Teach diff-apply-hunk to handle hunks with empty context

* lisp/vc/diff-mode.el (diff-find-source-location):
Consider the case when there is no diff context above or below
edited lines.  (bug#72556)

* test/lisp/vc/diff-mode-tests.el: Add tests for undoing hunks
from diffs with addtions only in the beginning or end of the
source file.

2 files changed, 101 insertions, 3 deletions

diff --git a/lisp/vc/diff-mode.el b/lisp/vc/diff-mode.el
index a99487b0370..459154f534b 100644
--- a/lisp/vc/diff-mode.el
+++ b/lisp/vc/diff-mode.el
@@ -2048,9 +2048,11 @@ SWITCHED is non-nil if the patch is already applied."
         (goto-char (point-min)) (forward-line (1- (string-to-number line)))
         (let* ((orig-pos (point))
                (switched nil)
-               ;; FIXME: Check for case where both OLD and NEW are found.
-               (pos (or (diff-find-text (car old))
-                        (progn (setq switched t) (diff-find-text (car new)))
+               (maybe-old (diff-find-text (car old)))
+               (maybe-new (diff-find-text (car new)))
+               (pos (or (and maybe-new maybe-old (null reverse) (setq switched t) maybe-new)
+                        maybe-old
+                        (progn (setq switched t) maybe-new)
                         (progn (setq switched nil)
                                (condition-case nil
                                    (diff-find-approx-text (car old))
diff --git a/test/lisp/vc/diff-mode-tests.el b/test/lisp/vc/diff-mode-tests.el
index bbd66824e48..5611e9abc79 100644
--- a/test/lisp/vc/diff-mode-tests.el
+++ b/test/lisp/vc/diff-mode-tests.el
@@ -597,5 +597,101 @@ baz"))))
      (should (eq (get-text-property (match-beginning 0) 'face)
                   'diff-context)))))
 
+(ert-deftest diff-mode-test-topmost-addition-undo ()
+  (let ((patch "diff --git a/fruits b/fruits
+index 0dcecd3..d0eb2e7 100644
+--- a/fruits
++++ b/fruits
+@@ -1,2 +1,3 @@
++fruits
+ apple
+ orange
+")
+        (text-before "apple
+orange
+")
+        (text-after "fruits
+apple
+orange
+"))
+    (ert-with-temp-directory temp-dir
+      (let ((buf-after
+             (find-file-noselect (format "%s/%s" temp-dir "fruits"))))
+        (cd temp-dir)
+
+        (with-current-buffer buf-after (insert text-after) (save-buffer))
+        (with-temp-buffer
+          (insert patch)
+          (goto-char (point-min))
+          (diff-hunk-next)
+          ;; Undo hunk by non-nil REVERSE argument (C-u C-c C-a)
+          (diff-apply-hunk t))
+        (with-current-buffer buf-after
+          (should (string-equal (buffer-string) text-before)))
+
+        (with-current-buffer buf-after
+          (erase-buffer) (insert text-after) (save-buffer))
+        (with-temp-buffer
+          (insert patch)
+          (goto-char (point-min))
+          (diff-hunk-next)
+          ;; Undo hunk by dwim behaviour
+          (cl-letf (((symbol-function 'y-or-n-p) #'always))
+            (diff-apply-hunk)))
+        (with-current-buffer buf-after
+          (should (string-equal (buffer-string) text-before)))
+
+        (with-current-buffer buf-after
+          (set-buffer-modified-p nil)
+          (kill-buffer buf-after))))))
+
+(ert-deftest diff-mode-test-bottommost-addition-undo ()
+  (let ((patch "diff --git a/fruits b/fruits
+index 0dcecd3..6f210ff 100644
+--- a/fruits
++++ b/fruits
+@@ -1,2 +1,3 @@
+ apple
+ orange
++plum
+")
+        (text-before "apple
+orange
+")
+        (text-after "apple
+orange
+plum
+"))
+    (ert-with-temp-directory temp-dir
+      (let ((buf-after
+             (find-file-noselect (format "%s/%s" temp-dir "fruits"))))
+        (cd temp-dir)
+
+        (with-current-buffer buf-after (insert text-after) (save-buffer))
+        (with-temp-buffer
+          (insert patch)
+          (goto-char (point-min))
+          (diff-hunk-next)
+          ;; Undo hunk by non-nil REVERSE argument (C-u C-c C-a)
+          (diff-apply-hunk t))
+        (with-current-buffer buf-after
+          (should (string-equal (buffer-string) text-before)))
+
+        (with-current-buffer buf-after
+          (erase-buffer) (insert text-after) (save-buffer))
+        (with-temp-buffer
+          (insert patch)
+          (goto-char (point-min))
+          (diff-hunk-next)
+          ;; Undo hunk by dwim behaviour
+          (cl-letf (((symbol-function 'y-or-n-p) #'always))
+            (diff-apply-hunk)))
+        (with-current-buffer buf-after
+          (should (string-equal (buffer-string) text-before)))
+
+        (with-current-buffer buf-after
+          (set-buffer-modified-p nil)
+          (kill-buffer buf-after))))))
+
 (provide 'diff-mode-tests)
 ;;; diff-mode-tests.el ends here