Allow ??= operator on auto-properties

[Pentadome]

Allow ??= operator on auto-properties

Summary
The ??= currently is a shorthand for:

_something ?? (_something = new Something(this));
// is the same as:
_something ??= new Something(this);
// However this operator can not be used on properties.

List<Something> Somethings { get; } ??= new List<Something>(); // Error

// Therefore we need to write this;

private List<Something> _somethings;

public List<Something> Somethings => _somethings ??= new List<Something>();

Motivation
Currently "lazy" properties take up too much space and require too much typing for something that should be able to be expressed more easily.

Detailed design
Allow ??= on properties. So that these two mean the same thing:

public ICommand DoSomethingCommand { get; } ??= new Command(this.DoSomething);
private void DoSomething() {}

private Command _doSomethingCommand;
public ICommand DoSomethingCommand => _doSomethingCommand ??= new Command(this.DoSomething);
private void DoSomething() {}

Drawbacks
People who don't know what ??= does might be confused.

Unresolved questions
Should the backing field generated be accessible by other members of the class? If so, how? Like DoSomethingCommand.field, fieldof(DoSomethingCommand) or not at all?

[Joe4evr]

This is superseded by #140, which will allow you to write

public ICommand DoSomethingCommand => field ??= new Command(this.DoSomething);

[yaakov-h]

Seems like another use-case for #140

public List<Thing> Things => field ??= new List<Thing>();

[Pentadome]

@Joe4evr It is not approved, is it?

[Joe4evr]

It's championed, discussed in LDM last month, and I think it was @jnm2 who did a lot of work for the feature already.

[jnm2]

Another one to add to https://gist.github.com/jnm2/d3b37a43894f1738273005e69824d089#140-mentioned-as-preferred-or-alleviating-the-pain

@Joe4evr I just started the prototype a few days ago and it looks like it will go relatively quickly