duplicates:

• SR-8878 Using Mirror to access a weak reference results in object being retained indefinitely

Issue Description:

The following code should print "Deinit!" before exiting. It does print this on the Swift 4.1 Snapshot 2018-05-31 toolchain, but does not on the Swift 4.2 toolchain that ships with Xcode 10.

class DeinitDetector {
    deinit {
        print("Deinit!")
    }
}
struct WeakHolder {
    weak var a: DeinitDetector!
}
do {
    let a = DeinitDetector()
    let b = WeakHolder(a: a)
    let _ = String(describing: b)
}

More info here https://forums.swift.org/t/struct-with-weak-ref-memory-issue/18040