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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"gh stack rebase --upstack doesn't detect squash-merged branches below the current branch","articleBody":"## Summary\n\nWhen `gh stack rebase --upstack` is run from a branch that has a squash-merged PR somewhere below it in the stack, the merged branch is never detected and skipped. Pre-squash commits from the merged branch stay in the history of every branch above, which (after push) shows up as bloated PR diffs containing unrelated code from the already-merged PR.\n\n## Reproducer\n\n1. Create a stack `main ← A ← B ← C` with PRs for each.\n2. Squash-merge A's PR into main.\n3. From B or C, run `gh stack rebase --upstack`.\n4. Observe: B and C are rebased but still contain A's pre-squash commits.\n5. The PRs for B and C now show a diff that includes A's changes in addition to their own.\n\nPlain `gh stack rebase` (no flag) handles the same scenario correctly — it detects A as merged, sets `needsOnto`, and rebases B and C with `--onto` to skip A's pre-squash commits.\n\n## Root cause\n\nIn `cmd/rebase.go`:\n\n```go\nif opts.upstack {\n    startIdx = currentIdx\n}\nbranchesToRebase := s.Branches[startIdx:endIdx]\n```\n\nThe squash-merge detection runs inside the loop that iterates over `branchesToRebase`:\n\n```go\nif br.IsMerged() {\n    ontoOldBase = originalRefs[br.Branch]\n    needsOnto = true\n    cfg.Successf(\"Skipping %s (PR %s merged)\", br.Branch, ...)\n    continue\n}\n```\n\n`--upstack` sets `startIdx = currentIdx`, so any merged branch below `currentIdx` is excluded from the slice. `IsMerged()` never fires on it, `needsOnto` stays `false`, and the subsequent rebases use the old (pre-squash) parent instead of `--onto` to the squashed base.\n\n## Suggested fixes\n\nTwo options:\n\n1. **Seed merge state before the upstack loop.** Scan `s.Branches[:startIdx]` for merged branches before starting the loop and, if any is found, pre-populate `ontoOldBase` / `needsOnto` with the appropriate old base so that the first branch in the upstack slice rebases with `--onto`.\n\n2. **Refuse `--upstack` when a merged branch exists below.** Detect the condition up front and error out with a message directing the user to run plain `gh stack rebase` instead. Simpler and surfaces the problem loudly rather than silently producing a bad rebase.\n\nOption 2 is probably cheapest and prevents silent data corruption. Option 1 is more user-friendly but needs more care around multiple merged branches in sequence.\n\n## Impact\n\nRepeatedly observed in the wild — the bad rebase is silent, the push succeeds, and the only signal is that PRs suddenly show hundreds of lines of unrelated diff. Easy to miss if you don't self-check every push.","author":{"url":"https://github.com/benschoel","@type":"Person","name":"benschoel"},"datePublished":"2026-04-15T20:22:41.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/31/gh-stack/issues/31"}