Cannot enable Major version roll-forward for 2.x apps

Steps to reproduce

I attempted to enable major-version roll-forward for the dotnetsay .NET Core tool.

I added <RollForward>LatestMajor</RollForward> within the PropertyGroup. I get this error:

C:\git\core\samples\dotnetsay>dotnet build
Microsoft (R) Build Engine version 16.3.0-preview-19377-01+dd8019d9e for .NET Core
Copyright (C) Microsoft Corporation. All rights reserved.

  Restore completed in 131.58 ms for C:\git\core\samples\dotnetsay\dotnetsay.csproj.
  You are using a preview version of .NET Core. See: https://aka.ms/dotnet-core-preview
C:\Program Files\dotnet\sdk\3.0.100-preview8-013656\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.Sdk.targets(221,5): error NETSDK1103: RollForward setting is only supported on .NET Core 3.0 or higher. [C:\git\core\samples\dotnetsay\dotnetsay.csproj]

Build FAILED.

Expected behavior

I understand why RollForward isn’t enabled in general for 2.x since the feature is new in 3.0. We should special-case the LatestMajor value to enable it to be used. It is an important scenario for enabling 3.0 users (on 3.x-only machines) to use 2.x tools.

The only other option to enable 2.x tools to run on 3.x is to multi-target. I’m not a fan of that being the only option (in the case you are only doing it for this scenario and not taking advantage of 3.x APIs).

Timeframe

Please consider for 3.0.

/cc @vitek-karas @livarcocc @KathleenDollard

[vitek-karas]

I think we should enable it for both Major and LatestMajor - the global tools may want LatestMajor but I think even Major would make sense as well.

[vitek-karas]

Fixed by dotnet/sdk#3542