bug#26026: Defining a method named zero? breaks primitive zero?

[Alejandro Sanchez]

If I define a ‘zero?’ predicate method for a custom class the primitive ‘zero?’ 
is lost. Here is a simple vector module:

        ;;; File vector2.scm
        (define-module (vector2)
          #:use-module (oop goops)
          #:export (<vector2> get-x get-y zero?))
        
        (define-class <vector2> ()
          (x #:init-value 0 #:getter get-x #:init-keyword #:x)
          (y #:init-value 0 #:getter get-y #:init-keyword #:y) )

        (define-generic zero?)
        (define-method (zero? (v <vector2>))
          (and (zero? (get-x v))
               (zero? (get-y v))))

In the Guile REPL try executing the following code:

        scheme@(guile-user)> (use-modules (oop goops) (vector2))
        scheme@(guile-user)> (zero? (make <vector2>))

This will display 

        WARNING: (guile-user): `zero?' imported from both (ice-9 r5rs) and 
(vector2)
        ERROR: In procedure scm-error:
        ERROR: No applicable method for #<<generic> zero? (1)> in call (zero? 0)
        
        Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
        scheme@(guile-user) [1]> ,bt
        In vector2.scm:
             11:7  2 (_ #<<vector2> 105e87e00>)
        In oop/goops.scm:
           1438:4  1 (cache-miss 0)
        In unknown file:
                   0 (scm-error goops-error #f "No applicable method for ~S in 
call ~S" (#<<gen…> …) …)

Apparently the problem is that ‘zero?’ is defined in two modules and the 
vector2 definition overrides it. This isn’t the case with other primitives like 
‘+’ or ‘*’, so this seems like a bug? I had built Guile from HEAD a few days 
ago, my package manager shows 6fff84d as the version number, so I guess that 
must be the hash of the commit HEAD was pointing to at that time.