[DCR]: Adjust compatibility rules for Apple TFMs in .NET

[rolfbjarne]

NuGet Product(s) Affected

NuGet.exe, dotnet.exe

Current Behavior

Current behavior is for the following TFMs to be compatible:

TFM	Compatible With
net6.0-ios	xamarin.ios
net6.0-macos	xamarin.mac
net6.0-maccatalyst	xamarin.ios
net6.0-tvos	xamarin.tvos

Desired Behavior

We've decided that the xamarin.* TFMs should not be compatible with the corresponding net6.0-* TFM (for the Apple platforms).

Note that there's no change for net6.0-android / monoandroid.

Additional Context

This is a design change that was approved here: dotnet/designs#252

Note that this is technically not a breaking change (yet), since the net6.0-* TFMs aren't considered stable (stable release is Q2 2022).

[JonDouglas]

Thank you kindly for filing an issue @rolfbjarne. We'll direct our attention towards it shortly.

[nkolev92]

@rolfbjarne

What's the timeline for this?
What SDK version do you need this change in?

[rolfbjarne]

@nkolev92 as soon as possible really. The absolute latest is for the GA MAUI release in Q2 2022, but the sooner the better (6.0.200?), since this will likely hit a lot of people and the earlier it happens the more prepared they'll be by GA.

[nkolev92]

PR out: NuGet/NuGet.Client#4361

We'd need to decide whether we want this in 6.0.200 preview 3 (feature stabilization).

Normally I'd say let's wait, but this shouldn't be something that's exercised at all.