✨ 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.

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>;