Caught exception is not the same as the one thrown in .flatMapLatest {}

[vkislicins]

I've got a simple Flow:

  fun doThings() {
    getSomeFlow()
      .flatMapLatest { getSomeOtherFlow() }
      .catch { e -> view.showError(e) }
      .launchIn(mainScope)
  }

It's paired with a simple unit test:

  fun doThings_error() {
    val error = IOException("some error")
    coEvery { getSomeOtherFlow() } returns flow { throw error }

    doThings()

    verify { view.showError(error) }
  }

However, the test will fail with:

Verification failed: call 1 of 1: View(#1).showError(eq(java.io.IOException), null())). Only one matching call to View(#1)/showError(Throwable, String) happened, but arguments are not matching:
[0]: argument: java.io.IOException, matcher: eq(java.io.IOException), result: -
[1]: argument: null, matcher: null(), result: +

So to confirm I captured the argument passed into showError

    val slot = slot<IOException>()

    doThings()

    verify { view.showError(capture(slot)) }
    with(slot.captured) {
      assertThat(this).isInstanceOf(error::class.java)
      assertThat(this.message).isEqualTo(error.message)
    }

Which made the test pass. So it seems that the exception object that gets passed to catch {} function is not the same object that gets thrown? I couldn't find this behavior documented anywhere, so I was naturally wondering if this is expected and why. Most likely this is WAD but I'm trying to understand why the object is not the same and come up with the least clunky way of testing this.

Some things I've tried (which didn't make any difference):

1. Running everything on MainScope to avoid using TestCoroutineScope normally used in my unit tests
2. Throwing an exception directly instead of mocking.

[qwwdfsad]

It seems like you've stepped into stacktrace recovery.

You can disable it via system property, but probably the most efficient solution will be to leverage #2551 when it's there and use it within mock verifiers