eval-defun ignores lexical-binding

Question

Today I discovered that if you re-evaluate a defun using eval-defun (C-M-x) in a buffer with lexical-binding set to t, the defun is recreated as an ordinary lambda, not a closure.

For example, after you evaluate this file:

;;; -*- lexical-binding: t; -*-

(defun foo (x) (lambda (y) (+ x y)))

Then (symbol-function 'foo) returns

(closure (t) (x) (function (lambda (y) (+ x y))))

But if you put point in the body of the function, press C-M-x, and evaluate (symbol-function 'foo) again, you get:

(lambda (x) (function (lambda (y) (+ x y))))

Certainly not the same thing, and in fact it throws a void-variable error when you call the returned function, if x is not globally bound.

Is this the intended behavior? Do I just need to remember not to use eval-defun but rather eval-buffer whenever I modify a function in a file where lexical-binding is t?

I'm using Emacs 25.0.50.2.

Answer 1

The -*- lexical-binding: t; -*- only takes effect when you open the file. So after adding it, you need to re-open the file otherwise your buffer is still in dynamic-binding mode.