Test whether variable is defined, but not initialized

Question

Q: How can I test (programmatically) whether a variable has been defined, but not initialized?

(defvar foo)

EXAMPLE:

(if (my-test foo)
  (message "%s has been defined, but not initialized." foo)
  (message "%s has been defined AND initialized." foo))

Answer 1

The following works only with lexical binding, because with dynamic binding (defvar foo) has no real effect. It checks whether let-binding the variable affects the dynamic value or not. I used the term "declared" instead of "defined", because (defvar foo) doesn't quite feel like a full definition to me (e.g., it only applies to the file it's in) so it's more like C's "declare".

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

(defvar foo 2)
(defvar bar)

(defmacro check-var (var)
  `(message "%s %s"
            ',var
            (cond ((boundp ',var) "has been initialized")
                  ((let ((,var t)) (boundp ',var))
                   "has been declared, but not initialized")
                  (t "has NOT been declared nor initialized"))))

(check-var foo)
(check-var bar)
(check-var qux)