✨ Reject a fallback for an exhaustive map

A fallback paired with a handler map that already covers `T` was still
accepted, even though its remainder is empty. Fold the guard into
`Handled`'s own (F-bounded) constraint so it is checked after inference;
a conditional in the fallback parameter is evaluated while `Handled` is
still its constraint and rejects every partial map whose handler callbacks
need contextual typing.

CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+- reject a fallback when the handler map already covers the universe
+
 ## [0.4.0] - 2026-09-20
 
 - move the `_` fallback out of handlers

backlog.tasks

@@ -44,8 +44,8 @@ Matcher:
     → 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
-    → `(handlers, fallback)` with `handlers` covering all of `T` is accepted; the redundant fallback should be rejected (currying would allow the guard)
+✔ 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
 
 Bugs:
 ✔ TS 7 LSP server logs `context canceled` on stderr at shutdown @done

development/library.md

@@ -31,8 +31,9 @@ Each factory is two overloads whose order is load-bearing:
 
 1. `Handlers<T, R>` — the exhaustive form, and the contextual type of the
    handler-map popup;
-2. `Handled extends Exact<Partial<Handlers<T, R>>, Handled>` plus
-   `Fallback<T, Handled, R>` — a partial handler map plus the fallback.
+2. `Handled extends Exact<Partial<Handlers<T, R>>, Handled>` intersected with
+   `MustBePartial<T, Handled>`, plus `Fallback<T, Handled, R>` — a partial
+   handler map plus the fallback, rejected when the map already covers `T`.
 
 #### Why
 
@@ -41,6 +42,14 @@ Each factory is two overloads whose order is load-bearing:
   handler map can only see all of `T`, never `Exclude<T, keyof Handled>`. A later
   argument is contextually typed from inference on an earlier one, so the split
   is what makes the remainder expressible.
+- **The redundant-fallback guard is an F-bounded constraint.** A map that
+  already covers `T` plus a fallback is rejected by folding
+  `MustBePartial<T, Handled>` into `Handled`'s own constraint. The guard is
+  checked _after_ `Handled` is inferred, so the contextual pass that types the
+  handler callbacks survives. The obvious conditional
+  `Exclude<T, keyof Handled> extends never ? …` in the fallback's parameter
+  type is evaluated while `Handled` is still its constraint and rejects every
+  partial map whose callbacks are context-sensitive.
 - **Overload order keeps both messages.** #1 supplies the contextual type
   (`a, b, c`); #2 accepts a partial map once a fallback is present, so its popup
   is optional (`a?, b?, c?`). A gap without a fallback is reported against #1.
@@ -57,9 +66,9 @@ Each factory is two overloads whose order is load-bearing:
 - **Single-object `_`** (the former shape). `_` sees only all of `T`; the
   remainder is not expressible there, and an exhaustive map plus `_` was
   accepted.
-- **Curried handlers-first** — `(handlers)(fallback)`. `Handled` is fixed before
-  the second call, so a redundant-fallback guard would work. Rejected: two calls
-  for the common case.
+- **Curried handlers-first** — `(handlers)(fallback)`. Rejected: two calls for
+  the common case. It is not needed for the redundant-fallback guard, which the
+  F-bounded constraint already provides (see Why).
 - **`this` / HKT self-reference.** `this` is post-construction (method bodies,
   return positions); a parameter's contextual type is pre-construction.
   `keyof this` in an interface method is the interface, not the literal.
@@ -76,11 +85,6 @@ Each factory is two overloads whose order is load-bearing:
 
 #### Known issue
 
-- A redundant fallback is accepted: when the handler map already covers `T`, the
-  fallback is still allowed. The guard would be
-  `Exclude<T, keyof Handled> extends never ? never : unknown`, but the
-  conditional is evaluated before `Handled` is inferred; currying is the only
-  encoding that fixes it (see Rejected).
 - `PatternReturns` must be
   `ReturnType<Extract<ValueOf<P>, (...args: never[]) => unknown>>` so it survives
   the closed, partly-optional `P` constraints.

src/primitive.test.ts

@@ -309,6 +309,26 @@ test("getMatcher factory rejects patterns outside its contract", () => {
     factory({ a: () => 1 });
     // @ts-expect-error a `string` fallback does not fit the `number` handler
     factory({ a: () => 1 }, () => "x");
+    // @ts-expect-error a fallback is redundant once the map covers the universe
+    factory({ a: () => 1, b: () => 2 }, () => 0);
+});
+
+test("getMatcher: an open universe keeps the fallback's remainder open", () => {
+    // Arrange
+    const factory = getMatcher<string>();
+
+    // 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>();
+        return 2 as const;
+    });
+
+    // Assert
+    expectTypeOf(matcher).toEqualTypeOf<(shape: string) => number>();
+    assert.equal(matcher("a"), 1);
+    assert.equal(matcher("b"), 2);
 });
 
 test("getMatcherW factory rejects patterns outside its contract", () => {
@@ -324,6 +344,8 @@ test("getMatcherW factory rejects patterns outside its contract", () => {
     });
     // @ts-expect-error a gap without a `_` fallback is not exhaustive
     factory({ x: () => 1 as const });
+    // @ts-expect-error a fallback is redundant once the map covers the universe
+    factory({ x: () => 1 as const, y: () => 2 as const }, () => 0);
 });
 
 // ============================================================================

src/primitive.ts

@@ -21,6 +21,21 @@ type Fallback<T extends string | number, Handled, R> = UnaryFn<
     R
 >;
 
+// A fallback is redundant once the handler map covers `T`. The guard is folded
+// into `Handled`'s own (self-referential) constraint so it is checked *after*
+// inference; a conditional in the fallback's parameter type is evaluated while
+// `Handled` is still its constraint and would reject context-sensitive partial
+// maps. That placement also fixes where the diagnostic lands: the constraint
+// failure is reported on the argument that inferred `Handled` (the handler
+// map), so the required property is spelled as the message instead of relying
+// on its position. See development/library.md.
+interface RedundantFallback {
+    readonly "every case is already handled, so the fallback is redundant": never;
+}
+type MustBePartial<T extends string | number, Handled> = T extends keyof Handled
+    ? RedundantFallback
+    : unknown;
+
 // TypeScript does not apply the excess-property check to a generic constraint,
 // so `Exact` restores it for the generic forms: a handler map can otherwise
 // carry keys outside `T`.
@@ -31,7 +46,11 @@ type Fallback<T extends string | number, Handled, R> = UnaryFn<
 //   #2 Fallback (last)  -> accepts a partial handler map plus a fallback
 interface MatcherStrict<T extends string | number> {
     <R>(handlers: Handlers<T, R>): UnaryFn<T, R>;
-    <R, Handled extends Exact<Partial<Handlers<T, R>>, Handled>>(
+    <
+        R,
+        Handled extends Exact<Partial<Handlers<T, R>>, Handled> &
+            MustBePartial<T, Handled>,
+    >(
         handlers: Handled & Partial<Handlers<T, R>>,
         fallback: Fallback<T, Handled, R>,
     ): UnaryFn<T, R>;
@@ -46,7 +65,11 @@ interface MatcherWidening<T extends string | number> {
     <P extends Exact<Handlers<T, unknown>, P>>(
         handlers: P,
     ): UnaryFn<T, PatternReturns<P>>;
-    <R, Handled extends Exact<Partial<Handlers<T, unknown>>, Handled>>(
+    <
+        R,
+        Handled extends Exact<Partial<Handlers<T, unknown>>, Handled> &
+            MustBePartial<T, Handled>,
+    >(
         handlers: Handled,
         fallback: Fallback<T, Handled, R>,
     ): UnaryFn<T, PatternReturns<Handled> | R>;