♻️ Adopt the three-overload matcher

strict/widened each take the exhaustive/fallback pattern shape in three
overloads ordered ExhaustiveLoose -> Fallback -> Handlers. TypeScript reads
the first overload for the object-literal popup (_?, a, b) and the last for
the missing-key error, so autocomplete and the error message are tuned
independently. The fallback is a pattern shape, not a factory, so the four
getPrimitiveUnionMatcher* factories collapse to two.

src/index.ts re-exports the two factories; the prototype scratch files that
explored the alternatives are removed.

src/primitive.ts

@@ -1,71 +1,79 @@
-import type { Simplify, ValueOf } from "type-fest";
+import type { 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>;
+// `Extract` drops optional handlers (`undefined`) so `PatternReturns` also works
+// when `P`'s constraint has optional keys.
+type PatternReturns<P> = ReturnType<
+    Extract<ValueOf<P>, (...args: never[]) => unknown>
+>;
+
+type Handlers<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<
-    P extends Record<string | number, UnaryFn<never, unknown>>,
-> = ReturnType<ValueOf<P>>;
+// Completion form: T-keys required, `_` optional. Overload #1, because
+// TypeScript takes the *first* overload as the contextual type for the popup.
+type ExhaustiveLoose<R, T extends string | number> = Handlers<R, T> & {
+    _?: UnaryFn<T, R>;
+};
 
-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.
+// The `P`-constraints are deliberately *closed* (no index signature): that is
+// what makes the excess-property check reject keys outside `T`/`_`.
+type ExhaustiveShape<T extends string | number> = Handlers<unknown, T> & {
+    _?: UnaryFn<T, unknown>;
+};
+type FallbackShape<T extends string | number> = Partial<
+    Handlers<unknown, T>
+> & {
+    _: UnaryFn<T, unknown>;
+};
 
-    // oxlint-disable-next-line typescript/no-explicit-any typescript/no-unsafe-type-assertion
-    (pattern[shape] as any)(shape);
+// Strict returns: one common `R`. Overload order is load-bearing:
+//   #1 ExhaustiveLoose -> autocomplete `_?, a, b`
+//   #2 Fallback        -> accepts a partial pattern
+//   #3 Handlers (last) -> "Property 'b' is missing" is the reported error
+interface StrictFn<T extends string | number> {
+    <R>(pattern: ExhaustiveLoose<R, T>): UnaryFn<T, R>;
+    <R>(pattern: Fallback<R, T>): UnaryFn<T, R>;
+    <R>(pattern: Handlers<R, T>): UnaryFn<T, R>;
+}
 
-// ============================================================================
-// ✔️ Exhaustive
-// ✔️ ReturnsStrict
-// ============================================================================
-export const getPrimitiveUnionMatcher: <T extends string | number>() => <R>(
-    pattern: Simplify<PatternPrimitiveUnion<R, T>>,
-) => UnaryFn<T, R> = getPrimitiveUnionMatcherW;
+// Widened returns: the union of every handler's return type. `P` is the whole
+// parameter (not an intersection), so its closed constraint both supplies the
+// contextual/autocomplete type and enforces the excess-property check.
+interface WidenedFn<T extends string | number> {
+    <P extends ExhaustiveShape<T>>(
+        pattern: P & (keyof P extends T | "_" ? unknown : never),
+    ): UnaryFn<T, PatternReturns<P>>;
+    <P extends FallbackShape<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>>;
+}
 
-// ============================================================================
-// ❌ Exhaustive
-// ✔️ ReturnsStrict
-// ============================================================================
-type PatternPrimitiveUnionPartial<R, T extends string | number> =
-    | PatternPrimitiveUnion<R, T>
-    | (Partial<PatternPrimitiveUnion<R, T>> & {
-          _: UnaryFn<T, R>;
-      });
+type HandlerMap = Record<string | number, UnaryFn<never, unknown> | undefined>;
 
-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.
+const dispatch =
+    (pattern: HandlerMap) =>
+    (shape: string | number): unknown => {
+        const handler = pattern[shape] ?? pattern["_"];
+        return handler === undefined
+            ? undefined
+            : Reflect.apply(handler, undefined, [shape]);
+    };
 
-    // oxlint-disable-next-line typescript/no-explicit-any typescript/no-unsafe-type-assertion
-    (pattern[shape] ?? (pattern as any)["_"])(shape);
+export const strict =
+    <T extends string | number>(): StrictFn<T> =>
+    (pattern: HandlerMap) =>
+        dispatch(pattern);
 
-// ============================================================================
-// ❌ 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;
+export const widened =
+    <T extends string | number>(): WidenedFn<T> =>
+    (pattern: HandlerMap) =>
+        dispatch(pattern);