Class mocks break when using mockReset / resetAllMocks

[simon-abbott]

Describe the bug

#4564 made class methods' mock instances independent, but it overlooked some important edge cases:

1. Resetting an automocked constructor causes calling that constructor to no longer isolate its called members
2. Resetting/overriding the implementation of an isolated function detaches it entirely from the parent, causing the prototype method to no longer register mock calls

This is causing a lot of headaches while I'm trying to update our version of Vitest past 1.0.0 as we used prototypes and resetAllMocks all over the place.

The second point I can forego a fix for if necessary, since I can rewrite most of our tests using instance accessing instead of prototypes (i.e. foo.bar instead of Foo.prototype.bar), but the first point is really painful, and both are very counterintuitive.

Reproduction

// file: src/class.ts
export class TestClass {
  public doAThing() {
    return 12;
  }
}

// file: src/__tests__/class.test.ts
import { expect, test, vitest } from "vitest";
import { TestClass } from "../class.js";

vitest.mock("../class.js");

test("isolation", () => {
  const inst1 = new TestClass();

  // Normally, using the prototype counts the same calls as the instance.
  void inst1.doAThing();
  expect(inst1.doAThing).toHaveBeenCalledTimes(1);
  expect(TestClass.prototype.doAThing).toHaveBeenCalledTimes(1);

  // However, mocking the implementation of the instance detaches calls from the
  // prototype.
  vitest.mocked(inst1.doAThing).mockReturnValue(21);
  void inst1.doAThing();
  expect(inst1.doAThing).toHaveBeenCalledTimes(2);
  expect.soft(TestClass.prototype.doAThing).toHaveBeenCalledTimes(2);

  // Resetting mocks should restore back to the original behavior, but it doesn't:
  vitest.resetAllMocks();

  void inst1.doAThing();
  expect(inst1.doAThing).toHaveBeenCalledTimes(1);
  expect.soft(TestClass.prototype.doAThing).toHaveBeenCalledTimes(1); // This is once again decoupled.

  // And now the constructor shim is also broken, so these two instances are no longer isolated.
  const inst2 = new TestClass();
  const inst3 = new TestClass();

  expect.soft(inst2.doAThing).not.toBe(inst3.doAThing);
  expect.soft(vitest.mocked(inst2.doAThing).mock).not.toBe(vitest.mocked(inst3.doAThing).mock);

  void inst2.doAThing();
  expect(inst2.doAThing).toHaveBeenCalledTimes(1);
  expect.soft(inst3.doAThing).toHaveBeenCalledTimes(0);
});

System Info

Irrelevant

Used Package Manager

npm

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