protocol DelegateProtocol : AnyObject {
    func foo()
}

protocol DelegateContainer : AnyObject {
    associatedtype ConcreteDelegate : AnyObject // <== THIS ONE HERE
    var delegate: ConcreteDelegate? { get set }
}

class SomeClass : DelegateContainer {
    // Typealias is needed to solve https://bugs.swift.org/browse/SR-6097
    typealias ConcreteDelegate = DelegateProtocol
    weak var delegate: DelegateProtocol?
}

class Delegate : DelegateProtocol {
    func foo() { print("foo") }
}

func connectDelegateContainer<T>(_ delegateContainer: T, with delegate: AnyObject) where T : DelegateContainer {
    if let aDelegate = delegate as? T.ConcreteDelegate {
        // Uncomment next code line to trigger the issue:
        //   error: Playground execution aborted: error: Execution was interrupted,
        //   reason: EXC_BAD_ACCESS (code=2, address=0x119a19180).
        //   The process has been left at the point where it was interrupted, use
        //   "thread return -x" to return to the state before expression evaluation.
        //
        // print("connected")
        delegateContainer.delegate = aDelegate
    }
}

// Test
let someClass = SomeClass()

let delegate = Delegate()

connectDelegateContainer(someClass, with: delegate)

print(someClass.delegate === delegate) // true

// Uncomment next code line to trigger the issue:
//   error: Playground execution aborted: error: Execution was interrupted,
//   reason: EXC_BAD_ACCESS (code=2, address=0x119a19180).
//   The process has been left at the point where it was interrupted, use
//   "thread return -x" to return to the state before expression evaluation.
//
// someClass.delegate?.foo()