init-cannot-assign-to-let.swift

init-cannot-assign-to-let.swift

public class Measurement {
    public let value: Double
    
    public required init(_ value:Double) {
        self.value = value
    }        
}

public struct Size {
    public let width:Measurement
    public let height:Measurement
    
    public init(measurement:Measurement.Type, size:Size) {
        self.width = Measurement(0.0)
        
        // with this line, both self.width and self.height assignments generate "cannot assign to property".
        self.height = measurement.init(size.height.value)
        
        // commenting out the above line and using this one instead, there is no complaint about assigning to the "let" property in init()
        //self.height = Measurement(0.0)
    }
}

Weirdly, if you move the troublesome code outside the body of Size.init()...

// insert at top level:
func makeMeasurement(m: Measurement.Type, size: Size) -> Measurement {
  return m.init(size.height.value)
}
// and replace line 17 with:
self.height = makeMeasurement(measurement, size: size)

...then it compiles just fine. But if you then move makeMeasurement() to be a local function inside Size.init(), it breaks again! So bizarre.

Oh, and this also works fine:

self.height = { measurement.init(size.height.value) }()

you are passing struct Size inside of Size's init : init(measurement:Measurement.Type, size:Size)
this seems like it would cause the compiler to think there is a circularity.