A stacked pull request cannot use auto-merge

从 gh stack link 命令开始，使用 issue 中的双层场景重现被拒绝的情况。结合相关 issue #174、#172、#502 和 #503，阅读 auto-merge 和 stack linking 的处理方式。完成的标准是：堆叠的 pull request 可以保留 auto-merge，或者命令和 stack 面板明确说明不兼容性以及需要手动入队的要求。

Summary

gh stack link refuses a pull request that has auto-merge enabled, so adopting a stack costs a capability that an unstacked pull request keeps. Every layer then needs a person at the keyboard at the moment the layer below lands.

Evidence

Both pull requests approved and green. #101 had auto-merge enabled, so linking failed:

Checking existing stacks...
Looking up PRs for 2 branches...
✗ PR #101 cannot be added to a stack: it has auto-merge enabled

Turning auto-merge off let the link succeed.

This compounds the behaviour in #174. When the bottom layer merges, the layer above leaves the queue and GitHub rewrites its head, which restarts its required checks. Nothing puts it back. With auto-merge unavailable, only a person can, and only once the restarted checks finish. A stack of three with a 15-minute required check needs three waits and three manual enqueues to land one change.

Expected

A stacked pull request can use auto-merge, so each layer enqueues itself once its base merges and its checks pass.

If the two genuinely cannot coexist, say so where the user meets it: gh stack link should explain why auto-merge and stacks are exclusive, and the stack panel should say that each layer needs a manual enqueue.

Actual

gh stack link rejects the pull request with no explanation of the conflict, and offers no alternative.

Environment

• gh 2.100.0, gh stack v0.0.8
• Trunk protected by a merge queue, merge_method: REBASE
• Two-layer stack, both pull requests approved with required checks passing

Reproduction

1. Open two pull requests that stack on each other, both approved and green.
2. Enable auto-merge on the bottom one.
3. Run gh stack link <bottom> <top>.

Note on scope

This reports only what was observed: gh stack link rejects a pull request that already has auto-merge enabled. The reverse direction, enabling auto-merge on a pull request that is already stacked, was not tested, because the repository in question has allow_auto_merge: false set at the repository level for unrelated reasons.

Related

• #174 — the layer above leaves the queue and is not put back, which is what makes the missing auto-merge costly
• #172 — "'Merge stack' appears to enqueue only the bottom PR"
• #502 — the merge queue rewrites a commit that needed no rewrite, which is what restarts the checks
• #503 — the panel gives no sign that a manual enqueue is needed