[boost] Policy for breaking changes

Subject: [boost] Policy for breaking changes
From: Louis Dionne (ldionne.2_at_[hidden])
Date: 2017-12-02 21:44:44

Hi all,

So far, I've been careful about not doing any breaking changes to Hana. 
Hana obeys semantic versioning, and I'm accumulating desirable breaking 
changes until I'm ready to publish a new major version.

I'm wondering what's the policy for major breaking changes with Boost 
libraries. For example, let's say I wanted to do an update of Hana with 
C++17 features; this would be a large breaking change, with all kinds 
of API changes, etc..

- Are such drastic changes prohibited altogether? Or perhaps they’re not
  prohibited but there’s historical evidence showing it’s always a bad
  idea?
- Do such changes require going through a mini review? 
- Is it customary to leave the old version of the library there in a 
  separate directory (or put the new one in a separate directory)? 
  I think Spirit does that, e.g. Spirit Classic is still available IIRC.

I think I know the answer to these questions from observing other author's
behavior, but I'd like to see what the community thinks about this. It has 
an impact on the Boost libraries as a whole, since a major breaking change 
in one library could prevent users from updating Boost as a whole, which 
has an impact on all of us.

I'm looking forward to hearing other people's thoughts on the matter.

Thanks,
Louis