bug#39395: GOOPS generic promotion fails for nary functions

I noticed that this fails in (at least) 2.0, 2.2 and 3.0:

  (use-modules (oop goops))
  (define (foo . rest) 'fallback)
  (define-method (foo (x <string>)) <string>)

e.g.:

  $ guile-3.0 -s test.scm
  Backtrace:
  In ice-9/boot-9.scm:
    1736:10  6 (with-exception-handler _ _ #:unwind? _ # _)
  In unknown file:
             5 (apply-smob/0 #<thunk 558f3c1bbb80>)
  In ice-9/boot-9.scm:
      718:2  4 (call-with-prompt _ _ #<procedure default-prompt-handle?>)
  In ice-9/eval.scm:
      619:8  3 (_ #(#(#<directory (guile-user) 558f3c28bf00>)))
  In ice-9/boot-9.scm:
     2806:4  2 (save-module-excursion _)
    4351:12  1 (_)
  In oop/goops.scm:
     1585:2  0 (_ _ _)

And then I found that the the manual says this:

  If symbol was previously bound to a Scheme procedure (or
  procedure-with-setter), the old procedure (and setter) is incorporated
  into the new generic function as its default procedure (and setter).

So I wondered if this might be a bug, or was expected behavior.  It's
also easy to work around -- just change the first define to a
define-method.

Thanks
-- 
Rob Browning
rlb @defaultvalue.org and @debian.org
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