Merge: Ensure idempotency so Merge<A, A> returns A (#1336)

Co-authored-by: Som Shekhar Mukherjee <49264891+som-sm@users.noreply.github.com>

Author: taiyakihitotsu

source/merge.d.ts

@@ -1,6 +1,8 @@
 import type {OmitIndexSignature} from './omit-index-signature.d.ts';
 import type {PickIndexSignature} from './pick-index-signature.d.ts';
 import type {Simplify} from './simplify.d.ts';
+import type {If} from './if.d.ts';
+import type {IsEqual} from './is-equal.d.ts';
 
 // Merges two objects without worrying about index signatures.
 type SimpleMerge<Destination, Source> = Simplify<{
@@ -47,11 +49,14 @@ Note: If you want a merge type that more accurately reflects the runtime behavio
 export type Merge<Destination, Source> =
 	Destination extends unknown // For distributing `Destination`
 		? Source extends unknown // For distributing `Source`
-			? Simplify<
-				SimpleMerge<PickIndexSignature<Destination>, PickIndexSignature<Source>>
-				& SimpleMerge<OmitIndexSignature<Destination>, OmitIndexSignature<Source>>
-			>
+			? If<IsEqual<Destination, Source>, Destination, _Merge<Destination, Source>>
 			: never // Should never happen
 		: never; // Should never happen
 
+export type _Merge<Destination, Source> =
+	Simplify<
+		SimpleMerge<PickIndexSignature<Destination>, PickIndexSignature<Source>>
+		& SimpleMerge<OmitIndexSignature<Destination>, OmitIndexSignature<Source>>
+	>;
+
 export {};

test-d/merge.ts

@@ -187,3 +187,13 @@ expectType<
 		{[x: number]: number; foo: number} | {bar: boolean; baz: boolean}
 	>,
 );
+
+// Idempotency
+type TestIntersectionObject = {a: string} & {b: string};
+// Note: If `Merge` simplified `TestIntersectionObject` to `{a: string; b: string}` then the following test would fail,
+// because `expectType` doesn't consider `{a: string; b: string}` equal to `{a: string} & {b: string}`.
+expectType<TestIntersectionObject>({} as Merge<TestIntersectionObject, TestIntersectionObject>);
+
+// Idempotency: Unions
+expectType<TestIntersectionObject | {a: string; b: string; c: string}>({} as Merge<TestIntersectionObject | {c: string}, TestIntersectionObject>);
+expectType<TestIntersectionObject | {a: string; b: string; c: string}>({} as Merge<TestIntersectionObject, TestIntersectionObject | {c: string}>);