[Suggestion] Allow optional opt-in for requiring null handling case

[AnthonyIacono]

Hello!

First of all, I absolutely love this library. I really appreciate your work on it :)

I ran into a case today where I had refactored some code that previously had not allowed null to be a valid value, but now I have switch cases throwing ExhaustiveMatch.Failed. I realized it was because I had not been handling the null case in the switch.

In the documentation, I see you have the following:

Since C# reference types are always nullable, but may be intended to never be null, exhaustiveness checking does not require a case for null. If a null value is expected it can be handled by a case null:. The analyzer will ignore this case for its analysis.

For nullable enum types, the analyzer requires that there be a case null: to handle the null value.

I understand that this might not be desired as a default behavior, but I would really personally like to be forced to handle null on reference types.

Is there any way that we can see this as an opt-in thing in the future? Perhap something we can decorate our assembly with as an attribute, or similar.

Thank you! Anthony

[WalkerCodeRanger]

I think this is a good idea and should probably be implemented in concert with #27 "Support C# 8 Nullable Reference Types" to make sure that it plays well with that.