🚀 Release 0.8.0

Add property-union tests for the completion popup, a broad value in the
union, and value/stringification collisions. Rename the existing
property-union tests to a `property union:` prefix so they can be
selected with `--test-name-pattern`, and extend the autocomplete helper
with a `key` option so a probe can complete on a property other than
`kind`.

CHANGELOG.md

@@ -7,6 +7,25 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## [0.8.0] - 2026-09-23
+
+- reject a universe that mixes a literal with a broad type (e.g.
+  `"a" | \`x-${number}\``), which the finite-literal gate let through
+- narrow the tagged-union matcher's handler parameters for a union-valued or
+  optional discriminant, and its fallback to the unhandled tags, instead of
+  passing `never`
+
+## [0.7.1] - 2026-09-23
+
+- upgrade dependencies
+
+## [0.7.0] - 2026-09-23
+
+- reject broad universes (`string`, `number`, template literals) and
+  value/stringification collisions (`true | "true"`, `1 | "1"`) at the factory
+- type handler parameters by the matched member, so a
+  standalone `"true"` universe is typed `"true"` rather than `true`
+
 ## [0.6.0] - 2026-09-22
 
 - add `getTaggedUnionMatcher` / `getTaggedUnionMatcherW` for discriminated unions
@@ -75,7 +94,10 @@ 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/0.6.0...main
+[Unreleased]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.8.0...main
+[0.8.0]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.7.1...0.8.0
+[0.7.1]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.7.0...0.7.1
+[0.7.0]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.6.0...0.7.0
 [0.6.0]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.5.0...0.6.0
 [0.5.0]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.4.0...0.5.0
 [0.4.0]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.3.0...0.4.0

README.md

@@ -25,10 +25,15 @@ 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.
+- **Only finite universes are supported.** The factory must be given a finite
+  union of literals; `string`, `number` and template literals are rejected. This
+  is what lets the exhaustive overload be proven, so the runtime `dispatch`
+  throw stays unreachable through the typed API.
+- **A value and its stringification must not both be present.** Object keys
+  stringify, so a universe containing both a member and the string it
+  stringifies to — `1 | "1"`, `true | "true"`, `null | "null"` — is rejected at
+  the factory. Either form alone is fine, and one value's string form may
+  coexist with a _different_ value's bare form (`"true" | false`).
 - **`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.

backlog.tasks

@@ -6,17 +6,6 @@ Backlog and tracking for tiny-pattern-ts. Managed in vscode-todotasks format.
 
 Setup:
 ☐ Split off template into separate package => pi --session 01a07dde-7050-7054-bb36-1606d7eb2bc3 @low
-✔ straighten oxc rules @done
-    ✔ oxc forbids ternary => remove oxc rule @done
-    ✔ oxc wants comments to start comments with a capital letter => remove oxc rule @done
-    ✔ there already exists a /* */ block comment in primitive.ts, => change to multi-line comment @done
-
-✔ Remove example code files and its documentation and its exports from index.ts @high @done
-    ✔ remove src/match.ts and its documentation @high @done
-    ✔ remove src/pattern.ts and its documentation @high @done
-    ✔ remove src/index.test.ts and its documentation @high @done
-    ✔ exports from `src/index.ts` should only be the public API surface @done
-
 
 v1.0:
 ☐ API surface is stable and fully typed
@@ -27,54 +16,11 @@ v1.0:
     ☐ Achieve 100% branch coverage on `src/primitive-union.ts`
     ☐ Achieve 100% branch coverage on `src/index.ts`
 
-Testing:
-✔ Cover autocomplete with real completion test cases in the suite @medium @done
-    → `src/util/__tests__/lsp-completion.ts` (`#test-utils/…`) is the helper; the suite asserts its labels (see development/testing.md § Autocomplete)
-    → wire it into `node --test` so a test asserts the offered labels
-    → note: `Parameters<typeof factory>[0]` resolves only the *last* overload; use `@ts-expect-error` call sites for factory negatives, not `not.toExtend<Parameters<…>>`
-✔ Drive the LSP completion helper through the LSP protocol library instead of a hand-rolled JSON-RPC client @medium @done
-    ✔ Add `vscode-languageserver-protocol` and record the tooling decision @done
-    ✔ Rewrite `src/util/__tests__/lsp-completion.ts` onto `createMessageConnection` and typed requests @done
-    ✔ Update `development/testing.md § Autocomplete` for the new client @done
-    ✔ Run `npm run verify` and check the task off @done
-☐ Test a literal union widened with `(string & {})` — `"red" | "green" | "yellow" | (string & {})` @medium
-    → 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
-    → `getMatcher` / `getMatcherW`, each with overloads `ExhaustiveLoose` → `Fallback` → `Handlers` (order is load-bearing)
-    → fold into `src/primitive.ts` / the public API; drop `src/prototype*.ts`
-✔ `_` should receive only the unhandled `T` keys, not all of `T` @medium @done
-    → the fallback is now a second argument: `(handlers, (s) => …)`, `s: Exclude<T, keyof handlers>`
-✔ A fallback for an already-exhaustive handler map must be a compile error @medium @done
-    → rejected by an F-bounded constraint on `Handled` (checked *after* inference); a conditional in the fallback parameter is evaluated too early and breaks contextual typing
-✔ Implement matcher with similar API like matcher from primitive.ts @high @done
-    → `getTaggedUnionMatcher` / `getTaggedUnionMatcherW`, curried on the discriminant key
-    → fallback is the second argument; `_` removed
-✔ Rename `primitive` to `primitive-union` and `getMatcher` to `getPrimitiveMatcher` @medium @done
-    → `src/primitive.ts` / `src/primitive.test.ts` → `primitive-union.*`
-    → `getMatcherW` → `getPrimitiveMatcherW` for symmetry with the tagged-union pair
-    → update `src/index.ts`, `development/library.md` and any README references
-    → shipped as `getPrimitiveUnionMatcher` / `getPrimitiveUnionMatcherW`; kept `Union` for symmetry with the tagged-union pair
-☐ when using a union type as a property, the current behavior of tagged union matcher is
+✔ when using a union type as a property, the current behavior of tagged union matcher is @done
     to pass never to handler parameters
     → new matcher function needed or can be fixed in tagged union matcher
-    ☐ optional discriminant (`{ type?: "x" }`) is the same hole: the boolean/nullish change now admits the `undefined` tag, so the factory accepts the key, but `Extract<T, Record<K, V>>` still passes `never` to both the `x` and `undefined` handlers
-
-Bugs:
-✔ TS 7 LSP server logs `context canceled` on stderr at shutdown @done
-    → `handleExit` returns `io.EOF`, cancelling the background context while `Session.updateWatches` is still in flight; the bare error is flushed to stderr and the server exits 1
-    → close stdin after `shutdown` instead of sending `exit`; the server exits cleanly (code 0, no output), kill kept as a fallback
-Enhancements:
-✔ Allow boolean literals in primitive union patterns (e.g. `true: () => "yes"`) @medium @done
-    ✔ Are there other primitive types that should be supported in union patterns? (e.g. `bigint`, `symbol`) @medium @done
-        → added boolean, null and undefined; rejected `symbol` (compile-time brand, nothing at runtime) and `bigint` (not a property key)
-✔ Allow boolean, null and undefined discriminant values in tagged-union patterns @medium @done
-    → moved the `PatternKey` / `PatternParam` projection to `matcher-shared.ts` and keyed the tagged-union handler map through it; see development/library.md § Tagged-union matcher
+    ✔ 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 @done
 
 Documentation:
 ☐ Bring README.md back to its previous form — synopsis and examples restored, in the correct place
@@ -84,12 +30,8 @@ Documentation:
 ☐ Add comparison section vs. other TS pattern-matching libs in Readme.md
 ☐ Write migration guide for users coming from discriminated unions
 ☐ Create backlog tasks for implementation
-✔ Document the matcher design paths in `development/` — union vs overload merge, inferred universe (`NoInfer`), conditional `RequireKeys`, cases-first — and why each was abandoned @done
 ☐ Validate code fences in Markdown (start with README.md) — compile the TypeScript examples against `src/` so the docs cannot drift from the API
 
-Workflow:
-✔ Resolve the finish/push tension: `create:finish` leaves `main` ahead of its upstream while `create:branch` refuses until `main` matches upstream — decide whether `finish` should push or `branch` should compare only `BEHIND` (see development/workflow.md) @done
-
 Maintenance:
 ☐ Serve CI coverage over a tiny self-hosted webserver (replace the zip artifact) @low
     ☐ Explore serving coverage for non-tag pushes (e.g. `main/coverage`, PR previews) @low

cspell.json

@@ -43,7 +43,9 @@
         "silverwind",
         "idris",
         "todotasks",
-        "connor"
+        "connor",
+        "injective",
+        "injectivity"
     ],
     "ignorePaths": ["dist", "node_modules", "coverage", "*.svg", ".gitignore"]
 }

development/library.md

@@ -97,9 +97,11 @@ Each factory is two overloads whose order is load-bearing:
 
 #### 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.
+`src/matcher-shared.ts` holds the universe-agnostic pieces both matchers use:
+`UnaryFn`, `PatternReturns`, `RedundantFallback`, `HandlerMap`, the shared
+`Matchable` universe, the `PatternKey` key projection and its `Member` inverse,
+and the `Stringified` / `Collisions` / `UnsupportedReason` / `UnsupportedUniverse`
+/ `UniverseGate` universe gate.
 
 #### Why
 
@@ -118,8 +120,69 @@ use: `UnaryFn`, `PatternReturns`, `RedundantFallback`, `HandlerMap`, the shared
   `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.
+  contextual typing it exists to preserve. `Matchable`, the `PatternKey` /
+  `Member` projection and the `UniverseGate` are the pieces both universes
+  genuinely share.
+
+## Supported universes
+
+#### Decision (2026-09)
+
+A universe must be a **finite union of literals** with **no
+value/stringification collision**. Broad types (`string`, `number`, a template
+literal) and `"true" | true` / `1 | "1"` are rejected; the factory intersects
+`UniverseGate<T>` (the `UnsupportedUniverse<Reason>` diagnostic) into the
+handler and fallback parameters. `Member<T, K>` replaces the `PatternParam<K>`
+inversion: the handler parameter is the member(s) of `T` whose `PatternKey` is
+`K`, so a standalone `"true"` is `"true"`, not `true`.
+
+#### Why
+
+- **`PatternKey` is not injective.** `"true"` and `true` (and `1` / `"1"`)
+  share a runtime key, so `PatternParam<K>` cannot recover the member.
+  `Member<T, K>` inverts against `T`, which is exact.
+- **Broad types cannot be proven exhaustive.** An index-like map lets a partial
+  object satisfy the exhaustive overload and reaches the `dispatch` throw.
+  Rejecting at the boundary avoids threading an open/closed branch through
+  `Handlers`, `Fallback` and `MustBePartial`.
+- **The finite-literal predicate is `IsLiteral<PatternKey<T>> extends true`.**
+  `IsLiteral` is `boolean` for a union that mixes a literal with a broad type
+  (`"a" | \`x-${number}\``), so `extends false`would treat the mix as
+supported;`extends true` is the check that rejects it.
+- **Collisions are rejected, not merged.** `Member<T, K>` would be sound (the
+  handler gets the union), but the API is one handler per member; rejecting
+  keeps `Member` a singleton and the remainder exact.
+- **The collision predicate is type-checkable.** `Collisions<T> =
+Extract<T, Stringified<T>>` catches numeric collisions too.
+- **The gate is an intersection, not a branch,** so `R` inference and the popup
+  survive; a conditional parameter type would not.
+
+#### Rejected
+
+- **Open universes with a required fallback** (`fix/open-universe-*`): sound,
+  but left the collision hole and added an `IsLiteral` /
+  `OpenUniverseNeedsFallback` branch through every handler type. Findings, kept
+  so they are not re-run: `{}` satisfies an index signature (and `Exact` misses
+  it); an index signature dominates contextual typing; `R` infers only from a
+  non-self-referential parameter type (`{ [K in keyof H]: … R … }` gives
+  `unknown`); an F-bounded guard referencing `keyof Handled` in `Handled`'s own
+  constraint sees the constraint, not the map; all handlers share one `R` (only
+  the fallback widens it); `IsLiteral` is the finite-literal predicate.
+- **`Member<T, K>` without the gate:** sound, but a colliding handler gets a
+  union and `1 | "1"` stays one runtime key.
+- **A round-trip injectivity gate** (`IsEqual<T, PatternParam<PatternKey<T>>>`):
+  over-rejects standalone `"true"` / `"false"` / `"null"` / `"undefined"`.
+- **A case-list / ts-pattern builder:** removes the collision class but drops
+  the object map (footprint, popup) and reimplements an existing library.
+- **Normalize numeric keys to strings:** makes `PatternKey` injective but
+  changes "numeric keys stay numbers" and defeats the numeric dispatch fast
+  path.
+
+#### Known issue
+
+- A multi-collision universe lists every collision in the diagnostic.
+- The `dispatch` throw is unreachable through the typed API; the throw tests
+  widen the factory to `Function` to reach it.
 
 ## Tagged-union matcher
 
@@ -149,28 +212,41 @@ The key is a separate call because `K` is inferred from its literal argument and
 
 #### Why
 
-- **Same fallback/remainder machinery as the primitive-union matcher.** `HandledMembers`
-  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.
+- **Same fallback/remainder machinery as the primitive-union matcher.** `HandledTags`
+  recovers the tag values the map handled (`Member<Tags, keyof Handled>`) and
+  `Narrowed<T, K, Exclude<Tags, …>>` 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.
+- **`Narrowed` distributes over `T`, narrowing `K` to the tag.** A member whose
+  `K` cannot take the tag drops out; a duplicated tag yields a union of members
+  instead of dropping one. `T[K] extends V` returns the exact member (a
+  discriminated union's declared interface) untouched, so the mapped form only
+  handles a property that is itself a union.
+- **A union-valued or optional discriminant is supported.** A single shape whose
+  property is a union (`{ color: "red" | "green" | "blue" }`) is narrowed per
+  handler instead of being passed `never`. Matching a defined tag on an optional
+  property (`{ type?: "x" }`) proves the key is present, so it becomes required
+  (`{ type: "x" }`); the `undefined` tag narrows it to `{ type?: never }` under
+  `exactOptionalPropertyTypes` (absence) or `{ type?: undefined }` when the
+  property explicitly admits `undefined`.
 - **A `boolean` / `null` / `undefined` tag goes through the shared
-  `PatternKey` / `PatternParam` projection.** `Discriminated` admits those tags
+  `PatternKey` / `Member` 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`.
+  keyed by the stringified form (`true` → `"true"`) and `Member` inverts it
+  against the tag set 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).
+  collapse to a union under one handler. A tag colliding with its
+  stringification (`true | "true"`) is rejected by the universe gate — see
+  § Supported universes.
 
 ## Primitive universe
 
@@ -180,11 +256,12 @@ 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.
+projection (`PatternKey`: each member stringified) and `Member` inverts it
+against the universe, so callbacks receive the real member (`true`, not
+`"true"`; the standalone string `"true"` stays `"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.
+The supported universes are constrained as described in § Supported universes.
 
 #### Why
 
@@ -194,5 +271,5 @@ same projection is shared with the tagged-union matcher; see
   `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
+  stringification is rejected by the gate: user-facing, stated once in
   [README § Caveats](../README.md#caveats).

development/testing.md

@@ -198,6 +198,11 @@ the CLI is for manual inspection.
   delay is needed.
 - The server answers some requests with a string id (`client/registerCapability`);
   the client must tolerate `string | number` ids or the server stalls.
+- `LspSession.close()` sends `shutdown` and then closes stdin instead of
+  sending `exit`. The TS 7 Go server's `handleExit` returns `io.EOF`, cancelling
+  the background context while a watch update is still in flight, and logs a
+  bare `context canceled` before exiting 1; EOF on stdin exits 0 with no output.
+  The kill stays as a fallback for a server that does not exit.
 
 ## Known issues
 

development/tooling.md

@@ -196,6 +196,28 @@ joining the oxfmt-superseded rules already off.
   are part of the public API.
 - The narrower scope keeps the signal high without config-file boilerplate.
 
+### `knip` lists `src/index.ts` as an entry
+
+#### Decision (2026-09)
+
+`knip.json` declares `"entry": ["src/index.ts", "scripts/*.ts"]`.
+
+#### Why
+
+- Supplying `entry` **replaces** knip's default entry detection, which otherwise
+  derives the public entry from `package.json` `exports`. Adding `scripts/*.ts`
+  there therefore dropped the library entry, so knip resolved the package through
+  its `dist/index.js` output and reported the unreferenced source entry file
+  `src/index.ts` as an unused file.
+- Naming the source entry restores the link between the public API and the
+  source graph without pointing knip at build output.
+
+#### Rejected
+
+- `paths` mapping `dist/index.*` back to `src/index.ts`: more config to model a
+  relation the explicit entry states directly, and it would break whenever the
+  build layout changes.
+
 ### `maintain:outdated` ignores `@types/node`
 
 #### Decision (2026-09)

knip.json

@@ -1,5 +1,5 @@
 {
     "$schema": "./node_modules/knip/schema.json",
-    "entry": ["scripts/*.ts"],
+    "entry": ["src/index.ts", "scripts/*.ts"],
     "ignoreDependencies": ["@runwisp/pubv"]
 }

package.json

@@ -1,6 +1,6 @@
 {
     "name": "tiny-pattern-ts",
-    "version": "0.6.0",
+    "version": "0.8.0",
     "description": "Pattern matching for TypeScript/ESM environments (F#-style, not regex)",
     "keywords": [
         "adt",
@@ -83,7 +83,7 @@
         "expect-type": "1.4.0",
         "knip": "^6.34.0",
         "lefthook": "^2.1.12",
-        "oxfmt": "^0.68.0",
+        "oxfmt": "^0.70.0",
         "oxlint": "^1.83.0",
         "oxlint-tsgolint": "^7.0.2001",
         "publint": "^0.3.24",

src/matcher-shared.ts

@@ -1,4 +1,4 @@
-import type { ValueOf } from "type-fest";
+import type { IsLiteral, IsNever, 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
@@ -16,11 +16,11 @@ 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.
+// stringification. `Member` inverts that projection against the universe, 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"
@@ -30,15 +30,56 @@ export type PatternKey<T> = T extends boolean
       : 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;
+
+// The member(s) of `T` whose `PatternKey` is `K`: the universe-keyed inverse of
+// `PatternKey`. The key alone cannot recover the member (`"true"` and `true`
+// share it), so the handler parameter is derived from `T` instead. For a
+// supported (injective) universe the result is a single member.
+export type Member<
+    T extends Matchable,
+    K extends PropertyKey,
+> = T extends Matchable ? (PatternKey<T> extends K ? T : never) : never;
+
+// The property key a `Matchable` member takes at runtime: booleans, `null` and
+// `undefined` stringify, and a numeric literal becomes its decimal string.
+export type Stringified<T> = T extends boolean
+    ? T extends true
+        ? "true"
+        : "false"
+    : T extends null
+      ? "null"
+      : T extends undefined
+        ? "undefined"
+        : T extends number
+          ? `${T}`
+          : never;
+
+// The members of `T` that are also the stringification of another member, so
+// `PatternKey` cannot invert them. `never` means the universe is injective.
+export type Collisions<T> = Extract<T, Stringified<T>>;
+
+// Why `T` is not a supported universe, or `never` when it is. A supported
+// universe is a finite union of literals with no value/stringification
+// collision: only then can `PatternKey` be inverted unambiguously.
+export type UnsupportedReason<T extends Matchable> =
+    IsLiteral<PatternKey<T>> extends true
+        ? [Collisions<T>] extends [never]
+            ? never
+            : `a value and its stringification collide. Value is "${Collisions<T> & string}"`
+        : "broad types like string, number and template literals are not supported";
+
+// The diagnostic for an unsupported universe. Extends `HandlerMap` so the
+// implementation's `handlers: HandlerMap` stays assignable when the gate is
+// intersected into a parameter; the property name is the message.
+export type UnsupportedUniverse<Reason extends string> = HandlerMap &
+    Readonly<Record<`unsupported universe: ${Reason}`, never>>;
+
+// `unknown` for a supported universe (an intersection no-op), the diagnostic
+// otherwise. Intersecting rather than branching keeps `R` inference intact.
+export type UniverseGate<T extends Matchable> =
+    IsNever<UnsupportedReason<T>> extends true
+        ? unknown
+        : UnsupportedUniverse<UnsupportedReason<T>>;
 
 // `Extract` drops optional handlers (`undefined`) so `PatternReturns` also works
 // when `P`'s constraint has optional keys.

src/primitive-union.test.ts

@@ -495,26 +495,394 @@ test("getPrimitiveUnionMatcher factory rejects non-exhaustive boolean and nullis
     );
 });
 
-test("getPrimitiveUnionMatcher: an open universe keeps the fallback's remainder open", () => {
+test("getPrimitiveUnionMatcher: a standalone stringified literal is typed by the member, not its key", () => {
+    // `"true"` is a string member whose `PatternKey` is `"true"`; inverting the
+    // key must recover the string, not the boolean that also stringifies to it.
     // Arrange
-    const factory = getPrimitiveUnionMatcher<string>();
+    const factory = getPrimitiveUnionMatcher<"true" | "1">();
 
     // 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;
+    const matcher = factory({
+        true: (s) => {
+            expectTypeOf(s).toEqualTypeOf<"true">();
+            assert.equal(s, "true");
+            return 1;
+        },
+        "1": (s) => {
+            expectTypeOf(s).toEqualTypeOf<"1">();
+            assert.equal(s, "1");
+            return 2;
+        },
     });
 
     // Assert
-    expectTypeOf(matcher).toEqualTypeOf<(shape: string) => number>();
-    assert.equal(matcher("a"), 1);
-    assert.equal(matcher("b"), 2);
+    assert.equal(matcher("true"), 1);
+    assert.equal(matcher("1"), 2);
+});
+
+test("getPrimitiveUnionMatcher: a broad universe is rejected", () => {
+    // Act / Assert — the calls below must not compile
+    // @ts-expect-error `string` is not a finite union of literals
+    getPrimitiveUnionMatcher<string>()({ a: () => 1 });
+    // @ts-expect-error `number` is not a finite union of literals
+    getPrimitiveUnionMatcher<number>()({ 1: () => 1 });
+    // @ts-expect-error a template literal is open
+    getPrimitiveUnionMatcher<`a${string}`>()({ a: () => 1 });
+    // @ts-expect-error a literal mixed with a template literal is open
+    getPrimitiveUnionMatcher<"a" | `x-${number}`>()({ a: () => 1 });
+    // @ts-expect-error `string | boolean` is open because of `string`
+    getPrimitiveUnionMatcher<string | boolean>()({ true: () => 1 });
+    // @ts-expect-error the widening factory rejects broad universes too
+    getPrimitiveUnionMatcherW<string>()({ a: () => 1 });
+});
+
+test("getPrimitiveUnionMatcher: a value/stringification collision is rejected", () => {
+    // Act / Assert — the calls below must not compile
+    // @ts-expect-error `true` collides with the string `"true"`
+    getPrimitiveUnionMatcher<"true" | true>()({ true: () => 1 });
+    // @ts-expect-error `false` collides with the string `"false"`
+    getPrimitiveUnionMatcher<"false" | false>()({ false: () => 1 });
+    // @ts-expect-error `null` collides with the string `"null"`
+    getPrimitiveUnionMatcher<"null" | null>()({ null: () => 1 });
+    // @ts-expect-error `undefined` collides with the string `"undefined"`
+    getPrimitiveUnionMatcher<"undefined" | undefined>()({
+        undefined: () => 1,
+    });
+    // @ts-expect-error the number `1` collides with the string `"1"`
+    getPrimitiveUnionMatcher<1 | "1">()({ 1: () => 1 });
+});
+
+test("getPrimitiveUnionMatcher: a collision is rejected beside legal members", () => {
+    // Act / Assert — the calls below must not compile; the legal members do not
+    // mask the collision. Each map is otherwise complete, so only the collision
+    // can make it invalid.
+    // @ts-expect-error `true` collides with `"true"` beside legal members
+    getPrimitiveUnionMatcher<"true" | true | "a" | 2>()({
+        true: () => 1,
+        a: () => 2,
+        2: () => 3,
+    });
+    // @ts-expect-error `false` collides with `"false"` beside legal members
+    getPrimitiveUnionMatcher<"false" | false | "a" | 2>()({
+        false: () => 1,
+        a: () => 2,
+        2: () => 3,
+    });
+    // @ts-expect-error `null` collides with `"null"` beside legal members
+    getPrimitiveUnionMatcher<"null" | null | "a" | 2>()({
+        null: () => 1,
+        a: () => 2,
+        2: () => 3,
+    });
+    // @ts-expect-error `undefined` collides with `"undefined"` beside legal members
+    getPrimitiveUnionMatcher<"undefined" | undefined | "a" | 2>()({
+        undefined: () => 1,
+        a: () => 2,
+        2: () => 3,
+    });
+    // @ts-expect-error `1` collides with `"1"` beside legal members
+    getPrimitiveUnionMatcher<1 | "1" | "a" | true>()({
+        1: () => 1,
+        a: () => 2,
+        true: () => 3,
+    });
+});
+
+test('getPrimitiveUnionMatcher: `"true"` and a bare `false` are legal together', () => {
+    // Arrange — `false` stringifies to `"false"`, so it does not collide with
+    // the string member `"true"`.
+    const factory = getPrimitiveUnionMatcher<"true" | false | "a" | 2>();
+
+    // Act
+    const matcher = factory({
+        true: (s) => {
+            expectTypeOf(s).toEqualTypeOf<"true">();
+            assert.equal(s, "true");
+            return 1;
+        },
+        false: (s) => {
+            expectTypeOf(s).toEqualTypeOf<false>();
+            assert.equal(s, false);
+            return 2;
+        },
+        a: (s) => {
+            expectTypeOf(s).toEqualTypeOf<"a">();
+            assert.equal(s, "a");
+            return 3;
+        },
+        2: (s) => {
+            expectTypeOf(s).toEqualTypeOf<2>();
+            assert.equal(s, 2);
+            return 4;
+        },
+    });
+
+    // Assert
+    assert.equal(matcher("true"), 1);
+    assert.equal(matcher(false), 2);
+    assert.equal(matcher("a"), 3);
+    assert.equal(matcher(2), 4);
+});
+
+test('getPrimitiveUnionMatcher: `"false"` and a bare `true` are legal together', () => {
+    // Arrange — `true` stringifies to `"true"`, not `"false"`.
+    const factory = getPrimitiveUnionMatcher<"false" | true | "a" | 2>();
+
+    // Act
+    const matcher = factory({
+        false: (s) => {
+            expectTypeOf(s).toEqualTypeOf<"false">();
+            assert.equal(s, "false");
+            return 1;
+        },
+        true: (s) => {
+            expectTypeOf(s).toEqualTypeOf<true>();
+            assert.equal(s, true);
+            return 2;
+        },
+        a: (s) => {
+            expectTypeOf(s).toEqualTypeOf<"a">();
+            assert.equal(s, "a");
+            return 3;
+        },
+        2: (s) => {
+            expectTypeOf(s).toEqualTypeOf<2>();
+            assert.equal(s, 2);
+            return 4;
+        },
+    });
+
+    // Assert
+    assert.equal(matcher("false"), 1);
+    assert.equal(matcher(true), 2);
+    assert.equal(matcher("a"), 3);
+    assert.equal(matcher(2), 4);
+});
+
+test('getPrimitiveUnionMatcher: `"null"` and a bare `undefined` are legal together', () => {
+    // Arrange — `undefined` stringifies to `"undefined"`, not `"null"`.
+    const factory = getPrimitiveUnionMatcher<"null" | undefined | "a" | 2>();
+
+    // Act
+    const matcher = factory({
+        null: (s) => {
+            expectTypeOf(s).toEqualTypeOf<"null">();
+            assert.equal(s, "null");
+            return 1;
+        },
+        undefined: (s) => {
+            expectTypeOf(s).toEqualTypeOf<undefined>();
+            assert.equal(s, undefined);
+            return 2;
+        },
+        a: (s) => {
+            expectTypeOf(s).toEqualTypeOf<"a">();
+            assert.equal(s, "a");
+            return 3;
+        },
+        2: (s) => {
+            expectTypeOf(s).toEqualTypeOf<2>();
+            assert.equal(s, 2);
+            return 4;
+        },
+    });
+
+    // Assert
+    assert.equal(matcher("null"), 1);
+    assert.equal(matcher(undefined), 2);
+    assert.equal(matcher("a"), 3);
+    assert.equal(matcher(2), 4);
+});
+
+test('getPrimitiveUnionMatcher: `"undefined"` and a bare `null` are legal together', () => {
+    // Arrange — `null` stringifies to `"null"`, not `"undefined"`.
+    const factory = getPrimitiveUnionMatcher<"undefined" | null | "a" | 2>();
+
+    // Act
+    const matcher = factory({
+        undefined: (s) => {
+            expectTypeOf(s).toEqualTypeOf<"undefined">();
+            assert.equal(s, "undefined");
+            return 1;
+        },
+        null: (s) => {
+            expectTypeOf(s).toEqualTypeOf<null>();
+            assert.equal(s, null);
+            return 2;
+        },
+        a: (s) => {
+            expectTypeOf(s).toEqualTypeOf<"a">();
+            assert.equal(s, "a");
+            return 3;
+        },
+        2: (s) => {
+            expectTypeOf(s).toEqualTypeOf<2>();
+            assert.equal(s, 2);
+            return 4;
+        },
+    });
+
+    // Assert
+    assert.equal(matcher("undefined"), 1);
+    assert.equal(matcher(null), 2);
+    assert.equal(matcher("a"), 3);
+    assert.equal(matcher(2), 4);
+});
+
+test('getPrimitiveUnionMatcher: `"1"` beside other numbers and strings is legal', () => {
+    // Arrange — the task's example: `"1"` is the string member, `2` and `3`
+    // are bare numbers, `"4"` is another string. No stringification lands on
+    // another member.
+    const factory = getPrimitiveUnionMatcher<"1" | 2 | 3 | "4">();
+
+    // Act
+    const matcher = factory({
+        "1": (s) => {
+            expectTypeOf(s).toEqualTypeOf<"1">();
+            assert.equal(s, "1");
+            return 1;
+        },
+        2: (s) => {
+            expectTypeOf(s).toEqualTypeOf<2>();
+            assert.equal(s, 2);
+            return 2;
+        },
+        3: (s) => {
+            expectTypeOf(s).toEqualTypeOf<3>();
+            assert.equal(s, 3);
+            return 3;
+        },
+        "4": (s) => {
+            expectTypeOf(s).toEqualTypeOf<"4">();
+            assert.equal(s, "4");
+            return 4;
+        },
+    });
+
+    // Assert
+    assert.equal(matcher("1"), 1);
+    assert.equal(matcher(2), 2);
+    assert.equal(matcher(3), 3);
+    assert.equal(matcher("4"), 4);
+});
+
+// The maximum legal universe needs one handler per member; the statement count
+// is inherent to the case.
+// oxlint-disable-next-line max-statements
+test("getPrimitiveUnionMatcher: the maximum legal universe is supported", () => {
+    // Arrange — every Matchable kind, one representation per value, plus extra
+    // literals. No member's stringification is another member.
+    type Legal =
+        | "true"
+        | false
+        | null
+        | undefined
+        | "1"
+        | 2
+        | 3
+        | "a"
+        | "b"
+        | "4";
+    const factory = getPrimitiveUnionMatcher<Legal>();
+
+    // Act
+    const matcher = factory({
+        true: (s) => {
+            expectTypeOf(s).toEqualTypeOf<"true">();
+            assert.equal(s, "true");
+            return 1;
+        },
+        false: (s) => {
+            expectTypeOf(s).toEqualTypeOf<false>();
+            assert.equal(s, false);
+            return 2;
+        },
+        null: (s) => {
+            expectTypeOf(s).toEqualTypeOf<null>();
+            assert.equal(s, null);
+            return 3;
+        },
+        undefined: (s) => {
+            expectTypeOf(s).toEqualTypeOf<undefined>();
+            assert.equal(s, undefined);
+            return 4;
+        },
+        "1": (s) => {
+            expectTypeOf(s).toEqualTypeOf<"1">();
+            assert.equal(s, "1");
+            return 5;
+        },
+        2: (s) => {
+            expectTypeOf(s).toEqualTypeOf<2>();
+            assert.equal(s, 2);
+            return 6;
+        },
+        3: (s) => {
+            expectTypeOf(s).toEqualTypeOf<3>();
+            assert.equal(s, 3);
+            return 7;
+        },
+        a: (s) => {
+            expectTypeOf(s).toEqualTypeOf<"a">();
+            assert.equal(s, "a");
+            return 8;
+        },
+        b: (s) => {
+            expectTypeOf(s).toEqualTypeOf<"b">();
+            assert.equal(s, "b");
+            return 9;
+        },
+        "4": (s) => {
+            expectTypeOf(s).toEqualTypeOf<"4">();
+            assert.equal(s, "4");
+            return 10;
+        },
+    });
+
+    // Assert
+    expectTypeOf(matcher).toEqualTypeOf<(shape: Legal) => number>();
+    assert.equal(matcher("true"), 1);
+    assert.equal(matcher(false), 2);
+    assert.equal(matcher(null), 3);
+    assert.equal(matcher(undefined), 4);
+    assert.equal(matcher("1"), 5);
+    assert.equal(matcher(2), 6);
+    assert.equal(matcher(3), 7);
+    assert.equal(matcher("a"), 8);
+    assert.equal(matcher("b"), 9);
+    assert.equal(matcher("4"), 10);
+});
+
+test("getPrimitiveUnionMatcher: the maximum illegal universe is rejected", () => {
+    // Arrange — every value paired with its stringification.
+    type Illegal =
+        | "true"
+        | true
+        | "false"
+        | false
+        | "null"
+        | null
+        | "undefined"
+        | undefined
+        | "1"
+        | 1;
+
+    // Act / Assert — the call below must not compile
+    // @ts-expect-error every value collides with its stringification
+    getPrimitiveUnionMatcher<Illegal>()({
+        true: () => 1,
+        false: () => 2,
+        null: () => 3,
+        undefined: () => 4,
+        "1": () => 5,
+    });
+});
+
+test("getPrimitiveUnionMatcher: an open universe is rejected with a fallback too", () => {
+    // Act / Assert — the call below must not compile; a broad universe is
+    // unsupported whether or not a fallback is supplied.
+    // @ts-expect-error `string` is not a finite union of literals
+    getPrimitiveUnionMatcher<string>()({ a: () => 1 }, () => 2);
 });
 
 test("getPrimitiveUnionMatcherW factory rejects patterns outside its contract", () => {
@@ -539,16 +907,11 @@ test("getPrimitiveUnionMatcherW factory rejects patterns outside its contract",
 // ============================================================================
 
 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);
+    // Arrange — a finite universe is exhaustive through the typed API, so the
+    // defensive throw is reachable only by widening the factory and dropping a
+    // handler.
+    const bypass: Function = getPrimitiveUnionMatcher<"a" | "b">();
+    const matcher: (shape: "a" | "b") => number = bypass({ a: () => 1 });
 
     // Act / Assert
     assert.equal(matcher("a"), 1);
@@ -556,17 +919,15 @@ test("getPrimitiveUnionMatcher: an unhandled shape throws without a fallback", (
 });
 
 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.
+    // Arrange — as above; `true` indexes the map as the property `"true"`, but
+    // the handler is still called with the boolean.
+    const bypass: Function = getPrimitiveUnionMatcher<true | false>();
+    const matcher: (shape: boolean) => number = bypass({
+        true: (shape: boolean) => {
             assert.equal(shape, true);
             return 1;
         },
-    };
-    const matcher = getPrimitiveUnionMatcher<string | boolean>()(handlers);
+    });
 
     // Act / Assert
     assert.equal(matcher(true), 1);

src/primitive-union.ts

@@ -3,25 +3,26 @@ import type { Exact } from "type-fest";
 import type {
     HandlerMap,
     Matchable,
+    Member,
     PatternKey,
-    PatternParam,
     PatternReturns,
     RedundantFallback,
     UnaryFn,
+    UniverseGate,
 } from "./matcher-shared.ts";
 
 type Handlers<T extends Matchable, R> = {
-    [K in PatternKey<T>]: UnaryFn<PatternParam<K>, R>;
+    [K in PatternKey<T>]: UnaryFn<Member<T, 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
+// because its parameter is the remainder `Exclude<T, Member<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>>,
+    Exclude<T, Member<T, keyof Handled>>,
     R
 >;
 
@@ -44,14 +45,14 @@ type MustBePartial<T extends Matchable, Handled> =
 //   #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>(handlers: Handlers<T, R> & UniverseGate<T>): 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>,
+        handlers: Handled & Partial<Handlers<T, R>> & UniverseGate<T>,
+        fallback: Fallback<T, Handled, R> & UniverseGate<T>,
     ): UnaryFn<T, R>;
 }
 
@@ -60,15 +61,15 @@ interface PrimitiveUnionMatcherStrict<T extends Matchable> {
 // contextual/autocomplete type.
 interface PrimitiveUnionMatcherWidening<T extends Matchable> {
     <P extends Exact<Handlers<T, unknown>, P>>(
-        handlers: P,
+        handlers: P & UniverseGate<T>,
     ): UnaryFn<T, PatternReturns<P>>;
     <
         R,
         Handled extends Exact<Partial<Handlers<T, unknown>>, Handled> &
             MustBePartial<T, Handled>,
     >(
-        handlers: Handled,
-        fallback: Fallback<T, Handled, R>,
+        handlers: Handled & UniverseGate<T>,
+        fallback: Fallback<T, Handled, R> & UniverseGate<T>,
     ): UnaryFn<T, PatternReturns<Handled> | R>;
 }
 

src/tagged-union.test.ts

@@ -273,27 +273,17 @@ test("getTaggedUnionMatcher: a fallback receives the unhandled members", () => {
     assert.equal(area({ kind: "triangle", base: 1, height: 1 }), 2);
 });
 
-test("getTaggedUnionMatcher: an open discriminant keeps the fallback open", () => {
+test("getTaggedUnionMatcher: an open discriminant is rejected with a fallback too", () => {
     // 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);
+    // Act / Assert — the call below must not compile; a broad tag is unsupported
+    // whether or not a fallback is supplied.
+    // @ts-expect-error `string` tags are not a finite union of literals
+    getTaggedUnionMatcher<Message>()("kind")({ info: () => 1 }, () => 2);
 });
 
 // ============================================================================
@@ -446,28 +436,841 @@ test("getTaggedUnionMatcherW factory rejects patterns outside its contract", ()
     );
 });
 
+test("getTaggedUnionMatcher: a standalone stringified tag is typed by the member, not its key", () => {
+    // `kind: "true"` is a string tag whose `PatternKey` is `"true"`; inverting
+    // the key must recover the member, not the boolean tag.
+    // Arrange
+    interface StringTag {
+        readonly kind: "true";
+        readonly a: number;
+    }
+    const factory = getTaggedUnionMatcher<StringTag>()("kind");
+
+    // Act
+    const matcher = factory({
+        true: (s) => {
+            expectTypeOf(s).toEqualTypeOf<StringTag>();
+            assert.equal(s.kind, "true");
+            return 1;
+        },
+    });
+
+    // Assert
+    assert.equal(matcher({ kind: "true", a: 1 }), 1);
+});
+
+test("getTaggedUnionMatcher: a broad tag is rejected", () => {
+    // Arrange
+    interface Message {
+        readonly kind: string;
+    }
+    interface Count {
+        readonly kind: number;
+    }
+
+    // Act / Assert — the calls below must not compile
+    // @ts-expect-error `string` tags are not a finite union of literals
+    getTaggedUnionMatcher<Message>()("kind")({ a: () => 1 });
+    // @ts-expect-error `number` tags are not a finite union of literals
+    getTaggedUnionMatcher<Count>()("kind")({ 1: () => 1 });
+    // @ts-expect-error the widening factory rejects broad tags too
+    getTaggedUnionMatcherW<Message>()("kind")({ a: () => 1 });
+});
+
+test("getTaggedUnionMatcher: a value/stringification tag collision is rejected", () => {
+    // Arrange
+    type Colliding =
+        | { readonly kind: "true"; readonly a: number }
+        | { readonly kind: true; readonly b: number };
+    type NumericColliding =
+        | { readonly kind: "1"; readonly a: number }
+        | { readonly kind: 1; readonly b: number };
+
+    // Act / Assert — the calls below must not compile
+    // @ts-expect-error `true` collides with the string tag `"true"`
+    getTaggedUnionMatcher<Colliding>()("kind")({ true: () => 1 });
+    // @ts-expect-error the number tag `1` collides with the string tag `"1"`
+    getTaggedUnionMatcher<NumericColliding>()("kind")({ "1": () => 1 });
+});
+
+test("getTaggedUnionMatcher: a tag collision is rejected beside legal members", () => {
+    // Act / Assert — the calls below must not compile. Each map is otherwise
+    // complete, so only the collision can make it invalid.
+    type TrueColliding =
+        | { readonly kind: "true"; readonly a: number }
+        | { readonly kind: true; readonly b: number }
+        | { readonly kind: "a"; readonly c: number };
+    // @ts-expect-error `true` collides with the string tag `"true"`
+    getTaggedUnionMatcher<TrueColliding>()("kind")({
+        true: () => 1,
+        a: () => 2,
+    });
+
+    type FalseColliding =
+        | { readonly kind: "false"; readonly a: number }
+        | { readonly kind: false; readonly b: number }
+        | { readonly kind: "a"; readonly c: number };
+    // @ts-expect-error `false` collides with the string tag `"false"`
+    getTaggedUnionMatcher<FalseColliding>()("kind")({
+        false: () => 1,
+        a: () => 2,
+    });
+
+    type NullColliding =
+        | { readonly kind: "null"; readonly a: number }
+        | { readonly kind: null; readonly b: number }
+        | { readonly kind: "a"; readonly c: number };
+    // @ts-expect-error `null` collides with the string tag `"null"`
+    getTaggedUnionMatcher<NullColliding>()("kind")({
+        null: () => 1,
+        a: () => 2,
+    });
+
+    type UndefinedColliding =
+        | { readonly kind: "undefined"; readonly a: number }
+        | { readonly kind: undefined; readonly b: number }
+        | { readonly kind: "a"; readonly c: number };
+    // @ts-expect-error `undefined` collides with the string tag `"undefined"`
+    getTaggedUnionMatcher<UndefinedColliding>()("kind")({
+        undefined: () => 1,
+        a: () => 2,
+    });
+
+    type NumericColliding =
+        | { readonly kind: "1"; readonly a: number }
+        | { readonly kind: 1; readonly b: number }
+        | { readonly kind: "a"; readonly c: number };
+    // @ts-expect-error the number tag `1` collides with the string tag `"1"`
+    getTaggedUnionMatcher<NumericColliding>()("kind")({
+        "1": () => 1,
+        a: () => 2,
+    });
+});
+
+test('getTaggedUnionMatcher: a `"true"` tag and a `false` tag are legal together', () => {
+    // Arrange
+    interface TrueString {
+        readonly kind: "true";
+        readonly a: number;
+    }
+    interface FalseBare {
+        readonly kind: false;
+        readonly b: number;
+    }
+    interface Extra {
+        readonly kind: "a";
+        readonly c: number;
+    }
+    type Legal = TrueString | FalseBare | Extra;
+
+    // Act
+    const matcher = getTaggedUnionMatcher<Legal>()("kind")({
+        true: (s) => {
+            expectTypeOf(s).toEqualTypeOf<TrueString>();
+            assert.equal(s.kind, "true");
+            return 1;
+        },
+        false: (s) => {
+            expectTypeOf(s).toEqualTypeOf<FalseBare>();
+            assert.equal(s.kind, false);
+            return 2;
+        },
+        a: (s) => {
+            expectTypeOf(s).toEqualTypeOf<Extra>();
+            assert.equal(s.kind, "a");
+            return 3;
+        },
+    });
+
+    // Assert
+    assert.equal(matcher({ kind: "true", a: 1 }), 1);
+    assert.equal(matcher({ kind: false, b: 2 }), 2);
+    assert.equal(matcher({ kind: "a", c: 3 }), 3);
+});
+
+test('getTaggedUnionMatcher: a `"false"` tag and a `true` tag are legal together', () => {
+    // Arrange
+    interface FalseString {
+        readonly kind: "false";
+        readonly a: number;
+    }
+    interface TrueBare {
+        readonly kind: true;
+        readonly b: number;
+    }
+    interface Extra {
+        readonly kind: "a";
+        readonly c: number;
+    }
+    type Legal = FalseString | TrueBare | Extra;
+
+    // Act
+    const matcher = getTaggedUnionMatcher<Legal>()("kind")({
+        false: (s) => {
+            expectTypeOf(s).toEqualTypeOf<FalseString>();
+            assert.equal(s.kind, "false");
+            return 1;
+        },
+        true: (s) => {
+            expectTypeOf(s).toEqualTypeOf<TrueBare>();
+            assert.equal(s.kind, true);
+            return 2;
+        },
+        a: (s) => {
+            expectTypeOf(s).toEqualTypeOf<Extra>();
+            assert.equal(s.kind, "a");
+            return 3;
+        },
+    });
+
+    // Assert
+    assert.equal(matcher({ kind: "false", a: 1 }), 1);
+    assert.equal(matcher({ kind: true, b: 2 }), 2);
+    assert.equal(matcher({ kind: "a", c: 3 }), 3);
+});
+
+test('getTaggedUnionMatcher: a `"null"` tag and an `undefined` tag are legal together', () => {
+    // Arrange
+    interface NullString {
+        readonly kind: "null";
+        readonly a: number;
+    }
+    interface UndefinedBare {
+        readonly kind: undefined;
+        readonly b: number;
+    }
+    interface Extra {
+        readonly kind: "a";
+        readonly c: number;
+    }
+    type Legal = NullString | UndefinedBare | Extra;
+
+    // Act
+    const matcher = getTaggedUnionMatcher<Legal>()("kind")({
+        null: (s) => {
+            expectTypeOf(s).toEqualTypeOf<NullString>();
+            assert.equal(s.kind, "null");
+            return 1;
+        },
+        undefined: (s) => {
+            expectTypeOf(s).toEqualTypeOf<UndefinedBare>();
+            assert.equal(s.kind, undefined);
+            return 2;
+        },
+        a: (s) => {
+            expectTypeOf(s).toEqualTypeOf<Extra>();
+            assert.equal(s.kind, "a");
+            return 3;
+        },
+    });
+
+    // Assert
+    assert.equal(matcher({ kind: "null", a: 1 }), 1);
+    assert.equal(matcher({ kind: undefined, b: 2 }), 2);
+    assert.equal(matcher({ kind: "a", c: 3 }), 3);
+});
+
+test('getTaggedUnionMatcher: an `"undefined"` tag and a `null` tag are legal together', () => {
+    // Arrange
+    interface UndefinedString {
+        readonly kind: "undefined";
+        readonly a: number;
+    }
+    interface NullBare {
+        readonly kind: null;
+        readonly b: number;
+    }
+    interface Extra {
+        readonly kind: "a";
+        readonly c: number;
+    }
+    type Legal = UndefinedString | NullBare | Extra;
+
+    // Act
+    const matcher = getTaggedUnionMatcher<Legal>()("kind")({
+        undefined: (s) => {
+            expectTypeOf(s).toEqualTypeOf<UndefinedString>();
+            assert.equal(s.kind, "undefined");
+            return 1;
+        },
+        null: (s) => {
+            expectTypeOf(s).toEqualTypeOf<NullBare>();
+            assert.equal(s.kind, null);
+            return 2;
+        },
+        a: (s) => {
+            expectTypeOf(s).toEqualTypeOf<Extra>();
+            assert.equal(s.kind, "a");
+            return 3;
+        },
+    });
+
+    // Assert
+    assert.equal(matcher({ kind: "undefined", a: 1 }), 1);
+    assert.equal(matcher({ kind: null, b: 2 }), 2);
+    assert.equal(matcher({ kind: "a", c: 3 }), 3);
+});
+
+test('getTaggedUnionMatcher: a `"1"` tag beside other tags is legal', () => {
+    // Arrange
+    interface OneString {
+        readonly kind: "1";
+        readonly a: number;
+    }
+    interface TwoBare {
+        readonly kind: 2;
+        readonly b: number;
+    }
+    interface ThreeBare {
+        readonly kind: 3;
+        readonly c: number;
+    }
+    interface FourString {
+        readonly kind: "4";
+        readonly d: number;
+    }
+    type Legal = OneString | TwoBare | ThreeBare | FourString;
+
+    // Act
+    const matcher = getTaggedUnionMatcher<Legal>()("kind")({
+        "1": (s) => {
+            expectTypeOf(s).toEqualTypeOf<OneString>();
+            assert.equal(s.kind, "1");
+            return 1;
+        },
+        2: (s) => {
+            expectTypeOf(s).toEqualTypeOf<TwoBare>();
+            assert.equal(s.kind, 2);
+            return 2;
+        },
+        3: (s) => {
+            expectTypeOf(s).toEqualTypeOf<ThreeBare>();
+            assert.equal(s.kind, 3);
+            return 3;
+        },
+        "4": (s) => {
+            expectTypeOf(s).toEqualTypeOf<FourString>();
+            assert.equal(s.kind, "4");
+            return 4;
+        },
+    });
+
+    // Assert
+    assert.equal(matcher({ kind: "1", a: 1 }), 1);
+    assert.equal(matcher({ kind: 2, b: 2 }), 2);
+    assert.equal(matcher({ kind: 3, c: 3 }), 3);
+    assert.equal(matcher({ kind: "4", d: 4 }), 4);
+});
+
+// The maximum legal tag universe needs one handler per member; the statement
+// count is inherent to the case.
+// oxlint-disable-next-line max-statements
+test("getTaggedUnionMatcher: the maximum legal tag universe is supported", () => {
+    // Arrange — every tag kind, one representation per value, plus extra tags.
+    interface M1 {
+        readonly kind: "true";
+        readonly a: 1;
+    }
+    interface M2 {
+        readonly kind: false;
+        readonly b: 2;
+    }
+    interface M3 {
+        readonly kind: null;
+        readonly c: 3;
+    }
+    interface M4 {
+        readonly kind: undefined;
+        readonly d: 4;
+    }
+    interface M5 {
+        readonly kind: "1";
+        readonly e: 5;
+    }
+    interface M6 {
+        readonly kind: 2;
+        readonly f: 6;
+    }
+    interface M7 {
+        readonly kind: 3;
+        readonly g: 7;
+    }
+    interface M8 {
+        readonly kind: "a";
+        readonly h: 8;
+    }
+    interface M9 {
+        readonly kind: "b";
+        readonly i: 9;
+    }
+    type Legal = M1 | M2 | M3 | M4 | M5 | M6 | M7 | M8 | M9;
+
+    // Act
+    const matcher = getTaggedUnionMatcher<Legal>()("kind")({
+        true: (s) => {
+            expectTypeOf(s).toEqualTypeOf<M1>();
+            assert.equal(s.kind, "true");
+            return 1;
+        },
+        false: (s) => {
+            expectTypeOf(s).toEqualTypeOf<M2>();
+            assert.equal(s.kind, false);
+            return 2;
+        },
+        null: (s) => {
+            expectTypeOf(s).toEqualTypeOf<M3>();
+            assert.equal(s.kind, null);
+            return 3;
+        },
+        undefined: (s) => {
+            expectTypeOf(s).toEqualTypeOf<M4>();
+            assert.equal(s.kind, undefined);
+            return 4;
+        },
+        "1": (s) => {
+            expectTypeOf(s).toEqualTypeOf<M5>();
+            assert.equal(s.kind, "1");
+            return 5;
+        },
+        2: (s) => {
+            expectTypeOf(s).toEqualTypeOf<M6>();
+            assert.equal(s.kind, 2);
+            return 6;
+        },
+        3: (s) => {
+            expectTypeOf(s).toEqualTypeOf<M7>();
+            assert.equal(s.kind, 3);
+            return 7;
+        },
+        a: (s) => {
+            expectTypeOf(s).toEqualTypeOf<M8>();
+            assert.equal(s.kind, "a");
+            return 8;
+        },
+        b: (s) => {
+            expectTypeOf(s).toEqualTypeOf<M9>();
+            assert.equal(s.kind, "b");
+            return 9;
+        },
+    });
+
+    // Assert
+    expectTypeOf(matcher).toEqualTypeOf<(shape: Legal) => number>();
+    assert.equal(matcher({ kind: "true", a: 1 }), 1);
+    assert.equal(matcher({ kind: false, b: 2 }), 2);
+    assert.equal(matcher({ kind: null, c: 3 }), 3);
+    assert.equal(matcher({ kind: undefined, d: 4 }), 4);
+    assert.equal(matcher({ kind: "1", e: 5 }), 5);
+    assert.equal(matcher({ kind: 2, f: 6 }), 6);
+    assert.equal(matcher({ kind: 3, g: 7 }), 7);
+    assert.equal(matcher({ kind: "a", h: 8 }), 8);
+    assert.equal(matcher({ kind: "b", i: 9 }), 9);
+});
+
+test("getTaggedUnionMatcher: the maximum illegal tag universe is rejected", () => {
+    // Arrange — every tag paired with its stringification.
+    type Illegal =
+        | { readonly kind: "true"; readonly a: 1 }
+        | { readonly kind: true; readonly a: 1 }
+        | { readonly kind: "false"; readonly b: 2 }
+        | { readonly kind: false; readonly b: 2 }
+        | { readonly kind: "null"; readonly c: 3 }
+        | { readonly kind: null; readonly c: 3 }
+        | { readonly kind: "undefined"; readonly d: 4 }
+        | { readonly kind: undefined; readonly d: 4 }
+        | { readonly kind: "1"; readonly e: 5 }
+        | { readonly kind: 1; readonly e: 5 };
+
+    // Act / Assert — the call below must not compile
+    // @ts-expect-error every tag collides with its stringification
+    getTaggedUnionMatcher<Illegal>()("kind")({
+        true: () => 1,
+        false: () => 2,
+        null: () => 3,
+        undefined: () => 4,
+        "1": () => 5,
+    });
+});
+
+// ============================================================================
+// Property unions
+// ============================================================================
+
+test("property union: narrows each handler", () => {
+    // Arrange — one shape, not a union of variants; `color` is a union.
+    interface Paint {
+        readonly color: "red" | "green" | "blue";
+        readonly value: number;
+    }
+    const factory = getTaggedUnionMatcher<Paint>()("color");
+
+    // Act
+    const describe = factory({
+        red: (s) => {
+            expectTypeOf(s).toEqualTypeOf<{
+                readonly color: "red";
+                readonly value: number;
+            }>();
+            assert.equal(s.color, "red");
+            return s.value;
+        },
+        green: (s) => {
+            expectTypeOf(s).toEqualTypeOf<{
+                readonly color: "green";
+                readonly value: number;
+            }>();
+            assert.equal(s.color, "green");
+            return s.value;
+        },
+        blue: (s) => {
+            expectTypeOf(s).toEqualTypeOf<{
+                readonly color: "blue";
+                readonly value: number;
+            }>();
+            assert.equal(s.color, "blue");
+            return s.value;
+        },
+    });
+
+    // Assert
+    expectTypeOf(describe).toEqualTypeOf<(shape: Paint) => number>();
+    assert.equal(describe({ color: "red", value: 1 }), 1);
+    assert.equal(describe({ color: "green", value: 2 }), 2);
+    assert.equal(describe({ color: "blue", value: 3 }), 3);
+});
+
+test("property union: a fallback receives the unhandled values", () => {
+    // Arrange
+    interface Paint {
+        color: "red" | "green" | "blue";
+        value: number;
+    }
+    const factory = getTaggedUnionMatcher<Paint>()("color");
+
+    // Act
+    const describe = factory(
+        {
+            red: (s) => {
+                expectTypeOf(s).toEqualTypeOf<{
+                    color: "red";
+                    value: number;
+                }>();
+                assert.equal(s.color, "red");
+                return 1 as const;
+            },
+        },
+        (s) => {
+            expectTypeOf(s).toEqualTypeOf<{
+                color: "green" | "blue";
+                value: number;
+            }>();
+            assert.ok(s.color === "green" || s.color === "blue");
+            return 2 as const;
+        },
+    );
+
+    // Assert
+    expectTypeOf(describe).toEqualTypeOf<(shape: Paint) => 1 | 2>();
+    assert.equal(describe({ color: "red", value: 1 }), 1);
+    assert.equal(describe({ color: "green", value: 2 }), 2);
+    assert.equal(describe({ color: "blue", value: 3 }), 2);
+});
+
+test("property union: a numeric property narrows each handler", () => {
+    // Arrange
+    interface Version {
+        code: 1 | 2 | 3;
+        value: number;
+    }
+    const factory = getTaggedUnionMatcher<Version>()("code");
+
+    // Act
+    const describe = factory({
+        1: (s) => {
+            expectTypeOf(s).toEqualTypeOf<{ code: 1; value: number }>();
+            assert.equal(s.code, 1);
+            return 1;
+        },
+        2: (s) => {
+            expectTypeOf(s).toEqualTypeOf<{ code: 2; value: number }>();
+            assert.equal(s.code, 2);
+            return 2;
+        },
+        3: (s) => {
+            expectTypeOf(s).toEqualTypeOf<{ code: 3; value: number }>();
+            assert.equal(s.code, 3);
+            return 3;
+        },
+    });
+
+    // Assert
+    expectTypeOf(describe).toEqualTypeOf<(shape: Version) => number>();
+    assert.equal(describe({ code: 1, value: 10 }), 1);
+    assert.equal(describe({ code: 2, value: 20 }), 2);
+    assert.equal(describe({ code: 3, value: 30 }), 3);
+});
+
+test("property union: an optional property is required for a defined tag", () => {
+    // Arrange — `type?` is `"x" | undefined`; both handlers must be typed,
+    // and matching `"x"` proves the key is present, so `type` becomes required.
+    interface Maybe {
+        type?: "x";
+        value: number;
+    }
+    const factory = getTaggedUnionMatcher<Maybe>()("type");
+
+    // Act
+    const describe = factory({
+        x: (s) => {
+            expectTypeOf(s).toEqualTypeOf<{ type: "x"; value: number }>();
+            assert.equal(s.type, "x");
+            return 1;
+        },
+        undefined: (s) => {
+            // The input only allows absence (exactOptionalPropertyTypes), so
+            // the `undefined` tag narrows the key to never-present.
+            expectTypeOf(s).toEqualTypeOf<{ type?: never; value: number }>();
+            assert.equal(s.type, undefined);
+            return 2;
+        },
+    });
+
+    // Assert
+    expectTypeOf(describe).toEqualTypeOf<(shape: Maybe) => number>();
+    assert.equal(describe({ type: "x", value: 1 }), 1);
+    assert.equal(describe({ value: 2 }), 2);
+});
+
+test("property union: an optional fallback receives the undefined tag", () => {
+    // Arrange
+    interface Maybe {
+        type?: "x";
+        value: number;
+    }
+    const factory = getTaggedUnionMatcher<Maybe>()("type");
+
+    // Act
+    const describe = factory(
+        {
+            x: (s) => {
+                expectTypeOf(s).toEqualTypeOf<{ type: "x"; value: number }>();
+                assert.equal(s.type, "x");
+                return 1 as const;
+            },
+        },
+        (s) => {
+            expectTypeOf(s).toEqualTypeOf<{
+                type?: never;
+                value: number;
+            }>();
+            assert.equal(s.type, undefined);
+            return 2 as const;
+        },
+    );
+
+    // Assert
+    expectTypeOf(describe).toEqualTypeOf<(shape: Maybe) => 1 | 2>();
+    assert.equal(describe({ type: "x", value: 1 }), 1);
+    assert.equal(describe({ value: 2 }), 2);
+});
+
+test("property union: explicit `undefined` stays distinct from absence", () => {
+    // Arrange — `type?: "x" | undefined` admits an explicit `undefined`, unlike
+    // the bare optional above; the two representations must not be conflated.
+    interface Explicit {
+        type?: "x" | undefined;
+        value: number;
+    }
+    const factory = getTaggedUnionMatcher<Explicit>()("type");
+
+    // Act
+    const describe = factory({
+        x: (s) => {
+            expectTypeOf(s).toEqualTypeOf<{ type: "x"; value: number }>();
+            assert.equal(s.type, "x");
+            return 1;
+        },
+        undefined: (s) => {
+            expectTypeOf(s).toEqualTypeOf<{
+                type?: undefined;
+                value: number;
+            }>();
+            assert.equal(s.type, undefined);
+            return 2;
+        },
+    });
+
+    // Assert
+    expectTypeOf(describe).toEqualTypeOf<(shape: Explicit) => number>();
+    assert.equal(describe({ type: "x", value: 1 }), 1);
+    assert.equal(describe({ type: undefined, value: 2 }), 2);
+    assert.equal(describe({ value: 3 }), 2);
+});
+
+test("property union: a union-valued member is split, not dropped", () => {
+    // A member whose tag is itself a union sits beside a singleton-tag member.
+    // Selecting members with `Extract<T, Record<K, V>>` keeps only the
+    // singleton; the distributive narrowing must keep both sides of the
+    // union-valued member.
+    // Arrange
+    type Mixed = { kind: "a"; a: number } | { kind: "a" | "b"; b: number };
+    const factory = getTaggedUnionMatcher<Mixed>()("kind");
+
+    // Act
+    const describe = factory({
+        a: (s) => {
+            expectTypeOf(s).toEqualTypeOf<
+                { kind: "a"; a: number } | { kind: "a"; b: number }
+            >();
+            assert.equal(s.kind, "a");
+            return 1;
+        },
+        b: (s) => {
+            expectTypeOf(s).toEqualTypeOf<{ kind: "b"; b: number }>();
+            assert.equal(s.kind, "b");
+            return 2;
+        },
+    });
+
+    // Assert
+    expectTypeOf(describe).toEqualTypeOf<(shape: Mixed) => number>();
+    assert.equal(describe({ kind: "a", a: 1 }), 1);
+    assert.equal(describe({ kind: "a", b: 2 }), 1);
+    assert.equal(describe({ kind: "b", b: 3 }), 2);
+});
+
+test("property union: getTaggedUnionMatcherW widens the returns", () => {
+    // Arrange
+    interface Paint {
+        color: "red" | "green" | "blue";
+        value: number;
+    }
+    const factory = getTaggedUnionMatcherW<Paint>()("color");
+
+    // Act
+    const describe = factory({
+        red: (s) => {
+            expectTypeOf(s).toEqualTypeOf<{ color: "red"; value: number }>();
+            assert.equal(s.color, "red");
+            return "r" as const;
+        },
+        green: (s) => {
+            expectTypeOf(s).toEqualTypeOf<{ color: "green"; value: number }>();
+            assert.equal(s.color, "green");
+            return 1 as const;
+        },
+        blue: (s) => {
+            expectTypeOf(s).toEqualTypeOf<{ color: "blue"; value: number }>();
+            assert.equal(s.color, "blue");
+            return true as const;
+        },
+    });
+
+    // Assert
+    expectTypeOf(describe).toEqualTypeOf<(shape: Paint) => "r" | 1 | true>();
+    assert.equal(describe({ color: "red", value: 1 }), "r");
+    assert.equal(describe({ color: "green", value: 2 }), 1);
+    assert.equal(describe({ color: "blue", value: 3 }), true);
+});
+
+test("property union: enforces its contract", () => {
+    // Arrange
+    interface Paint {
+        color: "red" | "green" | "blue";
+        value: number;
+    }
+    const factory = getTaggedUnionMatcher<Paint>()("color");
+
+    // Act / Assert — the calls below must not compile
+    factory({
+        red: () => 1,
+        green: () => 2,
+        blue: () => 3,
+        // @ts-expect-error `yellow` is not a value of `color`
+        yellow: () => 4,
+    });
+    // @ts-expect-error a gap without a fallback is not exhaustive
+    factory({ red: () => 1 });
+    // @ts-expect-error a fallback is redundant once every value is handled
+    factory({ red: () => 1, green: () => 2, blue: () => 3 }, () => 0);
+});
+
+test("property union: a broad value in the union is rejected", () => {
+    // Arrange — the property is a union, but not a finite union of literals: a
+    // template literal cannot be proven exhaustive.
+    interface Template {
+        kind: "a" | `x-${number}`;
+        a: number;
+    }
+
+    // Act / Assert — the calls below must not compile
+    // @ts-expect-error a template literal is not a finite literal
+    getTaggedUnionMatcher<Template>()("kind")({ a: () => 1 });
+    // @ts-expect-error the widening factory rejects broad values too
+    getTaggedUnionMatcherW<Template>()("kind")({ a: () => 1 });
+});
+
+test("property union: a value/stringification collision is rejected", () => {
+    // Arrange — one property holding both a value and its stringification; both
+    // would key the same handler.
+    interface BooleanColliding {
+        kind: "true" | true;
+        a: number;
+    }
+    interface NumericColliding {
+        kind: "1" | 1;
+        a: number;
+    }
+
+    // Act / Assert — the calls below must not compile
+    // @ts-expect-error `true` collides with the string tag `"true"`
+    getTaggedUnionMatcher<BooleanColliding>()("kind")({ true: () => 1 });
+    // @ts-expect-error the number tag `1` collides with the string tag `"1"`
+    getTaggedUnionMatcher<NumericColliding>()("kind")({ "1": () => 1 });
+    // @ts-expect-error the widening factory rejects the collision too
+    getTaggedUnionMatcherW<BooleanColliding>()("kind")({ true: () => 1 });
+});
+
+test("property union: a collision is rejected beside legal members", () => {
+    // Arrange — the map is otherwise complete, so only the collision can make
+    // it invalid.
+    interface Colliding {
+        kind: "true" | true | "a";
+        a: number;
+    }
+
+    // Act / Assert — the call below must not compile
+    // @ts-expect-error `true` collides with the string tag `"true"`
+    getTaggedUnionMatcher<Colliding>()("kind")({ true: () => 1, a: () => 2 });
+});
+
 // ============================================================================
 // 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.
+    // Arrange — a finite tag set is exhaustive through the typed API, so the
+    // defensive throw is reachable only by widening the factory and dropping a
+    // handler.
     interface Message {
-        readonly kind: string;
+        readonly kind: "info" | "warn";
+        readonly text: string;
     }
-    const handlers: Record<string, (shape: Message) => number> = {
-        info: (shape) => {
+    const bypass: Function = getTaggedUnionMatcher<Message>()("kind");
+    const matcher: (shape: Message) => number = bypass({
+        info: (shape: Message) => {
             // 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/);
+    assert.equal(matcher({ kind: "info", text: "" }), 1);
+    assert.throws(
+        () => matcher({ kind: "warn", text: "" }),
+        /Unhandled tag: warn/,
+    );
 });
 
 // ============================================================================
@@ -484,6 +1287,7 @@ interface LabelsProbe {
     readonly tail?: string;
     readonly typeName?: string;
     readonly typeSource?: string;
+    readonly key?: string;
 }
 
 const labelsFor = ({
@@ -493,6 +1297,7 @@ const labelsFor = ({
     tail = "",
     typeName = "Shape",
     typeSource = SHAPE_SOURCE,
+    key = "kind",
 }: LabelsProbe): Promise<readonly string[]> => {
     const session = new LspSession(REPO_ROOT);
     const target: CompletionTarget = {
@@ -500,7 +1305,7 @@ const labelsFor = ({
         source: [
             `import { ${factory} } from "./index.ts";`,
             typeSource,
-            `const m = ${factory}<${typeName}>()("kind")({`,
+            `const m = ${factory}<${typeName}>()("${key}")({`,
             body,
             `}${tail});`,
             "",
@@ -587,6 +1392,91 @@ test("autocomplete: boolean and nullish tags are offered by name", () => {
     });
 });
 
+// A single shape whose property is a union: the popup is still keyed by the
+// property's values, not by the members of a union type.
+const PALETTE_SOURCE = `interface Palette { color: "red" | "green" | "blue"; value: number }`;
+const FLAG_SOURCE = `interface Flag { kind: true | false | null | undefined; a: number }`;
+
+test("property union: autocomplete offers the property's values", () => {
+    // Arrange
+    const name = "property_union_fresh";
+
+    // Act
+    const labels = labelsFor({
+        name,
+        factory: "getTaggedUnionMatcher",
+        typeName: "Palette",
+        typeSource: PALETTE_SOURCE,
+        key: "color",
+        body: "    /*COMPLETE*/",
+    });
+
+    // Assert
+    return labels.then((result) => {
+        assert.deepEqual([...result], ["blue", "green", "red"]);
+    });
+});
+
+test("property union: autocomplete drops a handled value", () => {
+    // Arrange
+    const name = "property_union_after_key";
+
+    // Act
+    const labels = labelsFor({
+        name,
+        factory: "getTaggedUnionMatcher",
+        typeName: "Palette",
+        typeSource: PALETTE_SOURCE,
+        key: "color",
+        body: "    red: () => 1,\n    /*COMPLETE*/",
+    });
+
+    // Assert
+    return labels.then((result) => {
+        assert.deepEqual([...result], ["blue", "green"]);
+    });
+});
+
+test("property union: autocomplete makes the remaining values optional with a fallback", () => {
+    // Arrange
+    const name = "property_union_with_fallback";
+
+    // Act
+    const labels = labelsFor({
+        name,
+        factory: "getTaggedUnionMatcher",
+        typeName: "Palette",
+        typeSource: PALETTE_SOURCE,
+        key: "color",
+        body: "    /*COMPLETE*/",
+        tail: ", () => 0",
+    });
+
+    // Assert
+    return labels.then((result) => {
+        assert.deepEqual([...result], ["blue?", "green?", "red?"]);
+    });
+});
+
+test("property union: autocomplete offers boolean and nullish values by name", () => {
+    // Arrange
+    const name = "property_union_boolean_nullish";
+
+    // Act
+    const labels = labelsFor({
+        name,
+        factory: "getTaggedUnionMatcher",
+        typeName: "Flag",
+        typeSource: FLAG_SOURCE,
+        body: "    /*COMPLETE*/",
+    });
+
+    // Assert
+    return labels.then((result) => {
+        assert.deepEqual([...result], ["false", "null", "true", "undefined"]);
+    });
+});
+
 // `Mixed` has two common keys, but `id`'s value is not a tag, so only `kind`
 // may serve as the discriminant.
 const MIXED_SOURCE = `type Mixed = { id: Date; kind: "a"; a: number } | { id: Date; kind: "b"; b: number };`;

src/tagged-union.ts

@@ -1,13 +1,14 @@
-import type { Exact, UnknownRecord } from "type-fest";
+import type { Exact, SetRequired, UnknownRecord } from "type-fest";
 
 import type {
     HandlerMap,
     Matchable,
+    Member,
     PatternKey,
-    PatternParam,
     PatternReturns,
     RedundantFallback,
     UnaryFn,
+    UniverseGate,
 } from "./matcher-shared.ts";
 
 // A tagged union is discriminated by one property whose values are the tags (a
@@ -17,8 +18,8 @@ import type {
 // 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.
+// tags and leaves a widened `string`/`number` as itself; a broad tag is then
+// rejected by the universe gate.
 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>()`
@@ -27,33 +28,49 @@ type Discriminated<T extends object> = {
     [K in keyof T]: T[K] extends Matchable ? K : never;
 }[keyof T];
 
-// The member(s) of `T` tagged `V`. `Extract` distributes over the union, so a
+// The member(s) of `T` narrowed to the tag(s) `V`. Distributes over `T`, so a
 // duplicated tag maps to a union of members rather than silently dropping one.
+// A member whose `K` cannot take `V` drops out; the rest have `K` narrowed to
+// `Extract<T[K], V>`. `T[K] extends V` keeps an exact member (a discriminated
+// union's declared interface) untouched, so only a property that is itself a
+// union — or optional — goes through the mapped form. `SetRequired` makes `K`
+// required unless `V` admits `undefined`: a defined tag yields `{ type: "x" }`,
+// while the `undefined` tag keeps `{ type?: never }` (absence) or
+// `{ type?: undefined }` when the property admits an explicit `undefined`.
 // Keyed by `PatternKey`, so `boolean`/`null`/`undefined` tags can key a mapped
-// type; `PatternParam` inverts the projection to recover the member.
+// type; `Member` inverts the projection against the tag set to recover the tag.
+type Narrowed<T extends object, K extends keyof T, V> = T extends object
+    ? [Extract<T[K], V>] extends [never]
+        ? never
+        : T[K] extends V
+          ? T
+          : undefined extends V
+            ? { [P in keyof T]: P extends K ? Extract<T[P], V> : T[P] }
+            : SetRequired<
+                  { [P in keyof T]: P extends K ? Extract<T[P], V> : T[P] },
+                  K
+              >
+    : never;
+
 type MapTaggedUnion<T extends object, K extends keyof T> = {
-    [P in PatternKey<Tags<T, K>>]: Extract<T, Record<K, PatternParam<P>>>;
+    [P in PatternKey<Tags<T, K>>]: Narrowed<T, K, Member<Tags<T, K>, P>>;
 };
 
 type Handlers<T extends object, K extends keyof T, R> = {
     [P in PatternKey<Tags<T, K>>]: UnaryFn<MapTaggedUnion<T, K>[P], R>;
 };
 
-// The members `Handled` covers. Mapping over the projected keys keeps every
-// 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 tag values whose `PatternKey` is handled.
+type HandledTags<T extends object, K extends keyof T, Handled> = Member<
+    Tags<T, K>,
+    keyof Handled
+>;
 
 // 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.
+// its parameter can be the remainder the map left uncovered: the members
+// narrowed to the tags the map did not handle. See development/library.md.
 type Fallback<T extends object, K extends keyof T, Handled, R> = UnaryFn<
-    Exclude<T, HandledMembers<T, K, Handled>>,
+    Narrowed<T, K, Exclude<Tags<T, K>, HandledTags<T, K, Handled>>>,
     R
 >;
 
@@ -71,14 +88,16 @@ type MustBePartial<T extends object, K extends keyof T, Handled> =
 //   #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>(handlers: Handlers<T, K, R> & UniverseGate<Tags<T, K>>): 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>,
+        handlers: Handled &
+            Partial<Handlers<T, K, R>> &
+            UniverseGate<Tags<T, K>>,
+        fallback: Fallback<T, K, Handled, R> & UniverseGate<Tags<T, K>>,
     ): UnaryFn<T, R>;
 }
 
@@ -87,15 +106,15 @@ interface TaggedUnionMatcherStrict<T extends object, K extends keyof T> {
 // contextual/autocomplete type.
 interface TaggedUnionMatcherWidening<T extends object, K extends keyof T> {
     <P extends Exact<Handlers<T, K, unknown>, P>>(
-        handlers: P,
+        handlers: P & UniverseGate<Tags<T, K>>,
     ): 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>,
+        handlers: Handled & UniverseGate<Tags<T, K>>,
+        fallback: Fallback<T, K, Handled, R> & UniverseGate<Tags<T, K>>,
     ): UnaryFn<T, PatternReturns<Handled> | R>;
 }