duplicate-line: fix optional argument and add test (bug#46621)

The test assumes that the current semantics are intended and desired,
which may or may not be true, but it's better than not having any at
all.

* lisp/misc.el (duplicate-line): Don't crash if called with no argument.
* test/lisp/misc-tests.el (misc--duplicate-line): New test.

lisp/misc.el

@@ -69,6 +69,8 @@ Also see the `duplicate-line' command."
 Interactively, N is the prefix numeric argument, and defaults to 1.
 Also see the `copy-from-above-command' command."
   (interactive "p")
+  (unless n
+    (setq n 1))
   (let ((line (buffer-substring (line-beginning-position) (line-end-position))))
     (save-excursion
       (forward-line 1)

test/lisp/misc-tests.el

@@ -80,5 +80,21 @@
     (backward-to-word 3)
     (should (equal (point) 1))))
 
+(ert-deftest misc--duplicate-line ()
+  ;; Duplicate a line (twice).
+  (with-temp-buffer
+    (insert "abc\ndefg\nh\n")
+    (goto-char 7)
+    (duplicate-line 2)
+    (should (equal (buffer-string) "abc\ndefg\ndefg\ndefg\nh\n"))
+    (should (equal (point) 7)))
+  ;; Duplicate a non-terminated line.
+  (with-temp-buffer
+    (insert "abc")
+    (goto-char 2)
+    (duplicate-line)
+    (should (equal (buffer-string) "abc\nabc\n"))
+    (should (equal (point) 2))))
+
 (provide 'misc-tests)
 ;;; misc-tests.el ends here