bug#50719: Cannot add a no-applicable-method implementation for a generic with no method

[Vivien Kraus via "Bug reports for GUILE, GNU's Ubiquitous Extension Language" <bug-guile@gnu.org>]

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Dear guile,

The following program should just return 'foo, but it enters an
infinite loop:

(use-modules (oop goops))

(define-class <my-generic> (<generic>))

(define-method (no-method (generic <my-generic>) args)
  'foo)

(define-method (no-applicable-method (generic <my-generic>) args)
  'foo)

(define hello
  (make <my-generic>
    #:name 'hello))

(hello)

I don’t know GOOPS enough to understand what happens, but when I
slightly change the last line:

(use-modules (oop goops))

(define-class <my-generic> (<generic>))

(define-method (no-method (generic <my-generic>) args)
  'foo)

(define-method (no-applicable-method (generic <my-generic>) args)
  'foo)

(define hello
  (make <my-generic>
    #:name 'hello))

(apply-generic hello '())

Now it returns 'foo, as expected. So my guess is that the infinite
loops happens during the memoization phase. More specifically, the no-
applicable-method is called over and over again.

Please find a failing test case attached.

Best regards,

Vivien

[-- Attachment #2: 0001-goops-cannot-override-no-applicable-method-for-a-gen.patch --]
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From 435d4d7569a25dda05489c3eda3086e0dbf531af Mon Sep 17 00:00:00 2001
From: Vivien Kraus <vivien@planete-kraus.eu>
Date: Tue, 21 Sep 2021 11:40:43 +0200
Subject: [PATCH] goops: cannot override no-applicable-method for a generic
 that has no methods

---
 test-suite/tests/goops.test | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/test-suite/tests/goops.test b/test-suite/tests/goops.test
index b06ba98b2..626eeaacd 100644
--- a/test-suite/tests/goops.test
+++ b/test-suite/tests/goops.test
@@ -761,3 +761,22 @@
               #:metaclass <redefinable-meta>)))
       (pass-if-equal 123 (get-the-bar (make <foo>)))
       (pass-if-equal 123 (get-the-bar (make <redefinable-foo>))))))
+
+(with-test-prefix "Can override no-applicable-method in a generic without methods"
+  (let ((patience-no-method 10)
+        (patience-no-applicable-method 10))
+    (define-class <my-generic> (<generic>))
+    (define-method (no-method (generic <my-generic>) args)
+      (when (= patience-no-method 0)
+        (error "Infinite loop detected in no-method"))
+      (set! patience-no-method (- patience-no-method 1))
+      'foo)
+    (define-method (no-applicable-method (generic <my-generic>) args)
+      (when (= patience-no-applicable-method 0)
+        (error "Infinite loop detected in no-applicable-method"))
+      (set! patience-no-applicable-method (- patience-no-applicable-method 1))
+      'foo)
+    (define hello
+      (make <my-generic>
+        #:name 'hello))
+    (pass-if-equal (hello) 'foo)))
-- 
2.33.0