IsEqual: Fix generic inference (#1306)

Author: taiyakihitotsu

source/is-equal.d.ts

@@ -26,14 +26,10 @@ type Includes<Value extends readonly any[], Item> =
 @category Utilities
 */
 export type IsEqual<A, B> =
-	[A, B] extends [infer AA, infer BB]
-		? [AA] extends [never]
-			? [BB] extends [never]
-				? true
-				: false
-			: [BB] extends [never]
-				? false
-				: _IsEqual<AA, BB>
+	[A] extends [B]
+		? [B] extends [A]
+			? _IsEqual<A, B>
+			: false
 		: false;
 
 // This version fails the `equalWrappedTupleIntersectionToBeNeverAndNeverExpanded` test in `test-d/is-equal.ts`.

test-d/is-equal.ts

@@ -85,3 +85,7 @@ expectType<true>(equalTupleIntersectionToBeNeverAndNeverExpanded);
 
 declare const equalTupleIntersectionAndTuple: IsEqual<[{a: 1}] & [{a: 1}], [{a: 1}]>; // eslint-disable-line @typescript-eslint/no-duplicate-type-constituents
 expectType<true>(equalTupleIntersectionAndTuple);
+
+// Test for Issue https://github.com/sindresorhus/type-fest/issues/1305
+type Assignability<T, U, _V extends IsEqual<T, U>> = any;
+type TestAssignability<T> = Assignability<T, T, true>;