refactor: type the EOF-action seam with a sealed union#22
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Full-lane bundle 2026-06-26.03-type-eof-action-seam: replace the stringly-typed _detect_trailing/fix_file seam with a sealed union (Noop | AppendLf | Truncate) consumed by an exhaustive match; exhaustiveness enforced by ty via fix_file's bool return (no assert_never, 3.10-safe). Behavior-preserving; two-task plan with complete code. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Add a vendored `_assert_never(value: NoReturn) -> NoReturn` guard and a `case _: _assert_never(action)` wildcard arm in `fix_file`, so ty errors at the match site (not the return type) when a future `_EofAction` variant goes unhandled. Python 3.10 precludes `typing.assert_never` (3.11+); decision doc records the rationale and 2026-10-31 revisit trigger. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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What & why
A behavior-preserving deepening (architecture-review Candidate 2): replace the stringly-typed EOF-action seam in
eof_fixer/fixer.pywith a sealed union.Before:
_detect_trailing -> tuple[str, int]with magic strings"none"/"append_lf"/"truncate"and an offset meaningful only fortruncate;fix_filebranched on the strings with a comment-onlyelse: # truncate.After:
_detect_trailing -> _EofAction = Noop | AppendLf | Truncate(offset)(frozen dataclasses; offset only onTruncate— illegal states unrepresentable), consumed by an exhaustivematch. Magic strings and the dummy offset are gone. No behavior change.Exhaustiveness
The
matchis closed withcase _: _assert_never(action), pinning completeness at the match site: an unhandled future variant is a type error there (verified — ty reportsExpected Never, found <variant> & ~Noop & ~AppendLf & ~Truncate), not merely via the return type._assert_neveris vendored locally (def _assert_never(value: NoReturn) -> NoReturn) becausetyping.assert_neveris 3.11+ and the package supports 3.10. Rationale + revisit trigger recorded inplanning/decisions/2026-06-26-keep-py310-vendored-assert-never.md(revisit when 3.10 EOLs, 2026-10-31).Tests
Behavior-preserving, so existing tests stay green; added one coverage test (
test_check_mode_truncate_does_not_write) for theTruncate-under---checkbranch that the per-caseif not checksplit created. 36 passed, 100% coverage;just lint-ciclean (ruff select=["ALL"], ty).Planning artifacts (Full lane)
planning/changes/2026-06-26.03-type-eof-action-seam/planning/decisions/2026-06-26-keep-py310-vendored-assert-never.mdarchitecture/eof-normalization.mdBuilt via subagent-driven TDD (Task 1 review SHIP; final whole-branch review Ready to merge: Yes, which surfaced the structural-exhaustiveness subtlety now hardened here).
🤖 Generated with Claude Code