Single string literal in body is return value only, not doc string

A function or macro body consisting of a single string literal now only
uses it as a return value.  Previously, it had the dual uses as return
value and doc string, which was never what the programmer wanted and
had some inconvenient consequences (bug#69387).

This change applies to `lambda`, `defun`, `defsubst` and `defmacro`
forms; most other defining forms already worked in the sensible way.

* lisp/emacs-lisp/bytecomp.el (byte-compile-lambda):
Don't use a lone string literal as doc string.
* test/lisp/emacs-lisp/bytecomp-resources/warn-wide-docstring-defun.el
(foo): Update docstring warning test.
* doc/lispref/functions.texi (Function Documentation): Update.
* etc/NEWS: Announce.

1 files changed, 4 insertions, 5 deletions

diff --git a/lisp/emacs-lisp/bytecomp.el b/lisp/emacs-lisp/bytecomp.el
index c3355eedd75..cf0e6d600dd 100644
--- a/lisp/emacs-lisp/bytecomp.el
+++ b/lisp/emacs-lisp/bytecomp.el
@@ -3061,12 +3061,11 @@ lambda-expression."
 	  (append (if (not lexical-binding) arglistvars)
                   byte-compile-bound-variables))
 	 (body (cdr (cdr fun)))
-	 (doc (if (stringp (car body))
+         ;; Treat a final string literal as a value, not a doc string.
+	 (doc (if (and (cdr body) (stringp (car body)))
                   (prog1 (car body)
-                    ;; Discard the doc string from the body
-                    ;; unless it is the last element of the body.
-                    (if (cdr body)
-                        (setq body (cdr body))))))
+                    ;; Discard the doc string from the body.
+                    (setq body (cdr body)))))
 	 (int (assq 'interactive body))
          command-modes)
     (when lexical-binding