GHC 8.6.1 can't be bootstrapped with GHC 8.2.1
ghc: panic! (the 'impossible' happened) (GHC version 8.2.1 for x86_64-unknown-linux): runtimeRepPrimRep typePrimRep (r_aqbE :: TYPE rep_aqbD) rep_aqbD 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
This is a manifestation of #14393 (closed), which is fixed in 8.2.2 and later.
We should probably add a
configure check warning about this brokenness when the user tries to bootstrap with GHC 8.2.1.