Trying to represent generic trees in Swift

Tree.swift

public final class Box<T> {
    private let _value: () -> T
    public init(_ value: T) { self._value = {value} }
    public var value: T { return _value() }
}

enum Tree1<T> {
    case Leaf(T)
    case Left(Tree1<T>, T)
    case Right(T, Tree1<T>)
    case Both(Tree1<T>, T, Tree1<T>)
}
//=> swiftc segfaults with no explanation
// (I would have expected an error message saying that recursive value types aren't supported.)

enum Tree2<T> {
    case Leaf(T)
    case Left(Box<Tree2<T>>, T)
    case Right(T, Box<Tree2<T>>)
    case Both(Box<Tree2<T>>, T, Box<Tree2<T>>)
}
//=> swiftc segfaults with a trace containing:
// "(...) While emitting IR SIL function @_TFO4Tree5Tree24LeftU__fMGS0_Q__FTGCS_3BoxGS0_Q___Q__GS0_Q__ for 'Left' (...)"

enum Tree3<T> {
    case Leaf(Box<T>)
    case Left(Box<Tree3<T>>, Box<T>)
    case Right(Box<T>, Box<Tree3<T>>)
    case Both(Box<Tree3<T>>, Box<T>, Box<Tree3<T>>)
}
//=> This seems to work!

// The following compiles and runs. (Would of course make it easier to use with custom init etc.)
let leaf  = Tree3<Int>.Leaf(Box(1))
let left  = Tree3<Int>.Left(Box(leaf), Box(2))
let right = Tree3<Int>.Right(Box(3), Box(left))
let both  = Tree3<Int>.Both(Box(leaf), Box(4), Box(left))