fix(spy): fix type error when assigning vi.spyOn to MockInstance of function overload (#6086)

Author: hi-ogawa

packages/spy/src/index.ts

@@ -401,11 +401,11 @@ export function spyOn<T, G extends Properties<Required<T>>>(
 export function spyOn<T, M extends Classes<Required<T>> | Methods<Required<T>>>(
   obj: T,
   methodName: M
-): Required<T>[M] extends
-| { new (...args: infer A): infer R }
-| ((...args: infer A) => infer R)
+): Required<T>[M] extends { new (...args: infer A): infer R }
   ? MockInstance<(this: R, ...args: A) => R>
-  : never
+  : T[M] extends Procedure
+    ? MockInstance<T[M]>
+    : never
 export function spyOn<T, K extends keyof T>(
   obj: T,
   method: K,

test/core/test/vi.spec.ts

@@ -2,7 +2,7 @@
  * @vitest-environment jsdom
  */
 
-import type { Mock, MockedFunction, MockedObject } from 'vitest'
+import type { Mock, MockInstance, MockedFunction, MockedObject } from 'vitest'
 import { describe, expect, expectTypeOf, test, vi } from 'vitest'
 import { getWorkerState } from '../../../packages/vitest/src/utils'
 
@@ -118,6 +118,40 @@ describe('testing vi utils', () => {
     expect(someFn4).not.toBeCalled()
   })
 
+  test(`vi.spyOn for function overload types`, () => {
+    class MyElement {
+      scrollTo(options?: ScrollToOptions): void
+      scrollTo(x: number, y: number): void
+      scrollTo() {}
+    }
+
+    // verify `spyOn` is assignable to `MockInstance` with overload
+    const spy: MockInstance<MyElement['scrollTo']> = vi.spyOn(
+      MyElement.prototype,
+      'scrollTo',
+    )
+
+    // however `Parameters` only picks up the last overload
+    // due to typescript limitation
+    expectTypeOf(spy.mock.calls).toEqualTypeOf<
+      [x: number, y: number][]
+    >()
+  })
+
+  test(`mock.contexts types`, () => {
+    class TestClass {
+      f(this: TestClass) {}
+      g() {}
+    }
+
+    const fSpy = vi.spyOn(TestClass.prototype, 'f')
+    const gSpy = vi.spyOn(TestClass.prototype, 'g')
+
+    // contexts inferred only when `this` is explicitly annotated
+    expectTypeOf(fSpy.mock.contexts).toEqualTypeOf<TestClass[]>()
+    expectTypeOf(gSpy.mock.contexts).toEqualTypeOf<unknown[]>()
+  })
+
   test('can change config', () => {
     const state = getWorkerState()
     expect(state.config.hookTimeout).toBe(10000)