New nullable warnings when building runtime

[RikkiGibson]

Version Used: 81c5d71

Steps to Reproduce:

Build the Roslyn packages and set up your dotnet/runtime enlistment similar to #46575. Then run .\Build.cmd in your dotnet/runtime enlistment. You might also be able to use a method like https://github.com/dotnet/runtime/pull/40197/files#diff-8b8f08ffbf7b863fb3700c1718eeb4cbR20 using a recent build like 3.8.0-2.20404.11 to simply reference the most recently build release packages.

Expected Behavior:
Builds without nullable warnings

Actual Behavior:
src\libraries\System.Private.CoreLib\src\System\Collections\Generic\ArraySortHelper.cs(476,57): error CS8601: Possible null reference assignment. [src\coreclr\src\System.Private.CoreLib\System.Private.CoreLib.csproj]
src\libraries\System.Private.CoreLib\src\System\Collections\Generic\ArraySortHelper.cs(477,60): error CS8601: Possible null reference assignment. [src\coreclr\src\System.Private.CoreLib\System.Private.CoreLib.csproj]
src\libraries\System.Private.CoreLib\src\System\Collections\Generic\ArraySortHelper.cs(555,75): error CS8601: Possible null reference assignment. [src\coreclr\src\System.Private.CoreLib\System.Private.CoreLib.csproj]
src\libraries\System.Private.CoreLib\src\System\Collections\Generic\ArraySortHelper.cs(1057,31): error CS8601: Possible null reference assignment. [src\coreclr\src\System.Private.CoreLib\System.Private.CoreLib.csproj]
src\libraries\System.Private.CoreLib\src\System\Diagnostics\Tracing\EventCounter.cs(45,32): error CS8601: Possible null reference assignment. [src\coreclr\src\System.Private.CoreLib\System.Private.CoreLib.csproj]

The warnings seem to involve generic substitution or taking a ref to an unconstrained type parameter. @cston do you have any idea what could be happening here?

[cston]

The new warnings are for cases such as the following. The warning looks correct, although it was not reported with earlier builds.

#nullable enable

interface I { }

class Program
{
    static void F0<T>(ref T x, ref T y)
    {
    }
    static void F<T>(ref T x, ref T y) where T : I
    {
        if (x == null) { }
        F0(ref x, ref y); // warning: x may be null
    }
}

[RikkiGibson]

Oh, interesting, so is it because one of the Ts is constrained to non-nullable interface?

Would it make sense to send a patch to the runtime with a suggested fix, or just find a code owner and give a heads up?

[cston]

It looks like the new warnings are reported from #46405.

[cston]

Resolved in dotnet/runtime#41046.

[charsleysa]

@cston it seems that EventCounter.cs was missed in the PR dotnet/runtime#41046 and still has the issue in the release/5.0 branch but in master has been fixed by an unrelated PR dotnet/runtime#42174

[cston]

it seems that EventCounter.cs was missed in the PR dotnet/runtime#41046 and still has the issue in the release/5.0 branch but in master has been fixed

Thanks @charsleysa. It looks like the issue has been fixed in the release/5.0-rc2 branch in dotnet/runtime@4690f4e