Add spec for extension GetEnumerator

[333fred]

Added the spec for extension GetEnumerator. The only change from #3194 was the additional line about the refkind in overload resolution.

[333fred]

/cc @YairHalberstadt

[YairHalberstadt]

Should there be a section about async foreach?
Also there is a difference between extension foreach and instance foreach in that the former accepts params and optional arguments and the latter doesn't. I can't see any indication of that in the spec.

[333fred]

Should there be a section about async foreach?

Probably, will add.

Also there is a difference between extension foreach and instance foreach in that the former accepts params and optional arguments and the latter doesn't. I can't see any indication of that in the spec.

It doesn't need to be explicit. Overload resolution with a single argument will find methods with default parameters.

[YairHalberstadt]

I would expect it to be explicit for instance GetEnumerator then.

[gafter]

😷

[333fred]

I would expect it to be explicit for instance GetEnumerator then.

@YairHalberstadt thanks to some sleuthing from @gafter, there's yet another spec violation for compat that we kept: https://github.com/dotnet/roslyn/blob/master/src/Compilers/CSharp/Portable/Binder/ForEachLoopBinder.cs#L1006-L1016. It absolutely should be including those parameters, and it does for async enumerable.

[333fred]

Note: the spec I'm referring to in the update is https://github.com/dotnet/roslyn/blob/master/docs/features/async-streams.md, not the one checked into csharplang. @jcouv has taken a work item to update the one in this repo, which is out of date in several aspects (including whether extension methods are permitted).