♻️ Share the Matchable universe between both matchers

development/library.md

@@ -97,9 +97,9 @@ Each factory is two overloads whose order is load-bearing:
 
 #### Decision (2026-09)
 
-`src/matcher-shared.ts` holds the six universe-agnostic pieces both matchers
-use: `UnaryFn`, `PatternReturns`, `RedundantFallback`, `HandlerMap`, and the
-`PatternKey` / `PatternParam` key projection.
+`src/matcher-shared.ts` holds the seven universe-agnostic pieces both matchers
+use: `UnaryFn`, `PatternReturns`, `RedundantFallback`, `HandlerMap`, the shared
+`Matchable` universe, and the `PatternKey` / `PatternParam` key projection.
 
 #### Why
 
@@ -107,6 +107,10 @@ use: `UnaryFn`, `PatternReturns`, `RedundantFallback`, `HandlerMap`, and the
   keeps the two matchers' message from drifting; the other pieces appear
   verbatim in both public signatures or are the same projection over each
   matcher's universe.
+- **`Matchable` is one definition, not two.** The primitive-union matcher's
+  universe and the tagged-union matcher's allowed `Tag` values are the same set,
+  so aliasing them keeps the two matchers from drifting apart on what they
+  accept (`symbol`/`bigint` rejected once).
 
 #### Rejected
 
@@ -114,8 +118,8 @@ use: `UnaryFn`, `PatternReturns`, `RedundantFallback`, `HandlerMap`, and the
   `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. The `PatternKey` / `PatternParam`
-  projection is the one piece both universes genuinely share.
+  contextual typing it exists to preserve. `Matchable` and the `PatternKey` /
+  `PatternParam` projection are the pieces both universes genuinely share.
 
 ## Tagged-union matcher
 

src/matcher-shared.ts

@@ -8,6 +8,12 @@ import type { ValueOf } from "type-fest";
 // A handler: one universe member in, one return value out.
 export type UnaryFn<T, R> = (shape: T) => R;
 
+// The primitive universe a matcher can discriminate. The primitive-union matcher
+// uses it directly; the tagged-union matcher uses it as the set of allowed
+// discriminant (`Tag`) values. `boolean` is admitted as the pair `true | false`;
+// see README § Caveats for the unsupported members.
+export type Matchable = string | number | boolean | null | undefined;
+
 // `boolean`, `null` and `undefined` cannot be property keys, so a mapped type
 // over a universe that includes one keys each such member by its
 // stringification. `PatternParam` inverts that projection, so a handler callback

src/primitive-union.ts

@@ -2,6 +2,7 @@ import type { Exact } from "type-fest";
 
 import type {
     HandlerMap,
+    Matchable,
     PatternKey,
     PatternParam,
     PatternReturns,
@@ -9,10 +10,6 @@ import type {
     UnaryFn,
 } from "./matcher-shared.ts";
 
-// The primitive universe a matcher can discriminate. `boolean` is admitted as
-// the pair `true | false`; see README § Caveats for the unsupported members.
-type Matchable = string | number | boolean | null | undefined;
-
 type Handlers<T extends Matchable, R> = {
     [K in PatternKey<T>]: UnaryFn<PatternParam<K>, R>;
 };

src/tagged-union.ts

@@ -2,6 +2,7 @@ import type { Exact, UnknownRecord } from "type-fest";
 
 import type {
     HandlerMap,
+    Matchable,
     PatternKey,
     PatternParam,
     PatternReturns,
@@ -9,22 +10,21 @@ import type {
     UnaryFn,
 } from "./matcher-shared.ts";
 
-// A tagged union is discriminated by one property whose values are the tags.
-// `string` and `number` tags key a handler map directly; `boolean`, `null` and
-// `undefined` are admitted too but are not property keys, so they go through the
-// `PatternKey` projection. `symbol` has no literal syntax to write a handler
-// under, and `bigint` is not a property key.
-type Tag = string | number | boolean | null | undefined;
+// A tagged union is discriminated by one property whose values are the tags (a
+// `Matchable`). `string` and `number` tags key a handler map directly;
+// `boolean`, `null` and `undefined` are admitted too but are not property keys,
+// so they go through the `PatternKey` projection. `symbol` has no literal syntax
+// to write a handler under, and `bigint` is not a property key.
 
 // The discriminant values of `T` under `K`. `Extract` keeps the finite literal
 // tags and leaves a widened `string`/`number` as itself, so an open universe
 // keeps an open fallback.
-type Tags<T extends object, K extends keyof T> = Extract<T[K], Tag>;
+type Tags<T extends object, K extends keyof T> = Extract<T[K], Matchable>;
 
 // The keys of `T` that can act as a discriminant. `getTaggedUnionMatcher<T>()`
 // accepts only these, so the factory rejects a key whose values are not tags.
 type Discriminated<T extends object> = {
-    [K in keyof T]: T[K] extends Tag ? K : never;
+    [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