Feedback on potential change in boot library

[Ben Gamari]

tl;dr. We propose that GHC more aggressively bump its boot library
       dependencies.


Hello all,

Historically, GHC minor releases have been quite conservative in bumping
boot library versions, generally defaulting to retaining the versions
used in the GHC-x.y.1 release unless

 * a request is made by a boot library maintainer, or
 * we discover an issue which warrants a bump

The motivation for this policy is both to

 * minimize the potential for regressions in correctness and
   performance, and

 * reduce the potential for end-user breakage due to interface changes
   as even minor releases can result in considerable impact (e.g.
   #25411, #22633)

While this policy has served us well, its conservative nature demands
the vigilence of both GHC and upstream maintainers to ensure that bumps
that are truly necessary do not fall through the cracks.

As the flaws of this process have been more apparent recently (e.g.
#24597), we have done a bit of reflection on how we might improve the
status quo. Concretely, I would like feedback on the adopting the
following policy going forward:

> Unless guidance is provided otherwise by a library maintainer, a GHC
> x.y.z release will attempt to ship the newest minor version of each
> boot libray in the same major series shipped with GHC x.y.1.

I believe this policy would leave less room for human error and open
opportunities for automated checking. On the other hand, the more
aggressive bumping of submodules may mean that users are hit with more
(usually minor) compatibility issues when moving between minor GHC
releases.

How do others feel about this? We are particularly interested to hear
from boot library maintainers but feedback from packagers and users is
also very much welcome.

Cheers,

- Ben

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