Avoid using Omit in HydratedDocument when not needed

See microsoft/TypeScript#39556 and many other
related issues.

Fixes Automattic#13529.

Author: orgads

test/types/models.test.ts

@@ -622,3 +622,21 @@ function gh13206() {
     expectType<ChangeStreamInsertDocument<ITest>>(change);
   });
 }
+
+function gh13529() {
+  interface ResourceDoc {
+    foo: string;
+  }
+
+  type ResourceIdT<ResourceIdField extends string> = {
+    [key in ResourceIdField]: string;
+  };
+  type ResourceDocWithId<ResourceIdField extends string> = ResourceDoc & ResourceIdT<ResourceIdField>;
+
+  function test<
+    ResourceType extends string,
+    DocT extends ResourceDocWithId<`${ResourceType}Id`>>(dbModel: Model<DocT>) {
+    const resourceDoc = new dbModel();
+    resourceDoc.foo = 'bar';
+  }
+}

types/index.d.ts

@@ -141,7 +141,7 @@ declare module 'mongoose' {
   > = IfAny<
     DocType,
     any,
-    Document<unknown, TQueryHelpers, DocType> & MergeType<
+    Document<unknown, TQueryHelpers, DocType> & MergeUnlessEmpty<
       Require_id<DocType>,
       TOverrides extends Record<string, never> ?
         {} :

types/utility.d.ts

@@ -6,13 +6,12 @@ declare module 'mongoose' {
     U :
     T extends ReadonlyArray<infer U> ? U : T;
 
+  type MergeType<A, B> = Omit<A, keyof B> & B;
+  type MergeUnlessEmpty<A, B> = keyof B extends never ? A : Omit<A, keyof B> & B;
+
   type UnpackedIntersection<T, U> = T extends null ? null : T extends (infer A)[]
     ? (Omit<A, keyof U> & U)[]
-    : keyof U extends never
-      ? T
-      : Omit<T, keyof U> & U;
-
-  type MergeType<A, B> = Omit<A, keyof B> & B;
+    : MergeUnlessEmpty<T, U>;
 
   /**
    * @summary Converts Unions to one record "object".