fix(spy): correctly track constructor's "this" type

packages/spy/src/index.ts

@@ -404,7 +404,7 @@ export function spyOn<T, M extends Classes<Required<T>> | Methods<Required<T>>>(
 ): Required<T>[M] extends
 | { new (...args: infer A): infer R }
 | ((...args: infer A) => infer R)
-  ? MockInstance<(...args: A) => R>
+  ? MockInstance<(this: R, ...args: A) => R>
   : never
 export function spyOn<T, K extends keyof T>(
   obj: T,

test/core/test/jest-mock.test.ts

@@ -1,4 +1,4 @@
-import { describe, expect, it, vi } from 'vitest'
+import { describe, expect, expectTypeOf, it, vi } from 'vitest'
 
 describe('jest mock compat layer', () => {
   const returnFactory = (type: string) => (value: any) => ({ type, value })
@@ -72,6 +72,22 @@ describe('jest mock compat layer', () => {
     expect(Spy.mock.contexts[2]).toBe(ctx2)
   })
 
+  it('tracks spied class contexts and instances', () => {
+    interface SpyClass {}
+    interface SpyConstructor {
+      (): SpyClass
+      new (): SpyClass
+    }
+    const Spy = (function () {}) as SpyConstructor
+    const obj = { Spy }
+    const spy = vi.spyOn(obj, 'Spy')
+    const instance = new obj.Spy()
+
+    expectTypeOf(spy.mock.contexts[0]).toEqualTypeOf<SpyClass>()
+    expect(spy.mock.instances).toEqual([instance])
+    expect(spy.mock.contexts).toEqual([instance])
+  })
+
   it('implementation is set correctly on init', () => {
     const impl = () => 1
     const mock1 = vi.fn(impl)