Just got this during a build: "inplace/bin/ghc-stage1.exe" -hisuf hi -osuf o -hcsuf hc -static -H32m -O -Werror -Wall -H64m -O0 -this-package-key base_ESD4aQEEWwsHtYJVc1BwtJ -hide-all-packages -i -ilibraries/base/. -ilibraries/base/dist-install/build -ilibraries/base/dist-install/build/autogen -Ilibraries/base/dist-install/build -Ilibraries/base/dist-install/build/autogen -Ilibraries/base/include -optP-DOPTIMISE_INTEGER_GCD_LCM -optP-include -optPlibraries/base/dist-install/build/autogen/cabal_macros.h -package-key ghcpr_BE58KUgBe9ELCsPXiJ1Q2r -package-key integ_BcPVjqcazPNGsNFG4agFty -package-key rts -this-package-key base -XHaskell2010 -O2 -O -dcore-lint -fno-warn-deprecated-flags -fno-warn-tabs -no-user-package-db -rtsopts -odir libraries/base/dist-install/build -hidir libraries/base/dist-install/build -stubdir libraries/base/dist-install/build -c libraries/base/./GHC/TypeLits.hs -o libraries/base/dist-install/build/GHC/TypeLits.o ghc-stage1.exe: panic! (the 'impossible' happened) (GHC version 7.9.20141031 for x86_64-unknown-mingw32): Ix{Int}.index: Index (2053) out of range ((0,10)) Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug libraries/base/ghc.mk:4: recipe for target 'libraries/base/dist-install/build/GHC/TypeLits.o' failed I may have been building in a non-clean repository after a git pull though, could that have caused this? -- Gintautas Miliauskas