Correctly explain test failures with mixed uni/multibyte strings

* lisp/emacs-lisp/ert.el (ert--explain-equal-rec):
* test/lisp/emacs-lisp/ert-tests.el (ert-test-explain-equal):
When explaining a difference between a unibyte and a multibyte string,
first convert both to multibyte.  Otherwise, we might end up comparing
unibyte char C to multibyte char C, 127<C<256, and not detect the
difference (bug#30219).

2 files changed, 28 insertions, 0 deletions

diff --git a/lisp/emacs-lisp/ert.el b/lisp/emacs-lisp/ert.el
index 68762b0752c..47d20cb69e8 100644
--- a/lisp/emacs-lisp/ert.el
+++ b/lisp/emacs-lisp/ert.el
@@ -516,6 +516,11 @@ Returns nil if they are."
                      (cl-assert (equal a b) t)
                      nil))))))))
       ((pred arrayp)
+       ;; For mixed unibyte/multibyte string comparisons, make both multibyte.
+       (when (and (stringp a)
+                  (xor (multibyte-string-p a) (multibyte-string-p b)))
+         (setq a (string-to-multibyte a))
+         (setq b (string-to-multibyte b)))
        (if (/= (length a) (length b))
            `(arrays-of-different-length ,(length a) ,(length b)
                                         ,a ,b
diff --git a/test/lisp/emacs-lisp/ert-tests.el b/test/lisp/emacs-lisp/ert-tests.el
index 36db1eeb425..3a9e81595b1 100644
--- a/test/lisp/emacs-lisp/ert-tests.el
+++ b/test/lisp/emacs-lisp/ert-tests.el
@@ -627,6 +627,29 @@ This macro is used to test if macroexpansion in `should' works."
     (should (equal (ert--explain-equal 'a sym)
                    `(different-symbols-with-the-same-name a ,sym)))))
 
+(ert-deftest ert-test-explain-equal-strings ()
+  (should (equal (ert--explain-equal "abc" "axc")
+                 '(array-elt 1 (different-atoms
+                                (?b "#x62" "?b")
+                                (?x "#x78" "?x")))))
+  (should (equal (ert--explain-equal "abc" "abxc")
+                 '(arrays-of-different-length
+                   3 4 "abc" "abxc" first-mismatch-at 2)))
+  (should (equal (ert--explain-equal "xyA" "xyÅ")
+                 '(array-elt 2 (different-atoms
+                                (?A "#x41" "?A")
+                                (?Å "#xc5" "?Å")))))
+  (should (equal (ert--explain-equal "m\xff" "m\u00ff")
+                 `(array-elt
+                   1 (different-atoms
+                      (#x3fffff "#x3fffff" ,(string-to-multibyte "?\xff"))
+                      (#xff "#xff" "?ÿ")))))
+  (should (equal (ert--explain-equal (string-to-multibyte "m\xff") "m\u00ff")
+                 `(array-elt
+                   1 (different-atoms
+                      (#x3fffff "#x3fffff" ,(string-to-multibyte "?\xff"))
+                      (#xff "#xff" "?ÿ"))))))
+
 (ert-deftest ert-test-explain-equal-improper-list ()
   (should (equal (ert--explain-equal '(a . b) '(a . c))
                  '(cdr (different-atoms b c)))))