Subject: Re: bug#15786: 24.3; No warning on mal-formed let form when lexical-binding

From: Nathan Trapuzzano <[email-masked]>
Date: Sat, 02 Nov 2013 10:14:35 -0400

Leo Liu <[email-masked]> writes:

> Due to a typo in my code, I got a bug report on ggtags
> https://github.com/leoliu/ggtags/issues/17. I was fooled by the elisp
> compiler which warns mal-formed let forms without lexical-binding.
>
> To reproduce
> 1. Download the file in the attachment
> 2. byte-compile it
> 3. remove the first line and byte-compile it
>
> step 2 produces no warnings while step 3 does.

Under lexical binding, the malformed `let' is correctly reformed in
`cconv-convert' before it gets passed to the optimizer.  In other words,
it's assumed to be properly formed.  This looks like a bug.

On first thought, it seems that cconv-convert should check for proper
form and signal an _error_ if the check fails.  On the other hand, this
would be inconsistent with what happens under dynamic binding.  (On that
note, under dynamic binding, why is this considered a warning and not an
error?)

Patch attached.

>From 1cce762e1586ce9faa7fc41712973e22c1ea723c Mon Sep 17 00:00:00 2001
From: Nathan Trapuzzano <[email-masked]>
Date: Sat, 2 Nov 2013 10:12:35 -0400
Subject: [PATCH] Assert proper form of let binding during byte compilation
 under lexical binding.

---
 lisp/emacs-lisp/cconv.el | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/lisp/emacs-lisp/cconv.el b/lisp/emacs-lisp/cconv.el
index f24e503..6d87521 100644
--- a/lisp/emacs-lisp/cconv.el
+++ b/lisp/emacs-lisp/cconv.el
@@ -289,12 +289,14 @@ places where they originally did not directly appear."
 
        (dolist (binder binders)
          (let* ((value nil)
-                (var (if (not (consp binder))
-                         (prog1 binder (setq binder (list binder)))
-                       (setq value (cadr binder))
-                       (car binder)))
-                (new-val
-                 (cond
+		(var (if (not (consp binder))
+			 (prog1 binder (setq binder (list binder)))
+		       (cl-assert (= (length (cdr binder)) 1) nil
+				  "malformed let binding: `%s'" (prin1-to-string binder))
+		       (setq value (cadr binder))
+		       (car binder)))
+		(new-val
+		 (cond
                   ;; Check if var is a candidate for lambda lifting.
                   ((and (member (cons binder form) cconv-lambda-candidates)
                         (progn
-- 
1.8.4.2