It looks like the recent differential builds are all failing like this: compiler/main/DynamicLoading.hs:79:19: warning: [-Wunused-matches] Defined but not used: ‘hsc_env’ | 79 | initializePlugins hsc_env df | ^^^^^^^ ghc: panic! (the 'impossible' happened) (GHC version 8.2.1 for x86_64-unknown-linux): runtimeRepPrimRep typePrimRep (r_apc3 :: TYPE rep_apc2) rep_apc2 Call stack: CallStack (from HasCallStack): prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1133:58 in ghc:Outputable callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in ghc:Outputable pprPanic, called at compiler/simplStg/RepType.hs:360:5 in ghc:RepType Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug make[1]: *** [compiler/stage1/build/PrelRules.o] Error 1 make[1]: *** Waiting for unfinished jobs.... make: *** [all] Error 2 Oddly, I don't see a commit to master failing like this. What's going on? David