Paket dotnet tool doesn't run on 3.0 previews

[baronfel]

Description

running paket restore on the 3.0 preview7 runtime fails with the following error:

It was not possible to find any compatible framework version
The specified framework 'Microsoft.NETCore.App', version '2.1.0' was not found.
  - The following frameworks were found:
      3.0.0-preview7-27912-14 at [/usr/share/dotnet/shared/Microsoft.NETCore.App]

You can resolve the problem by installing the specified framework and/or SDK.

The .NET Core frameworks can be found at:
  - https://aka.ms/dotnet-download

I think we need a build of the paket tool that multi-targets to 3.0, because the default roll-forward strategy only supports going up a minor version (ie from 2.1 to 2.2).

[baronfel]

Documentation about roll-forward behaviors is here: https://github.com/dotnet/core-setup/blob/master/Documentation/design-docs/roll-forward-on-no-candidate-fx.md#roll-forward-in-absence-of-candidate-fx

[baronfel]

setting DOTNET_ROLL_FORWARD_ON_NO_CANDIDATE_FX=2 in my build container sidesteps the issue, but we should either have some documentation around this or publish a multi-targeted build soonish.

[baronfel]

Instead of the user having to set an environment variable, we could package a runtimesettings configuration file that opts-in to roll-forward on major versions: https://natemcmaster.com/blog/2019/01/09/netcore-primitives-3/#runtimeconfigtemplatejson

[matthid]

Same thing in fake: fsprojects/FAKE#2325

[baronfel]

I wondered why this felt like deja-vu. Would you like me to replicate my PR over there?

[matthid]

Would you like me to replicate my PR over there?

Sure, would love to merge it :).
Didn't realize there was a fix for this which doesn't need upgrading the sdk

[baronfel]

Closing, was fixed in #3635