Trying to build the repro case in Trac #14208, I get this cabal install --with-ghc=/home/simonpj/5builds/ghc-8.4-branch/inplace/bin/ghc-stage2 Warning: cannot determine version of /home/simonpj/.cabal/bin/c2hs : "" Resolving dependencies... cabal: Could not resolve dependencies: trying: exceptions-0.8.3 (dependency of streamly) next goal: template-haskell (dependency of exceptions) rejecting: template-haskell-2.13.0.0/installed-2.1... (conflict: exceptions => template-haskell>=2.2 && <2.13) rejecting: template-haskell-2.12.0.0, template-haskell-2.11.1.0, template-haskell-2.11.0.0, template-haskell-2.10.0.0, template-haskell-2.9.0.0, template-haskell-2.8.0.0, template-haskell-2.7.0.0, template-haskell-2.6.0.0, template-haskell-2.5.0.0, template-haskell-2.4.0.1, template-haskell-2.4.0.0, template-haskell-2.3.0.1, template-haskell-2.3.0.0, template-haskell-2.2.0.0 (constraint from non-upgradeable package requires installed instance) After searching the rest of the dependency tree exhaustively, these were the goals I've had most trouble fulfilling: template-haskell, exceptions, streamly, transformers, base This is with my in-place build of GHC-8.4 (in-place but not modified in any way). I've done a 'cabal upate'. What am I doing wrong? Thanks Simon