path: root/compiler/GHC/Tc/Deriv.hs

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

-}


{-# LANGUAGE MultiWayIf #-}
{-# LANGUAGE TypeFamilies #-}

{-# OPTIONS_GHC -Wno-incomplete-uni-patterns #-}

-- | Handles @deriving@ clauses on @data@ declarations.
module GHC.Tc.Deriv ( tcDeriving, DerivInfo(..) ) where

import GHC.Prelude

import GHC.Hs
import GHC.Driver.Session

import GHC.Tc.Errors.Types
import GHC.Tc.Utils.Monad
import GHC.Tc.Instance.Family
import GHC.Tc.Types.Origin
import GHC.Tc.Deriv.Infer
import GHC.Tc.Deriv.Utils
import GHC.Tc.TyCl.Class( instDeclCtxt3, tcATDefault )
import GHC.Tc.Utils.Env
import GHC.Tc.Deriv.Generate
import GHC.Tc.Validity( allDistinctTyVars, checkValidInstHead )
import GHC.Core.InstEnv
import GHC.Tc.Utils.Instantiate
import GHC.Core.FamInstEnv
import GHC.Tc.Gen.HsType
import GHC.Core.TyCo.Rep
import GHC.Core.TyCo.Ppr ( pprTyVars )

import GHC.Rename.Bind
import GHC.Rename.Env
import GHC.Rename.Module ( addTcgDUs )
import GHC.Rename.Utils

import GHC.Core.Unify( tcUnifyTy )
import GHC.Core.Class
import GHC.Core.Type
import GHC.Utils.Error
import GHC.Core.DataCon
import GHC.Data.Maybe
import GHC.Types.Name.Reader
import GHC.Types.Name
import GHC.Types.Name.Set as NameSet
import GHC.Core.TyCon
import GHC.Tc.Utils.TcType
import GHC.Types.Var as Var
import GHC.Types.Var.Env
import GHC.Types.Var.Set
import GHC.Builtin.Names
import GHC.Types.SrcLoc
import GHC.Utils.Misc
import GHC.Utils.Outputable as Outputable
import GHC.Utils.Panic
import GHC.Utils.Panic.Plain
import GHC.Utils.Logger
import GHC.Data.Bag
import GHC.Utils.FV as FV (fvVarList, unionFV, mkFVs)
import qualified GHC.LanguageExtensions as LangExt

import Control.Monad
import Control.Monad.Trans.Class
import Control.Monad.Trans.Reader
import Data.List (partition, find)

{-
************************************************************************
*                                                                      *
                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
-}

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 sets of
        --      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] in GHC.Tc.Deriv.Infer

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)

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

{-
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 [Unused constructors and deriving clauses]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
See #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 datatype's `deriving` clauses
data DerivInfo = DerivInfo { di_rep_tc  :: TyCon
                             -- ^ The data tycon for normal datatypes,
                             -- or the *representation* tycon for data families
                           , di_scoped_tvs :: ![(Name,TyVar)]
                             -- ^ Variables that scope over the deriving clause.
                             -- See @Note [Scoped tyvars in a TcTyCon]@ in
                             -- "GHC.Core.TyCon".
                           , di_clauses :: [LHsDerivingClause GhcRn]
                           , di_ctxt    :: SDoc -- ^ error context
                           }

{-

************************************************************************
*                                                                      *
Top-level function for \tr{derivings}
*                                                                      *
************************************************************************
-}

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

        ; let (infer_specs, given_specs) = splitEarlyDerivSpec early_specs
        ; insts1 <- mapM genInst given_specs
        ; insts2 <- mapM genInst infer_specs

        ; dflags <- getDynFlags
        ; logger <- getLogger

        ; let (_, deriv_stuff, fvs) = unzip3 (insts1 ++ insts2)
        ; loc <- getSrcSpanM
        ; let (binds, famInsts) = genAuxBinds dflags loc
                                    (unionManyBags deriv_stuff)

        ; let mk_inst_infos1 = map fstOf3 insts1
        ; inst_infos1 <- apply_inst_infos mk_inst_infos1 given_specs

          -- We must put all the derived type family instances (from both
          -- infer_specs and given_specs) in the local instance environment
          -- before proceeding, or else simplifyInstanceContexts might
          -- get stuck if it has to reason about any of those family instances.
          -- See Note [Staging of tcDeriving]
        ; tcExtendLocalFamInstEnv (bagToList famInsts) $
          -- NB: only call tcExtendLocalFamInstEnv once, as it performs
          -- validity checking for all of the family instances you give it.
          -- If the family instances have errors, calling it twice will result
          -- in duplicate error messages!

     do {
        -- the stand-alone derived instances (@inst_infos1@) are used when
        -- inferring the contexts for "deriving" clauses' instances
        -- (@infer_specs@)
        ; final_specs <- extendLocalInstEnv (map iSpec inst_infos1) $
                         simplifyInstanceContexts infer_specs

        ; let mk_inst_infos2 = map fstOf3 insts2
        ; inst_infos2 <- apply_inst_infos mk_inst_infos2 final_specs
        ; let inst_infos = inst_infos1 ++ inst_infos2

        ; (inst_info, rn_binds, rn_dus) <- renameDeriv inst_infos binds

        ; unless (isEmptyBag inst_info) $
             liftIO (putDumpFileMaybe logger Opt_D_dump_deriv "Derived instances"
                        FormatHaskell
                        (ddump_deriving inst_info rn_binds famInsts))

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

    hangP s x = text "" $$ hang s 2 x

    -- Apply the suspended computations given by genInst calls.
    -- See Note [Staging of tcDeriving]
    apply_inst_infos :: [ThetaType -> TcM (InstInfo GhcPs)]
                     -> [DerivSpec ThetaType] -> TcM [InstInfo GhcPs]
    apply_inst_infos = zipWithM (\f ds -> f (ds_theta ds))

-- 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 :: [InstInfo GhcPs]
            -> Bag (LHsBind GhcPs, LSig GhcPs)
            -> TcM (Bag (InstInfo GhcRn), HsValBinds GhcRn, DefUses)
renameDeriv inst_infos bagBinds
  = 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
    setXOptM LangExt.TypeApplications $
    -- GND/DerivingVia uses TypeApplications in generated code
    -- (See Note [Newtype-deriving instances] in GHC.Tc.Deriv.Generate)
    unsetXOptM LangExt.RebindableSyntax $
    -- See Note [Avoid RebindableSyntax when deriving]
    setXOptM LangExt.TemplateHaskellQuotes $
    -- DeriveLift makes uses of quotes
    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 = ValBinds NoAnnSortKey aux_binds (bagToList aux_sigs)
        -- Importantly, we use rnLocalValBindsLHS, not rnTopBindsLHS, to rename
        -- auxiliary bindings as if they were defined locally.
        -- See Note [Auxiliary binders] in GHC.Tc.Deriv.Generate.
        ; (bndrs, rn_aux_lhs) <- rnLocalValBindsLHS emptyFsEnv aux_val_binds
        ; bindLocalNames bndrs $
    do  { (rn_aux, dus_aux) <- rnLocalValBindsRHS (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 GhcPs -> TcM (InstInfo GhcRn, 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 } })
        =  do { (rn_binds, rn_sigs, fvs) <- rnMethodBinds False (is_cls_nm inst)
                                                          tyvars binds sigs
              ; let binds' = InstBindings { ib_binds = rn_binds
                                          , ib_tyvars = tyvars
                                          , ib_pragmas = rn_sigs
                                          , ib_extensions = exts
                                          , ib_derived = sa }
              ; return (inst_info { iBinds = binds' }, fvs) }

{-
Note [Staging of tcDeriving]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Here's a tricky corner case for deriving (adapted from #2721):

    class C a where
      type T a
      foo :: a -> T a

    instance C Int where
      type T Int = Int
      foo = id

    newtype N = N Int deriving C

This will produce an instance something like this:

    instance C N where
      type T N = T Int
      foo = coerce (foo :: Int -> T Int) :: N -> T N

We must be careful in order to typecheck this code. When determining the
context for the instance (in simplifyInstanceContexts), we need to determine
that T N and T Int have the same representation, but to do that, the T N
instance must be in the local family instance environment. Otherwise, GHC
would be unable to conclude that T Int is representationally equivalent to
T Int, and simplifyInstanceContexts would get stuck.

Previously, tcDeriving would defer adding any derived type family instances to
the instance environment until the very end, which meant that
simplifyInstanceContexts would get called without all the type family instances
it needed in the environment in order to properly simplify instance like
the C N instance above.

To avoid this scenario, we carefully structure the order of events in
tcDeriving. We first call genInst on the standalone derived instance specs and
the instance specs obtained from deriving clauses. Note that the return type of
genInst is a triple:

    TcM (ThetaType -> TcM (InstInfo RdrName), BagDerivStuff, Maybe Name)

The type family instances are in the BagDerivStuff. The first field of the
triple is a suspended computation which, given an instance context, produces
the rest of the instance. The fact that it is suspended is important, because
right now, we don't have ThetaTypes for the instances that use deriving clauses
(only the standalone-derived ones).

Now we can collect the type family instances and extend the local instance
environment. At this point, it is safe to run simplifyInstanceContexts on the
deriving-clause instance specs, which gives us the ThetaTypes for the
deriving-clause instances. Now we can feed all the ThetaTypes to the
suspended computations and obtain our InstInfos, at which point
tcDeriving is done.

An alternative design would be to split up genInst so that the
family instances are generated separately from the InstInfos. But this would
require carving up a lot of the GHC deriving internals to accommodate the
change. On the other hand, we can keep all of the InstInfo and type family
instance logic together in genInst simply by converting genInst to
continuation-returning style, so we opt for that route.

Note [Why we don't pass rep_tc into deriveTyData]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Down in the bowels of mk_deriv_inst_tys_maybe, 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 #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 :: [DerivInfo]
               -> [LDerivDecl GhcRn]
               -> TcM [EarlyDerivSpec]
makeDerivSpecs deriv_infos deriv_decls
  = do  { eqns1 <- sequenceA
                     [ deriveClause rep_tc scoped_tvs dcs (deriv_clause_preds dct) err_ctxt
                     | DerivInfo { di_rep_tc = rep_tc
                                 , di_scoped_tvs = scoped_tvs
                                 , di_clauses = clauses
                                 , di_ctxt = err_ctxt } <- deriv_infos
                     , L _ (HsDerivingClause { deriv_clause_strategy = dcs
                                             , deriv_clause_tys = dct })
                         <- clauses
                     ]
        ; eqns2 <- mapM (recoverM (pure Nothing) . deriveStandalone) deriv_decls
        ; return $ concat eqns1 ++ catMaybes eqns2 }
  where
    deriv_clause_preds :: LDerivClauseTys GhcRn -> [LHsSigType GhcRn]
    deriv_clause_preds (L _ dct) = case dct of
      DctSingle _ ty -> [ty]
      DctMulti _ tys -> tys

------------------------------------------------------------------
-- | Process the derived classes in a single @deriving@ clause.
deriveClause :: TyCon
             -> [(Name, TcTyVar)]  -- Scoped type variables taken from tcTyConScopedTyVars
                                   -- See Note [Scoped tyvars in a TcTyCon] in "GHC.Core.TyCon"
             -> Maybe (LDerivStrategy GhcRn)
             -> [LHsSigType GhcRn] -> SDoc
             -> TcM [EarlyDerivSpec]
deriveClause rep_tc scoped_tvs mb_lderiv_strat deriv_preds err_ctxt
  = addErrCtxt err_ctxt $ do
      traceTc "deriveClause" $ vcat
        [ text "tvs"             <+> ppr tvs
        , text "scoped_tvs"      <+> ppr scoped_tvs
        , text "tc"              <+> ppr tc
        , text "tys"             <+> ppr tys
        , text "mb_lderiv_strat" <+> ppr mb_lderiv_strat ]
      tcExtendNameTyVarEnv scoped_tvs $ do
        (mb_lderiv_strat', via_tvs) <- tcDerivStrategy mb_lderiv_strat
        tcExtendTyVarEnv via_tvs $
        -- Moreover, when using DerivingVia one can bind type variables in
        -- the `via` type as well, so these type variables must also be
        -- brought into scope.
          mapMaybeM (derivePred tc tys mb_lderiv_strat' via_tvs) deriv_preds
          -- After typechecking the `via` type once, we then typecheck all
          -- of the classes associated with that `via` type in the
          -- `deriving` clause.
          -- See also Note [Don't typecheck too much in DerivingVia].
  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

-- | Process a single predicate in a @deriving@ clause.
--
-- This returns a 'Maybe' because the user might try to derive 'Typeable',
-- which is a no-op nowadays.
derivePred :: TyCon -> [Type] -> Maybe (LDerivStrategy GhcTc) -> [TyVar]
           -> LHsSigType GhcRn -> TcM (Maybe EarlyDerivSpec)
derivePred tc tys mb_lderiv_strat via_tvs deriv_pred =
  -- We carefully set up uses of recoverM to minimize error message
  -- cascades. See Note [Recovering from failures in deriving clauses].
  recoverM (pure Nothing) $
  setSrcSpan (getLocA deriv_pred) $ do
    traceTc "derivePred" $ vcat
      [ text "tc"              <+> ppr tc
      , text "tys"             <+> ppr tys
      , text "deriv_pred"      <+> ppr deriv_pred
      , text "mb_lderiv_strat" <+> ppr mb_lderiv_strat
      , text "via_tvs"         <+> ppr via_tvs ]
    (cls_tvs, cls, cls_tys, cls_arg_kinds) <- tcHsDeriv deriv_pred
    when (cls_arg_kinds `lengthIsNot` 1) $
      failWithTc (TcRnNonUnaryTypeclassConstraint deriv_pred)
    let [cls_arg_kind] = cls_arg_kinds
        mb_deriv_strat = fmap unLoc mb_lderiv_strat
    if (className cls == typeableClassName)
    then do warnUselessTypeable
            return Nothing
    else let deriv_tvs = via_tvs ++ cls_tvs in
         Just <$> deriveTyData tc tys mb_deriv_strat
                               deriv_tvs cls cls_tys cls_arg_kind

{-
Note [Don't typecheck too much in DerivingVia]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider the following example:

  data D = ...
    deriving (A1 t, ..., A20 t) via T t

GHC used to be engineered such that it would typecheck the `deriving`
clause like so:

1. Take the first class in the clause (`A1`).
2. Typecheck the `via` type (`T t`) and bring its bound type variables
   into scope (`t`).
3. Typecheck the class (`A1`).
4. Move on to the next class (`A2`) and repeat the process until all
   classes have been typechecked.

This algorithm gets the job done most of the time, but it has two notable
flaws. One flaw is that it is wasteful: it requires that `T t` be typechecked
20 different times, once for each class in the `deriving` clause. This is
unnecessary because we only need to typecheck `T t` once in order to get
access to its bound type variable.

The other issue with this algorithm arises when there are no classes in the
`deriving` clause, like in the following example:

  data D2 = ...
    deriving () via Maybe Maybe

Because there are no classes, the algorithm above will simply do nothing.
As a consequence, GHC will completely miss the fact that `Maybe Maybe`
is ill-kinded nonsense (#16923).

To address both of these problems, GHC now uses this algorithm instead:

1. Typecheck the `via` type and bring its bound type variables into scope.
2. Take the first class in the `deriving` clause.
3. Typecheck the class.
4. Move on to the next class and repeat the process until all classes have been
   typechecked.

This algorithm ensures that the `via` type is always typechecked, even if there
are no classes in the `deriving` clause. Moreover, it typecheck the `via` type
/exactly/ once and no more, even if there are multiple classes in the clause.

Note [Recovering from failures in deriving clauses]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider what happens if you run this program (from #10684) without
DeriveGeneric enabled:

    data A = A deriving (Show, Generic)
    data B = B A deriving (Show)

Naturally, you'd expect GHC to give an error to the effect of:

    Can't make a derived instance of `Generic A':
      You need -XDeriveGeneric to derive an instance for this class

And *only* that error, since the other two derived Show instances appear to be
independent of this derived Generic instance. Yet GHC also used to give this
additional error on the program above:

    No instance for (Show A)
      arising from the 'deriving' clause of a data type declaration
    When deriving the instance for (Show B)

This was happening because when GHC encountered any error within a single
data type's set of deriving clauses, it would call recoverM and move on
to the next data type's deriving clauses. One unfortunate consequence of
this design is that if A's derived Generic instance failed, its derived
Show instance would be skipped entirely, leading to the "No instance for
(Show A)" error cascade.

The solution to this problem is to push through uses of recoverM to the
level of the individual derived classes in a particular data type's set of
deriving clauses. That is, if you have:

    newtype C = C D
      deriving (E, F, G)

Then instead of processing instances E through M under the scope of a single
recoverM, as in the following pseudocode:

  recoverM (pure Nothing) $ mapM derivePred [E, F, G]

We instead use recoverM in each iteration of the loop:

  mapM (recoverM (pure Nothing) . derivePred) [E, F, G]

And then process each class individually, under its own recoverM scope. That
way, failure to derive one class doesn't cancel out other classes in the
same set of clause-derived classes.
-}

------------------------------------------------------------------
deriveStandalone :: LDerivDecl GhcRn -> TcM (Maybe EarlyDerivSpec)
-- Process a single standalone deriving declaration
--  e.g.   deriving instance Show a => Show (T a)
-- Rather like tcLocalInstDecl
--
-- This returns a Maybe because the user might try to derive Typeable, which is
-- a no-op nowadays.
deriveStandalone (L loc (DerivDecl _ deriv_ty mb_lderiv_strat overlap_mode))
  = setSrcSpanA loc                       $
    addErrCtxt (standaloneCtxt deriv_ty)  $
    do { traceTc "Standalone deriving decl for" (ppr deriv_ty)
       ; let ctxt = GHC.Tc.Types.Origin.InstDeclCtxt True
       ; traceTc "Deriving strategy (standalone deriving)" $
           vcat [ppr mb_lderiv_strat, ppr deriv_ty]
       ; (mb_lderiv_strat, via_tvs) <- tcDerivStrategy mb_lderiv_strat
       ; (cls_tvs, deriv_ctxt, cls, inst_tys)
           <- tcExtendTyVarEnv via_tvs $
              tcStandaloneDerivInstType ctxt deriv_ty
       ; let mb_deriv_strat = fmap unLoc mb_lderiv_strat
             tvs            = via_tvs ++ cls_tvs
         -- See Note [Unify kinds in deriving]
       ; (tvs', deriv_ctxt', inst_tys', mb_deriv_strat') <-
           case mb_deriv_strat of
             -- Perform an additional unification with the kind of the `via`
             -- type and the result of the previous kind unification.
             Just (ViaStrategy via_ty)
                  -- This unification must be performed on the last element of
                  -- inst_tys, but we have not yet checked for this property.
                  -- (This is done later in expectNonNullaryClsArgs). For now,
                  -- simply do nothing if inst_tys is empty, since
                  -- expectNonNullaryClsArgs will error later if this
                  -- is the case.
               |  Just inst_ty <- lastMaybe inst_tys
               -> do
               let via_kind     = tcTypeKind via_ty
                   inst_ty_kind = tcTypeKind inst_ty
                   mb_match     = tcUnifyTy inst_ty_kind via_kind

               checkTc (isJust mb_match)
                       (TcRnCannotDeriveInstance cls mempty Nothing NoGeneralizedNewtypeDeriving $
                          DerivErrDerivingViaWrongKind inst_ty_kind via_ty via_kind)

               let Just kind_subst = mb_match
                   ki_subst_range  = getTCvSubstRangeFVs kind_subst
                   -- See Note [Unification of two kind variables in deriving]
                   unmapped_tkvs = filter (\v -> v `notElemTCvSubst` kind_subst
                                        && not (v `elemVarSet` ki_subst_range))
                                          tvs
                   (subst, _)    = substTyVarBndrs kind_subst unmapped_tkvs
                   (final_deriv_ctxt, final_deriv_ctxt_tys)
                     = case deriv_ctxt of
                         InferContext wc -> (InferContext wc, [])
                         SupplyContext theta ->
                           let final_theta = substTheta subst theta
                           in (SupplyContext final_theta, final_theta)
                   final_inst_tys   = substTys subst inst_tys
                   final_via_ty     = substTy  subst via_ty
                   -- See Note [Floating `via` type variables]
                   final_tvs        = tyCoVarsOfTypesWellScoped $
                                      final_deriv_ctxt_tys ++ final_inst_tys
                                        ++ [final_via_ty]
               pure ( final_tvs, final_deriv_ctxt, final_inst_tys
                    , Just (ViaStrategy final_via_ty) )

             _ -> pure (tvs, deriv_ctxt, inst_tys, mb_deriv_strat)
       ; traceTc "Standalone deriving;" $ vcat
              [ text "tvs':" <+> ppr tvs'
              , text "mb_deriv_strat':" <+> ppr mb_deriv_strat'
              , text "deriv_ctxt':" <+> ppr deriv_ctxt'
              , text "cls:" <+> ppr cls
              , text "inst_tys':" <+> ppr inst_tys' ]
                -- C.f. GHC.Tc.TyCl.Instance.tcLocalInstDecl1

       ; if className cls == typeableClassName
         then do warnUselessTypeable
                 return Nothing
         else Just <$> mkEqnHelp (fmap unLoc overlap_mode)
                                 tvs' cls inst_tys'
                                 deriv_ctxt' mb_deriv_strat' }

-- Typecheck the type in a standalone deriving declaration.
--
-- This may appear dense, but it's mostly huffing and puffing to recognize
-- the special case of a type with an extra-constraints wildcard context, e.g.,
--
--   deriving instance _ => Eq (Foo a)
--
-- If there is such a wildcard, we typecheck this as if we had written
-- @deriving instance Eq (Foo a)@, and return @'InferContext' ('Just' loc)@,
-- as the 'DerivContext', where loc is the location of the wildcard used for
-- error reporting. This indicates that we should infer the context as if we
-- were deriving Eq via a deriving clause
-- (see Note [Inferring the instance context] in GHC.Tc.Deriv.Infer).
--
-- If there is no wildcard, then proceed as normal, and instead return
-- @'SupplyContext' theta@, where theta is the typechecked context.
--
-- Note that this will never return @'InferContext' 'Nothing'@, as that can
-- only happen with @deriving@ clauses.
tcStandaloneDerivInstType
  :: UserTypeCtxt -> LHsSigWcType GhcRn
  -> TcM ([TyVar], DerivContext, Class, [Type])
tcStandaloneDerivInstType ctxt
    (HsWC { hswc_body = deriv_ty@(L loc (HsSig { sig_bndrs = outer_bndrs
                                               , sig_body = deriv_ty_body }))})
  | (theta, rho) <- splitLHsQualTy deriv_ty_body
  , [wc_pred] <- fromMaybeContext theta
  , L wc_span (HsWildCardTy _) <- ignoreParens wc_pred
  = do dfun_ty <- tcHsClsInstType ctxt $ L loc $
                  HsSig { sig_ext   = noExtField
                        , sig_bndrs = outer_bndrs
                        , sig_body  = rho }
       let (tvs, _theta, cls, inst_tys) = tcSplitDFunTy dfun_ty
       pure (tvs, InferContext (Just (locA wc_span)), cls, inst_tys)
  | otherwise
  = do dfun_ty <- tcHsClsInstType ctxt deriv_ty
       let (tvs, theta, cls, inst_tys) = tcSplitDFunTy dfun_ty
       pure (tvs, SupplyContext theta, cls, inst_tys)

warnUselessTypeable :: TcM ()
warnUselessTypeable = addDiagnosticTc TcRnUselessTypeable

------------------------------------------------------------------
deriveTyData :: TyCon -> [Type] -- LHS of data or data instance
                    -- Can be a data instance, hence [Type] args
                    -- and in that case the TyCon is the /family/ tycon
             -> Maybe (DerivStrategy GhcTc) -- The optional deriving strategy
             -> [TyVar] -- The type variables bound by the derived class
             -> Class   -- The derived class
             -> [Type]  -- The derived class's arguments
             -> Kind    -- The function argument in the derived class's kind.
                        -- (e.g., if `deriving Functor`, this would be
                        -- `Type -> Type` since
                        -- `Functor :: (Type -> Type) -> Constraint`)
             -> TcM EarlyDerivSpec
-- The deriving clause of a data or newtype declaration
-- I.e. not standalone deriving
deriveTyData tc tc_args mb_deriv_strat deriv_tvs cls cls_tys cls_arg_kind
   = 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  = length tc_args - n_args_to_drop
                                -- See Note [tc_args and tycon arity]
              (tc_args_to_keep, args_to_drop)
                              = splitAt n_args_to_keep tc_args
              inst_ty_kind    = tcTypeKind (mkTyConApp tc tc_args_to_keep)

              -- 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
              enough_args     = n_args_to_keep >= 0

        -- Check that the result really is well-kinded
        ; checkTc (enough_args && isJust mb_match)
                  (TcRnCannotDeriveInstance cls cls_tys Nothing NoGeneralizedNewtypeDeriving $
                     DerivErrNotWellKinded tc cls_arg_kind n_args_to_keep)

        ; let -- Returns a singleton-element list if using ViaStrategy and an
              -- empty list otherwise. Useful for free-variable calculations.
              deriv_strat_tys :: Maybe (DerivStrategy GhcTc) -> [Type]
              deriv_strat_tys = foldMap (foldDerivStrategy [] (:[]))

              propagate_subst kind_subst tkvs' cls_tys' tc_args' mb_deriv_strat'
                = (final_tkvs, final_cls_tys, final_tc_args, final_mb_deriv_strat)
                where
                  ki_subst_range  = getTCvSubstRangeFVs kind_subst
                  -- See Note [Unification of two kind variables in deriving]
                  unmapped_tkvs   = filter (\v -> v `notElemTCvSubst` kind_subst
                                         && not (v `elemVarSet` ki_subst_range))
                                           tkvs'
                  (subst, _)           = substTyVarBndrs kind_subst unmapped_tkvs
                  final_tc_args        = substTys subst tc_args'
                  final_cls_tys        = substTys subst cls_tys'
                  final_mb_deriv_strat = fmap (mapDerivStrategy (substTy subst))
                                              mb_deriv_strat'
                  -- See Note [Floating `via` type variables]
                  final_tkvs           = tyCoVarsOfTypesWellScoped $
                                         final_cls_tys ++ final_tc_args
                                           ++ deriv_strat_tys final_mb_deriv_strat

        ; let tkvs = scopedSort $ fvVarList $
                     unionFV (tyCoFVsOfTypes tc_args_to_keep)
                             (FV.mkFVs deriv_tvs)
              Just kind_subst = mb_match
              (tkvs', cls_tys', tc_args', mb_deriv_strat')
                = propagate_subst kind_subst tkvs cls_tys
                                  tc_args_to_keep mb_deriv_strat

          -- See Note [Unify kinds in deriving]
        ; (final_tkvs, final_cls_tys, final_tc_args, final_mb_deriv_strat) <-
            case mb_deriv_strat' of
              -- Perform an additional unification with the kind of the `via`
              -- type and the result of the previous kind unification.
              Just (ViaStrategy via_ty) -> do
                let via_kind = tcTypeKind via_ty
                    inst_ty_kind
                              = tcTypeKind (mkTyConApp tc tc_args')
                    via_match = tcUnifyTy inst_ty_kind via_kind

                checkTc (isJust via_match)
                        (TcRnCannotDeriveInstance cls mempty Nothing NoGeneralizedNewtypeDeriving $
                           DerivErrDerivingViaWrongKind inst_ty_kind via_ty via_kind)

                let Just via_subst = via_match
                pure $ propagate_subst via_subst tkvs' cls_tys'
                                       tc_args' mb_deriv_strat'

              _ -> pure (tkvs', cls_tys', tc_args', mb_deriv_strat')

        ; traceTc "deriveTyData 1" $ vcat
            [ ppr final_mb_deriv_strat, pprTyVars deriv_tvs, ppr tc, ppr tc_args
            , pprTyVars (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 ]

        ; traceTc "deriveTyData 2" $ vcat
            [ ppr final_tkvs ]

        ; let final_tc_app   = mkTyConApp tc final_tc_args
              final_cls_args = final_cls_tys ++ [final_tc_app]
        ; checkTc (allDistinctTyVars (mkVarSet final_tkvs) args_to_drop) -- (a, b, c)
                  (TcRnCannotDeriveInstance cls final_cls_tys Nothing NoGeneralizedNewtypeDeriving $
                     DerivErrNoEtaReduce final_tc_app)
                -- 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 instance K a a = ... deriving( Monad )
                --
                -- It is vital that the implementation of allDistinctTyVars
                -- expand any type synonyms.
                -- See Note [Eta-reducing type synonyms]

        ; checkValidInstHead DerivClauseCtxt cls final_cls_args
                -- Check that we aren't deriving an instance of a magical
                -- type like (~) or Coercible (#14916).

        ; spec <- mkEqnHelp Nothing final_tkvs cls final_cls_args
                            (InferContext Nothing) final_mb_deriv_strat
        ; traceTc "deriveTyData 3" (ppr spec)
        ; return spec }


{- Note [tc_args and tycon arity]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
You might wonder if we could use (tyConArity tc) at this point, rather
than (length tc_args).  But for data families the two can differ!  The
tc and tc_args passed into 'deriveTyData' come from 'deriveClause' which
in turn gets them from 'tyConFamInstSig_maybe' which in turn gets them
from DataFamInstTyCon:

| DataFamInstTyCon          -- See Note [Data type families]
      (CoAxiom Unbranched)
      TyCon   -- The family TyCon
      [Type]  -- Argument types (mentions the tyConTyVars of this TyCon)
              -- No shorter in length than the tyConTyVars of the family TyCon
              -- How could it be longer? See [Arity of data families] in GHC.Core.FamInstEnv

Notice that the arg tys might not be the same as the family tycon arity
(= length tyConTyVars).

Note [Unify kinds in deriving]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider (#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 (#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 #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 #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 by design: 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 #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 a 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 *)).

What happens with DerivingVia, when you have yet another type? Consider:

  newtype Foo (a :: Type) = MkFoo (Proxy a)
    deriving Functor via Proxy

As before, we unify the kind of Foo (* -> *) with the kind of the argument to
Functor (* -> *). But that's not enough: the `via` type, Proxy, has the kind
(k -> *), which is more general than what we want. So we must additionally
unify (k -> *) with (* -> *).

Currently, all of this unification is implemented kludgily with the pure
unifier, which is rather tiresome. #14331 lays out a plan for how this
might be made cleaner.

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 surprisingly 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, _)    = substTyVarBndrs 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 #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

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 #11416,
where this was first noticed). For this reason, we expand the type synonyms in
the eta-reduced types before doing any analysis.

Note [Floating `via` type variables]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When generating a derived instance, it will be of the form:

  instance forall ???. C c_args (D d_args) where ...

To fill in ???, GHC computes the free variables of `c_args` and `d_args`.
`DerivingVia` adds an extra wrinkle to this formula, since we must also
include the variables bound by the `via` type when computing the binders
used to fill in ???. This might seem strange, since if a `via` type binds
any type variables, then in almost all scenarios it will appear free in
`c_args` or `d_args`. There are certain corner cases where this does not hold,
however, such as in the following example (adapted from #15831):

  newtype Age = MkAge Int
    deriving Eq via Const Int a

In this example, the `via` type binds the type variable `a`, but `a` appears
nowhere in `Eq Age`. Nevertheless, we include it in the generated instance:

  instance forall a. Eq Age where
    (==) = coerce @(Const Int a -> Const Int a -> Bool)
                  @(Age         -> Age         -> Bool)
                  (==)

The use of `forall a` is certainly required here, since the `a` in
`Const Int a` would not be in scope otherwise. This instance is somewhat
strange in that nothing in the instance head `Eq Age` ever determines what `a`
will be, so any code that uses this instance will invariably instantiate `a`
to be `Any`. We refer to this property of `a` as being a "floating" `via`
type variable. Programs with floating `via` type variables are the only known
class of program in which the `via` type quantifies type variables that aren't
mentioned in the instance head in the generated instance.

Fortunately, the choice to instantiate floating `via` type variables to `Any`
is one that is completely transparent to the user (since the instance will
work as expected regardless of what `a` is instantiated to), so we decide to
permit them. An alternative design would make programs with floating `via`
variables illegal, by requiring that every variable mentioned in the `via` type
is also mentioned in the data header or the derived class. That restriction
would require the user to pick a particular type (the choice does not matter);
for example:

  newtype Age = MkAge Int
    -- deriving Eq via Const Int a  -- Floating 'a'
    deriving Eq via Const Int ()    -- Choose a=()
    deriving Eq via Const Int Any   -- Choose a=Any

No expressiveness would be lost thereby, but stylistically it seems preferable
to allow a type variable to indicate "it doesn't matter".

Note that by quantifying the `a` in `forall a. Eq Age`, we are deferring the
work of instantiating `a` to `Any` at every use site of the instance. An
alternative approach would be to generate an instance that directly defaulted
to `Any`:

  instance Eq Age where
    (==) = coerce @(Const Int Any -> Const Int Any -> Bool)
                  @(Age           -> Age           -> Bool)
                  (==)

We do not implement this approach since it would require a nontrivial amount
of implementation effort to substitute `Any` for the floating `via` type
variables, and since the end result isn't distinguishable from the former
instance (at least from the user's perspective), the amount of engineering
required to obtain the latter instance just isn't worth it.
-}

mkEqnHelp :: Maybe OverlapMode
          -> [TyVar]
          -> Class -> [Type]
          -> DerivContext
               -- SupplyContext => context supplied (standalone deriving)
               -- InferContext  => context inferred (deriving on data decl, or
               --                  standalone deriving decl with a wildcard)
          -> Maybe (DerivStrategy GhcTc)
          -> 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_args deriv_ctxt deriv_strat = do
  is_boot <- tcIsHsBootOrSig
  when is_boot $ bale_out DerivErrBootFileFound
  runReaderT mk_eqn deriv_env
  where
    deriv_env = DerivEnv { denv_overlap_mode = overlap_mode
                         , denv_tvs          = tvs
                         , denv_cls          = cls
                         , denv_inst_tys     = cls_args
                         , denv_ctxt         = deriv_ctxt
                         , denv_strat        = deriv_strat }

    bale_out =
      failWithTc . TcRnCannotDeriveInstance cls cls_args deriv_strat NoGeneralizedNewtypeDeriving

    mk_eqn :: DerivM EarlyDerivSpec
    mk_eqn = do
      DerivEnv { denv_inst_tys = cls_args
               , denv_strat    = mb_strat } <- ask
      case mb_strat of
        Just (StockStrategy _) -> do
          (cls_tys, inst_ty) <- expectNonNullaryClsArgs cls_args
          dit                <- expectAlgTyConApp cls_tys inst_ty
          mk_eqn_stock dit

        Just (AnyclassStrategy _) -> mk_eqn_anyclass

        Just (ViaStrategy via_ty) -> do
          (cls_tys, inst_ty) <- expectNonNullaryClsArgs cls_args
          mk_eqn_via cls_tys inst_ty via_ty

        Just (NewtypeStrategy _) -> do
          (cls_tys, inst_ty) <- expectNonNullaryClsArgs cls_args
          dit                <- expectAlgTyConApp cls_tys inst_ty
          unless (isNewTyCon (dit_rep_tc dit)) $
            derivingThingFailWith NoGeneralizedNewtypeDeriving DerivErrGNDUsedOnData
          mkNewTypeEqn True dit

        Nothing -> mk_eqn_no_strategy

-- @expectNonNullaryClsArgs inst_tys@ checks if @inst_tys@ is non-empty.
-- If so, return @(init inst_tys, last inst_tys)@.
-- Otherwise, throw an error message.
-- See @Note [DerivEnv and DerivSpecMechanism]@ in "GHC.Tc.Deriv.Utils" for why this
-- property is important.
expectNonNullaryClsArgs :: [Type] -> DerivM ([Type], Type)
expectNonNullaryClsArgs inst_tys =
  maybe (derivingThingFailWith NoGeneralizedNewtypeDeriving DerivErrNullaryClasses) pure $
  snocView inst_tys

-- @expectAlgTyConApp cls_tys inst_ty@ checks if @inst_ty@ is an application
-- of an algebraic type constructor. If so, return a 'DerivInstTys' consisting
-- of @cls_tys@ and the constituent pars of @inst_ty@.
-- Otherwise, throw an error message.
-- See @Note [DerivEnv and DerivSpecMechanism]@ in "GHC.Tc.Deriv.Utils" for why this
-- property is important.
expectAlgTyConApp :: [Type] -- All but the last argument to the class in a
                            -- derived instance
                  -> Type   -- The last argument to the class in a
                            -- derived instance
                  -> DerivM DerivInstTys
expectAlgTyConApp cls_tys inst_ty = do
  fam_envs <- lift tcGetFamInstEnvs
  case mk_deriv_inst_tys_maybe fam_envs cls_tys inst_ty of
    Nothing -> derivingThingFailWith NoGeneralizedNewtypeDeriving DerivErrLastArgMustBeApp
    Just dit -> do expectNonDataFamTyCon dit
                   pure dit

-- @expectNonDataFamTyCon dit@ checks if @dit_rep_tc dit@ is a representation
-- type constructor for a data family instance, and if not,
-- throws an error message.
-- See @Note [DerivEnv and DerivSpecMechanism]@ in "GHC.Tc.Deriv.Utils" for why this
-- property is important.
expectNonDataFamTyCon :: DerivInstTys -> DerivM ()
expectNonDataFamTyCon (DerivInstTys { dit_tc      = tc
                                    , dit_tc_args = tc_args
                                    , dit_rep_tc  = rep_tc }) =
  -- If it's still a data family, the lookup failed; i.e no instance exists
  when (isDataFamilyTyCon rep_tc) $
    derivingThingFailWith NoGeneralizedNewtypeDeriving $
      DerivErrNoFamilyInstance tc tc_args

mk_deriv_inst_tys_maybe :: FamInstEnvs
                        -> [Type] -> Type -> Maybe DerivInstTys
mk_deriv_inst_tys_maybe fam_envs cls_tys inst_ty =
  fmap lookup $ tcSplitTyConApp_maybe inst_ty
  where
    lookup :: (TyCon, [Type]) -> DerivInstTys
    lookup (tc, tc_args) =
      -- Find the instance of a data family
      -- Note [Looking up family instances for deriving]
      let (rep_tc, rep_tc_args, _co) = tcLookupDataFamInst fam_envs tc tc_args
      in DerivInstTys { dit_cls_tys     = cls_tys
                      , dit_tc          = tc
                      , dit_tc_args     = tc_args
                      , dit_rep_tc      = rep_tc
                      , dit_rep_tc_args = rep_tc_args }

{-
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 families 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 GHC.Core.Coercion.Axiom

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

-- Once the DerivSpecMechanism is known, we can finally produce an
-- EarlyDerivSpec from it.
mk_eqn_from_mechanism :: DerivSpecMechanism -> DerivM EarlyDerivSpec
mk_eqn_from_mechanism mechanism
  = do DerivEnv { denv_overlap_mode = overlap_mode
                , denv_tvs          = tvs
                , denv_cls          = cls
                , denv_inst_tys     = inst_tys
                , denv_ctxt         = deriv_ctxt } <- ask
       doDerivInstErrorChecks1 mechanism
       loc       <- lift getSrcSpanM
       dfun_name <- lift $ newDFunName cls inst_tys loc
       case deriv_ctxt of
        InferContext wildcard ->
          do { (inferred_constraints, tvs', inst_tys')
                 <- inferConstraints mechanism
             ; return $ InferTheta $ DS
                   { ds_loc = loc
                   , ds_name = dfun_name, ds_tvs = tvs'
                   , ds_cls = cls, ds_tys = inst_tys'
                   , ds_theta = inferred_constraints
                   , ds_overlap = overlap_mode
                   , ds_standalone_wildcard = wildcard
                   , ds_mechanism = mechanism } }

        SupplyContext theta ->
            return $ GivenTheta $ DS
                   { ds_loc = loc
                   , ds_name = dfun_name, ds_tvs = tvs
                   , ds_cls = cls, ds_tys = inst_tys
                   , ds_theta = theta
                   , ds_overlap = overlap_mode
                   , ds_standalone_wildcard = Nothing
                   , ds_mechanism = mechanism }

mk_eqn_stock :: DerivInstTys -- Information about the arguments to the class
             -> DerivM EarlyDerivSpec
mk_eqn_stock dit
  = do DerivEnv { denv_cls  = cls
                , denv_ctxt = deriv_ctxt } <- ask
       dflags <- getDynFlags
       let isDeriveAnyClassEnabled =
             deriveAnyClassEnabled (xopt LangExt.DeriveAnyClass dflags)
       case checkOriginativeSideConditions dflags deriv_ctxt cls dit of
         CanDeriveStock gen_fn -> mk_eqn_from_mechanism $
                                  DerivSpecStock { dsm_stock_dit    = dit
                                                 , dsm_stock_gen_fn = gen_fn }
         StockClassError why   -> derivingThingFailWith NoGeneralizedNewtypeDeriving why
         CanDeriveAnyClass     -> derivingThingFailWith NoGeneralizedNewtypeDeriving
                                    (DerivErrNotStockDeriveable isDeriveAnyClassEnabled)
         -- In the 'NonDerivableClass' case we can't derive with either stock or anyclass
         -- so we /don't want/ to suggest the user to enabled 'DeriveAnyClass', that's
         -- why we pass 'YesDeriveAnyClassEnabled', so that GHC won't attempt to suggest it.
         NonDerivableClass     -> derivingThingFailWith NoGeneralizedNewtypeDeriving
                                    (DerivErrNotStockDeriveable YesDeriveAnyClassEnabled)

mk_eqn_anyclass :: DerivM EarlyDerivSpec
mk_eqn_anyclass
  = do dflags <- getDynFlags
       let isDeriveAnyClassEnabled =
             deriveAnyClassEnabled (xopt LangExt.DeriveAnyClass dflags)
       case xopt LangExt.DeriveAnyClass dflags of
         True  -> mk_eqn_from_mechanism DerivSpecAnyClass
         False -> derivingThingFailWith NoGeneralizedNewtypeDeriving
                                        (DerivErrNotDeriveable isDeriveAnyClassEnabled)

mk_eqn_newtype :: DerivInstTys -- Information about the arguments to the class
               -> Type         -- The newtype's representation type
               -> DerivM EarlyDerivSpec
mk_eqn_newtype dit rep_ty =
  mk_eqn_from_mechanism $ DerivSpecNewtype { dsm_newtype_dit    = dit
                                           , dsm_newtype_rep_ty = rep_ty }

mk_eqn_via :: [Type] -- All arguments to the class besides the last
           -> Type   -- The last argument to the class
           -> Type   -- The @via@ type
           -> DerivM EarlyDerivSpec
mk_eqn_via cls_tys inst_ty via_ty =
  mk_eqn_from_mechanism $ DerivSpecVia { dsm_via_cls_tys = cls_tys
                                       , dsm_via_inst_ty = inst_ty
                                       , dsm_via_ty      = via_ty }

-- Derive an instance without a user-requested deriving strategy. This uses
-- heuristics to determine which deriving strategy to use.
-- See Note [Deriving strategies].
mk_eqn_no_strategy :: DerivM EarlyDerivSpec
mk_eqn_no_strategy = do
  DerivEnv { denv_cls      = cls
           , denv_inst_tys = cls_args } <- ask
  fam_envs <- lift tcGetFamInstEnvs

  -- First, check if the last argument is an application of a type constructor.
  -- If not, fall back to DeriveAnyClass.
  if |  Just (cls_tys, inst_ty) <- snocView cls_args
     ,  Just dit <- mk_deriv_inst_tys_maybe fam_envs cls_tys inst_ty
     -> if |  isNewTyCon (dit_rep_tc dit)
              -- We have a dedicated code path for newtypes (see the
              -- documentation for mkNewTypeEqn as to why this is the case)
           -> mkNewTypeEqn False dit

           |  otherwise
           -> do -- Otherwise, our only other options are stock or anyclass.
                 -- If it is stock, we must confirm that the last argument's
                 -- type constructor is algebraic.
                 -- See Note [DerivEnv and DerivSpecMechanism] in GHC.Tc.Deriv.Utils
                 whenIsJust (hasStockDeriving cls) $ \_ ->
                   expectNonDataFamTyCon dit
                 mk_eqn_originative dit

     |  otherwise
     -> mk_eqn_anyclass
  where
    -- Use heuristics (checkOriginativeSideConditions) to determine whether
    -- stock or anyclass deriving should be used.
    mk_eqn_originative :: DerivInstTys -> DerivM EarlyDerivSpec
    mk_eqn_originative dit@(DerivInstTys { dit_tc     = tc
                                         , dit_rep_tc = rep_tc }) = do
      DerivEnv { denv_cls  = cls
               , denv_ctxt = deriv_ctxt } <- ask
      dflags <- getDynFlags
      let isDeriveAnyClassEnabled =
            deriveAnyClassEnabled (xopt LangExt.DeriveAnyClass dflags)

      -- See Note [Deriving instances for classes themselves]
      let dac_error
            | isClassTyCon rep_tc
            = DerivErrOnlyAnyClassDeriveable tc isDeriveAnyClassEnabled
            | otherwise
            = DerivErrNotStockDeriveable isDeriveAnyClassEnabled

      case checkOriginativeSideConditions dflags deriv_ctxt cls dit of
        NonDerivableClass     -> derivingThingFailWith NoGeneralizedNewtypeDeriving dac_error
        StockClassError why   -> derivingThingFailWith NoGeneralizedNewtypeDeriving why
        CanDeriveStock gen_fn -> mk_eqn_from_mechanism $
                                 DerivSpecStock { dsm_stock_dit    = dit
                                                , dsm_stock_gen_fn = gen_fn }
        CanDeriveAnyClass     -> mk_eqn_from_mechanism DerivSpecAnyClass

{-
************************************************************************
*                                                                      *
            Deriving instances for newtypes
*                                                                      *
************************************************************************
-}

-- Derive an instance for a newtype. We put this logic into its own function
-- because
--
-- (a) When no explicit deriving strategy is requested, we have special
--     heuristics for newtypes to determine which deriving strategy should
--     actually be used. See Note [Deriving strategies].
-- (b) We make an effort to give error messages specifically tailored to
--     newtypes.
mkNewTypeEqn :: Bool -- Was this instance derived using an explicit @newtype@
                     -- deriving strategy?
             -> DerivInstTys -> DerivM EarlyDerivSpec
mkNewTypeEqn newtype_strat dit@(DerivInstTys { dit_cls_tys     = cls_tys
                                             , dit_rep_tc      = rep_tycon
                                             , dit_rep_tc_args = rep_tc_args })
-- Want: instance (...) => cls (cls_tys ++ [tycon tc_args]) where ...
  = do DerivEnv { denv_cls   = cls
                , denv_ctxt  = deriv_ctxt } <- ask
       dflags <- getDynFlags

       let newtype_deriving  = xopt LangExt.GeneralizedNewtypeDeriving dflags
           deriveAnyClass    = xopt LangExt.DeriveAnyClass             dflags

           bale_out = derivingThingFailWith (usingGeneralizedNewtypeDeriving newtype_deriving)

           -- 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 tyvars
           --       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

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

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

           -- Check that eta reduction is OK
           eta_ok = rep_tc_args `lengthAtLeast` nt_eta_arity
             -- 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.

       massert (cls_tys `lengthIs` (classArity cls - 1))
       if newtype_strat
       then
           -- Since the user explicitly asked for GeneralizedNewtypeDeriving,
           -- we don't need to perform all of the checks we normally would,
           -- such as if the class being derived is known to produce ill-roled
           -- coercions (e.g., Traversable), since we can just derive the
           -- instance and let it error if need be.
           -- See Note [Determining whether newtype-deriving is appropriate]
           if eta_ok && newtype_deriving
             then mk_eqn_newtype dit rep_inst_ty
             else bale_out (DerivErrCannotEtaReduceEnough eta_ok)
       else
         if might_be_newtype_derivable
             && ((newtype_deriving && not deriveAnyClass)
                  || std_class_via_coercible cls)
         then mk_eqn_newtype dit rep_inst_ty
         else case checkOriginativeSideConditions dflags deriv_ctxt cls dit of
               StockClassError why
                 -- There's a particular corner case where
                 --
                 -- 1. -XGeneralizedNewtypeDeriving and -XDeriveAnyClass are
                 --    both enabled at the same time
                 -- 2. We're deriving a particular stock derivable class
                 --    (such as Functor)
                 --
                 -- and the previous cases won't catch it. This fixes the bug
                 -- reported in #10598.
                 | might_be_newtype_derivable && newtype_deriving
                -> mk_eqn_newtype dit rep_inst_ty
                 -- Otherwise, throw an error for a stock class
                 | might_be_newtype_derivable && not newtype_deriving
                -> bale_out why
                 | otherwise
                -> bale_out why

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

               -- DeriveAnyClass
               CanDeriveAnyClass -> do
                 -- If both DeriveAnyClass and GeneralizedNewtypeDeriving are
                 -- enabled, we take the diplomatic approach of defaulting to
                 -- DeriveAnyClass, but emitting a warning about the choice.
                 -- See Note [Deriving strategies]
                 when (newtype_deriving && deriveAnyClass) $
                   lift $ addDiagnosticTc
                        $ TcRnDerivingDefaults cls
                 mk_eqn_from_mechanism DerivSpecAnyClass
               -- CanDeriveStock
               CanDeriveStock gen_fn -> mk_eqn_from_mechanism $
                                        DerivSpecStock { dsm_stock_dit    = dit
                                                       , dsm_stock_gen_fn = gen_fn }

{-
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.

"Glaring reasons not to" include trying to derive a class for which a
coercion-based instance doesn't make sense. These classes are listed in
the definition of non_coercible_class. They include Show (since it must
show the name of the datatype) and Traversable (since a coercion-based
Traversable instance is ill-roled).

However, non_coercible_class is ignored if the user explicitly requests
to derive an instance with GeneralizedNewtypeDeriving using the newtype
deriving strategy. In such a scenario, GHC will unquestioningly try to
derive the instance via coercions (even if the final generated code is
ill-roled!). See Note [Deriving strategies].

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.

Note [GND and associated type families]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
It's possible to use GeneralizedNewtypeDeriving (GND) to derive instances for
classes with associated type families. A general recipe is:

    class C x y z where
      type T y z x
      op :: x -> [y] -> z

    newtype N a = MkN <rep-type> deriving( C )

    =====>

    instance C x y <rep-type> => C x y (N a) where
      type T y (N a) x = T y <rep-type> x
      op = coerce (op :: x -> [y] -> <rep-type>)

However, we must watch out for three things:

(a) The class must not contain any data families. If it did, we'd have to
    generate a fresh data constructor name for the derived data family
    instance, and it's not clear how to do this.

(b) Each associated type family's type variables must mention the last type
    variable of the class. As an example, you wouldn't be able to use GND to
    derive an instance of this class:

      class C a b where
        type T a

    But you would be able to derive an instance of this class:

      class C a b where
        type T b

    The difference is that in the latter T mentions the last parameter of C
    (i.e., it mentions b), but the former T does not. If you tried, e.g.,

      newtype Foo x = Foo x deriving (C a)

    with the former definition of C, you'd end up with something like this:

      instance C a (Foo x) where
        type T a = T ???

    This T family instance doesn't mention the newtype (or its representation
    type) at all, so we disallow such constructions with GND.

(c) UndecidableInstances might need to be enabled. Here's a case where it is
    most definitely necessary:

      class C a where
        type T a
      newtype Loop = Loop MkLoop deriving C

      =====>

      instance C Loop where
        type T Loop = T Loop

    Obviously, T Loop would send the typechecker into a loop. Unfortunately,
    you might even need UndecidableInstances even in cases where the
    typechecker would be guaranteed to terminate. For example:

      instance C Int where
        type C Int = Int
      newtype MyInt = MyInt Int deriving C

      =====>

      instance C MyInt where
        type T MyInt = T Int

    GHC's termination checker isn't sophisticated enough to conclude that the
    definition of T MyInt terminates, so UndecidableInstances is required.

(d) For the time being, we do not allow the last type variable of the class to
    appear in a /kind/ of an associated type family definition. For instance:

    class C a where
      type T1 a        -- OK
      type T2 (x :: a) -- Illegal: a appears in the kind of x
      type T3 y :: a   -- Illegal: a appears in the kind of (T3 y)

    The reason we disallow this is because our current approach to deriving
    associated type family instances—i.e., by unwrapping the newtype's type
    constructor as shown above—is ill-equipped to handle the scenario when
    the last type variable appears as an implicit argument. In the worst case,
    allowing the last variable to appear in a kind can result in improper Core
    being generated (see #14728).

    There is hope for this feature being added some day, as one could
    conceivably take a newtype axiom (which witnesses a coercion between a
    newtype and its representation type) at lift that through each associated
    type at the Core level. See #14728, comment:3 for a sketch of how this
    might work. Until then, we disallow this featurette wholesale.

The same criteria apply to DerivingVia.

************************************************************************
*                                                                      *
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
-- plus any auxiliary bindings required
genInst :: DerivSpec theta
        -> TcM (ThetaType -> TcM (InstInfo GhcPs), BagDerivStuff, [Name])
-- We must use continuation-returning style here to get the order in which we
-- typecheck family instances and derived instances right.
-- See Note [Staging of tcDeriving]
genInst spec@(DS { ds_tvs = tvs, ds_mechanism = mechanism
                 , ds_tys = tys, ds_cls = clas, ds_loc = loc
                 , ds_standalone_wildcard = wildcard })
  = do (meth_binds, meth_sigs, deriv_stuff, unusedNames)
         <- set_span_and_ctxt $
            genDerivStuff mechanism loc clas tys tvs
       let mk_inst_info theta = set_span_and_ctxt $ do
             inst_spec <- newDerivClsInst theta spec
             doDerivInstErrorChecks2 clas inst_spec theta wildcard mechanism
             traceTc "newder" (ppr inst_spec)
             return $ InstInfo
                       { iSpec   = inst_spec
                       , iBinds  = InstBindings
                                     { ib_binds = meth_binds
                                     , ib_tyvars = map Var.varName tvs
                                     , ib_pragmas = meth_sigs
                                     , ib_extensions = extensions
                                     , ib_derived = True } }
       return (mk_inst_info, deriv_stuff, unusedNames)
  where
    extensions :: [LangExt.Extension]
    extensions
      | isDerivSpecNewtype mechanism || isDerivSpecVia mechanism
      = [
          -- Both these flags are needed for higher-rank uses of coerce...
          LangExt.ImpredicativeTypes, LangExt.RankNTypes
          -- ...and this flag is needed to support the instance signatures
          -- that bring type variables into scope.
          -- See Note [Newtype-deriving instances] in GHC.Tc.Deriv.Generate
        , LangExt.InstanceSigs
        ]
      | otherwise
      = []

    set_span_and_ctxt :: TcM a -> TcM a
    set_span_and_ctxt = setSrcSpan loc . addErrCtxt (instDeclCtxt3 clas tys)

-- Checks:
--
-- * All of the data constructors for a data type are in scope for a
--   standalone-derived instance (for `stock` and `newtype` deriving).
--
-- * All of the associated type families of a class are suitable for
--   GeneralizedNewtypeDeriving or DerivingVia (for `newtype` and `via`
--   deriving).
doDerivInstErrorChecks1 :: DerivSpecMechanism -> DerivM ()
doDerivInstErrorChecks1 mechanism =
  case mechanism of
    DerivSpecStock{dsm_stock_dit = dit}
      -> data_cons_in_scope_check dit
    DerivSpecNewtype{dsm_newtype_dit = dit}
      -> do atf_coerce_based_error_checks
            data_cons_in_scope_check dit
    DerivSpecAnyClass{}
      -> pure ()
    DerivSpecVia{}
      -> atf_coerce_based_error_checks
  where
    -- When processing a standalone deriving declaration, check that all of the
    -- constructors for the data type are in scope. For instance:
    --
    --   import M (T)
    --   deriving stock instance Eq T
    --
    -- This should be rejected, as the derived Eq instance would need to refer
    -- to the constructors for T, which are not in scope.
    --
    -- Note that the only strategies that require this check are `stock` and
    -- `newtype`. Neither `anyclass` nor `via` require it as the code that they
    -- generate does not require using data constructors.
    data_cons_in_scope_check :: DerivInstTys -> DerivM ()
    data_cons_in_scope_check (DerivInstTys { dit_tc     = tc
                                           , dit_rep_tc = rep_tc }) = do
      standalone <- isStandaloneDeriv
      when standalone $ do
        let bale_out msg = do err <- derivingThingErrMechanism mechanism msg
                              lift $ failWithTc err

        rdr_env <- lift getGlobalRdrEnv
        let data_con_names = map dataConName (tyConDataCons rep_tc)
            hidden_data_cons = not (isWiredIn rep_tc) &&
                               (isAbstractTyCon rep_tc ||
                                any not_in_scope data_con_names)
            not_in_scope dc  = isNothing (lookupGRE_Name rdr_env dc)

        -- Make sure to also mark the data constructors as used so that GHC won't
        -- mistakenly emit -Wunused-imports warnings about them.
        lift $ addUsedDataCons rdr_env rep_tc

        unless (not hidden_data_cons) $
          bale_out $ DerivErrDataConsNotAllInScope tc

    -- Ensure that a class's associated type variables are suitable for
    -- GeneralizedNewtypeDeriving or DerivingVia. Unsurprisingly, this check is
    -- only required for the `newtype` and `via` strategies.
    --
    -- See Note [GND and associated type families]
    atf_coerce_based_error_checks :: DerivM ()
    atf_coerce_based_error_checks = do
      cls <- asks denv_cls
      let bale_out msg = do err <- derivingThingErrMechanism mechanism msg
                            lift $ failWithTc err

          cls_tyvars = classTyVars cls

          ats_look_sensible
             =  -- Check (a) from Note [GND and associated type families]
                no_adfs
                -- Check (b) from Note [GND and associated type families]
             && isNothing at_without_last_cls_tv
                -- Check (d) from Note [GND and associated type families]
             && isNothing at_last_cls_tv_in_kinds

          (adf_tcs, atf_tcs) = partition isDataFamilyTyCon at_tcs
          no_adfs            = null adf_tcs
                 -- We cannot newtype-derive data family instances

          at_without_last_cls_tv
            = find (\tc -> last_cls_tv `notElem` tyConTyVars tc) atf_tcs
          at_last_cls_tv_in_kinds
            = find (\tc -> any (at_last_cls_tv_in_kind . tyVarKind)
                               (tyConTyVars tc)
                        || at_last_cls_tv_in_kind (tyConResKind tc)) atf_tcs
          at_last_cls_tv_in_kind kind
            = last_cls_tv `elemVarSet` exactTyCoVarsOfType kind
          at_tcs = classATs cls
          last_cls_tv = assert (notNull cls_tyvars )
                        last cls_tyvars

      unless ats_look_sensible $
        bale_out (DerivErrHasAssociatedDatatypes
                   (hasAssociatedDataFamInsts (not no_adfs))
                   (associatedTyLastVarInKind at_last_cls_tv_in_kinds)
                   (associatedTyNotParamOverLastTyVar at_without_last_cls_tv)
                 )

doDerivInstErrorChecks2 :: Class -> ClsInst -> ThetaType -> Maybe SrcSpan
                        -> DerivSpecMechanism -> TcM ()
doDerivInstErrorChecks2 clas clas_inst theta wildcard mechanism
  = do { traceTc "doDerivInstErrorChecks2" (ppr clas_inst)
       ; dflags <- getDynFlags
       ; xpartial_sigs <- xoptM LangExt.PartialTypeSignatures
       ; wpartial_sigs <- woptM Opt_WarnPartialTypeSignatures

         -- Error if PartialTypeSignatures isn't enabled when a user tries
         -- to write @deriving instance _ => Eq (Foo a)@. Or, if that
         -- extension is enabled, give a warning if -Wpartial-type-signatures
         -- is enabled.
       ; case wildcard of
           Nothing -> pure ()
           Just span -> setSrcSpan span $ do
             let suggParSigs = suggestPartialTypeSignatures xpartial_sigs
             let dia = TcRnPartialTypeSignatures suggParSigs theta
             checkTc xpartial_sigs dia
             diagnosticTc wpartial_sigs dia

         -- Check for Generic instances that are derived with an exotic
         -- deriving strategy like DAC
         -- See Note [Deriving strategies]
       ; when (exotic_mechanism && className clas `elem` genericClassNames) $
         do { failIfTc (safeLanguageOn dflags)
                       (TcRnCannotDeriveInstance clas mempty Nothing NoGeneralizedNewtypeDeriving $
                          DerivErrSafeHaskellGenericInst)
            ; when (safeInferOn dflags) (recordUnsafeInfer emptyMessages) } }
  where
    exotic_mechanism = not $ isDerivSpecStock mechanism

derivingThingFailWith :: UsingGeneralizedNewtypeDeriving
                         -- ^ If 'YesGeneralizedNewtypeDeriving', add a snippet about
                         -- how not even GeneralizedNewtypeDeriving would make this
                         -- declaration work. This only kicks in when
                         -- an explicit deriving strategy is not given.
                      -> DeriveInstanceErrReason -- The reason the derivation failed
                      -> DerivM a
derivingThingFailWith newtype_deriving msg = do
  err <- derivingThingErrM newtype_deriving msg
  lift $ failWithTc err

genDerivStuff :: DerivSpecMechanism -> SrcSpan -> Class
              -> [Type] -> [TyVar]
              -> TcM (LHsBinds GhcPs, [LSig GhcPs], BagDerivStuff, [Name])
genDerivStuff mechanism loc clas inst_tys tyvars
  = case mechanism of
      -- See Note [Bindings for Generalised Newtype Deriving]
      DerivSpecNewtype { dsm_newtype_rep_ty = rhs_ty}
        -> gen_newtype_or_via rhs_ty

      -- Try a stock deriver
      DerivSpecStock { dsm_stock_dit    = dit
                     , dsm_stock_gen_fn = gen_fn }
        -> gen_fn loc inst_tys dit

      -- Try DeriveAnyClass
      DerivSpecAnyClass -> do
        let mini_env   = mkVarEnv (classTyVars clas `zip` inst_tys)
            mini_subst = mkTvSubst (mkInScopeSet (mkVarSet tyvars)) mini_env
        dflags <- getDynFlags
        tyfam_insts <-
          -- canDeriveAnyClass should ensure that this code can't be reached
          -- unless -XDeriveAnyClass is enabled.
          assertPpr (xopt LangExt.DeriveAnyClass dflags)
                    (ppr "genDerivStuff: bad derived class" <+> ppr clas) $
          mapM (tcATDefault 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]
               , [] )

      -- Try DerivingVia
      DerivSpecVia{dsm_via_ty = via_ty}
        -> gen_newtype_or_via via_ty
  where
    gen_newtype_or_via ty = do
      (binds, sigs, faminsts) <- gen_Newtype_binds loc clas tyvars inst_tys ty
      return (binds, sigs, faminsts, [])

{-
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 Haskell".

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 GHC.Tc.TyCl.Class (which is also used in GHC.Tc.TyCl.Instance to
handle the empty instance declaration case).

Note [Deriving strategies]
~~~~~~~~~~~~~~~~~~~~~~~~~~
GHC has a notion of deriving strategies, which allow the user to explicitly
request which approach to use when deriving an instance (enabled with the
-XDerivingStrategies language extension). For more information, refer to the
original issue (#10598) or the associated wiki page:
https://gitlab.haskell.org/ghc/ghc/wikis/commentary/compiler/deriving-strategies

A deriving strategy can be specified in a deriving clause:

    newtype Foo = MkFoo Bar
      deriving newtype C

Or in a standalone deriving declaration:

    deriving anyclass instance C Foo

-XDerivingStrategies also allows the use of multiple deriving clauses per data
declaration so that a user can derive some instance with one deriving strategy
and other instances with another deriving strategy. For example:

    newtype Baz = Baz Quux
      deriving          (Eq, Ord)
      deriving stock    (Read, Show)
      deriving newtype  (Num, Floating)
      deriving anyclass C

Currently, the deriving strategies are:

* stock: Have GHC implement a "standard" instance for a data type, if possible
  (e.g., Eq, Ord, Generic, Data, Functor, etc.)

* anyclass: Use -XDeriveAnyClass

* newtype: Use -XGeneralizedNewtypeDeriving

* via: Use -XDerivingVia

The latter two strategies (newtype and via) are referred to as the
"coerce-based" strategies, since they generate code that relies on the `coerce`
function. See, for instance, GHC.Tc.Deriv.Infer.inferConstraintsCoerceBased.

The former two strategies (stock and anyclass), in contrast, are
referred to as the "originative" strategies, since they create "original"
instances instead of "reusing" old instances (by way of `coerce`).
See, for instance, GHC.Tc.Deriv.Utils.checkOriginativeSideConditions.

If an explicit deriving strategy is not given, GHC has an algorithm it uses to
determine which strategy it will actually use. The algorithm is quite long,
so it lives in the Haskell wiki at
https://gitlab.haskell.org/ghc/ghc/wikis/commentary/compiler/deriving-strategies
("The deriving strategy resolution algorithm" section).

Internally, GHC uses the DerivStrategy datatype to denote a user-requested
deriving strategy, and it uses the DerivSpecMechanism datatype to denote what
GHC will use to derive the instance after taking the above steps. In other
words, GHC will always settle on a DerivSpecMechnism, even if the user did not
ask for a particular DerivStrategy (using the algorithm linked to above).

Note [Deriving instances for classes themselves]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Much of the code in GHC.Tc.Deriv assumes that deriving only works on data types.
But this assumption doesn't hold true for DeriveAnyClass, since it's perfectly
reasonable to do something like this:

  {-# LANGUAGE DeriveAnyClass #-}
  class C1 (a :: Constraint) where
  class C2 where
  deriving instance C1 C2
    -- This is equivalent to `instance C1 C2`

If DeriveAnyClass isn't enabled in the code above (i.e., it defaults to stock
deriving), we throw a special error message indicating that DeriveAnyClass is
the only way to go. We don't bother throwing this error if an explicit 'stock'
or 'newtype' keyword is used, since both options have their own perfectly
sensible error messages in the case of the above code (as C1 isn't a stock
derivable class, and C2 isn't a newtype).

************************************************************************
*                                                                      *
What con2tag/tag2con functions are available?
*                                                                      *
************************************************************************
-}

derivingThingErrM :: UsingGeneralizedNewtypeDeriving
                  -> DeriveInstanceErrReason
                  -> DerivM TcRnMessage
derivingThingErrM newtype_deriving why
  = do DerivEnv { denv_cls      = cls
                , denv_inst_tys = cls_args
                , denv_strat    = mb_strat } <- ask
       pure $ TcRnCannotDeriveInstance cls cls_args mb_strat newtype_deriving why

derivingThingErrMechanism :: DerivSpecMechanism -> DeriveInstanceErrReason -> DerivM TcRnMessage
derivingThingErrMechanism mechanism why
  = do DerivEnv { denv_cls      = cls
                , denv_inst_tys = cls_args
                , denv_strat    = mb_strat } <- ask
       pure $ TcRnCannotDeriveInstance cls cls_args mb_strat newtype_deriving why
  where
    newtype_deriving :: UsingGeneralizedNewtypeDeriving
    newtype_deriving
      = if isDerivSpecNewtype mechanism then YesGeneralizedNewtypeDeriving
                                        else NoGeneralizedNewtypeDeriving

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