How to dynamically create sections in a SwiftUI List/ForEach and avoid "Unable to infer complex closure return type"

In reference to my comment on your question, the data should be put into sections before being displayed.

The idea would be to have an array of objects, where each object contains an array of occurrences. So we simplify your occurrence object (for this example) and create the following:

struct Occurrence: Identifiable {
    let id = UUID()
    let start: Date
    let title: String
}

Next we need an object to represent all the occurrences that occur on a given day. We'll call it a Day object, however the name is not too important for this example.

struct Day: Identifiable {
    let id = UUID()
    let title: String
    let occurrences: [Occurrence]
    let date: Date
}

So what we have to do is take an array of Occurrence objects and convert them into an array of Day objects.

I have created a simple struct that performs all the tasks that are needed to make this happen. Obviously you would want to modify this so that it matches the data that you have, but the crux of it is that you will have an array of Day objects that you can then easily display. I have added comments through the code so that you can clearly see what each thing is doing.

struct EventData {
    let sections: [Day]

    init() {
        // create some events
        let first = Occurrence(start: EventData.constructDate(day: 5, month: 5, year: 2019), title: "First Event")
        let second = Occurrence(start: EventData.constructDate(day: 5, month: 5, year: 2019, hour: 10), title: "Second Event")
        let third = Occurrence(start: EventData.constructDate(day: 5, month: 6, year: 2019), title: "Third Event")

        // Create an array of the occurrence objects and then sort them
        // this makes sure that they are in ascending date order
        let events = [third, first, second].sorted { $0.start < $1.start }

        // create a DateFormatter 
        let dateFormatter = DateFormatter()
        dateFormatter.dateStyle = .medium
        dateFormatter.timeStyle = .none

        // We use the Dictionary(grouping:) function so that all the events are 
        // group together, one downside of this is that the Dictionary keys may 
        // not be in order that we require, but we can fix that
        let grouped = Dictionary(grouping: events) { (occurrence: Occurrence) -> String in
            dateFormatter.string(from: occurrence.start)
        }

        // We now map over the dictionary and create our Day objects
        // making sure to sort them on the date of the first object in the occurrences array
        // You may want a protection for the date value but it would be 
        // unlikely that the occurrences array would be empty (but you never know)
        // Then we want to sort them so that they are in the correct order
        self.sections = grouped.map { day -> Day in
            Day(title: day.key, occurrences: day.value, date: day.value[0].start)
        }.sorted { $0.date < $1.date }
    }

    /// This is a helper function to quickly create dates so that this code will work. You probably don't need this in your code.
    static func constructDate(day: Int, month: Int, year: Int, hour: Int = 0, minute: Int = 0) -> Date {
        var dateComponents = DateComponents()
        dateComponents.year = year
        dateComponents.month = month
        dateComponents.day = day
        dateComponents.timeZone = TimeZone(abbreviation: "GMT")
        dateComponents.hour = hour
        dateComponents.minute = minute

        // Create date from components
        let userCalendar = Calendar.current // user calendar
        let someDateTime = userCalendar.date(from: dateComponents)
        return someDateTime!
    }

}

This then allows the ContentView to simply be two nested ForEach.

struct ContentView: View {

    // this mocks your data
    let events = EventData()

    var body: some View {
        NavigationView {
            List {
                ForEach(events.sections) { section in
                    Section(header: Text(section.title)) {
                        ForEach(section.occurrences) { occurrence in
                            NavigationLink(destination: OccurrenceDetail(occurrence: occurrence)) {
                                OccurrenceRow(occurrence: occurrence)
                            }
                        }
                    }
                }
            }.navigationBarTitle(Text("Events"))
        }
    }
}

// These are sample views so that the code will work
struct OccurrenceDetail: View {
    let occurrence: Occurrence

    var body: some View {
        Text(occurrence.title)
    }
}

struct OccurrenceRow: View {
    let occurrence: Occurrence

    var body: some View {
        Text(occurrence.title)
    }
}

This is the end result.

This is actually two questions.

In the data part, please upgrade userData.occurrences from [Occurrence] to [[ Occurrence ]] (I called it latestOccurrences, here)

   var self.userData.latestOccurrences = Dictionary(grouping: userData.occurrences) { (occurrence: Occurrence) -> String in
let dateFormatter = DateFormatter()
dateFormatter.dateStyle = .medium
dateFormatter.timeStyle = .none
return dateFormatter.string(from:  occurrence.start)
 }.values

In the swiftUI, you just reorganize the latest data:

    NavigationView {
    List {
        ForEach(userData.latestOccurrences, id:\.self) { occurrenceSameDate in
            Section(header: Text("\(occurrenceSameDate[0].start, formatter: DateFormatter.init())")) {
                ForEach(occurrenceSameDate){ occurrence in
                NavigationLink(
                    destination: OccurrenceDetail(occurrence: occurrence)
                        .environmentObject(self.userData)
                ) {
                    OccurrenceRow(occurrence: occurrence)
                    }
                }
            }
        }
    }
    .navigationBarTitle(Text("Events"))
}.onAppear(perform: populate)