WarningsAsErrors for nullable doesn't work as expected.

[furai]

Hey,

I'm creating new issue as requested:

@furai Please create a new one.

Originally posted by @JoeRobich in #2292 (comment)

I'm using latest omnisharp from the master branch with Visual Studio Code.
When setting nullable warnings as errors - msbuild fails but omnisharp still treats these warnings as warnings instead of errors.

Output from vscode:

Dotnet build:

$ dotnet build

MSBuild version 17.8.5+b5265ef37 for .NET
  Determining projects to restore...
  Restored test.csproj (in 140 ms).
Program.cs(8,22): error CS8600: Converting null literal or possible null value to non-nullable type. [test.csproj]

Build FAILED.

Program.cs(8,22): error CS8600: Converting null literal or possible null value to non-nullable type. [test.csproj]
    0 Warning(s)
    1 Error(s)

Time Elapsed 00:00:02.10

If I add CS8600 to WarnigngsAsErrors like this:

...
    <WarningsAsErrors>nullable,CS8600</WarningsAsErrors>
...

Then it shows up as error as expected:

Minimal reproducible example:

Program.cs

namespace test;

public class Program
{
    public static void Main(string[] args)
    {
        string? foo = null;
        string bar = null;
        Console.WriteLine(foo + bar);
    }
}

test.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
    <WarningsAsErrors>nullable</WarningsAsErrors>
  </PropertyGroup>

</Project>

Expected behaviour:

• Omnisharp should treat nullable warnings as errors when WarningsAsErrors is set to nullable in the project file without adding any extra warnings to WarningsAsErrors to match the behaviour of msbuild.

Thank you for your time, I really appreciate it.

Cheers,
Furai

PS At first I tried to tackle this issue on my own, I've cloned the repository and TBH I got stuck on build process for linux. It seems that base project is kind of outdated. Something should be done about using nuget on linux. Probably the best solution is to move from packages.config to PackageReference and use dotnet restore instead of nuget install. I'm not C# wizard and I know nearly nothing about this project so I'm not sure if this is the best solution. I'm just sharing my thoughts. I hope that's fine.

[JoeRobich]

What is odd is that we have a test for this scenario, but I am able to reproduce your bug locally. One difference is that the test is targeting net6.0. Changing your repro to net6.0 causes O# to return it as an error. Now to figure out why we are getting warnings when targeting net8.0.

[ryze312]

This issue also occurs with CS8509, Omnisharp treats it as a warning, while building fails as expected.