🚀 Release 0.9.0

🔀 Merge feature/ts-compat-ci into main
📝 Check off the CI TypeScript-floor task
2026-09-29 21:37:53 +00:00 · 2026-09-29 21:24:16 +00:00 · 2026-09-29 21:24:05 +00:00 · 2026-09-29 21:23:15 +00:00 · 2026-09-29 21:15:01 +00:00 · 2026-09-29 21:10:15 +00:00
34 changed files with 3070 additions and 613 deletions
@@ -77,6 +77,13 @@ jobs:
            - run: npm ci
            - run: npm run build
            - run: npm run check
            # Compile the prose examples into gitignored tests. Kept as an
            # explicit step (not a `pretest:ci` hook) so it is visible in the
            # job log and `test:ci` stays a plain command.
            - run: npm run create:doc-tests
            # Fails the build below 100% coverage on `src/` (`c8 --all --100`);
            # the same run produces the report published below. See
            # development/ci.md § Coverage threshold.
            - run: npm run test:ci
            # Publish this tag's coverage to the self-hosted pages server,
            # served read-only at
@@ -112,6 +119,35 @@ jobs:
                  name: dist
                  path: dist/
    # Consumer typecheck against the minimum supported TypeScript (README
    # § Requirements), run over `dist/`'s emitted declarations and the whole
    # suite. Deliberately a separate job, not a step in `build`: the compiler is
    # a different major picked by `npx`, and it must never enter
    # `devDependencies`, the local `check`/`verify` tiers, or the lockfile. See
    # development/ci.md § TypeScript compatibility.
    compat:
        needs: build
        runs-on: ubuntu-latest
        # Same baked image as `build` — without it this job re-downloads Node
        # per run (see docker/Dockerfile).
        container:
            image: gitea.e1nsnull.de/tmu/act-ci:26.8.2
        steps:
            - uses: actions/checkout@v4
            - uses: actions/setup-node@v4
              with:
                  node-version-file: .node-version
                  cache: "npm"
            - run: npm ci
            # Reuse the exact `dist/` that `check`, `test:ci` and `publint` were
            # run against, so the compat gate judges the shipped artifact and
            # pays no rebuild.
            - uses: actions/download-artifact@v4
              with:
                  name: dist
                  path: dist/
            - run: npm run test:compat
    # Advisory scans (dead code, dependency freshness). Non-blocking: surfaced in
    # the Actions tab for visibility, but must never gate a merge — so
    # continue-on-error and intentionally NOT in `publish`'s `needs`.
@@ -135,7 +171,9 @@ jobs:
    publish:
        if: startsWith(gitea.ref, 'refs/tags/')
-        needs: build
+        # `compat` gates the release: an artifact that is not consumable at the
        # claimed TypeScript floor must never ship.
        needs: [build, compat]
        runs-on: ubuntu-latest
        # Same baked image as `build` — setup-node still owns the registry-url
        # `.npmrc` rewrite here; only the Node download is skipped.
@@ -1,6 +1,10 @@
 node_modules
 dist
 coverage
 # Generated doc-tests (scripts/create-doc-tests.ts); the folder is kept via .gitkeep
 src/doc-test/__generated__/*.test.ts
 !src/doc-test/__generated__/.gitkeep
 *.log
 *.tsbuildinfo
 *.local
@@ -1,3 +1,3 @@
 {
-    "packages": ["npm:@spences10/pi-lsp@0.0.46"]
+    "packages": ["npm:@spences10/pi-lsp@0.0.47"]
 }
@@ -7,10 +7,11 @@ first-action facts. Do not restate evolving prose here — it will drift.
 ## First action
- Project: F#-style pattern matching for TypeScript/ESM. Node `>=26` (pinned via `.node-version`), ESM-only (no CommonJS shim).
+- Project: pattern matching for TypeScript/ESM. Node `>=26` (pinned via `.node-version`), ESM-only (no CommonJS shim).
 - **While iterating:** `npm run test` (`check:tsc` + the unit suite) for fast feedback on the files you changed.
 - **Optional code intelligence:** this repo installs `@spences10/pi-lsp` (pinned in `.pi/settings.json`) as a project-local pi extension. It talks to the repo's own TypeScript 7 via `tsc --lsp --stdio` and exposes **read-only** tools — `lsp_hover`, `lsp_definition`, `lsp_references`, `lsp_find_symbol`, `lsp_document_symbols`, `lsp_diagnostics(_many)`. **When you are looking for a symbol, reach for the LSP before `rg`/`grep`** — `lsp_references` / `lsp_find_symbol` / `lsp_definition` / `lsp_document_symbols` are semantic and cross-file, so they see shadowing, imports and overloads that a text search cannot; use `lsp_hover` to read inferred types on generic-heavy code. Use `rg` for what the LSP cannot see — doc prose, string literals, config, task lists, file discovery — and reconcile the two sets before editing (symbols from the LSP, strings and prose from `rg`). It has no rename / code-action / apply-edit surface — the write side is pi's `edit` tool + `check:tsc`. Treat empty LSP output as _inconclusive_, not success: **`npm run test` / `npm run verify` remain the sole authoritative gate** (see the next bullet). The server keeps running across that gate with a ~5 min idle timeout and registers no file watchers, so if you change `tsconfig.json` / `package.json` mid-session its diagnostics can be stale — when LSP output disagrees with `check:tsc`, trust `check:tsc` and restart pi (or wait out the idle timeout) before concluding the LSP is wrong.
 - **Definition of done — run this before you call the work finished:** `npm run verify`. If all green, commit. If red, look at the output, fix the root cause, and re-run.
 - **Touched a `ts`-tagged fence in `README.md` / `CONTRIBUTING.md`?** Run `npm run create:doc-tests` first: it regenerates the gitignored tests under `src/doc-test/__generated__/`, formats and type-checks them, so `verify` executes the documented example. CI runs it in an explicit step before `test:ci`; the fast local tiers do not. See [development/docs.md](./development/docs.md).
 - **On commit:** write a good message (see [CONTRIBUTING.md § Commit messages](./CONTRIBUTING.md#commit-messages)). Lefthook's pre-commit hook already runs the fast, offline, staged-file checks — don't run them by hand. If the hook fails on style, `npm run fix`, restage, recommit.
 - **Document decisions where the next maintainer will look:** rationale, rejected alternatives and known issues go in `development/<category>.md` (see [development/README.md](./development/README.md)); the actionable rule stays in [CONTRIBUTING.md](./CONTRIBUTING.md) and links to it. Write each fact once — never copy the rule into `development/` or the reason into `CONTRIBUTING.md` — and change both in the same commit when a rule changes.
 - **`npm run maintain` is NOT part of the feature loop.** Its scans are advisory, never a gate; run them only on an explicit maintenance / update-deps branch.
@@ -7,9 +7,53 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 ## [0.9.0] - 2026-09-29
 - add a CI `compat` job that type-checks the suite and a consumer fixture
  against the minimum supported TypeScript (5.9), consuming the built `dist/`
  and gating `publish`
 - correct the documented consumer floor to TypeScript >= 5.9 and drop the
  unsupported `node10` resolution claim
 ## [0.8.3] - 2026-09-28
 - upgrade dependencies: oxfmt 0.71 (the only range widened), oxlint 1.86,
  oxlint-tsgolint 7.0.2003, cspell 10.3.5 and the rest within their existing
  ranges; no rule or format fallout
 - prune the completed items from the backlog; their rationale already lives in
  the README, `development/` and the released changelog entries
 ## [0.8.2] - 2026-09-25
 - write the README's Synopsis and Examples sections
 - document the public API in the README
 - add TSDoc to the four public matcher factories
 - document alternatives
 ## [0.8.1] - 2026-09-23
 - gate CI at 100% coverage: `test:ci` runs c8 with `--all --100` over `src/`,
  and a test loads the `index.ts` barrel so it is measured
 - pin the duplicate-tag union collapse (members sharing a tag dispatch through
  one handler) with tests
 ## [0.8.0] - 2026-09-23
 - reject a universe that mixes a literal with a broad type (e.g.
  `"a" | \`x-${number}\``), which the finite-literal gate let through
 - narrow the tagged-union matcher's handler parameters for a union-valued or
  optional discriminant, and its fallback to the unhandled tags, instead of
  passing `never`
 ## [0.7.1] - 2026-09-23
 - upgrade dependencies
 ## [0.7.0] - 2026-09-23
 - reject broad universes (`string`, `number`, template literals) and
  value/stringification collisions (`true | "true"`, `1 | "1"`) at the factory
- type handler parameters by the matched member (`Member<T, K>`), so a
+- type handler parameters by the matched member, so a
  standalone `"true"` universe is typed `"true"` rather than `true`
 ## [0.6.0] - 2026-09-22
@@ -80,7 +124,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - basic setup
-[Unreleased]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.6.0...main
+[Unreleased]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.9.0...main
 [0.9.0]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.8.3...0.9.0
 [0.8.3]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.8.2...0.8.3
 [0.8.2]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.8.1...0.8.2
 [0.8.1]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.8.0...0.8.1
 [0.8.0]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.7.1...0.8.0
 [0.7.1]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.7.0...0.7.1
 [0.7.0]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.6.0...0.7.0
 [0.6.0]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.5.0...0.6.0
 [0.5.0]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.4.0...0.5.0
 [0.4.0]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.3.0...0.4.0
@@ -20,8 +20,14 @@ the rules so agents and humans don't diverge.
 - **Build:** `npm run build`
 - **Test:** `npm run test`, `npm run test:ci`
 - **Compat (CI-only, manual locally):** `npm run test:compat` — type-check the
  suite + a consumer fixture against the minimum supported TypeScript; needs a
  built `dist/` and network access (`npx`). Never part of a hook or
  `check`/`verify`
 - **Watch:** `npm run watch` - re-runs tests on file save, humans only
 - **Checks:** `npm run check`, `npm run fix`
 - **Doc tests:** `npm run create:doc-tests` — compile the `ts`-tagged fences
  in the prose docs into executed, gitignored tests
 - **Verify:** `npm run verify` — the definition of done
 - **Maintenance:** `npm run maintain` — advisory only
 - **Individual fixes:** `npm run fix:oxfmt`, `npm run fix:oxlint`
@@ -41,7 +47,8 @@ faster tiers catch less, slower tiers are more thorough":
 | `npm run verify`                 | manual                  | Definition of done: `npm run check` + unit tests, one shot                                                                                              | ~6s   |
 | `npm run fix`                    | manual                  | Auto-resolve fixable issues (lint, format)                                                                                                              | ~3s   |
 | `npm run maintain`               | manual / CI (advisory)  | `maintain:knip` + `maintain:outdated` (whole-project + network scans)                                                                                   | ~10s  |
-| CI build (auto)                  | on push to `main` / tag | `build` job (build + correctness + packaging) — see [.gitea/workflows/ci.yml](./.gitea/workflows/ci.yml)                                                | ~30s+ |
+| CI build (auto)                  | on push to `main` / tag | `build` job (build + correctness + coverage + packaging) — see [.gitea/workflows/ci.yml](./.gitea/workflows/ci.yml)                                     | ~30s+ |
 | CI compat (auto)                 | in CI only              | `compat` job — `test:compat` over the built `dist/`; never a local tier (run by hand) — see [compat/](./compat)                                         | ~15s  |
 | CI maintain (auto, non-blocking) | on push to `main`       | `npm run maintain` — reports, never fails the build                                                                                                     | ~10s  |
 | CI publish (auto)                | on tag                  | packaging checks + `publish:publint` / `publish:attw`, then the Gitea release page and `npm publish` (skipped, and the job failed, without `NPM_TOKEN`) | ~15s  |
@@ -87,6 +94,17 @@ Per [AGENTS.md § Never do](./AGENTS.md#never-do), reach green honestly — fix
 types, never suppress the checks you can't make pass. Full rationale:
 [development/testing.md](./development/testing.md).
 ## Documentation examples
 Every `ts`-tagged fence in `README.md` / `CONTRIBUTING.md` is compiled into an
 executed test, so a documented example cannot drift from the API. Describe each
 fence with the paragraph directly above it (that text becomes the test title),
 and keep its library import self-contained;
 `npm run create:doc-tests` regenerates, formats and type-checks the tests under
 `src/doc-test/__generated__/`. CI runs it in an explicit step before `test:ci`,
 so run it yourself before `npm run verify` when you touched a fence. Why:
 [development/docs.md](./development/docs.md).
 ## Code style and formatting
 `oxfmt` is the formatter and `oxlint` is the linter (with type-aware rules).
@@ -115,17 +133,23 @@ intended to run. A `<prefix>:<name>` script is implicitly aggregated by a
 `<prefix>` script (if one exists) and run by the corresponding lefthook hook or
 CI step. Pick the prefix that matches the script's lifecycle:
- `create:*` — front doors of the repo's own workflow; these mutate git state
+- `create:*` — front doors of the repo's own workflow; these produce or mutate
-  rather than the source. `create:branch` opens a unit of work, `create:finish`
+  workflow artifacts (git state, generated doc-tests) rather than the
  hand-written source. `create:branch` opens a unit of work, `create:finish`
  closes the branch half, `create:release` closes the release half
-  (maintainer-only). No bare `create` aggregator on purpose.
+  (maintainer-only), and `create:doc-tests` regenerates the compiled prose
  examples. No bare `create` aggregator on purpose.
 - `check:*` — read-only verification; never modifies files. Aggregated by
  `npm run check`.
 - `fix:*` — mutating counterpart of a `check:*` script. Aggregated by
  `npm run fix`; the diff is the review surface.
 - `test:*` — test scripts. `test` is the canonical entry point (`check:tsc` +
  unit tests); `test:unit` skips the typecheck for fast local iteration;
-  `test:ci` adds c8 coverage.
+  `test:coverage` runs c8 over the hand-written tests only; `test:doc` runs the
  generated doc examples without coverage; `test:ci` chains the two and fails
  below 100% coverage on `src/`; `test:compat` type-checks the suite + a
  consumer fixture against the minimum supported TypeScript via `npx` (both
  CI-only; `verify` stays coverage- and network-free).
 - `watch:*` — long-running watchers for the manual inner dev loop. Aggregated by
  `watch`.
 - `maintain:*` — advisory repo-maintenance scans: read-only, but whole-project
@@ -1,6 +1,6 @@
 MIT License
-Copyright (c) 2025 tmu
+Copyright (c) 2026 tmu
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal
@@ -1,27 +1,265 @@
 # tiny-pattern-ts
-Pattern matching for TypeScript/ESM environments (F#-style, not regex).
+Exhaustive, type-safe pattern matching for TypeScript.
 ## Synopsis
 ```ts
 import { getTaggedUnionMatcher } from "tiny-pattern-ts";
 // 1. We have a union type
 type Contact =
    | { kind: "email"; address: string }
    | { kind: "phone"; number: string }
    | { kind: "messenger"; username: string };
 // 2. Create a matcher providing the discriminant property
 const matchContact = getTaggedUnionMatcher<Contact>()("kind");
 // 3. Define handlers for each branch of the union
 const formatContact = matchContact({
    email: (e) => `MAIL: ${e.address}`,
    phone: (p) => `PHONE: ${p.number}`,
    messenger: (m) => `MESSENGER: @${m.username}`,
 });
 // 4. Call the matcher with a value
 const mailOutput = formatContact({ kind: "email", address: "ada@example.com" });
 assert.equal(mailOutput, "MAIL: ada@example.com");
 const phoneOutput = formatContact({ kind: "phone", number: "+1 555 0100" });
 assert.equal(phoneOutput, "PHONE: +1 555 0100");
 ```
 ## Description
-`tiny-pattern-ts` brings F#-style pattern matching to TypeScript. Patterns are
+`tiny-pattern-ts` is a pattern-matching library for TypeScript.
-ordinary objects whose `matches` method is a TypeScript type guard, so narrowing
+
-composes the way any other guard does. It is deliberately not a regex engine and
+The main goal of `tiny-pattern-ts` is to make pattern matching type-safe with a
-not a macro: there is no transpiler and no DSL to learn, and the type-level
+lean syntax. This is accomplished by being exhaustive and passing typed
-contract is the feature — see [development/library.md](./development/library.md)
+parameters per branch to the handlers — supported by an outstanding
-for the design decisions and [Caveats](#caveats) for the limits.
+autocomplete and a tiny footprint. The matchers are data last and pipe-friendly:
 build the handler map once, then apply the resulting matcher to values
 (`match(value)`, or `pipe(value, match)`).
 See [development/library.md](./development/library.md) for the design decisions
 and [Caveats](#caveats) for the limits.
 ## Installation
 ```sh
 npm install tiny-pattern-ts
 ```
 ## Requirements
 - **Node.js >= 26** (`engines` field; pinned via `.node-version`).
- **TypeScript >= 5.0** to consume the published declarations. The emitted `.d.ts`
+- **TypeScript >= 5.9** to consume the published declarations. The floor is set
-  use `const` type parameters (TS 5.0) and keep their relative `.ts` specifiers;
+  by the `type-fest` types the declarations use and is checked in CI against a
-  both resolve on TS >= 5.0 in `node10` / `node16` / `nodenext` / `bundler`.
+  consumer fixture; see [`compat/`](./compat) and
  [development/ci.md § TypeScript compatibility](./development/ci.md#typescript-compatibility).
  The emitted `.d.ts` keep their relative `.ts` specifiers, which resolve under
  `node16` / `nodenext` / `bundler`. The package exposes only an `exports` map
  (no `main` / top-level `types`), so the legacy `node10` resolver does not
  apply.
 - The package is **ESM-only** (no CommonJS shim).
 ## Examples
 A few real-world recipes. Each binds the handler-map function once and reuses
 it, so the matcher is allocated a single time.
 ### Dispatch on a primitive union
 A result code is itself a finite union, so `getPrimitiveUnionMatcher` keys a
 handler on each member:
 ```ts
 import { getPrimitiveUnionMatcher } from "tiny-pattern-ts";
 type ResultCode = "ok" | "created" | "no-content";
 const toStatus = getPrimitiveUnionMatcher<ResultCode>()({
    ok: () => 200,
    created: () => 201,
    "no-content": () => 204,
 });
 assert.equal(toStatus("ok"), 200);
 assert.equal(toStatus("created"), 201);
 assert.equal(toStatus("no-content"), 204);
 ```
 ### Leave cases to a fallback
 Pass a fallback as the second argument to handle only part of the universe; it
 receives the members the map leaves uncovered — here the parameter is
 `"deprecated" | "gateway-timeout"`:
 ```ts
 import { getPrimitiveUnionMatcher } from "tiny-pattern-ts";
 type Status = "active" | "beta" | "deprecated" | "gateway-timeout";
 const rollout = getPrimitiveUnionMatcher<Status>()(
    { active: () => "enabled", beta: () => "enabled" },
    (status) => `blocked (${status})`,
 );
 assert.equal(rollout("active"), "enabled");
 assert.equal(rollout("deprecated"), "blocked (deprecated)");
 ```
 ### Dispatch on a property union
 The value does not have to be the union itself. When a single property carries a
 finite union, the tagged-union matcher keys on it and narrows the whole record to
 the selected value:
 ```ts
 import { getTaggedUnionMatcher } from "tiny-pattern-ts";
 interface Invoice {
    readonly currency: "eur" | "usd" | "jpy";
    readonly amount: number;
 }
 const matchCurrency = getTaggedUnionMatcher<Invoice>()("currency");
 const symbolOf = matchCurrency({
    eur: (i) => `€${i.amount.toFixed(2)}`,
    usd: (i) => `$${i.amount.toFixed(2)}`,
    jpy: (i) => `¥${i.amount.toFixed(0)}`,
 });
 assert.equal(symbolOf({ currency: "usd", amount: 12.5 }), "$12.50");
 assert.equal(symbolOf({ currency: "jpy", amount: 900 }), "¥900");
 ```
 ### Widen the return type
 When the handlers return different types, reach for the widening `W` variant: the
 matcher's return is their union rather than one common type — here
 `string | string[] | undefined`:
 ```ts
 import { getPrimitiveUnionMatcherW } from "tiny-pattern-ts";
 type Field = "name" | "tags" | "note";
 const parse = getPrimitiveUnionMatcherW<Field>()({
    name: () => "Ada",
    tags: () => ["admin", "beta"],
    note: () => undefined,
 });
 assert.equal(parse("name"), "Ada");
 assert.deepEqual(parse("tags"), ["admin", "beta"]);
 assert.equal(parse("note"), undefined);
 ```
 ## API
-Yet to be implemented
+The package exports four factories. Two axes pick one:
 - **Universe** — a _primitive-union_ matcher matches a value that is itself a
  finite union (`"yes" | "no"`); a _tagged-union_ matcher matches an object
  discriminated by a property (`{ kind: … }`).
 - **Return** — the _strict_ variant gives every handler one common return type
  `R`; the _widening_ variant (`W`) widens the return value to the union of the
  handler returns.
 | Factory                          | Use when                                                                  | Return                |
 | -------------------------------- | ------------------------------------------------------------------------- | --------------------- |
 | `getPrimitiveUnionMatcher<T>()`  | the value is the union and all handlers return the same type              | one common `R`        |
 | `getPrimitiveUnionMatcherW<T>()` | the value is the union and handlers return different types                | union of the handlers |
 | `getTaggedUnionMatcher<T>()`     | the value is a discriminated object and all handlers return the same type | one common `R`        |
 | `getTaggedUnionMatcherW<T>()`    | the value is a discriminated object and handlers return different types   | union of the handlers |
 Bind the function that takes the handler map to a `match…` variable once and
 reuse it; the [Examples](#examples) do this, so the builder is allocated once.
 ### Primitive-union matchers
 `getPrimitiveUnionMatcher<T>()` takes the finite universe `T` and returns a
 builder. Calling the builder with a handler map keyed by `T`'s members returns a
 matcher: a function from `T` to the common return type.
 ```ts
 import { getPrimitiveUnionMatcher } from "tiny-pattern-ts";
 const matchAnswer = getPrimitiveUnionMatcher<"yes" | "no">();
 const reply = matchAnswer({
    yes: () => "agreed",
    no: () => "declined",
 });
 const answer = reply("yes");
 assert.equal(answer, "agreed");
 ```
 Add a fallback as the second argument to leave members unhandled; the fallback
 receives the remainder:
 ```ts
 import { getPrimitiveUnionMatcher } from "tiny-pattern-ts";
 const matchLabel = getPrimitiveUnionMatcher<"yes" | "no" | "maybe">();
 const label = matchLabel(
    { yes: () => "agreed", no: () => "declined" },
    (other) => `not sure: ${other}`, // other: "maybe"
 );
 const answer = label("yes");
 assert.equal(answer, "agreed");
 const fallback = label("maybe");
 assert.equal(fallback, "not sure: maybe");
 ```
 `getPrimitiveUnionMatcherW` is the same builder, but the matcher's return type
 is the union of the handler return types rather than one common `R`.
 ### Tagged-union matchers
 `getTaggedUnionMatcher<T>()` takes a discriminated union `T`. The returned
 function takes the discriminant property's name and returns the handler-map
 builder, keyed by that property's tags.
 ```ts
 import { getTaggedUnionMatcher } from "tiny-pattern-ts";
 type Shape =
    { kind: "circle"; radius: number } | { kind: "square"; side: number };
 const matchShape = getTaggedUnionMatcher<Shape>()("kind");
 const area = matchShape({
    circle: (s) => Math.PI * s.radius ** 2,
    square: (s) => s.side ** 2,
 });
 const circleArea = area({ kind: "circle", radius: 2 });
 assert.equal(circleArea, Math.PI * 4);
 const squareArea = area({ kind: "square", side: 3 });
 assert.equal(squareArea, 9);
 ```
 `getTaggedUnionMatcherW` is the widening counterpart, exactly as in the
 primitive-union pair. The discriminant key is restricted to properties whose
 values are tags; see [Caveats](#caveats) for the supported tags and the
 `boolean` / `null` / `undefined` key projection.
 ### The universe
 The primitive universe `T` must be a finite union of literals with no
 value/stringification collision. A broad member (`string`, `number`, a template
 literal) or a colliding pair (`true | "true"`, `1 | "1"`) is rejected at the
 factory. The reasons and the rejected alternatives are in
 [development/library.md](./development/library.md).
 ## Caveats
@@ -40,9 +278,59 @@ Yet to be implemented
 - **`NaN` and `-0` cannot be matched specifically.** They have no literal type,
  so both stay part of `number`.
 ### Why open universes are rejected
 An open universe — one carrying a broad member, as in
 `type Units = "s" | "ms" | "min" | (string & {})` — is not a dispatch concern.
 If the values arrive from outside the program, parse them at the boundary down
 to a finite union and match the narrowed result; the openness never reaches the
 matcher. If the domain is genuinely extensible, the right shape is a runtime
 `Map` of handlers, where "no handler" is a lookup, not a pattern. Either way an
 open matcher would abandon the one guarantee this library exists to give —
 provable exhaustiveness — to automate what a `switch` and a default arm already
 cover. The type-level cost of supporting open universes is recorded in
 [development/library.md](./development/library.md#supported-universes).
 ## Alternatives
 ### [`ts-pattern`](https://github.com/gvergnaud/ts-pattern)
 - is the full structural matcher — nested and partial patterns, guards, unions,
  captures — exhausting at `.exhaustive()`
 - has a fluent `.with(…)` chain that is heavy on syntax; tiny-pattern-ts is one
  handler map
 - is about 2 kB minified and gzipped; tiny-pattern-ts is 0.2 kB
 - reach for it when you need a feature tiny-pattern-ts does not cover
 ### [Effect's `Match`](https://effect.website/docs/code-style/pattern-matching/)
 - is the same piped matcher (`Match.type` / `Match.when` / `Match.exhaustive`),
  but only as part of the `effect` ecosystem
 - tiny-pattern-ts is standalone: no runtime dependency to buy into
 ### [`match-iz`](https://github.com/shuckster/match-iz)
 - expresses patterns in the TC39 proposal's style, deciding each case at runtime
 - is written in JavaScript with hand-maintained declarations, so its types do
  not prove the cases exhaustive
 - tiny-pattern-ts does: exhaustiveness is a compile-time guarantee, not an
  `otherwise` fallback
 ### plain `switch` (baseline)
 - is the zero-dependency baseline — pair it with
  [`eslint-plugin-strict-pattern-matching`](https://www.npmjs.com/package/eslint-plugin-strict-pattern-matching)
  for exhaustiveness
 - is a statement, not an expression, so it cannot produce a value directly
 - leaves the `never` guard to you; tiny-pattern-ts is an expression and does not
  need one
 The [TC39 pattern-matching proposal](https://github.com/tc39/proposal-pattern-matching)
 is still stage 1, so userland libraries remain the only option today.
 ## License
-MIT © 2025 tmu. See [LICENSE](./LICENSE).
+MIT © 2026 tmu. See [LICENSE](./LICENSE).
 ## Contributing
@@ -6,89 +6,9 @@ Backlog and tracking for tiny-pattern-ts. Managed in vscode-todotasks format.
 Setup:
 ☐ Split off template into separate package => pi --session 01a07dde-7050-7054-bb36-1606d7eb2bc3 @low
 ✔ straighten oxc rules @done
    ✔ oxc forbids ternary => remove oxc rule @done
    ✔ oxc wants comments to start comments with a capital letter => remove oxc rule @done
    ✔ there already exists a /* */ block comment in primitive.ts, => change to multi-line comment @done
 ✔ Remove example code files and its documentation and its exports from index.ts @high @done
    ✔ remove src/match.ts and its documentation @high @done
    ✔ remove src/pattern.ts and its documentation @high @done
    ✔ remove src/index.test.ts and its documentation @high @done
    ✔ exports from `src/index.ts` should only be the public API surface @done
 v1.0:
 ☐ API surface is stable and fully typed
    ☐ Finalize public exports in `src/index.ts`
    ☐ Document all exported types and functions
    ☐ Add JSDoc for public APIs
 ☐ Test coverage meets threshold
    ☐ Achieve 100% branch coverage on `src/primitive-union.ts`
    ☐ Achieve 100% branch coverage on `src/index.ts`
 Testing:
 ✔ Cover autocomplete with real completion test cases in the suite @medium @done
    → `src/util/__tests__/lsp-completion.ts` (`#test-utils/…`) is the helper; the suite asserts its labels (see development/testing.md § Autocomplete)
    → wire it into `node --test` so a test asserts the offered labels
    → note: `Parameters<typeof factory>[0]` resolves only the *last* overload; use `@ts-expect-error` call sites for factory negatives, not `not.toExtend<Parameters<…>>`
 ✔ Drive the LSP completion helper through the LSP protocol library instead of a hand-rolled JSON-RPC client @medium @done
    ✔ Add `vscode-languageserver-protocol` and record the tooling decision @done
    ✔ Rewrite `src/util/__tests__/lsp-completion.ts` onto `createMessageConnection` and typed requests @done
    ✔ Update `development/testing.md § Autocomplete` for the new client @done
    ✔ Run `npm run verify` and check the task off @done
 ✘ Test a literal union widened with `(string & {})` — `"red" | "green" | "yellow" | (string & {})` @medium @cancelled
    → broad universes are rejected; the rejection is covered by the broad-universe gate tests
 ✔ Type hole when using broad types like string as a universe @high @done
    → decided: broad universes are rejected at the factory, so the exhaustive overload can always be proven
    → value/stringification collisions (`true | "true"`, `1 | "1"`) are rejected too; handler params are typed by `Member<T, K>`
    → see development/library.md § Supported universes
 Matcher:
 ✔ Clean up: adopt the 3-overload matcher (`src/prototype-ac2.ts`) and delete the prototypes @high @done
    → `getMatcher` / `getMatcherW`, each with overloads `ExhaustiveLoose` → `Fallback` → `Handlers` (order is load-bearing)
    → fold into `src/primitive.ts` / the public API; drop `src/prototype*.ts`
 ✔ `_` should receive only the unhandled `T` keys, not all of `T` @medium @done
    → the fallback is now a second argument: `(handlers, (s) => …)`, `s: Exclude<T, keyof handlers>`
 ✔ A fallback for an already-exhaustive handler map must be a compile error @medium @done
    → rejected by an F-bounded constraint on `Handled` (checked *after* inference); a conditional in the fallback parameter is evaluated too early and breaks contextual typing
 ✔ Implement matcher with similar API like matcher from primitive.ts @high @done
    → `getTaggedUnionMatcher` / `getTaggedUnionMatcherW`, curried on the discriminant key
    → fallback is the second argument; `_` removed
 ✔ Rename `primitive` to `primitive-union` and `getMatcher` to `getPrimitiveMatcher` @medium @done
    → `src/primitive.ts` / `src/primitive.test.ts` → `primitive-union.*`
    → `getMatcherW` → `getPrimitiveMatcherW` for symmetry with the tagged-union pair
    → update `src/index.ts`, `development/library.md` and any README references
    → shipped as `getPrimitiveUnionMatcher` / `getPrimitiveUnionMatcherW`; kept `Union` for symmetry with the tagged-union pair
 ☐ when using a union type as a property, the current behavior of tagged union matcher is
    to pass never to handler parameters
    → new matcher function needed or can be fixed in tagged union matcher
    ☐ optional discriminant (`{ type?: "x" }`) is the same hole: the boolean/nullish change now admits the `undefined` tag, so the factory accepts the key, but `Extract<T, Record<K, V>>` still passes `never` to both the `x` and `undefined` handlers
 Bugs:
 ✔ TS 7 LSP server logs `context canceled` on stderr at shutdown @done
    → `handleExit` returns `io.EOF`, cancelling the background context while `Session.updateWatches` is still in flight; the bare error is flushed to stderr and the server exits 1
    → close stdin after `shutdown` instead of sending `exit`; the server exits cleanly (code 0, no output), kill kept as a fallback
 Enhancements:
 ✔ Allow boolean literals in primitive union patterns (e.g. `true: () => "yes"`) @medium @done
    ✔ Are there other primitive types that should be supported in union patterns? (e.g. `bigint`, `symbol`) @medium @done
        → added boolean, null and undefined; rejected `symbol` (compile-time brand, nothing at runtime) and `bigint` (not a property key)
 ✔ Allow boolean, null and undefined discriminant values in tagged-union patterns @medium @done
    → moved the `PatternKey` / `PatternParam` projection to `matcher-shared.ts` and keyed the tagged-union handler map through it; see development/library.md § Tagged-union matcher
 Documentation:
 ☐ Bring README.md back to its previous form — synopsis and examples restored, in the correct place
    → previous section order: title, tagline, Synopsis, Description, Requirements, Examples, API, License, Contributing
    → previous Examples order: literal/exhaustive, typeof, structural/discriminated unions, when, any
 ☐ Create `examples/` directory with runnable snippets
 ☐ Add comparison section vs. other TS pattern-matching libs in Readme.md
 ☐ Write migration guide for users coming from discriminated unions
 ☐ Create backlog tasks for implementation
 ✔ Document the matcher design paths in `development/` — union vs overload merge, inferred universe (`NoInfer`), conditional `RequireKeys`, cases-first — and why each was abandoned @done
 ☐ Validate code fences in Markdown (start with README.md) — compile the TypeScript examples against `src/` so the docs cannot drift from the API
 Workflow:
 ✔ Resolve the finish/push tension: `create:finish` leaves `main` ahead of its upstream while `create:branch` refuses until `main` matches upstream — decide whether `finish` should push or `branch` should compare only `BEHIND` (see development/workflow.md) @done
 Maintenance:
 ☐ Serve CI coverage over a tiny self-hosted webserver (replace the zip artifact) @low
@@ -102,3 +22,4 @@ Maintenance:
    ☐ Add a minimal dir-listing webserver to the gitea docker setup for serving landing page (reuse existing reverse proxy)
    ☐ CI writes landing page to a shared volume keyed by project + tag (e.g. `/landing/tiny-pattern-ts/<tag>/`)
    ☐ Browse to `…/tiny-pattern-ts/index.html` in the browser
 ✔ Add testing with the TypeScript floor in CI @done
@@ -0,0 +1,89 @@
 // Consumer smoke test for the minimum supported TypeScript (see README
 // § Requirements). It imports the package by name, so it resolves through the
 // `exports` map to the emitted `dist/*.d.ts` — including the relative `.ts`
 // specifiers they keep — rather than to the source. Run by `npm run test:compat`
 // and the CI `compat` job only; never by `check` / `verify`.
 //
 // Why the `expectTypeOf` assertions: a compile that merely succeeds is a weak
 // oracle. An `any`-typed declaration would compile, but `expect-type`'s exact
 // equality rejects `any`, so the assertions prove the emitted types are real.
 // Every handler *parameter* is asserted too, not just the matcher: the handler's
 // member type comes from the `Member` inversion in the declarations, and a wrong
 // or `any` parameter would otherwise slip through, since the matcher's own
 // `(shape: T) => R` type is built from `T` and the handler returns. See
 // development/ci.md § TypeScript compatibility.
 import { expectTypeOf } from "expect-type";
 import {
    getPrimitiveUnionMatcher,
    getPrimitiveUnionMatcherW,
    getTaggedUnionMatcher,
    getTaggedUnionMatcherW,
 } from "tiny-pattern-ts";
 type ResultCode = "ok" | "created";
 const toStatus = getPrimitiveUnionMatcher<ResultCode>()({
    ok: (s) => {
        expectTypeOf(s).toEqualTypeOf<"ok">();
        return "OK";
    },
    created: (s) => {
        expectTypeOf(s).toEqualTypeOf<"created">();
        return "CREATED";
    },
 });
 expectTypeOf(toStatus).toEqualTypeOf<(shape: ResultCode) => string>();
 // The widening twin keeps each handler's own return type in the union.
 const toStatusW = getPrimitiveUnionMatcherW<ResultCode>()(
    {
        ok: (s) => {
            expectTypeOf(s).toEqualTypeOf<"ok">();
            return "OK" as const;
        },
    },
    (rest) => {
        expectTypeOf(rest).toEqualTypeOf<"created">();
        return "CREATED" as const;
    },
 );
 expectTypeOf(toStatusW).toEqualTypeOf<
    (shape: ResultCode) => "OK" | "CREATED"
 >();
 type Contact =
    | { kind: "email"; address: string }
    | { kind: "phone"; number: string };
 const format = getTaggedUnionMatcher<Contact>()("kind")({
    email: (e) => {
        expectTypeOf(e).toEqualTypeOf<{ kind: "email"; address: string }>();
        return e.address;
    },
    phone: (p) => {
        expectTypeOf(p).toEqualTypeOf<{ kind: "phone"; number: string }>();
        return p.number;
    },
 });
 expectTypeOf(format).toEqualTypeOf<(shape: Contact) => string>();
 const formatW = getTaggedUnionMatcherW<Contact>()("kind")(
    {
        email: (e) => {
            expectTypeOf(e).toEqualTypeOf<{ kind: "email"; address: string }>();
            return e.address;
        },
    },
    (rest) => {
        expectTypeOf(rest).toEqualTypeOf<{ kind: "phone"; number: string }>();
        return rest.number;
    },
 );
 expectTypeOf(formatW).toEqualTypeOf<(shape: Contact) => string>();
 export { format, formatW, toStatus, toStatusW };
@@ -0,0 +1,15 @@
 {
    "extends": "@tsconfig/strictest/tsconfig.json",
    "compilerOptions": {
        "lib": ["es2024"],
        "module": "nodenext",
        "target": "es2024",
        "types": ["node"],
        "skipLibCheck": false,
        "noEmit": true,
        "allowImportingTsExtensions": true,
        "verbatimModuleSyntax": true
    },
    "include": ["../src", "../scripts", "*.ts"],
    "exclude": ["../src/doc-test"]
 }
@@ -45,7 +45,8 @@
        "todotasks",
        "connor",
        "injective",
-        "injectivity"
+        "injectivity",
        "userland"
    ],
    "ignorePaths": ["dist", "node_modules", "coverage", "*.svg", ".gitignore"]
 }
@@ -24,6 +24,7 @@ One file per category:
 | [workflow.md](./workflow.md)     | Branching and merging, script prefixes, feedback tiers, commit messages |
 | [tooling.md](./tooling.md)       | Toolchain choices and configuration, editor setup                       |
 | [testing.md](./testing.md)       | Test strategy and type-driven development                               |
 | [docs.md](./docs.md)             | Validating the Markdown code fences in the prose docs                   |
 | [ci.md](./ci.md)                 | CI pipeline, runner image, coverage serving                             |
 | [publishing.md](./publishing.md) | Release and npm publishing                                              |
@@ -6,6 +6,12 @@ the job graph; this file records why it is shaped the way it is.
 ## Pipeline
 - **`build`** (push to `main` / tag) — build + correctness + packaging.
 - **`compat`** (CI only) — type-check the suite and a consumer fixture against
  the minimum supported TypeScript; consumes `build`'s `dist/` and gates
  `publish`. It has **no local tier**: it never runs in a hook or in
  `check` / `verify` / `test:ci`; run it by hand with
  `npm run build && npm run test:compat`. See
  [§ TypeScript compatibility](#typescript-compatibility).
 - **`maintain`** (push to `main`, non-blocking) — `npm run maintain`; reports,
  never fails the build.
 - **`publish`** (tag) — packaging checks + `publish:publint` / `publish:attw`,
@@ -97,6 +103,106 @@ Leave `act_runner`'s `force_pull` disabled.
  (`docker rmi gitea.e1nsnull.de/tmu/act-ci:<version>`); do not reach for
  force-pull.
 ## Coverage threshold
 #### Decision (2026-09)
 `npm run test:ci` fails below 100% statements / branches / functions / lines
 across `src/**/*.ts` (`c8 --all --include "src/**/*.ts" --100`). The gate rides
 the `build` job; `npm run verify` stays coverage-free.
 #### Why
 - The types are the feature, so an untested branch is a hole in the contract,
  not a metric to trade off; 100% is the only threshold that means "no hole".
 - `--all` counts a `src/` file no test imports. Without it c8 reports only the
  files the suite happened to load, so a new untested module is invisible and
  the threshold passes vacuously.
 - The gate rides `test:ci`, which `build` already runs — no new job or step.
 - `verify` stays fast and local; the slower coverage run is a CI-only tier (see
  [workflow.md § Feedback tiers](./workflow.md#feedback-tiers)).
 #### Rejected
 - Per-file thresholds: a global 100% already forces every counted file to 100%.
 - A `check:coverage` script: it would re-run the suite or read c8's temp dir,
  and no `check:*` script runs tests.
 - `--all` without `--include`: it would also sweep `scripts/`, which is not the
  shipped surface.
 #### Known issue
 - `src/matcher-shared.ts` is types only, so its runtime image is empty; c8 still
  lists it under `--all`. It carries a file-level `/* c8 ignore start */` with
  the reason. Adding runtime code there means removing that directive.
 ## TypeScript compatibility
 #### Decision (2026-09)
 A dedicated `compat` job runs `npm run test:compat` — the `npx`-pinned
 TypeScript 5.9 compiler (`typescript@5.9.2`) over `compat/tsconfig.json` —
 against the `dist/` artifact `build` produced, and `publish` requires it. The
 floor is TypeScript 5.9, pinned in the `test:compat` script itself (the single
 source of truth) and documented in [README § Requirements](../README.md#requirements).
 #### Why
 - The compiler is a **different major** from the repo's TypeScript 7, so it is
  resolved by `npx` at run time. It must never appear in `devDependencies`: that
  would install it for every local `npm ci` and drift the lockfile, which would
  put a second compiler in the local `check` / `verify` loop and every editor.
 - **CI-only is the point.** `npx` fetches over the network — like `maintain`'s
  scans, a network-bound check is never a local feedback tier (see
  [workflow.md § Feedback tiers](./workflow.md#feedback-tiers)). It is not in
  `check`, `verify`, `test:ci` or any hook; locally it is run **by hand** with
  `npm run build && npm run test:compat`.
 - **`compat` consumes `build`'s artifact** rather than rebuilding, so it judges
  the exact bytes `check`, `test:ci` and `publint` saw.
 - **`compat` gates `publish`** because the types are the feature: an artifact
  that is not consumable at the advertised floor must not ship.
 - **The fixture is a consumer, not a unit test.** `compat/fixture.ts` imports
  the package by name (`tiny-pattern-ts`), so it resolves through the `exports`
  map to `dist/index.d.ts` and exercises the emitted declarations' relative
  `.ts` specifiers — not the source. `expectTypeOf`'s exact-equality assertions
  are load-bearing: a bare compile would also pass if a declaration collapsed to
  `any`; they reject that.
 #### Rejected
 - **A `devDependencies` alias** (`npm:typescript@5.9`): installs the legacy
  compiler locally, defeating "CI only".
 - **A second lockfile / sub-project** (`compat/` with its own `npm ci`): a
  pinned, reproducible matrix, but a whole extra lockfile to maintain for one
  compiler. `npx -p` is enough.
 - **A `paths` / `moduleSuffixes` redirect** to typecheck the _existing_ suite
  against `dist/` without touching it: `paths` cannot remap the relative
  `./index.ts` imports the tests use; `moduleSuffixes` only lets _missing_
  source resolve to suffixed copies, so it would need generated `.compat.ts`
  declarations staged into `src/` (plus excludes). Both spend more than the
  fixture buys. See the [handover](../backlog.tasks) discussion.
 - **Writing the fixture against source** (relative import): it would prove the
  source compiles under 5.9, not that the _published_ declarations do, which is
  the promise consumers rely on.
 - **Replacing `attw`**: `attw` owns the full resolution matrix
  (`node10`/`node16`/`nodenext`/`bundler`); `compat` answers only "does the
  documented floor compile the artifact".
 #### Known issue
 - `@tsconfig/node26` cannot be extended: its `lib: ["es2025", ...]` and
  `target: es2025` are rejected by 5.9 (`TS6046`). `compat/tsconfig.json`
  extends only `@tsconfig/strictest` and sets `lib` / `target: es2024`, the
  ceiling 5.9 accepts.
 - `skipLibCheck: false` is deliberate — it is what makes the floor honest
  (`type-fest` pins it at 5.9), rather than hiding a broken dependency d.ts
  behind `true`.
 - The version appears in both the `test:compat` script and the README; a floor
  bump is a two-file change. The script is authoritative.
 - `src/doc-test` is excluded from `compat/tsconfig.json`. The generated examples
  are checked against the source by their own project; `test:compat` covers the
  suite plus the fixture.
 ## Coverage serving
 #### Decision (2026-09)
@@ -0,0 +1,61 @@
 # Docs
 Why the prose documentation is maintained the way it is. The actionable rules
 are in [CONTRIBUTING.md](../CONTRIBUTING.md); this file records the rationale.
 ## Validating Markdown code fences
 #### Decision (2026-11)
 Compile every `ts / `typescript fence in the prose docs into a real
 `node:test` case under `src/doc-test/__generated__/`, typechecked by a scoped
 `tsc` project and executed by `node --test`.
 #### Why
 - A documented example is a promise about the API. Left unchecked it drifts the
  moment a signature changes, and a reader copies broken code.
 - The repo runs TypeScript 7, the native/Go compiler. It exposes **no legacy JS
  compiler API** (`ts.createProgram`, `ts.transpileModule`, `ts.createSourceFile`
  are all `undefined`; `Object.keys(require("typescript"))` is
  `["version", "versionMajorMinor"]`). So `@typescript/vfs`, the type-aware
  `eslint-plugin-markdown` and `docs-ts` / `@effect/docgen` cannot run here.
 - `node --check` parses as JS and rejects valid TS type annotations, so it is not
  a gate. The only faithful validator is the `tsc` **CLI**, which means emitting
  real `.ts` files and letting the existing `check:tsc` / `node --test` pipeline
  judge them.
 #### Rejected
 - **A packaged doc-test tool** (see above) — no usable compiler API on TS 7.
 - **Embedding a typecheck in the generator** — duplicates the gate, and would
  not exercise the repo's own resolution.
 - **`node --check`** — wrong language level.
 #### Known issue
 - Scanned sources are hard-coded to `README.md` and `CONTRIBUTING.md`. A
  `docs/` + `examples/` list is the natural extension; `development/` must never
  be scanned (its fences are illustrative, not compilable).
 - The generator hoists and merges leading imports, rewrites `tiny-pattern-ts` to
  the `#test-tiny-pattern-ts` source alias, and rejects an example that imports
  `node:assert` / `node:test` (the prelude already binds both). Titles are the
  immediately preceding paragraph; a fence with no such paragraph is a fatal
  error, which keeps every example described.
 - `oxlint src/doc-test` reports "No files found" because the generated
  `*.test.ts` are gitignored. That is cosmetic: the files are still typechecked
  and run.
 - The generated tests are `*.test.ts`, which c8's default excludes already keep
  out of the `--100` gate. Do not add an `--exclude` for them: passing any
  `--exclude` replaces the defaults, so every hand-written test file and
  `__tests__/` helper re-enters coverage and the gate fails.
 - Generated examples must not run under `c8`: an example could cover a line no
  hand-written test reaches, so the coverage gate would pass on documentation
  alone. `test:coverage` therefore runs c8 over the tracked tests only
  (`git ls-files 'src/*.test.ts'` — the generated files are untracked), and
  `test:doc` runs the examples in a separate process without c8. `test:ci`
  chains the two, so correctness and coverage stay independent.
 - The CI `build` job runs `npm run create:doc-tests` as an explicit step before
  `npm run test:ci`, not a `pretest:ci` lifecycle hook: the hook hides the step
  from the job log and makes `test:ci` behave differently under npm than when
  run directly.
@@ -5,8 +5,39 @@ user-facing reference is [README § API](../README.md#api).
 The matchers are implemented in `src/primitive-union.ts` (`getPrimitiveUnionMatcher` /
 `getPrimitiveUnionMatcherW`) and `src/tagged-union.ts` (`getTaggedUnionMatcher` /
-`getTaggedUnionMatcherW`), re-exported from `src/index.ts`; the rest of the
+`getTaggedUnionMatcherW`), re-exported from `src/index.ts`; the rest of `src/`
-library is placeholder code.
+is implementation detail.
 ## Public surface
 #### Decision (2026-09)
 `src/index.ts` exports the four factories and nothing else. Every exported
 function carries TSDoc; the builder types and the `matcher-shared.ts`
 vocabulary stay internal.
 #### Why
 - The factories are the whole contract: a consumer calls one and never needs to
  name the builder type it returns.
 - The builder interfaces are already the inferred return types, so they travel
  into the emitted `.d.ts` regardless. Exporting them would only make them
  nameable while freezing the internal `Strict` / `Widening` overload split as
  API.
 - TSDoc travels into the emitted declarations, so editor hovers and the
  published package document the API without a hand-written `.d.ts`.
 #### Rejected
 - **Exporting the builder types** (`PrimitiveUnionMatcher`, …). Nameable, but it
  grows the surface for no call-site benefit and pins the `Strict` / `Widening`
  split.
 - **Exporting the `matcher-shared.ts` vocabulary** (`Matchable`, `UnaryFn`,
  `PatternKey`, `Member`, `PatternReturns`, …). They appear in the public
  signatures, but a consumer never needs to name them; exporting them would
  freeze plumbing as API.
 - **A hand-written `.d.ts` or a separate API document.** It would drift from the
  implementation; TSDoc is generated from the source.
 ## Matcher shape
@@ -76,8 +107,8 @@ Each factory is two overloads whose order is load-bearing:
 - **Variance / `const` type parameters / `NoInfer` / `unique symbol` brands /
  defaulted type-param guards.** None change inference or evaluation order;
  `in`/`out` on the handler map broke contextual typing outright. `NoInfer`
-  specifically leaks into the emitted `.d.ts`, raising the consumer floor to
+  specifically leaks into the emitted `.d.ts`, which would raise the consumer
-  TypeScript 5.4 (README promises `>= 5.0`).
+  floor above the documented one (see [README § Requirements](../README.md#requirements)).
 - **Union merge**, **overload merge with only the exhaustive arm last**,
  **inferred universe**, **conditional `RequireKeys`**, **cases-first curried** —
  decided against while the API was single-object; their reasons (reported
@@ -145,6 +176,10 @@ inversion: the handler parameter is the member(s) of `T` whose `PatternKey` is
  object satisfy the exhaustive overload and reaches the `dispatch` throw.
  Rejecting at the boundary avoids threading an open/closed branch through
  `Handlers`, `Fallback` and `MustBePartial`.
 - **The finite-literal predicate is `IsLiteral<PatternKey<T>> extends true`.**
  `IsLiteral` is `boolean` for a union that mixes a literal with a broad type
  (`"a" | \`x-${number}\``), so `extends false`would treat the mix as
 supported;`extends true` is the check that rejects it.
 - **Collisions are rejected, not merged.** `Member<T, K>` would be sound (the
  handler gets the union), but the API is one handler per member; rejecting
  keeps `Member` a singleton and the remainder exact.
@@ -163,7 +198,9 @@ Extract<T, Stringified<T>>` catches numeric collisions too.
  non-self-referential parameter type (`{ [K in keyof H]: … R … }` gives
  `unknown`); an F-bounded guard referencing `keyof Handled` in `Handled`'s own
  constraint sees the constraint, not the map; all handlers share one `R` (only
-  the fallback widens it); `IsLiteral` is the finite-literal predicate.
+  the fallback widens it); `IsLiteral` is the finite-literal predicate. The
  user-facing consequence — open universes are a parsing or registry concern,
  not a dispatch one — is guidance in README § Why open universes are rejected.
 - **`Member<T, K>` without the gate:** sound, but a colliding handler gets a
  union and `1 | "1"` stays one runtime key.
 - **A round-trip injectivity gate** (`IsEqual<T, PatternParam<PatternKey<T>>>`):
@@ -208,16 +245,27 @@ The key is a separate call because `K` is inferred from its literal argument and
 #### Why
- **Same fallback/remainder machinery as the primitive-union matcher.** `HandledMembers`
+- **Same fallback/remainder machinery as the primitive-union matcher.** `HandledTags`
-  maps the handled tags to their members and `Exclude<T, …>` is the fallback's
+  recovers the tag values the map handled (`Member<Tags, keyof Handled>`) and
-  parameter; the redundant-fallback guard is the same F-bounded constraint. Only
+  `Narrowed<T, K, Exclude<Tags, …>>` is the fallback's parameter; the
-  the "universe" changes — `T`'s members instead of primitive values.
+  redundant-fallback guard is the same F-bounded constraint. Only the "universe"
  changes — `T`'s members instead of primitive values.
 - **`T extends object`, not `Record<PropertyKey, unknown>`.** An `interface` has
  no implicit index signature, so the `Record` constraint would reject
  interface-based unions. The runtime reads the tag off `object` with one
  assertion, the tagged twin of the primitive-union dispatch's `shape as string | number`.
- **`MapTaggedUnion` distributes with `Extract`.** A duplicated tag yields a
+- **`Narrowed` distributes over `T`, narrowing `K` to the tag.** A member whose
-  union of members instead of dropping one.
+  `K` cannot take the tag drops out; a duplicated tag yields a union of members
  instead of dropping one. `T[K] extends V` returns the exact member (a
  discriminated union's declared interface) untouched, so the mapped form only
  handles a property that is itself a union.
 - **A union-valued or optional discriminant is supported.** A single shape whose
  property is a union (`{ color: "red" | "green" | "blue" }`) is narrowed per
  handler instead of being passed `never`. Matching a defined tag on an optional
  property (`{ type?: "x" }`) proves the key is present, so it becomes required
  (`{ type: "x" }`); the `undefined` tag narrows it to `{ type?: never }` under
  `exactOptionalPropertyTypes` (absence) or `{ type?: undefined }` when the
  property explicitly admits `undefined`.
 - **A `boolean` / `null` / `undefined` tag goes through the shared
  `PatternKey` / `Member` projection.** `Discriminated` admits those tags
  (they are in `Tag`), but they cannot key a mapped type, so the handler map is
@@ -228,10 +276,13 @@ The key is a separate call because `K` is inferred from its literal argument and
 #### Known issue
- A member's tag must be unique across the union; two members with the same tag
+- A tag need not be unique across the union. Two members sharing one is not a
-  collapse to a union under one handler. A tag colliding with its
+  soundness hole: they select a single runtime key, so one handler receiving
-  stringification (`true | "true"`) is rejected by the universe gate — see
+  their union is the only correct behavior — the key is simply not a
-  § Supported universes.
+  discriminant. The gate rejects only the distinct-value collision
  (`true | "true"`), where two values share a key and `Member` can no longer
  invert it; see § Supported universes. Pinned by the duplicate-tag tests in
  `src/tagged-union.test.ts`.
 ## Primitive universe
@@ -35,7 +35,8 @@ in
 [CONTRIBUTING.md § Development commands](../CONTRIBUTING.md#development-commands).
 `c8` uses V8 coverage, so the `--strip-types` source is instrumented without a
 build step, and the runner relies on the `.ts` import-extension convention (see
-[tooling.md](./tooling.md#source-imports-use-ts-extensions)).
+[tooling.md](./tooling.md#source-imports-use-ts-extensions)). CI gates that
 coverage at 100% (see [ci.md § Coverage threshold](./ci.md#coverage-threshold)).
 ## Handler arguments
@@ -198,6 +199,11 @@ the CLI is for manual inspection.
  delay is needed.
 - The server answers some requests with a string id (`client/registerCapability`);
  the client must tolerate `string | number` ids or the server stalls.
 - `LspSession.close()` sends `shutdown` and then closes stdin instead of
  sending `exit`. The TS 7 Go server's `handleExit` returns `io.EOF`, cancelling
  the background context while a watch update is still in flight, and logs a
  bare `context canceled` before exiting 1; EOF on stdin exits 0 with no output.
  The kill stays as a fallback for a server that does not exit.
 ## Known issues
@@ -196,6 +196,29 @@ joining the oxfmt-superseded rules already off.
  are part of the public API.
 - The narrower scope keeps the signal high without config-file boilerplate.
 ### `knip` does not list the library entry
 #### Decision (2026-09)
 `knip.json` declares `"entry": ["scripts/*.ts"]`; the library entry
 `src/index.ts` is not listed.
 #### Why
 - knip derives the public entry from `package.json` `exports` and resolves it to
  `src/index.ts` itself, so naming the file is a redundant pattern and
  `maintain:knip` reports it as a configuration hint.
 - Listing `scripts/*.ts` does not switch that derivation off: the scope stays
  clean with or without `dist/`, and with the barrel's own test removed.
 #### Rejected
 - `"src/index.ts"` in `entry`: it silenced an `unused files` report for the
  barrel, which the derivation above no longer produces; keeping it only adds a
  hint.
 - `paths` mapping `dist/index.*` back to `src/index.ts`: more config to model a
  relation knip already resolves.
 ### `maintain:outdated` ignores `@types/node`
 #### Decision (2026-09)
@@ -305,7 +328,7 @@ of a hand-rolled JSON-RPC client.
 #### Decision (2026-09)
-`@spences10/pi-lsp` is pinned to `0.0.46` and used read-only.
+`@spences10/pi-lsp` is pinned to `0.0.47` and used read-only.
 #### Why
@@ -96,7 +96,8 @@ aggregator.
 #### Why
- Both members create something real: a branch, a release.
+- Every member creates something real: a branch, a release, the compiled
  doc-tests.
 - It joined both lists in [CONTRIBUTING.md](../CONTRIBUTING.md) alongside its
  first members, so it could not go invisible the way the retired `use:` prefix
  did.
@@ -1,5 +1,5 @@
 {
    "$schema": "./node_modules/knip/schema.json",
-    "entry": ["scripts/*.ts"],
+    "entry": ["scripts/*.ts", "compat/*.ts"],
    "ignoreDependencies": ["@runwisp/pubv"]
 }
@@ -1,7 +1,7 @@
 {
    "name": "tiny-pattern-ts",
-    "version": "0.6.0",
+    "version": "0.9.0",
-    "description": "Pattern matching for TypeScript/ESM environments (F#-style, not regex)",
+    "description": "Exhaustive, type-safe pattern matching for TypeScript",
    "keywords": [
        "adt",
        "algebraic-data-types",
@@ -28,7 +28,8 @@
    "type": "module",
    "sideEffects": false,
    "imports": {
-        "#test-utils/*": "./src/util/__tests__/*"
+        "#test-utils/*": "./src/util/__tests__/*",
        "#test-tiny-pattern-ts": "./src/index.ts"
    },
    "exports": {
        ".": {
@@ -47,7 +48,8 @@
        "check:oxfmt": "oxfmt --check ${LEFTHOOK_FILES:-.}",
        "check:oxlint": "oxlint ${LEFTHOOK_FILES:-src scripts}",
        "check:tsc": "tsc",
-        "clean": "rm -rf dist coverage",
+        "clean": "rm -rf dist coverage src/doc-test/__generated__/*.test.ts",
        "create:doc-tests": "node --strip-types scripts/create-doc-tests.ts && sh -c 'oxfmt src/doc-test/__generated__/*.test.ts' && tsc -p src/doc-test/tsconfig.json",
        "fix": "npm run fix:oxlint && npm run fix:oxfmt",
        "fix:oxfmt": "oxfmt ${LEFTHOOK_FILES:-.}",
        "fix:oxlint": "oxlint --fix src scripts",
@@ -58,7 +60,10 @@
        "maintain:knip": "knip --include dependencies,exports,files",
        "maintain:outdated": "check-outdated --ignore-pre-releases --ignore-packages @types/node",
        "test": "npm run check:tsc && node --test --strip-types \"src/**/*.test.ts\"",
-        "test:ci": "c8 --reporter=text --reporter=lcov --reporter=html node --test --strip-types \"src/**/*.test.ts\"",
+        "test:ci": "npm run test:coverage && npm run test:doc",
        "test:compat": "npx --yes --package typescript@5.9.2 tsc --project compat/tsconfig.json",
        "test:coverage": "c8 --all --include \"src/**/*.ts\" --reporter=text --reporter=lcov --reporter=html --100 node --test --strip-types $(git ls-files 'src/*.test.ts')",
        "test:doc": "node --test --strip-types \"src/doc-test/__generated__/*.test.ts\"",
        "test:unit": "node --test --strip-types \"src/**/*.test.ts\"",
        "verify": "npm run check && npm run test:unit",
        "watch": "npm run watch:test",
@@ -83,7 +88,8 @@
        "expect-type": "1.4.0",
        "knip": "^6.34.0",
        "lefthook": "^2.1.12",
-        "oxfmt": "^0.68.0",
+        "mdast-util-from-markdown": "^2.0.3",
        "oxfmt": "^0.71.0",
        "oxlint": "^1.83.0",
        "oxlint-tsgolint": "^7.0.2001",
        "publint": "^0.3.24",
@@ -0,0 +1,445 @@
 import fs from "node:fs";
 import path from "node:path";
 import type { Code, Heading, Paragraph, PhrasingContent, Root } from "mdast";
 import { fromMarkdown } from "mdast-util-from-markdown";
 /**
 * Compile the TypeScript examples embedded in the prose docs into runnable
 * tests, so a doc fence that drifts from the API fails CI. Every ```ts fence in
 * the scanned Markdown becomes a `test(...)` in a generated
 * `src/doc-test/__generated__/<Name>.test.ts`; the file is typechecked by
 * `tsc` and executed by `node --test` exactly like a hand-written test.
 *
 * Usage: node --strip-types scripts/create-doc-tests.ts
 *
 * Why regenerate-then-typecheck instead of a lint plugin: development/docs.md §
 * Validating Markdown code fences. The repo is TypeScript 7 (the native
 * compiler), which does not expose the legacy JS compiler API, so the only
 * faithful check is to emit real `.ts` files and let the existing `check:tsc` /
 * `node --test` pipeline judge them.
 */
 /** The package's public entry, as spelled inside the examples. */
 const LIBRARY_SOURCE = "tiny-pattern-ts";
 /** Matches the library specifier (single- or double-quoted) inside an import. */
 const LIBRARY_SPECIFIER = new RegExp(
    `(?<quote>["'])${LIBRARY_SOURCE}\\k<quote>`,
    "g",
 );
 /**
 * A source specifier resolved to `src/index.ts` by `package.json#imports`.
 * Examples import `from "tiny-pattern-ts"`, which neither `node` nor `tsc`
 * resolves to source before `dist/` exists, so the generator rewrites it.
 */
 const PRELUDE_IMPORT_SOURCE = "#test-tiny-pattern-ts";
 /** Markdown files to scan, relative to the repo root. */
 const SOURCES = ["README.md", "CONTRIBUTING.md"];
 /** Directory the generated tests are written to (gitignored contents). */
 const OUTPUT_DIR = "src/doc-test/__generated__";
 /** Fences whose info-string matches one of these are compiled; others skipped. */
 const TYPESCRIPT_LANGS: ReadonlySet<string> = new Set(["ts", "typescript"]);
 /**
 * Modules the generated file already imports. An example that imports one of
 * these would collide with the prelude binding (duplicate `test` / `assert`), so
 * it is surfaced as a fatal error and the example is rewritten.
 */
 const PRELUDE_MODULES: ReadonlySet<string> = new Set([
    "node:assert",
    "node:test",
 ]);
 /** One line of the prelude every generated file starts with. */
 const PRELUDE_TEST = 'import { test } from "node:test";';
 const PRELUDE_ASSERT = 'import { strict as assert } from "node:assert";';
 /** Indentation applied to every fence body line inside the `test` callback. */
 const INDENT = "    ";
 const EMPTY = "";
 const INITIAL_COUNT = 0;
 const WRITE_INCREMENT = 1;
 const FIRST_ITEM = 0;
 const FAILURE_EXIT_CODE = 1;
 /** Strip all leading blank lines so the import scan starts at real content. */
 const LEADING_BLANK_LINES = /^(?:[ \t]*\r?\n)+/;
 /** Collapse the plain text of a title paragraph to one line. */
 const WHITESPACE = /\s+/g;
 /** Split an info-string like `ts title="x"` into its bare language. */
 const LANG_SEPARATOR = /\s+/;
 /** One leading `import` statement, including a multi-line named import. */
 const IMPORT_STATEMENT =
    /^import\s+(?:(?:type\s+)?[\w$*{}\s,]+?\s+from\s+)?["'][^"'\n]+["']\s*;?[ \t]*(?:\r?\n|$)/;
 /** `import { a, b } from "src";`, with an optional `type` keyword. */
 const NAMED_IMPORT =
    /^import\s+(?<isType>type\s+)?\{(?<specifiers>[^}]*)\}\s+from\s+["'](?<source>[^"']+)["']\s*;?$/;
 /** The module in a `… from "src"` statement. */
 const FROM_SOURCE = /from\s+["'](?<source>[^"']+)["']/;
 /** The module in a side-effect `import "src"` statement. */
 const SIDE_EFFECT_SOURCE = /^import\s+["'](?<source>[^"']+)["']/;
 type Block = Root["children"][number];
 class DocTestError extends Error {
    public constructor(message: string) {
        super(message);
        this.name = "DocTestError";
    }
 }
 /** Merge named specifiers that target the same module into one import. */
 interface NamedImportGroup {
    readonly source: string;
    readonly isType: boolean;
    readonly names: string[];
 }
 /** One compiled fence, ready to be wrapped in a `test(...)`. */
 interface DocCase {
    readonly title: string;
    readonly body: string;
 }
 /** Accumulator threaded through the Markdown walk. */
 interface BuilderState {
    readonly named: Map<string, NamedImportGroup>;
    readonly passthrough: Set<string>;
    readonly cases: DocCase[];
    title: string | undefined;
 }
 /** Read one capture group, tolerating the type-level `groups` optionality. */
 const groupOf = (match: RegExpExecArray, name: string): string | undefined => {
    const { groups } = match;
    return groups === undefined ? undefined : groups[name];
 };
 /** Recursively concatenate the literal text of a single inline node. */
 const inlineText = (node: PhrasingContent): string => {
    if ("value" in node) {
        return node.value;
    }
    if ("children" in node) {
        return node.children.map(inlineText).join(EMPTY);
    }
    return EMPTY;
 };
 /** Render a paragraph/heading node to its plain text for use as a test title. */
 const textOf = (node: Paragraph | Heading): string =>
    node.children.map(inlineText).join(EMPTY).replace(WHITESPACE, " ").trim();
 /**
 * Split a fence into its leading `import` statements and the executable body.
 * Only a contiguous run of imports at the very top is hoisted; anything after
 * the first non-import line stays in the body verbatim.
 */
 const splitImports = (code: string): { imports: string[]; body: string } => {
    const imports: string[] = [];
    let rest = code.replace(LEADING_BLANK_LINES, EMPTY);
    let match = IMPORT_STATEMENT.exec(rest);
    while (rest.startsWith("import") && match !== null) {
        const statement = match[FIRST_ITEM] ?? EMPTY;
        imports.push(statement.trim());
        rest = rest.slice(statement.length).replace(LEADING_BLANK_LINES, EMPTY);
        match = IMPORT_STATEMENT.exec(rest);
    }
    return { imports, body: rest.trim() };
 };
 /** The module an import statement points at, or `undefined` if unreadable. */
 const importSource = (statement: string): string | undefined => {
    const fromMatch = FROM_SOURCE.exec(statement);
    if (fromMatch !== null) {
        return groupOf(fromMatch, "source");
    }
    const sideEffectMatch = SIDE_EFFECT_SOURCE.exec(statement);
    return sideEffectMatch === null
        ? undefined
        : groupOf(sideEffectMatch, "source");
 };
 /**
 * Reject an example that imports a harness-provided module, then rewrite the
 * library specifier to the source alias so the emitted file resolves.
 */
 const normalizeImport = (statement: string): string => {
    const source = importSource(statement);
    if (source !== undefined && PRELUDE_MODULES.has(source)) {
        throw new DocTestError(
            `example imports "${source}", which the harness injects; remove the line:\n  ${statement.trim()}`,
        );
    }
    return statement.replace(
        LIBRARY_SPECIFIER,
        `$<quote>${PRELUDE_IMPORT_SOURCE}$<quote>`,
    );
 };
 /** Split the `{ a, b }` contents of a named import into trimmed specifiers. */
 const specifiersOf = (raw: string): string[] =>
    raw
        .split(",")
        .map((name) => name.trim())
        .filter((name) => name.length > INITIAL_COUNT);
 /** Add named specifiers to the group for `(isType, source)`, merging. */
 const addNamedImport = (
    named: Map<string, NamedImportGroup>,
    ref: Pick<NamedImportGroup, "source" | "isType">,
    names: readonly string[],
 ): void => {
    const key = `${ref.isType ? "type:" : "value:"}${ref.source}`;
    const existing = named.get(key);
    if (existing === undefined) {
        named.set(key, {
            source: ref.source,
            isType: ref.isType,
            names: [...names],
        });
        return;
    }
    for (const name of names) {
        if (!existing.names.includes(name)) {
            existing.names.push(name);
        }
    }
 };
 /** Sort each hoisted import into a merged named group or a passthrough set. */
 const collectImports = (
    statements: readonly string[],
    named: Map<string, NamedImportGroup>,
    passthrough: Set<string>,
 ): void => {
    for (const raw of statements) {
        const statement = normalizeImport(raw);
        const match = NAMED_IMPORT.exec(statement);
        if (match === null) {
            passthrough.add(statement);
        } else {
            addNamedImport(
                named,
                {
                    source: groupOf(match, "source") ?? EMPTY,
                    isType: groupOf(match, "isType") !== undefined,
                },
                specifiersOf(groupOf(match, "specifiers") ?? EMPTY),
            );
        }
    }
 };
 /** Render the hoisted imports: merged named groups first, then the rest. */
 const renderImports = (
    named: ReadonlyMap<string, NamedImportGroup>,
    passthrough: ReadonlySet<string>,
 ): string[] => {
    const lines: string[] = [];
    for (const group of named.values()) {
        const keyword = group.isType ? "import type" : "import";
        lines.push(
            `${keyword} { ${group.names.join(", ")} } from "${group.source}";`,
        );
    }
    for (const statement of passthrough) {
        lines.push(statement);
    }
    return lines;
 };
 /** The bare language of a fence's info-string, e.g. `ts` in `ts title="x"`. */
 const typescriptLang = (code: Code): string =>
    (code.lang ?? EMPTY).trim().split(LANG_SEPARATOR)[FIRST_ITEM] ?? EMPTY;
 /** Set the title a following fence will inherit. */
 const handleTitleNode = (
    state: BuilderState,
    node: Paragraph | Heading,
 ): void => {
    state.title = textOf(node);
 };
 /** Report and reset a non-TS fence that was skipped. */
 const skipFence = (state: BuilderState, name: string, code: Code): void => {
    process.stderr.write(
        `${name}: skipped non-TypeScript fence (lang="${code.lang ?? EMPTY}")\n`,
    );
    state.title = undefined;
 };
 /** Compile a described TS fence into a case, or reject it. */
 const compileFence = (
    state: BuilderState,
    name: string,
    code: Code,
 ): DocCase => {
    const { title } = state;
    if (title === undefined) {
        const lang = typescriptLang(code);
        throw new DocTestError(
            `${name}: a \`\`\`${lang} fence has no preceding paragraph or ` +
                `heading to use as its test title — describe the example.`,
        );
    }
    const { imports, body } = splitImports(code.value);
    collectImports(imports, state.named, state.passthrough);
    return { title, body };
 };
 /** Compile one TS fence into a case, or reject/skip it. */
 const handleCodeNode = (
    state: BuilderState,
    name: string,
    code: Code,
 ): void => {
    const lang = typescriptLang(code);
    if (!TYPESCRIPT_LANGS.has(lang)) {
        skipFence(state, name, code);
        return;
    }
    state.cases.push(compileFence(state, name, code));
 };
 /** Route one Markdown block, preserving the "described fence" invariant. */
 const handleNode = (state: BuilderState, name: string, node: Block): void => {
    if (node.type === "paragraph" || node.type === "heading") {
        handleTitleNode(state, node);
    } else if (node.type === "code") {
        handleCodeNode(state, name, node);
    } else {
        // Only a paragraph/heading introduces a fence; any other block breaks
        // the "immediately preceded" chain.
        state.title = undefined;
    }
 };
 /** Walk one Markdown file and collect its cases and hoisted imports. */
 const parseDoc = (
    name: string,
    markdown: string,
 ): Omit<BuilderState, "title"> => {
    const state: BuilderState = {
        named: new Map(),
        passthrough: new Set(),
        cases: [],
        title: undefined,
    };
    for (const node of fromMarkdown(markdown).children) {
        handleNode(state, name, node);
    }
    return {
        named: state.named,
        passthrough: state.passthrough,
        cases: state.cases,
    };
 };
 /** Indent a fence body one level for the body of the `test` callback. */
 const indentBody = (body: string): string =>
    body
        .split("\n")
        .map((line) =>
            line.length > INITIAL_COUNT ? `${INDENT}${line}` : line,
        )
        .join("\n");
 /** Wrap one compiled fence in an executed `test(...)`. */
 const renderCase = (docCase: DocCase): string => {
    const indented = indentBody(docCase.body);
    return `test(${JSON.stringify(docCase.title)}, () => {\n${indented}\n});`;
 };
 /** Build the prelude + hoisted imports that top every generated file. */
 const buildHeader = (name: string, imports: readonly string[]): string[] => {
    const header = [
        "// Generated by scripts/create-doc-tests.ts — do not edit by hand.",
        `// Source: ${name}`,
        EMPTY,
        PRELUDE_TEST,
        PRELUDE_ASSERT,
    ];
    if (imports.length > INITIAL_COUNT) {
        header.push(EMPTY, ...imports);
    }
    header.push(EMPTY);
    return header;
 };
 /** Turn one Markdown file into the source of its generated test file. */
 const buildTestFile = (name: string, markdown: string): string => {
    const parsed = parseDoc(name, markdown);
    if (parsed.cases.length === INITIAL_COUNT) {
        return EMPTY;
    }
    const imports = renderImports(parsed.named, parsed.passthrough);
    const header = buildHeader(name, imports);
    const blocks = parsed.cases.map(renderCase);
    return `${header.join("\n")}\n${blocks.join("\n\n")}\n`;
 };
 /** Map a source Markdown path to its generated test path under OUTPUT_DIR. */
 const outputFor = (source: string): string =>
    path.join(
        OUTPUT_DIR,
        `${path.basename(source, path.extname(source))}.test.ts`,
    );
 /** Write a generated file when there is content, else clear a stale one. */
 const writeIfAny = (dest: string, name: string, out: string): boolean => {
    if (out === EMPTY) {
        process.stderr.write(`${name}: no TypeScript fences\n`);
        fs.rmSync(dest, { force: true });
        return false;
    }
    fs.writeFileSync(dest, out);
    return true;
 };
 /** Generate the doc-test for one source; return whether a file was written. */
 const generateFor = (root: string, source: string): boolean => {
    const abs = path.join(root, source);
    if (!fs.existsSync(abs)) {
        process.stderr.write(`skipped missing ${source}\n`);
        return false;
    }
    const name = path.basename(source);
    const out = buildTestFile(name, fs.readFileSync(abs, "utf8"));
    return writeIfAny(path.join(root, outputFor(source)), name, out);
 };
 const main = (): void => {
    const root = process.cwd();
    fs.mkdirSync(path.join(root, OUTPUT_DIR), { recursive: true });
    let written = INITIAL_COUNT;
    for (const source of SOURCES) {
        if (generateFor(root, source)) {
            written += WRITE_INCREMENT;
        }
    }
    process.stdout.write(
        `created doc-tests for ${written} file(s) under ${OUTPUT_DIR}/\n`,
    );
 };
 try {
    main();
 } catch (error) {
    if (error instanceof DocTestError) {
        process.stderr.write(`${error.message}\n`);
        process.exitCode = FAILURE_EXIT_CODE;
    } else {
        throw error;
    }
 }
@@ -0,0 +1,12 @@
 # Generated doc-tests
 `__generated__/` is the output of [`scripts/create-doc-tests.ts`](../../scripts/create-doc-tests.ts):
 each `*.test.ts` is compiled from the ```ts fences in a prose doc and is
 **gitignored**, not edited by hand. `npm run create:doc-tests` regenerates them,
 formats them with `oxfmt`, then type-checks them with
 [`tsconfig.json`](./tsconfig.json) — the scoped config that relaxes
 `noUnusedLocals` so example-only locals (and the injected `assert`) compile.
 CI regenerates them in an explicit step before `test:ci`; the fast local `test` /
 `verify` tiers do not. See [development/docs.md](../../development/docs.md) for why the
 generator is shaped this way.
@@ -0,0 +1,10 @@
 {
    "extends": "../../tsconfig.json",
    "compilerOptions": {
        "noEmit": true,
        "noUnusedLocals": false,
        "noUnusedParameters": false
    },
    "include": ["__generated__/**/*.test.ts"],
    "exclude": []
 }
@@ -0,0 +1,27 @@
 import { strict as assert } from "node:assert";
 import { test } from "node:test";
 import { expectTypeOf } from "expect-type";
 import {
    getPrimitiveUnionMatcher,
    getPrimitiveUnionMatcherW,
    getTaggedUnionMatcher,
    getTaggedUnionMatcherW,
 } from "./index.ts";
 // The published entry point is the barrel (`package.json` exports
 // `./dist/index.js`), so every factory must be reachable from here. Importing it
 // also loads the module, which is what lets c8's `--all` measure it — see
 // development/ci.md § Coverage threshold.
 test("index: the public entry point re-exports every matcher factory", () => {
    // Assert
    expectTypeOf(getPrimitiveUnionMatcher).toBeFunction();
    assert.equal(typeof getPrimitiveUnionMatcher, "function");
    expectTypeOf(getPrimitiveUnionMatcherW).toBeFunction();
    assert.equal(typeof getPrimitiveUnionMatcherW, "function");
    expectTypeOf(getTaggedUnionMatcher).toBeFunction();
    assert.equal(typeof getTaggedUnionMatcher, "function");
    expectTypeOf(getTaggedUnionMatcherW).toBeFunction();
    assert.equal(typeof getTaggedUnionMatcherW, "function");
 });
@@ -1,3 +1,11 @@
 /**
 * The public entry point of `tiny-pattern-ts`.
 *
 * Exports the four matcher factories and nothing else; the builder types they
 * return and the rest of `src/` are implementation detail.
 *
 * @module
 */
 export {
    getPrimitiveUnionMatcher,
    getPrimitiveUnionMatcherW,
@@ -1,3 +1,4 @@
 /* c8 ignore start -- types only: the module has no runtime image to cover */
 import type { IsLiteral, IsNever, ValueOf } from "type-fest";
 // The primitive-union and tagged-union matchers differ in their universe, but the
@@ -62,11 +63,11 @@ export type Collisions<T> = Extract<T, Stringified<T>>;
 // universe is a finite union of literals with no value/stringification
 // collision: only then can `PatternKey` be inverted unambiguously.
 export type UnsupportedReason<T extends Matchable> =
-    IsLiteral<PatternKey<T>> extends false
+    IsLiteral<PatternKey<T>> extends true
-        ? "broad types like string, number and template literals are not supported"
+        ? [Collisions<T>] extends [never]
        : [Collisions<T>] extends [never]
            ? never
-          : `a value and its stringification collide. Value is "${Collisions<T> & string}"`;
+            : `a value and its stringification collide. Value is "${Collisions<T> & string}"`
        : "broad types like string, number and template literals are not supported";
 // The diagnostic for an unsupported universe. Extends `HandlerMap` so the
 // implementation's `handlers: HandlerMap` stays assignable when the gate is
@@ -528,6 +528,8 @@ test("getPrimitiveUnionMatcher: a broad universe is rejected", () => {
    getPrimitiveUnionMatcher<number>()({ 1: () => 1 });
    // @ts-expect-error a template literal is open
    getPrimitiveUnionMatcher<`a${string}`>()({ a: () => 1 });
    // @ts-expect-error a literal mixed with a template literal is open
    getPrimitiveUnionMatcher<"a" | `x-${number}`>()({ a: () => 1 });
    // @ts-expect-error `string | boolean` is open because of `string`
    getPrimitiveUnionMatcher<string | boolean>()({ true: () => 1 });
    // @ts-expect-error the widening factory rejects broad universes too
@@ -96,9 +96,50 @@ const dispatch =
            shape as never,
        );
 /**
 * Create a matcher for a finite primitive universe, with one common return
 * type.
 *
 * Use it when the value itself is the union (`"yes" | "no"`) and every handler
 * returns the same type.
 *
 * The returned builder takes a handler map keyed by the members; supplying a
 * second fallback argument allows a partial map and receives the unhandled
 * remainder.
 *
 * @typeParam T - The finite universe of primitive members to match.
 * @returns A builder for the handler map, or the handler map plus a fallback.
 * @example
 *     const matchAnswer = getPrimitiveUnionMatcher<"yes" | "no">();
 *     const describe = matchAnswer({
 *         yes: () => "agreed",
 *         no: () => "declined",
 *     });
 *     describe("yes"); // "agreed"
 */
 export const getPrimitiveUnionMatcher = <
    T extends Matchable,
 >(): PrimitiveUnionMatcherStrict<T> => dispatch;
 /**
 * Create a matcher for a finite primitive universe whose return type is the
 * union of every handler's return type.
 *
 * Use it when the value itself is the union (`"yes" | "no"`) and the handlers
 * return different types. The `W` (widening) counterpart of
 * {@link getPrimitiveUnionMatcher}; the universe constraint and the optional
 * fallback are identical.
 *
 * @typeParam T - The finite universe of primitive members to match.
 * @returns A builder for the handler map, or the handler map plus a fallback.
 * @example
 *     const matchReply = getPrimitiveUnionMatcherW<"yes" | "no">();
 *     const reply = matchReply({
 *         yes: () => 1,
 *         no: () => "declined",
 *     });
 *     // reply: (shape: "yes" | "no") => number | string
 */
 export const getPrimitiveUnionMatcherW = <
    T extends Matchable,
 >(): PrimitiveUnionMatcherWidening<T> => dispatch;
@@ -892,6 +892,434 @@ test("getTaggedUnionMatcher: the maximum illegal tag universe is rejected", () =
    });
 });
 // ============================================================================
 // Property unions
 // ============================================================================
 test("property union: narrows each handler", () => {
    // Arrange — one shape, not a union of variants; `color` is a union.
    interface Paint {
        readonly color: "red" | "green" | "blue";
        readonly value: number;
    }
    const factory = getTaggedUnionMatcher<Paint>()("color");
    // Act
    const describe = factory({
        red: (s) => {
            expectTypeOf(s).toEqualTypeOf<{
                readonly color: "red";
                readonly value: number;
            }>();
            assert.equal(s.color, "red");
            return s.value;
        },
        green: (s) => {
            expectTypeOf(s).toEqualTypeOf<{
                readonly color: "green";
                readonly value: number;
            }>();
            assert.equal(s.color, "green");
            return s.value;
        },
        blue: (s) => {
            expectTypeOf(s).toEqualTypeOf<{
                readonly color: "blue";
                readonly value: number;
            }>();
            assert.equal(s.color, "blue");
            return s.value;
        },
    });
    // Assert
    expectTypeOf(describe).toEqualTypeOf<(shape: Paint) => number>();
    assert.equal(describe({ color: "red", value: 1 }), 1);
    assert.equal(describe({ color: "green", value: 2 }), 2);
    assert.equal(describe({ color: "blue", value: 3 }), 3);
 });
 test("property union: a fallback receives the unhandled values", () => {
    // Arrange
    interface Paint {
        color: "red" | "green" | "blue";
        value: number;
    }
    const factory = getTaggedUnionMatcher<Paint>()("color");
    // Act
    const describe = factory(
        {
            red: (s) => {
                expectTypeOf(s).toEqualTypeOf<{
                    color: "red";
                    value: number;
                }>();
                assert.equal(s.color, "red");
                return 1 as const;
            },
        },
        (s) => {
            expectTypeOf(s).toEqualTypeOf<{
                color: "green" | "blue";
                value: number;
            }>();
            assert.ok(s.color === "green" || s.color === "blue");
            return 2 as const;
        },
    );
    // Assert
    expectTypeOf(describe).toEqualTypeOf<(shape: Paint) => 1 | 2>();
    assert.equal(describe({ color: "red", value: 1 }), 1);
    assert.equal(describe({ color: "green", value: 2 }), 2);
    assert.equal(describe({ color: "blue", value: 3 }), 2);
 });
 test("property union: a numeric property narrows each handler", () => {
    // Arrange
    interface Version {
        code: 1 | 2 | 3;
        value: number;
    }
    const factory = getTaggedUnionMatcher<Version>()("code");
    // Act
    const describe = factory({
        1: (s) => {
            expectTypeOf(s).toEqualTypeOf<{ code: 1; value: number }>();
            assert.equal(s.code, 1);
            return 1;
        },
        2: (s) => {
            expectTypeOf(s).toEqualTypeOf<{ code: 2; value: number }>();
            assert.equal(s.code, 2);
            return 2;
        },
        3: (s) => {
            expectTypeOf(s).toEqualTypeOf<{ code: 3; value: number }>();
            assert.equal(s.code, 3);
            return 3;
        },
    });
    // Assert
    expectTypeOf(describe).toEqualTypeOf<(shape: Version) => number>();
    assert.equal(describe({ code: 1, value: 10 }), 1);
    assert.equal(describe({ code: 2, value: 20 }), 2);
    assert.equal(describe({ code: 3, value: 30 }), 3);
 });
 test("property union: an optional property is required for a defined tag", () => {
    // Arrange — `type?` is `"x" | undefined`; both handlers must be typed,
    // and matching `"x"` proves the key is present, so `type` becomes required.
    interface Maybe {
        type?: "x";
        value: number;
    }
    const factory = getTaggedUnionMatcher<Maybe>()("type");
    // Act
    const describe = factory({
        x: (s) => {
            expectTypeOf(s).toEqualTypeOf<{ type: "x"; value: number }>();
            assert.equal(s.type, "x");
            return 1;
        },
        undefined: (s) => {
            // The input only allows absence (exactOptionalPropertyTypes), so
            // the `undefined` tag narrows the key to never-present.
            expectTypeOf(s).toEqualTypeOf<{ type?: never; value: number }>();
            assert.equal(s.type, undefined);
            return 2;
        },
    });
    // Assert
    expectTypeOf(describe).toEqualTypeOf<(shape: Maybe) => number>();
    assert.equal(describe({ type: "x", value: 1 }), 1);
    assert.equal(describe({ value: 2 }), 2);
 });
 test("property union: an optional fallback receives the undefined tag", () => {
    // Arrange
    interface Maybe {
        type?: "x";
        value: number;
    }
    const factory = getTaggedUnionMatcher<Maybe>()("type");
    // Act
    const describe = factory(
        {
            x: (s) => {
                expectTypeOf(s).toEqualTypeOf<{ type: "x"; value: number }>();
                assert.equal(s.type, "x");
                return 1 as const;
            },
        },
        (s) => {
            expectTypeOf(s).toEqualTypeOf<{
                type?: never;
                value: number;
            }>();
            assert.equal(s.type, undefined);
            return 2 as const;
        },
    );
    // Assert
    expectTypeOf(describe).toEqualTypeOf<(shape: Maybe) => 1 | 2>();
    assert.equal(describe({ type: "x", value: 1 }), 1);
    assert.equal(describe({ value: 2 }), 2);
 });
 test("property union: explicit `undefined` stays distinct from absence", () => {
    // Arrange — `type?: "x" | undefined` admits an explicit `undefined`, unlike
    // the bare optional above; the two representations must not be conflated.
    interface Explicit {
        type?: "x" | undefined;
        value: number;
    }
    const factory = getTaggedUnionMatcher<Explicit>()("type");
    // Act
    const describe = factory({
        x: (s) => {
            expectTypeOf(s).toEqualTypeOf<{ type: "x"; value: number }>();
            assert.equal(s.type, "x");
            return 1;
        },
        undefined: (s) => {
            expectTypeOf(s).toEqualTypeOf<{
                type?: undefined;
                value: number;
            }>();
            assert.equal(s.type, undefined);
            return 2;
        },
    });
    // Assert
    expectTypeOf(describe).toEqualTypeOf<(shape: Explicit) => number>();
    assert.equal(describe({ type: "x", value: 1 }), 1);
    assert.equal(describe({ type: undefined, value: 2 }), 2);
    assert.equal(describe({ value: 3 }), 2);
 });
 test("property union: a union-valued member is split, not dropped", () => {
    // A member whose tag is itself a union sits beside a singleton-tag member.
    // Selecting members with `Extract<T, Record<K, V>>` keeps only the
    // singleton; the distributive narrowing must keep both sides of the
    // union-valued member.
    // Arrange
    type Mixed = { kind: "a"; a: number } | { kind: "a" | "b"; b: number };
    const factory = getTaggedUnionMatcher<Mixed>()("kind");
    // Act
    const describe = factory({
        a: (s) => {
            expectTypeOf(s).toEqualTypeOf<
                { kind: "a"; a: number } | { kind: "a"; b: number }
            >();
            assert.equal(s.kind, "a");
            return 1;
        },
        b: (s) => {
            expectTypeOf(s).toEqualTypeOf<{ kind: "b"; b: number }>();
            assert.equal(s.kind, "b");
            return 2;
        },
    });
    // Assert
    expectTypeOf(describe).toEqualTypeOf<(shape: Mixed) => number>();
    assert.equal(describe({ kind: "a", a: 1 }), 1);
    assert.equal(describe({ kind: "a", b: 2 }), 1);
    assert.equal(describe({ kind: "b", b: 3 }), 2);
 });
 test("property union: getTaggedUnionMatcherW widens the returns", () => {
    // Arrange
    interface Paint {
        color: "red" | "green" | "blue";
        value: number;
    }
    const factory = getTaggedUnionMatcherW<Paint>()("color");
    // Act
    const describe = factory({
        red: (s) => {
            expectTypeOf(s).toEqualTypeOf<{ color: "red"; value: number }>();
            assert.equal(s.color, "red");
            return "r" as const;
        },
        green: (s) => {
            expectTypeOf(s).toEqualTypeOf<{ color: "green"; value: number }>();
            assert.equal(s.color, "green");
            return 1 as const;
        },
        blue: (s) => {
            expectTypeOf(s).toEqualTypeOf<{ color: "blue"; value: number }>();
            assert.equal(s.color, "blue");
            return true as const;
        },
    });
    // Assert
    expectTypeOf(describe).toEqualTypeOf<(shape: Paint) => "r" | 1 | true>();
    assert.equal(describe({ color: "red", value: 1 }), "r");
    assert.equal(describe({ color: "green", value: 2 }), 1);
    assert.equal(describe({ color: "blue", value: 3 }), true);
 });
 test("property union: enforces its contract", () => {
    // Arrange
    interface Paint {
        color: "red" | "green" | "blue";
        value: number;
    }
    const factory = getTaggedUnionMatcher<Paint>()("color");
    // Act / Assert — the calls below must not compile
    factory({
        red: () => 1,
        green: () => 2,
        blue: () => 3,
        // @ts-expect-error `yellow` is not a value of `color`
        yellow: () => 4,
    });
    // @ts-expect-error a gap without a fallback is not exhaustive
    factory({ red: () => 1 });
    // @ts-expect-error a fallback is redundant once every value is handled
    factory({ red: () => 1, green: () => 2, blue: () => 3 }, () => 0);
 });
 test("property union: a broad value in the union is rejected", () => {
    // Arrange — the property is a union, but not a finite union of literals: a
    // template literal cannot be proven exhaustive.
    interface Template {
        kind: "a" | `x-${number}`;
        a: number;
    }
    // Act / Assert — the calls below must not compile
    // @ts-expect-error a template literal is not a finite literal
    getTaggedUnionMatcher<Template>()("kind")({ a: () => 1 });
    // @ts-expect-error the widening factory rejects broad values too
    getTaggedUnionMatcherW<Template>()("kind")({ a: () => 1 });
 });
 test("property union: a value/stringification collision is rejected", () => {
    // Arrange — one property holding both a value and its stringification; both
    // would key the same handler.
    interface BooleanColliding {
        kind: "true" | true;
        a: number;
    }
    interface NumericColliding {
        kind: "1" | 1;
        a: number;
    }
    // Act / Assert — the calls below must not compile
    // @ts-expect-error `true` collides with the string tag `"true"`
    getTaggedUnionMatcher<BooleanColliding>()("kind")({ true: () => 1 });
    // @ts-expect-error the number tag `1` collides with the string tag `"1"`
    getTaggedUnionMatcher<NumericColliding>()("kind")({ "1": () => 1 });
    // @ts-expect-error the widening factory rejects the collision too
    getTaggedUnionMatcherW<BooleanColliding>()("kind")({ true: () => 1 });
 });
 test("property union: a collision is rejected beside legal members", () => {
    // Arrange — the map is otherwise complete, so only the collision can make
    // it invalid.
    interface Colliding {
        kind: "true" | true | "a";
        a: number;
    }
    // Act / Assert — the call below must not compile
    // @ts-expect-error `true` collides with the string tag `"true"`
    getTaggedUnionMatcher<Colliding>()("kind")({ true: () => 1, a: () => 2 });
 });
 // ============================================================================
 // Duplicate tags
 // ============================================================================
 // A tag is a discriminant only while it is unique across the union. Two members
 // may still share one: the tag set `Tags<T, K>` dedupes, so a single handler is
 // exhaustive for both and receives their union. This is the documented known
 // issue (development/library.md § Tagged-union matcher); the tests below pin the
 // behavior so it cannot change silently.
 test("duplicate tag: members sharing a tag collapse to one handler", () => {
    // Arrange — `kind` is not a true discriminant: both members carry `"a"`.
    interface First {
        readonly kind: "a";
        readonly first: number;
    }
    interface Second {
        readonly kind: "a";
        readonly second: string;
    }
    type Clashing = First | Second;
    const factory = getTaggedUnionMatcher<Clashing>()("kind");
    // Act
    const pick = factory({
        a: (s) => {
            // The shared tag cannot be split, so the handler sees both members.
            expectTypeOf(s).toEqualTypeOf<First | Second>();
            assert.equal(s.kind, "a");
            return "first" in s ? s.first : s.second.length;
        },
    });
    // Assert — the one `a` key is exhaustive and both members reach it.
    expectTypeOf(pick).toEqualTypeOf<(shape: Clashing) => number>();
    assert.equal(pick({ kind: "a", first: 1 }), 1);
    assert.equal(pick({ kind: "a", second: "abc" }), 3);
 });
 test("duplicate tag: handling one tag consumes every member that shares it", () => {
    // Arrange — `"a"` selects two members; `"c"` selects one.
    type Mixed =
        | { readonly kind: "a"; readonly a: number }
        | { readonly kind: "a"; readonly b: string }
        | { readonly kind: "c"; readonly c: boolean };
    const factory = getTaggedUnionMatcher<Mixed>()("kind");
    // Act
    const describe = factory(
        {
            a: (s) => {
                expectTypeOf(s).toEqualTypeOf<
                    | { readonly kind: "a"; readonly a: number }
                    | { readonly kind: "a"; readonly b: string }
                >();
                return 1 as const;
            },
        },
        (s) => {
            // Handling `"a"` removes both of its members, not just one.
            expectTypeOf(s).toEqualTypeOf<{
                readonly kind: "c";
                readonly c: boolean;
            }>();
            assert.equal(s.kind, "c");
            return 2 as const;
        },
    );
    // Assert
    expectTypeOf(describe).toEqualTypeOf<(shape: Mixed) => 1 | 2>();
    assert.equal(describe({ kind: "a", a: 1 }), 1);
    assert.equal(describe({ kind: "a", b: "x" }), 1);
    assert.equal(describe({ kind: "c", c: true }), 2);
 });
 // ============================================================================
 // Dispatch — runtime behavior
 // ============================================================================
@@ -935,6 +1363,7 @@ interface LabelsProbe {
    readonly tail?: string;
    readonly typeName?: string;
    readonly typeSource?: string;
    readonly key?: string;
 }
 const labelsFor = ({
@@ -944,6 +1373,7 @@ const labelsFor = ({
    tail = "",
    typeName = "Shape",
    typeSource = SHAPE_SOURCE,
    key = "kind",
 }: LabelsProbe): Promise<readonly string[]> => {
    const session = new LspSession(REPO_ROOT);
    const target: CompletionTarget = {
@@ -951,7 +1381,7 @@ const labelsFor = ({
        source: [
            `import { ${factory} } from "./index.ts";`,
            typeSource,
-            `const m = ${factory}<${typeName}>()("kind")({`,
+            `const m = ${factory}<${typeName}>()("${key}")({`,
            body,
            `}${tail});`,
            "",
@@ -1038,6 +1468,91 @@ test("autocomplete: boolean and nullish tags are offered by name", () => {
    });
 });
 // A single shape whose property is a union: the popup is still keyed by the
 // property's values, not by the members of a union type.
 const PALETTE_SOURCE = `interface Palette { color: "red" | "green" | "blue"; value: number }`;
 const FLAG_SOURCE = `interface Flag { kind: true | false | null | undefined; a: number }`;
 test("property union: autocomplete offers the property's values", () => {
    // Arrange
    const name = "property_union_fresh";
    // Act
    const labels = labelsFor({
        name,
        factory: "getTaggedUnionMatcher",
        typeName: "Palette",
        typeSource: PALETTE_SOURCE,
        key: "color",
        body: "    /*COMPLETE*/",
    });
    // Assert
    return labels.then((result) => {
        assert.deepEqual([...result], ["blue", "green", "red"]);
    });
 });
 test("property union: autocomplete drops a handled value", () => {
    // Arrange
    const name = "property_union_after_key";
    // Act
    const labels = labelsFor({
        name,
        factory: "getTaggedUnionMatcher",
        typeName: "Palette",
        typeSource: PALETTE_SOURCE,
        key: "color",
        body: "    red: () => 1,\n    /*COMPLETE*/",
    });
    // Assert
    return labels.then((result) => {
        assert.deepEqual([...result], ["blue", "green"]);
    });
 });
 test("property union: autocomplete makes the remaining values optional with a fallback", () => {
    // Arrange
    const name = "property_union_with_fallback";
    // Act
    const labels = labelsFor({
        name,
        factory: "getTaggedUnionMatcher",
        typeName: "Palette",
        typeSource: PALETTE_SOURCE,
        key: "color",
        body: "    /*COMPLETE*/",
        tail: ", () => 0",
    });
    // Assert
    return labels.then((result) => {
        assert.deepEqual([...result], ["blue?", "green?", "red?"]);
    });
 });
 test("property union: autocomplete offers boolean and nullish values by name", () => {
    // Arrange
    const name = "property_union_boolean_nullish";
    // Act
    const labels = labelsFor({
        name,
        factory: "getTaggedUnionMatcher",
        typeName: "Flag",
        typeSource: FLAG_SOURCE,
        body: "    /*COMPLETE*/",
    });
    // Assert
    return labels.then((result) => {
        assert.deepEqual([...result], ["false", "null", "true", "undefined"]);
    });
 });
 // `Mixed` has two common keys, but `id`'s value is not a tag, so only `kind`
 // may serve as the discriminant.
 const MIXED_SOURCE = `type Mixed = { id: Date; kind: "a"; a: number } | { id: Date; kind: "b"; b: number };`;
@@ -1,4 +1,4 @@
-import type { Exact, UnknownRecord } from "type-fest";
+import type { Exact, SetRequired, UnknownRecord } from "type-fest";
 import type {
    HandlerMap,
@@ -28,34 +28,49 @@ type Discriminated<T extends object> = {
    [K in keyof T]: T[K] extends Matchable ? K : never;
 }[keyof T];
-// The member(s) of `T` tagged `V`. `Extract` distributes over the union, so a
+// The member(s) of `T` narrowed to the tag(s) `V`. Distributes over `T`, so a
 // duplicated tag maps to a union of members rather than silently dropping one.
 // A member whose `K` cannot take `V` drops out; the rest have `K` narrowed to
 // `Extract<T[K], V>`. `T[K] extends V` keeps an exact member (a discriminated
 // union's declared interface) untouched, so only a property that is itself a
 // union — or optional — goes through the mapped form. `SetRequired` makes `K`
 // required unless `V` admits `undefined`: a defined tag yields `{ type: "x" }`,
 // while the `undefined` tag keeps `{ type?: never }` (absence) or
 // `{ type?: undefined }` when the property admits an explicit `undefined`.
 // Keyed by `PatternKey`, so `boolean`/`null`/`undefined` tags can key a mapped
-// type; `Member` inverts the projection against the tag set to recover the
+// type; `Member` inverts the projection against the tag set to recover the tag.
-// member.
+type Narrowed<T extends object, K extends keyof T, V> = T extends object
    ? [Extract<T[K], V>] extends [never]
        ? never
        : T[K] extends V
          ? T
          : undefined extends V
            ? { [P in keyof T]: P extends K ? Extract<T[P], V> : T[P] }
            : SetRequired<
                  { [P in keyof T]: P extends K ? Extract<T[P], V> : T[P] },
                  K
              >
    : never;
 type MapTaggedUnion<T extends object, K extends keyof T> = {
-    [P in PatternKey<Tags<T, K>>]: Extract<T, Record<K, Member<Tags<T, K>, P>>>;
+    [P in PatternKey<Tags<T, K>>]: Narrowed<T, K, Member<Tags<T, K>, P>>;
 };
 type Handlers<T extends object, K extends keyof T, R> = {
    [P in PatternKey<Tags<T, K>>]: UnaryFn<MapTaggedUnion<T, K>[P], R>;
 };
-// The members `Handled` covers. Mapping over the projected keys keeps every
+// The tag values whose `PatternKey` is handled.
-// index within `MapTaggedUnion`'s keys, and the conditional drops a stray key
+type HandledTags<T extends object, K extends keyof T, Handled> = Member<
-// outside `T` so it cannot widen the remainder. The remainder is
+    Tags<T, K>,
-// `Exclude<T, …>`, mirroring the primitive-union matcher's
+    keyof Handled
-// `Exclude<T, Member<T, keyof Handled>>`.
+>;
 type HandledMembers<T extends object, K extends keyof T, Handled> = {
    [P in PatternKey<Tags<T, K>>]: P extends keyof Handled
        ? MapTaggedUnion<T, K>[P]
        : never;
 }[PatternKey<Tags<T, K>>];
 // The fallback is a *second argument*, not a property of the handler map, so
-// its parameter can be the remainder the map left uncovered. See development/library.md.
+// its parameter can be the remainder the map left uncovered: the members
 // narrowed to the tags the map did not handle. See development/library.md.
 type Fallback<T extends object, K extends keyof T, Handled, R> = UnaryFn<
-    Exclude<T, HandledMembers<T, K, Handled>>,
+    Narrowed<T, K, Exclude<Tags<T, K>, HandledTags<T, K, Handled>>>,
    R
 >;
@@ -137,10 +152,56 @@ const dispatch =
        );
    };
 /**
 * Create a matcher for a discriminated union, with one common return type.
 *
 * Use it when the value is an object discriminated by a property
 * (`{ kind: "circle" } | { kind: "square" }`) and every handler returns the
 * same type.
 *
 * The first call fixes the union `T`; the returned function takes the
 * discriminant property's name (`K`, restricted to properties whose values are
 * tags), and that returns the handler-map builder. Supplying a second fallback
 * argument to the builder allows a partial map and receives the members whose
 * tag was not handled. A `boolean`, `null` or `undefined` tag is keyed by its
 * stringified form (`true` -> `"true"`); see README § Caveats.
 *
 * @typeParam T - The discriminated-union type to match.
 * @returns A function that takes the discriminant property's name.
 * @example
 *     type Shape =
 *         | { kind: "circle"; radius: number }
 *         | { kind: "square"; side: number };
 *
 *     const matchShape = getTaggedUnionMatcher<Shape>()("kind");
 *     const area = matchShape({
 *         circle: (s) => Math.PI * s.radius ** 2,
 *         square: (s) => s.side ** 2,
 *     });
 */
 export const getTaggedUnionMatcher = <
    T extends object,
 >(): TaggedUnionMatcherFactory<T> => dispatch;
 /**
 * Create a matcher for a discriminated union whose return type is the union of
 * every handler's return type.
 *
 * Use it when the value is a discriminated object and the handlers return
 * different types. The `W` (widening) counterpart of
 * {@link getTaggedUnionMatcher}; the curried key step and the optional fallback
 * are identical.
 *
 * @typeParam T - The discriminated-union type to match.
 * @returns A function that takes the discriminant property's name.
 * @example
 *     const matchShape = getTaggedUnionMatcherW<Shape>()("kind");
 *     const describe = matchShape({
 *         circle: () => "round",
 *         square: () => 4,
 *     });
 *     // describe: (shape: Shape) => string | number
 */
 export const getTaggedUnionMatcherW = <
    T extends object,
 >(): TaggedUnionMatcherWideningFactory<T> => dispatch;
@@ -8,5 +8,6 @@
        "allowImportingTsExtensions": true,
        "verbatimModuleSyntax": true
    },
-    "include": ["src", "scripts"]
+    "include": ["src", "scripts"],
    "exclude": ["src/doc-test"]
 }