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By default, Swift guarantees thread safety for lazy initialization, ensuring that only one instance is created even in concurrent environments.

This is a wrong assumption. Initialization is not thread-safe, the initialization block can be called multiple times in a concurrent environment. For example:

class Example {
    lazy var value: Int = {
        print("Initializing value")
        return 42
    }()
}

let instance = Example()

DispatchQueue.global().async {
    print("Thread 1: \(instance.value)")
}

DispatchQueue.global().async {
    print("Thread 2: \(instance.value)")
}

"Initializing value" will be printed twice. See also this discussion - forums.swift.org/t/lazy-property-initialization-question/51085/8 for deeper understanding.