☂ Review nullability reference annotations

[atifaziz]

This is a follow-up “umbrella issue” to track review of nullability reference annotations applied in PR #582. The annotations were slated for release 3.4.0 but that milestone missed its desired schedule by a big margin. On the positive side, C# 9 got released meanwhile with improvements that could help in revising some of the annotations to be more “honest” (especially with regards to public signatures). The review should therefore focus on:

• benefitting from unconstrained type parameter annotations
• remove uses of null-forgiving operator (!), if possible, based on improvements in static analysis
• pattern matches on {} versus var.

PR #582 touched on a number of methods, but the following are the priority for this review:

• Await
• CountBy
• EquiZip (Enable nullable context for remaining *Zip* tests #908)
• FallbackIfEmpty (Cleanup ! operator usage in FallbackIfEmpty #808)
• Flatten (Fix Flatten signature parts to be null-oblivious #847)
• Fold
• FullGroupJoin (Fix "FullGroupJoin" signature with more honest nullable annotations #806)
• GroupAdjacent (GroupAdjacent no longer works for null keys #809)
• Lag (Fix "Lag" & "Lead" signatures with more honest nullable annotations #805)
• Lead (Fix "Lag" & "Lead" signatures with more honest nullable annotations #805)
• LeftJoin
• MaxBy (Replace MayBeNull with ? in IExtremaEnumerable members #872)
• MinBy (Replace MayBeNull with ? in IExtremaEnumerable members #872)
• Memoize
• OrderBy
• OrderedMerge
• Pad (Fix "Pad*" signatures with more honest nullable annotations #804)
• PadStart (Fix "Pad*" signatures with more honest nullable annotations #804)
• PartialSort (PartialSort: #nullable enable on tests #888)
• Partition (Cleanup Partition Nullability #881)
• Permutations
• Rank (Fix nullability of Rank comparer argument #913)
• RightJoin
• ScanBy
• ToDataTable (Fix nullability of ToDataTable expressions argument #914)
• ToDelimitedString
• ToDictionary (Constrain key type param in ToDictionary to notnull #852)
• TrySingle (Fix "TrySingle" signature with honest nullable annotations #801)
• ZipLongest (Fix nullability for ZipLongest #900)
• ZipShortest (Enable nullable context for remaining *Zip* tests #908)

/cc @sid-6581

[viceroypenguin]

Await does not have any nullability concerns as far as I can tell.