GHC 9.12.5-rc2 is now available

The GHC developers are very pleased to announce the availability of the second release candidate for GHC 9.12.5, ghc-9.12.5-rc2. The first release candidate did not receive an announcement since promptly after publishing it, an urgent issue with distribution of the process library on macos was discovered. This is now fixed. Binary distributions, source distributions, and documentation are available at downloads.haskell.org and via GHCup.

GHC 9.12.5 is a bug-fix release fixing many issues of a variety of severities and scopes, including:

• Implement version 2 of the ghc semaphore protocol for GHC’s -jsem jobserver on Linux and other POSIX platforms (#25087, #27253)
• several improvements to the pattern match checker (#25926, #27124)
• several improvements to the demand analyser (#27261, #27106, #26416)
• several fixes regarding the interaction of ticks and coercions (#27121, #26929, #26642, #26693)
• several fixes for blackholes (#27261)

… and many more

Mind that the very important overhaul of the semaphore-compat library to v2 is included in this release and that the semaphore feature will only start working with cabal-install 3.18 again, which is sadly a breaking but necessary change.

A full accounting of these fixes can be found in the release notes. As always, GHC’s release status, including planned future releases, can be found on the GHC Wiki status.

This release candidate will have a two-week testing period. If all goes well the final release will be available the week of 8 July 2026.

GHC development is sponsored by:

• Juspay
• QBayLogic
• Channable
• Haskell Foundation
• Serokell
• Well-Typed
• Tweag
• Dotcom-Monitor
• LoadView
• Web Hosting Buddy
• Find My Electric
• Standard Chartered
• UpCloud
• Mercury

We would like to thank these sponsors and other anonymous contributors whose on-going financial and in-kind support has facilitated GHC maintenance and release management over the years. Finally, this release would not have been possible without the hundreds of open-source contributors whose work comprise this release.

As always, do give this release a try and open a ticket if you see anything amiss.