gerrit: check default upload revisions are rewritable

The implicit revision selection path for `jj gerrit upload` bypassed the
usual rewritability check. This leads to a panic if the selected revision
was immutable. Run the check before continuing so the command fails with a
user error instead.

Fixes: #9398

CHANGELOG.md

@@ -67,6 +67,9 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 * Pre-existing Git submodule directories are no longer considered conflicts in
   checkouts. [#8065](https://github.com/jj-vcs/jj/issues/8065).
 
+* Fixed a panic in `jj gerrit upload` when run without `-r` and the
+  inferred revision was immutable. [#9398](https://github.com/jj-vcs/jj/issues/9398)
+
 ## [0.40.0] - 2026-04-01
 
 ### Release highlights

cli/src/commands/gerrit/upload.rs

@@ -430,7 +430,7 @@ pub async fn cmd_gerrit_upload(
             }
             // This distinguishes between the "squash workflow" and "edit workflow".
             Some(commit) => {
-                if commit.description().is_empty() {
+                let revisions = if commit.description().is_empty() {
                     let parents = commit.parent_ids();
                     if parents.len() != 1 {
                         return Err(user_error(
@@ -447,7 +447,14 @@ pub async fn cmd_gerrit_upload(
                 } else {
                     writeln!(ui.status(), "No revision provided. Defaulting to @")?;
                     vec![commit.id().clone()]
-                }
+                };
+
+                let revisions_expr = RevsetExpression::commits(revisions.clone());
+                workspace_command
+                    .check_rewritable_expr(&revisions_expr)
+                    .await?;
+
+                revisions
             }
         }
     } else {

cli/tests/test_gerrit_upload.rs

@@ -149,6 +149,43 @@ fn test_gerrit_upload_default_revision() {
     ");
 }
 
+#[test]
+fn test_gerrit_upload_default_revision_already_in_trunk() {
+    let test_env = TestEnvironment::default();
+    test_env
+        .run_jj_in(".", ["git", "init", "--colocate", "remote"])
+        .success();
+    let remote_dir = test_env.work_dir("remote");
+    create_commit(&remote_dir, "main", &[]);
+
+    test_env
+        .run_jj_in(".", ["git", "clone", "remote", "local"])
+        .success();
+    let local_dir = test_env.work_dir("local");
+    test_env.add_config(r#"gerrit.default-remote="origin""#);
+    test_env.add_config(r#"gerrit.default-remote-branch="main""#);
+    test_env.add_config(r#"revset-aliases."trunk()" = "main@origin""#);
+
+    // Make @ an empty working-copy commit over immutable main@origin.
+    local_dir.run_jj(["new", "main@origin"]).success();
+    local_dir.run_jj(["describe", "-m="]).success();
+
+    let output = local_dir.run_jj(["gerrit", "upload", "--dry-run"]);
+    insta::assert_snapshot!(output, @"
+    ------- stderr -------
+    No revision provided and @ has no description. Defaulting to @-
+    Error: Commit 57df4838fd85 is immutable
+    Hint: Could not modify commit: rlvkpnrz 57df4838 main@origin | main
+    Hint: Immutable commits are used to protect shared history.
+    Hint: For more information, see:
+          - https://docs.jj-vcs.dev/latest/config/#set-of-immutable-commits
+          - `jj help -k config`, \"Set of immutable commits\"
+    Hint: This operation would rewrite 1 immutable commits.
+    [EOF]
+    [exit status: 1]
+    ");
+}
+
 #[test]
 fn test_gerrit_upload_option_failure() {
     let test_env = TestEnvironment::default();