title: Cannot use mutating getter on immutable value: 'self' is immutable error
body: <p>I'm trying to reuse an older piece of Swift code, but getting an error 'Cannot use mutating getter on immutable value: 'self' is immutable error'. Xcode wanted to add 'mutating' before the func, and offered to do so through a 'fix'. So the error is gone there but still remains at the 'Text' statements.</p>
  
  <pre><code>import SwiftUI
  
  struct ContentView: View {
  
       typealias PointTuple = (day: Double, mW: Double)
      let points: [PointTuple] = [(0.0, 31.98), (1.0, 31.89), (2.0, 31.77), (4.0, 31.58), (6.0, 31.46)]
  
      lazy var meanDays = points.reduce(0) { $0 + $1.0 } / Double(points.count)
      lazy var meanMW   = points.reduce(0) { $0 + $1.1 } / Double(points.count)
  
      lazy var a = points.reduce(0) { $0 + ($1.day - meanDays) * ($1.mW - meanMW) }
      lazy var b = points.reduce(0) { $0 + pow($1.day - meanDays, 2) }
  
      lazy var m = a / b
      lazy var c = meanMW - m * meanDays        
      lazy var x : Double = bG(day: 3.0)
      lazy var y : Double = bG(day: 5.0)
      lazy var z : Double = bG(day: 7.0)
  
      mutating func bG(day: Double) -&gt; Double {
          return m * day + c
      }
  
      var body: some View {
          VStack {
              Text("\(x)")
              Text("\(y)")
              Text("\(z)")
          }
      }
  }
  
  #if DEBUG
  struct ContentView_Previews: PreviewProvider {
      static var previews: some View {
          ContentView()
      }
  }
  #endif
  </code></pre>