[swift-evolution] SE-0030 Property behaviours

[Антон Жилин]

var behaviour computed<Value>: Value {
    accessor get() -> Value
    accessor set(newValue: Value) {
        fatalError()
    }
    get {
        return get()
    }
    set {
        set(newValue)
    }
}

There seems to be no way we can disable `set` if `accessor set` is
defaulted, based on current property behaviour rules. We could add both
@computed and @computedReadOnly, for example.

Behaviors already support read-only getters—you just omit the set {} block.
>

And I've just discovered a gaping hole in this design: mutability of
`accessor get` cannot be automatically forwarded to `get`. We would have to
always declare mutable `get`. Or have @computed, @computedMutable,
@computedReadOnly, @computedMutableReadOnly.

So computed properties should remain untouched for now, I guess.
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