RFC: std.sumtype (adding sumtype to Phobos)

[Paul Backus]

Following the recent 1.0.0 release [1], I have decided to submit 
sumtype [2] for inclusion in Phobos, D's standard library. The PR 
can be found here:

https://github.com/dlang/phobos/pull/7702

This thread is for you, the D community, to give your feedback, 
comments, reviews, and constructive criticism regarding both 
sumtype itself, and my proposal to include it in D's standard 
library. Please keep your replies on-topic. If you are unsure 
what to include in your comments, the Boost reviewer guidelines 
[3] have some good advice.

THIS IS NOT A VOTING THREAD. IF YOU WISH TO SHOW 
SUPPORT/OPPOSIION, GIVE THE PR A "THUMBS UP" OR "THUMBS DOWN" ON 
GITHUB.

[1] 
https://forum.dlang.org/post/zagxftjoxxzirneavymj@forum.dlang.org
[2] https://code.dlang.org/packages/sumtype
[3] https://www.boost.org/community/reviews.html#Comments