When NetPrevious is set to empty, some projects have unexpected behavior

[mmitche]

• This issue is blocking
• This issue is causing unreasonable pain

When NetPrevious and NetMinimum are the same, the current behavior is to null out NetPrevious. This avoids duplicate-TFM conflicts for projects that target both NetPrevious and NetMinimum. The bad news is that this will cause errors for single-targeted projects that just target NetPrevious. It's worse new for projects that multi-target target NetPrevious and NetCurrent/some other non NetMinimum TFM.

A solution here is that:

• NetPrevious should always be set
• The only set of properties you can always combine are NetPrevious and NetCurrent
• A property should be introduced that is the union of NetPrevious and NetMinimum.

[mmitche]

/cc @ViktorHofer @jaredpar @akoeplinger

[jaredpar]

A property should be introduced that is the union of NetPrevious and NetMinimum.

I think this is the key. Given that NetPrevious and NetCurrent can change, and will change in various builds, there is no safe way to combine them with other properties. They can be safely combined with each other but no one else.

For the combinations of NetPrevious/NetCurrent and other Net propreties that we want to support we should have a named property defined that safely combines them.