Really convert to multibyte in Fdirectory_append

* src/fileio.c (Fdirectory_append): Fix check for whether we need
to convert to multibyte.
(Fdirectory_append):

2 files changed, 6 insertions, 2 deletions

diff --git a/src/fileio.c b/src/fileio.c
index 60f5650302c..6d505fd0f01 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -799,8 +799,7 @@ usage: (record DIRECTORY &rest COMPONENTS) */)
 	  if (STRING_MULTIBYTE (arg) || string_ascii_p (arg))
 	    elements[i] = arg;
 	  else
-	    elements[i] = make_multibyte_string (SSDATA (arg), SCHARS (arg),
-						 SCHARS (arg));
+	    elements[i] = Fstring_to_multibyte (arg);
 	  arg = elements[i];
 	  /* We have to recompute the number of bytes. */
 	  if (i == nargs - 1
diff --git a/test/src/fileio-tests.el b/test/src/fileio-tests.el
index 702659fa395..73a7775279a 100644
--- a/test/src/fileio-tests.el
+++ b/test/src/fileio-tests.el
@@ -170,6 +170,11 @@ Also check that an encoding error can appear in a symlink."
   (should (equal (directory-append "fóo" "bar") "fóo/bar"))
   (should (equal (directory-append "foo" "bár") "foo/bár"))
   (should (equal (directory-append "fóo" "bár") "fóo/bár"))
+  (let ((string (make-string 5 ?a)))
+    (should (not (multibyte-string-p string)))
+    (aset string 2 255)
+    (should (not (multibyte-string-p string)))
+    (should (equal (directory-append "fóo" string) "fóo/aa\377aa")))
   (should-error (directory-append "foo" ""))
   (should-error (directory-append "" "bar"))
   (should-error (directory-append "" "")))