Nullable: Compiler issues

[krwq]

Joint issue for compiler issues related to Nullable feature.

Design questions:

• Virtual/Abstract members: is it safe to mark them non-nullable?
  • mark according to what user would expect (i.e. object.ToString should return non-nullable, even though it is possible to return null)
  • Nullable: System.Object coreclr#23466 (comment)

Bugs:

• initialization of fields is not being correctly tracked
• nullable foo + foo == null || foo.Bar()
• initialization using Interlocked.CompareExchange
• marking abstract/virtual member nullable does not let you restrict it in derived classes
• caching the result of a null comparison value forces you to use ! operator.

cc: @jaredpar @jcouv @stephentoub

[buyaa-n]

Bug: Compiler complaining when assagning nullable value to non nullable after checking it was not null

if (c[i] != null)
        ret[cnt++] = c[i]!;

dotnet/coreclr#23489 (comment)

[danmoseley]

For compiler issues we should presumably be opening bugs (with mini reproes) against Roslyn as we go along -- @jaredpar ?

Amongst other reasons if they fix them promptly, it might make future work easier.

Also I assume it is easier to construct a mini repro while reproing the issue, rather later from an old PR comment

[buyaa-n]

Similar bug: assigning nullable value after indirect null check throwing compiler error dotnet/coreclr#23489 (comment)

[jcouv]

For compiler issues we should presumably be opening bugs (with mini reproes) against Roslyn as we go along

Yes, please.

[krwq]

some of them might be fixed in the latest compiler version. @safern and @tannergooding will update coreclr compiler soon and we will try again and see if still repros - if still does then we will file issues.

[krwq]

ref: dotnet/coreclr#23526 (comment)

[krwq]

ref: dotnet/coreclr#23526 (comment)

[joperezr]

@krwq what is the difference between this issue and #29072 ?

[stephentoub]

I think this can be closed. All of these issues are tracked individually in the roslyn and csharplang repos, and we've been linking to those directly from the code.