Nested exceptions are incorrectly recovered

[vadimsemenov]

See #3714 (comment)

This test asserts wrong behaviour.

Correct test:

        val result = runCatching {
            coroutineScope<Unit> {
                throw RuntimeException("outer", IllegalStateException("inner"))
            }
        }
        val ex = result.exceptionOrNull() ?: error("Expected to fail")
        assertIs<RuntimeException>(ex)
        assertEquals("outer", ex.message)
        val originalCause = ex?.cause
        assertIs<IllegalStateException>(originalCause)
        assertEquals("inner", originalCause.message)

[dkhalanskyjb]

RuntimeException does not implement CopyableThrowable, so it's not copied, but instead wrapped into a new RuntimeException. The original and the copy have different stack traces and object identities, and accessing the original is occasionally useful. This behavior is documented and intentional: https://github.com/Kotlin/kotlinx.coroutines/blob/master/docs/topics/debugging.md#stacktrace-recovery-machinery

stacktrace recovery machinery tries to create a copy of the original exception (with the original exception as the cause)

[dkhalanskyjb]

By the way, if you need to access the original exception regardless of whether stacktrace recovery was enabled, please describe your use cases in a reply to #2551 (comment)