;;; -*- emacs-lisp -*-

(defun my-font-lock-composite-character-matcher (limit)
  (when auto-composition-mode
    (let ((composition (find-composition (point) limit)))
      (when composition
        (let ((begin (copy-marker (car composition)))
              (end (copy-marker (cadr composition))))
          (goto-char end)
          (set-match-data (list begin end))
          t)))))

(dolist (hook '(text-mode-hook
                prog-mode-hook))
  (add-hook hook
            (lambda ()
              (font-lock-add-keywords nil
                '((my-font-lock-composite-character-matcher
                   (0 font-lock-warning-face t)))))))

;; ズ   U+30B9 U+3099
;; ズ  U+30BA
;; Ō  U+004F U+0304
;; Ō   U+014C