🚀 Release 0.6.0

Rename `MatcherStrict` / `MatcherWidening` to
`PrimitiveUnionMatcherStrict` / `PrimitiveUnionMatcherWidening`, matching
the exported `getPrimitiveUnionMatcher(W)` names and the
`TaggedUnionMatcher*` pair.

Folds in the working-tree lint cleanups: drop the now-unneeded
`typescript/unified-signatures` disables and turn the tagged-union
factory interfaces into type aliases.

.oxlintrc.json

@@ -34,7 +34,9 @@
                 "no-unused-expressions": "off",
                 "no-empty-file": "off",
                 "import/no-nodejs-modules": "off",
-                "eslint/no-magic-numbers": "off"
+                "eslint/no-magic-numbers": "off",
+                "unicorn/no-null": "off",
+                "typescript/no-floating-promises": "off"
             }
         },
         {

AGENTS.md

@@ -9,7 +9,7 @@ first-action facts. Do not restate evolving prose here — it will drift.
 
 - Project: F#-style 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)`. Prefer `lsp_references` over `grep -w` for widely-colliding identifiers (`matches`, `type`, …); use `lsp_hover` to read inferred types on generic-heavy code. 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.
+- **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.
 - **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.
@@ -25,10 +25,11 @@ first-action facts. Do not restate evolving prose here — it will drift.
 Don't silence the type system to force a green run. As an agent these are forbidden:
 
 - `// @ts-nocheck`, `// @ts-ignore`, `// @ts-expect-error`
-- `// oxlint-disable` / `// oxlint-disable-next-line` — the sole exception is the fixed file-level `typescript/no-floating-promises` header at the top of a `*.test.ts` file, spelled exactly as [CONTRIBUTING.md § Rules the tools don't enforce](./CONTRIBUTING.md#rules-the-tools-dont-enforce) prescribes
+- `// oxlint-disable` / `// oxlint-disable-next-line`
+- editing `.oxlintrc.json` to silence a finding (e.g. turning `typescript/no-floating-promises` off)
 - `as` casts used to push an expression through (type-aware oxlint already flags unsafe assertions)
 
-Fix the root cause with the type system instead — narrowing, generics, `satisfies`, conditional / mapped types, utility types (`NonNullable`, `Exclude`, …). TypeScript can express it; that's the intended tool. The `oxlint-disable`-location rule in [CONTRIBUTING.md § Rules the tools don't enforce](./CONTRIBUTING.md#rules-the-tools-dont-enforce) is a **human** last-resort convention (so a reviewer can spot a deliberate suppression) — it is not permission for you to add one, except the one fixed `*.test.ts` file-level header named there. If the types genuinely cannot express something, stop and surface the conflict (commit message / handover) rather than suppress it.
+Fix the root cause with the type system instead — narrowing, generics, `satisfies`, conditional / mapped types, utility types (`NonNullable`, `Exclude`, …). TypeScript can express it; that's the intended tool. Suppressions — a source `oxlint-disable` **or** a `.oxlintrc.json` entry — are a **human** last resort, not a tool for you. If the types genuinely cannot express something, stop and surface the conflict (commit message / handover) rather than suppress it.
 
 The same applies to the checks themselves: **never `git commit --no-verify`** (or otherwise skip a pre-commit / pre-push hook). The checks are fast and offline, so a redundant run is fine — bypassing a hook to get green is the identical anti-pattern. If a commit already skipped a hook, redo it through one: `git reset --soft HEAD~1 && git commit -C <skipped-sha>`.
 
@@ -42,7 +43,9 @@ Never start a long-lived / blocking process such as `npm run watch`. It runs unt
 
 ### Working on tasks
 
-- **Task with subtasks** (a task that has indented children): create the branch with `npm run create:branch -- <prefix>/<desc>`, inferring the prefix from the task content (`feature/…` / `fix/…` / `chore/…`) — do not hand-write `git switch -c`, the command enforces the clean-tree / current-`main` / green-baseline precondition. Work on each subtask with commits, then present a concise handover for the user to review. Use this fixed shape:
+Every task — with or without subtasks — goes through the complete branching model:
+
+- Create the branch with `npm run create:branch -- <prefix>/<desc>`, inferring the prefix from the task content (`feature/…` / `fix/…` / `chore/…`) — do not hand-write `git switch -c`, the command enforces the clean-tree / current-`main` / green-baseline precondition. Work with commits (each subtask gets one or more), then present a concise handover for the user to review. Use this fixed shape:
 
     ```md
     ## Handover — <branch>
@@ -54,9 +57,7 @@ Never start a long-lived / blocking process such as `npm run watch`. It runs unt
 
     Once the user has no further objections, merge back: `npm run create:finish` (on the branch — it merges `--no-ff`, runs `npm run verify`, and deletes the branch). The branching model is documented in [CONTRIBUTING.md § Branching model](./CONTRIBUTING.md#branching-model).
 
-- **Leaf task** (no indented children): implement on the current branch and commit.
+Follow [CONTRIBUTING.md § Testing discipline (type-driven)](./CONTRIBUTING.md#testing-discipline-type-driven) throughout.
-
-In both cases, follow [CONTRIBUTING.md § Testing discipline (type-driven)](./CONTRIBUTING.md#testing-discipline-type-driven). Each subtask gets one or more commits.
 
 ## Read these
 

CHANGELOG.md

@@ -7,7 +7,21 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
-- move the `_` fallback to a second argument typed `Exclude<T, keyof handlers>`, so it receives only the unhandled keys, and infer handler returns from short, unannotated handlers
+## [0.6.0] - 2026-09-22
+
+- add `getTaggedUnionMatcher` / `getTaggedUnionMatcherW` for discriminated unions
+- rename the primitive matcher to primitive-union: `getMatcher` /
+  `getMatcherW` → `getPrimitiveUnionMatcher` / `getPrimitiveUnionMatcherW`, and
+  `src/primitive.ts` / `src/primitive.test.ts` → `src/primitive-union.*`
+
+## [0.5.0] - 2026-09-21
+
+- reject a fallback when the handler map already covers the universe
+- allow boolean, null and undefined in the matcher universe
+
+## [0.4.0] - 2026-09-20
+
+- move the `_` fallback out of handlers
 
 ## [0.3.0] - 2026-09-18
 
@@ -61,7 +75,10 @@ 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/0.3.0...main
+[Unreleased]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.6.0...main
+[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
 [0.3.0]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.2.0...0.3.0
 [0.2.0]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.1.8...0.2.0
 [0.1.8]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.1.7...0.1.8

CONTRIBUTING.md

@@ -66,9 +66,12 @@ before the implementation. The loop is **type → red → green → refactor**:
 4. **Refactor** — with the type system and the tests as the safety net, then
    `npm run verify` as the definition-of-done gate.
 
-Every test pairs an `expectTypeOf(...)` with an `assert.*`; keep them together.
+Every test pairs an `expectTypeOf(...)` with an `assert.*`; keep them together
+— including the expectations inside handler bodies, which pair with an
+assertion on the value dispatch passed, not only the type it inferred (see
+[development/testing.md § Handler arguments](./development/testing.md#handler-arguments)).
 The autocomplete tests (`src/util/__tests__/lsp-completion.test.ts` for the
-helper, `src/primitive.test.ts` for the matcher's popup) are the
+helper, `src/primitive-union.test.ts` for the matcher's popup) are the
 exception —
 the language server, not the type system, is the oracle (see
 [development/testing.md § Autocomplete](./development/testing.md#autocomplete)).
@@ -155,12 +158,9 @@ reaches for by default:
   [Script prefix convention](#script-prefix-convention).
 - **`oxlint-disable` directives live in source, not `.oxlintrc.json`.** The
   trade-off must sit next to the code it silences. This is a _human_ last-resort
-  convention; agents must not add these — see
+  convention; agents must not add one, nor edit `.oxlintrc.json` to silence a
-  [AGENTS.md § Never do](./AGENTS.md#never-do). The sole agent exception is the
+  finding (e.g. `typescript/no-floating-promises`) — see
-  fixed file-level header at the top of a `*.test.ts` file, spelled exactly:
+  [AGENTS.md § Never do](./AGENTS.md#never-do). (why:
-  `/* oxlint-disable typescript/no-floating-promises -- expectTypeOf() is a sync
-type-assertion library that the type-aware linter misidentifies as a promise
-*/` — any other suppression stays human-last-resort. (why:
   [development/tooling.md](./development/tooling.md#oxlint-disable-directives-live-next-to-the-code))
 - **Don't put slow / network / whole-project scans in `check` or pre-commit.**
   Advisory scans are not correctness gates; they belong under `maintain:`. (why:

README.md

@@ -9,7 +9,7 @@ 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
 not a macro: there is no transpiler and no DSL to learn, and the type-level
 contract is the feature — see [development/library.md](./development/library.md)
-for the design decisions and the known limitations.
+for the design decisions and [Caveats](#caveats) for the limits.
 
 ## Requirements
 
@@ -23,6 +23,18 @@ for the design decisions and the known limitations.
 
 Yet to be implemented
 
+## Caveats
+
+- **A value and its stringification collide.** Object keys stringify, so a
+  universe that mixes a member with the string it stringifies to — `1 | "1"`,
+  `true | "true"`, `null | "null"` — collapses to a single handler key and both
+  members are routed to it. Use one form or the other.
+- **`symbol` and `bigint` are not supported.** A `symbol` brand is a
+  compile-time phantom with nothing to match at runtime, and a `bigint` is not a
+  valid property key; neither satisfies the matcher's universe constraint.
+- **`NaN` and `-0` cannot be matched specifically.** They have no literal type,
+  so both stay part of `number`.
+
 ## License
 
 MIT © 2025 tmu. See [LICENSE](./LICENSE).

backlog.tasks

@@ -24,7 +24,7 @@ v1.0:
     ☐ Document all exported types and functions
     ☐ Add JSDoc for public APIs
 ☐ Test coverage meets threshold
-    ☐ Achieve 100% branch coverage on `src/primitive.ts`
+    ☐ Achieve 100% branch coverage on `src/primitive-union.ts`
     ☐ Achieve 100% branch coverage on `src/index.ts`
 
 Testing:
@@ -37,6 +37,12 @@ Testing:
     ✔ 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
+    → autocomplete should still offer the literals; arbitrary strings stay assignable and fall through to the fallback
+☐ Type hole when using broad types like string as a universe @high
+    ☐ handlers cannot be exhaustive, yet it is possible to not have a fallback
+    ☐ one test even exploits this, I think for a negative test
+    ☐ does the same type hole exist for tagged-union matchers or primitive-union matchers only?
 
 Matcher:
 ✔ Clean up: adopt the 3-overload matcher (`src/prototype-ac2.ts`) and delete the prototypes @high @done
@@ -44,16 +50,31 @@ Matcher:
     → 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
+✔ A fallback for an already-exhaustive handler map must be a compile error @medium @done
-    → `(handlers, fallback)` with `handlers` covering all of `T` is accepted; the redundant fallback should be rejected (currying would allow the guard)
+    → 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
+✔ 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
+    ✔ 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

development/library.md

@@ -3,9 +3,10 @@
 The type-level design of the public API and the limitations it carries. The
 user-facing reference is [README § API](../README.md#api).
 
-The matcher is implemented in `src/primitive.ts` and re-exported from
+The matchers are implemented in `src/primitive-union.ts` (`getPrimitiveUnionMatcher` /
-`src/index.ts` as `getMatcher` / `getMatcherW`; the rest of the library is
+`getPrimitiveUnionMatcherW`) and `src/tagged-union.ts` (`getTaggedUnionMatcher` /
-placeholder code.
+`getTaggedUnionMatcherW`), re-exported from `src/index.ts`; the rest of the
+library is placeholder code.
 
 ## Matcher shape
 
@@ -15,8 +16,8 @@ A factory takes the universe and returns a builder; the builder takes a handler
 map and an optional fallback:
 
 ```ts
-const matcher = getMatcher<"a" | "b">()({ a: (s) => …, b: (s) => … });
+const matcher = getPrimitiveUnionMatcher<"a" | "b">()({ a: (s) => …, b: (s) => … });
-const fallback = getMatcher<"a" | "b" | "c">()({ a: (s) => … }, (s) => …);
+const fallback = getPrimitiveUnionMatcher<"a" | "b" | "c">()({ a: (s) => … }, (s) => …);
 ```
 
 Exhaustive or fallback is decided **at the call site**, by whether the second
@@ -24,15 +25,16 @@ argument is present. The fallback's parameter is the remainder
 `Exclude<T, keyof Handled>`. Only the return-strictness axis remains, so there
 are two factories:
 
-- `getMatcher` — one common `R`; the fallback must fit it;
+- `getPrimitiveUnionMatcher` — one common `R`; the fallback must fit it;
-- `getMatcherW` — the union `PatternReturns<Handled> | R`.
+- `getPrimitiveUnionMatcherW` — the union `PatternReturns<Handled> | R`.
 
 Each factory is two overloads whose order is load-bearing:
 
 1. `Handlers<T, R>` — the exhaustive form, and the contextual type of the
    handler-map popup;
-2. `Handled extends Exact<Partial<Handlers<T, R>>, Handled>` plus
+2. `Handled extends Exact<Partial<Handlers<T, R>>, Handled>` intersected with
-   `Fallback<T, Handled, R>` — a partial handler map plus the fallback.
+   `MustBePartial<T, Handled>`, plus `Fallback<T, Handled, R>` — a partial
+   handler map plus the fallback, rejected when the map already covers `T`.
 
 #### Why
 
@@ -41,6 +43,14 @@ Each factory is two overloads whose order is load-bearing:
   handler map can only see all of `T`, never `Exclude<T, keyof Handled>`. A later
   argument is contextually typed from inference on an earlier one, so the split
   is what makes the remainder expressible.
+- **The redundant-fallback guard is an F-bounded constraint.** A map that
+  already covers `T` plus a fallback is rejected by folding
+  `MustBePartial<T, Handled>` into `Handled`'s own constraint. The guard is
+  checked _after_ `Handled` is inferred, so the contextual pass that types the
+  handler callbacks survives. The obvious conditional
+  `Exclude<T, keyof Handled> extends never ? …` in the fallback's parameter
+  type is evaluated while `Handled` is still its constraint and rejects every
+  partial map whose callbacks are context-sensitive.
 - **Overload order keeps both messages.** #1 supplies the contextual type
   (`a, b, c`); #2 accepts a partial map once a fallback is present, so its popup
   is optional (`a?, b?, c?`). A gap without a fallback is reported against #1.
@@ -57,9 +67,9 @@ Each factory is two overloads whose order is load-bearing:
 - **Single-object `_`** (the former shape). `_` sees only all of `T`; the
   remainder is not expressible there, and an exhaustive map plus `_` was
   accepted.
-- **Curried handlers-first** — `(handlers)(fallback)`. `Handled` is fixed before
+- **Curried handlers-first** — `(handlers)(fallback)`. Rejected: two calls for
-  the second call, so a redundant-fallback guard would work. Rejected: two calls
+  the common case. It is not needed for the redundant-fallback guard, which the
-  for the common case.
+  F-bounded constraint already provides (see Why).
 - **`this` / HKT self-reference.** `this` is post-construction (method bodies,
   return positions); a parameter's contextual type is pre-construction.
   `keyof this` in an interface method is the interface, not the literal.
@@ -76,14 +86,113 @@ Each factory is two overloads whose order is load-bearing:
 
 #### Known issue
 
-- A redundant fallback is accepted: when the handler map already covers `T`, the
-  fallback is still allowed. The guard would be
-  `Exclude<T, keyof Handled> extends never ? never : unknown`, but the
-  conditional is evaluated before `Handled` is inferred; currying is the only
-  encoding that fixes it (see Rejected).
 - `PatternReturns` must be
   `ReturnType<Extract<ValueOf<P>, (...args: never[]) => unknown>>` so it survives
   the closed, partly-optional `P` constraints.
 - `Parameters<typeof factory>[0]` resolves only the **last** overload, so it is
   not a sound "rejected" oracle for a factory. Factory-negative tests use
   `@ts-expect-error` call sites (the test file only — the general ban stands).
+
+## Shared internals
+
+#### Decision (2026-09)
+
+`src/matcher-shared.ts` holds the seven universe-agnostic pieces both matchers
+use: `UnaryFn`, `PatternReturns`, `RedundantFallback`, `HandlerMap`, the shared
+`Matchable` universe, and the `PatternKey` / `PatternParam` key projection.
+
+#### Why
+
+- `RedundantFallback`'s property name is the diagnostic, so one definition
+  keeps the two matchers' message from drifting; the other pieces appear
+  verbatim in both public signatures or are the same projection over each
+  matcher's universe.
+- **`Matchable` is one definition, not two.** The primitive-union matcher's
+  universe and the tagged-union matcher's allowed `Tag` values are the same set,
+  so aliasing them keeps the two matchers from drifting apart on what they
+  accept (`symbol`/`bigint` rejected once).
+
+#### Rejected
+
+- **A generic `Matcher<Universe>` over the interface pair, `Handlers`,
+  `Fallback` and `MustBePartial`.** Each is built from its own universe
+  (`Tags`/`MapTaggedUnion` vs the primitive values); abstracting over the
+  F-bounded `Handled` constraint that makes the remainder work risks the
+  contextual typing it exists to preserve. `Matchable` and the `PatternKey` /
+  `PatternParam` projection are the pieces both universes genuinely share.
+
+## Tagged-union matcher
+
+#### Decision (2026-09)
+
+`getTaggedUnionMatcher` / `getTaggedUnionMatcherW` mirror the primitive-union pair
+with one extra curried step for the discriminant key:
+
+```ts
+type Shape =
+    | { kind: "circle"; radius: number }
+    | { kind: "square"; side: number };
+
+const area = getTaggedUnionMatcher<Shape>()("kind")({
+    circle: (s) => Math.PI * s.radius ** 2,
+    square: (s) => s.side ** 2,
+});
+const fallback = getTaggedUnionMatcher<Shape>()("kind")(
+    { circle: (s) => … },
+    (s) => …, // s: { kind: "square"; side: number }
+);
+```
+
+The key is a separate call because `K` is inferred from its literal argument and
+`T` is fixed by the first factory; one call could not infer both.
+`Discriminated<T>` restricts the key to properties whose values are tags.
+
+#### Why
+
+- **Same fallback/remainder machinery as the primitive-union matcher.** `HandledMembers`
+  maps the handled tags to their members and `Exclude<T, …>` is the fallback's
+  parameter; the 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
+  union of members instead of dropping one.
+- **A `boolean` / `null` / `undefined` tag goes through the shared
+  `PatternKey` / `PatternParam` projection.** `Discriminated` admits those tags
+  (they are in `Tag`), but they cannot key a mapped type, so the handler map is
+  keyed by the stringified form (`true` → `"true"`) and `PatternParam` inverts
+  it to recover the member. This is the same projection the primitive-union
+  matcher uses over its universe, which is why it lives in `matcher-shared.ts`.
+
+#### Known issue
+
+- A member's tag must be unique across the union; two members with the same tag
+  collapse to a union under one handler. The same holds for a tag colliding with
+  its stringification (`true | "true"`) — see [README § Caveats](../README.md#caveats).
+
+## Primitive universe
+
+#### Decision (2026-09)
+
+The universe (`Matchable`) is `string | number | boolean | null | undefined`,
+with `boolean` admitted as `true | false`.
+
+`boolean`/`null`/`undefined` are not property keys, so handler-map keys are a
+projection (`PatternKey`: each member stringified) and `PatternParam` inverts
+it, so callbacks receive the real member (`true`, not `"true"`). The popup
+offers `true`, `false`, `null`, `undefined` by name (verified over LSP). The
+same projection is shared with the tagged-union matcher; see
+§ Tagged-union matcher.
+
+#### Why
+
+- Runtime dispatch indexes with the raw `shape`; `handlers[true]` coerces to
+  `"true"` at runtime exactly as `String` would. The `shape as string | number`
+  assertion only placates `TS2538` and buys the number fast path (an explicit
+  `String()` defeats V8's numeric-key path: measured ~2× on number-keyed
+  dispatch).
+- `symbol`/`bigint`/`NaN`/`-0` are rejected, and a member colliding with its
+  stringification is unguarded: user-facing, stated once in
+  [README § Caveats](../README.md#caveats).

development/testing.md

@@ -28,7 +28,7 @@ before the implementation.
 - Runtime-first (classic red/green): it verifies the value, not the contract,
   and the contract is the product.
 - Testing the type only: it would not catch handler dispatch or the `_`
-  fallback (see `src/primitive.test.ts`).
+  fallback (see `src/primitive-union.test.ts`).
 
 The runner is `node --test --strip-types "src/**/*.test.ts"` and the tiers are
 in
@@ -37,6 +37,40 @@ in
 build step, and the runner relies on the `.ts` import-extension convention (see
 [tooling.md](./tooling.md#source-imports-use-ts-extensions)).
 
+## Handler arguments
+
+#### Decision (2026-09)
+
+A handler's `expectTypeOf(shape)` is always paired with an assertion on the
+argument `dispatch` actually passed: `assert.equal` where the handler runs for
+one shape, `assert.ok(s === … || s === …)` over the set a `_` fallback accepts
+(`assert` is imported as `strict`, so each comparison is `Object.is`). Where a
+test should also prove that the _exact_ value reached the handler unchanged,
+the fallback returns the shape verbatim and the call site asserts it.
+
+#### Why
+
+- The parameter's type is what the compiler inferred from the pattern; the
+  argument is what the runtime passed. Only the second can drift, and the keys
+  whose property name differs from their value (`true`, `null`, `1`) are
+  exactly where it can — see [library.md](./library.md).
+- `String(shape)` at the call site keeps every type expectation and every
+  return-value assertion green; the argument assertions fail (10 of the 33
+  tests). Without them the suite never looks at the passed argument.
+- Returning the shape verbatim costs a widening pattern nothing: the remainder
+  type joins the union of handler returns in place of a marker literal, so the
+  test still shows the widening it is named for.
+
+#### Rejected
+
+- A recorded `unknown[]` of every fallback call compared with `deepEqual`:
+  strong, but it couples the assertion to call order, and the sink sits three
+  blocks away from the value it observes.
+- `typeof` checks: they cannot separate `2` from its key text `"2"`, which is
+  the drift a fallback with a numeric remainder can hit.
+- One expected value asserted inline in a fallback: its argument is a _set_ of
+  shapes, so only the disjunction holds on every call.
+
 ## AAA ordering
 
 The rule is in
@@ -133,7 +167,7 @@ The script prints the labels the server offers at a `/*COMPLETE*/` marker inside
 `<file>` (the marker is stripped before the document is sent). Its `LspSession`
 is imported by `src/util/__tests__/lsp-completion.test.ts` — which tests the
 helper itself against inline documents, never the library's code — and by
-`src/primitive.test.ts`, where the same probe asserts the matcher's popup;
+`src/primitive-union.test.ts`, where the same probe asserts the matcher's popup;
 the CLI is for manual inspection.
 
 #### Why
@@ -167,8 +201,8 @@ the CLI is for manual inspection.
 
 ## Known issues
 
-- The type-aware linter misidentifies `expectTypeOf()` as a floating promise, so
+- The type-aware linter misidentifies `expectTypeOf()` as a floating promise.
-  test files that use it (`src/primitive.test.ts`) carry a file-level
+  It is a known false positive, so `typescript/no-floating-promises` is off for
-  `oxlint-disable typescript/no-floating-promises` with an explanatory comment.
+  `**/*.test.ts` in the `.oxlintrc.json` override rather than repeated as a
-  It is a known false positive, not a rule worth disabling project-wide (see
+  file-level header (see
   [tooling.md § oxlint-disable directives live next to the code](./tooling.md#oxlint-disable-directives-live-next-to-the-code)).

development/tooling.md

@@ -106,26 +106,34 @@ Type-aware oxlint is enabled via `options.typeAware: true` in `.oxlintrc.json`
 
 #### Decision (2026-09)
 
-A type-aware rule that false-positives is silenced with a source-level
+A type-aware rule that false-positives **at one site** is silenced with a
-`oxlint-disable` directive (see `src/primitive.ts`,
+source-level `oxlint-disable` directive (see `src/primitive-union.ts`). A rule that is
-`src/primitive.test.ts`), not by turning the rule off in `.oxlintrc.json`.
+wrong for a whole **file class** is turned off in a `.oxlintrc.json` `overrides`
+entry instead — e.g. `typescript/no-floating-promises` (synchronous
+`expectTypeOf` reads as an unhandled promise) and `unicorn/no-null` (intentional
+`null` inputs) for `**/*.test.ts`. The same exemption is not repeated as a
+file-level header in every affected file.
 
 #### Why
 
-- The disable sits next to the code it silences, visible to anyone reading the
+- A one-site disable sits next to the code it silences, visible to anyone
-  source.
+  reading the source, and the rule stays on everywhere else.
-- The rule stays on everywhere else, so only the mis-firing line is exempted.
+- A file-class rule is a property of the file class, not of one line; the
+  override states it once, where the rest of the file-class config lives.
 
 #### Rejected
 
-- A project-wide disable in `.oxlintrc.json` for a false positive: it hides the
+- A project-wide disable in `.oxlintrc.json` for a one-site false positive: it
-  exemption from the reader of the affected code and switches the rule off
+  hides the exemption from the reader of the affected code and switches the rule
-  repo-wide for a one-site problem.
+  off repo-wide for a one-site problem.
+- A repeated file-level `oxlint-disable` header for a file-class false positive:
+  the copies drift and scatter one config decision across the tree.
 
 #### Known issue
 
-- A source-level disable is a _human_ last resort. AI agents must not add one;
+- Both placements are _human_ last resorts. AI agents must neither add a source
-  they fix the type at its root (see [AGENTS.md § Never do](../AGENTS.md#never-do)).
+  disable nor edit `.oxlintrc.json`; they fix the type at its root (see
+  [AGENTS.md § Never do](../AGENTS.md#never-do)).
 
 ### Unwanted stylistic rules are turned off in the config
 

package-lock.json

@@ -1,12 +1,12 @@
 {
     "name": "tiny-pattern-ts",
-    "version": "0.3.0",
+    "version": "0.6.0",
     "lockfileVersion": 3,
     "requires": true,
     "packages": {
         "": {
             "name": "tiny-pattern-ts",
-            "version": "0.3.0",
+            "version": "0.6.0",
             "license": "MIT",
             "dependencies": {
                 "type-fest": "^5.9.0"

package.json

@@ -1,6 +1,6 @@
 {
     "name": "tiny-pattern-ts",
-    "version": "0.3.0",
+    "version": "0.6.0",
     "description": "Pattern matching for TypeScript/ESM environments (F#-style, not regex)",
     "keywords": [
         "adt",

src/index.ts

@@ -1 +1,8 @@
-export { getMatcher, getMatcherW } from "./primitive.ts";
+export {
+    getPrimitiveUnionMatcher,
+    getPrimitiveUnionMatcherW,
+} from "./primitive-union.ts";
+export {
+    getTaggedUnionMatcher,
+    getTaggedUnionMatcherW,
+} from "./tagged-union.ts";

src/matcher-shared.ts

@@ -0,0 +1,60 @@
+import type { ValueOf } from "type-fest";
+
+// The primitive-union and tagged-union matchers differ in their universe, but the
+// handler/fallback plumbing is identical; these are the shared pieces. The
+// boundary is deliberate: `Handlers`, `Fallback` and `MustBePartial` stay with
+// each matcher because they are built from its universe. See development/library.md.
+
+// A handler: one universe member in, one return value out.
+export type UnaryFn<T, R> = (shape: T) => R;
+
+// The primitive universe a matcher can discriminate. The primitive-union matcher
+// uses it directly; the tagged-union matcher uses it as the set of allowed
+// discriminant (`Tag`) values. `boolean` is admitted as the pair `true | false`;
+// see README § Caveats for the unsupported members.
+export type Matchable = string | number | boolean | null | undefined;
+
+// `boolean`, `null` and `undefined` cannot be property keys, so a mapped type
+// over a universe that includes one keys each such member by its
+// stringification. `PatternParam` inverts that projection, so a handler callback
+// still receives the *real* member (`true`, not `"true"`). Both matchers use the
+// projection: the primitive-union matcher over its universe, the tagged-union
+// matcher over a discriminant property's values. See README § Caveats for the
+// limits.
+export type PatternKey<T> = T extends boolean
+    ? T extends true
+        ? "true"
+        : "false"
+    : T extends null
+      ? "null"
+      : T extends undefined
+        ? "undefined"
+        : T;
+export type PatternParam<K> = K extends "true"
+    ? true
+    : K extends "false"
+      ? false
+      : K extends "null"
+        ? null
+        : K extends "undefined"
+          ? undefined
+          : K;
+
+// `Extract` drops optional handlers (`undefined`) so `PatternReturns` also works
+// when `P`'s constraint has optional keys.
+export type PatternReturns<P> = ReturnType<
+    Extract<ValueOf<P>, (...args: never[]) => unknown>
+>;
+
+// The diagnostic raised when a fallback is supplied for an already-exhaustive
+// handler map. Each matcher's `MustBePartial` folds it into `Handled`'s
+// constraint so the guard is checked after inference.
+export interface RedundantFallback {
+    readonly "every case is already handled, so the fallback is redundant": never;
+}
+
+// The runtime dispatch map the handler maps and fallback erase to.
+export type HandlerMap = Record<
+    string | number,
+    UnaryFn<never, unknown> | undefined
+>;

src/primitive-union.test.ts

@@ -0,0 +1,761 @@
+import { strict as assert } from "node:assert";
+import path from "node:path";
+import { test } from "node:test";
+
+import { expectTypeOf } from "expect-type";
+
+import {
+    type CompletionTarget,
+    LspSession,
+} from "#test-utils/lsp-completion.ts";
+
+import {
+    getPrimitiveUnionMatcher,
+    getPrimitiveUnionMatcherW,
+} from "./primitive-union.ts";
+
+// ============================================================================
+// API: getPrimitiveUnionMatcher — ✔️ Exhaustive / ✔️ ReturnsStrict
+// ============================================================================
+
+test("getPrimitiveUnionMatcher: exhaustive pattern infers one common return type", () => {
+    // Arrange
+    const factory = getPrimitiveUnionMatcher<"a" | "b">();
+
+    // Act
+    const matcher = factory({
+        a: (s) => {
+            expectTypeOf(s).toEqualTypeOf<"a">();
+            assert.equal(s, "a");
+            return 1;
+        },
+        b: (s) => {
+            expectTypeOf(s).toEqualTypeOf<"b">();
+            assert.equal(s, "b");
+            return 2;
+        },
+    });
+
+    // Assert
+    // ✔️ ReturnsStrict: R is the best common return type, not a widening union.
+    expectTypeOf(matcher).toEqualTypeOf<(shape: "a" | "b") => number>();
+    // ❌ a value outside T must not be accepted by the matcher
+    expectTypeOf<"c">().not.toExtend<Parameters<typeof matcher>[0]>();
+    assert.equal(matcher("a"), 1);
+    assert.equal(matcher("b"), 2);
+});
+
+test("getPrimitiveUnionMatcher: dispatches on numeric literal keys", () => {
+    // Arrange
+    const factory = getPrimitiveUnionMatcher<1 | 2>();
+
+    // Act
+    const matcher = factory({
+        1: (n) => {
+            expectTypeOf(n).toEqualTypeOf<1>();
+            // A numeric key must not reach its handler as the string `"1"`.
+            assert.equal(n, 1);
+            return n + 1;
+        },
+        2: (n) => {
+            expectTypeOf(n).toEqualTypeOf<2>();
+            assert.equal(n, 2);
+            return n * 10;
+        },
+    });
+
+    // Assert
+    expectTypeOf(matcher).toEqualTypeOf<(shape: 1 | 2) => number>();
+    assert.equal(matcher(1), 2);
+    assert.equal(matcher(2), 20);
+});
+
+test("getPrimitiveUnionMatcher: dispatches on mixed string and numeric keys", () => {
+    // Arrange
+    const factory = getPrimitiveUnionMatcher<"a" | "b" | 1 | 2>();
+
+    // Act
+    const matcher = factory({
+        a: (s) => {
+            expectTypeOf(s).toEqualTypeOf<"a">();
+            assert.equal(s, "a");
+            return 1;
+        },
+        b: (s) => {
+            expectTypeOf(s).toEqualTypeOf<"b">();
+            assert.equal(s, "b");
+            return 2;
+        },
+        1: (n) => {
+            expectTypeOf(n).toEqualTypeOf<1>();
+            assert.equal(n, 1);
+            return 10;
+        },
+        2: (n) => {
+            expectTypeOf(n).toEqualTypeOf<2>();
+            assert.equal(n, 2);
+            return 20;
+        },
+    });
+
+    // Assert
+    expectTypeOf(matcher).toEqualTypeOf<(shape: "a" | "b" | 1 | 2) => number>();
+    assert.equal(matcher("a"), 1);
+    assert.equal(matcher("b"), 2);
+    assert.equal(matcher(1), 10);
+    assert.equal(matcher(2), 20);
+});
+
+test("getPrimitiveUnionMatcher: dispatches on boolean literals", () => {
+    // Arrange
+    const factory = getPrimitiveUnionMatcher<boolean>();
+
+    // Act
+    const matcher = factory({
+        true: (s) => {
+            expectTypeOf(s).toEqualTypeOf<true>();
+            // The `true` key is a property name; the handler must still be
+            // called with the boolean `true`, not the string `"true"`.
+            assert.equal(s, true);
+            return 1;
+        },
+        false: (s) => {
+            expectTypeOf(s).toEqualTypeOf<false>();
+            assert.equal(s, false);
+            return 2;
+        },
+    });
+
+    // Assert
+    expectTypeOf(matcher).toEqualTypeOf<(shape: boolean) => number>();
+    assert.equal(matcher(true), 1);
+    assert.equal(matcher(false), 2);
+});
+
+test("getPrimitiveUnionMatcher: dispatches on null and undefined", () => {
+    // Arrange
+    const factory = getPrimitiveUnionMatcher<null | undefined>();
+
+    // Act
+    const matcher = factory({
+        null: (s) => {
+            expectTypeOf(s).toEqualTypeOf<null>();
+            assert.equal(s, null);
+            return "null";
+        },
+        undefined: (s) => {
+            expectTypeOf(s).toEqualTypeOf<undefined>();
+            assert.equal(s, undefined);
+            return "undefined";
+        },
+    });
+
+    // Assert
+    expectTypeOf(matcher).toEqualTypeOf<(shape: null | undefined) => string>();
+    assert.equal(matcher(null), "null");
+    assert.equal(matcher(undefined), "undefined");
+});
+
+// ============================================================================
+// API: getPrimitiveUnionMatcher — ❌ Exhaustive (fallback) / ✔️ ReturnsStrict
+// ============================================================================
+
+test("getPrimitiveUnionMatcher: a `_` fallback receives the unhandled keys", () => {
+    // Arrange
+    const factory = getPrimitiveUnionMatcher<"a" | "b" | "c">();
+
+    // Act
+    const matcher = factory(
+        {
+            a: (s) => {
+                expectTypeOf(s).toEqualTypeOf<"a">();
+                assert.equal(s, "a");
+                return 1 as const;
+            },
+        },
+        (s) => {
+            // The fallback sees only the keys `a` did not handle.
+            expectTypeOf(s).toEqualTypeOf<"b" | "c">();
+            assert.ok(s === "b" || s === "c");
+            return 2 as const;
+        },
+    );
+
+    // Assert
+    expectTypeOf(matcher).toEqualTypeOf<(shape: "a" | "b" | "c") => 1 | 2>();
+    // ❌ a value outside T must not be accepted by the matcher
+    expectTypeOf<"d">().not.toExtend<Parameters<typeof matcher>[0]>();
+    assert.equal(matcher("a"), 1);
+    assert.equal(matcher("b"), 2);
+    assert.equal(matcher("c"), 2);
+});
+
+test("getPrimitiveUnionMatcher: a `_` fallback routes mixed string and numeric gaps", () => {
+    // Arrange
+    const factory = getPrimitiveUnionMatcher<"a" | "b" | 1 | 2>();
+
+    // Act
+    const matcher = factory(
+        {
+            a: (s) => {
+                expectTypeOf(s).toEqualTypeOf<"a">();
+                assert.equal(s, "a");
+                return "A";
+            },
+            1: (n) => {
+                expectTypeOf(n).toEqualTypeOf<1>();
+                assert.equal(n, 1);
+                return "one";
+            },
+        },
+        (s) => {
+            expectTypeOf(s).toEqualTypeOf<"b" | 2>();
+            // The gap mixes a string and a number, so the number must reach the
+            // fallback as `2`, never as its key text `"2"`.
+            assert.ok(s === "b" || s === 2);
+            return "fallback";
+        },
+    );
+
+    // Assert
+    expectTypeOf(matcher).toEqualTypeOf<(shape: "a" | "b" | 1 | 2) => string>();
+    assert.equal(matcher("a"), "A");
+    assert.equal(matcher("b"), "fallback");
+    assert.equal(matcher(1), "one");
+    assert.equal(matcher(2), "fallback");
+});
+
+test("getPrimitiveUnionMatcher: a `_` fallback receives unhandled boolean and nullish keys", () => {
+    // Arrange
+    const factory = getPrimitiveUnionMatcher<
+        "a" | true | false | null | undefined
+    >();
+
+    // Act
+    const matcher = factory(
+        {
+            a: (s) => {
+                expectTypeOf(s).toEqualTypeOf<"a">();
+                assert.equal(s, "a");
+                return 1 as const;
+            },
+            null: (s) => {
+                expectTypeOf(s).toEqualTypeOf<null>();
+                assert.equal(s, null);
+                return 2 as const;
+            },
+        },
+        (s) => {
+            // `true` and `false` are keyed as `"true"`/`"false"` but the
+            // fallback still sees them as booleans.
+            expectTypeOf(s).toEqualTypeOf<true | false | undefined>();
+            assert.ok(s === true || s === false || s === undefined);
+            return 3 as const;
+        },
+    );
+
+    // Assert
+    expectTypeOf(matcher).toEqualTypeOf<
+        (shape: "a" | true | false | null | undefined) => 1 | 2 | 3
+    >();
+    assert.equal(matcher("a"), 1);
+    assert.equal(matcher(true), 3);
+    assert.equal(matcher(false), 3);
+    assert.equal(matcher(null), 2);
+    assert.equal(matcher(undefined), 3);
+});
+
+// ============================================================================
+// API: getPrimitiveUnionMatcherW — ✔️ Exhaustive / ❌ ReturnsStrict
+// ============================================================================
+
+test("getPrimitiveUnionMatcherW: exhaustive pattern widens to the union of handler returns", () => {
+    // Arrange
+    const factory = getPrimitiveUnionMatcherW<"x" | "y">();
+
+    // Act
+    const matcher = factory({
+        x: (s) => {
+            expectTypeOf(s).toEqualTypeOf<"x">();
+            assert.equal(s, "x");
+            return 1 as const;
+        },
+        y: (s) => {
+            expectTypeOf(s).toEqualTypeOf<"y">();
+            assert.equal(s, "y");
+            return "two" as const;
+        },
+    });
+
+    // Assert
+    // ❌ ReturnsStrict: mixed handler returns widen to their union.
+    expectTypeOf(matcher).toEqualTypeOf<(shape: "x" | "y") => 1 | "two">();
+    // ❌ a value outside T must not be accepted by the matcher
+    expectTypeOf<"z">().not.toExtend<Parameters<typeof matcher>[0]>();
+    assert.equal(matcher("x"), 1);
+    assert.equal(matcher("y"), "two");
+});
+
+test("getPrimitiveUnionMatcherW: dispatches on mixed string and numeric keys", () => {
+    // Arrange
+    const factory = getPrimitiveUnionMatcherW<"a" | "b" | 1 | 2>();
+
+    // Act
+    const matcher = factory({
+        a: (s) => {
+            expectTypeOf(s).toEqualTypeOf<"a">();
+            assert.equal(s, "a");
+            return "A" as const;
+        },
+        b: (s) => {
+            expectTypeOf(s).toEqualTypeOf<"b">();
+            assert.equal(s, "b");
+            return "B" as const;
+        },
+        1: (n) => {
+            expectTypeOf(n).toEqualTypeOf<1>();
+            assert.equal(n, 1);
+            return 10 as const;
+        },
+        2: (n) => {
+            expectTypeOf(n).toEqualTypeOf<2>();
+            assert.equal(n, 2);
+            return 20 as const;
+        },
+    });
+
+    // Assert
+    expectTypeOf(matcher).toEqualTypeOf<
+        (shape: "a" | "b" | 1 | 2) => "A" | "B" | 10 | 20
+    >();
+    assert.equal(matcher("a"), "A");
+    assert.equal(matcher("b"), "B");
+    assert.equal(matcher(1), 10);
+    assert.equal(matcher(2), 20);
+});
+
+test("getPrimitiveUnionMatcherW: exhaustive boolean and nullish widen to the union", () => {
+    // Arrange
+    const factory = getPrimitiveUnionMatcherW<boolean | null | undefined>();
+
+    // Act
+    const matcher = factory({
+        true: (s) => {
+            expectTypeOf(s).toEqualTypeOf<true>();
+            assert.equal(s, true);
+            return "yes" as const;
+        },
+        false: (s) => {
+            expectTypeOf(s).toEqualTypeOf<false>();
+            assert.equal(s, false);
+            return "no" as const;
+        },
+        null: (s) => {
+            expectTypeOf(s).toEqualTypeOf<null>();
+            assert.equal(s, null);
+            return 0 as const;
+        },
+        undefined: (s) => {
+            expectTypeOf(s).toEqualTypeOf<undefined>();
+            assert.equal(s, undefined);
+            return 1 as const;
+        },
+    });
+
+    // Assert
+    expectTypeOf(matcher).toEqualTypeOf<
+        (shape: boolean | null | undefined) => "yes" | "no" | 0 | 1
+    >();
+    assert.equal(matcher(true), "yes");
+    assert.equal(matcher(false), "no");
+    assert.equal(matcher(null), 0);
+    assert.equal(matcher(undefined), 1);
+});
+
+// ============================================================================
+// API: getPrimitiveUnionMatcherW — ❌ Exhaustive (fallback) / ❌ ReturnsStrict
+// ============================================================================
+
+test("getPrimitiveUnionMatcherW: a `_` fallback widens gaps into the union", () => {
+    // Arrange
+    const factory = getPrimitiveUnionMatcherW<"x" | "y" | "z">();
+
+    // Act
+    const matcher = factory(
+        {
+            x: (s) => {
+                expectTypeOf(s).toEqualTypeOf<"x">();
+                assert.equal(s, "x");
+                return 1 as const;
+            },
+        },
+        (s) => {
+            // The fallback sees only the keys `x` did not handle.
+            expectTypeOf(s).toEqualTypeOf<"y" | "z">();
+            // Returning the shape verbatim lets the `Assert` block check the
+            // exact value dispatch passed.
+            return s;
+        },
+    );
+
+    // Assert
+    expectTypeOf(matcher).toEqualTypeOf<
+        (shape: "x" | "y" | "z") => 1 | "y" | "z"
+    >();
+    // ❌ a value outside T must not be accepted by the matcher
+    expectTypeOf<"w">().not.toExtend<Parameters<typeof matcher>[0]>();
+    assert.equal(matcher("x"), 1);
+    assert.equal(matcher("y"), "y");
+    assert.equal(matcher("z"), "z");
+});
+
+test("getPrimitiveUnionMatcherW: a `_` fallback widens mixed string and numeric returns", () => {
+    // Arrange
+    const factory = getPrimitiveUnionMatcherW<"a" | "b" | 1 | 2>();
+
+    // Act
+    const matcher = factory(
+        {
+            a: (s) => {
+                expectTypeOf(s).toEqualTypeOf<"a">();
+                assert.equal(s, "a");
+                return "A" as const;
+            },
+            1: (n) => {
+                expectTypeOf(n).toEqualTypeOf<1>();
+                assert.equal(n, 1);
+                return 10 as const;
+            },
+        },
+        (s) => {
+            expectTypeOf(s).toEqualTypeOf<"b" | 2>();
+            // The number must arrive as `2`, not as its key text `"2"`.
+            assert.ok(s === "b" || s === 2);
+            // Returning the shape verbatim lets the `Assert` block check the
+            // exact value dispatch passed.
+            return s;
+        },
+    );
+
+    // Assert
+    // ❌ ReturnsStrict: mixed handler returns widen to their union.
+    expectTypeOf(matcher).toEqualTypeOf<
+        (shape: "a" | "b" | 1 | 2) => "A" | 10 | "b" | 2
+    >();
+    assert.equal(matcher("a"), "A");
+    assert.equal(matcher("b"), "b");
+    assert.equal(matcher(1), 10);
+    assert.equal(matcher(2), 2);
+});
+
+// ============================================================================
+// Factory contracts — calls that must not compile
+// ============================================================================
+
+test("getPrimitiveUnionMatcher factory rejects patterns outside its contract", () => {
+    // Arrange
+    const factory = getPrimitiveUnionMatcher<"a" | "b">();
+
+    // Act / Assert — the calls below must not compile.
+    // `Parameters<typeof factory>[0]` resolves only the *last* overload, so it
+    // is not a sound "rejected" oracle (an accepted partial pattern does not
+    // extend it either); the call sites are the oracle instead.
+    factory({
+        a: () => 1,
+        b: () => 2,
+        // @ts-expect-error `c` is not part of the universe `"a" | "b"`
+        c: () => 3,
+    });
+    // @ts-expect-error a gap without a `_` fallback is not exhaustive
+    factory({ a: () => 1 });
+    // @ts-expect-error a `string` fallback does not fit the `number` handler
+    factory({ a: () => 1 }, () => "x");
+    // @ts-expect-error a fallback is redundant once the map covers the universe
+    factory({ a: () => 1, b: () => 2 }, () => 0);
+});
+
+test("getPrimitiveUnionMatcher factory rejects non-exhaustive boolean and nullish maps", () => {
+    // Arrange
+    const factory = getPrimitiveUnionMatcher<boolean | null | undefined>();
+
+    // Act / Assert — the calls below must not compile
+    // @ts-expect-error only `true` is handled; `false`, `null`, `undefined` are not
+    factory({ true: () => 1 });
+    // @ts-expect-error `null` and `undefined` are not handled
+    factory({ true: () => 1, false: () => 2 });
+    factory(
+        // @ts-expect-error a fallback is redundant once the map covers the universe
+        {
+            true: () => 1,
+            false: () => 2,
+            null: () => 3,
+            undefined: () => 4,
+        },
+        () => 0,
+    );
+});
+
+test("getPrimitiveUnionMatcher: an open universe keeps the fallback's remainder open", () => {
+    // Arrange
+    const factory = getPrimitiveUnionMatcher<string>();
+
+    // Act
+    const matcher = factory({ a: () => 1 }, (s) => {
+        // The map's literal keys do not close an open universe, so the
+        // remainder stays `string` and the fallback is not redundant.
+        expectTypeOf(s).toEqualTypeOf<string>();
+        // A strict pattern fixes one `R` for every handler, so this fallback
+        // cannot return its shape; `typeof` is the strongest claim the value
+        // makes on its own — any string passes, including `""`.
+        assert.equal(typeof s, "string");
+        return 2 as const;
+    });
+
+    // Assert
+    expectTypeOf(matcher).toEqualTypeOf<(shape: string) => number>();
+    assert.equal(matcher("a"), 1);
+    assert.equal(matcher("b"), 2);
+});
+
+test("getPrimitiveUnionMatcherW factory rejects patterns outside its contract", () => {
+    // Arrange
+    const factory = getPrimitiveUnionMatcherW<"x" | "y">();
+
+    // Act / Assert — the calls below must not compile
+    factory({
+        x: () => 1 as const,
+        y: () => 2 as const,
+        // @ts-expect-error `z` is not part of the universe `"x" | "y"`
+        z: () => 3 as const,
+    });
+    // @ts-expect-error a gap without a `_` fallback is not exhaustive
+    factory({ x: () => 1 as const });
+    // @ts-expect-error a fallback is redundant once the map covers the universe
+    factory({ x: () => 1 as const, y: () => 2 as const }, () => 0);
+});
+
+// ============================================================================
+// Dispatch — runtime behavior
+// ============================================================================
+
+test("getPrimitiveUnionMatcher: an unhandled shape throws without a fallback", () => {
+    // Arrange — an open universe types its handler map as an index signature,
+    // so the type system cannot prove the runtime map is exhaustive.
+    const handlers: Record<string, (shape: unknown) => number> = {
+        a: (shape) => {
+            // The raw shape reaches the handler, not the property key's text.
+            assert.equal(shape, "a");
+            return 1;
+        },
+    };
+    const matcher = getPrimitiveUnionMatcher<string>()(handlers);
+
+    // Act / Assert
+    assert.equal(matcher("a"), 1);
+    assert.throws(() => matcher("b"), /Unhandled shape: b/);
+});
+
+test("getPrimitiveUnionMatcher: an unhandled boolean shape throws without a fallback", () => {
+    // Arrange — an `string | boolean` universe widens its handler map to an
+    // index signature, so the runtime map's exhaustiveness is not provable.
+    const handlers: Record<string, (shape: unknown) => number> = {
+        true: (shape) => {
+            // `true` indexes the map as the property `"true"`, but the handler
+            // is still called with the boolean.
+            assert.equal(shape, true);
+            return 1;
+        },
+    };
+    const matcher = getPrimitiveUnionMatcher<string | boolean>()(handlers);
+
+    // Act / Assert
+    assert.equal(matcher(true), 1);
+    assert.throws(() => matcher(false), /Unhandled shape: false/);
+});
+
+// ============================================================================
+// Autocomplete — the language server is the oracle, not the type system
+// ============================================================================
+
+// Completion is a contextual-type property that the type system cannot observe,
+// so the cases below read the popup from the repo's language server (via the
+// test helper) rather than pairing `expectTypeOf` with `assert` — see
+// development/testing.md § Autocomplete. They probe the *matcher's* overloads,
+// which is why they live with the matcher and not with the helper.
+const REPO_ROOT = path.resolve(import.meta.dirname, "..");
+const UNIVERSE = `"a" | "b" | "c"`;
+
+// One session per probe: the tests share no language-server state (open
+// documents, project membership), so they pass in any order.
+interface LabelsProbe {
+    readonly name: string;
+    readonly factory: "getPrimitiveUnionMatcher" | "getPrimitiveUnionMatcherW";
+    readonly body: string;
+    readonly universe?: string;
+    readonly tail?: string;
+}
+
+const labelsFor = ({
+    name,
+    factory,
+    body,
+    universe = UNIVERSE,
+    tail = "",
+}: LabelsProbe): Promise<readonly string[]> => {
+    const session = new LspSession(REPO_ROOT);
+    const target: CompletionTarget = {
+        file: `src/__autocomplete_${name}.ts`,
+        source: [
+            `import { ${factory} } from "./index.ts";`,
+            `const m = ${factory}<${universe}>()({`,
+            body,
+            `}${tail});`,
+            "",
+        ].join("\n"),
+    };
+    return session
+        .completionLabelsAt(target)
+        .then((result) => result.labels)
+        .finally(() => session.close());
+};
+
+test("autocomplete: an exhaustive pattern requires the universe", () => {
+    // Arrange
+    const name = "getPrimitiveUnionMatcher_fresh";
+
+    // Act
+    const labels = labelsFor({
+        name,
+        factory: "getPrimitiveUnionMatcher",
+        body: "    /*COMPLETE*/",
+    });
+
+    // Assert
+    return labels.then((result) => {
+        assert.deepEqual([...result], ["a", "b", "c"]);
+    });
+});
+
+test("autocomplete: handled keys drop out of the popup", () => {
+    // Arrange
+    const name = "getPrimitiveUnionMatcher_after_key";
+
+    // Act
+    const labels = labelsFor({
+        name,
+        factory: "getPrimitiveUnionMatcher",
+        body: "    a: () => 1,\n    /*COMPLETE*/",
+    });
+
+    // Assert
+    return labels.then((result) => {
+        assert.deepEqual([...result], ["b", "c"]);
+    });
+});
+
+test("autocomplete: a fallback makes the remaining keys optional", () => {
+    // Arrange
+    const name = "getPrimitiveUnionMatcher_with_fallback";
+
+    // Act
+    const labels = labelsFor({
+        name,
+        factory: "getPrimitiveUnionMatcher",
+        body: "    /*COMPLETE*/",
+        tail: ", () => 0",
+    });
+
+    // Assert
+    return labels.then((result) => {
+        assert.deepEqual([...result], ["a?", "b?", "c?"]);
+    });
+});
+
+test("autocomplete: with a fallback, handled keys stay optional", () => {
+    // Arrange
+    const name = "getPrimitiveUnionMatcher_with_fallback_after_key";
+
+    // Act
+    const labels = labelsFor({
+        name,
+        factory: "getPrimitiveUnionMatcher",
+        body: "    a: () => 1,\n    /*COMPLETE*/",
+        tail: ", () => 0",
+    });
+
+    // Assert
+    return labels.then((result) => {
+        assert.deepEqual([...result], ["b?", "c?"]);
+    });
+});
+
+test("autocomplete: `getPrimitiveUnionMatcherW` offers the same popup as `getPrimitiveUnionMatcher`", () => {
+    // Arrange
+    const name = "getPrimitiveUnionMatcherW_fresh";
+
+    // Act
+    const labels = labelsFor({
+        name,
+        factory: "getPrimitiveUnionMatcherW",
+        body: "    /*COMPLETE*/",
+    });
+
+    // Assert
+    return labels.then((result) => {
+        assert.deepEqual([...result], ["a", "b", "c"]);
+    });
+});
+
+test("autocomplete: `getPrimitiveUnionMatcherW` also offers optional keys with a fallback", () => {
+    // Arrange
+    const name = "getPrimitiveUnionMatcherW_with_fallback";
+
+    // Act
+    const labels = labelsFor({
+        name,
+        factory: "getPrimitiveUnionMatcherW",
+        body: "    /*COMPLETE*/",
+        tail: ", () => 0",
+    });
+
+    // Assert
+    return labels.then((result) => {
+        assert.deepEqual([...result], ["a?", "b?", "c?"]);
+    });
+});
+
+test("autocomplete: boolean and nullish keys are offered by name", () => {
+    // Arrange
+    const name = "getPrimitiveUnionMatcher_boolean_nullish";
+
+    // Act
+    const labels = labelsFor({
+        name,
+        factory: "getPrimitiveUnionMatcher",
+        universe: "boolean | null | undefined",
+        body: "    /*COMPLETE*/",
+    });
+
+    // Assert
+    return labels.then((result) => {
+        assert.deepEqual([...result], ["false", "null", "true", "undefined"]);
+    });
+});
+
+test("autocomplete: a handled boolean literal drops out of the popup", () => {
+    // Arrange
+    const name = "getPrimitiveUnionMatcher_boolean_after_key";
+
+    // Act
+    const labels = labelsFor({
+        name,
+        factory: "getPrimitiveUnionMatcher",
+        universe: "boolean | null | undefined",
+        body: "    true: () => 1,\n    /*COMPLETE*/",
+    });
+
+    // Assert
+    return labels.then((result) => {
+        assert.deepEqual([...result], ["false", "null", "undefined"]);
+    });
+});

src/primitive-union.ts

@@ -0,0 +1,103 @@
+import type { Exact } from "type-fest";
+
+import type {
+    HandlerMap,
+    Matchable,
+    PatternKey,
+    PatternParam,
+    PatternReturns,
+    RedundantFallback,
+    UnaryFn,
+} from "./matcher-shared.ts";
+
+type Handlers<T extends Matchable, R> = {
+    [K in PatternKey<T>]: UnaryFn<PatternParam<K>, R>;
+};
+
+// The fallback is a *second argument*, not a property of the handler map,
+// because its parameter is the remainder `Exclude<T, keyof Handled>` and TypeScript
+// fixes a property's contextual type before it infers its sibling keys. A later
+// argument, by contrast, is contextually typed from inference on an earlier
+// one, so the split is what makes the remainder expressible at all.
+// See development/library.md.
+type Fallback<T extends Matchable, Handled, R> = UnaryFn<
+    Exclude<T, PatternParam<keyof Handled>>,
+    R
+>;
+
+// A fallback is redundant once the handler map covers `T`. The guard is folded
+// into `Handled`'s own (self-referential) constraint so it is checked *after*
+// inference; a conditional in the fallback's parameter type is evaluated while
+// `Handled` is still its constraint and would reject context-sensitive partial
+// maps. That placement also fixes where the diagnostic lands: the constraint
+// failure is reported on the argument that inferred `Handled` (the handler
+// map), so the required property is spelled as the message instead of relying
+// on its position. See development/library.md.
+type MustBePartial<T extends Matchable, Handled> =
+    PatternKey<T> extends keyof Handled ? RedundantFallback : unknown;
+
+// TypeScript does not apply the excess-property check to a generic constraint,
+// so `Exact` restores it for the generic forms: a handler map can otherwise
+// carry keys outside `T`.
+
+// Strict returns: one common `R`. Overload order is load-bearing:
+//   #1 Handlers (first) -> the exhaustive form and the autocomplete popup
+//   #2 Fallback (last)  -> accepts a partial handler map plus a fallback
+interface PrimitiveUnionMatcherStrict<T extends Matchable> {
+    <R>(handlers: Handlers<T, R>): UnaryFn<T, R>;
+    <
+        R,
+        Handled extends Exact<Partial<Handlers<T, R>>, Handled> &
+            MustBePartial<T, Handled>,
+    >(
+        handlers: Handled & Partial<Handlers<T, R>>,
+        fallback: Fallback<T, Handled, R>,
+    ): UnaryFn<T, R>;
+}
+
+// Widened returns: the union of every handler's return type. `P` is inferred
+// from the whole handler map, whose closed constraint supplies the
+// contextual/autocomplete type.
+interface PrimitiveUnionMatcherWidening<T extends Matchable> {
+    <P extends Exact<Handlers<T, unknown>, P>>(
+        handlers: P,
+    ): UnaryFn<T, PatternReturns<P>>;
+    <
+        R,
+        Handled extends Exact<Partial<Handlers<T, unknown>>, Handled> &
+            MustBePartial<T, Handled>,
+    >(
+        handlers: Handled,
+        fallback: Fallback<T, Handled, R>,
+    ): UnaryFn<T, PatternReturns<Handled> | R>;
+}
+
+const dispatch =
+    (handlers: HandlerMap, fallback?: UnaryFn<never, unknown>) =>
+    (shape: Matchable): unknown =>
+        // `handlers[true]` already coerces to the `"true"` property at runtime,
+        // identical to `handlers[String(shape)]`, so indexing with `shape`
+        // directly is sound: `shape` is a facade-checked universe member and
+        // `PatternKey` only ever produces valid property keys. The assertion is
+        // needed solely because TypeScript forbids indexing with
+        // `boolean`/`null`/`undefined` (TS2538); it buys the number fast path.
+        (
+            handlers[
+                // oxlint-disable-next-line typescript/no-unsafe-type-assertion
+                shape as string | number
+            ] ??
+            fallback ??
+            (() => {
+                throw new Error(`Unhandled shape: ${String(shape)}`);
+            })
+        )(
+            // oxlint-disable-next-line typescript/no-unsafe-type-assertion
+            shape as never,
+        );
+
+export const getPrimitiveUnionMatcher = <
+    T extends Matchable,
+>(): PrimitiveUnionMatcherStrict<T> => dispatch;
+export const getPrimitiveUnionMatcherW = <
+    T extends Matchable,
+>(): PrimitiveUnionMatcherWidening<T> => dispatch;

src/primitive.test.ts

@@ -1,491 +0,0 @@
-/* oxlint-disable typescript/no-floating-promises -- expectTypeOf() is a sync type-assertion library that the type-aware linter misidentifies as a promise */
-import { strict as assert } from "node:assert";
-import path from "node:path";
-import { test } from "node:test";
-
-import { expectTypeOf } from "expect-type";
-
-import {
-    type CompletionTarget,
-    LspSession,
-} from "#test-utils/lsp-completion.ts";
-
-import { getMatcher, getMatcherW } from "./primitive.ts";
-
-// ============================================================================
-// API: getMatcher — ✔️ Exhaustive / ✔️ ReturnsStrict
-// ============================================================================
-
-test("getMatcher: exhaustive pattern infers one common return type", () => {
-    // Arrange
-    const factory = getMatcher<"a" | "b">();
-
-    // Act
-    const matcher = factory({
-        a: (s) => {
-            expectTypeOf(s).toEqualTypeOf<"a">();
-            return 1;
-        },
-        b: (s) => {
-            expectTypeOf(s).toEqualTypeOf<"b">();
-            return 2;
-        },
-    });
-
-    // Assert
-    // ✔️ ReturnsStrict: R is the best common return type, not a widening union.
-    expectTypeOf(matcher).toEqualTypeOf<(shape: "a" | "b") => number>();
-    // ❌ a value outside T must not be accepted by the matcher
-    expectTypeOf<"c">().not.toExtend<Parameters<typeof matcher>[0]>();
-    assert.equal(matcher("a"), 1);
-    assert.equal(matcher("b"), 2);
-});
-
-test("getMatcher: dispatches on numeric literal keys", () => {
-    // Arrange
-    const factory = getMatcher<1 | 2>();
-
-    // Act
-    const matcher = factory({
-        1: (n) => {
-            expectTypeOf(n).toEqualTypeOf<1>();
-            return n + 1;
-        },
-        2: (n) => {
-            expectTypeOf(n).toEqualTypeOf<2>();
-            return n * 10;
-        },
-    });
-
-    // Assert
-    expectTypeOf(matcher).toEqualTypeOf<(shape: 1 | 2) => number>();
-    assert.equal(matcher(1), 2);
-    assert.equal(matcher(2), 20);
-});
-
-test("getMatcher: dispatches on mixed string and numeric keys", () => {
-    // Arrange
-    const factory = getMatcher<"a" | "b" | 1 | 2>();
-
-    // Act
-    const matcher = factory({
-        a: (s) => {
-            expectTypeOf(s).toEqualTypeOf<"a">();
-            return 1;
-        },
-        b: (s) => {
-            expectTypeOf(s).toEqualTypeOf<"b">();
-            return 2;
-        },
-        1: (n) => {
-            expectTypeOf(n).toEqualTypeOf<1>();
-            return 10;
-        },
-        2: (n) => {
-            expectTypeOf(n).toEqualTypeOf<2>();
-            return 20;
-        },
-    });
-
-    // Assert
-    expectTypeOf(matcher).toEqualTypeOf<(shape: "a" | "b" | 1 | 2) => number>();
-    assert.equal(matcher("a"), 1);
-    assert.equal(matcher("b"), 2);
-    assert.equal(matcher(1), 10);
-    assert.equal(matcher(2), 20);
-});
-
-// ============================================================================
-// API: getMatcher — ❌ Exhaustive (fallback) / ✔️ ReturnsStrict
-// ============================================================================
-
-test("getMatcher: a `_` fallback receives the unhandled keys", () => {
-    // Arrange
-    const factory = getMatcher<"a" | "b" | "c">();
-
-    // Act
-    const matcher = factory(
-        {
-            a: (s) => {
-                expectTypeOf(s).toEqualTypeOf<"a">();
-                return 1 as const;
-            },
-        },
-        (s) => {
-            // The fallback sees only the keys `a` did not handle.
-            expectTypeOf(s).toEqualTypeOf<"b" | "c">();
-            return 2 as const;
-        },
-    );
-
-    // Assert
-    expectTypeOf(matcher).toEqualTypeOf<(shape: "a" | "b" | "c") => 1 | 2>();
-    // ❌ a value outside T must not be accepted by the matcher
-    expectTypeOf<"d">().not.toExtend<Parameters<typeof matcher>[0]>();
-    assert.equal(matcher("a"), 1);
-    assert.equal(matcher("b"), 2);
-    assert.equal(matcher("c"), 2);
-});
-
-test("getMatcher: a `_` fallback routes mixed string and numeric gaps", () => {
-    // Arrange
-    const factory = getMatcher<"a" | "b" | 1 | 2>();
-
-    // Act
-    const matcher = factory(
-        {
-            a: (s) => {
-                expectTypeOf(s).toEqualTypeOf<"a">();
-                return "A";
-            },
-            1: (n) => {
-                expectTypeOf(n).toEqualTypeOf<1>();
-                return "one";
-            },
-        },
-        (s) => {
-            expectTypeOf(s).toEqualTypeOf<"b" | 2>();
-            return "fallback";
-        },
-    );
-
-    // Assert
-    expectTypeOf(matcher).toEqualTypeOf<(shape: "a" | "b" | 1 | 2) => string>();
-    assert.equal(matcher("a"), "A");
-    assert.equal(matcher("b"), "fallback");
-    assert.equal(matcher(1), "one");
-    assert.equal(matcher(2), "fallback");
-});
-
-// ============================================================================
-// API: getMatcherW — ✔️ Exhaustive / ❌ ReturnsStrict
-// ============================================================================
-
-test("getMatcherW: exhaustive pattern widens to the union of handler returns", () => {
-    // Arrange
-    const factory = getMatcherW<"x" | "y">();
-
-    // Act
-    const matcher = factory({
-        x: (s) => {
-            expectTypeOf(s).toEqualTypeOf<"x">();
-            return 1 as const;
-        },
-        y: (s) => {
-            expectTypeOf(s).toEqualTypeOf<"y">();
-            return "two" as const;
-        },
-    });
-
-    // Assert
-    // ❌ ReturnsStrict: mixed handler returns widen to their union.
-    expectTypeOf(matcher).toEqualTypeOf<(shape: "x" | "y") => 1 | "two">();
-    // ❌ a value outside T must not be accepted by the matcher
-    expectTypeOf<"z">().not.toExtend<Parameters<typeof matcher>[0]>();
-    assert.equal(matcher("x"), 1);
-    assert.equal(matcher("y"), "two");
-});
-
-test("getMatcherW: dispatches on mixed string and numeric keys", () => {
-    // Arrange
-    const factory = getMatcherW<"a" | "b" | 1 | 2>();
-
-    // Act
-    const matcher = factory({
-        a: (s) => {
-            expectTypeOf(s).toEqualTypeOf<"a">();
-            return "A" as const;
-        },
-        b: (s) => {
-            expectTypeOf(s).toEqualTypeOf<"b">();
-            return "B" as const;
-        },
-        1: (n) => {
-            expectTypeOf(n).toEqualTypeOf<1>();
-            return 10 as const;
-        },
-        2: (n) => {
-            expectTypeOf(n).toEqualTypeOf<2>();
-            return 20 as const;
-        },
-    });
-
-    // Assert
-    expectTypeOf(matcher).toEqualTypeOf<
-        (shape: "a" | "b" | 1 | 2) => "A" | "B" | 10 | 20
-    >();
-    assert.equal(matcher("a"), "A");
-    assert.equal(matcher("b"), "B");
-    assert.equal(matcher(1), 10);
-    assert.equal(matcher(2), 20);
-});
-
-// ============================================================================
-// API: getMatcherW — ❌ Exhaustive (fallback) / ❌ ReturnsStrict
-// ============================================================================
-
-test("getMatcherW: a `_` fallback widens gaps into the union", () => {
-    // Arrange
-    const factory = getMatcherW<"x" | "y" | "z">();
-
-    // Act
-    const matcher = factory(
-        {
-            x: (s) => {
-                expectTypeOf(s).toEqualTypeOf<"x">();
-                return 1 as const;
-            },
-        },
-        (s) => {
-            // The fallback sees only the keys `x` did not handle.
-            expectTypeOf(s).toEqualTypeOf<"y" | "z">();
-            return "fallback" as const;
-        },
-    );
-
-    // Assert
-    expectTypeOf(matcher).toEqualTypeOf<
-        (shape: "x" | "y" | "z") => 1 | "fallback"
-    >();
-    // ❌ a value outside T must not be accepted by the matcher
-    expectTypeOf<"w">().not.toExtend<Parameters<typeof matcher>[0]>();
-    assert.equal(matcher("x"), 1);
-    assert.equal(matcher("y"), "fallback");
-    assert.equal(matcher("z"), "fallback");
-});
-
-test("getMatcherW: a `_` fallback widens mixed string and numeric returns", () => {
-    // Arrange
-    const factory = getMatcherW<"a" | "b" | 1 | 2>();
-
-    // Act
-    const matcher = factory(
-        {
-            a: (s) => {
-                expectTypeOf(s).toEqualTypeOf<"a">();
-                return "A" as const;
-            },
-            1: (n) => {
-                expectTypeOf(n).toEqualTypeOf<1>();
-                return 10 as const;
-            },
-        },
-        (s) => {
-            expectTypeOf(s).toEqualTypeOf<"b" | 2>();
-            return "fallback" as const;
-        },
-    );
-
-    // Assert
-    // ❌ ReturnsStrict: mixed handler returns widen to their union.
-    expectTypeOf(matcher).toEqualTypeOf<
-        (shape: "a" | "b" | 1 | 2) => "A" | 10 | "fallback"
-    >();
-    assert.equal(matcher("a"), "A");
-    assert.equal(matcher("b"), "fallback");
-    assert.equal(matcher(1), 10);
-    assert.equal(matcher(2), "fallback");
-});
-
-// ============================================================================
-// Factory contracts — calls that must not compile
-// ============================================================================
-
-test("getMatcher factory rejects patterns outside its contract", () => {
-    // Arrange
-    const factory = getMatcher<"a" | "b">();
-
-    // Act / Assert — the calls below must not compile.
-    // `Parameters<typeof factory>[0]` resolves only the *last* overload, so it
-    // is not a sound "rejected" oracle (an accepted partial pattern does not
-    // extend it either); the call sites are the oracle instead.
-    factory({
-        a: () => 1,
-        b: () => 2,
-        // @ts-expect-error `c` is not part of the universe `"a" | "b"`
-        c: () => 3,
-    });
-    // @ts-expect-error a gap without a `_` fallback is not exhaustive
-    factory({ a: () => 1 });
-    // @ts-expect-error a `string` fallback does not fit the `number` handler
-    factory({ a: () => 1 }, () => "x");
-});
-
-test("getMatcherW factory rejects patterns outside its contract", () => {
-    // Arrange
-    const factory = getMatcherW<"x" | "y">();
-
-    // Act / Assert — the calls below must not compile
-    factory({
-        x: () => 1 as const,
-        y: () => 2 as const,
-        // @ts-expect-error `z` is not part of the universe `"x" | "y"`
-        z: () => 3 as const,
-    });
-    // @ts-expect-error a gap without a `_` fallback is not exhaustive
-    factory({ x: () => 1 as const });
-});
-
-// ============================================================================
-// Dispatch — runtime behavior
-// ============================================================================
-
-test("getMatcher: an unhandled shape throws without a fallback", () => {
-    // Arrange — an open universe types its handler map as an index signature,
-    // so the type system cannot prove the runtime map is exhaustive.
-    const handlers: Record<string, () => number> = { a: () => 1 };
-    const matcher = getMatcher<string>()(handlers);
-
-    // Act / Assert
-    assert.equal(matcher("a"), 1);
-    assert.throws(() => matcher("b"), /Unhandled shape: b/);
-});
-
-// ============================================================================
-// Autocomplete — the language server is the oracle, not the type system
-// ============================================================================
-
-// Completion is a contextual-type property that the type system cannot observe,
-// so the cases below read the popup from the repo's language server (via the
-// test helper) rather than pairing `expectTypeOf` with `assert` — see
-// development/testing.md § Autocomplete. They probe the *matcher's* overloads,
-// which is why they live with the matcher and not with the helper.
-const REPO_ROOT = path.resolve(import.meta.dirname, "..");
-const UNIVERSE = `"a" | "b" | "c"`;
-
-// One session per probe: the tests share no language-server state (open
-// documents, project membership), so they pass in any order.
-interface LabelsProbe {
-    readonly name: string;
-    readonly factory: "getMatcher" | "getMatcherW";
-    readonly body: string;
-    readonly tail?: string;
-}
-
-const labelsFor = ({
-    name,
-    factory,
-    body,
-    tail = "",
-}: LabelsProbe): Promise<readonly string[]> => {
-    const session = new LspSession(REPO_ROOT);
-    const target: CompletionTarget = {
-        file: `src/__autocomplete_${name}.ts`,
-        source: [
-            `import { ${factory} } from "./index.ts";`,
-            `const m = ${factory}<${UNIVERSE}>()({`,
-            body,
-            `}${tail});`,
-            "",
-        ].join("\n"),
-    };
-    return session
-        .completionLabelsAt(target)
-        .then((result) => result.labels)
-        .finally(() => session.close());
-};
-
-test("autocomplete: an exhaustive pattern requires the universe", () => {
-    // Arrange
-    const name = "getMatcher_fresh";
-
-    // Act
-    const labels = labelsFor({
-        name,
-        factory: "getMatcher",
-        body: "    /*COMPLETE*/",
-    });
-
-    // Assert
-    return labels.then((result) => {
-        assert.deepEqual([...result], ["a", "b", "c"]);
-    });
-});
-
-test("autocomplete: handled keys drop out of the popup", () => {
-    // Arrange
-    const name = "getMatcher_after_key";
-
-    // Act
-    const labels = labelsFor({
-        name,
-        factory: "getMatcher",
-        body: "    a: () => 1,\n    /*COMPLETE*/",
-    });
-
-    // Assert
-    return labels.then((result) => {
-        assert.deepEqual([...result], ["b", "c"]);
-    });
-});
-
-test("autocomplete: a fallback makes the remaining keys optional", () => {
-    // Arrange
-    const name = "getMatcher_with_fallback";
-
-    // Act
-    const labels = labelsFor({
-        name,
-        factory: "getMatcher",
-        body: "    /*COMPLETE*/",
-        tail: ", () => 0",
-    });
-
-    // Assert
-    return labels.then((result) => {
-        assert.deepEqual([...result], ["a?", "b?", "c?"]);
-    });
-});
-
-test("autocomplete: with a fallback, handled keys stay optional", () => {
-    // Arrange
-    const name = "getMatcher_with_fallback_after_key";
-
-    // Act
-    const labels = labelsFor({
-        name,
-        factory: "getMatcher",
-        body: "    a: () => 1,\n    /*COMPLETE*/",
-        tail: ", () => 0",
-    });
-
-    // Assert
-    return labels.then((result) => {
-        assert.deepEqual([...result], ["b?", "c?"]);
-    });
-});
-
-test("autocomplete: `getMatcherW` offers the same popup as `getMatcher`", () => {
-    // Arrange
-    const name = "getMatcherW_fresh";
-
-    // Act
-    const labels = labelsFor({
-        name,
-        factory: "getMatcherW",
-        body: "    /*COMPLETE*/",
-    });
-
-    // Assert
-    return labels.then((result) => {
-        assert.deepEqual([...result], ["a", "b", "c"]);
-    });
-});
-
-test("autocomplete: `getMatcherW` also offers optional keys with a fallback", () => {
-    // Arrange
-    const name = "getMatcherW_with_fallback";
-
-    // Act
-    const labels = labelsFor({
-        name,
-        factory: "getMatcherW",
-        body: "    /*COMPLETE*/",
-        tail: ", () => 0",
-    });
-
-    // Assert
-    return labels.then((result) => {
-        assert.deepEqual([...result], ["a?", "b?", "c?"]);
-    });
-});

src/primitive.ts

@@ -1,75 +0,0 @@
-import type { Exact, ValueOf } from "type-fest";
-
-type UnaryFn<T, R> = (shape: T) => R;
-
-// `Extract` drops optional handlers (`undefined`) so `PatternReturns` also works
-// when `P`'s constraint has optional keys.
-type PatternReturns<P> = ReturnType<
-    Extract<ValueOf<P>, (...args: never[]) => unknown>
->;
-
-type Handlers<T extends string | number, R> = { [K in T]: UnaryFn<K, R> };
-
-// The fallback is a *second argument*, not a property of the handler map,
-// because its parameter is the remainder `Exclude<T, keyof Handled>` and TypeScript
-// fixes a property's contextual type before it infers its sibling keys. A later
-// argument, by contrast, is contextually typed from inference on an earlier
-// one, so the split is what makes the remainder expressible at all.
-// See development/library.md.
-type Fallback<T extends string | number, Handled, R> = UnaryFn<
-    Exclude<T, keyof Handled>,
-    R
->;
-
-// TypeScript does not apply the excess-property check to a generic constraint,
-// so `Exact` restores it for the generic forms: a handler map can otherwise
-// carry keys outside `T`.
-
-// oxlint-disable typescript/unified-signatures
-// Strict returns: one common `R`. Overload order is load-bearing:
-//   #1 Handlers (first) -> the exhaustive form and the autocomplete popup
-//   #2 Fallback (last)  -> accepts a partial handler map plus a fallback
-interface MatcherStrict<T extends string | number> {
-    <R>(handlers: Handlers<T, R>): UnaryFn<T, R>;
-    <R, Handled extends Exact<Partial<Handlers<T, R>>, Handled>>(
-        handlers: Handled & Partial<Handlers<T, R>>,
-        fallback: Fallback<T, Handled, R>,
-    ): UnaryFn<T, R>;
-}
-// oxlint-enable typescript/unified-signatures
-
-// Widened returns: the union of every handler's return type. `P` is inferred
-// from the whole handler map, whose closed constraint supplies the
-// contextual/autocomplete type.
-// oxlint-disable typescript/unified-signatures
-interface MatcherWidening<T extends string | number> {
-    <P extends Exact<Handlers<T, unknown>, P>>(
-        handlers: P,
-    ): UnaryFn<T, PatternReturns<P>>;
-    <R, Handled extends Exact<Partial<Handlers<T, unknown>>, Handled>>(
-        handlers: Handled,
-        fallback: Fallback<T, Handled, R>,
-    ): UnaryFn<T, PatternReturns<Handled> | R>;
-}
-// oxlint-enable typescript/unified-signatures
-
-type HandlerMap = Record<string | number, UnaryFn<never, unknown> | undefined>;
-
-const dispatch =
-    (handlers: HandlerMap, fallback?: UnaryFn<never, unknown>) =>
-    (shape: string | number): unknown =>
-        (
-            handlers[shape] ??
-            fallback ??
-            (() => {
-                throw new Error(`Unhandled shape: ${shape}`);
-            })
-        )(
-            // oxlint-disable-next-line typescript/no-unsafe-type-assertion
-            shape as never,
-        );
-
-export const getMatcher = <T extends string | number>(): MatcherStrict<T> =>
-    dispatch;
-export const getMatcherW = <T extends string | number>(): MatcherWidening<T> =>
-    dispatch;

src/tagged-union.test.ts

@@ -0,0 +1,640 @@
+import { strict as assert } from "node:assert";
+import path from "node:path";
+import { test } from "node:test";
+
+import { expectTypeOf } from "expect-type";
+
+import {
+    type CompletionTarget,
+    LspSession,
+} from "#test-utils/lsp-completion.ts";
+
+import {
+    getTaggedUnionMatcher,
+    getTaggedUnionMatcherW,
+} from "./tagged-union.ts";
+
+interface Circle {
+    readonly kind: "circle";
+    readonly radius: number;
+}
+interface Square {
+    readonly kind: "square";
+    readonly side: number;
+}
+interface Triangle {
+    readonly kind: "triangle";
+    readonly base: number;
+    readonly height: number;
+}
+type Shape = Circle | Square | Triangle;
+
+// ============================================================================
+// API: getTaggedUnionMatcher — ✔️ Exhaustive / ✔️ ReturnsStrict
+// ============================================================================
+
+test("getTaggedUnionMatcher: exhaustive pattern infers one common return type", () => {
+    // Arrange
+    const factory = getTaggedUnionMatcher<Shape>()("kind");
+
+    // Act
+    const area = factory({
+        circle: (s) => {
+            // Each handler receives the member its tag selects, not the union.
+            expectTypeOf(s).toEqualTypeOf<Circle>();
+            assert.equal(s.kind, "circle");
+            return Math.PI * s.radius ** 2;
+        },
+        square: (s) => {
+            expectTypeOf(s).toEqualTypeOf<Square>();
+            assert.equal(s.kind, "square");
+            return s.side ** 2;
+        },
+        triangle: (s) => {
+            expectTypeOf(s).toEqualTypeOf<Triangle>();
+            assert.equal(s.kind, "triangle");
+            return (s.base * s.height) / 2;
+        },
+    });
+
+    // Assert
+    // ✔️ ReturnsStrict: R is the best common return type, not a widening union.
+    expectTypeOf(area).toEqualTypeOf<(shape: Shape) => number>();
+    assert.equal(area({ kind: "square", side: 2 }), 4);
+    assert.equal(area({ kind: "triangle", base: 2, height: 3 }), 3);
+});
+
+test("getTaggedUnionMatcher: dispatches on numeric tags", () => {
+    // Arrange
+    type Version =
+        | { readonly kind: 1; readonly a: number }
+        | {
+              readonly kind: 2;
+              readonly b: number;
+          };
+    const factory = getTaggedUnionMatcher<Version>()("kind");
+
+    // Act
+    const pick = factory({
+        1: (v) => {
+            expectTypeOf(v).toEqualTypeOf<{
+                readonly kind: 1;
+                readonly a: number;
+            }>();
+            assert.equal(v.kind, 1);
+            return v.a;
+        },
+        2: (v) => {
+            expectTypeOf(v).toEqualTypeOf<{
+                readonly kind: 2;
+                readonly b: number;
+            }>();
+            assert.equal(v.kind, 2);
+            return v.b;
+        },
+    });
+
+    // Assert
+    expectTypeOf(pick).toEqualTypeOf<(shape: Version) => number>();
+    assert.equal(pick({ kind: 1, a: 10 }), 10);
+    assert.equal(pick({ kind: 2, b: 20 }), 20);
+});
+
+test("getTaggedUnionMatcher: dispatches on boolean tags", () => {
+    // Arrange — a boolean discriminant is not a property key, so the handler
+    // map is keyed `true`/`false` and the callback still receives the boolean.
+    interface Ok {
+        readonly status: true;
+        readonly value: number;
+    }
+    interface Err {
+        readonly status: false;
+        readonly message: string;
+    }
+    type Result = Ok | Err;
+    const factory = getTaggedUnionMatcher<Result>()("status");
+
+    // Act
+    const pick = factory({
+        true: (s) => {
+            expectTypeOf(s).toEqualTypeOf<Ok>();
+            assert.equal(s.status, true);
+            return s.value;
+        },
+        false: (s) => {
+            expectTypeOf(s).toEqualTypeOf<Err>();
+            assert.equal(s.status, false);
+            return s.message.length;
+        },
+    });
+
+    // Assert
+    expectTypeOf(pick).toEqualTypeOf<(shape: Result) => number>();
+    assert.equal(pick({ status: true, value: 3 }), 3);
+    assert.equal(pick({ status: false, message: "hi" }), 2);
+});
+
+test("getTaggedUnionMatcher: dispatches on null and undefined tags", () => {
+    // Arrange
+    type Maybe =
+        | { readonly kind: null; readonly text: string }
+        | { readonly kind: undefined; readonly code: number };
+    const factory = getTaggedUnionMatcher<Maybe>()("kind");
+
+    // Act
+    const pick = factory({
+        null: (s) => {
+            expectTypeOf(s).toEqualTypeOf<{
+                readonly kind: null;
+                readonly text: string;
+            }>();
+            assert.equal(s.kind, null);
+            return s.text.length;
+        },
+        undefined: (s) => {
+            expectTypeOf(s).toEqualTypeOf<{
+                readonly kind: undefined;
+                readonly code: number;
+            }>();
+            assert.equal(s.kind, undefined);
+            return s.code;
+        },
+    });
+
+    // Assert
+    expectTypeOf(pick).toEqualTypeOf<(shape: Maybe) => number>();
+    assert.equal(pick({ kind: null, text: "hi" }), 2);
+    assert.equal(pick({ kind: undefined, code: 7 }), 7);
+});
+
+test("getTaggedUnionMatcher: a fallback receives unhandled boolean and nullish members", () => {
+    // Arrange
+    type Mixed =
+        | { readonly kind: "a"; readonly a: number }
+        | { readonly kind: true; readonly b: number }
+        | { readonly kind: false; readonly c: number }
+        | { readonly kind: null; readonly d: number };
+    const factory = getTaggedUnionMatcher<Mixed>()("kind");
+
+    // Act
+    const pick = factory({ a: () => 1 as const }, (s) => {
+        // The fallback sees the real members, not the projected keys.
+        expectTypeOf(s).toEqualTypeOf<
+            | { readonly kind: true; readonly b: number }
+            | { readonly kind: false; readonly c: number }
+            | { readonly kind: null; readonly d: number }
+        >();
+        // `true`/`false`/`null` are keyed as `"true"`/`"false"`/`"null"`,
+        // but the fallback still receives the boolean/null, not the string.
+        assert.ok(s.kind === true || s.kind === false || s.kind === null);
+        return 2 as const;
+    });
+
+    // Assert
+    expectTypeOf(pick).toEqualTypeOf<(shape: Mixed) => 1 | 2>();
+    assert.equal(pick({ kind: "a", a: 1 }), 1);
+    assert.equal(pick({ kind: true, b: 1 }), 2);
+    assert.equal(pick({ kind: false, c: 1 }), 2);
+    assert.equal(pick({ kind: null, d: 1 }), 2);
+});
+
+test("getTaggedUnionMatcherW: boolean and nullish tags widen to the union", () => {
+    // Arrange
+    type Maybe =
+        | { readonly kind: true; readonly a: number }
+        | { readonly kind: false; readonly b: number }
+        | { readonly kind: null; readonly c: number };
+    const factory = getTaggedUnionMatcherW<Maybe>()("kind");
+
+    // Act
+    const matcher = factory({
+        true: (s) => {
+            expectTypeOf(s).toEqualTypeOf<{
+                readonly kind: true;
+                readonly a: number;
+            }>();
+            assert.equal(s.kind, true);
+            return "yes" as const;
+        },
+        false: (s) => {
+            expectTypeOf(s).toEqualTypeOf<{
+                readonly kind: false;
+                readonly b: number;
+            }>();
+            assert.equal(s.kind, false);
+            return 0 as const;
+        },
+        null: (s) => {
+            expectTypeOf(s).toEqualTypeOf<{
+                readonly kind: null;
+                readonly c: number;
+            }>();
+            assert.equal(s.kind, null);
+            return null;
+        },
+    });
+
+    // Assert
+    expectTypeOf(matcher).toEqualTypeOf<(shape: Maybe) => "yes" | 0 | null>();
+    assert.equal(matcher({ kind: true, a: 1 }), "yes");
+    assert.equal(matcher({ kind: false, b: 1 }), 0);
+    assert.equal(matcher({ kind: null, c: 1 }), null);
+});
+
+// ============================================================================
+// API: getTaggedUnionMatcher — ❌ Exhaustive (fallback) / ✔️ ReturnsStrict
+// ============================================================================
+
+test("getTaggedUnionMatcher: a fallback receives the unhandled members", () => {
+    // Arrange
+    const factory = getTaggedUnionMatcher<Shape>()("kind");
+
+    // Act
+    const area = factory(
+        {
+            circle: (s) => {
+                expectTypeOf(s).toEqualTypeOf<Circle>();
+                assert.equal(s.kind, "circle");
+                return 1 as const;
+            },
+        },
+        (s) => {
+            // The fallback sees only the members `circle` did not handle.
+            expectTypeOf(s).toEqualTypeOf<Square | Triangle>();
+            assert.ok(s.kind === "square" || s.kind === "triangle");
+            return 2 as const;
+        },
+    );
+
+    // Assert
+    expectTypeOf(area).toEqualTypeOf<(shape: Shape) => 1 | 2>();
+    assert.equal(area({ kind: "circle", radius: 1 }), 1);
+    assert.equal(area({ kind: "square", side: 1 }), 2);
+    assert.equal(area({ kind: "triangle", base: 1, height: 1 }), 2);
+});
+
+test("getTaggedUnionMatcher: an open discriminant keeps the fallback open", () => {
+    // Arrange
+    interface Message {
+        readonly kind: string;
+        readonly text: string;
+    }
+    const factory = getTaggedUnionMatcher<Message>()("kind");
+
+    // Act
+    const matcher = factory({ info: () => 1 as const }, (s) => {
+        // The map's literal key does not close an open discriminant, so the
+        // remainder stays `Message` and the fallback is not redundant.
+        expectTypeOf(s).toEqualTypeOf<Message>();
+        assert.equal(typeof s.text, "string");
+        return 2 as const;
+    });
+
+    // Assert
+    expectTypeOf(matcher).toEqualTypeOf<(shape: Message) => 1 | 2>();
+    assert.equal(matcher({ kind: "info", text: "" }), 1);
+    assert.equal(matcher({ kind: "warn", text: "" }), 2);
+});
+
+// ============================================================================
+// API: getTaggedUnionMatcherW — ✔️ Exhaustive / ❌ ReturnsStrict
+// ============================================================================
+
+test("getTaggedUnionMatcherW: exhaustive pattern widens to the union of returns", () => {
+    // Arrange
+    const factory = getTaggedUnionMatcherW<Shape>()("kind");
+
+    // Act
+    const describe = factory({
+        circle: (s) => {
+            expectTypeOf(s).toEqualTypeOf<Circle>();
+            assert.equal(s.kind, "circle");
+            return "round" as const;
+        },
+        square: (s) => {
+            expectTypeOf(s).toEqualTypeOf<Square>();
+            assert.equal(s.kind, "square");
+            return 4 as const;
+        },
+        triangle: (s) => {
+            expectTypeOf(s).toEqualTypeOf<Triangle>();
+            assert.equal(s.kind, "triangle");
+            return true as const;
+        },
+    });
+
+    // Assert
+    // ❌ ReturnsStrict: mixed handler returns widen to their union.
+    expectTypeOf(describe).toEqualTypeOf<
+        (shape: Shape) => "round" | 4 | true
+    >();
+    assert.equal(describe({ kind: "circle", radius: 1 }), "round");
+    assert.equal(describe({ kind: "square", side: 1 }), 4);
+    assert.equal(describe({ kind: "triangle", base: 1, height: 1 }), true);
+});
+
+// ============================================================================
+// API: getTaggedUnionMatcherW — ❌ Exhaustive (fallback) / ❌ ReturnsStrict
+// ============================================================================
+
+test("getTaggedUnionMatcherW: a fallback widens gaps into the union", () => {
+    // Arrange
+    const factory = getTaggedUnionMatcherW<Shape>()("kind");
+
+    // Act
+    const matcher = factory({ circle: () => 1 as const }, (s) => {
+        expectTypeOf(s).toEqualTypeOf<Square | Triangle>();
+        // Returning the member verbatim lets the `Assert` block check the
+        // exact value dispatch passed.
+        return s;
+    });
+
+    // Assert
+    expectTypeOf(matcher).toEqualTypeOf<
+        (shape: Shape) => 1 | Square | Triangle
+    >();
+    assert.equal(matcher({ kind: "circle", radius: 1 }), 1);
+    assert.deepEqual(matcher({ kind: "square", side: 1 }), {
+        kind: "square",
+        side: 1,
+    });
+    assert.deepEqual(matcher({ kind: "triangle", base: 1, height: 1 }), {
+        kind: "triangle",
+        base: 1,
+        height: 1,
+    });
+});
+
+// ============================================================================
+// Factory contracts — calls that must not compile
+// ============================================================================
+
+test("getTaggedUnionMatcher factory rejects patterns outside its contract", () => {
+    // Arrange
+    const factory = getTaggedUnionMatcher<Shape>()("kind");
+
+    // Act / Assert — the calls below must not compile
+    factory({
+        circle: () => 1,
+        square: () => 2,
+        triangle: () => 3,
+        // @ts-expect-error `hexagon` is not a tag of Shape
+        hexagon: () => 4,
+    });
+    // @ts-expect-error a gap without a fallback is not exhaustive
+    factory({ circle: () => 1 });
+    // @ts-expect-error a `string` fallback does not fit the `number` handlers
+    factory({ circle: () => 1 }, () => "x");
+    // @ts-expect-error a fallback is redundant once the map covers the union
+    factory({ circle: () => 1, square: () => 2, triangle: () => 3 }, () => 0);
+});
+
+test("getTaggedUnionMatcher factory rejects a non-discriminant key", () => {
+    // Arrange
+    type Mixed =
+        | { readonly id: Date; readonly kind: "a" }
+        | { readonly id: Date; readonly kind: "b" };
+
+    // Act / Assert — the calls below must not compile
+    // @ts-expect-error `id` is a common key but its value is not a tag
+    getTaggedUnionMatcher<Mixed>()("id");
+    getTaggedUnionMatcher<Mixed>()("kind");
+});
+
+test("getTaggedUnionMatcher factory rejects non-exhaustive boolean and nullish maps", () => {
+    // Arrange
+    type Result =
+        | { readonly status: true; readonly a: number }
+        | { readonly status: false; readonly b: number };
+    type Maybe =
+        | { readonly status: null; readonly c: number }
+        | { readonly status: undefined; readonly d: number };
+    const resultFactory = getTaggedUnionMatcher<Result>()("status");
+    const maybeFactory = getTaggedUnionMatcher<Maybe>()("status");
+
+    // Act / Assert — the calls below must not compile
+    // @ts-expect-error only `true` is handled; `false` is not
+    resultFactory({ true: () => 1 });
+    // @ts-expect-error only `null` is handled; `undefined` is not
+    maybeFactory({ null: () => 1 });
+    // @ts-expect-error a fallback is redundant once both tags are handled
+    resultFactory({ true: () => 1, false: () => 2 }, () => 0);
+});
+
+test("getTaggedUnionMatcherW factory rejects patterns outside its contract", () => {
+    // Arrange
+    const factory = getTaggedUnionMatcherW<Shape>()("kind");
+
+    // Act / Assert — the calls below must not compile
+    factory({
+        circle: () => 1 as const,
+        square: () => 2 as const,
+        triangle: () => 3 as const,
+        // @ts-expect-error `hexagon` is not a tag of Shape
+        hexagon: () => 4 as const,
+    });
+    // @ts-expect-error a gap without a fallback is not exhaustive
+    factory({ circle: () => 1 as const });
+    factory(
+        // @ts-expect-error a fallback is redundant once the map covers the union
+        {
+            circle: () => 1 as const,
+            square: () => 2 as const,
+            triangle: () => 3 as const,
+        },
+        () => 0,
+    );
+});
+
+// ============================================================================
+// Dispatch — runtime behavior
+// ============================================================================
+
+test("getTaggedUnionMatcher: an unhandled tag throws without a fallback", () => {
+    // Arrange — an open discriminant widens its handler map to an index
+    // signature, so the type system cannot prove the runtime map is exhaustive.
+    interface Message {
+        readonly kind: string;
+    }
+    const handlers: Record<string, (shape: Message) => number> = {
+        info: (shape) => {
+            // The full member reaches the handler, not its tag.
+            assert.equal(shape.kind, "info");
+            return 1;
+        },
+    };
+    const matcher = getTaggedUnionMatcher<Message>()("kind")(handlers);
+
+    // Act / Assert
+    assert.equal(matcher({ kind: "info" }), 1);
+    assert.throws(() => matcher({ kind: "warn" }), /Unhandled tag: warn/);
+});
+
+// ============================================================================
+// Autocomplete — the language server is the oracle, not the type system
+// ============================================================================
+
+const REPO_ROOT = path.resolve(import.meta.dirname, "..");
+const SHAPE_SOURCE = `type Shape = { kind: "a"; a: number } | { kind: "b"; b: number } | { kind: "c"; c: number };`;
+
+interface LabelsProbe {
+    readonly name: string;
+    readonly factory: "getTaggedUnionMatcher" | "getTaggedUnionMatcherW";
+    readonly body: string;
+    readonly tail?: string;
+    readonly typeName?: string;
+    readonly typeSource?: string;
+}
+
+const labelsFor = ({
+    name,
+    factory,
+    body,
+    tail = "",
+    typeName = "Shape",
+    typeSource = SHAPE_SOURCE,
+}: LabelsProbe): Promise<readonly string[]> => {
+    const session = new LspSession(REPO_ROOT);
+    const target: CompletionTarget = {
+        file: `src/__autocomplete_${name}.ts`,
+        source: [
+            `import { ${factory} } from "./index.ts";`,
+            typeSource,
+            `const m = ${factory}<${typeName}>()("kind")({`,
+            body,
+            `}${tail});`,
+            "",
+        ].join("\n"),
+    };
+    return session
+        .completionLabelsAt(target)
+        .then((result) => result.labels)
+        .finally(() => session.close());
+};
+
+test("autocomplete: a tagged-union pattern requires the tags", () => {
+    // Arrange
+    const name = "tagged_union_fresh";
+
+    // Act
+    const labels = labelsFor({
+        name,
+        factory: "getTaggedUnionMatcher",
+        body: "    /*COMPLETE*/",
+    });
+
+    // Assert
+    return labels.then((result) => {
+        assert.deepEqual([...result], ["a", "b", "c"]);
+    });
+});
+
+test("autocomplete: a handled tag drops out of the popup", () => {
+    // Arrange
+    const name = "tagged_union_after_key";
+
+    // Act
+    const labels = labelsFor({
+        name,
+        factory: "getTaggedUnionMatcher",
+        body: "    a: () => 1,\n    /*COMPLETE*/",
+    });
+
+    // Assert
+    return labels.then((result) => {
+        assert.deepEqual([...result], ["b", "c"]);
+    });
+});
+
+test("autocomplete: a fallback makes the remaining tags optional", () => {
+    // Arrange
+    const name = "tagged_union_with_fallback";
+
+    // Act
+    const labels = labelsFor({
+        name,
+        factory: "getTaggedUnionMatcher",
+        body: "    /*COMPLETE*/",
+        tail: ", () => 0",
+    });
+
+    // Assert
+    return labels.then((result) => {
+        assert.deepEqual([...result], ["a?", "b?", "c?"]);
+    });
+});
+
+// `true`/`false`/`null`/`undefined` tags are not property keys, so the popup
+// must offer them by name through the `PatternKey` projection.
+const STATUS_SOURCE = `type Status = { kind: true; a: number } | { kind: false; b: number } | { kind: null; c: number } | { kind: undefined; d: number };`;
+
+test("autocomplete: boolean and nullish tags are offered by name", () => {
+    // Arrange
+    const name = "tagged_union_boolean_nullish";
+
+    // Act
+    const labels = labelsFor({
+        name,
+        factory: "getTaggedUnionMatcher",
+        typeName: "Status",
+        typeSource: STATUS_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 };`;
+
+const discriminantLabelsFor = ({
+    name,
+    factory,
+    typeName,
+    typeSource,
+}: {
+    readonly name: string;
+    readonly factory: "getTaggedUnionMatcher" | "getTaggedUnionMatcherW";
+    readonly typeName: string;
+    readonly typeSource: string;
+}): Promise<readonly string[]> => {
+    const session = new LspSession(REPO_ROOT);
+    const target: CompletionTarget = {
+        file: `src/__autocomplete_${name}.ts`,
+        source: [
+            `import { ${factory} } from "./index.ts";`,
+            typeSource,
+            `const m = ${factory}<${typeName}>()(/*COMPLETE*/);`,
+            "",
+        ].join("\n"),
+    };
+    return session
+        .completionLabelsAt(target)
+        .then((result) => result.labels)
+        .finally(() => session.close());
+};
+
+test("autocomplete: the discriminant key is offered at the key argument", () => {
+    // Arrange
+    const name = "tagged_union_discriminant";
+
+    // Act
+    const labels = discriminantLabelsFor({
+        name,
+        factory: "getTaggedUnionMatcher",
+        typeName: "Mixed",
+        typeSource: MIXED_SOURCE,
+    });
+
+    // Assert
+    return labels.then((result) => {
+        // The argument position also offers every global identifier, so assert
+        // inclusion of the discriminant and exclusion of the non-tag key.
+        assert.ok(result.includes('"kind"'));
+        assert.ok(!result.includes('"id"'));
+    });
+});

src/tagged-union.ts

@@ -0,0 +1,142 @@
+import type { Exact, UnknownRecord } from "type-fest";
+
+import type {
+    HandlerMap,
+    Matchable,
+    PatternKey,
+    PatternParam,
+    PatternReturns,
+    RedundantFallback,
+    UnaryFn,
+} from "./matcher-shared.ts";
+
+// A tagged union is discriminated by one property whose values are the tags (a
+// `Matchable`). `string` and `number` tags key a handler map directly;
+// `boolean`, `null` and `undefined` are admitted too but are not property keys,
+// so they go through the `PatternKey` projection. `symbol` has no literal syntax
+// to write a handler under, and `bigint` is not a property key.
+
+// The discriminant values of `T` under `K`. `Extract` keeps the finite literal
+// tags and leaves a widened `string`/`number` as itself, so an open universe
+// keeps an open fallback.
+type Tags<T extends object, K extends keyof T> = Extract<T[K], Matchable>;
+
+// The keys of `T` that can act as a discriminant. `getTaggedUnionMatcher<T>()`
+// accepts only these, so the factory rejects a key whose values are not tags.
+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
+// duplicated tag maps to a union of members rather than silently dropping one.
+// Keyed by `PatternKey`, so `boolean`/`null`/`undefined` tags can key a mapped
+// type; `PatternParam` inverts the projection to recover the member.
+type MapTaggedUnion<T extends object, K extends keyof T> = {
+    [P in PatternKey<Tags<T, K>>]: Extract<T, Record<K, PatternParam<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
+// index within `MapTaggedUnion`'s keys, and the conditional drops a stray key
+// outside `T` so it cannot widen the remainder. The remainder is
+// `Exclude<T, …>`, mirroring the primitive-union matcher's
+// `Exclude<T, PatternParam<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 open. See development/library.md.
+type Fallback<T extends object, K extends keyof T, Handled, R> = UnaryFn<
+    Exclude<T, HandledMembers<T, K, Handled>>,
+    R
+>;
+
+// A fallback is redundant once the handler map covers every tag of `T`. Folded
+// into `Handled`'s own (self-referential) constraint so it is checked *after*
+// inference; see the primitive-union matcher for why a conditional in the fallback's
+// parameter is evaluated too early.
+type MustBePartial<T extends object, K extends keyof T, Handled> =
+    PatternKey<Tags<T, K>> extends keyof Handled ? RedundantFallback : unknown;
+
+// TypeScript does not apply the excess-property check to a generic constraint,
+// so `Exact` restores it for the generic forms.
+
+// Strict returns: one common `R`. Overload order is load-bearing:
+//   #1 Handlers (first) -> the exhaustive form and the autocomplete popup
+//   #2 Fallback (last)  -> accepts a partial handler map plus a fallback
+interface TaggedUnionMatcherStrict<T extends object, K extends keyof T> {
+    <R>(handlers: Handlers<T, K, R>): UnaryFn<T, R>;
+    <
+        R,
+        Handled extends Exact<Partial<Handlers<T, K, R>>, Handled> &
+            MustBePartial<T, K, Handled>,
+    >(
+        handlers: Handled & Partial<Handlers<T, K, R>>,
+        fallback: Fallback<T, K, Handled, R>,
+    ): UnaryFn<T, R>;
+}
+
+// Widened returns: the union of every handler's return type. `P` is inferred
+// from the whole handler map, whose closed constraint supplies the
+// contextual/autocomplete type.
+interface TaggedUnionMatcherWidening<T extends object, K extends keyof T> {
+    <P extends Exact<Handlers<T, K, unknown>, P>>(
+        handlers: P,
+    ): UnaryFn<T, PatternReturns<P>>;
+    <
+        R,
+        Handled extends Exact<Partial<Handlers<T, K, unknown>>, Handled> &
+            MustBePartial<T, K, Handled>,
+    >(
+        handlers: Handled,
+        fallback: Fallback<T, K, Handled, R>,
+    ): UnaryFn<T, PatternReturns<Handled> | R>;
+}
+
+// The key-taking step of the curried factory. Naming it lets the factory return
+// `dispatch` directly, the tacit twin of the primitive-union factory's bare
+// `=> dispatch`.
+type TaggedUnionMatcherFactory<T extends object> = <K extends Discriminated<T>>(
+    k: K,
+) => TaggedUnionMatcherStrict<T, K>;
+
+type TaggedUnionMatcherWideningFactory<T extends object> = <
+    K extends Discriminated<T>,
+>(
+    k: K,
+) => TaggedUnionMatcherWidening<T, K>;
+
+const dispatch =
+    (k: PropertyKey) =>
+    (handlers: HandlerMap, fallback?: UnaryFn<never, unknown>) =>
+    (shape: object): unknown => {
+        // `object` carries no index signature, so the read needs the assertion;
+        // the factory admits only keys whose values are tags, and the map keys
+        // them by `PatternKey`, so the result is narrowed to the map's key space.
+        // oxlint-disable-next-line typescript/no-unsafe-type-assertion
+        const tag = (shape as UnknownRecord)[k] as string | number;
+        return (
+            handlers[tag] ??
+            fallback ??
+            (() => {
+                throw new Error(`Unhandled tag: ${String(tag)}`);
+            })
+        )(
+            // oxlint-disable-next-line typescript/no-unsafe-type-assertion
+            shape as never,
+        );
+    };
+
+export const getTaggedUnionMatcher = <
+    T extends object,
+>(): TaggedUnionMatcherFactory<T> => dispatch;
+
+export const getTaggedUnionMatcherW = <
+    T extends object,
+>(): TaggedUnionMatcherWideningFactory<T> => dispatch;

src/util/__tests__/lsp-completion.test.ts

@@ -1,4 +1,3 @@
-/* oxlint-disable typescript/no-floating-promises -- expectTypeOf() is a sync type-assertion library that the type-aware linter misidentifies as a promise */
 import { strict as assert } from "node:assert";
 import path from "node:path";
 import { test } from "node:test";