Add support for init accessors

[RexJaeschke]

Notes:

1. New section "§init-accessors Init accessors" contains the place-holder link ([§xxx](plug in here link to v9 records grammar)), which will need to be replaced by the corresponding grammar section number that results from the addition of the records feature (also in V9).
2. The edits to the final para of "15.9.2 Indexer and Property Differences" rely on the corresponding text being changed by PR Reverse some V7 edits w.r.t Indexers #970, which at the time of this writing had not been merged.

[KalleOlaviNiemitalo]

Should this say something about referencing an init accessor in an expression tree (System.Linq.Expressions)?

[KalleOlaviNiemitalo]

Apparently, it can also make a writable reference to a readonly field, but I'm not sure whether that is a C# 9 feature. However, unlike a constructor, it cannot assign to a get-only property. SharpLab:

public class C {
    private readonly int x;
    
    public int Y { get; }

    public int Z {
        get {
            // error CS0192: A readonly field cannot be used as a ref or out value (except in a constructor)
            ref int r = ref this.x;
            return 0;
        }
        init {
            // OK, despite readonly
            ref int r = ref this.x;
            
            // error CS0200: Property or indexer 'C.Y' cannot be assigned to -- it is read only
            this.Y = 0; 
        }
    }
}

[KalleOlaviNiemitalo]

Does nameof count as referencing a property in an expression?

[KalleOlaviNiemitalo]

Expression tree seems to be working as expected. SharpLab:

using System;
using System.Linq.Expressions;

public class C {
    public int M { get; init; }
}

public static class D {
    public static void Main()
    {
        Expression<Func<C>> expression1 = () => new C { M = 1 }; // OK
        ////Expression<Action<C>> expression2 = (c) => { c.M = 2; }; // error CS8852
        C c = expression1.Compile().Invoke();
        Console.WriteLine(c.M); // outputs 1
    }
}

[KalleOlaviNiemitalo]

Please add init to the interface_accessors grammar rule. Although an interface cannot be named in a new expression, the init accessor can be called via a type parameter constrained to that interface. SharpLab:

public interface I {
    int P { get; init; }
    int this[int index] { get; init; }
}

public class C : I {
    public int P { get; init; }
    public int this[int index] { get => 0; init {} }
}

static class Program
{
    static void M<T>() where T : I, new() {
        T t = new T() { P = 1, [5] = 6 };
    }
    
    static void Main()
    {
        M<C>();
    }
}

[RexJaeschke]

Re @KalleOlaviNiemitalo's comment #978 (comment):

This suggests the following changes to the grammar:

interface_accessors
    : attributes? 'get' ';'
    | attributes? 'set' ';'
    | attributes? 'init' ';'
    | attributes? 'get' ';' attributes? ('set'|'init') ';'
    | attributes? ('set'|'init') ';' attributes? 'get' ';'
    ;

However, that will be superseded by changes to this same grammar by V8's PR #681, which adds support for default interface function members. We should revisit this once that PR has been merged.