MergeDeep: Fix optional key when value type is any or never (#777)

source/merge-deep.d.ts

@@ -1,7 +1,6 @@
 import type {ConditionalSimplifyDeep} from './conditional-simplify';
 import type {OmitIndexSignature} from './omit-index-signature';
 import type {PickIndexSignature} from './pick-index-signature';
-import type {EnforceOptional} from './enforce-optional';
 import type {Merge} from './merge';
 import type {
 	ArrayTail,
@@ -30,23 +29,28 @@ type MergeDeepRecordProperty<
 
 /**
 Walk through the union of the keys of the two objects and test in which object the properties are defined.
-- If the source does not contain the key, the value of the destination is returned.
-- If the source contains the key and the destination does not contain the key, the value of the source is returned.
-- If both contain the key, try to merge according to the chosen {@link MergeDeepOptions options} or return the source if unable to merge.
+Rules:
+1. If the source does not contain the key, the value of the destination is returned.
+2. If the source contains the key and the destination does not contain the key, the value of the source is returned.
+3. If both contain the key, try to merge according to the chosen {@link MergeDeepOptions options} or return the source if unable to merge.
 */
 type DoMergeDeepRecord<
 	Destination extends UnknownRecord,
 	Source extends UnknownRecord,
 	Options extends MergeDeepInternalOptions,
-> = EnforceOptional<{
-	[Key in keyof Destination | keyof Source]: Key extends keyof Source
-		? Key extends keyof Destination
-			? MergeDeepRecordProperty<Destination[Key], Source[Key], Options>
-			: Source[Key]
-		: Key extends keyof Destination
-			? Destination[Key]
-			: never;
-}>;
+> =
+// Case in rule 1: The destination contains the key but the source doesn't.
+{
+	[Key in keyof Destination as Key extends keyof Source ? never : Key]: Destination[Key];
+}
+// Case in rule 2: The source contains the key but the destination doesn't.
+& {
+	[Key in keyof Source as Key extends keyof Destination ? never : Key]: Source[Key];
+}
+// Case in rule 3: Both the source and the destination contain the key.
+& {
+	[Key in keyof Source as Key extends keyof Destination ? Key : never]: MergeDeepRecordProperty<Destination[Key], Source[Key], Options>;
+};
 
 /**
 Wrapper around {@link DoMergeDeepRecord} which preserves index signatures.

test-d/merge-deep.ts

@@ -33,9 +33,9 @@ expectType<never>(mergeDeep(undefined, {}));
 
 // Should merge simple objects
 expectType<{a: string; b: number}>(mergeDeep({a: 'life'}, {b: 42}));
-expectType<{a: 'life'; b: number}>(mergeDeep({a: 'life'} as const, {b: 42}));
-expectType<{a: string; b: 42}>(mergeDeep({a: 'life'}, {b: 42} as const));
-expectType<{a: 'life'; b: 42}>(mergeDeep({a: 'life'} as const, {b: 42} as const));
+expectType<{readonly a: 'life'; b: number}>(mergeDeep({a: 'life'} as const, {b: 42}));
+expectType<{a: string; readonly b: 42}>(mergeDeep({a: 'life'}, {b: 42} as const));
+expectType<{readonly a: 'life'; readonly b: 42}>(mergeDeep({a: 'life'} as const, {b: 42} as const));
 
 // Should spread simple arrays/tuples (default mode)
 expectType<Array<string | number>>(mergeDeep(['life'], [42]));
@@ -157,14 +157,34 @@ expectType<{
 		fooBar: boolean;
 		items: number[];
 	};
-	fooBarOptional?: {
+	fooBarOptional: {
 		foo: string;
 		bar: number;
 		fooBar: boolean;
 		items: number[];
-	};
+	} | undefined;
 }>(fooBarWithOptional);
 
+// Test for optional
+type FooOptional = {
+	string?: string;
+	any?: any;
+	never?: never;
+};
+type BarOptional = {
+	number?: number;
+};
+type MergedFooBar = {
+	string?: string;
+	any?: any;
+	never?: never;
+	number?: number;
+};
+declare const mergedFooBar: MergeDeep<FooOptional, BarOptional>;
+expectType<MergedFooBar>(mergedFooBar);
+declare const mergedBarFoo: MergeDeep<FooOptional, BarOptional>;
+expectType<MergedFooBar>(mergedBarFoo);
+
 // Should merge arrays with object entries
 type FooArray = Foo[];
 type BarArray = Bar[];