This PR adds a CI check that fails when a PR introduces no changes
compared to its base branch. This catches cases where a duplicate PR is
queued for merge after an identical PR has already landed (as happened
with https://github.com/leanprover/lean4/pull/12876 and
https://github.com/leanprover/lean4/pull/12877).
The check is added as a second job in the existing `check-stage0.yml`
workflow, which already has the same trigger conditions and git setup
pattern. On `pull_request` events it diffs against the merge base; on
`merge_group` events it diffs `HEAD^1..HEAD` (the PR's contribution to
the synthetic merge commit). Note that batched merge groups are treated
as a unit — if the entire group is non-empty the check passes, which is
the right behaviour for lean4's typical single-PR queuing.
🤖 Prepared with Claude Code
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
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