* src/eval.c (Fprogn): Do not check that BODY is a proper list.

This undoes the previous change.  The check slows down the
interpreter, and is not needed to prevent a crash.  See
<http://lists.gnu.org/archive/html/emacs-devel/2013-07/msg00693.html>.
* doc/lispref/eval.texi (Special Forms): Mention 'lambda'.  Also, say that
non-well-formed expressions result in unspecified behavior, though
Emacs will not crash.

doc/lispref/ChangeLog

@@ -1,3 +1,9 @@
+2013-07-24  Paul Eggert  <eggert@cs.ucla.edu>
+
+	* eval.texi (Special Forms): Mention 'lambda'.  Also, say that
+	non-well-formed expressions result in unspecified behavior, though
+	Emacs will not crash.
+
 2013-07-22  Michael Albinus  <michael.albinus@gmx.de>
 
 	* files.texi (Magic File Names): Add file-notify-add-watch,

doc/lispref/eval.texi

@@ -432,6 +432,14 @@ do.
 and which are used without evaluation.  Whether a particular argument is
 evaluated may depend on the results of evaluating other arguments.
 
+  If an expression's first symbol is that of a special form, the
+expression should follow the rules of that special form; otherwise,
+Emacs's behavior is not well-defined (though it will not crash).  For
+example, @code{((lambda (x) x . 3) 4)} contains a subexpression that
+begins with @code{lambda} but is not a well-formed @code{lambda}
+expression, so Emacs may signal an error, or may return 3 or 4 or
+@code{nil}, or may behave in other ways.
+
   Here is a list, in alphabetical order, of all of the special forms in
 Emacs Lisp with a reference to where each is described.
 
@@ -463,6 +471,9 @@ Emacs Lisp with a reference to where each is described.
 @item interactive
 @pxref{Interactive Call}
 
+@item lambda
+@pxref{Lambda Expressions}
+
 @item let
 @itemx let*
 @pxref{Local Variables}

etc/NEWS

@@ -538,14 +538,6 @@ file using `set-file-extended-attributes'.
 ** `visited-file-modtime' now returns -1 for nonexistent files.
 Formerly it returned a list (-1 LOW USEC PSEC), but this was ambiguous
 in the presence of files with negative time stamps.
-
-** Special forms with implied progn now check for proper lists.
-Starting in Emacs 21.4, a special form with an implied progn of an
-improper list ignored the trailing value, treating it as nil.  For
-example, (cond (t (message "hello") . "there")) ignored the "there".
-This inadvertent change to Emacs's behavior has been reverted, and
-Emacs now signals an error for these improper forms, as it did in
-version 21.3 and earlier.
 
 * Lisp Changes in Emacs 24.4
 

src/ChangeLog

@@ -1,3 +1,10 @@
+2013-07-24  Paul Eggert  <eggert@cs.ucla.edu>
+
+	* eval.c (Fprogn): Do not check that BODY is a proper list.
+	This undoes the previous change.  The check slows down the
+	interpreter, and is not needed to prevent a crash.  See
+	<http://lists.gnu.org/archive/html/emacs-devel/2013-07/msg00693.html>.
+
 2013-07-23  Glenn Morris  <rgm@gnu.org>
 
 	* Makefile.in ($(etc)/DOC, temacs$(EXEEXT)): Ensure etc/ exists.

src/eval.c

@@ -454,12 +454,6 @@ usage: (progn BODY...)  */)
       body = XCDR (body);
     }
 
-  if (!NILP (body))
-    {
-      /* This can happen if functions like Fcond are the caller.  */
-      wrong_type_argument (Qlistp, body);
-    }
-
   UNGCPRO;
   return val;
 }