Hi Richard, along with this or some other change it seems you broke the test suite when run with DEBUG: T2478.hs:1:41: warning: -XDatatypeContexts is deprecated: It was widely considered a misfeature, and has been removed from the Haskell language. ghc-stage2: panic! (the 'impossible' happened) (GHC version 8.1.20160321 for x86_64-unknown-linux): ASSERT failed! CallStack (from HasCallStack): assertPprPanic, called at compiler/typecheck/TcType.hs:931:47 in ghc:TcType t_ap0 Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug *** unexpected failure for T2478(normal) at least according to travis: https://s3.amazonaws.com/archive.travis-ci.org/jobs/117548750/log.txt Greetings, Joachim -- Joachim “nomeata” Breitner mail@joachim-breitner.de • https://www.joachim-breitner.de/ XMPP: nomeata@joachim-breitner.de • OpenPGP-Key: 0xF0FBF51F Debian Developer: nomeata@debian.org