mockReturnValue does not override class methods

[cexbrayat]

Describe the bug

It looks like mockReturnValue does not always work as expected (at least when used with classes).

Let's say you have an App class, that calls a method run on each of its deps:

  class Dep {
    run(): boolean {
      // does something random
    }
  }

  class App {
    constructor(private deps: Array<Dep>) {}

    start(): Array<Dep> {
      return (
        this.deps
          // run every dep
          // and only keeps the ones that started
          .filter((dep) => dep.run())
      );
    }
  }

The following simple test fails:

describe('App', () => {
    let dep1: Dep;
    let dep2: Dep;
    let app: App;

    beforeEach(() => {
      dep1 = new Dep();
      // first dep starts
      vi.spyOn(dep1, 'run').mockReturnValue(true);

      dep2 = new Dep();
      // second dep does not
      vi.spyOn(dep2, 'run').mockReturnValue(false);

      app = new App([dep1, dep2]);
    });

    test('should return only the deps that started', () => {
      const runningDeps: Array<Dep> = app.start();
      expect(dep1.run).toHaveBeenCalled();
      expect(dep2.run).toHaveBeenCalled();
      // fails because `runningDeps` is empty
      expect(runningDeps).toEqual([dep1]);
    });
  });

Reproduction

https://stackblitz.com/edit/vitest-dev-vitest-iucjaa?file=test/suite.test.ts

Note that the same test in Jest does succeed

System Info

System:
    OS: macOS 12.1
    CPU: (10) arm64 Apple M1 Max
    Memory: 12.57 GB / 64.00 GB
    Shell: 5.8 - /bin/zsh
  Binaries:
    Node: 16.13.2 - ~/.volta/tools/image/node/16.13.2/bin/node
    Yarn: 1.22.17 - ~/.volta/tools/image/yarn/1.22.17/bin/yarn
    npm: 8.3.2 - ~/.volta/tools/image/npm/8.3.2/bin/npm
  Browsers:
    Chrome: 97.0.4692.99
    Firefox: 96.0.2
    Safari: 15.2
  npmPackages:
    @vitejs/plugin-vue: 2.1.0 => 2.1.0
    vite: 2.7.13 => 2.7.13
    vitest: 0.2.4 => 0.2.4

Used Package Manager

yarn

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• Read the Contributing Guidelines.
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• The provided reproduction is a minimal reproducible example of the bug.