NuGet restore improperly NoOps when 2 projects are sharing obj directory

[BrennanConroy]

@emgarten

1. dotnet new console
2. dotnet run (just to make sure it runs)
3. change name of csproj and downgrade the netcoreapp version to 1.0 or 1.1
4. dotnet restore
5. dotnet run (make sure it runs too)
6. rename csproj to original name
7. undo the previous downgrade
8. dotnet restore (does nothing)
9. dotnet run
10. See error:

C:\Program Files\dotnet\sdk\15.5.0-preview-007044\Sdks\Microsoft.NET.Sdk\build\Microsoft.PackageDependencyResolution.targets(167,5):
error : Assets file 'C:\app\obj\project.assets.json' doesn't have a target for '.NETCoreApp,Version=v2.0'.
Ensure that restore has run and that you have included 'netcoreapp2.0' in the TargetFrameworks for your project. [C:\app\app.csproj]

The build failed. Please fix the build errors and run again.

[mishra14]

@BrennanConroy Thanks for reporting this issue. I think this is an issue with some no-op check that we have to prevent restore if no files have changed. Requesting @nkolev92 to investigate.

in the mean time please use dotnet restore --force to ensure that a new restore is done if you run into such an issue.

[nkolev92]

@BrennanConroy

This scenario is equivalent to having ProjectA (NetCore20) and ProjectB (NetCore10) in the same directory.

They both create the same assets file (project.assets.json), while they each create a separate cache file that's used for no-op.

This is not exactly a supported scenario, not sure if we want to do something about it.

The workaround as @mishra14 mentioned is very simple, you can do a restore force the time.

But the reality is that you have leftover build files in the obj folder that should not be there.

cc @emgarten

[rrelyea]

Although the title says edge case, no-op is important enough to make this issue a p1 backlog issue.

[emgarten]

Related: #4463