Show a warning when undoing an undo operation

CHANGELOG.md

@@ -77,6 +77,9 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ### New features
 
+* `jj undo` now shows a hint when undoing an undo operation that the user may
+   be looking for `jj restore` instead.
+
 * `jj git {push,clone,fetch}` can now spawn an external `git` subprocess, via
    the `git.subprocess = true` config knob. This provides an alternative that,
    when turned on, fixes SSH bugs when interacting with Git remotes due to

cli/src/commands/operation/undo.rs

@@ -75,6 +75,20 @@ pub fn cmd_op_undo(
         let template = tx.base_workspace_helper().operation_summary_template();
         template.format(&bad_op, formatter.as_mut())?;
         writeln!(formatter)?;
+
+        let was_undo_op = *tx.repo().view() == parent_op.view()?;
+        if args.operation == "@" && was_undo_op {
+            writeln!(
+                ui.hint_default(),
+                "This action reverted an 'undo' operation. The repository is now in the same \
+                 state as it was before the original 'undo'."
+            )?;
+            writeln!(
+                ui.hint_default(),
+                "If your goal is to undo multiple operations, consider using `jj op log` to see \
+                 past states, and `jj op restore` to restore one of these states."
+            )?;
+        }
     }
     tx.finish(ui, format!("undo operation {}", bad_op.id().hex()))?;
 

cli/tests/test_undo.rs

@@ -385,6 +385,44 @@ fn test_bookmark_track_untrack_undo() {
     "###);
 }
 
+#[test]
+fn test_shows_a_warning_when_undoing_an_undo_operation_as_bare_jj_undo() {
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_ok(test_env.env_root(), &["git", "init", "repo"]);
+    let repo_path = test_env.env_root().join("repo");
+
+    test_env.jj_cmd_ok(&repo_path, &["new"]);
+    test_env.jj_cmd_ok(&repo_path, &["undo"]);
+    let (stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["undo"]);
+    insta::assert_snapshot!(stdout, @"");
+    insta::assert_snapshot!(stderr, @r"
+    Undid operation: 2d5b73a97567 (2001-02-03 08:05:09) undo operation 289cb69a8458456474a77cc432e8009b99f039cdcaf19ba4526753e97d70fee3fd0f410ff2b7c1d10cf0c2501702e7a85d58f9d813cdca567c377431ec4d2b97
+    Hint: This action reverted an 'undo' operation. The repository is now in the same state as it was before the original 'undo'.
+    Hint: If your goal is to undo multiple operations, consider using `jj op log` to see past states, and `jj op restore` to restore one of these states.
+    Working copy now at: rlvkpnrz 65b6b74e (empty) (no description set)
+    Parent commit      : qpvuntsm 230dd059 (empty) (no description set)
+    ");
+}
+
+#[test]
+fn test_shows_no_warning_when_undoing_a_specific_undo_change() {
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_ok(test_env.env_root(), &["git", "init", "repo"]);
+    let repo_path = test_env.env_root().join("repo");
+
+    test_env.jj_cmd_ok(&repo_path, &["new"]);
+    test_env.jj_cmd_ok(&repo_path, &["undo"]);
+    let stdout = test_env.jj_cmd_success(&repo_path, &["op", "log"]);
+    let op_id_hex = stdout[3..15].to_string();
+    let (stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["undo", &op_id_hex]);
+    insta::assert_snapshot!(stdout, @"");
+    insta::assert_snapshot!(stderr, @r"
+    Undid operation: 2d5b73a97567 (2001-02-03 08:05:09) undo operation 289cb69a8458456474a77cc432e8009b99f039cdcaf19ba4526753e97d70fee3fd0f410ff2b7c1d10cf0c2501702e7a85d58f9d813cdca567c377431ec4d2b97
+    Working copy now at: rlvkpnrz 65b6b74e (empty) (no description set)
+    Parent commit      : qpvuntsm 230dd059 (empty) (no description set)
+    ");
+}
+
 fn get_bookmark_output(test_env: &TestEnvironment, repo_path: &Path) -> String {
     // --quiet to suppress deleted bookmarks hint
     test_env.jj_cmd_success(repo_path, &["bookmark", "list", "--all-remotes", "--quiet"])