The following code demonstrates that when byte-compiling (but not when
interpreting), the call (- x) is transformed into (- 0 x). This results
in failure of custom - methods defined via GOOPS.

I think this says it all:


$ guile3.0
GNU Guile 3.0.7 [...]
scheme@(guile-user)> ,optimize (- x)
$1 = (- 0 x)


The longer version:


$ cat minus.scm 
(use-modules (oop goops))

(define-class <Foo> ()
  (bar #:init-keyword #:bar))

(define-method (- (x <Foo>))
  (make <Foo> #:bar (- (slot-ref x 'bar))))

(display (slot-ref (- (make <Foo> #:bar 5))
                   'bar))
(newline)



$ guile3.0 --no-auto-compile minus.scm 
-5


$ guile3.0 minus.scm 
;;; note: auto-compilation is enabled, set GUILE_AUTO_COMPILE=0
;;;       or pass the --no-auto-compile argument to disable.
;;; compiling /home/jean/tmp/minus.scm
;;; compiled /home/jean/.cache/guile/ccache/3.0-LE-8-
4.5/home/jean/tmp/minus.scm.go
Backtrace:
In ice-9/boot-9.scm:
  1752:10  8 (with-exception-handler _ _ #:unwind? _ # _)
In unknown file:
           7 (apply-smob/0 #<thunk 7f569a4a0f60>)
In ice-9/boot-9.scm:
    724:2  6 (call-with-prompt _ _ #<procedure default-prompt-handle…>)
In ice-9/eval.scm:
    619:8  5 (_ #(#(#<directory (guile-user) 7f569a499c80>)))
In ice-9/boot-9.scm:
   2835:4  4 (save-module-excursion _)
  4380:12  3 (_)
In /home/jean/tmp/minus.scm:
     9:19  2 (_)
In oop/goops.scm:
  1567:11  1 (cache-miss 0 #<<Foo> 7f56944e70b0>)
   1585:2  0 (_ _ _)

oop/goops.scm:1585:2: No applicable method for #<<generic> - (1)> in call (- 0
#<<Foo> 7f56944e70b0>)