Fix Merge with optional any value (#583)

source/merge.d.ts

@@ -2,14 +2,10 @@ import type {OmitIndexSignature} from './omit-index-signature';
 import type {PickIndexSignature} from './pick-index-signature';
 import type {EnforceOptional} from './enforce-optional';
 
-// Merges two objects without worrying about index signatures or optional keys.
+// Merges two objects without worrying about index signatures.
 type SimpleMerge<Destination, Source> = {
- [Key in keyof Destination | keyof Source]: Key extends keyof Source
- ? Source[Key]
- : Key extends keyof Destination
- ? Destination[Key]
- : never;
-};
+ [Key in keyof Destination as Key extends keyof Source ? never : Key]: Destination[Key];
+} & Source;
 
 /**
 Merge two types into a new type. Keys of the second type overrides keys of the first type.

test-d/merge.ts

@@ -134,3 +134,17 @@ expectType<{
  bar: string;
  fooBar: string;
 }>(fooBarWithIndexSignature);
+
+declare const destinationWithAny: Merge<{foo?: any}, {bar: true}>;
+
+expectType<{
+ foo?: any;
+ bar: true;
+}>(destinationWithAny);
+
+declare const sourceWithAny: Merge<{foo: true}, {bar?: any}>;
+
+expectType<{
+ foo: true;
+ bar?: any;
+}>(sourceWithAny);