Checkdoc: use syntax functions instead of regex

In checkdoc.el, get rid of the error-prone regex to find definition
forms, and use existing syntax-based navigation functions instead.
This fixes a corner case with one-argument `defvar' forms.

* lisp/emacs-lisp/checkdoc.el (checkdoc--next-docstring): New function.
(checkdoc-next-docstring, checkdoc-defun): Use it.
* test/lisp/emacs-lisp/checkdoc-tests.el (checkdoc-tests--next-docstring):
Add unit test.

1 files changed, 13 insertions, 0 deletions

diff --git a/test/lisp/emacs-lisp/checkdoc-tests.el b/test/lisp/emacs-lisp/checkdoc-tests.el
index 18b5a499e08..02db88c17e2 100644
--- a/test/lisp/emacs-lisp/checkdoc-tests.el
+++ b/test/lisp/emacs-lisp/checkdoc-tests.el
@@ -37,4 +37,17 @@
     (insert "(defun foo())")
     (should-error (checkdoc-defun) :type 'user-error)))
 
+(ert-deftest checkdoc-tests--next-docstring ()
+  "Checks that the one-argument form of `defvar' works.
+See the comments in Bug#24998."
+  (with-temp-buffer
+    (emacs-lisp-mode)
+    (insert "(defvar foo)
+\(defvar foo bar \"baz\")
+\(require 'foo)")
+    (goto-char (point-min))
+    (should (checkdoc-next-docstring))
+    (should (looking-at-p "\"baz\")"))
+    (should-not (checkdoc-next-docstring))))
+
 ;;; checkdoc-tests.el ends here