Jest 25 invokes setters during toEqual comparison, causing side effects

This is similar to https://github.com/facebook/jest/issues/9531.

It is not safe to assign a bunch of setters in the passed object. They may throw or they may have side effects. This isn’t theoretical because we may add setters like this to React.

Example: https://repl.it/repls/HuskyCandidConfiguration

test('setter object comparison', () => {
  expect({
    get a() {
      return {};
    },
    set a(value) {
      throw new Error('noo');
    },
    b: {},
  }).toEqual({ 
    a: {},
   });
});

This used to work fine in 24: https://repl.it/repls/AllMediumorchidDeals

This regression is currently blocking upgrading React to Jest 25.

[SimenB]

Ooh, just dropping the setter from the descriptor sounds good. We never want to run them anyways, as we only do this stuff after an assertion has failed and we generate a diff

[SimenB]

We don’t mutate the actual object passed in - we make a deep copy and mutate that copy. The reason is that we want to replace the properties that use asymmetric matchers and pass with the value, so that it’s not highlighted as failing in the diff view.

It’s not an optimal solution, ideally we’d have a smarter object diffing than a full serialization then comparing strings