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

[Alejandro Sanchez <hiphish@openmailbox.org>]

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.