C#: Allow ref this

[BillWagner]

There are two related features that will affect the same documents:

1. Allow ref this (where only this ref is allowed in 7.2) See Discussion: ref this vs this ref in ref extension methods (15.6, as a 7.2 bug fix) csharplang#1022
2. Relax ordering on ref and partial modifiers. See Champion: relax ordering constraints around ref and partial modifiers on type declarations csharplang#946

The first fixes a grammar issue with ref. Extension methods whose first argument is a struct type currently have to write that argument as ref this T. That looks clumsy and gives the wrong impression upon reading it.

This work would allow this ref T as the preferred syntax.

The second would relax the ordering constraint on partial ref struct.

Neither would not introduce new articles, but would update the grammar for extension methods, argument modifiers, and ref value types.

[BillWagner]

Question for @jcouv Will either of these be targetting LangVersion=7.2 once they ship?

[jcouv]

Only the first one (order of ref and this modifiers in ref extensions) will target C# 7.2.
The second one (order of ref and partial in type declarations) would be tied to C# 7.3.

There is also the by-val vs. in tiebreaker that will be targeting C# 7.2.

[BillWagner]

Thanks @jcouv The tiebreaker feature is issue #3949 for docs.

[BillWagner]

This is being done in a community PR dotnet/roslyn#23533

[BillWagner]

Changing the title as this did not make the 7.3 release

[BillWagner]

Closing pending acceptance of the feature.