Swift: Failed to assign value to a property of protocol?

by

You have to define the protocol as a class protocol:

protocol ValueProvider : class {
    var value: String {get set}
}

Then

var value: String {
    get { return v.value }
    set { v.value = newValue }
}

compiles and works as expected (i.e. assigns the new value to the
object referenced by v1 if v1 != nil, and to the object
referenced by v2 otherwise).

v is a read-only computed property of the type ValueProvider.
By defining the protocol as a class protocol the compiler knows
that v is a reference type, and therefore its v.value
property can be modified even if the reference itself is a constant.

Your initial code example works because there the v property has
the type A which is a reference type.

And your workaround 

set {
    var tmp = v1 ?? v2
    tmp.value = newValue
}

works because (read-write) properties of variables can be set in
any case (value type or reference type).

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