I noticed recently that Template Haskell reifies Constraint as Type:

$ ~/Software/ghc2/inplace/bin/ghc-stage2 --interactive
GHCi, version 8.5.20180221: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
λ> :set -XTypeFamilies -XTemplateHaskell
λ> :m + Data.Kind Language.Haskell.TH
λ> type family Foo :: Constraint
λ> putStrLn $(reify ''Foo >>= stringE . pprint)
type family Ghci1.Foo :: *

The root of the issue can be traced back to the isLiftedTypeKind function, which TcSplice uses to distinguish Type from Constraint. At least, that's what its documentation claims:

 -- | This version considers Constraint to be distinct from *. Returns True
 -- if the argument is equivalent to Type and False otherwise.
 isLiftedTypeKind :: Kind -> Bool
 isLiftedTypeKind = is_TYPE is_lifted
   where
     is_lifted (TyConApp lifted_rep []) = lifted_rep `hasKey` liftedRepDataConKey
     is_lifted _                        = False

However, in practice this claim about treating Constraint and Type as distinct is false:

$ ~/Software/ghc2/inplace/bin/ghc-stage2 --interactive -package ghc
GHCi, version 8.5.20180221: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
λ> :m + TyCoRep TysWiredIn
λ> isLiftedTypeKind liftedTypeKind
True
λ> isLiftedTypeKind constraintKind
True

Either we should change the implementation of isLiftedTypeKind to match the documentation's claim, or change the documentation.