fix: Merge, return arg if both are equal

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<{
@@ -44,7 +46,9 @@ Note: If you want a merge type that more accurately reflects the runtime behavio
 @see {@link ObjectMerge}
 @category Object
 */
-export type Merge<Destination, Source> =
+export type Merge<Destination, Source> = If<IsEqual<Destination, Source>, Destination, _Merge<Destination, Source>>;
+
+export type _Merge<Destination, Source> =
 	Destination extends unknown // For distributing `Destination`
 		? Source extends unknown // For distributing `Source`
 			? Simplify<

test-d/merge.ts

@@ -1,5 +1,5 @@
 import {expectType} from 'tsd';
-import type {Merge, FixedLengthArray} from '../index.d.ts';
+import type {Merge, FixedLengthArray, IsEqual} from '../index.d.ts';
 
 type Foo = {
 	a: number;
@@ -191,3 +191,9 @@ expectType<
 // Idempotency
 expectType<BarWithIndexSignatureOverwrite>({} as Merge<BarWithIndexSignatureOverwrite, BarWithIndexSignatureOverwrite>);
 expectType<FixedLengthArray<string, 3>>({} as Merge<FixedLengthArray<string, 3>, FixedLengthArray<string, 3>>);
+
+// Idempotency
+type TestIntersectionObject = {a: string} & {b: string};
+type IDMerge = Merge<TestIntersectionObject, TestIntersectionObject>;
+expectType<TestIntersectionObject>({} as IDMerge);
+expectType<IDMerge>({} as TestIntersectionObject);