Extend SDK version entry in global.json

[jaredpar]

This is a feature request. Would really like to see an extension to the global.json SDK entry that allows me to specify that the SDK must match exactly what is in global.json.

The ability to specify a sdk version in global.json is only useful if it can guarantee that I have an SDK of at least the version specified. The behavior today allows for SDKs with a lower version to be chosen even when a higher SDK is available. This means that I cannot use global.json to ensure that I'm using an sdk which has the bug fixes necessary to build my repository.

The current behavior leads to issues like #3070 which will be closed as "By Design". There are bug fixes our repository absolutely depends on. Yet even though I have specified the correct minimum SDK, and have SDKS on the machine of a higher version, dotnet inexplicably silently picks a lower one. Instead of a polite error message about missing SDKs I have to close my console window because there are so many errors that even Ctrl-C is ineffective.

I'm guessing this is a compat problem at this point. Hence I recommend extending the entry something like the following:

{
  "sdk": {
    "version": "2.1.300-preview1-008174",
    "enforceVersion": "true"
  }
}

[nguerrera]

cc @KathleenDollard I think there's overlap with this and other things being discussed.

[nguerrera]

I reopened #3070. My quick reading of "enforceVersion" was that it should never pick anything but exact version (this has been on the table before), but #3070 shows a case where a roll-backward occured and that shouldn't happen regardless of flags.

[vitek-karas]

Proposed design which should include this feature request is discussed here: dotnet/designs#71

There's also ongoing implementation in dotnet/core-setup#6953

[nguerrera]

Done by dotnet/core-setup#6953