🔀 Merge chore/straighten-oxc-rules into main

Note the finished work under [Unreleased] and mark the task and its
subtasks done.

.gitignore

@@ -10,5 +10,6 @@ coverage
 !.vscode/extensions.json
 !.vscode/settings.json
 !.vscode/tasks.json
+!.vscode/launch.json
 .idea
 .DS_Store

.oxlintrc.json

@@ -13,6 +13,8 @@
         "eslint/no-undefined": "off",
         "eslint/sort-keys": "off",
         "eslint/id-length": "off",
+        "eslint/capitalized-comments": "off",
+        "eslint/no-ternary": "off",
         "import/no-named-export": "off",
         "eslint/one-var": "off",
         "import/group-exports": "off",

.vscode/extensions.json

@@ -1,5 +1,6 @@
 {
     "recommendations": [
+        "connor4312.nodejs-testing",
         "oxc.oxc-vscode",
         "streetsidesoftware.code-spell-checker",
         "typescriptteam.native-preview",

.vscode/launch.json

@@ -0,0 +1,15 @@
+{
+    "version": "0.2.0",
+    "configurations": [
+        {
+            "type": "node",
+            "request": "launch",
+            "name": "Debug current test file",
+            "runtimeExecutable": "node",
+            "runtimeArgs": ["--test", "--strip-types"],
+            "args": ["${file}"],
+            "cwd": "${workspaceFolder}",
+            "console": "integratedTerminal"
+        }
+    ]
+}

.vscode/settings.json

@@ -1,4 +1,14 @@
 {
+    "nodejs-testing.extensions": [
+        {
+            "extensions": ["mjs", "cjs", "js"],
+            "parameters": []
+        },
+        {
+            "extensions": ["ts"],
+            "parameters": ["--strip-types"]
+        }
+    ],
     "[typescript]": {
         "editor.defaultFormatter": "oxc.oxc-vscode",
         "editor.formatOnSave": true

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+- allow ternaries and lowercase comments in oxlint
+- switch `src/primitive.ts` prose from block comments to line comments
+
 ## [0.1.8] - 2026-09-16
 
 - upgrade dependencies

CONTRIBUTING.md

@@ -67,6 +67,12 @@ 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.
+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
+throwaway call), `// Assert` holds every check — type expectations first,
+runtime assertions last; an empty block drops its label (see
+[development/testing.md § AAA ordering](./development/testing.md#aaa-ordering)).
 Type-first is enforced structurally: `npm test` runs `check:tsc` before the
 test runner, so a wrong type can never be papered over by a passing assertion.
 Per [AGENTS.md § Never do](./AGENTS.md#never-do), reach green honestly — fix the

backlog.tasks

@@ -6,6 +6,10 @@ 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 @high
+✔ 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
 
 v1.0:
 ☐ API surface is stable and fully typed
@@ -20,6 +24,8 @@ v1.0:
 Bugs:
 
 Enhancements:
+☐ Allow boolean literals in primitive union patterns (e.g. `true: () => "yes"`) @medium
+☐ Are there other primitive types that should be supported in union patterns? (e.g. `bigint`, `symbol`) @medium
 
 Documentation:
 ☐ Create `examples/` directory with runnable snippets

cspell.json

@@ -42,7 +42,8 @@
         "bestikk",
         "silverwind",
         "idris",
-        "todotasks"
+        "todotasks",
+        "connor"
     ],
     "ignorePaths": ["dist", "node_modules", "coverage", "*.svg", ".gitignore"]
 }

development/testing.md

@@ -36,10 +36,38 @@ The runner is `node --test --strip-types "src/**/*.test.ts"` and the tiers are i
 build step, and the runner relies on the `.ts` import-extension convention (see
 [tooling.md](./tooling.md#source-imports-use-ts-extensions)).
 
+## AAA ordering
+
+The rule is in
+[CONTRIBUTING.md § Testing discipline (type-driven)](../CONTRIBUTING.md#testing-discipline-type-driven).
+
+#### Decision (2026-09)
+
+Test bodies read arrange → act → assert: inputs (the factory) set up first, the
+subject exercised once from them, all checks last — types then runtime. The
+blocks are labeled with `// Arrange` / `// Act` / `// Assert` comments and
+separated by a blank line; an empty block drops its label.
+
+#### Why
+
+- Interleaved setup/checks hide what runs vs. what is observed; the eye
+  re-reads the block to find the seams.
+- A factory built mid-test invites a second throwaway call of the subject;
+  arranging it once makes the positive construction and the negative
+  `Parameters<…>` check share one source of truth.
+- Labels make the seams explicit, not inferred — grep-able and reviewable
+  without reading the statements.
+
+#### Rejected
+
+- Unlabeled ordering (bare blank lines): the seams still have to be found by
+  reading; the labels cost nothing.
+
 ## Known issues
 
 - The type-aware linter misidentifies `expectTypeOf()` as a floating promise, so
-  `src/index.test.ts` carries a file-level `oxlint-disable
+  test files that use it (`src/index.test.ts`, `src/primitive.test.ts`) carry a
+  file-level `oxlint-disable
 typescript/no-floating-promises` with an explanatory comment. It is a known
   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)).

development/tooling.md

@@ -104,25 +104,48 @@ Type-aware oxlint is enabled via `options.typeAware: true` in `.oxlintrc.json`
 
 #### Decision (2026-09)
 
-Known type-aware false positives are silenced with source-level `oxlint-disable`
-directives (see `src/pattern.ts`, `src/match.ts`, `src/index.test.ts`), not with
-rules disabled in `.oxlintrc.json`.
+A type-aware rule that false-positives is silenced with a source-level
+`oxlint-disable` directive (see `src/pattern.ts`, `src/match.ts`,
+`src/index.test.ts`), not by turning the rule off in `.oxlintrc.json`.
 
 #### Why
 
 - The disable sits next to the code it silences, visible to anyone reading the
   source.
+- The rule stays on everywhere else, so only the mis-firing line is exempted.
 
 #### Rejected
 
-- A project-wide disable in `.oxlintrc.json`: it hides the suppression from the
-  reader of the affected code.
+- A project-wide disable in `.oxlintrc.json` for a false positive: it hides the
+  exemption from the reader of the affected code and switches the rule off
+  repo-wide for a one-site problem.
 
 #### Known issue
 
 - A source-level disable is a _human_ last resort. AI agents must not add one;
   they fix the type at its root (see [AGENTS.md § Never do](../AGENTS.md#never-do)).
 
+### Unwanted stylistic rules are turned off in the config
+
+#### Decision (2026-09)
+
+A stylistic rule the project rejects is `"off"` in the `.oxlintrc.json` `rules`
+map, not silenced at a use site. Current entries: `eslint/capitalized-comments`
+(comments may start lowercase) and `eslint/no-ternary` (ternaries are allowed),
+joining the oxfmt-superseded rules already off.
+
+#### Why
+
+- The rule is wrong for the whole project, not mis-firing at one site, so there
+  is no line to annotate.
+- Keeping the two mechanisms separate keeps a source-level `oxlint-disable`
+  meaningful: it marks a lone exception.
+
+#### Rejected
+
+- A source-level `oxlint-disable` per use: the same exemption repeated at every
+  site, and oxfmt can move the site.
+
 ### `check:tsc` runs first
 
 #### Decision (2026-09)

package-lock.json

@@ -8,6 +8,9 @@
             "name": "tiny-pattern-ts",
             "version": "0.1.8",
             "license": "MIT",
+            "dependencies": {
+                "type-fest": "^5.9.0"
+            },
             "devDependencies": {
                 "@arethetypeswrong/cli": "^0.18.5",
                 "@runwisp/pubv": "^1.5.1",
@@ -4622,6 +4625,18 @@
                 "url": "https://github.com/chalk/supports-hyperlinks?sponsor=1"
             }
         },
+        "node_modules/tagged-tag": {
+            "version": "1.0.0",
+            "resolved": "https://registry.npmjs.org/tagged-tag/-/tagged-tag-1.0.0.tgz",
+            "integrity": "sha512-yEFYrVhod+hdNyx7g5Bnkkb0G6si8HJurOoOEgC8B/O0uXLHlaey/65KRv6cuWBNhBgHKAROVpc7QyYqE5gFng==",
+            "license": "MIT",
+            "engines": {
+                "node": ">=20"
+            },
+            "funding": {
+                "url": "https://github.com/sponsors/sindresorhus"
+            }
+        },
         "node_modules/test-exclude": {
             "version": "8.0.0",
             "resolved": "https://registry.npmjs.org/test-exclude/-/test-exclude-8.0.0.tgz",
@@ -4705,6 +4720,21 @@
             "license": "0BSD",
             "optional": true
         },
+        "node_modules/type-fest": {
+            "version": "5.9.0",
+            "resolved": "https://registry.npmjs.org/type-fest/-/type-fest-5.9.0.tgz",
+            "integrity": "sha512-yANm3Jr3GiJ1qgJlxGAVxTOIcEOk1rhQHamlXtnrCK7EHP4HeM9OGxtMg/W7HFdrVzw/ZWJKGVIJusVH85sLtw==",
+            "license": "(MIT OR CC0-1.0)",
+            "dependencies": {
+                "tagged-tag": "^1.0.0"
+            },
+            "engines": {
+                "node": ">=20"
+            },
+            "funding": {
+                "url": "https://github.com/sponsors/sindresorhus"
+            }
+        },
         "node_modules/typescript": {
             "version": "7.0.2",
             "resolved": "https://registry.npmjs.org/typescript/-/typescript-7.0.2.tgz",

package.json

@@ -65,6 +65,9 @@
         "setup": "npm run setup:git-commit-message",
         "setup:git-commit-message": "git config commit.template commit-message-template"
     },
+    "dependencies": {
+        "type-fest": "^5.9.0"
+    },
     "devDependencies": {
         "@arethetypeswrong/cli": "^0.18.5",
         "@runwisp/pubv": "^1.5.1",

src/index.test.ts

@@ -7,44 +7,59 @@ import { expectTypeOf } from "expect-type";
 import { type Matcher, P, match } from "./index.ts";
 
 test("match returns a builder", () => {
+    // Act
     const builder = match("x");
+
+    // Assert
     expectTypeOf(builder).toHaveProperty("with");
     expectTypeOf(builder).toHaveProperty("exhaustive");
     expectTypeOf(builder).toHaveProperty("otherwise");
 });
 
 test("P.literal narrows to its literal type", () => {
+    // Act
     const matcher = P.literal("yes");
-    expectTypeOf(matcher).toMatchTypeOf<Matcher<"yes">>();
+
+    // Assert
+    expectTypeOf(matcher).toExtend<Matcher<"yes">>();
     assert.equal(matcher.matches("yes"), true);
     assert.equal(matcher.matches("no"), false);
 });
 
 test("P.type narrows to the typeof target", () => {
+    // Act
     const matcher = P.type<string>("string");
-    expectTypeOf(matcher).toMatchTypeOf<Matcher<string>>();
+
+    // Assert
+    expectTypeOf(matcher).toExtend<Matcher<string>>();
     assert.equal(matcher.matches("hi"), true);
     assert.equal(matcher.matches(42), false);
 });
 
 test("exhaustive() returns the union of handler return types", () => {
+    // Act
     const result = match<"a" | "b">("a")
         .with(P.literal("a"), () => 1 as const)
         .with(P.literal("b"), () => "two" as const)
         .exhaustive();
 
+    // Assert
     expectTypeOf(result).toEqualTypeOf<1 | "two">();
     assert.equal(result, 1);
 });
 
 test("otherwise() falls back when no case matches", () => {
+    // Act
     const result = match<"x" | "y" | "z">("z")
         .with(P.literal("x"), (v): string => `got ${v}`)
         .otherwise((v): string => `fallback ${v}`);
+
+    // Assert
     assert.equal(result, "fallback z");
 });
 
 test("exhaustive throws when no case matches", () => {
+    // Assert
     assert.throws(
         () =>
             match<"a" | "b" | "c">("c")

src/primitive.test.ts

@@ -0,0 +1,357 @@
+/* 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 { test } from "node:test";
+
+import { expectTypeOf } from "expect-type";
+
+import {
+    getPrimitiveUnionMatcher,
+    getPrimitiveUnionMatcherPartial,
+    getPrimitiveUnionMatcherPartialW,
+    getPrimitiveUnionMatcherW,
+} from "./primitive.ts";
+
+// ============================================================================
+// API: getPrimitiveUnionMatcherW — ✔️ Exhaustive / ❌ ReturnsStrict
+// ============================================================================
+
+test("getPrimitiveUnionMatcherW requires every literal key", () => {
+    // Arrange
+    const factory = getPrimitiveUnionMatcherW<"a" | "b">();
+
+    // Act
+    const matcher = factory({
+        a: (s) => {
+            expectTypeOf(s).toEqualTypeOf<"a">();
+            return 1 as const;
+        },
+        b: (s) => {
+            expectTypeOf(s).toEqualTypeOf<"b">();
+            return "two" as const;
+        },
+    });
+
+    // Assert
+    // ❌ ReturnsStrict: mixed handler returns widen to their union.
+    expectTypeOf(matcher).toEqualTypeOf<(shape: "a" | "b") => 1 | "two">();
+    // A pattern missing a key must not satisfy the parameter type.
+    expectTypeOf<{
+        a: () => number;
+    }>().not.toExtend<Parameters<typeof factory>[0]>();
+    assert.equal(matcher("a"), 1);
+    assert.equal(matcher("b"), "two");
+});
+
+test("getPrimitiveUnionMatcherW dispatches on numeric literal keys", () => {
+    // Arrange
+    const factory = getPrimitiveUnionMatcherW<1 | 2>();
+
+    // Act
+    const matcher = factory({
+        1: (n) => {
+            expectTypeOf(n).toEqualTypeOf<1>();
+            return n + 1;
+        },
+        2: (n) => {
+            expectTypeOf(n).toEqualTypeOf<2>();
+            return n * 10;
+        },
+    });
+
+    // Assert
+    expectTypeOf(matcher).toEqualTypeOf<(shape: 1 | 2) => number>();
+    assert.equal(matcher(1), 2);
+    assert.equal(matcher(2), 20);
+});
+
+test("getPrimitiveUnionMatcherW dispatches on mixed string and numeric keys", () => {
+    // Arrange
+    const factory = getPrimitiveUnionMatcherW<"a" | "b" | 1 | 2>();
+
+    // Act
+    const matcher = factory({
+        a: (s) => {
+            expectTypeOf(s).toEqualTypeOf<"a">();
+            return "A" as const;
+        },
+        b: (s) => {
+            expectTypeOf(s).toEqualTypeOf<"b">();
+            return "B" as const;
+        },
+        1: (n) => {
+            expectTypeOf(n).toEqualTypeOf<1>();
+            return 10 as const;
+        },
+        2: (n) => {
+            expectTypeOf(n).toEqualTypeOf<2>();
+            return 20 as const;
+        },
+    });
+
+    // Assert
+    expectTypeOf(matcher).toEqualTypeOf<
+        (shape: "a" | "b" | 1 | 2) => "A" | "B" | 10 | 20
+    >();
+    assert.equal(matcher("a"), "A");
+    assert.equal(matcher("b"), "B");
+    assert.equal(matcher(1), 10);
+    assert.equal(matcher(2), 20);
+});
+
+// ============================================================================
+// API: getPrimitiveUnionMatcher — ✔️ Exhaustive / ✔️ ReturnsStrict
+// ============================================================================
+
+test("getPrimitiveUnionMatcher infers a single return type shared by all handlers", () => {
+    // Arrange
+    const factory = getPrimitiveUnionMatcher<"a" | "b">();
+
+    // 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({
+        x: (s) => {
+            expectTypeOf(s).toEqualTypeOf<"x">();
+            return 1 as const;
+        },
+        _: (s) => {
+            // The fallback sees the whole union, not a single literal.
+            expectTypeOf(s).toEqualTypeOf<"x" | "y" | "z">();
+            return "fallback" as const;
+        },
+    });
+
+    // Assert
+    expectTypeOf(matcher).toEqualTypeOf<
+        (shape: "x" | "y" | "z") => 1 | "fallback"
+    >();
+    // ❌ Exhaustive: gaps are allowed, but only with a `_` fallback.
+    expectTypeOf<{
+        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", () => {
+    // Arrange
+    const factory = getPrimitiveUnionMatcherPartialW<"a" | "b" | 1 | 2>();
+
+    // Act
+    const matcher = factory({
+        a: (s) => {
+            expectTypeOf(s).toEqualTypeOf<"a">();
+            return "A" as const;
+        },
+        1: (n) => {
+            expectTypeOf(n).toEqualTypeOf<1>();
+            return 10 as const;
+        },
+        _: (s) => {
+            // The fallback sees the whole union, not a single literal.
+            expectTypeOf(s).toEqualTypeOf<"a" | "b" | 1 | 2>();
+            return "fallback" as const;
+        },
+    });
+
+    // Assert
+    // ❌ ReturnsStrict: mixed handler returns widen to their union.
+    expectTypeOf(matcher).toEqualTypeOf<
+        (shape: "a" | "b" | 1 | 2) => "A" | 10 | "fallback"
+    >();
+    assert.equal(matcher("a"), "A");
+    assert.equal(matcher("b"), "fallback");
+    assert.equal(matcher(1), 10);
+    assert.equal(matcher(2), "fallback");
+});
+
+test("getPrimitiveUnionMatcherPartialW also accepts an exhaustive pattern", () => {
+    // Arrange
+    const factory = getPrimitiveUnionMatcherPartialW<"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">();
+    assert.equal(matcher("x"), 1);
+    assert.equal(matcher("y"), "two");
+});

src/primitive.ts

@@ -0,0 +1,71 @@
+import type { Simplify, ValueOf } from "type-fest";
+
+type UnaryFn<T, R> = (shape: T) => R;
+
+// ============================================================================
+// ✔️ Exhaustive
+// ❌ ReturnsStrict
+// ============================================================================
+type PatternPrimitiveUnion<R, T extends string | number> = {
+    [K in T]: UnaryFn<K, R>;
+};
+
+type PatternReturns<
+    P extends Record<string | number, UnaryFn<never, unknown>>,
+> = ReturnType<ValueOf<P>>;
+
+export const getPrimitiveUnionMatcherW: <T extends string | number>() => <
+    P extends PatternPrimitiveUnion<unknown, T>,
+>(
+    pattern: Simplify<P>,
+) => UnaryFn<T, PatternReturns<P>> = () => (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] as any)(shape);
+
+// ============================================================================
+// ✔️ Exhaustive
+// ✔️ ReturnsStrict
+// ============================================================================
+export const getPrimitiveUnionMatcher: <T extends string | number>() => <R>(
+    pattern: Simplify<PatternPrimitiveUnion<R, T>>,
+) => UnaryFn<T, R> = getPrimitiveUnionMatcherW;
+
+// ============================================================================
+// ❌ Exhaustive
+// ✔️ ReturnsStrict
+// ============================================================================
+type PatternPrimitiveUnionPartial<R, T extends string | number> =
+    | PatternPrimitiveUnion<R, T>
+    | (Partial<PatternPrimitiveUnion<R, T>> & {
+          _: UnaryFn<T, R>;
+      });
+
+export const getPrimitiveUnionMatcherPartial: <T extends string | number>() => <
+    R,
+>(
+    pattern: Simplify<PatternPrimitiveUnionPartial<R, T>>,
+) => 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;