From 628ab32f18636c3f82ac6882e5338d8f833c2938 Mon Sep 17 00:00:00 2001
From: Rayan Salhab <r.salhab@aiyexpertsolutions.com>
Date: Thu, 30 Apr 2026 10:01:34 +0300
Subject: [PATCH] fix(spy): support private method spy types (fix #10172)
 (#10213)

---
 packages/spy/src/index.ts             | 26 ++++++++++++++++++++++----
 test/typescript/test-d/test.test-d.ts | 22 ++++++++++++++++++++++
 2 files changed, 44 insertions(+), 4 deletions(-)

diff --git a/packages/spy/src/index.ts b/packages/spy/src/index.ts
index bd50f730b..125724845 100644
--- a/packages/spy/src/index.ts
+++ b/packages/spy/src/index.ts
@@ -259,6 +259,20 @@ export function fn<T extends Procedure | Constructable = Procedure>(
   }) as Mock<T>
 }
 
+type SpyOnValue<T extends object, K extends keyof any> = K extends keyof Required<T>
+  ? Required<T>[K]
+  : (T & Record<K, unknown>)[K]
+
+type SpyOnMethod<T extends object, K extends keyof any>
+  = SpyOnValue<T, K> extends Constructable | Procedure
+    ? SpyOnValue<T, K>
+    : never
+
+type SpyOnMethodKey<T extends object, K extends keyof any>
+  = SpyOnValue<T, K> extends Constructable | Procedure
+    ? K
+    : never
+
 export function spyOn<T extends object, S extends Properties<Required<T>>>(
   object: T,
   key: S,
@@ -275,7 +289,11 @@ export function spyOn<T extends object, M extends Classes<Required<T>> | Methods
 ): Required<T>[M] extends Constructable | Procedure
   ? Mock<Required<T>[M]>
   : never
-export function spyOn<T extends object, K extends keyof T>(
+export function spyOn<T extends object, K extends keyof any>(
+  object: T,
+  key: SpyOnMethodKey<T, K>,
+): Mock<SpyOnMethod<T, K>>
+export function spyOn<T extends object, K extends keyof any>(
   object: T,
   key: K,
   accessor?: 'get' | 'set',
@@ -317,14 +335,14 @@ export function spyOn<T extends object, K extends keyof T>(
     // but there's still a value on the object when called
     // https://github.com/vitest-dev/vitest/issues/9439
     if (original == null && accessType === 'value') {
-      original = object[key] as unknown as Procedure
+      original = object[key as unknown as keyof T] as unknown as Procedure
     }
   }
   else if (accessType !== 'value') {
-    original = () => object[key]
+    original = () => object[key as unknown as keyof T]
   }
   else {
-    original = object[key] as unknown as Procedure
+    original = object[key as unknown as keyof T] as unknown as Procedure
   }
 
   const originalImplementation = ssr && original ? original() : original
diff --git a/test/typescript/test-d/test.test-d.ts b/test/typescript/test-d/test.test-d.ts
index c940a29b9..56e6e2a93 100644
--- a/test/typescript/test-d/test.test-d.ts
+++ b/test/typescript/test-d/test.test-d.ts
@@ -44,6 +44,28 @@ describe('test', () => {
     // @ts-expect-error
     vi.spyOn(google, 'sheets').mockReturnValue({ foo: 1234 })
   })
+
+  test('spyOn private and protected methods compiles', () => {
+    class TestClass {
+      private privateMethod(): number {
+        return 42
+      }
+
+      protected protectedMethod(): string {
+        return '42'
+      }
+    }
+
+    const instance = new TestClass()
+    vi.spyOn(instance, 'privateMethod').mockReturnValue(1)
+    vi.spyOn(instance, 'protectedMethod').mockReturnValue('1')
+    // @ts-expect-error private method returns number
+    vi.spyOn(instance, 'privateMethod').mockReturnValue('1')
+    // @ts-expect-error protected method returns string
+    vi.spyOn(instance, 'protectedMethod').mockReturnValue(1)
+    // @ts-expect-error unknown methods are rejected
+    vi.spyOn(instance, 'unknownMethod')
+  })
 })
 
 expectTypeOf({ wolk: 'true' }).toHaveProperty('wolk')
-- 
2.51.2