Hello, 

The current implementation of the tramp-compat-file-name-concat 
from lisp/net/tramp-compat.el fails test fileio-tests/file-name-concat
from file test/src/fileio-tests.el for file-name-concat on Emacs where no
file-name-concat.

#+begin_src elisp
(ert-deftest test-tramp-compat-file-name-concat ()
  (should (equal (tramp-compat-file-name-concat "foo" "bar") "foo/bar"))
  (should (equal (tramp-compat-file-name-concat "foo" "bar") "foo/bar"))
  (should (equal (tramp-compat-file-name-concat "foo" "bar" "zot") "foo/bar/zot"))
  (should (equal (tramp-compat-file-name-concat "foo/" "bar") "foo/bar"))
  (should (equal (tramp-compat-file-name-concat "foo//" "bar") "foo//bar"))
  (should (equal (tramp-compat-file-name-concat "foo/" "bar/" "zot") "foo/bar/zot"))
  (should (equal (tramp-compat-file-name-concat "fóo" "bar") "fóo/bar"))
  (should (equal (tramp-compat-file-name-concat "foo" "bár") "foo/bár"))
  (should (equal (tramp-compat-file-name-concat "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 (tramp-compat-file-name-concat "fóo" string) "fóo/aa\377aa")))
  (should (equal (tramp-compat-file-name-concat "foo") "foo"))
  (should (equal (tramp-compat-file-name-concat "foo/") "foo/"))
  (should (equal (tramp-compat-file-name-concat "foo" "") "foo"))
  (should (equal (tramp-compat-file-name-concat "foo" "" "" "" nil) "foo"))
  (should (equal (tramp-compat-file-name-concat "" "bar") "bar"))
  (should (equal (tramp-compat-file-name-concat "" "") "")))

(ert 'test-tramp-compat-file-name-concat)
#+end_src