Stop Testcover from producing spurious 1value errors

Fix bug#25351 by copying results of form evaluations for later
comparison.
* lisp/emacs-lisp/testcover.el (testcover-after): Copy the result
of a form's first evaluation and compare subsequent evaluations to
the copy. Improve the error message used when a form's value
changes.
(testcover--copy-object, testcover--copy-object1): New functions.
* test/lisp/emacs-lisp/testcover-resources/testcases.el
(by-value-vs-by-reference-bug-25351): Remove expected failure tag.
(circular-lists-bug-24402): Add another circular list case.

1 files changed, 11 insertions, 4 deletions

diff --git a/test/lisp/emacs-lisp/testcover-resources/testcases.el b/test/lisp/emacs-lisp/testcover-resources/testcases.el
index d8b8192748d..6a9612db05a 100644
--- a/test/lisp/emacs-lisp/testcover-resources/testcases.el
+++ b/test/lisp/emacs-lisp/testcover-resources/testcases.el
@@ -357,7 +357,6 @@
 
 ;; ==== by-value-vs-by-reference-bug-25351 ====
 "An object created by a 1value expression may be modified by other code."
-:expected-result :failed
 ;; ====
 (defun testcover-testcase-ab ()
   (list 'a 'b))
@@ -491,10 +490,18 @@ regarding the odd-looking coverage result for the quoted form."
 "Testcover captures and ignores circular list errors."
 ;; ====
 (defun testcover-testcase-cyc1 (a)
-  (let ((ls (make-list 10 a%%%)))
-    (nconc ls ls)
-    ls))
+  (let ((ls (make-list 10 a%%%)%%%))
+    (nconc ls%%% ls%%%)
+    ls)) ; The lack of a mark here is due to an ignored circular list error.
 (testcover-testcase-cyc1 1)
 (testcover-testcase-cyc1 1)
+(defun testcover-testcase-cyc2 (a b)
+  (let ((ls1 (make-list 10 a%%%)%%%)
+        (ls2 (make-list 10 b)))
+    (nconc ls2 ls2)
+    (nconc ls1%%% ls2)
+    ls1))
+(testcover-testcase-cyc2 1 2)
+(testcover-testcase-cyc2 1 4)
 
 ;; testcases.el ends here.