Fix assignability of wrong message type in create

packages/protobuf-test/extra/ts-types-proto2.proto

@@ -0,0 +1,26 @@
+// Copyright 2021-2024 Buf Technologies, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+syntax = "proto2";
+package spec;
+
+message TsTypeA {
+  optional string str = 1;
+  optional TsTypeA child = 2;
+}
+
+message TsTypeB {
+  optional string str = 1;
+  optional TsTypeB child = 2;
+}

packages/protobuf-test/src/create.test.ts

@@ -27,6 +27,7 @@ import * as proto3_ts from "./gen/ts/extra/proto3_pb.js";
 import * as proto2_ts from "./gen/ts/extra/proto2_pb.js";
 import * as edition2023_ts from "./gen/ts/extra/edition2023_pb.js";
 import * as test_messages_proto3_editions_ts from "./gen/ts/editions/golden/test_messages_proto3_editions_pb.js";
+import * as ts_types from "./gen/ts/extra/ts-types-proto2_pb.js";
 import { fillProto3Message, fillProto3MessageNames } from "./helpers-proto3.js";
 import {
   fillEdition2023Message,
@@ -549,6 +550,12 @@ describe("create()", () => {
       const notAUser = create(proto3_ts.Proto3MessageSchema, user);
       expect(notAUser).toBeDefined();
     });
+    test("rejects foreign typed message with assignable properties as a type error", () => {
+      const a = create(ts_types.TsTypeASchema);
+      // @ts-expect-error TS2345: Argument of type TsTypeA is not assignable to parameter of type MessageInit<TsTypeB> | undefined
+      const b = create(ts_types.TsTypeBSchema, a);
+      expect(b).toBeDefined();
+    });
     test("rejects extra properties in the object literal as a type error", () => {
       const msg = create(proto3_ts.Proto3MessageSchema, {
         // @ts-expect-error TS2353

packages/protobuf/src/types.ts

@@ -50,7 +50,7 @@ export type MessageJsonType<Desc extends DescMessage> =
  */
 export type MessageInitShape<Desc extends DescMessage> =
   Desc extends GenMessage<infer RuntimeShape>
-    ? RuntimeShape | MessageInit<RuntimeShape>
+    ? MessageInit<RuntimeShape>
     : Record<string, unknown>;
 
 /**
@@ -93,10 +93,11 @@ export type UnknownField = {
  * The init type for a message, which makes all fields optional.
  * The init type is accepted by the function create().
  */
-type MessageInit<T extends Message> = {
-  [P in keyof T as P extends "$typeName" | "$unknown" ? never : P]?: FieldInit<
-    T[P]
-  >;
+// prettier-ignore
+type MessageInit<T extends Message> = T | {
+  [P in keyof T as P extends "$unknown" ? never : P]?: P extends "$typeName"
+    ? never
+    : FieldInit<T[P]>;
 };
 
 // prettier-ignore