.NET SDK 6.0 can't be used anymore to build C# solutions in a project

[Calinou]

Tested versions

• Reproducible in: 4.3.dev f040a35

System information

Godot v4.3.dev (f040a35) - Fedora Linux 39 (KDE Plasma) - Wayland - GLES3 (Compatibility) - NVIDIA GeForce RTX 4090 (nvidia; 545.29.06) - 13th Gen Intel(R) Core(TM) i9-13900K (32 Threads)

Issue description

I've been trying to run https://github.com/godotengine/godot-benchmarks on the benchmarks server (which runs Fedora 39 just like my PC), but I'm getting an error:

  Determining projects to restore...
  Restored /home/godot/godot-benchmarks/godot/modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Sample/Godot.SourceGenerators.Sample.csproj (in 82 ms).
  Restored /home/godot/godot-benchmarks/godot/modules/mono/editor/Godot.NET.Sdk/Godot.NET.Sdk/Godot.NET.Sdk.csproj (in 119 ms).
  Restored /home/godot/godot-benchmarks/godot/modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Tests/Godot.SourceGenerators.Tests.csproj (in 5.04 sec).
  3 of 6 projects are up-to-date for restore.
  Godot.NET.Sdk ->
  Godot.SourceGenerators -> /home/godot/godot-benchmarks/godot/modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators/bin/Release/netstandard2.0/Godot.SourceGenerators.dll
  Successfully created package '/home/godot/godot-benchmarks/godot/modules/mono/editor/Godot.NET.Sdk/Godot.NET.Sdk/bin/Release/Godot.NET.Sdk.4.3.0-dev.nupkg'.
  Godot.SourceGenerators.Internal -> /home/godot/godot-benchmarks/godot/modules/mono/glue/GodotSharp/Godot.SourceGenerators.Internal/bin/Debug/netstandard2.0/Godot.SourceGenerators.Internal.dll
  GodotSharp -> /home/godot/godot-benchmarks/godot/modules/mono/glue/GodotSharp/GodotSharp/bin/Release/GodotSharp.dll
CSC : error CS1617: Invalid option '11' for /langversion. Use '/langversion:?' to list supported values. [/home/godot/godot-benchmarks/godot/modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Sample/Godot.SourceGenerators.Sample.csproj]
CSC : error CS1617: Invalid option '11' for /langversion. Use '/langversion:?' to list supported values. [/home/godot/godot-benchmarks/godot/modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Tests/Godot.SourceGenerators.Tests.csproj]

I have dotnet-sdk-6.0 installed from the Fedora repositories on the server. On my system, I also have dotnet-sdk-7.0 and dotnet-sdk-8.0 installed and it works.

As far as I know, we still try to support .NET 6.0 as a baseline, so it should be possible to build solutions for desktop platforms without having to install a newer version.

According to @paulloz, this may be related to #86528. cc @avilches

Steps to reproduce

git clone --depth=1 https://github.com/godotengine/godot-benchmarks.git
cd godot-benchmarks
path/to/godot.binary --editor --build-solutions --quit-after 100

# If you want to run C# benchmarks to see whether the built solution works:
path/to/godot.binary -- --run-benchmarks --include-benchmarks="csharp/*"

Minimal reproduction project (MRP)

https://github.com/godotengine/godot-benchmarks

[paulloz]

We could go back to the previous langversion for the problematic projects (Godot.SourceGenerators.Tests and Godot.SourceGenerators.Sample), or simply say that we build with SDK 8 now (we've actually been building with that for some time now IINW). Keeping in mind that SDK 6 will be out of LTS in a few months anyway.