On Wed, 2 Dec 2020 at 17:58, Jeff Garland via Boost
It doesn't need to define ABI for it to be real -- the library implementers are very aggressive in limiting ABI breaks. The former LEWG chair (Titus) was quite vocal in wanting to remove this constraint leading to vigorous debates and a vote which basically affirmed that the committee was very split on the matter, but preferred stability.
ABI breaks shift the burden and pain downstream. There's a curious correlation between the amount of downstream levels that some developers have and their reluctance to break ABI when they can avoid it. There's a non-zero amount of users to whom the lack of ABI stability in boost is a reason not to touch boost with a ten-foot pole. "Breaking ABI like mad" is nice and fine if all your users are developers that like rebuilding their world often. When the vast majority of your users are not developers, the situation may be quite different. And that difference multiplies when you're trying to provide libraries for users who provide them combined with something else to other downstream users further apart from you the upstream library vendor.