When using the reify template-haskell operation on a data family, the returned data instances lose the relationship between the type variables in the parameters and those in the value constructors.

In this paste I show that reify forgets the relationship, but that declaration splices preserve it. Note the two occurrences of a_6989586621679027007 .

I would specifically expect the occurences of a_6989586621679015819 and a_6989586621679015790 to be the same.

The code that implements this reification is in compiler/typecheck/TcSplice.hs , the template-haskell package merely exposes the functionality, so this is why I've specified "compiler (type checker)"

I've confirmed that this bug exists in GHC 8.0.2 and 8.2.1-rc1. I suspect it exists in older versions, too.

$ /Users/emertens/Tools/ghc-8.2.1-rc1/bin/ghci
GHCi, version 8.2.0.20170404: http://www.haskell.org/ghc/  :? for help
Prelude> :set -XTypeFamilies
Prelude> :set -XTemplateHaskell
Prelude> import Language.Haskell.TH
Prelude Language.Haskell.TH> import Text.Show.Pretty (ppShow)
Prelude Language.Haskell.TH Text.Show.Pretty> data family DF a; data instance DF [a] = DFList a
Prelude Language.Haskell.TH Text.Show.Pretty> putStrLn $(stringE . ppShow =<< reify ''DF)
FamilyI
  (DataFamilyD
     Ghci1.DF [ KindedTV a_6989586621679015789 StarT ] (Just StarT))
  [ DataInstD
      []
      Ghci1.DF
      [ AppT ListT (VarT a_6989586621679015819) ]
      Nothing
      [ NormalC
          Ghci1.DFList
          [ ( Bang NoSourceUnpackedness NoSourceStrictness
            , VarT a_6989586621679015790
            )
          ]
      ]
      []
  ]
Prelude Language.Haskell.TH Text.Show.Pretty> putStrLn $(stringE . ppShow =<< [d| data family DF a; data instance DF [a] = DFList a |])
[ DataFamilyD
    DF_6989586621679027004 [ PlainTV a_6989586621679027006 ] Nothing
, DataInstD
    []
    DF_6989586621679027004
    [ AppT ListT (VarT a_6989586621679027007) ]
    Nothing
    [ NormalC
        DFList_6989586621679027005
        [ ( Bang NoSourceUnpackedness NoSourceStrictness
          , VarT a_6989586621679027007
          )
        ]
    ]
    []
]