[Nullable] Compiler doesn't consider Where(x => x != null) to be a null check

[maxkoshevoi]

Background and Motivation

Improve nullability feature by enabling compiler to understand null checks done through LINQ.

Currently lines 3-9 produce nullable warnings, but shouldn't:

List<string?> list = new() { "a", null };

IEnumerable<string> nonNullableList = list.Where(s => s != null);
string nonNullableValue =             list.First(s => s != null);
nonNullableValue =                    list.Single(s => s != null);

nonNullableList =                     list.Where(s => !string.IsNullOrEmpty(s));
nonNullableValue =                    list.First(s => !string.IsNullOrEmpty(s));
nonNullableValue =                    list.Single(s => !string.IsNullOrEmpty(s));

[Dreamescaper]

Related: dotnet/csharplang#3951

[CyrusNajmabadi]

I believe this is currently working as expected from the compiler perspective (teh dotnet/roslyn repo). We'd need a language change at dotnet/csharplang in order to support this. I'm going to close this out as i believe the corresponding lang issue is already being tracked.