Roslyn: WarningsAsErrors for CS8602 is not returning an error

[J0rgeSerran0]

Version Used:
Visual Studio 2019 16.2.3
.NET Core: 3.0.0-preview8-28405-07

Steps to Reproduce:

For CS8602 I am not getting the expected errors when I add this code in the WarningsAsErrors section.

1. Modify the csproj as:

<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<OutputType>Exe</OutputType>
<TargetFramework>netcoreapp3.0</TargetFramework>
<Nullable>enable</Nullable>
<WarningsAsErrors>CS8602</WarningsAsErrors>
</PropertyGroup>
</Project>

1. My sample application code is:

private static void PosibleDereferenceNullableReferenceTypes()
{
string data = null;
Console.WriteLine(data.Length);
}

Expected Behavior:
An error when we build the project.
Error CS8602 Dereference of a possibly null reference.

Note: When I put the codes CS8600, CS8603 or CS8604 into the WarningsAsErrors section, I receive the expected error messages in compilation time.

Actual Behavior:
Warning CS8602 Dereference of a possibly null reference.

Updated information about this issue and its behavior:

If I write into the csproj:
<WarningsAsErrors>CS8600;CS8602</WarningsAsErrors>
I get the error messages for CS8600 (as expected) and for CS8602

[J0rgeSerran0]

Additional notes

If I change my csproj as:

<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<OutputType>Exe</OutputType>
<TargetFramework>netcoreapp3.0</TargetFramework>
<TreatWarningsAsErrors>true</TreatWarningsAsErrors>
</PropertyGroup>
</Project>

And my application code is:

private static void PosibleDereferenceNullableReferenceTypes()
{
#nullable enable
string data = null;
Console.WriteLine(data.Length);
#nullable restore
}

Here I get the error for CS8602 as expected

[RikkiGibson]

For some reason, the command line build fails, but we don't get an error in the VS error list. Looking into whether there is maybe a bug in the VS workspace.

[RikkiGibson]

This may be an instance of dotnet/project-system#3680, but relating to the WarningsAsErrors property instead of the WarningsNotAsErrors property.

[RikkiGibson]

Basically, it seems like for some values of WarningsAsErrors, the project system sends us a command line with ... /warnAsError- ... /warnAsError+:CS0105 .... For other values it sends us a command line like ... /warnAsError+:CS8602 ... /warnAsError- .... The latter case is where the bug happens-- warnAsError- wins and we don't get the right behavior in the IDE.

[RikkiGibson]

As far as I am aware, this is a project system bug, and can't be fixed within Roslyn. /cc @davkean