public protocol Proto {}
public struct Struct: Proto {}

public struct Indicator<T> where T : Proto {}
extension Indicator where T == MyStruct {
  public static var activity: Indicator<MyStruct> { fatalError() }
}

public struct WebImage {
  public func indicator<T>(_ indicator: Indicator<T>) where T : Proto {
    fatalError()
  }
}

WebImage()
  .indicator(./*complete here*/activity) // activity not suggested