GHC 9.12.5-rc2 is now available

I have modified Stack (in its fix6934 branch) to depend on semaphore-compat-2.0.0 and build with GHC 9.12.5-rc2/Cabal-3.14.2.0. That works fine on Windows 11 and Linux (Ubuntu via WSL2) but on macOS 26.5.1 (on AArch64) I am getting a failure when I build stack with build --semaphore.

The error report is (reformatted, for ease of reading):

...
[217 of 217] Compiling Stack
CallStack (from HasCallStack):
  fdReadByte, called at libraries/semaphore-compat/src/System/Semaphore/Internal/Posix.hs:159:9 in semaphore-compat-2.0.0-068d:System.Semaphore.Internal.Posix
  waitOnSemaphore, called at compiler/GHC/Driver/MakeSem.hs:321:51 in ghc-9.12.4.20260614-868c:GHC.Driver.MakeSem: end of file (fd=13)

Error: [S-7282]
       Stack failed to execute the build plan.

       While executing the build plan, Stack encountered the error:

       [S-7011]
       While building package stack-3.12.0 (scroll up to its section to see the error) using:
       .../ghc-9.12.4.20260614/setup/setup
         --verbose=1
         --builddir=.../ghc-9.12.4.20260614
         build lib:stack exe:stack
         --semaphore v2-stack_1sPonmdUx
         --ghc-options " -fdiagnostics-color=always"

There is no macOS-specific code involved at Stack’s end, so my hypothesis is that there may be something up with semaphore-combat-2.0.0 (perhaps in combination with Cabal-3.14.2.0) on macOS.

I turned on Stack’s and Cabal’s verbose reporting, and it ends with:

2026-06-26 14:09:22.878725: [info] [217 of 217] Compiling Stack
2026-06-26 14:09:23.030317: [warn] fdWriteByte(fd=13): resource vanished (Broken pipe)
2026-06-26 14:09:23.118799: [debug] Loading project config file stack-ghc-9.12.5.yaml
2026-06-26 14:09:23.120822: [debug] Use of Casa server enabled: (CasaRepoPrefix "https://casa.stackage.org", 1280).
2026-06-26 14:09:23.122446: [debug] (SQL) SELECT COUNT(*) FROM "last_performed" WHERE ("action"=?) AND ("timestamp">=?); [PersistInt64 1,PersistUTCTime 2026-06-25 13:09:23.122421 UTC]
2026-06-26 14:09:23.122775: [error]
...

The [info] and [warn] lines are Cabal’s reporting. The [debug] lines reflect Stack’s usual verbose reporting.

EDIT: There was a bug and (thanks to @wz1000) there is a fix:

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