bug#26106: Defining a method named '-' with one parameter

[Alejandro Sanchez]

If I define a method named ‘-‘ which only takes in one parameter, the 
expression ‘(- v)’ gets rewritten to ‘(- 0 v)’. Here is a minimal example:

        (use-modules (oop goops))
        
        (define-class <vector2> ()
          (x #:init-value 0 #:getter get-x #:init-keyword #:x)
          (y #:init-value 0 #:getter get-y #:init-keyword #:y))
        
        (define-method (* (n <number>) (v <vector2>))
          (make <vector2> #:x (* n (get-x v)) #:y (* n (get-y v))))
        
        (define-method (- (v <vector2>))
          (* -1 v))
        
        (define v (make <vector2> #:x 1 #:y 2))
        (* -1 v)  ; Works fine
        (- v)  ; Throws error

Here is the error message:

        scheme@(guile-user)> (- v)
        ERROR: In procedure scm-error:
        ERROR: No applicable method for #<<generic> - (2)> in call (- 0 
#<<vector2> 10a8e4020>)
        
        Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
        scheme@(guile-user) [1]> ,bt
        In current input:
             23:0  2 (_)
        In oop/goops.scm:
           1438:4  1 (cache-miss 0 #<<vector2> 10a8e4020>)
        In unknown file:
                   0 (scm-error goops-error #f "No applicable method for ~S in 
call ~S" (#<<generic> - (2)> (- 0 #<<vec…>)) #)