A problemtic code

main.swift

// With Swift 5.8, it crashes on runtime due to reference count problem.
// With Swift 5.9-dev (LLVM d59f7189924eff9, Swift 12f2c2eb92c4c6a), today's nightly, it crashes due to an assertion.

class Box<T> {
    private var value: T

    init(_ value: T) {
        self.value = value
    }

    func mutate(_ mutation: (inout T) -> Void) {
        mutation(&self.value)
    }
}


func f() {
    let box: Box<() -> Void> = Box {
    }

    let s = "meow"

    box.mutate { value in
        value = {
            s
        }
    }
}

f()