* lisp/files-x.el (add-file-local-variable):

Special-case 'lexical-binding'.  (Bug#20641)

1 files changed, 7 insertions, 0 deletions

diff --git a/lisp/files-x.el b/lisp/files-x.el
index 248c3073307..a130ffcf928 100644
--- a/lisp/files-x.el
+++ b/lisp/files-x.el
@@ -247,7 +247,14 @@ then this function adds the first line containing the string
 `Local Variables:' and the last line containing the string `End:'."
   (interactive
    (let ((variable (read-file-local-variable "Add file-local variable")))
+     ;; Error before reading value.
+     (if (equal variable 'lexical-binding)
+         (user-error "The `%s' variable must be set at the start of the file"
+                     variable))
      (list variable (read-file-local-variable-value variable) t)))
+  (if (equal variable 'lexical-binding)
+      (user-error "The `%s' variable must be set at the start of the file"
+                  variable))
   (modify-file-local-variable variable value 'add-or-replace interactive))
 
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