types: correctly handle union types in BufferToBinary and related helpers

Fix Automattic#15057

Author: vkarpov15

test/types/lean.test.ts

@@ -206,3 +206,34 @@ async function gh13382() {
   const res = await Test.updateOne({}, { name: 'bar' }).lean();
   expectAssignable<{ matchedCount: number, modifiedCount: number }>(res);
 }
+
+async function gh15057() {
+  type Attachment =
+    | {
+        type: 'foo';
+        value?: undefined;
+      }
+    | {
+        type: 'string';
+        value?: string;
+      };
+
+  const TestSchema = new Schema<Attachment>({
+    type: { type: String, required: true },
+    value: { type: String }
+  });
+
+  const AttachmentModel = model<Attachment>('test', TestSchema);
+
+  const main = async() => {
+    const item = await AttachmentModel.findOne().lean();
+
+    if (!item) return;
+
+    doSomeThing(item);
+  };
+
+  const doSomeThing = (item: Attachment) => {
+    console.log(item);
+  };
+}

types/index.d.ts

@@ -712,76 +712,82 @@ declare module 'mongoose' {
   /**
    * Converts any Buffer properties into mongodb.Binary instances, which is what `lean()` returns
    */
-  export type BufferToBinary<T> = T extends TreatAsPrimitives ? T : T extends Record<string, any> ? {
-    [K in keyof T]: T[K] extends Buffer
-      ? mongodb.Binary
-      : T[K] extends (Buffer | null | undefined)
-        ? mongodb.Binary | null | undefined
-        : T[K] extends Types.DocumentArray<infer ItemType>
+  export type BufferToBinary<T> = T extends Buffer
+    ? mongodb.Binary
+    : T extends TreatAsPrimitives
+      ? T
+      : T extends Record<string, any> ? {
+        [K in keyof T]: T[K] extends Buffer
+          ? mongodb.Binary
+          : T[K] extends Types.DocumentArray<infer ItemType>
             ? Types.DocumentArray<BufferToBinary<ItemType>>
             : T[K] extends Types.Subdocument<unknown, unknown, infer SubdocType>
               ? HydratedSingleSubdocument<SubdocType>
               : BufferToBinary<T[K]>;
-  } : T;
+      } : T;
 
   /**
    * Converts any Buffer properties into { type: 'buffer', data: [1, 2, 3] } format for JSON serialization
    */
-  export type BufferToJSON<T> = T extends TreatAsPrimitives ? T : T extends Record<string, any> ? {
-    [K in keyof T]: T[K] extends Buffer
-      ? { type: 'buffer', data: number[] }
-      : T[K] extends (Buffer | null | undefined)
-        ? { type: 'buffer', data: number[] } | null | undefined
-        : T[K] extends Types.DocumentArray<infer ItemType>
-            ? Types.DocumentArray<BufferToBinary<ItemType>>
-            : T[K] extends Types.Subdocument<unknown, unknown, infer SubdocType>
-              ? HydratedSingleSubdocument<SubdocType>
-              : BufferToBinary<T[K]>;
-  } : T;
+  export type BufferToJSON<T> = T extends Buffer
+    ? { type: 'buffer', data: number[] }
+    : T extends TreatAsPrimitives
+      ? T
+      : T extends Record<string, any> ? {
+        [K in keyof T]: T[K] extends Buffer
+          ? { type: 'buffer', data: number[] }
+          : T[K] extends Types.DocumentArray<infer ItemType>
+              ? Types.DocumentArray<BufferToBinary<ItemType>>
+              : T[K] extends Types.Subdocument<unknown, unknown, infer SubdocType>
+                ? HydratedSingleSubdocument<SubdocType>
+                : BufferToBinary<T[K]>;
+      } : T;
 
   /**
    * Converts any ObjectId properties into strings for JSON serialization
    */
-  export type ObjectIdToString<T> = T extends TreatAsPrimitives ? T : T extends Record<string, any> ? {
-    [K in keyof T]: T[K] extends mongodb.ObjectId
-      ? string
-      : T[K] extends (mongodb.ObjectId | null | undefined)
-        ? string | null | undefined
-        : T[K] extends Types.DocumentArray<infer ItemType>
-            ? Types.DocumentArray<ObjectIdToString<ItemType>>
-            : T[K] extends Types.Subdocument<unknown, unknown, infer SubdocType>
-              ? HydratedSingleSubdocument<ObjectIdToString<SubdocType>>
-              : ObjectIdToString<T[K]>;
-  } : T;
+  export type ObjectIdToString<T> = T extends mongodb.ObjectId
+    ? string
+    : T extends TreatAsPrimitives
+      ? T
+      : T extends Record<string, any> ? {
+        [K in keyof T]: T[K] extends mongodb.ObjectId
+          ? string
+          : T[K] extends Types.DocumentArray<infer ItemType>
+              ? Types.DocumentArray<ObjectIdToString<ItemType>>
+              : T[K] extends Types.Subdocument<unknown, unknown, infer SubdocType>
+                ? HydratedSingleSubdocument<ObjectIdToString<SubdocType>>
+                : ObjectIdToString<T[K]>;
+      } : T;
 
   /**
    * Converts any Date properties into strings for JSON serialization
    */
-  export type DateToString<T> = T extends TreatAsPrimitives ? T : T extends Record<string, any> ? {
-    [K in keyof T]: T[K] extends NativeDate
-      ? string
-      : T[K] extends (NativeDate | null | undefined)
-        ? string | null | undefined
-        : T[K] extends Types.DocumentArray<infer ItemType>
-            ? Types.DocumentArray<DateToString<ItemType>>
-            : T[K] extends Types.Subdocument<unknown, unknown, infer SubdocType>
-              ? HydratedSingleSubdocument<DateToString<SubdocType>>
-              : DateToString<T[K]>;
-  } : T;
+  export type DateToString<T> = T extends NativeDate
+    ? string
+    : T extends TreatAsPrimitives
+      ? T
+      : T extends Record<string, any> ? {
+        [K in keyof T]: T[K] extends NativeDate
+          ? string
+          : T[K] extends (NativeDate | null | undefined)
+            ? string | null | undefined
+            : T[K] extends Types.DocumentArray<infer ItemType>
+                ? Types.DocumentArray<DateToString<ItemType>>
+                : T[K] extends Types.Subdocument<unknown, unknown, infer SubdocType>
+                  ? HydratedSingleSubdocument<DateToString<SubdocType>>
+                  : DateToString<T[K]>;
+      } : T;
 
   /**
    * Converts any Mongoose subdocuments (single nested or doc arrays) into POJO equivalents
    */
   export type SubdocsToPOJOs<T> = T extends TreatAsPrimitives ? T : T extends Record<string, any> ? {
-    [K in keyof T]: T[K] extends NativeDate
-      ? string
-      : T[K] extends (NativeDate | null | undefined)
-        ? string | null | undefined
-        : T[K] extends Types.DocumentArray<infer ItemType>
-            ? ItemType[]
-            : T[K] extends Types.Subdocument<unknown, unknown, infer SubdocType>
-              ? SubdocType
-              : SubdocsToPOJOs<T[K]>;
+    [K in keyof T]: T[K] extends Types.DocumentArray<infer ItemType>
+      ? ItemType[]
+      : T[K] extends Types.Subdocument<unknown, unknown, infer SubdocType>
+        ? SubdocType
+        : SubdocsToPOJOs<T[K]>;
   } : T;
 
   export type JSONSerialized<T> = SubdocsToPOJOs<