New dependency resolver erroneously reports NU1605 (downgrade) when using transitive pinning a direct dependency and a downgrade exists in a package graph

[jeffkl]

NuGet Product Used

dotnet.exe, MSBuild.exe, NuGet.exe

Product Version

6.12

Worked before?

Yes

Impact

It's more difficult to complete my work

Repro Steps & Context

If you have a package with a downgrade in its graph, you have a direct package reference, and you have transitive pinning enabled, the new dependency resolution algorithm does not properly treat it as "pinned" and instead logs a downgrade warning. The legacy dependency resolution algorithm treats pinned dependencies as special, even if they are direct dependencies and will use higher versions, working around the downgrade. This results in erroneous NU1605 warnings being logged when they weren't before.

Verbose Logs

NU1605: Warning As Error: Detected package downgrade: PackageC from 2.0.0 to 1.0.0. Reference the package directly from the project to select a different version.
 Project -> PackageA 1.0.0 -> PackageB 1.0.0 -> PackageC (>= 1.0.0)
 Project -> PackageA 1.0.0 -> PackageC (>= 2.0.0)

Workaround

Users can workaround the issue by setting an MSBuild property in their Directory.Build.props or an environment variable:

<Project>
  <PropertyGroup>
    <RestoreUseLegacyDependencyResolver>true</RestoreUseLegacyDependencyResolver>
  </PropertyGroup>
</Project>