formatter functions should be lazy in arg count checking

[Douglas Katzman]

Given:
* (defstruct foo a)
* (defmethod print-object ((self foo) stream) (when (eql (foo-a self) 42) (throw 'dog 'bone)))

Other implementations are able to process :
* (catch 'dog (format t "Hi there ~S ~S~%" (make-foo :a 42)))

But the compiler turns that format into a lambda which never executes its body because it insists on receiving 2 args.
(macroexpand-1 '(formatter "Hi there ~S ~S~%"))
#'(LAMBDA
      (STREAM
       &OPTIONAL
       (#:FORMAT-ARG669
        (ERROR 'SB-FORMAT:FORMAT-ERROR :COMPLAINT "required argument missing" :CONTROL-STRING
               "Hi there ~S ~S~%" :OFFSET 10))
       (#:FORMAT-ARG670
        (ERROR 'SB-FORMAT:FORMAT-ERROR :COMPLAINT "required argument missing" :CONTROL-STRING
               "Hi there ~S ~S~%" :OFFSET 13))
       &REST SB-FORMAT::ARGS)
    (DECLARE (IGNORABLE STREAM))
    (BLOCK NIL
      (WRITE-STRING "Hi there " STREAM)
      (PRIN1 #:FORMAT-ARG669 STREAM)
      (WRITE-STRING " " STREAM)
      (PRIN1 #:FORMAT-ARG670 STREAM)
      (TERPRI STREAM))
    SB-FORMAT::ARGS)

Probably we should have the lambda accept a dx &rest arg and use guarded expressions of the form
  (output-something (if (> (length args) K) (NTH ...) (error ...)))
which should compile not too much worse if either &rest is converted to &more or is really stack-allocated.

[Douglas Katzman (dougk)]

actually, since you have to return args anyway, there's no additional overhead imposed by
  (output-something (if args (pop args) (error ...))