Support versionless PackageReference for AspNetCore when targeting .NET Core 3.0

[dsplaisted]

In .NET Core 3.0, the syntax for depending on the ASP.NET Core shared framework is:

<FrameworkReference Include="Microsoft.AspNetCore" />

In .NET Core 2.1 and 2.2, a versionless PackageReference is used instead:

<PackageReference Include="Microsoft.AspNetCore.App" />

In order to support retargeting 2.x apps to 3.0, we should also support the PackageReference syntax when targeting 3.0. We might also want a warning or info message suggesting a change to the newer syntax.

[dsplaisted]

FYI @natemcmaster @DamianEdwards

[natemcmaster]

I think a PackageReference to aspnetcore in 3.0 should be an error. It's not an actual package reference anymore, right?

If that's all we did, the upgrade experience would be a bummer. How hard would it be to have VS switch the item type for us when users upgrade to .NET Core 3.0?

[DamianEdwards]

Can't we support it but make it a warning?

[dasMulli]

to clarify: the scenario is using 3.0 tools to build a 2.1/2.2 app?

[dsplaisted]

@dasMulli That's not the scenario for this bug. The scenario for this bug is that you have a 2.1/2.2 app with a versionless AspNetCore PackageReference, and then you change the TargetFramework to netcoreapp3.0. Per @DamianEdwards' suggestion, current POR is to build correctly but generate a warning.