[2.0] Describe AssetTargetFallback

[guardrex]

Similar to the request to surface AssetTargetFallback in https://github.com/NuGet/docs.microsoft.com-nuget/issues/410, which was prompted by a discussion regarding PackageTargetFallback here, there are a few topics over here that might (probably) need a smidgen of work ... a note? a section? a replacement? versioned content? ... something to surface it:

https://docs.microsoft.com/en-us/dotnet/core/tools/csproj#packagetargetfallback
https://docs.microsoft.com/en-us/dotnet/core/tools/project-json-to-csproj#imports

Cross ref: Documenting .NET Core MSBuild targets and properties/CSPROJ (2642)

[dasMulli]

Note that especially when upgrading to 2.0, users might see this error if they have been using PackageTargetFallback :

error NU1003: PackageTargetFallback and AssetTargetFallback cannot be used together. Remove PackageTargetFallback(deprecated) references from the project environment.

see https://github.com/dotnet/cli/issues/6952.
TL;DR unless DisableImplicitAssetTargetFallback is set to true, .NET Core and .NET Standard >= 2.0 projects will implicitly have their AssetTargetFallback set to include net461 (see source).

The only docs I found are in a table at the bottom of https://github.com/NuGet/Home/wiki/Enable-.NET-Core-2.0-projects-to-work-with-.NET-Framework-4.6.1-compatible-packages

[dasMulli]

Some more context from dotnet/core#711 (comment)

by @rrelyea:

We have since improved that error to:
PackageTargetFallback is deprecated. Replace PackageTargetFallback references with AssetTargetFallback in the project environment.

More details...PackageTargetFallback worked like imports did in Project.json.
The teams have realized that our implementation of that wasn't ideal.
In order to not break old projects/packages -- we created AssetTargetFallback which is implicitly set for netcoreapp/netstandard2.0 projects via targets.
Some project templates, in the past, used PackageTargetFallback.
When upgrading those projects to v2.0, you end up with both PTF and ATF in effect. Thus we error.
Ideally, you just move to ATF from your existing PTF.

[mairaw]

Thank you @guardrex and @dasMulli for the info. I'll try to squeeze this one in asap after I'm done with some other high pri work.