Describe the impact of ? on arrays of arrays

[jskeet]

Alternative to #1386.

Fixes #1385.

[jskeet]

I'm fully expecting there to be some more work here, but I'm hoping it's at least close.

[jskeet]

@Nigel-Ecma: I believe the grammar test failure is now a matter of changing the expected tokens (due to the introduction of non_array_non_nullable_type) - let me know if this is something I can fix myself, or whether it needs your help :)

[BillWagner]

I like this approach. I have a couple comments to consider and discuss.

[Nigel-Ecma]

I’ve made a few comments but obviously there is a fair bit to go yet – e.g. conversions. Some of it is still confusing, but it seems a good start to dressing this up ;-)

[Nigel-Ecma]

@Nigel-Ecma: I believe the grammar test failure is now a matter of changing the expected tokens (due to the introduction of non_array_non_nullable_type) - let me know if this is something I can fix myself, or whether it needs your help :)

With the proviso that I haven’t downloaded this and run the failing tests it does look like the errors reported are valid changes in the expect parse due to the revised grammar.

If this is the case then it will just need a few updates to the “reference” parse files (the ones that are machine generated and ignored in PR reviews). You can generate them yourself but you might prefer one of us with the tools already downloaded to run and produce them does it! It can wait until the PR is approved and before it is merged.

[jskeet]

Next steps:

• Nigel to email Jon the files to add to this PR to get the grammar checker passing
• Jon will update this PR, then merge.
• Jon to file a new issue around mapping between nullable and non-nullable arrays, see Allow arrays of nullable reference types #1386, to also include conversions - and anything else folks think is missing
• Jon will hopefully create a new PR to address that

[BillWagner]

Let's based on our discussion in the meeting.

Per meeting discussion