fix(spy): preserve class mock prototype methods

[hi-ogawa]

Description

• closes vi.spyOn(...).mockImplementation(class {...}) produces instances with no prototype methods #10553

Root cause

Vitest constructs class mocks by running the original/mock implementation constructor, but it uses the mock wrapper as newTarget. That means the constructor body and class fields run, but the allocated object inherits from MockClass.prototype instead of ActualClass.prototype. The gap is easiest to see by splitting construction into two conceptual phases: object allocation with a prototype, then constructor/class-field initialization.

class ActualClass {
  constructor(...ctorArgs) {
    // In normal construction, `this` already inherits from ActualClass.prototype.
    // So `this.method1` is visible here.
  }

  method1() {} // lives on ActualClass.prototype
  method2 = () => {} // assigned as an own property during construction
}

const actualInstance = new ActualClass(...ctorArgs)
//
// This is equivalent to:
//
//   Reflect.construct(ActualClass, ctorArgs, ActualClass)
//
// which then semantically almost same as:
//
//   const actualInstance = Object.create(ActualClass.prototype)
//   // Creates this prototype chain:
//   //   actualInstance -> ActualClass.prototype -> Object.prototype
//   // This is why:
//   //   actualInstance.method1                 // found via ActualClass.prototype
//   //   actualInstance instanceof ActualClass  // true
//
//   ActualClass[[constructorBody]].apply(actualInstance, ctorArgs)
//   // Runs constructor/class-field initialization on `actualInstance`.
//   // This is why:
//   //   actualInstance.method2                 // own property assigned during construction

const MockClass = vi.fn(ActualClass)
const mockInstance = new MockClass(...ctorArgs)
//
// Vitest currently implements this as:
//
//   Reflect.construct(ActualClass, ctorArgs, MockClass)
//
// which mostly means:
//
//   const mockInstance = Object.create(MockClass.prototype)
//   // Creates this prototype chain:
//   //   mockInstance -> MockClass.prototype -> Object.prototype
//   //
//   // This is why:
//   //   mockInstance.method1                 // missing: ActualClass.prototype is not in the chain
//   //   mockInstance instanceof MockClass    // true
//   //   mockInstance instanceof ActualClass  // false
//
//   ActualClass[[constructorBody]].apply(mockInstance, ctorArgs)
//   // Runs constructor/class-field initialization on `mockInstance`.
//   // This is why:
//   //   mockInstance.method2                 // own property assigned during construction

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[sheremet-va]

Is there any explanation why Reflect.construct just doesn't work? Does it behave the same outside of Vitest?

[hi-ogawa]

It's is quite mind-bending, but I got the gist. Added root cause in the PR description. I'm not settled with the fix. There is still some divergence and probably need to pick some trade off.

[sheremet-va]

Ah, I remember this now. I don't think there is a good solution for this, all of them break something, it just needs to be documented