RazorClassLib template offline restore fails

[mikeharder]

Repro Steps

1. dotnet new razorclasslib --no-restore
2. Add NuGet.config to clear package sources and simulate offline restore:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <packageSources>
    <clear />
  </packageSources>
</configuration>

3. dotnet restore

Error

error NU1100: Unable to resolve 'System.Numerics.Vectors (>= 4.4.0)' for '.NETStandard,Version=v2.0'.
error NU1100: Unable to resolve 'System.Security.Cryptography.Cng (>= 4.4.0)' for '.NETStandard,Version=v2.0'.

CC: @muratg, @mkArtakMSFT, @pranavkm

[natemcmaster]

If this is a scenario that should be supported offline, it's new in 2.1. See https://github.com/aspnet/Universe/blob/dev/docs/FallbackPackageCache.md for list of scenarios we currently support.

[JunTaoLuo]

We do generate LZMAs with System.Numerics.Vectors and System.Security.Cryptography.Cng. However, since we generate the LZMA with the netcoreapp2.1 tfm, the latest versions (4.5.0-rc1-*) are what we include in the fallback archive. For class libraries that target netstandard2.0, such as our razor class libraries, the versions that are referenced via NETStandard.Library are the stable versions, in this case 4.4.0. These stable released versions are not included in the latest SDKs and we end up seeing restore failures. The question is whether this is something we want to address and how do we determine what goes into out LZMA. I'll also start an email thread on this.