fix: properly infer parameter and return types in server.boundary()

src/node/SetupServerApi.ts

@@ -60,10 +60,10 @@ export class SetupServerApi
     this.handlersController = new AsyncHandlersController(handlers)
   }
 
-  public boundary<Fn extends (...args: Array<any>) => unknown>(
-    callback: Fn,
-  ): (...args: Parameters<Fn>) => ReturnType<Fn> {
-    return (...args: Parameters<Fn>): ReturnType<Fn> => {
+  public boundary<Args extends Array<any>, R>(
+    callback: (...args: Args) => R,
+  ): (...args: Args) => R {
+    return (...args: Args): R => {
       return store.run<any, any>(
         {
           initialHandlers: this.handlersController.currentHandlers(),

src/node/glossary.ts

@@ -70,7 +70,7 @@ export interface SetupServer extends SetupServerCommon {
    *
    * @see {@link https://mswjs.io/docs/api/setup-server/boundary `server.boundary()` API reference}
    */
-  boundary<Fn extends (...args: Array<any>) => unknown>(
-    callback: Fn,
-  ): (...args: Parameters<Fn>) => ReturnType<Fn>
+  boundary<Args extends Array<any>, R>(
+    callback: (...args: Args) => R,
+  ): (...args: Args) => R
 }

test/typings/server.boundary.test-d.ts

@@ -0,0 +1,27 @@
+import { setupServer } from 'msw/node'
+import { test } from 'vitest'
+
+const fn = (_args: { a: number }): string => 'hello'
+
+const server = setupServer()
+const bound = server.boundary(fn)
+
+bound({ a: 1 }).toUpperCase()
+
+bound({
+  // @ts-expect-error Expected number, got string.
+  a: '1',
+})
+
+bound({ a: 1 })
+  // @ts-expect-error Unknown method ".fooBar()" on string.
+  .fooBar()
+
+test(
+  'should work',
+  server.boundary(({ expect }) => {
+    expect(true).toBe(true)
+    // @ts-expect-error Property 'doesntExist' does not exist on type 'ExpectStatic'
+    expect.doesntExist
+  }),
+)