fix: type issue with spyOn method (fix #2365)

packages/vitest/src/integrations/spy.ts

@@ -154,10 +154,10 @@ export function spyOn<T, G extends Properties<Required<T>>>(
   methodName: G,
   accessType: 'set',
 ): SpyInstance<[T[G]], void>
-export function spyOn<T, M extends (Methods<Required<T>> | Classes<Required<T>>)>(
+export function spyOn<T, M extends (Classes<Required<T>> | Methods<Required<T>>)>(
   obj: T,
   methodName: M,
-): Required<T>[M] extends (...args: infer A) => infer R | (new (...args: infer A) => infer R) ? SpyInstance<A, R> : never
+): Required<T>[M] extends ({ new (...args: infer A): infer R }) | ((...args: infer A) => infer R) ? SpyInstance<A, R> : never
 export function spyOn<T, K extends keyof T>(
   obj: T,
   method: K,

test/core/test/fixtures/hello-mock.ts

@@ -0,0 +1,5 @@
+export class HelloWorld {
+  hello() {
+    return 'Hello World!'
+  }
+}

test/core/test/spy.test.ts

@@ -1,12 +1,32 @@
 import { describe, expect, test, vi } from 'vitest'
+import * as mock from './fixtures/hello-mock'
 
 /**
  * @vitest-environment happy-dom
  */
 
 describe('spyOn', () => {
+  const hw = new mock.HelloWorld()
+
   test('correctly infers method types', async () => {
     vi.spyOn(localStorage, 'getItem').mockReturnValue('world')
     expect(window.localStorage.getItem('hello')).toEqual('world')
   })
+
+  test('infers a class correctly', () => {
+    vi.spyOn(mock, 'HelloWorld').mockImplementationOnce(() => {
+      const Mock = vi.fn()
+      Mock.prototype.hello = vi.fn(() => 'hello world')
+      return new Mock()
+    })
+
+    const mockedHelloWorld = new mock.HelloWorld()
+    expect(mockedHelloWorld.hello()).toEqual('hello world')
+  })
+
+  test('infers a method correctly', () => {
+    vi.spyOn(hw, 'hello').mockImplementationOnce(() => 'hello world')
+
+    expect(hw.hello()).toEqual('hello world')
+  })
 })