storage.swift

final class Storage<StoredValue> {
    private var storage: [StoredValue] = []

    func addFailingValue<T: ModelRepresentable>(_ model: T) where T.Model == StoredValue {
        self.storage.append(model.value)
    }

    func addValue<T: ModelRepresentable>(_ model: T) {
        guard let value = model.value as? StoredValue else { return }
        self.storage.append(value)
    }
}

protocol ModelRepresentable {
    associatedtype Model
    var value: Model { get }
}

struct SimpleModel: ModelRepresentable {
    let value = "test"
}

let storage = Storage<SimpleModel>()
storage.addValue(SimpleModel()) // works as expected
storage.addFailingValue(SimpleModel()) // generic parameter 'T' could not be inferred