Indirect payloads

Normally, enums can’t be recursive (because they would require infinite storage):

enum Tree<T> {
    case leaf(T)
    case branch(Tree<T>, Tree<T>)  // error: recursive enum 'Tree<T>' is not marked 'indirect'
}

The indirect keyword makes the enum store its payload with a layer of indirection, rather than storing it inline. You can use this keyword on a single case:

enum Tree<T> {
    case leaf(T)
    indirect case branch(Tree<T>, Tree<T>)
}

let tree = Tree.branch(.leaf(1), .branch(.leaf(2), .leaf(3)))

indirect also works on the whole enum, making any case indirect when necessary:

indirect enum Tree<T> {
    case leaf(T)
    case branch(Tree<T>, Tree<T>)
}

Found a mistake? Have a question or improvement idea? Let me know.

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