System.Linq.Enumerable.OrderBy keySelector should be ItemCanBeNull

[drewnoakes]

It's ok for the Func<TSource, TKey> to return null. R# warns if you try to (when using implicit not null).

Same for OrderByDescending, ThenBy and ThenByDescending.

[sirduke]

Func<> does not support ItemNotNulll/ItemCanBeNull annotations. Request: https://youtrack.jetbrains.com/issue/RSRP-463008