🔀 Merge chore/adopt-three-overload-matcher into main

CONTRIBUTING.md

@@ -67,6 +67,11 @@ before the implementation. The loop is **type → red → green → refactor**:
    `npm run verify` as the definition-of-done gate.
 
 Every test pairs an `expectTypeOf(...)` with an `assert.*`; keep them together.
+The autocomplete tests (`src/util/__tests__/lsp-completion.test.ts` for the
+helper, `src/primitive.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)).
 Each test body follows **AAA (Arrange–Act–Assert)** with labeled blocks
 separated by a blank line: `// Arrange` sets up the inputs (e.g. the matcher
 factory), `// Act` exercises the subject once from them (not a second

backlog.tasks

@@ -27,6 +27,21 @@ v1.0:
     ☐ Achieve 100% branch coverage on `src/primitive.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<…>>`
+
+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
+    → today `_: (shape: T) => R`; desired `_: (shape: Exclude<T, handledKeys>) => R`
+☐ An exhaustive pattern that also carries `_` must be a compile error @medium
+    → `{ a, b, _ }` for `T = "a" | "b"` is accepted today; the redundant `_` should be rejected
+
 Bugs:
 
 Enhancements:
@@ -41,6 +56,7 @@ 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:

development/library.md

@@ -3,4 +3,91 @@
 The type-level design of the public API and the limitations it carries. The
 user-facing reference is [README § API](../README.md#api).
 
-Currently the library is placeholder code.
+The matcher below is implemented in `src/primitive.ts` and re-exported from
+`src/index.ts` as `getMatcher` / `getMatcherW`; the rest of the library is
+placeholder code.
+
+## Matcher shape
+
+#### Decision (2026-09)
+
+A matcher is built by a factory and applied to a pattern:
+
+```ts
+const matcher = getMatcher<"a" | "b">()({ a: (s) => …, b: (s) => … });
+```
+
+Whether the pattern is exhaustive or has a fallback is decided **at the call
+site**, by whether it carries `_` — F#'s `| _ ->`. Only the return-strictness
+axis remains, so there are two factories:
+
+- `getMatcher` — one common `R`, the best common return type of every handler;
+- `getMatcherW` — the union of every handler's return type.
+
+Both are three overloads whose order is load-bearing:
+
+1. `ExhaustiveLoose<R, T>` = `{ [K in T]: UnaryFn<K, R> } & { _?: UnaryFn<T, R> }`
+2. `Fallback<R, T>` = `Partial<…> & { _: UnaryFn<T, R> }`
+3. `Handlers<R, T>` — the pure exhaustive shape
+
+#### Why
+
+- **Autocomplete reads the first overload, the error reads the last.**
+  TypeScript takes the first overload signature as the contextual type for the
+  object-literal popup, and the last for `No overload matches this call`. So
+  `ExhaustiveLoose` first yields the popup `_?, a, b` (T-keys required, `_`
+  optional) while `Handlers` last yields `Property 'b' is missing`. The two can be
+  tuned independently.
+- Two factories, not four: the fallback is a pattern _shape_, not a separate API.
+- The widened return union is derived from the pattern's handler types, so it
+  needs no fourth signature.
+
+#### Rejected
+
+- **Four factories** (exhaustive and fallback each split by return handling).
+  The exhaustive/fallback axis is expressible as one pattern type; four
+  signatures duplicate it.
+- **Union merge** — one type `Exhaustive<R,T> | (Partial<…> & { _: … })`,
+  explicit `<T>()`. Type-safe and completable, but TypeScript reports the
+  near-miss union member, so a missing key reads `Property '_' is missing`
+  instead of naming the key. Arm order does not change the report; the overload
+  split does.
+- **Overload merge with only the exhaustive arm last.** Fixes the missing-key
+  message, but a wrong `_` parameter is then reported against the exhaustive
+  arm, and `Parameters<typeof factory>` sees only one arm.
+- **Inferred universe** — `match(pattern)` with `T` taken from the keys
+  (exhaustive) or from `_`'s annotated parameter (fallback), via `NoInfer<T>` and
+  `_?: never`, split by overloads (a plain union merges inference; measured
+  `T = "_" | "a"`). No explicit `<T>`, and pipe-friendly. Rejected because: with
+  no declared universe the exhaustive popup offers only `_`; an unannotated `_`
+  widens `T` to `string | number`; and `NoInfer` leaks into the emitted `.d.ts`,
+  raising the consumer floor to TypeScript 5.4 (README promises `>= 5.0`).
+- **Conditional `RequireKeys`** — parameter
+  `P & ("_" extends keyof P ? unknown : Handlers<R, T>)`. Gives the good
+  missing-key message, but `keyof P` counts _optional_ keys: a widened value
+  whose declared type has `_?:` bypasses the completeness check. Demanding a
+  required `_` instead rejects that case but breaks `P` inference — `P` falls back
+  to its constraint and partial literals then demand every key. Typos also need a
+  `NoExtra` guard, whose message degrades to `not assignable to never`.
+- **Cases-first curried** — `match(["a", "b"])({ a: …, b: … })`. Completion works
+  for exhaustive patterns, and the array is a single source of truth for the
+  runtime list and the union. Rejected as not pipe-friendly; it needs a runtime
+  array; and the single-call form `match(cases, pattern)` cannot infer `R` (the
+  mapped key type `K[number]` stays deferred, so `R` widens to `unknown`).
+
+#### Known issue
+
+- The `_` handler receives **all** of `T`, not the unhandled subset
+  (`Exclude<T, handledKeys>`).
+- An exhaustive pattern that also carries `_` is accepted; the redundant `_`
+  should be a compile error.
+- The widened overloads carry a completeness guard
+  `keyof P extends T | "_" ? unknown : never`, because TypeScript does not apply
+  the excess-property check to a generic constraint: a generic parameter accepts
+  extra keys, a parameter typed as a concrete object type does not.
+  `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).

development/testing.md

@@ -30,7 +30,8 @@ before the implementation.
 - Testing the type only: it would not catch handler dispatch or the `_`
   fallback (see `src/primitive.test.ts`).
 
-The runner is `node --test --strip-types "src/**/*.test.ts"` and the tiers are in
+The runner is `node --test --strip-types "src/**/*.test.ts"` and the tiers are
+in
 [CONTRIBUTING.md § Development commands](../CONTRIBUTING.md#development-commands).
 `c8` uses V8 coverage, so the `--strip-types` source is instrumented without a
 build step, and the runner relies on the `.ts` import-extension convention (see
@@ -95,7 +96,11 @@ import { LspSession } from "#test-utils/lsp-completion.ts";
   loader needed.
 - The helper uses `node:` builtins (it drives a language server), so
   `import/no-nodejs-modules` is off for `src/util/__tests__/**` in
-  `.oxlintrc.json` — the same exception the old `scripts/**` scope carried.
+  `.oxlintrc.json` — the same exception the `scripts/**` scope carries.
+- Helpers carry no library coupling: their tests probe in-memory documents
+  whose contextual types are written inline, so only the helper's own contract
+  (marker handling, position, label extraction) is under test. A test that
+  asserts a _matcher's_ popup belongs with the matcher.
 
 #### Rejected
 
@@ -111,11 +116,59 @@ import { LspSession } from "#test-utils/lsp-completion.ts";
   still guards non-test helpers importing each other, and the exemption is
   only needed for the one specifier the mapping already solves cleanly.
 
+## Autocomplete
+
+#### Decision (2026-09)
+
+Completion is verified by driving the repo's own language server
+(`tsc --lsp --stdio`, the same server pi's LSP extension talks to) through
+the test helper `#test-utils/lsp-completion.ts`
+(`src/util/__tests__/lsp-completion.ts`), not through the type system:
+
+```sh
+node --strip-types src/util/__tests__/lsp-completion.ts <file> [<marker>]
+```
+
+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;
+the CLI is for manual inspection.
+
+#### Why
+
+- Completion is a contextual-type property: it depends on which overload
+  signature TypeScript picks for the object literal, and no type-level assertion
+  observes that.
+- `Parameters<typeof factory>[0]` resolves only the _last_ overload, so it is
+  not the popup's contextual type either — see
+  [library.md § Matcher shape](./library.md#matcher-shape).
+- The server is the only ground truth; the script reproduces what the editor
+  shows.
+
+#### Rejected
+
+- **`expect-type` would not work**: there is no operator for “the popup offers
+  these labels”. `toExtend` / `toEqualTypeOf` test assignability and cannot say
+  which overload supplied the contextual type.
+- **Checking by hand in the editor**: not reproducible in review or by an agent.
+- **`@ts-expect-error` at a completion position**: it asserts the absence of a
+  compile error, not the presence of specific labels.
+
+#### Known issue
+
+- Each test spawns its own `tsc` server so the tests share no state and pass in
+  any order; the file is an integration test (~1.6 s) that needs `node_modules`.
+  `didOpen` is handled in order before the completion request, so no settle
+  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.
+
 ## Known issues
 
 - The type-aware linter misidentifies `expectTypeOf()` as a floating promise, so
-  test files that use it (`src/primitive.test.ts`) carry a
+  test files that use it (`src/primitive.test.ts`) carry a file-level
-  file-level `oxlint-disable
+  `oxlint-disable typescript/no-floating-promises` with an explanatory comment.
-typescript/no-floating-promises` with an explanatory comment. It is a known
+  It is a known false positive, not a rule worth disabling project-wide (see
-  false positive, not a rule worth disabling project-wide (see
   [tooling.md § oxlint-disable directives live next to the code](./tooling.md#oxlint-disable-directives-live-next-to-the-code)).

src/index.ts

@@ -1,6 +1 @@
-export {
+export { getMatcher, getMatcherW } from "./primitive.ts";
-    getPrimitiveUnionMatcher,
-    getPrimitiveUnionMatcherPartial,
-    getPrimitiveUnionMatcherPartialW,
-    getPrimitiveUnionMatcherW,
-} from "./primitive.ts";

src/primitive.test.ts

@@ -1,50 +1,49 @@
 /* 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 {
-    getPrimitiveUnionMatcher,
+    type CompletionTarget,
-    getPrimitiveUnionMatcherPartial,
+    LspSession,
-    getPrimitiveUnionMatcherPartialW,
+} from "#test-utils/lsp-completion.ts";
-    getPrimitiveUnionMatcherW,
+
-} from "./primitive.ts";
+import { getMatcher, getMatcherW } from "./primitive.ts";
 
 // ============================================================================
-// API: getPrimitiveUnionMatcherW — ✔️ Exhaustive / ❌ ReturnsStrict
+// API: getMatcher — ✔️ Exhaustive / ✔️ ReturnsStrict
 // ============================================================================
 
-test("getPrimitiveUnionMatcherW requires every literal key", () => {
+test("getMatcher: exhaustive pattern infers one common return type", () => {
     // Arrange
-    const factory = getPrimitiveUnionMatcherW<"a" | "b">();
+    const factory = getMatcher<"a" | "b">();
 
     // Act
     const matcher = factory({
-        a: (s) => {
+        a: (s): number => {
             expectTypeOf(s).toEqualTypeOf<"a">();
-            return 1 as const;
+            return 1;
         },
-        b: (s) => {
+        b: (s): 1 | 2 => {
             expectTypeOf(s).toEqualTypeOf<"b">();
-            return "two" as const;
+            return 2;
         },
     });
 
     // Assert
-    // ❌ ReturnsStrict: mixed handler returns widen to their union.
+    // ✔️ ReturnsStrict: R is the best common return type, not a widening union.
-    expectTypeOf(matcher).toEqualTypeOf<(shape: "a" | "b") => 1 | "two">();
+    expectTypeOf(matcher).toEqualTypeOf<(shape: "a" | "b") => number>();
-    // A pattern missing a key must not satisfy the parameter type.
+    // ❌ a value outside T must not be accepted by the matcher
-    expectTypeOf<{
+    expectTypeOf<"c">().not.toExtend<Parameters<typeof matcher>[0]>();
-        a: () => number;
-    }>().not.toExtend<Parameters<typeof factory>[0]>();
     assert.equal(matcher("a"), 1);
-    assert.equal(matcher("b"), "two");
+    assert.equal(matcher("b"), 2);
 });
 
-test("getPrimitiveUnionMatcherW dispatches on numeric literal keys", () => {
+test("getMatcher: dispatches on numeric literal keys", () => {
     // Arrange
-    const factory = getPrimitiveUnionMatcherW<1 | 2>();
+    const factory = getMatcher<1 | 2>();
 
     // Act
     const matcher = factory({
@@ -64,9 +63,150 @@ test("getPrimitiveUnionMatcherW dispatches on numeric literal keys", () => {
     assert.equal(matcher(2), 20);
 });
 
-test("getPrimitiveUnionMatcherW dispatches on mixed string and numeric keys", () => {
+test("getMatcher: dispatches on mixed string and numeric keys", () => {
     // Arrange
-    const factory = getPrimitiveUnionMatcherW<"a" | "b" | 1 | 2>();
+    const factory = getMatcher<"a" | "b" | 1 | 2>();
+
+    // Act
+    const matcher = factory({
+        a: (s): number => {
+            expectTypeOf(s).toEqualTypeOf<"a">();
+            return 1;
+        },
+        b: (s): 1 | 2 => {
+            expectTypeOf(s).toEqualTypeOf<"b">();
+            return 2;
+        },
+        1: (n): number => {
+            expectTypeOf(n).toEqualTypeOf<1>();
+            return 10;
+        },
+        2: (n): number => {
+            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 makes the universe keys optional", () => {
+    // Arrange
+    const factory = getMatcher<"a" | "b" | "c">();
+
+    // Act
+    const matcher = factory({
+        a: (s): 1 | 2 => {
+            expectTypeOf(s).toEqualTypeOf<"a">();
+            return 1;
+        },
+        _: (s): 1 | 2 => {
+            // The fallback sees the whole union, not a single literal.
+            expectTypeOf(s).toEqualTypeOf<"a" | "b" | "c">();
+            return 2;
+        },
+    });
+
+    // 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 also accepts an exhaustive pattern", () => {
+    // Arrange
+    const factory = getMatcher<"a" | "b">();
+
+    // Act
+    const matcher = factory({
+        a: (s) => {
+            expectTypeOf(s).toEqualTypeOf<"a">();
+            return "A";
+        },
+        b: (s) => {
+            expectTypeOf(s).toEqualTypeOf<"b">();
+            return "B";
+        },
+    });
+
+    // Assert
+    expectTypeOf(matcher).toEqualTypeOf<(shape: "a" | "b") => string>();
+    assert.equal(matcher("a"), "A");
+    assert.equal(matcher("b"), "B");
+});
+
+test("getMatcher: a `_` fallback routes mixed string and numeric gaps", () => {
+    // Arrange
+    const factory = getMatcher<"a" | "b" | 1 | 2>();
+
+    // Act
+    const matcher = factory({
+        a: (s): string => {
+            expectTypeOf(s).toEqualTypeOf<"a">();
+            return "A";
+        },
+        1: (n): string => {
+            expectTypeOf(n).toEqualTypeOf<1>();
+            return "one";
+        },
+        _: (s): string => {
+            expectTypeOf(s).toEqualTypeOf<"a" | "b" | 1 | 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({
@@ -99,181 +239,12 @@ test("getPrimitiveUnionMatcherW dispatches on mixed string and numeric keys", ()
 });
 
 // ============================================================================
-// API: getPrimitiveUnionMatcher — ✔️ Exhaustive / ✔️ ReturnsStrict
+// API: getMatcherW — ❌ Exhaustive (fallback) / ❌ ReturnsStrict
 // ============================================================================
 
-test("getPrimitiveUnionMatcher infers a single return type shared by all handlers", () => {
+test("getMatcherW: a `_` fallback widens gaps into the union", () => {
     // Arrange
-    const factory = getPrimitiveUnionMatcher<"a" | "b">();
+    const factory = getMatcherW<"x" | "y" | "z">();
-
-    // Act
-    const matcher = factory({
-        a: (s): number => {
-            expectTypeOf(s).toEqualTypeOf<"a">();
-            return 1;
-        },
-        b: (s): 1 | 2 => {
-            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>();
-    assert.equal(matcher("a"), 1);
-    assert.equal(matcher("b"), 2);
-});
-
-test("getPrimitiveUnionMatcher handlers receive the matched literal", () => {
-    // Arrange
-    const factory = getPrimitiveUnionMatcher<"on" | "off">();
-
-    // Act
-    const matcher = factory({
-        on: (s) => {
-            expectTypeOf(s).toEqualTypeOf<"on">();
-            return `handler ${s}`;
-        },
-        off: (s) => {
-            expectTypeOf(s).toEqualTypeOf<"off">();
-            return `handler ${s}`;
-        },
-    });
-
-    // Assert
-    expectTypeOf(matcher).toEqualTypeOf<(shape: "on" | "off") => string>();
-    assert.equal(matcher("on"), "handler on");
-    assert.equal(matcher("off"), "handler off");
-});
-
-test("getPrimitiveUnionMatcher infers one return type across mixed string and numeric keys", () => {
-    // Arrange
-    const factory = getPrimitiveUnionMatcher<"a" | "b" | 1 | 2>();
-
-    // Act
-    const matcher = factory({
-        a: (s): number => {
-            expectTypeOf(s).toEqualTypeOf<"a">();
-            return 1;
-        },
-        b: (s): 1 | 2 => {
-            expectTypeOf(s).toEqualTypeOf<"b">();
-            return 2;
-        },
-        1: (n): number => {
-            expectTypeOf(n).toEqualTypeOf<1>();
-            return 10;
-        },
-        2: (n): number => {
-            expectTypeOf(n).toEqualTypeOf<2>();
-            return 20;
-        },
-    });
-
-    // Assert
-    // ✔️ ReturnsStrict: R is the best common return type, not a widening union.
-    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: getPrimitiveUnionMatcherPartial — ❌ Exhaustive / ✔️ ReturnsStrict
-// ============================================================================
-
-test("getPrimitiveUnionMatcherPartial routes shapes without a handler to _", () => {
-    // Arrange
-    const factory = getPrimitiveUnionMatcherPartial<"a" | "b" | "c">();
-    // Two keys, so `{ a }` lacks only the `_` fallback, nothing else.
-    const sparseFactory = getPrimitiveUnionMatcherPartial<"a" | "b">();
-
-    // Act
-    const matcher = factory({
-        a: (s): 1 | 2 => {
-            expectTypeOf(s).toEqualTypeOf<"a">();
-            return 1;
-        },
-        _: (s): 1 | 2 => {
-            // The fallback sees the whole union, not a single literal.
-            expectTypeOf(s).toEqualTypeOf<"a" | "b" | "c">();
-            return 2;
-        },
-    });
-
-    // Assert
-    expectTypeOf(matcher).toEqualTypeOf<(shape: "a" | "b" | "c") => 1 | 2>();
-    // ❌ Exhaustive: gaps are allowed, but only with a `_` fallback.
-    expectTypeOf<{
-        a: () => number;
-    }>().not.toExtend<Parameters<typeof sparseFactory>[0]>();
-    assert.equal(matcher("a"), 1);
-    assert.equal(matcher("b"), 2);
-    assert.equal(matcher("c"), 2);
-});
-
-test("getPrimitiveUnionMatcherPartial also accepts an exhaustive pattern", () => {
-    // Arrange
-    const factory = getPrimitiveUnionMatcherPartial<"a" | "b">();
-
-    // Act
-    const matcher = factory({
-        a: (s) => {
-            expectTypeOf(s).toEqualTypeOf<"a">();
-            return "A";
-        },
-        b: (s) => {
-            expectTypeOf(s).toEqualTypeOf<"b">();
-            return "B";
-        },
-    });
-
-    // Assert
-    expectTypeOf(matcher).toEqualTypeOf<(shape: "a" | "b") => string>();
-    assert.equal(matcher("a"), "A");
-    assert.equal(matcher("b"), "B");
-});
-
-test("getPrimitiveUnionMatcherPartial routes mixed string and numeric keys, gaps go to _", () => {
-    // Arrange
-    const factory = getPrimitiveUnionMatcherPartial<"a" | "b" | 1 | 2>();
-
-    // Act
-    const matcher = factory({
-        a: (s): string => {
-            expectTypeOf(s).toEqualTypeOf<"a">();
-            return "A";
-        },
-        1: (n): string => {
-            expectTypeOf(n).toEqualTypeOf<1>();
-            return "one";
-        },
-        _: (s): string => {
-            // The fallback sees the whole union, not a single literal.
-            expectTypeOf(s).toEqualTypeOf<"a" | "b" | 1 | 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: getPrimitiveUnionMatcherPartialW — ❌ Exhaustive / ❌ ReturnsStrict
-// ============================================================================
-
-test("getPrimitiveUnionMatcherPartialW allows gaps and widens to the union of handler returns", () => {
-    // Arrange
-    const factory = getPrimitiveUnionMatcherPartialW<"x" | "y" | "z">();
-    // Two keys, so `{ x }` lacks only the `_` fallback, nothing else.
-    const sparseFactory = getPrimitiveUnionMatcherPartialW<"x" | "y">();
 
     // Act
     const matcher = factory({
@@ -292,18 +263,16 @@ test("getPrimitiveUnionMatcherPartialW allows gaps and widens to the union of ha
     expectTypeOf(matcher).toEqualTypeOf<
         (shape: "x" | "y" | "z") => 1 | "fallback"
     >();
-    // ❌ Exhaustive: gaps are allowed, but only with a `_` fallback.
+    // ❌ a value outside T must not be accepted by the matcher
-    expectTypeOf<{
+    expectTypeOf<"w">().not.toExtend<Parameters<typeof matcher>[0]>();
-        x: () => number;
-    }>().not.toExtend<Parameters<typeof sparseFactory>[0]>();
     assert.equal(matcher("x"), 1);
     assert.equal(matcher("y"), "fallback");
     assert.equal(matcher("z"), "fallback");
 });
 
-test("getPrimitiveUnionMatcherPartialW widens mixed string and numeric key returns to their union", () => {
+test("getMatcherW: a `_` fallback widens mixed string and numeric returns", () => {
     // Arrange
-    const factory = getPrimitiveUnionMatcherPartialW<"a" | "b" | 1 | 2>();
+    const factory = getMatcherW<"a" | "b" | 1 | 2>();
 
     // Act
     const matcher = factory({
@@ -316,7 +285,6 @@ test("getPrimitiveUnionMatcherPartialW widens mixed string and numeric key retur
             return 10 as const;
         },
         _: (s) => {
-            // The fallback sees the whole union, not a single literal.
             expectTypeOf(s).toEqualTypeOf<"a" | "b" | 1 | 2>();
             return "fallback" as const;
         },
@@ -333,9 +301,9 @@ test("getPrimitiveUnionMatcherPartialW widens mixed string and numeric key retur
     assert.equal(matcher(2), "fallback");
 });
 
-test("getPrimitiveUnionMatcherPartialW also accepts an exhaustive pattern", () => {
+test("getMatcherW: a `_` fallback also accepts an exhaustive pattern", () => {
     // Arrange
-    const factory = getPrimitiveUnionMatcherPartialW<"x" | "y">();
+    const factory = getMatcherW<"x" | "y">();
 
     // Act
     const matcher = factory({
@@ -350,8 +318,140 @@ test("getPrimitiveUnionMatcherPartialW also accepts an exhaustive pattern", () =
     });
 
     // Assert
-    // ❌ ReturnsStrict: mixed handler returns widen to their union.
     expectTypeOf(matcher).toEqualTypeOf<(shape: "x" | "y") => 1 | "two">();
     assert.equal(matcher("x"), 1);
     assert.equal(matcher("y"), "two");
 });
+
+// ============================================================================
+// 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 `_` is not exhaustive
+    factory({ a: () => 1 });
+});
+
+test("getMatcherW factory rejects patterns outside its contract", () => {
+    // Arrange
+    const factory = getMatcherW<"x" | "y">();
+
+    // Act / Assert — the calls below must not compile
+    // @ts-expect-error `z` is not part of the universe `"x" | "y"`
+    factory({
+        x: () => 1 as const,
+        y: () => 2 as const,
+        z: () => 3 as const,
+    });
+    // @ts-expect-error a gap without `_` is not exhaustive
+    factory({ x: () => 1 as const });
+});
+
+// ============================================================================
+// 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.
+const labelsFor = (
+    name: string,
+    factory: "getMatcher" | "getMatcherW",
+    body: string,
+): 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,
+            "});",
+            "",
+        ].join("\n"),
+    };
+    return session
+        .completionLabelsAt(target)
+        .then((result) => result.labels)
+        .finally(() => session.close());
+};
+
+test("autocomplete: an exhaustive pattern requires the universe, `_` optional", () => {
+    // Arrange
+    const name = "getMatcher_fresh";
+
+    // Act
+    const labels = labelsFor(name, "getMatcher", "    /*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,
+        "getMatcher",
+        "    a: () => 1,\n    /*COMPLETE*/",
+    );
+
+    // Assert
+    return labels.then((result) => {
+        assert.deepEqual([...result], ["_?", "b", "c"]);
+    });
+});
+
+test("autocomplete: `_` makes the remaining keys optional", () => {
+    // Arrange
+    const name = "getMatcher_after_fallback";
+
+    // Act
+    const labels = labelsFor(
+        name,
+        "getMatcher",
+        "    _: () => 0,\n    /*COMPLETE*/",
+    );
+
+    // Assert
+    return labels.then((result) => {
+        assert.deepEqual([...result], ["a?", "b?", "c?"]);
+    });
+});
+
+test("autocomplete: `getMatcherW` offers the same popup as `getMatcher`", () => {
+    // Arrange
+    const name = "getMatcherW_fresh";
+
+    // Act
+    const labels = labelsFor(name, "getMatcherW", "    /*COMPLETE*/");
+
+    // Assert
+    return labels.then((result) => {
+        assert.deepEqual([...result], ["_?", "a", "b", "c"]);
+    });
+});

src/primitive.ts

@@ -1,71 +1,63 @@
-import type { Simplify, ValueOf } from "type-fest";
+import type { ValueOf } from "type-fest";
 
 type UnaryFn<T, R> = (shape: T) => R;
 
-// ============================================================================
+// `Extract` drops optional handlers (`undefined`) so `PatternReturns` also works
-// ✔️ Exhaustive
+// when `P`'s constraint has optional keys.
-// ❌ ReturnsStrict
+type PatternReturns<P> = ReturnType<
-// ============================================================================
+    Extract<ValueOf<P>, (...args: never[]) => unknown>
-type PatternPrimitiveUnion<R, T extends string | number> = {
+>;
-    [K in T]: UnaryFn<K, R>;
+
+type Handlers<R, T extends string | number> = { [K in T]: UnaryFn<K, R> };
+
+// Fallback form: `_` is a required key, the T-keys are optional.
+type Fallback<R, T extends string | number> = Partial<Handlers<R, T>> & {
+    _: UnaryFn<T, R>;
 };
 
-type PatternReturns<
+// Completion form: T-keys required, `_` optional. Overload #1, because
-    P extends Record<string | number, UnaryFn<never, unknown>>,
+// TypeScript takes the *first* overload as the contextual type for the popup.
-> = ReturnType<ValueOf<P>>;
+type ExhaustiveLoose<R, T extends string | number> = Handlers<R, T> & {
+    _?: UnaryFn<T, R>;
+};
 
-export const getPrimitiveUnionMatcherW: <T extends string | number>() => <
+// oxlint-disable typescript/unified-signatures
-    P extends PatternPrimitiveUnion<unknown, T>,
+// Strict returns: one common `R`. Overload order is load-bearing:
->(
+//   #1 ExhaustiveLoose -> autocomplete `_?, a, b`
-    pattern: Simplify<P>,
+//   #2 Fallback        -> accepts a partial pattern
-) => UnaryFn<T, PatternReturns<P>> = () => (pattern) => (shape) =>
+//   #3 Handlers (last) -> "Property 'b' is missing" is the reported error
-    // Rewrite not to use any is possible, was evaluated and solutions were
+interface MatcherStrict<T extends string | number> {
-    // more complex than the current solution.
+    <R>(pattern: ExhaustiveLoose<R, T>): UnaryFn<T, R>;
+    <R>(pattern: Fallback<R, T>): UnaryFn<T, R>;
+    <R>(pattern: Handlers<R, T>): UnaryFn<T, R>;
+}
+// oxlint-enable typescript/unified-signatures
 
-    // oxlint-disable-next-line typescript/no-explicit-any typescript/no-unsafe-type-assertion
+// Widened returns: the union of every handler's return type. `P` is inferred
-    (pattern[shape] as any)(shape);
+// from the whole parameter, whose closed constraint supplies the
+// contextual/autocomplete type; the `keyof P` guard appended to each overload
+// rejects keys outside `T`/`_`.
+interface MatcherWidening<T extends string | number> {
+    <P extends ExhaustiveLoose<unknown, T>>(
+        pattern: P & (keyof P extends T | "_" ? unknown : never),
+    ): UnaryFn<T, PatternReturns<P>>;
+    <P extends Fallback<unknown, T>>(
+        pattern: P & (keyof P extends T | "_" ? unknown : never),
+    ): UnaryFn<T, PatternReturns<P>>;
+    <P extends Handlers<unknown, T>>(
+        pattern: P & (keyof P extends T | "_" ? unknown : never),
+    ): UnaryFn<T, PatternReturns<P>>;
+}
 
-// ============================================================================
+type HandlerMap = Record<string | number, UnaryFn<never, unknown> | undefined>;
-// ✔️ Exhaustive
-// ✔️ ReturnsStrict
-// ============================================================================
-export const getPrimitiveUnionMatcher: <T extends string | number>() => <R>(
-    pattern: Simplify<PatternPrimitiveUnion<R, T>>,
-) => UnaryFn<T, R> = getPrimitiveUnionMatcherW;
 
-// ============================================================================
+const dispatch =
-// ❌ Exhaustive
+    (pattern: HandlerMap) =>
-// ✔️ ReturnsStrict
+    (shape: string | number): unknown =>
-// ============================================================================
+        // oxlint-disable-next-line typescript/no-non-null-assertion typescript/no-unsafe-type-assertion
-type PatternPrimitiveUnionPartial<R, T extends string | number> =
+        (pattern[shape] ?? pattern["_"]!)(shape as never);
-    | PatternPrimitiveUnion<R, T>
-    | (Partial<PatternPrimitiveUnion<R, T>> & {
-          _: UnaryFn<T, R>;
-      });
 
-export const getPrimitiveUnionMatcherPartial: <T extends string | number>() => <
+export const getMatcher = <T extends string | number>(): MatcherStrict<T> =>
-    R,
+    dispatch;
->(
+export const getMatcherW = <T extends string | number>(): MatcherWidening<T> =>
-    pattern: Simplify<PatternPrimitiveUnionPartial<R, T>>,
+    dispatch;
-) => UnaryFn<T, R> = () => (pattern) => (shape) =>
-    // Rewrite not to use any is possible, was evaluated and solutions were
-    // more complex than the current solution.
-
-    // oxlint-disable-next-line typescript/no-explicit-any typescript/no-unsafe-type-assertion
-    (pattern[shape] ?? (pattern as any)["_"])(shape);
-
-// ============================================================================
-// ❌ Exhaustive
-// ❌ ReturnsStrict
-// ============================================================================
-export const getPrimitiveUnionMatcherPartialW: <
-    T extends string | number,
->() => <P extends PatternPrimitiveUnionPartial<unknown, T>>(
-    // `Simplify<P>` is the inference hook: callers infer `P` from the argument.
-    // the second half pins the impl parameter's `R` to `PatternReturns<P>`.
-    // that makes the `= getPrimitiveUnionMatcherPartial` assignment type-check.
-    // neither half works alone.
-    // without the witness the union's `_` arm demands `_ ∈ keyof P`.
-    // without `Simplify<P>` the parameter types do not compare.
-    pattern: Simplify<P> & PatternPrimitiveUnionPartial<PatternReturns<P>, T>,
-) => UnaryFn<T, PatternReturns<P>> = getPrimitiveUnionMatcherPartial;

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

@@ -1,5 +1,6 @@
 /* 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";
@@ -10,6 +11,8 @@ import {
     LspSession,
 } from "#test-utils/lsp-completion.ts";
 
+const REPO_ROOT = path.resolve(import.meta.dirname, "../../..");
+
 // The `#test-utils/*` self-reference (package.json#imports) is the one route
 // scattered test files take to reach test helpers; this pins that it resolves
 // and types without starting a language server
@@ -30,3 +33,116 @@ test("test-helper home: `#test-utils/…` resolves to the helper and types it",
     assert.equal(typeof LspSession, "function");
     assert.equal(target.file, "src/util/__tests__/lsp-completion.ts");
 });
+
+// The helper drives a language server, so its contract is asserted against the
+// server. Every document below is probed from memory and carries its
+// contextual type inline, so the helper is tested without the library's code.
+const probe = (source: string, marker?: string): Promise<CompletionResult> => {
+    const session = new LspSession(REPO_ROOT);
+    const target: CompletionTarget =
+        marker === undefined
+            ? { file: "src/__probe.ts", source }
+            : { file: "src/__probe.ts", source, marker };
+    return session.completionLabelsAt(target).finally(() => session.close());
+};
+
+const HANDLERS = [
+    "type Handlers = { a: () => number; b: () => number; _?: () => number };",
+    "declare const apply: (handlers: Handlers) => Handlers;",
+];
+
+test("helper: a fresh object literal completes with its contextual keys", () => {
+    // Arrange
+    const source = [
+        ...HANDLERS,
+        "const done = apply({",
+        "    /*COMPLETE*/",
+        "});",
+        "export { done };",
+    ].join("\n");
+
+    // Act
+    const probed = probe(source);
+
+    // Assert — the position is the marker's, which the helper strips
+    expectTypeOf(probed).toEqualTypeOf<Promise<CompletionResult>>();
+    return probed.then((result) => {
+        assert.deepEqual(result.position, { line: 3, character: 4 });
+        assert.deepEqual([...result.labels], ["_?", "a", "b"]);
+    });
+});
+
+test("helper: a handled key drops out of the popup", () => {
+    // Arrange
+    const source = [
+        ...HANDLERS,
+        "const done = apply({",
+        "    b: () => 1,",
+        "    /*COMPLETE*/",
+        "});",
+        "export { done };",
+    ].join("\n");
+
+    // Act
+    const probed = probe(source);
+
+    // Assert
+    expectTypeOf<CompletionResult["labels"]>().toEqualTypeOf<
+        readonly string[]
+    >();
+    return probed.then((result) => {
+        assert.deepEqual([...result.labels], ["_?", "a"]);
+    });
+});
+
+test("helper: a custom marker is located at the line start", () => {
+    // Arrange
+    const source = [
+        "type Handlers = { a: () => number; _?: () => number };",
+        "declare const apply: (handlers: Handlers) => Handlers;",
+        "const done = apply({",
+        "@@@",
+        "});",
+        "export { done };",
+    ].join("\n");
+
+    // Act
+    const probed = probe(source, "@@@");
+
+    // Assert
+    expectTypeOf(probed).resolves.toEqualTypeOf<CompletionResult>();
+    return probed.then((result) => {
+        assert.deepEqual(result.position, { line: 3, character: 0 });
+        assert.deepEqual([...result.labels], ["_?", "a"]);
+    });
+});
+
+test("helper: labels are read from the server, not from a pattern literal", () => {
+    // Arrange
+    const source = [
+        'const text = "x";',
+        "const upper = text./*COMPLETE*/;",
+        "export { upper };",
+    ].join("\n");
+
+    // Act
+    const probed = probe(source);
+
+    // Assert
+    expectTypeOf(probed).resolves.toHaveProperty("labels");
+    return probed.then((result) => {
+        assert.ok(result.labels.includes("toUpperCase"));
+    });
+});
+
+test("helper: a source without the marker rejects", () => {
+    // Arrange
+    const source = "const text = 1;\nexport { text };\n";
+
+    // Act
+    const probed = probe(source);
+
+    // Assert
+    expectTypeOf(probed).resolves.toEqualTypeOf<CompletionResult>();
+    return assert.rejects(probed, /marker not found/);
+});