fix(core): preserve exhaustiveness for widened value patterns

[baptou12]

What & why

A value pattern whose type has widened can make an incomplete error match compile as exhaustive. When the unhandled error actually arrives, the matcher throws NonExhaustiveError, which the combinator converts to a Defect. An application mapping domain failures to 4xx and defects to 500 can therefore report an internal failure for an anticipated domain error.

Before

type DomainError = { _tag: "NotFound" } | { _tag: "Forbidden" };
const pattern = { _tag: "NotFound" }; // inferred as { _tag: string }

const result = Err<DomainError>({ _tag: "Forbidden" })
  .mapErrCases(m => m.with(pattern, () => "missing"));
// Previously compiled, but produced Defect(NonExhaustiveError) at runtime.

MatchedOf<typeof pattern> contains _tag: string. Subtracting that shape with Exclude removed both domain variants, although the runtime pattern only compares against "NotFound".

After

The incomplete example is a compile error. MatchedOf continues to narrow the handler input; a separate CoveredBy calculation determines which cases a pattern can safely remove from the remaining cases. Widened primitives and union-typed values prove no coverage, recursively through object fields. Grouped patterns are evaluated separately, so grouping literal patterns still covers each named alternative.

const result = Err<DomainError>({ _tag: "Forbidden" })
  .mapErrCases(m => m
    .with(P.tag("NotFound"), () => 404)
    .with(P.tag("Forbidden"), () => 403));
// Err(403)

Dynamic patterns remain usable when followed by sufficient covering arms. Inline literals, as const, P.tag, and branded predicate patterns retain their coverage. Runtime matching is unchanged. Previously accepted incomplete matches now require additional arms.

Validation

• Added negative type regressions for sync/async combinators, standalone termination, widened primitives, union values, nested fields, and grouped dynamic patterns; positive checks preserve handler narrowing, literal groups, and predicate coverage.
• Added a runtime regression covering both domain variants on sync and async surfaces.
• 378 core tests / 12 files passed, including documentation examples and bundled ESM/CJS declaration emit, on a temporary copy with a standalone configuration. Coverage: 100% lines/functions, 99.8% statements, 99.16% branches.
• The isolated new type regressions pass with the locally installed TypeScript 6.0.3. The complete core type suite has the same four diagnostics before and after: two existing ensure negative tests place their directives for TypeScript 7's diagnostic locations.
• Formatting checked for the changed TypeScript files, guide, and changeset; git diff --check passes.
• Local commit hooks could not load their shared configuration (format/lint jobs had no command); the commit was made with LEFTHOOK=0 after the explicit formatting checks.
• Full repository gate has not passed locally. The installation lacks shared @btravstack/tsconfig and @btravstack/oxlint presets and has TypeScript 6 rather than the pinned 7.0.2. CI with the lockfile toolchain must validate the complete change. The full local gate remains unverified; CI results are recorded below.

CI follow-up

• Registered CoveredByPatterns in TypeDoc's intentionallyNotExported list. CI on 4de0467 passed build, bundle size, both test jobs, type checking, lint, format, Knip, and CodeQL. The only remaining failure was the dependency audit.
• Commit 20d531a raises the existing fast-uri override from 3.1.6 to 3.1.7 and regenerates the lockfile with pnpm 12.4.1. This fixes GHSA-qw65-cvwx-89v3 and GHSA-58mr-gqgx-xq4g. No other dependency resolutions changed.
• pnpm audit --audit-level=high now reports No known vulnerabilities found; workspace YAML formatting and git diff --check pass.

Checklist

• The full gate passes locally: pnpm format --check && pnpm lint && pnpm typecheck && pnpm knip && pnpm test && pnpm build
• Added/updated tests (and invariant guards / type-level tests if behaviour changed)
• Added a changeset (pnpm changeset) for any user-facing change
• Updated TSDoc and CLAUDE.md if the public surface or a design rule changed
• Commits follow Conventional Commits

Summary by CodeRabbit

• Bug Fixes

  • Exhaustiveness checks now count only cases a pattern can guarantee it covers. Widened values, unions, arrays, and functions no longer incorrectly prove full coverage; literal patterns and predicates with declared coverage continue to count.
  • Type errors now identify cases that remain unhandled.

• Documentation

  • Clarified how pattern types affect handler inputs and exhaustiveness, with guidance for preserving literal types and handling class instances.

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📚 Code guidelines (1)

CLAUDE.md — auto-discovered

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 6df8963 and 7b00f12.

📒 Files selected for processing (6)

• .changeset/tidy-pattern-coverage.md
• CLAUDE.md
• docs/explanation/exhaustive-error-matching.md
• packages/core/src/matcher.ts
• packages/core/src/types.test-d.ts
• skills/unthrown/SKILL.md

🚧 Files skipped from review as they are similar to previous changes (3)

• .changeset/tidy-pattern-coverage.md
• docs/explanation/exhaustive-error-matching.md
• CLAUDE.md

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---

📝 Walkthrough

Walkthrough

The matcher now separates handler narrowing from guaranteed exhaustiveness coverage. Value patterns count toward coverage only when their types meet the defined literal or object-shape rules. The change adds type-level tests, documentation, a changeset, and a gRPC dependency override.

Changes

Pattern coverage

Layer / File(s)	Summary
Calculate guaranteed coverage packages/core/src/matcher.ts	The matcher calculates guaranteed coverage separately from handler narrowing. It uses guaranteed coverage to update Remaining.
Validate and document coverage packages/core/src/types.test-d.ts, CLAUDE.md, docs/explanation/exhaustive-error-matching.md, skills/unthrown/SKILL.md, .changeset/tidy-pattern-coverage.md, docs/typedoc.core.json	Type-level tests cover widened and literal patterns, including the documented class-instance limitation. Documentation and the changeset describe the coverage rules and literal-preserving alternatives. TypeDoc lists CoveredByPatterns as intentionally not exported.

gRPC dependency override

Layer / File(s)	Summary
Set gRPC version floor pnpm-workspace.yaml	The workspace resolves @grpc/grpc-js versions below 1.14.5 to 1.14.5.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix

Suggested reviewers: btravers

Merge Risk: 🔵 Low · up to 7b00f

The matcher change makes exhaustiveness checks stricter. Code that previously compiled but could fail at runtime now needs extra arms, which is the intended behavior. A minor open dependency-advisory concern about the fast-uri override remains and should be confirmed before merge.

Security Architecture Review

Security architecture risk: 🔵 Low · up to 7b00f

The change strengthens compile-time error coverage without changing matcher execution or privileges. Previously accepted incomplete matches may stop compiling. No introduced security attack path was identified, but downstream compatibility and the dependency upgrade were not validated by execution.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The matcher change affects consumers’ compile-time error-handling contracts, not a newly exposed network entrypoint or privileged operation. The separate dependency update affects workspace resolution, including the documented Testcontainers Docker transport path.

Security Findings and Attack Paths

• inferred — The inspected base-to-head matcher change does not expand input reachability, execution authority, or bypass the existing matcher-failure containment paths. This bounded conclusion does not establish complete security coverage for downstream applications.

Trust Boundaries and Controls

• observed — Compile-time exhaustiveness is not runtime validation of arbitrary input. Branded predicates retain coverage based on their declared matched type, while runtime execution still invokes the predicate and can throw when no arm matches.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 3 files. (4 skipped: 4…	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly identifies the main change: correcting exhaustiveness checking for widened value patterns.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 3 files. (4 skipped: 4 unsupported.)

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Copilot review overview

🟡 Changes recommended

The new CoveredByPatterns type is referenced by the public Matcher.with signature but is not added to docs/typedoc.core.json's intentionallyNotExported, which will fail the TypeDoc docs build gate.

Review effort: Balanced
Findings: 1

Open (1)

• Unexported CoveredByPatterns breaks TypeDoc build · New

What changed in this PR

This PR fixes an unsoundness in the built-in error matcher (packages/core/src/matcher.ts). A value pattern whose type has widened (e.g. const pattern = { _tag: "A" }, inferred as { _tag: string }) could make an incomplete error match compile as exhaustive, because the exhaustiveness subtraction used MatchedOf, which returns { _tag: string } and therefore Excluded every tagged variant. At runtime the pattern only matches one tag, so an unhandled error later throws NonExhaustiveError, which the combinators convert to a Defect — turning an anticipated domain error into an unmodeled failure. The fix splits the two concerns: MatchedOf still computes handler-input narrowing, while a new CoveredBy/CoveredByPatterns computes only the cases a pattern is guaranteed to cover (rejecting widened primitives, union-typed values, and recursing through object fields), and with now subtracts CoveredByPatterns from Remaining.

Changes:

• Add IsUnion, CoveredBy, and CoveredByPatterns helper types and change Matcher.with's return to Exclude<Remaining, CoveredByPatterns<Pts>> (runtime matching unchanged).
• Add type-level regressions (widened primitives, union values, nested fields, grouped dynamic patterns, sync/async surfaces, standalone termination) and a runtime regression covering both domain variants.
• Update the exhaustive-error-matching guide, CLAUDE.md, and add a patch changeset.

File	Description
packages/​core/​src/​matcher.ts	Introduces CoveredBy/CoveredByPatterns/IsUnion and uses them for exhaustiveness subtraction while keeping MatchedOf for narrowing.
packages/​core/​src/​types.test-d.ts	Adds negative/positive type regressions for widened value patterns across sync/async and standalone builders.
packages/​core/​src/​matcher.spec.ts	Adds a runtime regression asserting uncovered tags stay reachable after a widened value pattern.
docs/​explanation/​exhaustive-error-matching.md	Documents that value patterns must preserve their literals; as const/P.tag/predicates cover.
CLAUDE.md	Updates the spec to describe the MatchedOf vs CoveredBy split and widened-pattern rule.
.changeset/​tidy-pattern-coverage.md	Adds a patch changeset describing the exhaustiveness fix.

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Actionable comments posted: 1

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Inline comments:
Review comments at @pnpm-workspace.yaml:
- Line 123: Update the fast-uri override in the pnpm overrides configuration
from 3.1.7 to 3.1.8, and widen its version selector to cover versions below
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ℹ️ Review info

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Run ID: ec05e6e5-8f2e-4df6-aea1-7b68626f7dac

📥 Commits

Reviewing files that changed from the base of the PR and between 714d47b and 20d531a.

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📒 Files selected for processing (8)

• .changeset/tidy-pattern-coverage.md
• CLAUDE.md
• docs/explanation/exhaustive-error-matching.md
• docs/typedoc.core.json
• packages/core/src/matcher.spec.ts
• packages/core/src/matcher.ts
• packages/core/src/types.test-d.ts
• pnpm-workspace.yaml

Included review availability: This review used your included allowance. Your plan provides up to 1 included review per hour; 0 remain after this review.

[btravers]

Verified every case below with tsc (and a runtime run where relevant) against 6df8963. The fix closes plain widened string/number/bigint/symbol/union values, but several other pattern shapes still prove coverage they don't have at runtime — inline comments have a repro each.

Not in the diff — skills/unthrown/SKILL.md (~L257-263) needs updating. CLAUDE.md asks for the skill to change in the same PR as the docs. It still says only that a widened E breaks exhaustiveness; it doesn't say a widened pattern no longer discharges a case, so agents following it will write:

const notFound = { _tag: "NotFound" };          // widened to { _tag: string }
r.mapErrCases((m) => m.with(notFound, h1).with(P.tag("Other"), h2));
// ❌ now UnhandledCases<{ _tag: "NotFound" }> — fix: `as const` or P.tag("NotFound")

Excluded (pre-existing on main, not this PR): a branded field (id: string & { __brand }) in an object pattern is rejected by NoEmptyPattern, so it never reaches coverage.

[baptou12]

Addressed the review in 7b00f12: CoveredBy is an allow-list of unit types, IsUnion runs before PatternMatcher, the class-instance case is pinned as a known limitation (structural typing — see the thread), the runtime spec that passed on main is dropped, and the changeset is minor with the typecheck migration spelled out.

The skill now carries the widened-pattern paragraph with your { _tag: "NotFound" } example (skills/unthrown/SKILL.md), and CLAUDE.md / the guide describe the allow-list rule.

Full gate green locally, including the workspace-wide typecheck — no satellite or example relied on a widened pattern.