Swift - weak self in didSet

I rarely see people using [weak self] in didSet. Is there a reason for this?

I tried to use [weak self] in my didSet of a variable:

var data: Dictionary<String, Any>! { // [1]
        didSet { [2]
            self?.layoutSubviews()
        }
    }

Either I put [weak self] in at [1] or [2], I still get the error: Use of unresolved identifier weak

Why is that? Is it illegal to use [weak self] for a didSet ?

Regards,