typecheck/TcDeriv.hs

    {-
    (c) The University of Glasgow 2006
    (c) The GRASP/AQUA Project, Glasgow University, 1992-1998

    Handles @deriving@ clauses on @data@ declarations.
    -}

    {-# LANGUAGE CPP #-}
    {-# LANGUAGE ImplicitParams #-}

    module TcDeriv ( tcDeriving, DerivInfo(..), mkDerivInfos ) where

    #include "HsVersions.h"

    import HsSyn
    import DynFlags

    import TcRnMonad
    import FamInst
    import TcErrors( reportAllUnsolved )
    import TcValidity( validDerivPred )
    import TcClassDcl( tcATDefault, tcMkDeclCtxt )
    import TcEnv
    import TcGenDeriv                       -- Deriv stuff
    import TcGenGenerics
    import InstEnv
    import Inst
    import FamInstEnv
    import TcHsType
    import TcMType
    import TcSimplify
    import TcUnify( buildImplicationFor )
    import LoadIface( loadInterfaceForName )
    import Module( getModule )

    import RnNames( extendGlobalRdrEnvRn )
    import RnBinds
    import RnEnv
    import RnSource   ( addTcgDUs )
    import HscTypes
    import Avail

    import Unify( tcUnifyTy )
    import Class
    import Type
    import ErrUtils
    import DataCon
    import Maybes
    import RdrName
    import Name
    import NameSet
    import TyCon
    import TcType
    import Var
    import VarEnv
    import VarSet
    import PrelNames
    import THNames ( liftClassKey )
    import SrcLoc
    import Util
    import Outputable
    import FastString
    import Bag
    import Pair
    import FV (fvVarList, unionFV, mkFVs)
    import qualified GHC.LanguageExtensions as LangExt

    import Control.Monad
    import Data.List
    #if __GLASGOW_HASKELL__ > 710
    import GHC.Stack (CallStack)
    #endif

    {-
    ************************************************************************
    *                                                                      *
                    Overview
    *                                                                      *
    ************************************************************************

    Overall plan
    ~~~~~~~~~~~~
    1.  Convert the decls (i.e. data/newtype deriving clauses,
        plus standalone deriving) to [EarlyDerivSpec]

    2.  Infer the missing contexts for the InferTheta's

    3.  Add the derived bindings, generating InstInfos
    -}

    -- DerivSpec is purely  local to this module
    data DerivSpec theta = DS { ds_loc     :: SrcSpan
                              , ds_name    :: Name           -- DFun name
                              , ds_tvs     :: [TyVar]
                              , ds_theta   :: theta
                              , ds_cls     :: Class
                              , ds_tys     :: [Type]
                              , ds_tc      :: TyCon
                              , ds_overlap :: Maybe OverlapMode
                              , ds_newtype :: Maybe Type }  -- The newtype rep type
            -- This spec implies a dfun declaration of the form
            --       df :: forall tvs. theta => C tys
            -- The Name is the name for the DFun we'll build
            -- The tyvars bind all the variables in the theta
            -- For type families, the tycon in
            --       in ds_tys is the *family* tycon
            --       in ds_tc is the *representation* type
            -- For non-family tycons, both are the same

            -- the theta is either the given and final theta, in standalone deriving,
            -- or the not-yet-simplified list of constraints together with their origin

            -- ds_newtype = Just rep_ty  <=> Generalised Newtype Deriving (GND)
            --              Nothing      <=> Vanilla deriving

    {-
    Example:

         newtype instance T [a] = MkT (Tree a) deriving( C s )
    ==>
         axiom T [a] = :RTList a
         axiom :RTList a = Tree a

         DS { ds_tvs = [a,s], ds_cls = C, ds_tys = [s, T [a]]
            , ds_tc = :RTList, ds_newtype = Just (Tree a) }
    -}

    type DerivContext = Maybe ThetaType
       -- Nothing    <=> Vanilla deriving; infer the context of the instance decl
       -- Just theta <=> Standalone deriving: context supplied by programmer

    data PredOrigin = PredOrigin PredType CtOrigin TypeOrKind
    type ThetaOrigin = [PredOrigin]

    mkPredOrigin :: CtOrigin -> TypeOrKind -> PredType -> PredOrigin
    mkPredOrigin origin t_or_k pred = PredOrigin pred origin t_or_k

    mkThetaOrigin :: CtOrigin -> TypeOrKind -> ThetaType -> ThetaOrigin
    mkThetaOrigin origin t_or_k = map (mkPredOrigin origin t_or_k)

    substPredOrigin ::
    -- CallStack wasn't present in GHC 7.10.1, disable callstacks in stage 1
    #if __GLASGOW_HASKELL__ > 710
        (?callStack :: CallStack) =>
    #endif
        TCvSubst -> PredOrigin -> PredOrigin
    substPredOrigin subst (PredOrigin pred origin t_or_k)
      = PredOrigin (substTy subst pred) origin t_or_k

    substThetaOrigin ::
    -- CallStack wasn't present in GHC 7.10.1, disable callstacks in stage 1
    #if __GLASGOW_HASKELL__ > 710
        (?callStack :: CallStack) =>
    #endif
        TCvSubst -> ThetaOrigin -> ThetaOrigin
    substThetaOrigin subst = map (substPredOrigin subst)

    data EarlyDerivSpec = InferTheta (DerivSpec ThetaOrigin)
                        | GivenTheta (DerivSpec ThetaType)
            -- InferTheta ds => the context for the instance should be inferred
            --      In this case ds_theta is the list of all the constraints
            --      needed, such as (Eq [a], Eq a), together with a suitable CtLoc
            --      to get good error messages.
            --      The inference process is to reduce this to a
            --      simpler form (e.g. Eq a)
            --
            -- GivenTheta ds => the exact context for the instance is supplied
            --                  by the programmer; it is ds_theta
            -- See Note [Inferring the instance context]

    earlyDSLoc :: EarlyDerivSpec -> SrcSpan
    earlyDSLoc (InferTheta spec) = ds_loc spec
    earlyDSLoc (GivenTheta spec) = ds_loc spec

    splitEarlyDerivSpec :: [EarlyDerivSpec] -> ([DerivSpec ThetaOrigin], [DerivSpec ThetaType])
    splitEarlyDerivSpec [] = ([],[])
    splitEarlyDerivSpec (InferTheta spec : specs) =
        case splitEarlyDerivSpec specs of (is, gs) -> (spec : is, gs)
    splitEarlyDerivSpec (GivenTheta spec : specs) =
        case splitEarlyDerivSpec specs of (is, gs) -> (is, spec : gs)

    pprDerivSpec :: Outputable theta => DerivSpec theta -> SDoc
    pprDerivSpec (DS { ds_loc = l, ds_name = n, ds_tvs = tvs,
                       ds_cls = c, ds_tys = tys, ds_theta = rhs })
      = hang (text "DerivSpec")
           2 (vcat [ text "ds_loc   =" <+> ppr l
                   , text "ds_name  =" <+> ppr n
                   , text "ds_tvs   =" <+> ppr tvs
                   , text "ds_cls   =" <+> ppr c
                   , text "ds_tys   =" <+> ppr tys
                   , text "ds_theta =" <+> ppr rhs ])

    instance Outputable theta => Outputable (DerivSpec theta) where
      ppr = pprDerivSpec

    instance Outputable EarlyDerivSpec where
      ppr (InferTheta spec) = ppr spec <+> text "(Infer)"
      ppr (GivenTheta spec) = ppr spec <+> text "(Given)"

    instance Outputable PredOrigin where
      ppr (PredOrigin ty _ _) = ppr ty -- The origin is not so interesting when debugging

    {- Note [Inferring the instance context]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    There are two sorts of 'deriving':

      * InferTheta: the deriving clause for a data type
          data T a = T1 a deriving( Eq )
        Here we must infer an instance context,
        and generate instance declaration
          instance Eq a => Eq (T a) where ...

      * CheckTheta: standalone deriving
          deriving instance Eq a => Eq (T a)
        Here we only need to fill in the bindings;
        the instance context is user-supplied

    For a deriving clause (InferTheta) we must figure out the
    instance context (inferConstraints). Suppose we are inferring
    the instance context for
        C t1 .. tn (T s1 .. sm)
    There are two cases

      * (T s1 .. sm) :: *         (the normal case)
        Then we behave like Eq and guess (C t1 .. tn t)
        for each data constructor arg of type t.  More
        details below.

      * (T s1 .. sm) :: * -> *    (the functor-like case)
        Then we behave like Functor.

    In both cases we produce a bunch of un-simplified constraints
    and them simplify them in simplifyInstanceContexts; see
    Note [Simplifying the instance context].

    In the functor-like case, we may need to unify some kind variables with * in
    order for the generated instance to be well-kinded. An example from
    Trac #10524:

      newtype Compose (f :: k2 -> *) (g :: k1 -> k2) (a :: k1)
        = Compose (f (g a)) deriving Functor

    Earlier in the deriving pipeline, GHC unifies the kind of Compose f g
    (k1 -> *) with the kind of Functor's argument (* -> *), so k1 := *. But this
    alone isn't enough, since k2 wasn't unified with *:

      instance (Functor (f :: k2 -> *), Functor (g :: * -> k2)) =>
        Functor (Compose f g) where ...

    The two Functor constraints are ill-kinded. To ensure this doesn't happen, we:

      1. Collect all of a datatype's subtypes which require functor-like
         constraints.
      2. For each subtype, create a substitution by unifying the subtype's kind
         with (* -> *).
      3. Compose all the substitutions into one, then apply that substitution to
         all of the in-scope type variables and the instance types.

    Note [Data decl contexts]
    ~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider

            data (RealFloat a) => Complex a = !a :+ !a deriving( Read )

    We will need an instance decl like:

            instance (Read a, RealFloat a) => Read (Complex a) where
              ...

    The RealFloat in the context is because the read method for Complex is bound
    to construct a Complex, and doing that requires that the argument type is
    in RealFloat.

    But this ain't true for Show, Eq, Ord, etc, since they don't construct
    a Complex; they only take them apart.

    Our approach: identify the offending classes, and add the data type
    context to the instance decl.  The "offending classes" are

            Read, Enum?

    FURTHER NOTE ADDED March 2002.  In fact, Haskell98 now requires that
    pattern matching against a constructor from a data type with a context
    gives rise to the constraints for that context -- or at least the thinned
    version.  So now all classes are "offending".

    Note [Newtype deriving]
    ~~~~~~~~~~~~~~~~~~~~~~~
    Consider this:
        class C a b
        instance C [a] Char
        newtype T = T Char deriving( C [a] )

    Notice the free 'a' in the deriving.  We have to fill this out to
        newtype T = T Char deriving( forall a. C [a] )

    And then translate it to:
        instance C [a] Char => C [a] T where ...

    Note [Newtype deriving superclasses]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    (See also Trac #1220 for an interesting exchange on newtype
    deriving and superclasses.)

    The 'tys' here come from the partial application in the deriving
    clause. The last arg is the new instance type.

    We must pass the superclasses; the newtype might be an instance
    of them in a different way than the representation type
    E.g.            newtype Foo a = Foo a deriving( Show, Num, Eq )
    Then the Show instance is not done via Coercible; it shows
            Foo 3 as "Foo 3"
    The Num instance is derived via Coercible, but the Show superclass
    dictionary must the Show instance for Foo, *not* the Show dictionary
    gotten from the Num dictionary. So we must build a whole new dictionary
    not just use the Num one.  The instance we want is something like:
         instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
            (+) = ((+)@a)
            ...etc...
    There may be a coercion needed which we get from the tycon for the newtype
    when the dict is constructed in TcInstDcls.tcInstDecl2

    Note [Unused constructors and deriving clauses]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    See Trac #3221.  Consider
       data T = T1 | T2 deriving( Show )
    Are T1 and T2 unused?  Well, no: the deriving clause expands to mention
    both of them.  So we gather defs/uses from deriving just like anything else.

    -}

    -- | Stuff needed to process a `deriving` clause
    data DerivInfo = DerivInfo { di_rep_tc :: TyCon
                                 -- ^ The data tycon for normal datatypes,
                                 -- or the *representation* tycon for data families
                               , di_preds  :: [LHsSigType Name]
                               , di_ctxt   :: SDoc -- ^ error context
                               }

    -- | Extract `deriving` clauses of proper data type (skips data families)
    mkDerivInfos :: [TyClGroup Name] -> TcM [DerivInfo]
    mkDerivInfos tycls = concatMapM mk_derivs tycls
      where
        mk_derivs (TyClGroup { group_tyclds = decls })
          = concatMapM (mk_deriv . unLoc) decls

        mk_deriv decl@(DataDecl { tcdLName = L _ data_name
                                , tcdDataDefn =
                                    HsDataDefn { dd_derivs = Just (L _ preds) } })
          = do { tycon <- tcLookupTyCon data_name
               ; return [DerivInfo { di_rep_tc = tycon, di_preds = preds
                                   , di_ctxt = tcMkDeclCtxt decl }] }
        mk_deriv _ = return []

    {-

    ************************************************************************
    *                                                                      *
    \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
    *                                                                      *
    ************************************************************************
    -}

    tcDeriving  :: [DerivInfo]       -- All `deriving` clauses
                -> [LDerivDecl Name] -- All stand-alone deriving declarations
                -> TcM (TcGblEnv, Bag (InstInfo Name), HsValBinds Name)
    tcDeriving deriv_infos deriv_decls
      = recoverM (do { g <- getGblEnv
                     ; return (g, emptyBag, emptyValBindsOut)}) $
        do  {       -- Fish the "deriving"-related information out of the TcEnv
                    -- And make the necessary "equations".
              is_boot <- tcIsHsBootOrSig
            ; traceTc "tcDeriving" (ppr is_boot)

            ; early_specs <- makeDerivSpecs is_boot deriv_infos deriv_decls
            ; traceTc "tcDeriving 1" (ppr early_specs)

            ; let (infer_specs, given_specs) = splitEarlyDerivSpec early_specs
            ; insts1 <- mapM genInst given_specs

            -- the stand-alone derived instances (@insts1@) are used when inferring
            -- the contexts for "deriving" clauses' instances (@infer_specs@)
            ; final_specs <- extendLocalInstEnv (map (iSpec . fstOf3) insts1) $
                             simplifyInstanceContexts infer_specs

            ; insts2 <- mapM genInst final_specs

            ; let (inst_infos, deriv_stuff, maybe_fvs) = unzip3 (insts1 ++ insts2)
            ; loc <- getSrcSpanM
            ; let (binds, famInsts, extraInstances) =
                    genAuxBinds loc (unionManyBags deriv_stuff)

            ; dflags <- getDynFlags

            ; (inst_info, rn_binds, rn_dus) <-
                renameDeriv is_boot (inst_infos ++ (bagToList extraInstances)) binds

            ; unless (isEmptyBag inst_info) $
                 liftIO (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
                            (ddump_deriving inst_info rn_binds famInsts))

            ; gbl_env <- tcExtendLocalFamInstEnv (bagToList famInsts) $
                         tcExtendLocalInstEnv (map iSpec (bagToList inst_info)) getGblEnv
            ; let all_dus = rn_dus `plusDU` usesOnly (NameSet.mkFVs $ catMaybes maybe_fvs)
            ; return (addTcgDUs gbl_env all_dus, inst_info, rn_binds) }
      where
        ddump_deriving :: Bag (InstInfo Name) -> HsValBinds Name
                       -> Bag FamInst             -- ^ Rep type family instances
                       -> SDoc
        ddump_deriving inst_infos extra_binds repFamInsts
          =    hang (text "Derived instances:")
                  2 (vcat (map (\i -> pprInstInfoDetails i $$ text "") (bagToList inst_infos))
                     $$ ppr extra_binds)
            $$ hangP "GHC.Generics representation types:"
                 (vcat (map pprRepTy (bagToList repFamInsts)))

        hangP s x = text "" $$ hang (ptext (sLit s)) 2 x

    -- Prints the representable type family instance
    pprRepTy :: FamInst -> SDoc
    pprRepTy fi@(FamInst { fi_tys = lhs })
      = text "type" <+> ppr (mkTyConApp (famInstTyCon fi) lhs) <+>
          equals <+> ppr rhs
      where rhs = famInstRHS fi

    renameDeriv :: Bool
                -> [InstInfo RdrName]
                -> Bag (LHsBind RdrName, LSig RdrName)
                -> TcM (Bag (InstInfo Name), HsValBinds Name, DefUses)
    renameDeriv is_boot inst_infos bagBinds
      | is_boot     -- If we are compiling a hs-boot file, don't generate any derived bindings
                    -- The inst-info bindings will all be empty, but it's easier to
                    -- just use rn_inst_info to change the type appropriately
      = do  { (rn_inst_infos, fvs) <- mapAndUnzipM rn_inst_info inst_infos
            ; return ( listToBag rn_inst_infos
                     , emptyValBindsOut, usesOnly (plusFVs fvs)) }

      | otherwise
      = discardWarnings $
        -- Discard warnings about unused bindings etc
        setXOptM LangExt.EmptyCase $
        -- Derived decls (for empty types) can have
        --    case x of {}
        setXOptM LangExt.ScopedTypeVariables $
        setXOptM LangExt.KindSignatures $
        -- Derived decls (for newtype-deriving) can use ScopedTypeVariables &
        -- KindSignatures
        unsetXOptM LangExt.RebindableSyntax $
        -- See Note [Avoid RebindableSyntax when deriving]
        do  {
            -- Bring the extra deriving stuff into scope
            -- before renaming the instances themselves
            ; traceTc "rnd" (vcat (map (\i -> pprInstInfoDetails i $$ text "") inst_infos))
            ; (aux_binds, aux_sigs) <- mapAndUnzipBagM return bagBinds
            ; let aux_val_binds = ValBindsIn aux_binds (bagToList aux_sigs)
            ; rn_aux_lhs <- rnTopBindsLHS emptyFsEnv aux_val_binds
            ; let bndrs = collectHsValBinders rn_aux_lhs
            ; envs <- extendGlobalRdrEnvRn (map avail bndrs) emptyFsEnv ;
            ; setEnvs envs $
        do  { (rn_aux, dus_aux) <- rnValBindsRHS (TopSigCtxt (mkNameSet bndrs)) rn_aux_lhs
            ; (rn_inst_infos, fvs_insts) <- mapAndUnzipM rn_inst_info inst_infos
            ; return (listToBag rn_inst_infos, rn_aux,
                      dus_aux `plusDU` usesOnly (plusFVs fvs_insts)) } }

      where
        rn_inst_info :: InstInfo RdrName -> TcM (InstInfo Name, FreeVars)
        rn_inst_info
          inst_info@(InstInfo { iSpec = inst
                              , iBinds = InstBindings
                                { ib_binds = binds
                                , ib_tyvars = tyvars
                                , ib_pragmas = sigs
                                , ib_extensions = exts -- Only for type-checking
                                , ib_derived = sa } })
            =  ASSERT( null sigs )
               bindLocalNamesFV tyvars $
               do { (rn_binds,_, fvs) <- rnMethodBinds False (is_cls_nm inst) [] binds []
                  ; let binds' = InstBindings { ib_binds = rn_binds
                                              , ib_tyvars = tyvars
                                              , ib_pragmas = []
                                              , ib_extensions = exts
                                              , ib_derived = sa }
                  ; return (inst_info { iBinds = binds' }, fvs) }

    {-
    Note [Newtype deriving and unused constructors]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider this (see Trac #1954):

      module Bug(P) where
      newtype P a = MkP (IO a) deriving Monad

    If you compile with -Wunused-binds you do not expect the warning
    "Defined but not used: data consructor MkP". Yet the newtype deriving
    code does not explicitly mention MkP, but it should behave as if you
    had written
      instance Monad P where
         return x = MkP (return x)
         ...etc...

    So we want to signal a user of the data constructor 'MkP'.
    This is the reason behind the (Maybe Name) part of the return type
    of genInst.

    Note [Why we don't pass rep_tc into deriveTyData]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Down in the bowels of mkEqnHelp, we need to convert the fam_tc back into
    the rep_tc by means of a lookup. And yet we have the rep_tc right here!
    Why look it up again? Answer: it's just easier this way.
    We drop some number of arguments from the end of the datatype definition
    in deriveTyData. The arguments are dropped from the fam_tc.
    This action may drop a *different* number of arguments
    passed to the rep_tc, depending on how many free variables, etc., the
    dropped patterns have.

    Also, this technique carries over the kind substitution from deriveTyData
    nicely.

    Note [Avoid RebindableSyntax when deriving]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The RebindableSyntax extension interacts awkwardly with the derivation of
    any stock class whose methods require the use of string literals. The Show
    class is a simple example (see Trac #12688):

      {-# LANGUAGE RebindableSyntax, OverloadedStrings #-}
      newtype Text = Text String
      fromString :: String -> Text
      fromString = Text

      data Foo = Foo deriving Show

    This will generate code to the effect of:

      instance Show Foo where
        showsPrec _ Foo = showString "Foo"

    But because RebindableSyntax and OverloadedStrings are enabled, the "Foo"
    string literal is now of type Text, not String, which showString doesn't
    accept! This causes the generated Show instance to fail to typecheck.

    To avoid this kind of scenario, we simply turn off RebindableSyntax entirely
    in derived code.

    ************************************************************************
    *                                                                      *
                    From HsSyn to DerivSpec
    *                                                                      *
    ************************************************************************

    @makeDerivSpecs@ fishes around to find the info about needed derived instances.
    -}

    makeDerivSpecs :: Bool
                   -> [DerivInfo]
                   -> [LDerivDecl Name]
                   -> TcM [EarlyDerivSpec]
    makeDerivSpecs is_boot deriv_infos deriv_decls
      = do  { eqns1 <- concatMapM (recoverM (return []) . deriveDerivInfo)  deriv_infos
            ; eqns2 <- concatMapM (recoverM (return []) . deriveStandalone) deriv_decls
            ; let eqns = eqns1 ++ eqns2

            ; if is_boot then   -- No 'deriving' at all in hs-boot files
                  do { unless (null eqns) (add_deriv_err (head eqns))
                     ; return [] }
              else return eqns }
      where
        add_deriv_err eqn
           = setSrcSpan (earlyDSLoc eqn) $
             addErr (hang (text "Deriving not permitted in hs-boot file")
                        2 (text "Use an instance declaration instead"))

    ------------------------------------------------------------------
    -- | Process a `deriving` clause
    deriveDerivInfo :: DerivInfo -> TcM [EarlyDerivSpec]
    deriveDerivInfo (DerivInfo { di_rep_tc = rep_tc, di_preds = preds
                               , di_ctxt = err_ctxt })
      = addErrCtxt err_ctxt $
        concatMapM (deriveTyData tvs tc tys) preds
      where
        tvs = tyConTyVars rep_tc
        (tc, tys) = case tyConFamInstSig_maybe rep_tc of
                            -- data family:
                      Just (fam_tc, pats, _) -> (fam_tc, pats)
          -- NB: deriveTyData wants the *user-specified*
          -- name. See Note [Why we don't pass rep_tc into deriveTyData]

                      _ -> (rep_tc, mkTyVarTys tvs)     -- datatype

    ------------------------------------------------------------------
    deriveStandalone :: LDerivDecl Name -> TcM [EarlyDerivSpec]
    -- Standalone deriving declarations
    --  e.g.   deriving instance Show a => Show (T a)
    -- Rather like tcLocalInstDecl
    deriveStandalone (L loc (DerivDecl deriv_ty overlap_mode))
      = setSrcSpan loc                   $
        addErrCtxt (standaloneCtxt deriv_ty)  $
        do { traceTc "Standalone deriving decl for" (ppr deriv_ty)
           ; (tvs, theta, cls, inst_tys) <- tcHsClsInstType TcType.InstDeclCtxt deriv_ty
           ; traceTc "Standalone deriving;" $ vcat
                  [ text "tvs:" <+> ppr tvs
                  , text "theta:" <+> ppr theta
                  , text "cls:" <+> ppr cls
                  , text "tys:" <+> ppr inst_tys ]
                    -- C.f. TcInstDcls.tcLocalInstDecl1
           ; checkTc (not (null inst_tys)) derivingNullaryErr

           ; let cls_tys = take (length inst_tys - 1) inst_tys
                 inst_ty = last inst_tys
           ; traceTc "Standalone deriving:" $ vcat
                  [ text "class:" <+> ppr cls
                  , text "class types:" <+> ppr cls_tys
                  , text "type:" <+> ppr inst_ty ]

           ; let bale_out msg = failWithTc (derivingThingErr False cls cls_tys
                                  inst_ty msg)

           ; case tcSplitTyConApp_maybe inst_ty of
               Just (tc, tc_args)
                  | className cls == typeableClassName
                  -> do warnUselessTypeable
                        return []

                  | isUnboxedTupleTyCon tc
                  -> bale_out $ unboxedTyConErr "tuple"

                  | isAlgTyCon tc || isDataFamilyTyCon tc  -- All other classes
                  -> do { spec <- mkEqnHelp (fmap unLoc overlap_mode)
                                            tvs cls cls_tys tc tc_args
                                            (Just theta)
                        ; return [spec] }

               _  -> -- Complain about functions, primitive types, etc,
                     bale_out $
                     text "The last argument of the instance must be a data or newtype application"
            }

    warnUselessTypeable :: TcM ()
    warnUselessTypeable
      = do { warn <- woptM Opt_WarnDerivingTypeable
           ; when warn $ addWarnTc (Reason Opt_WarnDerivingTypeable)
                       $ text "Deriving" <+> quotes (ppr typeableClassName) <+>
                         text "has no effect: all types now auto-derive Typeable" }

    ------------------------------------------------------------------
    deriveTyData :: [TyVar] -> TyCon -> [Type]   -- LHS of data or data instance
                                                 --   Can be a data instance, hence [Type] args
                 -> LHsSigType Name              -- The deriving predicate
                 -> TcM [EarlyDerivSpec]
    -- The deriving clause of a data or newtype declaration
    -- I.e. not standalone deriving
    deriveTyData tvs tc tc_args deriv_pred
      = setSrcSpan (getLoc (hsSigType deriv_pred)) $  -- Use loc of the 'deriving' item
        do  { (deriv_tvs, cls, cls_tys, cls_arg_kinds)
                    <- tcExtendTyVarEnv tvs $
                       tcHsDeriv deriv_pred
                    -- Deriving preds may (now) mention
                    -- the type variables for the type constructor, hence tcExtendTyVarenv
                    -- The "deriv_pred" is a LHsType to take account of the fact that for
                    -- newtype deriving we allow deriving (forall a. C [a]).

                    -- Typeable is special, because Typeable :: forall k. k -> Constraint
                    -- so the argument kind 'k' is not decomposable by splitKindFunTys
                    -- as is the case for all other derivable type classes
            ; when (length cls_arg_kinds /= 1) $
                failWithTc (nonUnaryErr deriv_pred)
            ; let [cls_arg_kind] = cls_arg_kinds
            ; if className cls == typeableClassName
              then do warnUselessTypeable
                      return []
              else

         do {  -- Given data T a b c = ... deriving( C d ),
               -- we want to drop type variables from T so that (C d (T a)) is well-kinded
              let (arg_kinds, _)  = splitFunTys cls_arg_kind
                  n_args_to_drop  = length arg_kinds
                  n_args_to_keep  = tyConArity tc - n_args_to_drop
                  (tc_args_to_keep, args_to_drop)
                                  = splitAt n_args_to_keep tc_args
                  inst_ty_kind    = typeKind (mkTyConApp tc tc_args_to_keep)
                  -- Use exactTyCoVarsOfTypes, not tyCoVarsOfTypes, so that we
                  -- don't mistakenly grab a type variable mentioned in a type
                  -- synonym that drops it.
                  -- See Note [Eta-reducing type synonyms].
                  dropped_tvs     = exactTyCoVarsOfTypes args_to_drop

                  -- Match up the kinds, and apply the resulting kind substitution
                  -- to the types.  See Note [Unify kinds in deriving]
                  -- We are assuming the tycon tyvars and the class tyvars are distinct
                  mb_match        = tcUnifyTy inst_ty_kind cls_arg_kind
                  Just kind_subst = mb_match
                  ki_subst_range  = getTCvSubstRangeFVs kind_subst

                  all_tkvs        = toposortTyVars $
                                    fvVarList $ unionFV
                                      (tyCoFVsOfTypes tc_args_to_keep)
                                      (FV.mkFVs deriv_tvs)

                  -- See Note [Unification of two kind variables in deriving]
                  unmapped_tkvs   = filter (\v -> v `notElemTCvSubst` kind_subst
                                          && not (v `elemVarSet` ki_subst_range))
                                           all_tkvs
                  (subst, _)      = mapAccumL substTyVarBndr
                                              kind_subst unmapped_tkvs
                  final_tc_args   = substTys subst tc_args_to_keep
                  final_cls_tys   = substTys subst cls_tys
                  tkvs            = tyCoVarsOfTypesWellScoped $
                                    final_cls_tys ++ final_tc_args

            ; traceTc "derivTyData1" (vcat [ pprTvBndrs tvs, ppr tc, ppr tc_args, ppr deriv_pred
                                           , pprTvBndrs (tyCoVarsOfTypesList tc_args)
                                           , ppr n_args_to_keep, ppr n_args_to_drop
                                           , ppr inst_ty_kind, ppr cls_arg_kind, ppr mb_match
                                           , ppr final_tc_args, ppr final_cls_tys ])

            -- Check that the result really is well-kinded
            ; checkTc (n_args_to_keep >= 0 && isJust mb_match)
                      (derivingKindErr tc cls cls_tys cls_arg_kind)

            ; traceTc "derivTyData2" (vcat [ ppr tkvs ])

            ; checkTc (allDistinctTyVars args_to_drop &&              -- (a) and (b)
                       not (any (`elemVarSet` dropped_tvs) tkvs))     -- (c)
                      (derivingEtaErr cls final_cls_tys (mkTyConApp tc final_tc_args))
                    -- Check that
                    --  (a) The args to drop are all type variables; eg reject:
                    --              data instance T a Int = .... deriving( Monad )
                    --  (b) The args to drop are all *distinct* type variables; eg reject:
                    --              class C (a :: * -> * -> *) where ...
                    --              data instance T a a = ... deriving( C )
                    --  (c) The type class args, or remaining tycon args,
                    --      do not mention any of the dropped type variables
                    --              newtype T a s = ... deriving( ST s )
                    --              newtype K a a = ... deriving( Monad )

            ; spec <- mkEqnHelp Nothing tkvs
                                cls final_cls_tys tc final_tc_args Nothing
            ; traceTc "derivTyData" (ppr spec)
            ; return [spec] } }

    {-
    Note [Unify kinds in deriving]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider (Trac #8534)
        data T a b = MkT a deriving( Functor )
        -- where Functor :: (*->*) -> Constraint

    So T :: forall k. * -> k -> *.   We want to get
        instance Functor (T * (a:*)) where ...
    Notice the '*' argument to T.

    Moreover, as well as instantiating T's kind arguments, we may need to instantiate
    C's kind args.  Consider (Trac #8865):
      newtype T a b = MkT (Either a b) deriving( Category )
    where
      Category :: forall k. (k -> k -> *) -> Constraint
    We need to generate the instance
      instance Category * (Either a) where ...
    Notice the '*' argument to Category.

    So we need to
     * drop arguments from (T a b) to match the number of
       arrows in the (last argument of the) class;
     * and then *unify* kind of the remaining type against the
       expected kind, to figure out how to instantiate C's and T's
       kind arguments.

    In the two examples,
     * we unify   kind-of( T k (a:k) ) ~ kind-of( Functor )
             i.e.      (k -> *) ~ (* -> *)   to find k:=*.
             yielding  k:=*

     * we unify   kind-of( Either ) ~ kind-of( Category )
             i.e.      (* -> * -> *)  ~ (k -> k -> k)
             yielding  k:=*

    Now we get a kind substitution.  We then need to:

      1. Remove the substituted-out kind variables from the quantified kind vars

      2. Apply the substitution to the kinds of quantified *type* vars
         (and extend the substitution to reflect this change)

      3. Apply that extended substitution to the non-dropped args (types and
         kinds) of the type and class

    Forgetting step (2) caused Trac #8893:
      data V a = V [a] deriving Functor
      data P (x::k->*) (a:k) = P (x a) deriving Functor
      data C (x::k->*) (a:k) = C (V (P x a)) deriving Functor

    When deriving Functor for P, we unify k to *, but we then want
    an instance   $df :: forall (x:*->*). Functor x => Functor (P * (x:*->*))
    and similarly for C.  Notice the modified kind of x, both at binding
    and occurrence sites.

    This can lead to some surprising results when *visible* kind binder is
    unified (in contrast to the above examples, in which only non-visible kind
    binders were considered). Consider this example from Trac #11732:

        data T k (a :: k) = MkT deriving Functor

    Since unification yields k:=*, this results in a generated instance of:

        instance Functor (T *) where ...

    which looks odd at first glance, since one might expect the instance head
    to be of the form Functor (T k). Indeed, one could envision an alternative
    generated instance of:

        instance (k ~ *) => Functor (T k) where

    But this does not typecheck as the result of a -XTypeInType design decision:
    kind equalities are not allowed to be bound in types, only terms. But in
    essence, the two instance declarations are entirely equivalent, since even
    though (T k) matches any kind k, the only possibly value for k is *, since
    anything else is ill-typed. As a result, we can just as comfortably use (T *).

    Another way of thinking about is: deriving clauses often infer constraints.
    For example:

        data S a = S a deriving Eq

    infers an (Eq a) constraint in the derived instance. By analogy, when we
    are deriving Functor, we might infer an equality constraint (e.g., k ~ *).
    The only distinction is that GHC instantiates equality constraints directly
    during the deriving process.

    Another quirk of this design choice manifests when typeclasses have visible
    kind parameters. Consider this code (also from Trac #11732):

        class Cat k (cat :: k -> k -> *) where
          catId   :: cat a a
          catComp :: cat b c -> cat a b -> cat a c

        instance Cat * (->) where
          catId   = id
          catComp = (.)

        newtype Fun a b = Fun (a -> b) deriving (Cat k)

    Even though we requested an derived instance of the form (Cat k Fun), the
    kind unification will actually generate (Cat * Fun) (i.e., the same thing as if
    the user wrote deriving (Cat *)).

    Note [Unification of two kind variables in deriving]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    As a special case of the Note above, it is possible to derive an instance of
    a poly-kinded typeclass for a poly-kinded datatype. For example:

        class Category (cat :: k -> k -> *) where
        newtype T (c :: k -> k -> *) a b = MkT (c a b) deriving Category

    This case is suprisingly tricky. To see why, let's write out what instance GHC
    will attempt to derive (using -fprint-explicit-kinds syntax):

        instance Category k1 (T k2 c) where ...

    GHC will attempt to unify k1 and k2, which produces a substitution (kind_subst)
    that looks like [k2 :-> k1]. Importantly, we need to apply this substitution to
    the type variable binder for c, since its kind is (k2 -> k2 -> *).

    We used to accomplish this by doing the following:

        unmapped_tkvs = filter (`notElemTCvSubst` kind_subst) all_tkvs
        (subst, _)    = mapAccumL substTyVarBndr kind_subst unmapped_tkvs

    Where all_tkvs contains all kind variables in the class and instance types (in
    this case, all_tkvs = [k1,k2]). But since kind_subst only has one mapping,
    this results in unmapped_tkvs being [k1], and as a consequence, k1 gets mapped
    to another kind variable in subst! That is, subst = [k2 :-> k1, k1 :-> k_new].
    This is bad, because applying that substitution yields the following instance:

       instance Category k_new (T k1 c) where ...

    In other words, keeping k1 in unmapped_tvks taints the substitution, resulting
    in an ill-kinded instance (this caused Trac #11837).

    To prevent this, we need to filter out any variable from all_tkvs which either

    1. Appears in the domain of kind_subst. notElemTCvSubst checks this.
    2. Appears in the range of kind_subst. To do this, we compute the free
       variable set of the range of kind_subst with getTCvSubstRangeFVs, and check
       if a kind variable appears in that set.

    Note [Eta-reducing type synonyms]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    One can instantiate a type in a data family instance with a type synonym that
    mentions other type variables:

      type Const a b = a
      data family Fam (f :: * -> *) (a :: *)
      newtype instance Fam f (Const a f) = Fam (f a) deriving Functor

    With -XTypeInType, it is also possible to define kind synonyms, and they can
    mention other types in a datatype declaration. For example,

      type Const a b = a
      newtype T f (a :: Const * f) = T (f a) deriving Functor

    When deriving, we need to perform eta-reduction analysis to ensure that none of
    the eta-reduced type variables are mentioned elsewhere in the declaration. But
    we need to be careful, because if we don't expand through the Const type
    synonym, we will mistakenly believe that f is an eta-reduced type variable and
    fail to derive Functor, even though the code above is correct (see Trac #11416,
    where this was first noticed).

    For this reason, we call exactTyCoVarsOfTypes on the eta-reduced types so that
    we only consider the type variables that remain after expanding through type
    synonyms.
    -}

    mkEqnHelp :: Maybe OverlapMode
              -> [TyVar]
              -> Class -> [Type]
              -> TyCon -> [Type]
              -> DerivContext       -- Just    => context supplied (standalone deriving)
                                    -- Nothing => context inferred (deriving on data decl)
              -> TcRn EarlyDerivSpec
    -- Make the EarlyDerivSpec for an instance
    --      forall tvs. theta => cls (tys ++ [ty])
    -- where the 'theta' is optional (that's the Maybe part)
    -- Assumes that this declaration is well-kinded

    mkEqnHelp overlap_mode tvs cls cls_tys tycon tc_args mtheta
      = do {      -- Find the instance of a data family
                  -- Note [Looking up family instances for deriving]
             fam_envs <- tcGetFamInstEnvs
           ; let (rep_tc, rep_tc_args, _co) = tcLookupDataFamInst fam_envs tycon tc_args
                  -- If it's still a data family, the lookup failed; i.e no instance exists
           ; when (isDataFamilyTyCon rep_tc)
                  (bale_out (text "No family instance for" <+> quotes (pprTypeApp tycon tc_args)))

           -- For standalone deriving (mtheta /= Nothing),
           -- check that all the data constructors are in scope.
           ; rdr_env <- getGlobalRdrEnv
           ; let data_con_names = map dataConName (tyConDataCons rep_tc)
                 hidden_data_cons = not (isWiredInName (tyConName rep_tc)) &&
                                    (isAbstractTyCon rep_tc ||
                                     any not_in_scope data_con_names)
                 not_in_scope dc  = null (lookupGRE_Name rdr_env dc)

           ; addUsedDataCons rdr_env rep_tc
           ; unless (isNothing mtheta || not hidden_data_cons)
                    (bale_out (derivingHiddenErr tycon))

           ; dflags <- getDynFlags
           ; if isDataTyCon rep_tc then
                mkDataTypeEqn dflags overlap_mode tvs cls cls_tys
                              tycon tc_args rep_tc rep_tc_args mtheta
             else
                mkNewTypeEqn dflags overlap_mode tvs cls cls_tys
                             tycon tc_args rep_tc rep_tc_args mtheta }
      where
         bale_out msg = failWithTc (derivingThingErr False cls cls_tys (mkTyConApp tycon tc_args) msg)

    {-
    Note [Looking up family instances for deriving]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    tcLookupFamInstExact is an auxiliary lookup wrapper which requires
    that looked-up family instances exist.  If called with a vanilla
    tycon, the old type application is simply returned.

    If we have
      data instance F () = ... deriving Eq
      data instance F () = ... deriving Eq
    then tcLookupFamInstExact will be confused by the two matches;
    but that can't happen because tcInstDecls1 doesn't call tcDeriving
    if there are any overlaps.

    There are two other things that might go wrong with the lookup.
    First, we might see a standalone deriving clause
       deriving Eq (F ())
    when there is no data instance F () in scope.

    Note that it's OK to have
      data instance F [a] = ...
      deriving Eq (F [(a,b)])
    where the match is not exact; the same holds for ordinary data types
    with standalone deriving declarations.

    Note [Deriving, type families, and partial applications]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When there are no type families, it's quite easy:

        newtype S a = MkS [a]
        -- :CoS :: S  ~ []  -- Eta-reduced

        instance Eq [a] => Eq (S a)         -- by coercion sym (Eq (:CoS a)) : Eq [a] ~ Eq (S a)
        instance Monad [] => Monad S        -- by coercion sym (Monad :CoS)  : Monad [] ~ Monad S

    When type familes are involved it's trickier:

        data family T a b
        newtype instance T Int a = MkT [a] deriving( Eq, Monad )
        -- :RT is the representation type for (T Int a)
        --  :Co:RT    :: :RT ~ []          -- Eta-reduced!
        --  :CoF:RT a :: T Int a ~ :RT a   -- Also eta-reduced!

        instance Eq [a] => Eq (T Int a)     -- easy by coercion
           -- d1 :: Eq [a]
           -- d2 :: Eq (T Int a) = d1 |> Eq (sym (:Co:RT a ; :coF:RT a))

        instance Monad [] => Monad (T Int)  -- only if we can eta reduce???
           -- d1 :: Monad []
           -- d2 :: Monad (T Int) = d1 |> Monad (sym (:Co:RT ; :coF:RT))

    Note the need for the eta-reduced rule axioms.  After all, we can
    write it out
        instance Monad [] => Monad (T Int)  -- only if we can eta reduce???
          return x = MkT [x]
          ... etc ...

    See Note [Eta reduction for data families] in FamInstEnv

    %************************************************************************
    %*                                                                      *
                    Deriving data types
    *                                                                      *
    ************************************************************************
    -}

    mkDataTypeEqn :: DynFlags
                  -> Maybe OverlapMode
                  -> [TyVar]                -- Universally quantified type variables in the instance
                  -> Class                  -- Class for which we need to derive an instance
                  -> [Type]                 -- Other parameters to the class except the last
                  -> TyCon                  -- Type constructor for which the instance is requested
                                            --    (last parameter to the type class)
                  -> [Type]                 -- Parameters to the type constructor
                  -> TyCon                  -- rep of the above (for type families)
                  -> [Type]                 -- rep of the above
                  -> DerivContext        -- Context of the instance, for standalone deriving
                  -> TcRn EarlyDerivSpec    -- Return 'Nothing' if error

    mkDataTypeEqn dflags overlap_mode tvs cls cls_tys
                  tycon tc_args rep_tc rep_tc_args mtheta
      = case checkSideConditions dflags mtheta cls cls_tys rep_tc of
            -- NB: pass the *representation* tycon to checkSideConditions
            NonDerivableClass   msg -> bale_out (nonStdErr cls $$ msg)
            DerivableClassError msg -> bale_out msg
            CanDerive               -> go_for_it
            DerivableViaInstance    -> go_for_it
      where
        go_for_it    = mk_data_eqn overlap_mode tvs cls cls_tys tycon tc_args rep_tc rep_tc_args mtheta
        bale_out msg = failWithTc (derivingThingErr False cls cls_tys (mkTyConApp tycon tc_args) msg)

    mk_data_eqn :: Maybe OverlapMode -> [TyVar] -> Class -> [Type]
                -> TyCon -> [TcType] -> TyCon -> [TcType] -> DerivContext
                -> TcM EarlyDerivSpec
    mk_data_eqn overlap_mode tvs cls cls_tys tycon tc_args rep_tc rep_tc_args mtheta
      = do loc                  <- getSrcSpanM
           dfun_name            <- newDFunName' cls tycon
           case mtheta of
            Nothing -> -- Infer context
              inferConstraints tvs cls cls_tys
                               inst_ty rep_tc rep_tc_args
                $ \inferred_constraints tvs' inst_tys' ->
                return $ InferTheta $ DS
                       { ds_loc = loc
                       , ds_name = dfun_name, ds_tvs = tvs'
                       , ds_cls = cls, ds_tys = inst_tys'
                       , ds_tc = rep_tc
                       , ds_theta = inferred_constraints
                       , ds_overlap = overlap_mode
                       , ds_newtype = Nothing }
            Just theta -> do -- Specified context
                return $ GivenTheta $ DS
                       { ds_loc = loc
                       , ds_name = dfun_name, ds_tvs = tvs
                       , ds_cls = cls, ds_tys = inst_tys
                       , ds_tc = rep_tc
                       , ds_theta = theta
                       , ds_overlap = overlap_mode
                       , ds_newtype = Nothing }
      where
        inst_ty  = mkTyConApp tycon tc_args
        inst_tys = cls_tys ++ [inst_ty]

    ----------------------

    inferConstraints :: [TyVar] -> Class -> [TcType] -> TcType
                     -> TyCon -> [TcType]
                     -> (ThetaOrigin -> [TyVar] -> [TcType] -> TcM a)
                     -> TcM a
    -- inferConstraints figures out the constraints needed for the
    -- instance declaration generated by a 'deriving' clause on a
    -- data type declaration. It also returns the new in-scope type
    -- variables and instance types, in case they were changed due to
    -- the presence of functor-like constraints.
    -- See Note [Inferring the instance context]

    -- e.g. inferConstraints
    --        C Int (T [a])    -- Class and inst_tys
    --        :RTList a        -- Rep tycon and its arg tys
    -- where T [a] ~R :RTList a
    --
    -- Generate a sufficiently large set of constraints that typechecking the
    -- generated method definitions should succeed.   This set will be simplified
    -- before being used in the instance declaration
    inferConstraints tvs main_cls cls_tys inst_ty rep_tc rep_tc_args mkTheta
      | main_cls `hasKey` genClassKey    -- Generic constraints are easy
      = mkTheta [] tvs inst_tys

      | main_cls `hasKey` gen1ClassKey   -- Gen1 needs Functor
      = ASSERT( length rep_tc_tvs > 0 )   -- See Note [Getting base classes]
        ASSERT( null cls_tys )
        do { functorClass <- tcLookupClass functorClassName
           ; con_arg_constraints (get_gen1_constraints functorClass) mkTheta }

      | otherwise  -- The others are a bit more complicated
      = ASSERT2( equalLength rep_tc_tvs all_rep_tc_args
               , ppr main_cls <+> ppr rep_tc
                 $$ ppr rep_tc_tvs $$ ppr all_rep_tc_args )
        con_arg_constraints get_std_constrained_tys
          $ \arg_constraints tvs' inst_tys' ->
          do { traceTc "inferConstraints" $ vcat
                    [ ppr main_cls <+> ppr inst_tys'
                    , ppr arg_constraints
                    ]
             ; mkTheta (stupid_constraints ++ extra_constraints
                         ++ sc_constraints ++ arg_constraints)
                       tvs' inst_tys' }
      where
        tc_binders = tyConBinders rep_tc
        choose_level bndr
          | isNamedBinder bndr = KindLevel
          | otherwise          = TypeLevel
        t_or_ks = map choose_level tc_binders ++ repeat TypeLevel
           -- want to report *kind* errors when possible

           -- Constraints arising from the arguments of each constructor
        con_arg_constraints :: (CtOrigin -> TypeOrKind
                                         -> Type
                                         -> [(ThetaOrigin, Maybe TCvSubst)])
                            -> (ThetaOrigin -> [TyVar] -> [TcType] -> TcM a)
                            -> TcM a
        con_arg_constraints get_arg_constraints mkTheta
          = let (predss, mbSubsts) = unzip
                  [ preds_and_mbSubst
                  | data_con <- tyConDataCons rep_tc
                  , (arg_n, arg_t_or_k, arg_ty)
                      <- zip3 [1..] t_or_ks $
                         dataConInstOrigArgTys data_con all_rep_tc_args
                    -- No constraints for unlifted types
                    -- See Note [Deriving and unboxed types]
                  , not (isUnliftedType arg_ty)
                  , let orig = DerivOriginDC data_con arg_n
                  , preds_and_mbSubst <- get_arg_constraints orig arg_t_or_k arg_ty
                  ]
                preds = concat predss
                -- If the constraints require a subtype to be of kind (* -> *)
                -- (which is the case for functor-like constraints), then we
                -- explicitly unify the subtype's kinds with (* -> *).
                -- See Note [Inferring the instance context]
                subst          = foldl' composeTCvSubst
                                        emptyTCvSubst (catMaybes mbSubsts)
                subst_range    = getTCvSubstRangeFVs subst
                unmapped_tvs   = filter (\v -> v `notElemTCvSubst` subst
                                       && not (v `elemVarSet` subst_range)) tvs
                (subst', _)    = mapAccumL substTyVarBndr subst unmapped_tvs
                preds'         = substThetaOrigin subst' preds
                inst_tys'      = substTys subst' inst_tys
                tvs'           = tyCoVarsOfTypesWellScoped inst_tys'
            in mkTheta preds' tvs' inst_tys'

        -- is_functor_like: see Note [Inferring the instance context]
        is_functor_like = typeKind inst_ty `tcEqKind` typeToTypeKind

        get_gen1_constraints :: Class -> CtOrigin -> TypeOrKind -> Type
                             -> [(ThetaOrigin, Maybe TCvSubst)]
        get_gen1_constraints functor_cls orig t_or_k ty
           = mk_functor_like_constraints orig t_or_k functor_cls $
             get_gen1_constrained_tys last_tv ty

        get_std_constrained_tys :: CtOrigin -> TypeOrKind -> Type
                                -> [(ThetaOrigin, Maybe TCvSubst)]
        get_std_constrained_tys orig t_or_k ty
            | is_functor_like = mk_functor_like_constraints orig t_or_k main_cls $
                                deepSubtypesContaining last_tv ty
            | otherwise       = [( [mk_cls_pred orig t_or_k main_cls ty]
                                 , Nothing )]

        mk_functor_like_constraints :: CtOrigin -> TypeOrKind
                                    -> Class -> [Type]
                                    -> [(ThetaOrigin, Maybe TCvSubst)]
        -- 'cls' is usually main_cls (Functor or Traversable etc), but if
        -- main_cls = Generic1, then 'cls' can be Functor; see get_gen1_constraints
        --
        -- For each type, generate two constraints, [cls ty, kind(ty) ~ (*->*)],
        -- and a kind substitution that results from unifying kind(ty) with * -> *.
        -- If the unification is successful, it will ensure that the resulting
        -- instance is well kinded. If not, the second constraint will result
        -- in an error message which points out the kind mismatch.
        -- See Note [Inferring the instance context]
        mk_functor_like_constraints orig t_or_k cls
           = map $ \ty -> let ki = typeKind ty in
                          ( [ mk_cls_pred orig t_or_k cls ty
                            , mkPredOrigin orig KindLevel
                                (mkPrimEqPred ki typeToTypeKind) ]
                          , tcUnifyTy ki typeToTypeKind
                          )

        rep_tc_tvs      = tyConTyVars rep_tc
        last_tv         = last rep_tc_tvs
        all_rep_tc_args | is_functor_like = rep_tc_args ++ [mkTyVarTy last_tv]
                        | otherwise       = rep_tc_args

            -- Constraints arising from superclasses
            -- See Note [Superclasses of derived instance]
        cls_tvs  = classTyVars main_cls
        inst_tys = cls_tys ++ [inst_ty]
        sc_constraints = ASSERT2( equalLength cls_tvs inst_tys, ppr main_cls <+> ppr rep_tc)
                         mkThetaOrigin DerivOrigin TypeLevel $
                         substTheta cls_subst (classSCTheta main_cls)
        cls_subst = ASSERT( equalLength cls_tvs inst_tys )
                    zipTvSubst cls_tvs inst_tys

            -- Stupid constraints
        stupid_constraints = mkThetaOrigin DerivOrigin TypeLevel $
                             substTheta tc_subst (tyConStupidTheta rep_tc)
        tc_subst = ASSERT( equalLength rep_tc_tvs all_rep_tc_args )
                   zipTvSubst rep_tc_tvs all_rep_tc_args

            -- Extra Data constraints
            -- The Data class (only) requires that for
            --    instance (...) => Data (T t1 t2)
            -- IF   t1:*, t2:*
            -- THEN (Data t1, Data t2) are among the (...) constraints
            -- Reason: when the IF holds, we generate a method
            --             dataCast2 f = gcast2 f
            --         and we need the Data constraints to typecheck the method
        extra_constraints
          | main_cls `hasKey` dataClassKey
          , all (isLiftedTypeKind . typeKind) rep_tc_args
          = [ mk_cls_pred DerivOrigin t_or_k main_cls ty
            | (t_or_k, ty) <- zip t_or_ks rep_tc_args]
          | otherwise
          = []

        mk_cls_pred orig t_or_k cls ty   -- Don't forget to apply to cls_tys too
                                  -- In the awkward Generic1 casde, cls_tys is empty
           = mkPredOrigin orig t_or_k (mkClassPred cls (cls_tys ++ [ty]))

    {- Note [Getting base classes]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Functor and Typeable are defined in package 'base', and that is not available
    when compiling 'ghc-prim'.  So we must be careful that 'deriving' for stuff in
    ghc-prim does not use Functor or Typeable implicitly via these lookups.

    Note [Deriving and unboxed types]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We have some special hacks to support things like
       data T = MkT Int# deriving ( Show )

    Specifically, we use TcGenDeriv.box to box the Int# into an Int
    (which we know how to show), and append a '#'. Parenthesis are not required
    for unboxed values (`MkT -3#` is a valid expression).

    Note [Deriving any class]
    ~~~~~~~~~~~~~~~~~~~~~~~~~
    Classic uses of a deriving clause, or a standalone-deriving declaration, are
    for:
      * a built-in class like Eq or Show, for which GHC knows how to generate
        the instance code
      * a newtype, via the mechanism enabled by GeneralizedNewtypeDeriving

    The DeriveAnyClass extension adds a third way to derive instances, based on
    empty instance declarations.

    The canonical use case is in combination with GHC.Generics and default method
    signatures. These allow us to have instance declarations being empty, but still
    useful, e.g.

      data T a = ...blah..blah... deriving( Generic )
      instance C a => C (T a)  -- No 'where' clause

    where C is some "random" user-defined class.

    This boilerplate code can be replaced by the more compact

      data T a = ...blah..blah... deriving( Generic, C )

    if DeriveAnyClass is enabled.

    This is not restricted to Generics; any class can be derived, simply giving
    rise to an empty instance.

    Unfortunately, it is not clear how to determine the context (in case of
    standard deriving; in standalone deriving, the user provides the context).
    GHC uses the same heuristic for figuring out the class context that it uses for
    Eq in the case of *-kinded classes, and for Functor in the case of
    * -> *-kinded classes. That may not be optimal or even wrong. But in such
    cases, standalone deriving can still be used.
    -}

    ------------------------------------------------------------------
    -- Check side conditions that dis-allow derivability for particular classes
    -- This is *apart* from the newtype-deriving mechanism
    --
    -- Here we get the representation tycon in case of family instances as it has
    -- the data constructors - but we need to be careful to fall back to the
    -- family tycon (with indexes) in error messages.

    data DerivStatus = CanDerive                 -- Standard class, can derive
                     | DerivableClassError SDoc  -- Standard class, but can't do it
                     | DerivableViaInstance      -- See Note [Deriving any class]
                     | NonDerivableClass SDoc    -- Non-standard class

    -- A "standard" class is one defined in the Haskell report which GHC knows how
    -- to generate code for, such as Eq, Ord, Ix, etc.

    checkSideConditions :: DynFlags -> DerivContext -> Class -> [TcType]
                        -> TyCon -- tycon
                        -> DerivStatus
    checkSideConditions dflags mtheta cls cls_tys rep_tc
      | Just cond <- sideConditions mtheta cls
      = case (cond (dflags, rep_tc)) of
            NotValid err -> DerivableClassError err  -- Class-specific error
            IsValid  | null cls_tys -> CanDerive     -- All derivable classes are unary, so
                                                     -- cls_tys (the type args other than last)
                                                     -- should be null
                     | otherwise    -> DerivableClassError (classArgsErr cls cls_tys)  -- e.g. deriving( Eq s )

      | Just err <- canDeriveAnyClass dflags rep_tc cls
      = NonDerivableClass err  -- DeriveAnyClass does not work

      | otherwise
      = DerivableViaInstance   -- DeriveAnyClass should work

    classArgsErr :: Class -> [Type] -> SDoc
    classArgsErr cls cls_tys = quotes (ppr (mkClassPred cls cls_tys)) <+> text "is not a class"

    nonUnaryErr :: LHsSigType Name -> SDoc
    nonUnaryErr ct = quotes (ppr ct)
      <+> text "is not a unary constraint, as expected by a deriving clause"

    nonStdErr :: Class -> SDoc
    nonStdErr cls =
          quotes (ppr cls)
      <+> text "is not a standard derivable class (Eq, Show, etc.)"

    sideConditions :: DerivContext -> Class -> Maybe Condition
    -- Side conditions for classes that GHC knows about,
    -- that is, "deriviable classes"
    -- Returns Nothing for a non-derivable class
    sideConditions mtheta cls
      | cls_key == eqClassKey          = Just (cond_std `andCond` cond_args cls)
      | cls_key == ordClassKey         = Just (cond_std `andCond` cond_args cls)
      | cls_key == showClassKey        = Just (cond_std `andCond` cond_args cls)
      | cls_key == readClassKey        = Just (cond_std `andCond` cond_args cls)
      | cls_key == enumClassKey        = Just (cond_std `andCond` cond_isEnumeration)
      | cls_key == ixClassKey          = Just (cond_std `andCond` cond_enumOrProduct cls)
      | cls_key == boundedClassKey     = Just (cond_std `andCond` cond_enumOrProduct cls)
      | cls_key == dataClassKey        = Just (checkFlag LangExt.DeriveDataTypeable `andCond`
                                               cond_std `andCond`
                                               cond_args cls)
      | cls_key == functorClassKey     = Just (checkFlag LangExt.DeriveFunctor `andCond`
                                               cond_vanilla `andCond`
                                               cond_functorOK True False)
      | cls_key == foldableClassKey    = Just (checkFlag LangExt.DeriveFoldable `andCond`
                                               cond_vanilla `andCond`
                                               cond_functorOK False True)
                                               -- Functor/Fold/Trav works ok
                                               -- for rank-n types
      | cls_key == traversableClassKey = Just (checkFlag LangExt.DeriveTraversable `andCond`
                                               cond_vanilla `andCond`
                                               cond_functorOK False False)
      | cls_key == genClassKey         = Just (checkFlag LangExt.DeriveGeneric `andCond`
                                               cond_vanilla `andCond`
                                               cond_RepresentableOk)
      | cls_key == gen1ClassKey        = Just (checkFlag LangExt.DeriveGeneric `andCond`
                                               cond_vanilla `andCond`
                                               cond_Representable1Ok)
      | cls_key == liftClassKey        = Just (checkFlag LangExt.DeriveLift `andCond`
                                               cond_vanilla `andCond`
                                               cond_args cls)
      | otherwise                      = Nothing
      where
        cls_key = getUnique cls
        cond_std     = cond_stdOK mtheta False  -- Vanilla data constructors, at least one,
                                                --    and monotype arguments
        cond_vanilla = cond_stdOK mtheta True   -- Vanilla data constructors but
                                                --   allow no data cons or polytype arguments

    canDeriveAnyClass :: DynFlags -> TyCon -> Class -> Maybe SDoc
    -- Nothing: we can (try to) derive it via an empty instance declaration
    -- Just s:  we can't, reason s
    -- Precondition: the class is not one of the standard ones
    canDeriveAnyClass dflags _tycon clas
      | not (xopt LangExt.DeriveAnyClass dflags)
      = Just (text "Try enabling DeriveAnyClass")
      | not (any (target_kind `tcEqKind`) [ liftedTypeKind, typeToTypeKind ])
      = Just (text "The last argument of class" <+> quotes (ppr clas)
              <+> text "does not have kind * or (* -> *)")
      | otherwise
      = Nothing   -- OK!
      where
        -- We are making an instance  (C t1 .. tn (T s1 .. sm))
        -- and we can only do so if the kind of C's last argument
        -- is * or (* -> *).  Because only then can we make a reasonable
        -- guess at the instance context
        target_kind = tyVarKind (last (classTyVars clas))

    typeToTypeKind :: Kind
    typeToTypeKind = liftedTypeKind `mkFunTy` liftedTypeKind

    type Condition = (DynFlags, TyCon) -> Validity
            -- TyCon is the *representation* tycon if the data type is an indexed one
            -- Nothing => OK

    orCond :: Condition -> Condition -> Condition
    orCond c1 c2 tc
      = case (c1 tc, c2 tc) of
         (IsValid,    _)          -> IsValid    -- c1 succeeds
         (_,          IsValid)    -> IsValid    -- c21 succeeds
         (NotValid x, NotValid y) -> NotValid (x $$ text "  or" $$ y)
                                                -- Both fail

    andCond :: Condition -> Condition -> Condition
    andCond c1 c2 tc = c1 tc `andValid` c2 tc

    cond_stdOK :: DerivContext -- Says whether this is standalone deriving or not;
                               --     if standalone, we just say "yes, go for it"
               -> Bool         -- True <=> permissive: allow higher rank
                               --          args and no data constructors
               -> Condition
    cond_stdOK (Just _) _ _
      = IsValid     -- Don't check these conservative conditions for
                    -- standalone deriving; just generate the code
                    -- and let the typechecker handle the result
    cond_stdOK Nothing permissive (_, rep_tc)
      | null data_cons
      , not permissive      = NotValid (no_cons_why rep_tc $$ suggestion)
      | not (null con_whys) = NotValid (vcat con_whys $$ suggestion)
      | otherwise           = IsValid
      where
        suggestion = text "Possible fix: use a standalone deriving declaration instead"
        data_cons  = tyConDataCons rep_tc
        con_whys   = getInvalids (map check_con data_cons)

        check_con :: DataCon -> Validity
        check_con con
          | not (isVanillaDataCon con)
          = NotValid (badCon con (text "has existentials or constraints in its type"))
          | not (permissive || all isTauTy (dataConOrigArgTys con))
          = NotValid (badCon con (text "has a higher-rank type"))
          | otherwise
          = IsValid

    no_cons_why :: TyCon -> SDoc
    no_cons_why rep_tc = quotes (pprSourceTyCon rep_tc) <+>
                         text "must have at least one data constructor"

    cond_RepresentableOk :: Condition
    cond_RepresentableOk (_, tc) = canDoGenerics tc

    cond_Representable1Ok :: Condition
    cond_Representable1Ok (_, tc) = canDoGenerics1 tc

    cond_enumOrProduct :: Class -> Condition
    cond_enumOrProduct cls = cond_isEnumeration `orCond`
                             (cond_isProduct `andCond` cond_args cls)

    cond_args :: Class -> Condition
    -- For some classes (eg Eq, Ord) we allow unlifted arg types
    -- by generating specialised code.  For others (eg Data) we don't.
    cond_args cls (_, tc)
      = case bad_args of
          []     -> IsValid
          (ty:_) -> NotValid (hang (text "Don't know how to derive" <+> quotes (ppr cls))
                                 2 (text "for type" <+> quotes (ppr ty)))
      where
        bad_args = [ arg_ty | con <- tyConDataCons tc
                            , arg_ty <- dataConOrigArgTys con
                            , isUnliftedType arg_ty
                            , not (ok_ty arg_ty) ]

        cls_key = classKey cls
        ok_ty arg_ty
         | cls_key == eqClassKey   = check_in arg_ty ordOpTbl
         | cls_key == ordClassKey  = check_in arg_ty ordOpTbl
         | cls_key == showClassKey = check_in arg_ty boxConTbl
         | cls_key == liftClassKey = check_in arg_ty litConTbl
         | otherwise               = False    -- Read, Ix etc

        check_in :: Type -> [(Type,a)] -> Bool
        check_in arg_ty tbl = any (eqType arg_ty . fst) tbl

    cond_isEnumeration :: Condition
    cond_isEnumeration (_, rep_tc)
      | isEnumerationTyCon rep_tc = IsValid
      | otherwise                 = NotValid why
      where
        why = sep [ quotes (pprSourceTyCon rep_tc) <+>
                      text "must be an enumeration type"
                  , text "(an enumeration consists of one or more nullary, non-GADT constructors)" ]
                      -- See Note [Enumeration types] in TyCon

    cond_isProduct :: Condition
    cond_isProduct (_, rep_tc)
      | isProductTyCon rep_tc = IsValid
      | otherwise             = NotValid why
      where
        why = quotes (pprSourceTyCon rep_tc) <+>
              text "must have precisely one constructor"

    cond_functorOK :: Bool -> Bool -> Condition
    -- OK for Functor/Foldable/Traversable class
    -- Currently: (a) at least one argument
    --            (b) don't use argument contravariantly
    --            (c) don't use argument in the wrong place, e.g. data T a = T (X a a)
    --            (d) optionally: don't use function types
    --            (e) no "stupid context" on data type
    cond_functorOK allowFunctions allowExQuantifiedLastTyVar (_, rep_tc)
      | null tc_tvs
      = NotValid (text "Data type" <+> quotes (ppr rep_tc)
                  <+> text "must have some type parameters")

      | not (null bad_stupid_theta)
      = NotValid (text "Data type" <+> quotes (ppr rep_tc)
                  <+> text "must not have a class context:" <+> pprTheta bad_stupid_theta)

      | otherwise
      = allValid (map check_con data_cons)
      where
        tc_tvs            = tyConTyVars rep_tc
        Just (_, last_tv) = snocView tc_tvs
        bad_stupid_theta  = filter is_bad (tyConStupidTheta rep_tc)
        is_bad pred       = last_tv `elemVarSet` tyCoVarsOfType pred

        data_cons = tyConDataCons rep_tc
        check_con con = allValid (check_universal con : foldDataConArgs (ft_check con) con)

        check_universal :: DataCon -> Validity
        check_universal con
          | allowExQuantifiedLastTyVar
          = IsValid -- See Note [DeriveFoldable with ExistentialQuantification]
                    -- in TcGenDeriv
          | Just tv <- getTyVar_maybe (last (tyConAppArgs (dataConOrigResTy con)))
          , tv `elem` dataConUnivTyVars con
          , not (tv `elemVarSet` tyCoVarsOfTypes (dataConTheta con))
          = IsValid   -- See Note [Check that the type variable is truly universal]
          | otherwise
          = NotValid (badCon con existential)

        ft_check :: DataCon -> FFoldType Validity
        ft_check con = FT { ft_triv = IsValid, ft_var = IsValid
                          , ft_co_var = NotValid (badCon con covariant)
                          , ft_fun = \x y -> if allowFunctions then x `andValid` y
                                                               else NotValid (badCon con functions)
                          , ft_tup = \_ xs  -> allValid xs
                          , ft_ty_app = \_ x   -> x
                          , ft_bad_app = NotValid (badCon con wrong_arg)
                          , ft_forall = \_ x   -> x }

        existential = text "must be truly polymorphic in the last argument of the data type"
        covariant   = text "must not use the type variable in a function argument"
        functions   = text "must not contain function types"
        wrong_arg   = text "must use the type variable only as the last argument of a data type"

    checkFlag :: LangExt.Extension -> Condition
    checkFlag flag (dflags, _)
      | xopt flag dflags = IsValid
      | otherwise        = NotValid why
      where
        why = text "You need " <> text flag_str
              <+> text "to derive an instance for this class"
        flag_str = case [ flagSpecName f | f <- xFlags , flagSpecFlag f == flag ] of
                     [s]   -> s
                     other -> pprPanic "checkFlag" (ppr other)

    std_class_via_coercible :: Class -> Bool
    -- These standard classes can be derived for a newtype
    -- using the coercible trick *even if no -XGeneralizedNewtypeDeriving
    -- because giving so gives the same results as generating the boilerplate
    std_class_via_coercible clas
      = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
            -- Not Read/Show/Lift because they respect the type
            -- Not Enum, because newtypes are never in Enum

    non_coercible_class :: Class -> Bool
    -- *Never* derive Read, Show, Typeable, Data, Generic, Generic1, Lift
    -- by Coercible, even with -XGeneralizedNewtypeDeriving
    -- Also, avoid Traversable, as the Coercible-derived instance and the "normal"-derived
    -- instance behave differently if there's a non-lawful Applicative out there.
    -- Besides, with roles, Coercible-deriving Traversable is ill-roled.
    non_coercible_class cls
      = classKey cls `elem` ([ readClassKey, showClassKey, dataClassKey
                             , genClassKey, gen1ClassKey, typeableClassKey
                             , traversableClassKey, liftClassKey ])

    badCon :: DataCon -> SDoc -> SDoc
    badCon con msg = text "Constructor" <+> quotes (ppr con) <+> msg

    {-
    Note [Check that the type variable is truly universal]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    For Functor and Traversable instances, we must check that the *last argument*
    of the type constructor is used truly universally quantified.  Example

       data T a b where
         T1 :: a -> b -> T a b      -- Fine! Vanilla H-98
         T2 :: b -> c -> T a b      -- Fine! Existential c, but we can still map over 'b'
         T3 :: b -> T Int b         -- Fine! Constraint 'a', but 'b' is still polymorphic
         T4 :: Ord b => b -> T a b  -- No!  'b' is constrained
         T5 :: b -> T b b           -- No!  'b' is constrained
         T6 :: T a (b,b)            -- No!  'b' is constrained

    Notice that only the first of these constructors is vanilla H-98. We only
    need to take care about the last argument (b in this case).  See Trac #8678.
    Eg. for T1-T3 we can write

         fmap f (T1 a b) = T1 a (f b)
         fmap f (T2 b c) = T2 (f b) c
         fmap f (T3 x)   = T3 (f x)

    We need not perform these checks for Foldable instances, however, since
    functions in Foldable can only consume existentially quantified type variables,
    rather than produce them (as is the case in Functor and Traversable functions.)
    As a result, T can have a derived Foldable instance:

        foldr f z (T1 a b) = f b z
        foldr f z (T2 b c) = f b z
        foldr f z (T3 x)   = f x z
        foldr f z (T4 x)   = f x z
        foldr f z (T5 x)   = f x z
        foldr _ z T6       = z

    See Note [DeriveFoldable with ExistentialQuantification] in TcGenDeriv.

    Note [Superclasses of derived instance]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In general, a derived instance decl needs the superclasses of the derived
    class too.  So if we have
            data T a = ...deriving( Ord )
    then the initial context for Ord (T a) should include Eq (T a).  Often this is
    redundant; we'll also generate an Ord constraint for each constructor argument,
    and that will probably generate enough constraints to make the Eq (T a) constraint
    be satisfied too.  But not always; consider:

     data S a = S
     instance Eq (S a)
     instance Ord (S a)

     data T a = MkT (S a) deriving( Ord )
     instance Num a => Eq (T a)

    The derived instance for (Ord (T a)) must have a (Num a) constraint!
    Similarly consider:
            data T a = MkT deriving( Data, Typeable )
    Here there *is* no argument field, but we must nevertheless generate
    a context for the Data instances:
            instance Typable a => Data (T a) where ...

    ************************************************************************
    *                                                                      *
                    Deriving newtypes
    *                                                                      *
    ************************************************************************
    -}

    mkNewTypeEqn :: DynFlags -> Maybe OverlapMode -> [TyVar] -> Class
                 -> [Type] -> TyCon -> [Type] -> TyCon -> [Type]
                 -> DerivContext
                 -> TcRn EarlyDerivSpec
    mkNewTypeEqn dflags overlap_mode tvs
                 cls cls_tys tycon tc_args rep_tycon rep_tc_args mtheta
    -- Want: instance (...) => cls (cls_tys ++ [tycon tc_args]) where ...
      | ASSERT( length cls_tys + 1 == classArity cls )
        might_derive_via_coercible && ((newtype_deriving && not deriveAnyClass)
                                      || std_class_via_coercible cls)
      = do traceTc "newtype deriving:" (ppr tycon <+> ppr rep_tys <+> ppr all_preds)
           dfun_name <- newDFunName' cls tycon
           loc <- getSrcSpanM
           case mtheta of
            Just theta -> return $ GivenTheta $ DS
                { ds_loc = loc
                , ds_name = dfun_name, ds_tvs = dfun_tvs
                , ds_cls = cls, ds_tys = inst_tys
                , ds_tc = rep_tycon
                , ds_theta = theta
                , ds_overlap = overlap_mode
                , ds_newtype = Just rep_inst_ty }
            Nothing -> return $ InferTheta $ DS
                { ds_loc = loc
                , ds_name = dfun_name, ds_tvs = dfun_tvs
                , ds_cls = cls, ds_tys = inst_tys
                , ds_tc = rep_tycon
                , ds_theta = all_preds
                , ds_overlap = overlap_mode
                , ds_newtype = Just rep_inst_ty }
      | otherwise
      = case checkSideConditions dflags mtheta cls cls_tys rep_tycon of
          -- Error with standard class
          DerivableClassError msg
            | might_derive_via_coercible -> bale_out (msg $$ suggest_gnd)
            | otherwise                  -> bale_out msg

          -- Must use newtype deriving or DeriveAnyClass
          NonDerivableClass _msg
            -- Too hard, even with newtype deriving
            | newtype_deriving           -> bale_out cant_derive_err
            -- Try newtype deriving!
            -- Here we suggest GeneralizedNewtypeDeriving even in cases where it may
            -- not be applicable. See Trac #9600.
            | otherwise                  -> bale_out (non_std $$ suggest_gnd)

          -- CanDerive/DerivableViaInstance
          _ -> do when (newtype_deriving && deriveAnyClass) $
                    addWarnTc NoReason
                              (sep [ text "Both DeriveAnyClass and GeneralizedNewtypeDeriving are enabled"
                                   , text "Defaulting to the DeriveAnyClass strategy for instantiating" <+> ppr cls ])
                  go_for_it
      where
            newtype_deriving  = xopt LangExt.GeneralizedNewtypeDeriving dflags
            deriveAnyClass    = xopt LangExt.DeriveAnyClass             dflags
            go_for_it         = mk_data_eqn overlap_mode tvs cls cls_tys tycon tc_args
                                  rep_tycon rep_tc_args mtheta
            bale_out    = bale_out' newtype_deriving
            bale_out' b = failWithTc . derivingThingErr b cls cls_tys inst_ty

            non_std     = nonStdErr cls
            suggest_gnd = text "Try GeneralizedNewtypeDeriving for GHC's newtype-deriving extension"

            -- Here is the plan for newtype derivings.  We see
            --        newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
            -- where t is a type,
            --       ak+1...an is a suffix of a1..an, and are all tyars
            --       ak+1...an do not occur free in t, nor in the s1..sm
            --       (C s1 ... sm) is a  *partial applications* of class C
            --                      with the last parameter missing
            --       (T a1 .. ak) matches the kind of C's last argument
            --              (and hence so does t)
            -- The latter kind-check has been done by deriveTyData already,
            -- and tc_args are already trimmed
            --
            -- We generate the instance
            --       instance forall ({a1..ak} u fvs(s1..sm)).
            --                C s1 .. sm t => C s1 .. sm (T a1...ak)
            -- where T a1...ap is the partial application of
            --       the LHS of the correct kind and p >= k
            --
            --      NB: the variables below are:
            --              tc_tvs = [a1, ..., an]
            --              tyvars_to_keep = [a1, ..., ak]
            --              rep_ty = t ak .. an
            --              deriv_tvs = fvs(s1..sm) \ tc_tvs
            --              tys = [s1, ..., sm]
            --              rep_fn' = t
            --
            -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
            -- We generate the instance
            --      instance Monad (ST s) => Monad (T s) where

            nt_eta_arity = newTyConEtadArity rep_tycon
                    -- For newtype T a b = MkT (S a a b), the TyCon machinery already
                    -- eta-reduces the representation type, so we know that
                    --      T a ~ S a a
                    -- That's convenient here, because we may have to apply
                    -- it to fewer than its original complement of arguments

            -- Note [Newtype representation]
            -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
            -- Need newTyConRhs (*not* a recursive representation finder)
            -- to get the representation type. For example
            --      newtype B = MkB Int
            --      newtype A = MkA B deriving( Num )
            -- We want the Num instance of B, *not* the Num instance of Int,
            -- when making the Num instance of A!
            rep_inst_ty = newTyConInstRhs rep_tycon rep_tc_args
            rep_tys     = cls_tys ++ [rep_inst_ty]
            rep_pred    = mkClassPred cls rep_tys
            rep_pred_o  = mkPredOrigin DerivOrigin TypeLevel rep_pred
                    -- rep_pred is the representation dictionary, from where
                    -- we are gong to get all the methods for the newtype
                    -- dictionary

            -- Next we figure out what superclass dictionaries to use
            -- See Note [Newtype deriving superclasses] above
            cls_tyvars = classTyVars cls
            dfun_tvs   = tyCoVarsOfTypesWellScoped inst_tys
            inst_ty    = mkTyConApp tycon tc_args
            inst_tys   = cls_tys ++ [inst_ty]
            sc_theta   = mkThetaOrigin DerivOrigin TypeLevel $
                         substTheta (zipTvSubst cls_tyvars inst_tys) $
                         classSCTheta cls

            -- Next we collect Coercible constraints between
            -- the Class method types, instantiated with the representation and the
            -- newtype type; precisely the constraints required for the
            -- calls to coercible that we are going to generate.
            coercible_constraints =
                [ let (Pair t1 t2) = mkCoerceClassMethEqn cls dfun_tvs inst_tys rep_inst_ty meth
                  in mkPredOrigin (DerivOriginCoerce meth t1 t2) TypeLevel
                                  (mkReprPrimEqPred t1 t2)
                | meth <- classMethods cls ]

                    -- If there are no tyvars, there's no need
                    -- to abstract over the dictionaries we need
                    -- Example:     newtype T = MkT Int deriving( C )
                    -- We get the derived instance
                    --              instance C T
                    -- rather than
                    --              instance C Int => C T
            all_preds = rep_pred_o : coercible_constraints ++ sc_theta -- NB: rep_pred comes first

            -------------------------------------------------------------------
            --  Figuring out whether we can only do this newtype-deriving thing

            -- See Note [Determining whether newtype-deriving is appropriate]
            might_derive_via_coercible
               =  not (non_coercible_class cls)
               && eta_ok
               && ats_ok
    --         && not (isRecursiveTyCon tycon)      -- Note [Recursive newtypes]

            -- Check that eta reduction is OK
            eta_ok = nt_eta_arity <= length rep_tc_args
                    -- The newtype can be eta-reduced to match the number
                    --     of type argument actually supplied
                    --        newtype T a b = MkT (S [a] b) deriving( Monad )
                    --     Here the 'b' must be the same in the rep type (S [a] b)
                    --     And the [a] must not mention 'b'.  That's all handled
                    --     by nt_eta_rity.

            ats_ok = null (classATs cls)
                   -- No associated types for the class, because we don't
                   -- currently generate type 'instance' decls; and cannot do
                   -- so for 'data' instance decls

            cant_derive_err
               = vcat [ ppUnless eta_ok eta_msg
                      , ppUnless ats_ok ats_msg ]
            eta_msg   = text "cannot eta-reduce the representation type enough"
            ats_msg   = text "the class has associated types"

    {-
    Note [Recursive newtypes]
    ~~~~~~~~~~~~~~~~~~~~~~~~~
    Newtype deriving works fine, even if the newtype is recursive.
    e.g.    newtype S1 = S1 [T1 ()]
            newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
    Remember, too, that type families are currently (conservatively) given
    a recursive flag, so this also allows newtype deriving to work
    for type famillies.

    We used to exclude recursive types, because we had a rather simple
    minded way of generating the instance decl:
       newtype A = MkA [A]
       instance Eq [A] => Eq A      -- Makes typechecker loop!
    But now we require a simple context, so it's ok.

    Note [Determining whether newtype-deriving is appropriate]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When we see
      newtype NT = MkNT Foo
        deriving C
    we have to decide how to perform the deriving. Do we do newtype deriving,
    or do we do normal deriving? In general, we prefer to do newtype deriving
    wherever possible. So, we try newtype deriving unless there's a glaring
    reason not to.

    Note that newtype deriving might fail, even after we commit to it. This
    is because the derived instance uses `coerce`, which must satisfy its
    `Coercible` constraint. This is different than other deriving scenarios,
    where we're sure that the resulting instance will type-check.

    ************************************************************************
    *                                                                      *
             Finding the fixed point of deriving equations
    *                                                                      *
    ************************************************************************

    Note [Simplifying the instance context]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider

            data T a b = C1 (Foo a) (Bar b)
                       | C2 Int (T b a)
                       | C3 (T a a)
                       deriving (Eq)

    We want to come up with an instance declaration of the form

            instance (Ping a, Pong b, ...) => Eq (T a b) where
                    x == y = ...

    It is pretty easy, albeit tedious, to fill in the code "...".  The
    trick is to figure out what the context for the instance decl is,
    namely Ping, Pong and friends.

    Let's call the context reqd for the T instance of class C at types
    (a,b, ...)  C (T a b).  Thus:

            Eq (T a b) = (Ping a, Pong b, ...)

    Now we can get a (recursive) equation from the data decl.  This part
    is done by inferConstraints.

            Eq (T a b) = Eq (Foo a) u Eq (Bar b)    -- From C1
                       u Eq (T b a) u Eq Int        -- From C2
                       u Eq (T a a)                 -- From C3

    Foo and Bar may have explicit instances for Eq, in which case we can
    just substitute for them.  Alternatively, either or both may have
    their Eq instances given by deriving clauses, in which case they
    form part of the system of equations.

    Now all we need do is simplify and solve the equations, iterating to
    find the least fixpoint.  This is done by simplifyInstanceConstraints.
    Notice that the order of the arguments can
    switch around, as here in the recursive calls to T.

    Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.

    We start with:

            Eq (T a b) = {}         -- The empty set

    Next iteration:
            Eq (T a b) = Eq (Foo a) u Eq (Bar b)    -- From C1
                       u Eq (T b a) u Eq Int        -- From C2
                       u Eq (T a a)                 -- From C3

            After simplification:
                       = Eq a u Ping b u {} u {} u {}
                       = Eq a u Ping b

    Next iteration:

            Eq (T a b) = Eq (Foo a) u Eq (Bar b)    -- From C1
                       u Eq (T b a) u Eq Int        -- From C2
                       u Eq (T a a)                 -- From C3

            After simplification:
                       = Eq a u Ping b
                       u (Eq b u Ping a)
                       u (Eq a u Ping a)

                       = Eq a u Ping b u Eq b u Ping a

    The next iteration gives the same result, so this is the fixpoint.  We
    need to make a canonical form of the RHS to ensure convergence.  We do
    this by simplifying the RHS to a form in which

            - the classes constrain only tyvars
            - the list is sorted by tyvar (major key) and then class (minor key)
            - no duplicates, of course

    Note [Deterministic simplifyInstanceContexts]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Canonicalisation uses cmpType which is nondeterministic. Sorting
    with cmpType puts the returned lists in a nondeterministic order.
    If we were to return them, we'd get class constraints in
    nondeterministic order.

    Consider:

      data ADT a b = Z a b deriving Eq

    The generated code could be either:

      instance (Eq a, Eq b) => Eq (Z a b) where

    Or:

      instance (Eq b, Eq a) => Eq (Z a b) where

    To prevent the order from being nondeterministic we only
    canonicalize when comparing and return them in the same order as
    simplifyDeriv returned them.
    See also Note [cmpType nondeterminism]
    -}

    simplifyInstanceContexts :: [DerivSpec ThetaOrigin] -> TcM [DerivSpec ThetaType]
    -- Used only for deriving clauses (InferTheta)
    -- not for standalone deriving
    -- See Note [Simplifying the instance context]

    simplifyInstanceContexts [] = return []

    simplifyInstanceContexts infer_specs
      = do  { traceTc "simplifyInstanceContexts" $ vcat (map pprDerivSpec infer_specs)
            ; iterate_deriv 1 initial_solutions }
      where
        ------------------------------------------------------------------
            -- The initial solutions for the equations claim that each
            -- instance has an empty context; this solution is certainly
            -- in canonical form.
        initial_solutions :: [ThetaType]
        initial_solutions = [ [] | _ <- infer_specs ]

        ------------------------------------------------------------------
            -- iterate_deriv calculates the next batch of solutions,
            -- compares it with the current one; finishes if they are the
            -- same, otherwise recurses with the new solutions.
            -- It fails if any iteration fails
        iterate_deriv :: Int -> [ThetaType] -> TcM [DerivSpec ThetaType]
        iterate_deriv n current_solns
          | n > 20  -- Looks as if we are in an infinite loop
                    -- This can happen if we have -XUndecidableInstances
                    -- (See TcSimplify.tcSimplifyDeriv.)
          = pprPanic "solveDerivEqns: probable loop"
                     (vcat (map pprDerivSpec infer_specs) $$ ppr current_solns)
          | otherwise
          = do {      -- Extend the inst info from the explicit instance decls
                      -- with the current set of solutions, and simplify each RHS
                 inst_specs <- zipWithM newDerivClsInst current_solns infer_specs
               ; new_solns <- checkNoErrs $
                              extendLocalInstEnv inst_specs $
                              mapM gen_soln infer_specs

               ; if (current_solns `eqSolution` new_solns) then
                    return [ spec { ds_theta = soln }
                           | (spec, soln) <- zip infer_specs current_solns ]
                 else
                    iterate_deriv (n+1) new_solns }

        eqSolution a b = eqListBy (eqListBy eqType) (canSolution a) (canSolution b)
           -- Canonicalise for comparison
           -- See Note [Deterministic simplifyInstanceContexts]
        canSolution = map (sortBy cmpType)
        ------------------------------------------------------------------
        gen_soln :: DerivSpec ThetaOrigin -> TcM ThetaType
        gen_soln (DS { ds_loc = loc, ds_tvs = tyvars
                     , ds_cls = clas, ds_tys = inst_tys, ds_theta = deriv_rhs })
          = setSrcSpan loc  $
            addErrCtxt (derivInstCtxt the_pred) $
            do { theta <- simplifyDeriv the_pred tyvars deriv_rhs
                    -- checkValidInstance tyvars theta clas inst_tys
                    -- Not necessary; see Note [Exotic derived instance contexts]

               ; traceTc "TcDeriv" (ppr deriv_rhs $$ ppr theta)
                    -- Claim: the result instance declaration is guaranteed valid
                    -- Hence no need to call:
                    --   checkValidInstance tyvars theta clas inst_tys
               ; return theta }
          where
            the_pred = mkClassPred clas inst_tys

    ------------------------------------------------------------------
    newDerivClsInst :: ThetaType -> DerivSpec theta -> TcM ClsInst
    newDerivClsInst theta (DS { ds_name = dfun_name, ds_overlap = overlap_mode
                              , ds_tvs = tvs, ds_cls = clas, ds_tys = tys })
      = newClsInst overlap_mode dfun_name tvs theta clas tys

    extendLocalInstEnv :: [ClsInst] -> TcM a -> TcM a
    -- Add new locally-defined instances; don't bother to check
    -- for functional dependency errors -- that'll happen in TcInstDcls
    extendLocalInstEnv dfuns thing_inside
     = do { env <- getGblEnv
          ; let  inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
                 env'      = env { tcg_inst_env = inst_env' }
          ; setGblEnv env' thing_inside }

    {-
    ***********************************************************************************
    *                                                                                 *
    *            Simplify derived constraints
    *                                                                                 *
    ***********************************************************************************
    -}

    simplifyDeriv :: PredType
                  -> [TyVar]
                  -> ThetaOrigin      -- Wanted
                  -> TcM ThetaType  -- Needed
    -- Given  instance (wanted) => C inst_ty
    -- Simplify 'wanted' as much as possibles
    -- Fail if not possible
    simplifyDeriv pred tvs theta
      = do { (skol_subst, tvs_skols) <- tcInstSkolTyVars tvs -- Skolemize
                    -- The constraint solving machinery
                    -- expects *TcTyVars* not TyVars.
                    -- We use *non-overlappable* (vanilla) skolems
                    -- See Note [Overlap and deriving]

           ; let skol_set  = mkVarSet tvs_skols
                 skol_info = DerivSkol pred
                 doc = text "deriving" <+> parens (ppr pred)
                 mk_ct (PredOrigin t o t_or_k)
                     = newWanted o (Just t_or_k) (substTy skol_subst t)

           ; (wanted, tclvl) <- pushTcLevelM (mapM mk_ct theta)

           ; traceTc "simplifyDeriv" $
             vcat [ pprTvBndrs tvs $$ ppr theta $$ ppr wanted, doc ]
           ; residual_wanted <- simplifyWantedsTcM wanted
                -- Result is zonked

           ; let residual_simple = wc_simple residual_wanted
                 (good, bad) = partitionBagWith get_good residual_simple
                 unsolved    = residual_wanted { wc_simple = bad }

                             -- See Note [Exotic derived instance contexts]

                 get_good :: Ct -> Either PredType Ct
                 get_good ct | validDerivPred skol_set p
                             , isWantedCt ct
                             = Left p
                              -- NB: residual_wanted may contain unsolved
                              -- Derived and we stick them into the bad set
                              -- so that reportUnsolved may decide what to do with them
                             | otherwise
                             = Right ct
                               where p = ctPred ct

           ; traceTc "simplifyDeriv 2" $
             vcat [ ppr tvs_skols, ppr residual_simple, ppr good, ppr bad ]

           -- If we are deferring type errors, simply ignore any insoluble
           -- constraints.  They'll come up again when we typecheck the
           -- generated instance declaration
           ; defer <- goptM Opt_DeferTypeErrors
           ; (implic, _) <- buildImplicationFor tclvl skol_info tvs_skols [] unsolved
                       -- The buildImplicationFor is just to bind the skolems,
                       -- in case they are mentioned in error messages
                       -- See Trac #11347
           ; unless defer (reportAllUnsolved (mkImplicWC implic))

           ; let min_theta  = mkMinimalBySCs (bagToList good)
                 subst_skol = zipTvSubst tvs_skols $ mkTyVarTys tvs
                              -- The reverse substitution (sigh)
           ; return (substTheta subst_skol min_theta) }

    {-
    Note [Overlap and deriving]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider some overlapping instances:
      data Show a => Show [a] where ..
      data Show [Char] where ...

    Now a data type with deriving:
      data T a = MkT [a] deriving( Show )

    We want to get the derived instance
      instance Show [a] => Show (T a) where...
    and NOT
      instance Show a => Show (T a) where...
    so that the (Show (T Char)) instance does the Right Thing

    It's very like the situation when we're inferring the type
    of a function
       f x = show [x]
    and we want to infer
       f :: Show [a] => a -> String

    BOTTOM LINE: use vanilla, non-overlappable skolems when inferring
                 the context for the derived instance.
                 Hence tcInstSkolTyVars not tcInstSuperSkolTyVars

    Note [Exotic derived instance contexts]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In a 'derived' instance declaration, we *infer* the context.  It's a
    bit unclear what rules we should apply for this; the Haskell report is
    silent.  Obviously, constraints like (Eq a) are fine, but what about
            data T f a = MkT (f a) deriving( Eq )
    where we'd get an Eq (f a) constraint.  That's probably fine too.

    One could go further: consider
            data T a b c = MkT (Foo a b c) deriving( Eq )
            instance (C Int a, Eq b, Eq c) => Eq (Foo a b c)

    Notice that this instance (just) satisfies the Paterson termination
    conditions.  Then we *could* derive an instance decl like this:

            instance (C Int a, Eq b, Eq c) => Eq (T a b c)
    even though there is no instance for (C Int a), because there just
    *might* be an instance for, say, (C Int Bool) at a site where we
    need the equality instance for T's.

    However, this seems pretty exotic, and it's quite tricky to allow
    this, and yet give sensible error messages in the (much more common)
    case where we really want that instance decl for C.

    So for now we simply require that the derived instance context
    should have only type-variable constraints.

    Here is another example:
            data Fix f = In (f (Fix f)) deriving( Eq )
    Here, if we are prepared to allow -XUndecidableInstances we
    could derive the instance
            instance Eq (f (Fix f)) => Eq (Fix f)
    but this is so delicate that I don't think it should happen inside
    'deriving'. If you want this, write it yourself!

    NB: if you want to lift this condition, make sure you still meet the
    termination conditions!  If not, the deriving mechanism generates
    larger and larger constraints.  Example:
      data Succ a = S a
      data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show

    Note the lack of a Show instance for Succ.  First we'll generate
      instance (Show (Succ a), Show a) => Show (Seq a)
    and then
      instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a)
    and so on.  Instead we want to complain of no instance for (Show (Succ a)).

    The bottom line
    ~~~~~~~~~~~~~~~
    Allow constraints which consist only of type variables, with no repeats.

    ************************************************************************
    *                                                                      *
    \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
    *                                                                      *
    ************************************************************************

    After all the trouble to figure out the required context for the
    derived instance declarations, all that's left is to chug along to
    produce them.  They will then be shoved into @tcInstDecls2@, which
    will do all its usual business.

    There are lots of possibilities for code to generate.  Here are
    various general remarks.

    PRINCIPLES:
    \begin{itemize}
    \item
    We want derived instances of @Eq@ and @Ord@ (both v common) to be
    ``you-couldn't-do-better-by-hand'' efficient.

    \item
    Deriving @Show@---also pretty common--- should also be reasonable good code.

    \item
    Deriving for the other classes isn't that common or that big a deal.
    \end{itemize}

    PRAGMATICS:

    \begin{itemize}
    \item
    Deriving @Ord@ is done mostly with the 1.3 @compare@ method.

    \item
    Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.

    \item
    We {\em normally} generate code only for the non-defaulted methods;
    there are some exceptions for @Eq@ and (especially) @Ord@...

    \item
    Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
    constructor's numeric (@Int#@) tag.  These are generated by
    @gen_tag_n_con_binds@, and the heuristic for deciding if one of
    these is around is given by @hasCon2TagFun@.

    The examples under the different sections below will make this
    clearer.

    \item
    Much less often (really just for deriving @Ix@), we use a
    @_tag2con_<tycon>@ function.  See the examples.

    \item
    We use the renamer!!!  Reason: we're supposed to be
    producing @LHsBinds Name@ for the methods, but that means
    producing correctly-uniquified code on the fly.  This is entirely
    possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
    So, instead, we produce @MonoBinds RdrName@ then heave 'em through
    the renamer.  What a great hack!
    \end{itemize}
    -}

    -- Generate the InstInfo for the required instance paired with the
    --   *representation* tycon for that instance,
    -- plus any auxiliary bindings required
    --
    -- Representation tycons differ from the tycon in the instance signature in
    -- case of instances for indexed families.
    --
    genInst :: DerivSpec ThetaType
            -> TcM (InstInfo RdrName, BagDerivStuff, Maybe Name)
    genInst spec@(DS { ds_tvs = tvs, ds_tc = rep_tycon
                     , ds_theta = theta, ds_newtype = is_newtype, ds_tys = tys
                     , ds_name = dfun_name, ds_cls = clas, ds_loc = loc })
      | Just rhs_ty <- is_newtype   -- See Note [Bindings for Generalised Newtype Deriving]
      = do { inst_spec <- newDerivClsInst theta spec
           ; traceTc "genInst/is_newtype" (vcat [ppr loc, ppr clas, ppr tvs, ppr tys, ppr rhs_ty])
           ; return ( InstInfo
                        { iSpec   = inst_spec
                        , iBinds  = InstBindings
                            { ib_binds      = gen_Newtype_binds loc clas tvs tys rhs_ty
                            , ib_tyvars     = map Var.varName tvs   -- Scope over bindings
                            , ib_pragmas    = []
                            , ib_extensions = [ LangExt.ImpredicativeTypes
                                              , LangExt.RankNTypes ]
                              -- Both these flags are needed for higher-rank uses of coerce
                              -- See Note [Newtype-deriving instances] in TcGenDeriv
                            , ib_derived    = True } }
                    , emptyBag
                    , Just $ getName $ head $ tyConDataCons rep_tycon ) }
                  -- See Note [Newtype deriving and unused constructors]

      | otherwise
      = do { (meth_binds, deriv_stuff) <- genDerivStuff loc clas
                                            dfun_name rep_tycon
                                            tys tvs
           ; inst_spec <- newDerivClsInst theta spec
           ; traceTc "newder" (ppr inst_spec)
           ; let inst_info = InstInfo { iSpec   = inst_spec
                                      , iBinds  = InstBindings
                                                    { ib_binds = meth_binds
                                                    , ib_tyvars = map Var.varName tvs
                                                    , ib_pragmas = []
                                                    , ib_extensions = []
                                                    , ib_derived = True } }
           ; return ( inst_info, deriv_stuff, Nothing ) }

    -- Generate the bindings needed for a derived class that isn't handled by
    -- -XGeneralizedNewtypeDeriving.
    genDerivStuff :: SrcSpan -> Class -> Name -> TyCon -> [Type] -> [TyVar]
                  -> TcM (LHsBinds RdrName, BagDerivStuff)
    genDerivStuff loc clas dfun_name tycon inst_tys tyvars
      -- Special case for DeriveGeneric
      | let ck = classKey clas
      , ck `elem` [genClassKey, gen1ClassKey]
      = let gk = if ck == genClassKey then Gen0 else Gen1
            -- TODO NSF: correctly identify when we're building Both instead of One
        in do
          let inst_ty = ASSERT(not $ null inst_tys)
                              head inst_tys
          (binds, faminst) <- gen_Generic_binds gk tycon inst_ty
                                                (nameModule dfun_name)
          return (binds, unitBag (DerivFamInst faminst))

      -- Not deriving Generic(1), so we first check if the compiler has built-in
      -- support for deriving the class in question.
      | otherwise
      = do { dflags <- getDynFlags
           ; fix_env <- getDataConFixityFun tycon
           ; case hasBuiltinDeriving dflags fix_env clas of
                  Just gen_fn -> return (gen_fn loc tycon)
                  Nothing -> genDerivAnyClass dflags }

      where
        genDerivAnyClass :: DynFlags -> TcM (LHsBinds RdrName, BagDerivStuff)
        genDerivAnyClass dflags =
          do { -- If there isn't compiler support for deriving the class, our last
               -- resort is -XDeriveAnyClass (since -XGeneralizedNewtypeDeriving
               -- fell through).
              let mini_env   = mkVarEnv (classTyVars clas `zip` inst_tys)
                  mini_subst = mkTvSubst (mkInScopeSet (mkVarSet tyvars)) mini_env

             ; tyfam_insts <-
                 ASSERT2( isNothing (canDeriveAnyClass dflags tycon clas)
                        , ppr "genDerivStuff: bad derived class" <+> ppr clas )
                 mapM (tcATDefault False loc mini_subst emptyNameSet)
                      (classATItems clas)
             ; return ( emptyBag -- No method bindings are needed...
                      , listToBag (map DerivFamInst (concat tyfam_insts))
                      -- ...but we may need to generate binding for associated type
                      -- family default instances.
                      -- See Note [DeriveAnyClass and default family instances]
                      ) }

    getDataConFixityFun :: TyCon -> TcM (Name -> Fixity)
    -- If the TyCon is locally defined, we want the local fixity env;
    -- but if it is imported (which happens for standalone deriving)
    -- we need to get the fixity env from the interface file
    -- c.f. RnEnv.lookupFixity, and Trac #9830
    getDataConFixityFun tc
      = do { this_mod <- getModule
           ; if nameIsLocalOrFrom this_mod name
             then do { fix_env <- getFixityEnv
                     ; return (lookupFixity fix_env) }
             else do { iface <- loadInterfaceForName doc name
                                -- Should already be loaded!
                     ; return (mi_fix iface . nameOccName) } }
      where
        name = tyConName tc
        doc = text "Data con fixities for" <+> ppr name

    {-
    Note [Bindings for Generalised Newtype Deriving]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
      class Eq a => C a where
         f :: a -> a
      newtype N a = MkN [a] deriving( C )
      instance Eq (N a) where ...

    The 'deriving C' clause generates, in effect
      instance (C [a], Eq a) => C (N a) where
         f = coerce (f :: [a] -> [a])

    This generates a cast for each method, but allows the superclasse to
    be worked out in the usual way.  In this case the superclass (Eq (N
    a)) will be solved by the explicit Eq (N a) instance.  We do *not*
    create the superclasses by casting the superclass dictionaries for the
    representation type.

    See the paper "Safe zero-cost coercions for Hsakell".

    Note [DeriveAnyClass and default family instances]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    When a class has a associated type family with a default instance, e.g.:

      class C a where
        type T a
        type T a = Char

    then there are a couple of scenarios in which a user would expect T a to
    default to Char. One is when an instance declaration for C is given without
    an implementation for T:

      instance C Int

    Another scenario in which this can occur is when the -XDeriveAnyClass extension
    is used:

      data Example = Example deriving (C, Generic)

    In the latter case, we must take care to check if C has any associated type
    families with default instances, because -XDeriveAnyClass will never provide
    an implementation for them. We "fill in" the default instances using the
    tcATDefault function from TcClsDcl (which is also used in TcInstDcls to handle
    the empty instance declaration case).

    ************************************************************************
    *                                                                      *
    \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
    *                                                                      *
    ************************************************************************
    -}

    derivingNullaryErr :: MsgDoc
    derivingNullaryErr = text "Cannot derive instances for nullary classes"

    derivingKindErr :: TyCon -> Class -> [Type] -> Kind -> MsgDoc
    derivingKindErr tc cls cls_tys cls_kind
      = hang (text "Cannot derive well-kinded instance of form"
                    <+> quotes (pprClassPred cls cls_tys <+> parens (ppr tc <+> text "...")))
           2 (text "Class" <+> quotes (ppr cls)
                <+> text "expects an argument of kind" <+> quotes (pprKind cls_kind))

    derivingEtaErr :: Class -> [Type] -> Type -> MsgDoc
    derivingEtaErr cls cls_tys inst_ty
      = sep [text "Cannot eta-reduce to an instance of form",
             nest 2 (text "instance (...) =>"
                    <+> pprClassPred cls (cls_tys ++ [inst_ty]))]

    derivingThingErr :: Bool -> Class -> [Type] -> Type -> MsgDoc -> MsgDoc
    derivingThingErr newtype_deriving clas tys ty why
      = sep [(hang (text "Can't make a derived instance of")
                 2 (quotes (ppr pred))
              $$ nest 2 extra) <> colon,
             nest 2 why]
      where
        extra | newtype_deriving = text "(even with cunning GeneralizedNewtypeDeriving)"
              | otherwise        = Outputable.empty
        pred = mkClassPred clas (tys ++ [ty])

    derivingHiddenErr :: TyCon -> SDoc
    derivingHiddenErr tc
      = hang (text "The data constructors of" <+> quotes (ppr tc) <+> ptext (sLit "are not all in scope"))
           2 (text "so you cannot derive an instance for it")

    standaloneCtxt :: LHsSigType Name -> SDoc
    standaloneCtxt ty = hang (text "In the stand-alone deriving instance for")
                           2 (quotes (ppr ty))

    derivInstCtxt :: PredType -> MsgDoc
    derivInstCtxt pred
      = text "When deriving the instance for" <+> parens (ppr pred)

    unboxedTyConErr :: String -> MsgDoc
    unboxedTyConErr thing =
      text "The last argument of the instance cannot be an unboxed" <+> text thing