When trying to build GHC 9.14.2-rc1 with GHC 9.12.4 as the boot compiler the build failed due to a missing "Binary" instance for "HalfWord". I see that Ben Gamari already had to implement a work-around for this: commit b965fa766c6bdf1bf3c8be63a6187e1100051599 Author: Ben Gamari <bgamari.foss@gmail.com> Date: Sat Oct 4 11:45:32 2025 -0400 ghci: Fix CPP guarding Binary HalfWord instance diff --git a/libraries/ghci/GHCi/Message.hs b/libraries/ghci/GHCi/Message.hs index 072f46cd32..c731d33e31 100644 --- a/libraries/ghci/GHCi/Message.hs +++ b/libraries/ghci/GHCi/Message.hs @@ -522,7 +522,7 @@ instance Binary (FunPtr a) where put = put . castFunPtrToPtr get = castPtrToFunPtr <$> get -#if MIN_VERSION_ghc_internal(9,1500,0) +#if MIN_VERSION_ghc_internal(9,1400,0) instance Binary Heap.HalfWord where put x = put (fromIntegral x :: Word32) get = fromIntegral <$> (get :: Get Word32) I had to further reduce this from 1400 to 1204. -- Viktor. 🇺🇦 Слава Україні!