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).

1 files changed, 5 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