New NU1701 errors appearing after May 10th release of SDK (6.0.300)

Describe the bug

Our github actions started failing today with the following error: error NU1701: Package ‘RelaxNG 3.2.3’ was restored using ‘.NETFramework,Version=v4.6.1, .NETFramework,Version=v4.6.2, .NETFramework,Version=v4.7, .NETFramework,Version=v4.7.1, .NETFramework,Version=v4.7.2, .NETFramework,Version=v4.8’ instead of the project target framework ‘net6.0’. This package may not be fully compatible with your project.

There were no changes to any code or project files to explain the sudden failure. I think this behavior used to be a warning?

If this was an intentional change I would expect to find it in the release notes but I didn’t see anything mentioning NU1701 or any update to the restore behavior,

To Reproduce

The workflow log for the first failure can be viewed here: https://github.com/sillsdev/TheCombine/runs/6376161726?check_suite_focus=true The PR that triggered it is here: https://github.com/sillsdev/TheCombine/pull/1659/files

Further technical details

Our log had one sdk sha on the build that worked: [builder 1/6] FROM mcr.microsoft.com/dotnet/sdk:6.0@sha256:fde93347d1cc74a03f1804f113ce85add00c6f0af15881181165ef04bc76bd00 and an updated one when it began to fail: [builder 1/6] FROM mcr.microsoft.com/dotnet/sdk:6.0@sha256:27372653b52f0e0215574be2d1e74db2e9e6b3f09ad74c5618f0f5788cd848dd

[zivkan]

.NET SDK 6.0.300 brings in NuGet 6.2, which fixed a long-standing bug with asset target fallback not bringing in dependencies, which causes runtime failures for affected customers. This only happens when package authors mismatch the package dependencies vs package assets (usually lib/ folder). NuGet has been warning package authors about this for 2-3 years, but page authors who use old versions of NuGet or ignore the warning might still be generating potentially problematic packages, plus older packages, the authors might not have been aware. There’s a good chance that’s the cause of the error you’re seeing. We have a sample which explains it better, with some links to issues if that help: https://github.com/NuGet/Samples/tree/main/AssetTargetFallbackTransitiveDependencies

Note, as @kenans points out, NuGet treats it as a warning. If your project uses <TreatWarningsAsErrors>true, or similar, then your project is asking to be broken by warnings. As documented in a few places, you can NoWarn those NuGet codes and they’ll neither warning nor error your build.

cc @nkolev92

[kenans]

Checking out the commit history, seems Nuget.Client dependency was upgraded recently, which might be related to the issue.

NU1701 used to be a warning and is still being documented so. Now it seems to be reported as an error.