go back to overloads instead of union

Author: Brian Chen

packages/firebase/index.d.ts

@@ -15,7 +15,11 @@
  * limitations under the License.
  */
 
-import { DocumentData } from '@firebase/firestore-types';
+import {
+  DocumentData,
+  QueryDocumentSnapshot,
+  SetOptions
+} from '@firebase/firestore-types';
 
 /**
  * <code>firebase</code> is a global namespace from which all Firebase
@@ -7864,9 +7868,8 @@ declare namespace firebase.firestore {
      * Firestore database). To use set() with `merge` and `mergeFields,
      * toFirestore() must be defined with `Partial<T>`.
      */
-    toFirestore:
-      | ((modelObject: T) => DocumentData)
-      | ((modelObject: Partial<T>, options?: SetOptions) => DocumentData);
+    toFirestore(modelObject: T): DocumentData;
+    toFirestore(modelObject: Partial<T>, options: SetOptions): DocumentData;
 
     /**
      * Called by the Firestore SDK to convert Firestore data into an object of
@@ -7875,10 +7878,7 @@ declare namespace firebase.firestore {
      * @param snapshot A QueryDocumentSnapshot containing your data and metadata.
      * @param options The SnapshotOptions from the initial call to `data()`.
      */
-    fromFirestore: (
-      snapshot: QueryDocumentSnapshot,
-      options: SnapshotOptions
-    ) => T;
+    fromFirestore(snapshot: QueryDocumentSnapshot, options: SnapshotOptions): T;
   }
 
   /**

packages/firestore-types/index.d.ts

@@ -49,14 +49,10 @@ export type LogLevel =
 export function setLogLevel(logLevel: LogLevel): void;
 
 export interface FirestoreDataConverter<T> {
-  toFirestore:
-    | ((modelObject: T) => DocumentData)
-    | ((modelObject: Partial<T>, options?: SetOptions) => DocumentData);
-
-  fromFirestore: (
-    snapshot: QueryDocumentSnapshot,
-    options: SnapshotOptions
-  ) => T;
+  toFirestore(modelObject: T): DocumentData;
+  toFirestore(modelObject: Partial<T>, options: SetOptions): DocumentData;
+
+  fromFirestore(snapshot: QueryDocumentSnapshot, options: SnapshotOptions): T;
 }
 
 export class FirebaseFirestore {