* lisp/progmodes/tcl.el (tcl-filter):

Handle comint-prompt-read-only like gud.el does.  (Bug#20549)

lisp/progmodes/tcl.el

@@ -1028,7 +1028,8 @@ Returns nil if line starts inside a string, t if in a comment."
     (with-current-buffer (process-buffer proc)
       ;; Delete prompt if requested.
       (when (marker-buffer inferior-tcl-delete-prompt-marker)
-        (delete-region (process-mark proc) inferior-tcl-delete-prompt-marker)
+	(let ((inhibit-read-only t))
+	  (delete-region (process-mark proc) inferior-tcl-delete-prompt-marker))
         (set-marker inferior-tcl-delete-prompt-marker nil))))
   (comint-output-filter proc string))