draft proposal for Sum Types for D

[cc]

On Thursday, 1 December 2022 at 05:49:56 UTC, cc wrote:
> I imagine some stubbed toes writing `if (ret.Error)` without 
> the `?` for a while, and potential unnoticed bugs when it works 
> by accident.  I like the recentish trend of D compiler errors 
> suggesting "Did you mean to use..." but would that be suitable 
> for a runtime error message?

Nevermind, I see now it's just a straight up termination with no 
message.  Ouch.