After using Package Refs, retargeting project produces a NuGet Error

[rido-min]

using d15.7

Repro Steps

1. Create Console App Targeting .NET 4.6.1
2. Add NewtonSoft using PR
3. Remove NewtonSoft
4. Retarget to .NET 4.6.2
5. Build

Expected. Build succeeds.

Observed. Build fails with

Microsoft.NuGet.targets(186,5): error : Your project is not referencing the ".NETFramework,Version=v4.6.2" framework. 
Add a reference to ".NETFramework,Version=v4.6.2" 
in the "frameworks" section of your project.json, and then re-run NuGet restore.

weird error message since the project is not using any packages, and project.json was never used.

[anangaur]

Looks like we are not cleaning up properly upon uninstall of the last and only packageref. The error does not seem to get fixed with the proposed workaround. Is it due to the choice of package format being persisted somewhere in the obj dir? Where is the choice about the package format persisted?

[nkolev92]

Step 3 creates an assets file, which is the tell for this is PR, but then when the retargeting happens, there are no PackageReference and not RestoreProjectStyle so it just breaks.

I'd say we should add a RestoreProjectStyle property to legacy projects when installing the first PackageReference.

Low priority given the lack of additional input.