB-KERNEL::DATUM = "The variable ~S is defined but never used."
        #:G7 = SB-INT:SIMPLE-STYLE-WARNING
        SB-KERNEL::SUPER = #<SB-KERNEL::CONDITION-CLASSOID STYLE-WARNING>
        SB-KERNEL::SUPERCLASSOID-NAME = STYLE-WARNING
        SB-DEBUG::MORE = (BAR)
  5: (SB-C:COMPILER-STYLE-WARN "The variable ~S is defined but never used." BAR)
      Locals:
        #:G54315 = "The variable ~S is defined but never used."
        SB-DEBUG::MORE = (BAR)
  6: (SB-C::NOTE-UNREFERENCED-VARS (#<SB-C::LAMBDA-VAR :%SOURCE-NAME BAR {100E152FE3}>) #<SB-KERNEL:BIND :LAMBDA #<SB-C::CLAMBDA :%SOURCE-NAME #:METHOD-FUNCTION :%DEBUG-NAME (FLET "METHOD-FUNCTION") :KIND ..
      Locals:
        POLICY = #<SB-KERNEL:BIND :LAMBDA #<SB-C::CLAMBDA :%SOURCE-NAME #:METHOD-FUNCTION :%DEBUG-NAME # :KIND NIL :TYPE # :WHERE-FROM :DEFINED :VARS # {100E1531D3}>>
        VARS = (#<SB-C::LAMBDA-VAR :%SOURCE-NAME BAR {100E152FE3}>)
  7: (SB-C::NOTE-UNREFERENCED-FUN-VARS #<SB-C::CLAMBDA :%SOURCE-NAME #:METHOD-FUNCTION :%DEBUG-NAME (FLET "METHOD-FUNCTION") :KIND NIL :TYPE #<SB-KERNEL:FUN-TYPE (FUNCTION (T) (VALUES NULL &OPTIONAL))> :WH..
      Locals:
        FUN = #<SB-C::CLAMBDA :%SOURCE-NAME #:METHOD-FUNCTION :%DEBUG-NAME (FLET "METHOD-FUNCTION") :KIND NIL :TYPE #<SB-KERNEL:FUN-TYPE (FUNCTION # #)> :WHERE-FROM :DEFINED :VARS (BAR) {100E1531D3}>
  8: (SB-C::%ADD-LAMBDA-VARS-TO-CLOSURES #<SB-C::CLAMBDA :%SOURCE-NAME #:METHOD-FUNCTION :%DEBUG-NAME (FLET "METHOD-FUNCTION") :KIND NIL :TYPE #<SB-KERNEL:FUN-TYPE (FUNCTION (T) (VALUES NULL &OPTIONAL))> :..
      Locals:
        CLAMBDA = #<SB-C::CLAMBDA :%SOURCE-NAME #:METHOD-FUNCTION :%DEBUG-NAME (FLET "METHOD-FUNCTION") :KIND NIL :TYPE #<SB-KERNEL:FUN-TYPE (FUNCTION # #)> :WHERE-FROM :DEFINED :VARS (BAR) {100E1531D3}>
        DID-SOMETHING = NIL
        PHYSENV = #<SB-C::PHYSENV :LAMBDA #<SB-C::CLAMBDA :%SOURCE-NAME #:METHOD-FUNCTION :%DEBUG-NAME # :KIND NIL :TYPE # :WHERE-FROM :DEFINED :VARS # {100E1531D3}> {100E177173}>
  9: (SB-C::ADD-LAMBDA-VARS-AND-LET-VARS-TO-CLOSURES #<SB-C::CLAMBDA :%SOURCE-NAME #:METHOD-FUNCTION :%DEBUG-NAME (FLET "METHOD-FUNCTION") :KIND NIL :TYPE #<SB-KERNEL:FUN-TYPE (FUNCTION (T) (VALUES NULL &O..
      Locals:
        CLAMBDA = #<SB-C::CLAMBDA :%SOURCE-NAME #:METHOD-FUNCTION :%DEBUG-NAME (FLET "METHOD-FUNCTION") :KIND NIL :TYPE #<SB-KERNEL:FUN-TYPE (FUNCTION # #)> :WHERE-FROM :DEFINED :VARS (BAR) {100E1531D3}>
        DID-SOMETHING = NIL
 10: (SB-C::PHYSENV-ANALYZE #<SB-C:COMPONENT :NAME (FLET CALL-NEXT-METHOD) :REANALYZE T {100E165EB3}>)
      Locals:
        COMPONENT = #<SB-C:COMPONENT :NAME (FLET CALL-NEXT-METHOD) :REANALYZE T {100E165EB3}>
 11: (SB-C::COMPILE-COMPONENT #<SB-C:COMPONENT :NAME (FLET CALL-NEXT-METHOD) :REANALYZE T {100E165EB3}>)
      Locals:
        COMPONENT = #<SB-C:COMPONENT :NAME (FLET CALL-NEXT-METHOD) :REANALYZE T {100E165EB3}>
 12: (SB-C::%COMPILE (SB-INT:NAMED-LAMBDA (SB-PCL::SLOW-METHOD FROB (T)) (#1=#:ARGS #2=#:NEXT-METHODS &REST #3=#:MORE-ARGS) (DECLARE (IGNORABLE #1# #2# #3#)) (FLET (# #) (DECLARE # #) (FLET # # #))) #<SB-C..
      Locals:
        COMPONENT-FROM-DFO = #<SB-C:COMPONENT :NAME (FLET CALL-NEXT-METHOD) :REANALYZE T {100E165EB3}>
        COMPONENTS-FROM-DFO = (#<SB-C:COMPONENT :NAME (FLET CALL-NEXT-METHOD) :REANALYZE T {100E165EB3}>)
        FUN = #<SB-C::CLAMBDA ..>
        LAMBDA-EXPRESSION = (SB-INT:NAMED-LAMBDA (SB-PCL::SLOW-METHOD FROB (T)) ..)
        NAME = NIL
        OBJECT = #<SB-C::CORE-OBJECT {100E136DB3}>
        PATH = (SB-C::ORIGINAL-SOURCE-START 0 0)
 13: ((FLET "LAMBDA0" :IN "SYS:SRC;COMPILER;TARGET-MAIN.LISP"))
      Locals:
        SB-C::OOPS = NIL
 14: ((FLET SB-C::WITH-IT :IN SB-C::%WITH-COMPILATION-UNIT))
      Locals:
        SUCCEEDED-P = NIL
 15: (SB-C:COMPILE-IN-LEXENV (SB-INT:NAMED-LAMBDA (SB-PCL::SLOW-METHOD FROB (T)) (#1=#:ARGS #2=#:NEXT-METHODS &REST #3=#:MORE-ARGS) (DECLARE (IGNORABLE #1# #2# #3#)) (FLET (# #) (DECLARE # #) (FLET # # #))..
      Locals:
        .ANONYMOUS. = #<NULL-LEXENV>
        EPHEMERAL = NIL
        ERRORP = T
        FORM = (SB-INT:NAMED-LAMBDA (SB-PCL::SLOW-METHOD FROB (T)) ..)
        #:G63329 = NIL
        NAME = NIL
        SOURCE-INFO = NIL
 16: (SB-IMPL::%SIMPLE-EVAL (SB-INT:NAMED-LAMBDA (SB-PCL::SLOW-METHOD FROB (T)) (#1=#:ARGS #2=#:NEXT-METHODS &REST #3=#:MORE-ARGS) (DECLARE (IGNORABLE #1# #2# #3#)) (FLET (# #) (DECLARE # #) (FLET # # #)))..
      Locals:
        EXPR = (SB-INT:NAMED-LAMBDA (SB-PCL::SLOW-METHOD FROB (T)) ..)
        LEXENV = #<NULL-LEXENV>
 17: (SB-INT:SIMPLE-EVAL-IN-LEXENV (SB-INT:NAMED-LAMBDA (SB-PCL::SLOW-METHOD FROB (T)) (#1=#:ARGS #2=#:NEXT-METHODS &REST #3=#:MORE-ARGS) (DECLARE (IGNORABLE #1# #2# #3#)) (FLET (# #) (DECLARE # #) (FLET #..
      Locals:
        SB-KERNEL:LEXENV = #<NULL-LEXENV>
        SB-IMPL::ORIGINAL-EXP = (SB-INT:NAMED-LAMBDA (SB-PCL::SLOW-METHOD FROB (T)) ..)
 18: (SB-INT:SIMPLE-EVAL-IN-LEXENV (LIST* :FUNCTION (SB-INT:NAMED-LAMBDA (SB-PCL::SLOW-METHOD FROB #) (#:ARGS #:NEXT-METHODS &REST #:MORE-ARGS) (DECLARE #) (FLET # # #)) (QUOTE NIL)) #<NULL-LEXENV>)
      Locals:
        SB-KERNEL:LEXENV = #<NULL-LEXENV>
        SB-IMPL::NAME#2 = LIST*
        SB-IMPL::ORIGINAL-EXP = (LIST* :FUNCTION ..)
 19: (SB-INT:SIMPLE-EVAL-IN-LEXENV (SB-PCL::%DEFMETHOD-EXPANDER FROB NIL (BAR) NIL) #<NULL-LEXENV>)
      Locals:
        SB-KERNEL:LEXENV = #<NULL-LEXENV>
        SB-IMPL::NAME#2 = SB-PCL::LOAD-DEFMETHOD
        SB-IMPL::ORIGINAL-EXP = (SB-PCL::%DEFMETHOD-EXPANDER FROB NIL (BAR) NIL)
 20: (SB-INT:SIMPLE-EVAL-IN-LEXENV (EVAL-WHEN (:EXECUTE) (SB-PCL::%DEFMETHOD-EXPANDER FROB NIL (BAR) NIL)) #<NULL-LEXENV>)
      Locals:
        SB-KERNEL:LEXENV = #<NULL-LEXENV>
        SB-IMPL::ORIGINAL-EXP = (EVAL-WHEN (:EXECUTE) (SB-PCL::%DEFMETHOD-EXPANDER FROB NIL (BAR) NIL))
 21: (SB-INT:SIMPLE-EVAL-IN-LEXENV (DEFMETHOD FROB (BAR)) #<NULL-LEXENV>)
      Locals:
        SB-KERNEL:LEXENV = #<NULL-LEXENV>
        SB-IMPL::ORIGINAL-EXP = (DEFMETHOD FROB (BAR))
 22: (SB-INT:SIMPLE-EVAL-IN-LEXENV (LIST (DEFMETHOD FROB (BAR))) #<NULL-LEXENV>)
      Locals:
        SB-KERNEL:LEXENV = #<NULL-LEXENV>
        SB-IMPL::NAME#2 = LIST
        SB-IMPL::ORIGINAL-EXP = (LIST (DEFMETHOD FROB (BAR)))
 23: (SB-INT:SIMPLE-EVAL-IN-LEXENV (SB-PCL::SET-INITIAL-METHODS (LIST (DEFMETHOD FROB #)) (FDEFINITION (QUOTE FROB))) #<NULL-LEXENV>)
      Locals:
        SB-KERNEL:LEXENV = #<NULL-LEXENV>
        SB-IMPL::NAME#2 = SB-PCL::SET-INITIAL-METHODS
        SB-IMPL::ORIGINAL-EXP = (SB-PCL::SET-INITIAL-METHODS (LIST (DEFMETHOD FROB (BAR))) (FDEFINITION 'FROB))
 24: (SB-IMPL::SIMPLE-EVAL-PROGN-BODY ((EVAL-WHEN (:COMPILE-TOPLEVEL :LOAD-TOPLEVEL :EXECUTE) (SB-PCL::COMPILE-OR-LOAD-DEFGENERIC #)) (SB-PCL::LOAD-DEFGENERIC (QUOTE FROB) (QUOTE #) (SB-C:SOURCE-LOCATION) ..
      Locals:
        LEXENV = #<NULL-LEXENV>
        PROGN-BODY = ((EVAL-WHEN (:COMPILE-TOPLEVEL :LOAD-TOPLEVEL :EXECUTE) (SB-PCL::COMPILE-OR-LOAD-DEFGENERIC 'FROB)) ..)
        REST-I = ((FDEFINITION 'FROB))
 25: (SB-INT:SIMPLE-EVAL-IN-LEXENV (DEFGENERIC FROB (BAR) (:GENERIC-FUNCTION-CLASS FILTERED-FUNCTIONS:FILTERED-FUNCTION) (:METHOD (BAR))) #<NULL-LEXENV>)
      Locals:
        SB-KERNEL:LEXENV = #<NULL-LEXENV>
        SB-IMPL::ORIGINAL-EXP = (DEFGENERIC FROB ..)
 26: (EVAL (DEFGENERIC FROB (BAR) (:GENERIC-FUNCTION-CLASS FILTERED-FUNCTIONS:FILTERED-FUNCTION) (:METHOD (BAR))))
      Locals:
        SB-IMPL::ORIGINAL-EXP = (DEFGENERIC FROB ..)
 27: ((LAMBDA ()))
 28: (SB-INT:SIMPLE-EVAL-IN-LEXENV (HANDLER-BIND ((STYLE-WARNING #)) (EVAL (QUOTE #))) #<NULL-LEXENV>)
 29: (EVAL (HANDLER-BIND ((STYLE-WARNING #)) (EVAL (QUOTE #))))
 30: (SWANK::EVAL-REGION "(handler-bind ((style-warning #'break)) ..)
 31: ((LAMBDA NIL :IN SWANK-REPL::REPL-EVAL))
 32: (SWANK-REPL::TRACK-PACKAGE #<CLOSURE (LAMBDA NIL :IN SWANK-REPL::REPL-EVAL) {100E0DFE4B}>)
 33: ((LAMBDA NIL :IN SWANK-REPL::REPL-EVAL))
 34: (SWANK::CALL-WITH-RETRY-RESTART "Retry SLIME REPL evaluation request." #<CLOSURE (LAMBDA NIL :IN SWANK-REPL::REPL-EVAL) {100E0DFDDB}>)
 35: ((LAMBDA NIL :IN SWANK-REPL::REPL-EVAL))
 36: (SWANK/BACKEND:CALL-WITH-SYNTAX-HOOKS #<CLOSURE (LAMBDA NIL :IN SWANK-REPL::REPL-EVAL) {100E0DFDBB}>)
 37: (SWANK::CALL-WITH-BUFFER-SYNTAX NIL #<CLOSURE (LAMBDA NIL :IN SWANK-REPL::REPL-EVAL) {100E0DFDBB}>)
 38: (SWANK-REPL::REPL-EVAL "(handler-bind ((style-warning #'break)) ..)
 39: (SWANK-REPL:LISTENER-EVAL "(handler-bind ((style-warning #'break)) ..)
 40: (SB-INT:SIMPLE-EVAL-IN-LEXENV (SWANK-REPL:LISTENER-EVAL "(handler-bind ((style-warning #'break)) ..)
 41: (EVAL (SWANK-REPL:LISTENER-EVAL "(handler-bind ((style-warning #'break)) ..)
 42: (SWANK:EVAL-FOR-EMACS (SWANK-REPL:LISTENER-EVAL "(handler-bind ((style-warning #'break)) ..)
 43: (SWANK::PROCESS-REQUESTS NIL)
 44: ((LAMBDA NIL :IN SWANK::HANDLE-REQUESTS))
 45: ((LAMBDA NIL :IN SWANK::HANDLE-REQUESTS))
 46: (SWANK/SBCL::CALL-WITH-BREAK-HOOK #<FUNCTION SWANK:SWANK-DEBUGGER-HOOK> #<CLOSURE (LAMBDA NIL :IN SWANK::HANDLE-REQUESTS) {100BF4818B}>)
 47: ((FLET SWANK/BACKEND:CALL-WITH-DEBUGGER-HOOK :IN "/home/phoe/.emacs.d/elpa/26.1/develop/slime-20190930.1713/swank/sbcl.lisp") #<FUNCTION SWANK:SWANK-DEBUGGER-HOOK> #<CLOSURE (LAMBDA NIL :IN SWANK::HAN..
 48: (SWANK/BACKEND:CALL-WITH-DEBUGGER-HOOK #<FUNCTION SWANK:SWANK-DEBUGGER-HOOK> #<CLOSURE (LAMBDA NIL :IN SWANK::HANDLE-REQUESTS) {100BF4818B}>)
 49: ((LAMBDA NIL :IN SWANK::HANDLE-REQUESTS))
 50: (SWANK::CALL-WITH-BINDINGS ((*STANDARD-INPUT* . #<SWANK/GRAY::SLIME-INPUT-STREAM {100BE1B7F3}>)) #<CLOSURE (LAMBDA NIL :IN SWANK::HANDLE-REQUESTS) {100BF481AB}>)
 51: ((FLET "FORM-FUN-5" :IN SWANK::HANDLE-REQUESTS))
 52: (SWANK::HANDLE-REQUESTS #<SWANK::MULTITHREADED-CONNECTION {100AC34733}> NIL)
 53: (SWANK-REPL::REPL-LOOP #<SWANK::MULTITHREADED-CONNECTION {100AC34733}>)
 54: ((LAMBDA NIL :IN SWANK-REPL::SPAWN-REPL-THREAD))
 55: (SWANK::CALL-WITH-BINDINGS NIL #<CLOSURE (LAMBDA NIL :IN SWANK-REPL::SPAWN-REPL-THREAD) {100BF4816B}>)
 56: ((LAMBDA NIL :IN SWANK-REPL::SPAWN-REPL-THREAD))
 57: ((FLET SB-UNIX::BODY :IN SB-THREAD::NEW-LISP-THREAD-TRAMPOLINE))
 58: ((FLET "WITHOUT-INTERRUPTS-BODY-4" :IN SB-THREAD::NEW-LISP-THREAD-TRAMPOLINE))
 59: ((FLET SB-THREAD::WITH-MUTEX-THUNK :IN SB-THREAD::NEW-LISP-THREAD-TRAMPOLINE))
 --more--