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


Functions for inferring (and simplifying) the context for derived instances.
-}

{-# LANGUAGE CPP #-}
{-# LANGUAGE MultiWayIf #-}

module TcDerivInfer (inferConstraints, simplifyInstanceContexts) where

#include "HsVersions.h"

import GhcPrelude

import Bag
import BasicTypes
import Class
import DataCon
import ErrUtils
import Inst
import Outputable
import PrelNames
import TcDerivUtils
import TcEnv
import TcGenFunctor
import TcGenGenerics
import TcMType
import TcRnMonad
import TcType
import TyCon
import Type
import TcSimplify
import TcValidity (validDerivPred)
import TcUnify (buildImplicationFor, checkConstraints)
import Unify (tcUnifyTy)
import Util
import Var
import VarSet

import Control.Monad
import Control.Monad.Trans.Class  (lift)
import Control.Monad.Trans.Reader (ask)
import Data.List
import Data.Maybe

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

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

-- e.g. inferConstraints
--        C Int (T [a])    -- Class and inst_tys
--        :RTList a        -- Rep tycon and its arg tys
-- where T [a] ~R :RTList a
--
-- Generate a sufficiently large set of constraints that typechecking the
-- generated method definitions should succeed.   This set will be simplified
-- before being used in the instance declaration
inferConstraints :: DerivSpecMechanism -> DerivM ([ThetaOrigin], [TyVar], [TcType])
inferConstraints mechanism :: DerivSpecMechanism
mechanism
  = do { DerivEnv { denv_tc :: DerivEnv -> TyCon
denv_tc          = TyCon
tc
                  , denv_tc_args :: DerivEnv -> [TcType]
denv_tc_args     = [TcType]
tc_args
                  , denv_cls :: DerivEnv -> Class
denv_cls         = Class
main_cls
                  , denv_cls_tys :: DerivEnv -> [TcType]
denv_cls_tys     = [TcType]
cls_tys } <- ReaderT DerivEnv TcRn DerivEnv
forall (m :: * -> *) r. Monad m => ReaderT r m r
ask
       ; Bool
wildcard <- DerivM Bool
isStandaloneWildcardDeriv
       ; let is_anyclass :: Bool
is_anyclass = DerivSpecMechanism -> Bool
isDerivSpecAnyClass DerivSpecMechanism
mechanism
             infer_constraints :: DerivM ([ThetaOrigin], [TyVar], [TcType])
infer_constraints
               | Bool
is_anyclass = [TcType] -> DerivM ([ThetaOrigin], [TyVar], [TcType])
inferConstraintsDAC [TcType]
inst_tys
               | Bool
otherwise   = TcType -> [TcType] -> DerivM ([ThetaOrigin], [TyVar], [TcType])
inferConstraintsDataConArgs TcType
inst_ty [TcType]
inst_tys

             inst_ty :: TcType
inst_ty  = TyCon -> [TcType] -> TcType
mkTyConApp TyCon
tc [TcType]
tc_args
             inst_tys :: [TcType]
inst_tys = [TcType]
cls_tys [TcType] -> [TcType] -> [TcType]
forall a. [a] -> [a] -> [a]
++ [TcType
inst_ty]

             -- Constraints arising from superclasses
             -- See Note [Superclasses of derived instance]
             cls_tvs :: [TyVar]
cls_tvs  = Class -> [TyVar]
classTyVars Class
main_cls
             sc_constraints :: [ThetaOrigin]
sc_constraints = ASSERT2( equalLength cls_tvs inst_tys
                                     , ppr main_cls <+> ppr inst_tys )
                              [ CtOrigin
-> TypeOrKind
-> [TyVar]
-> [TyVar]
-> [TcType]
-> [TcType]
-> ThetaOrigin
mkThetaOrigin (Bool -> CtOrigin
mkDerivOrigin Bool
wildcard)
                                              TypeOrKind
TypeLevel [] [] [] ([TcType] -> ThetaOrigin) -> [TcType] -> ThetaOrigin
forall a b. (a -> b) -> a -> b
$
                                HasCallStack => TCvSubst -> [TcType] -> [TcType]
TCvSubst -> [TcType] -> [TcType]
substTheta TCvSubst
cls_subst (Class -> [TcType]
classSCTheta Class
main_cls) ]
             cls_subst :: TCvSubst
cls_subst = ASSERT( equalLength cls_tvs inst_tys )
                         [TyVar] -> [TcType] -> TCvSubst
HasDebugCallStack => [TyVar] -> [TcType] -> TCvSubst
zipTvSubst [TyVar]
cls_tvs [TcType]
inst_tys

       ; (inferred_constraints :: [ThetaOrigin]
inferred_constraints, tvs' :: [TyVar]
tvs', inst_tys' :: [TcType]
inst_tys') <- DerivM ([ThetaOrigin], [TyVar], [TcType])
infer_constraints
       ; IOEnv (Env TcGblEnv TcLclEnv) () -> ReaderT DerivEnv TcRn ()
forall (t :: (* -> *) -> * -> *) (m :: * -> *) a.
(MonadTrans t, Monad m) =>
m a -> t m a
lift (IOEnv (Env TcGblEnv TcLclEnv) () -> ReaderT DerivEnv TcRn ())
-> IOEnv (Env TcGblEnv TcLclEnv) () -> ReaderT DerivEnv TcRn ()
forall a b. (a -> b) -> a -> b
$ String -> SDoc -> IOEnv (Env TcGblEnv TcLclEnv) ()
traceTc "inferConstraints" (SDoc -> IOEnv (Env TcGblEnv TcLclEnv) ())
-> SDoc -> IOEnv (Env TcGblEnv TcLclEnv) ()
forall a b. (a -> b) -> a -> b
$ [SDoc] -> SDoc
vcat
              [ Class -> SDoc
forall a. Outputable a => a -> SDoc
ppr Class
main_cls SDoc -> SDoc -> SDoc
<+> [TcType] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [TcType]
inst_tys'
              , [ThetaOrigin] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [ThetaOrigin]
inferred_constraints
              ]
       ; ([ThetaOrigin], [TyVar], [TcType])
-> DerivM ([ThetaOrigin], [TyVar], [TcType])
forall (m :: * -> *) a. Monad m => a -> m a
return ( [ThetaOrigin]
sc_constraints [ThetaOrigin] -> [ThetaOrigin] -> [ThetaOrigin]
forall a. [a] -> [a] -> [a]
++ [ThetaOrigin]
inferred_constraints
                , [TyVar]
tvs', [TcType]
inst_tys' ) }

-- | Like 'inferConstraints', but used only in the case of deriving strategies
-- where the constraints are inferred by inspecting the fields of each data
-- constructor (i.e., stock- and newtype-deriving).
inferConstraintsDataConArgs :: TcType -> [TcType]
                            -> DerivM ([ThetaOrigin], [TyVar], [TcType])
inferConstraintsDataConArgs :: TcType -> [TcType] -> DerivM ([ThetaOrigin], [TyVar], [TcType])
inferConstraintsDataConArgs inst_ty :: TcType
inst_ty inst_tys :: [TcType]
inst_tys
  = do DerivEnv { denv_tvs :: DerivEnv -> [TyVar]
denv_tvs         = [TyVar]
tvs
                , denv_rep_tc :: DerivEnv -> TyCon
denv_rep_tc      = TyCon
rep_tc
                , denv_rep_tc_args :: DerivEnv -> [TcType]
denv_rep_tc_args = [TcType]
rep_tc_args
                , denv_cls :: DerivEnv -> Class
denv_cls         = Class
main_cls
                , denv_cls_tys :: DerivEnv -> [TcType]
denv_cls_tys     = [TcType]
cls_tys } <- ReaderT DerivEnv TcRn DerivEnv
forall (m :: * -> *) r. Monad m => ReaderT r m r
ask
       Bool
wildcard <- DerivM Bool
isStandaloneWildcardDeriv

       let tc_binders :: [TyConBinder]
tc_binders = TyCon -> [TyConBinder]
tyConBinders TyCon
rep_tc
           choose_level :: TyConBinder -> TypeOrKind
choose_level bndr :: TyConBinder
bndr
             | TyConBinder -> Bool
isNamedTyConBinder TyConBinder
bndr = TypeOrKind
KindLevel
             | Bool
otherwise               = TypeOrKind
TypeLevel
           t_or_ks :: [TypeOrKind]
t_or_ks = (TyConBinder -> TypeOrKind) -> [TyConBinder] -> [TypeOrKind]
forall a b. (a -> b) -> [a] -> [b]
map TyConBinder -> TypeOrKind
choose_level [TyConBinder]
tc_binders [TypeOrKind] -> [TypeOrKind] -> [TypeOrKind]
forall a. [a] -> [a] -> [a]
++ TypeOrKind -> [TypeOrKind]
forall a. a -> [a]
repeat TypeOrKind
TypeLevel
              -- want to report *kind* errors when possible

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

           is_generic :: Bool
is_generic  = Class
main_cls Class -> Unique -> Bool
forall a. Uniquable a => a -> Unique -> Bool
`hasKey` Unique
genClassKey
           is_generic1 :: Bool
is_generic1 = Class
main_cls Class -> Unique -> Bool
forall a. Uniquable a => a -> Unique -> Bool
`hasKey` Unique
gen1ClassKey
           -- is_functor_like: see Note [Inferring the instance context]
           is_functor_like :: Bool
is_functor_like = HasDebugCallStack => TcType -> TcType
TcType -> TcType
tcTypeKind TcType
inst_ty HasDebugCallStack => TcType -> TcType -> Bool
TcType -> TcType -> Bool
`tcEqKind` TcType
typeToTypeKind
                          Bool -> Bool -> Bool
|| Bool
is_generic1

           get_gen1_constraints :: Class -> CtOrigin -> TypeOrKind -> Type
                                -> [([PredOrigin], Maybe TCvSubst)]
           get_gen1_constraints :: Class
-> CtOrigin
-> TypeOrKind
-> TcType
-> [([PredOrigin], Maybe TCvSubst)]
get_gen1_constraints functor_cls :: Class
functor_cls orig :: CtOrigin
orig t_or_k :: TypeOrKind
t_or_k ty :: TcType
ty
              = CtOrigin
-> TypeOrKind
-> Class
-> [TcType]
-> [([PredOrigin], Maybe TCvSubst)]
mk_functor_like_constraints CtOrigin
orig TypeOrKind
t_or_k Class
functor_cls ([TcType] -> [([PredOrigin], Maybe TCvSubst)])
-> [TcType] -> [([PredOrigin], Maybe TCvSubst)]
forall a b. (a -> b) -> a -> b
$
                TyVar -> TcType -> [TcType]
get_gen1_constrained_tys TyVar
last_tv TcType
ty

           get_std_constrained_tys :: CtOrigin -> TypeOrKind -> Type
                                   -> [([PredOrigin], Maybe TCvSubst)]
           get_std_constrained_tys :: CtOrigin
-> TypeOrKind -> TcType -> [([PredOrigin], Maybe TCvSubst)]
get_std_constrained_tys orig :: CtOrigin
orig t_or_k :: TypeOrKind
t_or_k ty :: TcType
ty
               | Bool
is_functor_like
               = CtOrigin
-> TypeOrKind
-> Class
-> [TcType]
-> [([PredOrigin], Maybe TCvSubst)]
mk_functor_like_constraints CtOrigin
orig TypeOrKind
t_or_k Class
main_cls ([TcType] -> [([PredOrigin], Maybe TCvSubst)])
-> [TcType] -> [([PredOrigin], Maybe TCvSubst)]
forall a b. (a -> b) -> a -> b
$
                 TyVar -> TcType -> [TcType]
deepSubtypesContaining TyVar
last_tv TcType
ty
               | Bool
otherwise
               = [( [CtOrigin -> TypeOrKind -> Class -> TcType -> PredOrigin
mk_cls_pred CtOrigin
orig TypeOrKind
t_or_k Class
main_cls TcType
ty]
                  , Maybe TCvSubst
forall a. Maybe a
Nothing )]

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

           rep_tc_tvs :: [TyVar]
rep_tc_tvs      = TyCon -> [TyVar]
tyConTyVars TyCon
rep_tc
           last_tv :: TyVar
last_tv         = [TyVar] -> TyVar
forall a. [a] -> a
last [TyVar]
rep_tc_tvs
           -- When we first gather up the constraints to solve, most of them
           -- contain rep_tc_tvs, i.e., the type variables from the derived
           -- datatype's type constructor. We don't want these type variables
           -- to appear in the final instance declaration, so we must
           -- substitute each type variable with its counterpart in the derived
           -- instance. rep_tc_args lists each of these counterpart types in
           -- the same order as the type variables.
           all_rep_tc_args :: [TcType]
all_rep_tc_args
             = [TcType]
rep_tc_args [TcType] -> [TcType] -> [TcType]
forall a. [a] -> [a] -> [a]
++ (TyVar -> TcType) -> [TyVar] -> [TcType]
forall a b. (a -> b) -> [a] -> [b]
map TyVar -> TcType
mkTyVarTy
                                  (Int -> [TyVar] -> [TyVar]
forall a. Int -> [a] -> [a]
drop ([TcType] -> Int
forall (t :: * -> *) a. Foldable t => t a -> Int
length [TcType]
rep_tc_args) [TyVar]
rep_tc_tvs)

               -- Stupid constraints
           stupid_constraints :: [ThetaOrigin]
stupid_constraints
             = [ CtOrigin
-> TypeOrKind
-> [TyVar]
-> [TyVar]
-> [TcType]
-> [TcType]
-> ThetaOrigin
mkThetaOrigin CtOrigin
deriv_origin TypeOrKind
TypeLevel [] [] [] ([TcType] -> ThetaOrigin) -> [TcType] -> ThetaOrigin
forall a b. (a -> b) -> a -> b
$
                 HasCallStack => TCvSubst -> [TcType] -> [TcType]
TCvSubst -> [TcType] -> [TcType]
substTheta TCvSubst
tc_subst (TyCon -> [TcType]
tyConStupidTheta TyCon
rep_tc) ]
           tc_subst :: TCvSubst
tc_subst = -- See the comment with all_rep_tc_args for an
                      -- explanation of this assertion
                      ASSERT( equalLength rep_tc_tvs all_rep_tc_args )
                      [TyVar] -> [TcType] -> TCvSubst
HasDebugCallStack => [TyVar] -> [TcType] -> TCvSubst
zipTvSubst [TyVar]
rep_tc_tvs [TcType]
all_rep_tc_args

           -- Extra Data constraints
           -- The Data class (only) requires that for
           --    instance (...) => Data (T t1 t2)
           -- IF   t1:*, t2:*
           -- THEN (Data t1, Data t2) are among the (...) constraints
           -- Reason: when the IF holds, we generate a method
           --             dataCast2 f = gcast2 f
           --         and we need the Data constraints to typecheck the method
           extra_constraints :: [ThetaOrigin]
extra_constraints = [[PredOrigin] -> ThetaOrigin
mkThetaOriginFromPreds [PredOrigin]
constrs]
             where
               constrs :: [PredOrigin]
constrs
                 | Class
main_cls Class -> Unique -> Bool
forall a. Uniquable a => a -> Unique -> Bool
`hasKey` Unique
dataClassKey
                 , (TcType -> Bool) -> [TcType] -> Bool
forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
all (TcType -> Bool
isLiftedTypeKind (TcType -> Bool) -> (TcType -> TcType) -> TcType -> Bool
forall b c a. (b -> c) -> (a -> b) -> a -> c
. HasDebugCallStack => TcType -> TcType
TcType -> TcType
tcTypeKind) [TcType]
rep_tc_args
                 = [ CtOrigin -> TypeOrKind -> Class -> TcType -> PredOrigin
mk_cls_pred CtOrigin
deriv_origin TypeOrKind
t_or_k Class
main_cls TcType
ty
                   | (t_or_k :: TypeOrKind
t_or_k, ty :: TcType
ty) <- [TypeOrKind] -> [TcType] -> [(TypeOrKind, TcType)]
forall a b. [a] -> [b] -> [(a, b)]
zip [TypeOrKind]
t_or_ks [TcType]
rep_tc_args]
                 | Bool
otherwise
                 = []

           mk_cls_pred :: CtOrigin -> TypeOrKind -> Class -> TcType -> PredOrigin
mk_cls_pred orig :: CtOrigin
orig t_or_k :: TypeOrKind
t_or_k cls :: Class
cls ty :: TcType
ty
                -- Don't forget to apply to cls_tys' too
              = CtOrigin -> TypeOrKind -> TcType -> PredOrigin
mkPredOrigin CtOrigin
orig TypeOrKind
t_or_k (Class -> [TcType] -> TcType
mkClassPred Class
cls ([TcType]
cls_tys' [TcType] -> [TcType] -> [TcType]
forall a. [a] -> [a] -> [a]
++ [TcType
ty]))
           cls_tys' :: [TcType]
cls_tys' | Bool
is_generic1 = []
                      -- In the awkward Generic1 case, cls_tys' should be
                      -- empty, since we are applying the class Functor.

                    | Bool
otherwise   = [TcType]
cls_tys

           deriv_origin :: CtOrigin
deriv_origin = Bool -> CtOrigin
mkDerivOrigin Bool
wildcard

       if    -- Generic constraints are easy
          |  Bool
is_generic
           -> ([ThetaOrigin], [TyVar], [TcType])
-> DerivM ([ThetaOrigin], [TyVar], [TcType])
forall (m :: * -> *) a. Monad m => a -> m a
return ([], [TyVar]
tvs, [TcType]
inst_tys)

             -- Generic1 needs Functor
             -- See Note [Getting base classes]
          |  Bool
is_generic1
           -> ASSERT( rep_tc_tvs `lengthExceeds` 0 )
              -- Generic1 has a single kind variable
              ASSERT( cls_tys `lengthIs` 1 )
              do { Class
functorClass <- IOEnv (Env TcGblEnv TcLclEnv) Class -> ReaderT DerivEnv TcRn Class
forall (t :: (* -> *) -> * -> *) (m :: * -> *) a.
(MonadTrans t, Monad m) =>
m a -> t m a
lift (IOEnv (Env TcGblEnv TcLclEnv) Class
 -> ReaderT DerivEnv TcRn Class)
-> IOEnv (Env TcGblEnv TcLclEnv) Class
-> ReaderT DerivEnv TcRn Class
forall a b. (a -> b) -> a -> b
$ Name -> IOEnv (Env TcGblEnv TcLclEnv) Class
tcLookupClass Name
functorClassName
                 ; ([ThetaOrigin], [TyVar], [TcType])
-> DerivM ([ThetaOrigin], [TyVar], [TcType])
forall (f :: * -> *) a. Applicative f => a -> f a
pure (([ThetaOrigin], [TyVar], [TcType])
 -> DerivM ([ThetaOrigin], [TyVar], [TcType]))
-> ([ThetaOrigin], [TyVar], [TcType])
-> DerivM ([ThetaOrigin], [TyVar], [TcType])
forall a b. (a -> b) -> a -> b
$ (CtOrigin
 -> TypeOrKind -> TcType -> [([PredOrigin], Maybe TCvSubst)])
-> ([ThetaOrigin], [TyVar], [TcType])
con_arg_constraints
                        ((CtOrigin
  -> TypeOrKind -> TcType -> [([PredOrigin], Maybe TCvSubst)])
 -> ([ThetaOrigin], [TyVar], [TcType]))
-> (CtOrigin
    -> TypeOrKind -> TcType -> [([PredOrigin], Maybe TCvSubst)])
-> ([ThetaOrigin], [TyVar], [TcType])
forall a b. (a -> b) -> a -> b
$ Class
-> CtOrigin
-> TypeOrKind
-> TcType
-> [([PredOrigin], Maybe TCvSubst)]
get_gen1_constraints Class
functorClass }

             -- The others are a bit more complicated
          |  Bool
otherwise
           -> -- See the comment with all_rep_tc_args for an explanation of
              -- this assertion
              ASSERT2( equalLength rep_tc_tvs all_rep_tc_args
                     , ppr main_cls <+> ppr rep_tc
                       $$ ppr rep_tc_tvs $$ ppr all_rep_tc_args )
                do { let (arg_constraints :: [ThetaOrigin]
arg_constraints, tvs' :: [TyVar]
tvs', inst_tys' :: [TcType]
inst_tys')
                           = (CtOrigin
 -> TypeOrKind -> TcType -> [([PredOrigin], Maybe TCvSubst)])
-> ([ThetaOrigin], [TyVar], [TcType])
con_arg_constraints CtOrigin
-> TypeOrKind -> TcType -> [([PredOrigin], Maybe TCvSubst)]
get_std_constrained_tys
                   ; IOEnv (Env TcGblEnv TcLclEnv) () -> ReaderT DerivEnv TcRn ()
forall (t :: (* -> *) -> * -> *) (m :: * -> *) a.
(MonadTrans t, Monad m) =>
m a -> t m a
lift (IOEnv (Env TcGblEnv TcLclEnv) () -> ReaderT DerivEnv TcRn ())
-> IOEnv (Env TcGblEnv TcLclEnv) () -> ReaderT DerivEnv TcRn ()
forall a b. (a -> b) -> a -> b
$ String -> SDoc -> IOEnv (Env TcGblEnv TcLclEnv) ()
traceTc "inferConstraintsDataConArgs" (SDoc -> IOEnv (Env TcGblEnv TcLclEnv) ())
-> SDoc -> IOEnv (Env TcGblEnv TcLclEnv) ()
forall a b. (a -> b) -> a -> b
$ [SDoc] -> SDoc
vcat
                          [ Class -> SDoc
forall a. Outputable a => a -> SDoc
ppr Class
main_cls SDoc -> SDoc -> SDoc
<+> [TcType] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [TcType]
inst_tys'
                          , [ThetaOrigin] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [ThetaOrigin]
arg_constraints
                          ]
                   ; ([ThetaOrigin], [TyVar], [TcType])
-> DerivM ([ThetaOrigin], [TyVar], [TcType])
forall (m :: * -> *) a. Monad m => a -> m a
return ( [ThetaOrigin]
stupid_constraints [ThetaOrigin] -> [ThetaOrigin] -> [ThetaOrigin]
forall a. [a] -> [a] -> [a]
++ [ThetaOrigin]
extra_constraints
                                                 [ThetaOrigin] -> [ThetaOrigin] -> [ThetaOrigin]
forall a. [a] -> [a] -> [a]
++ [ThetaOrigin]
arg_constraints
                            , [TyVar]
tvs', [TcType]
inst_tys') }

typeToTypeKind :: Kind
typeToTypeKind :: TcType
typeToTypeKind = TcType
liftedTypeKind TcType -> TcType -> TcType
`mkFunTy` TcType
liftedTypeKind

-- | Like 'inferConstraints', but used only in the case of @DeriveAnyClass@,
-- which gathers its constraints based on the type signatures of the class's
-- methods instead of the types of the data constructor's field.
--
-- See Note [Gathering and simplifying constraints for DeriveAnyClass]
-- for an explanation of how these constraints are used to determine the
-- derived instance context.
inferConstraintsDAC :: [TcType] -> DerivM ([ThetaOrigin], [TyVar], [TcType])
inferConstraintsDAC :: [TcType] -> DerivM ([ThetaOrigin], [TyVar], [TcType])
inferConstraintsDAC inst_tys :: [TcType]
inst_tys
  = do { DerivEnv { denv_tvs :: DerivEnv -> [TyVar]
denv_tvs = [TyVar]
tvs
                  , denv_cls :: DerivEnv -> Class
denv_cls = Class
cls } <- ReaderT DerivEnv TcRn DerivEnv
forall (m :: * -> *) r. Monad m => ReaderT r m r
ask
       ; Bool
wildcard <- DerivM Bool
isStandaloneWildcardDeriv

       ; let gen_dms :: [(TyVar, TcType)]
gen_dms = [ (TyVar
sel_id, TcType
dm_ty)
                       | (sel_id :: TyVar
sel_id, Just (_, GenericDM dm_ty :: TcType
dm_ty)) <- Class -> [(TyVar, DefMethInfo)]
classOpItems Class
cls ]

             cls_tvs :: [TyVar]
cls_tvs = Class -> [TyVar]
classTyVars Class
cls

             do_one_meth :: (Id, Type) -> TcM ThetaOrigin
               -- (Id,Type) are the selector Id and the generic default method type
               -- NB: the latter is /not/ quantified over the class variables
               -- See Note [Gathering and simplifying constraints for DeriveAnyClass]
             do_one_meth :: (TyVar, TcType) -> TcM ThetaOrigin
do_one_meth (sel_id :: TyVar
sel_id, gen_dm_ty :: TcType
gen_dm_ty)
               = do { let (sel_tvs :: [TyVar]
sel_tvs, _cls_pred :: TcType
_cls_pred, meth_ty :: TcType
meth_ty)
                                   = TcType -> ([TyVar], TcType, TcType)
tcSplitMethodTy (TyVar -> TcType
varType TyVar
sel_id)
                          meth_ty' :: TcType
meth_ty' = HasCallStack => [TyVar] -> [TcType] -> TcType -> TcType
[TyVar] -> [TcType] -> TcType -> TcType
substTyWith [TyVar]
sel_tvs [TcType]
inst_tys TcType
meth_ty
                          (meth_tvs :: [TyVar]
meth_tvs, meth_theta :: [TcType]
meth_theta, meth_tau :: TcType
meth_tau)
                                   = TcType -> ([TyVar], [TcType], TcType)
tcSplitNestedSigmaTys TcType
meth_ty'

                          gen_dm_ty' :: TcType
gen_dm_ty' = HasCallStack => [TyVar] -> [TcType] -> TcType -> TcType
[TyVar] -> [TcType] -> TcType -> TcType
substTyWith [TyVar]
cls_tvs [TcType]
inst_tys TcType
gen_dm_ty
                          (dm_tvs :: [TyVar]
dm_tvs, dm_theta :: [TcType]
dm_theta, dm_tau :: TcType
dm_tau)
                                     = TcType -> ([TyVar], [TcType], TcType)
tcSplitNestedSigmaTys TcType
gen_dm_ty'
                          tau_eq :: TcType
tau_eq     = TcType -> TcType -> TcType
mkPrimEqPred TcType
meth_tau TcType
dm_tau
                    ; ThetaOrigin -> TcM ThetaOrigin
forall (m :: * -> *) a. Monad m => a -> m a
return (CtOrigin
-> TypeOrKind
-> [TyVar]
-> [TyVar]
-> [TcType]
-> [TcType]
-> ThetaOrigin
mkThetaOrigin (Bool -> CtOrigin
mkDerivOrigin Bool
wildcard) TypeOrKind
TypeLevel
                                [TyVar]
meth_tvs [TyVar]
dm_tvs [TcType]
meth_theta (TcType
tau_eqTcType -> [TcType] -> [TcType]
forall a. a -> [a] -> [a]
:[TcType]
dm_theta)) }

       ; [ThetaOrigin]
theta_origins <- IOEnv (Env TcGblEnv TcLclEnv) [ThetaOrigin]
-> ReaderT DerivEnv TcRn [ThetaOrigin]
forall (t :: (* -> *) -> * -> *) (m :: * -> *) a.
(MonadTrans t, Monad m) =>
m a -> t m a
lift (IOEnv (Env TcGblEnv TcLclEnv) [ThetaOrigin]
 -> ReaderT DerivEnv TcRn [ThetaOrigin])
-> IOEnv (Env TcGblEnv TcLclEnv) [ThetaOrigin]
-> ReaderT DerivEnv TcRn [ThetaOrigin]
forall a b. (a -> b) -> a -> b
$ ((TyVar, TcType) -> TcM ThetaOrigin)
-> [(TyVar, TcType)] -> IOEnv (Env TcGblEnv TcLclEnv) [ThetaOrigin]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
mapM (TyVar, TcType) -> TcM ThetaOrigin
do_one_meth [(TyVar, TcType)]
gen_dms
       ; ([ThetaOrigin], [TyVar], [TcType])
-> DerivM ([ThetaOrigin], [TyVar], [TcType])
forall (m :: * -> *) a. Monad m => a -> m a
return ([ThetaOrigin]
theta_origins, [TyVar]
tvs, [TcType]
inst_tys) }

{- Note [Inferring the instance context]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
There are two sorts of 'deriving', as represented by the two constructors
for DerivContext:

  * InferContext mb_wildcard: This can either be:
    - The deriving clause for a data type.
        (e.g, data T a = T1 a deriving( Eq ))
      In this case, mb_wildcard = Nothing.
    - A standalone declaration with an extra-constraints wildcard
        (e.g., deriving instance _ => Eq (Foo a))
      In this case, mb_wildcard = Just loc, where loc is the location
      of the extra-constraints wildcard.

    Here we must infer an instance context,
    and generate instance declaration
      instance Eq a => Eq (T a) where ...

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

For the InferContext case, we must figure out the
instance context (inferConstraintsDataConArgs). Suppose we are inferring
the instance context for
    C t1 .. tn (T s1 .. sm)
There are two cases

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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


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

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

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

We start with:

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

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

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

Next iteration:

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

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

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

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

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

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

Consider:

  data ADT a b = Z a b deriving Eq

The generated code could be either:

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

Or:

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

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


simplifyInstanceContexts :: [DerivSpec [ThetaOrigin]]
                         -> TcM [DerivSpec ThetaType]
-- Used only for deriving clauses or standalone deriving with an
-- extra-constraints wildcard (InferContext)
-- See Note [Simplifying the instance context]

simplifyInstanceContexts :: [DerivSpec [ThetaOrigin]] -> TcM [DerivSpec [TcType]]
simplifyInstanceContexts [] = [DerivSpec [TcType]] -> TcM [DerivSpec [TcType]]
forall (m :: * -> *) a. Monad m => a -> m a
return []

simplifyInstanceContexts infer_specs :: [DerivSpec [ThetaOrigin]]
infer_specs
  = do  { String -> SDoc -> IOEnv (Env TcGblEnv TcLclEnv) ()
traceTc "simplifyInstanceContexts" (SDoc -> IOEnv (Env TcGblEnv TcLclEnv) ())
-> SDoc -> IOEnv (Env TcGblEnv TcLclEnv) ()
forall a b. (a -> b) -> a -> b
$ [SDoc] -> SDoc
vcat ((DerivSpec [ThetaOrigin] -> SDoc)
-> [DerivSpec [ThetaOrigin]] -> [SDoc]
forall a b. (a -> b) -> [a] -> [b]
map DerivSpec [ThetaOrigin] -> SDoc
forall theta. Outputable theta => DerivSpec theta -> SDoc
pprDerivSpec [DerivSpec [ThetaOrigin]]
infer_specs)
        ; Int -> [[TcType]] -> TcM [DerivSpec [TcType]]
iterate_deriv 1 [[TcType]]
initial_solutions }
  where
    ------------------------------------------------------------------
        -- The initial solutions for the equations claim that each
        -- instance has an empty context; this solution is certainly
        -- in canonical form.
    initial_solutions :: [ThetaType]
    initial_solutions :: [[TcType]]
initial_solutions = [ [] | DerivSpec [ThetaOrigin]
_ <- [DerivSpec [ThetaOrigin]]
infer_specs ]

    ------------------------------------------------------------------
        -- iterate_deriv calculates the next batch of solutions,
        -- compares it with the current one; finishes if they are the
        -- same, otherwise recurses with the new solutions.
        -- It fails if any iteration fails
    iterate_deriv :: Int -> [ThetaType] -> TcM [DerivSpec ThetaType]
    iterate_deriv :: Int -> [[TcType]] -> TcM [DerivSpec [TcType]]
iterate_deriv n :: Int
n current_solns :: [[TcType]]
current_solns
      | Int
n Int -> Int -> Bool
forall a. Ord a => a -> a -> Bool
> 20  -- Looks as if we are in an infinite loop
                -- This can happen if we have -XUndecidableInstances
                -- (See TcSimplify.tcSimplifyDeriv.)
      = String -> SDoc -> TcM [DerivSpec [TcType]]
forall a. HasCallStack => String -> SDoc -> a
pprPanic "solveDerivEqns: probable loop"
                 ([SDoc] -> SDoc
vcat ((DerivSpec [ThetaOrigin] -> SDoc)
-> [DerivSpec [ThetaOrigin]] -> [SDoc]
forall a b. (a -> b) -> [a] -> [b]
map DerivSpec [ThetaOrigin] -> SDoc
forall theta. Outputable theta => DerivSpec theta -> SDoc
pprDerivSpec [DerivSpec [ThetaOrigin]]
infer_specs) SDoc -> SDoc -> SDoc
$$ [[TcType]] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [[TcType]]
current_solns)
      | Bool
otherwise
      = do {      -- Extend the inst info from the explicit instance decls
                  -- with the current set of solutions, and simplify each RHS
             [ClsInst]
inst_specs <- ([TcType]
 -> DerivSpec [ThetaOrigin]
 -> IOEnv (Env TcGblEnv TcLclEnv) ClsInst)
-> [[TcType]]
-> [DerivSpec [ThetaOrigin]]
-> IOEnv (Env TcGblEnv TcLclEnv) [ClsInst]
forall (m :: * -> *) a b c.
Applicative m =>
(a -> b -> m c) -> [a] -> [b] -> m [c]
zipWithM [TcType]
-> DerivSpec [ThetaOrigin] -> IOEnv (Env TcGblEnv TcLclEnv) ClsInst
forall theta.
[TcType]
-> DerivSpec theta -> IOEnv (Env TcGblEnv TcLclEnv) ClsInst
newDerivClsInst [[TcType]]
current_solns [DerivSpec [ThetaOrigin]]
infer_specs
           ; [[TcType]]
new_solns <- TcM [[TcType]] -> TcM [[TcType]]
forall r. TcM r -> TcM r
checkNoErrs (TcM [[TcType]] -> TcM [[TcType]])
-> TcM [[TcType]] -> TcM [[TcType]]
forall a b. (a -> b) -> a -> b
$
                          [ClsInst] -> TcM [[TcType]] -> TcM [[TcType]]
forall a. [ClsInst] -> TcM a -> TcM a
extendLocalInstEnv [ClsInst]
inst_specs (TcM [[TcType]] -> TcM [[TcType]])
-> TcM [[TcType]] -> TcM [[TcType]]
forall a b. (a -> b) -> a -> b
$
                          (DerivSpec [ThetaOrigin] -> IOEnv (Env TcGblEnv TcLclEnv) [TcType])
-> [DerivSpec [ThetaOrigin]] -> TcM [[TcType]]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
mapM DerivSpec [ThetaOrigin] -> IOEnv (Env TcGblEnv TcLclEnv) [TcType]
gen_soln [DerivSpec [ThetaOrigin]]
infer_specs

           ; if ([[TcType]]
current_solns [[TcType]] -> [[TcType]] -> Bool
`eqSolution` [[TcType]]
new_solns) then
                [DerivSpec [TcType]] -> TcM [DerivSpec [TcType]]
forall (m :: * -> *) a. Monad m => a -> m a
return [ DerivSpec [ThetaOrigin]
spec { ds_theta :: [TcType]
ds_theta = [TcType]
soln }
                       | (spec :: DerivSpec [ThetaOrigin]
spec, soln :: [TcType]
soln) <- [DerivSpec [ThetaOrigin]]
-> [[TcType]] -> [(DerivSpec [ThetaOrigin], [TcType])]
forall a b. [a] -> [b] -> [(a, b)]
zip [DerivSpec [ThetaOrigin]]
infer_specs [[TcType]]
current_solns ]
             else
                Int -> [[TcType]] -> TcM [DerivSpec [TcType]]
iterate_deriv (Int
nInt -> Int -> Int
forall a. Num a => a -> a -> a
+1) [[TcType]]
new_solns }

    eqSolution :: [[TcType]] -> [[TcType]] -> Bool
eqSolution a :: [[TcType]]
a b :: [[TcType]]
b = ([TcType] -> [TcType] -> Bool) -> [[TcType]] -> [[TcType]] -> Bool
forall a. (a -> a -> Bool) -> [a] -> [a] -> Bool
eqListBy ((TcType -> TcType -> Bool) -> [TcType] -> [TcType] -> Bool
forall a. (a -> a -> Bool) -> [a] -> [a] -> Bool
eqListBy TcType -> TcType -> Bool
eqType) ([[TcType]] -> [[TcType]]
canSolution [[TcType]]
a) ([[TcType]] -> [[TcType]]
canSolution [[TcType]]
b)
       -- Canonicalise for comparison
       -- See Note [Deterministic simplifyInstanceContexts]
    canSolution :: [[TcType]] -> [[TcType]]
canSolution = ([TcType] -> [TcType]) -> [[TcType]] -> [[TcType]]
forall a b. (a -> b) -> [a] -> [b]
map ((TcType -> TcType -> Ordering) -> [TcType] -> [TcType]
forall a. (a -> a -> Ordering) -> [a] -> [a]
sortBy TcType -> TcType -> Ordering
nonDetCmpType)
    ------------------------------------------------------------------
    gen_soln :: DerivSpec [ThetaOrigin] -> TcM ThetaType
    gen_soln :: DerivSpec [ThetaOrigin] -> IOEnv (Env TcGblEnv TcLclEnv) [TcType]
gen_soln (DS { ds_loc :: forall theta. DerivSpec theta -> SrcSpan
ds_loc = SrcSpan
loc, ds_tvs :: forall theta. DerivSpec theta -> [TyVar]
ds_tvs = [TyVar]
tyvars
                 , ds_cls :: forall theta. DerivSpec theta -> Class
ds_cls = Class
clas, ds_tys :: forall theta. DerivSpec theta -> [TcType]
ds_tys = [TcType]
inst_tys, ds_theta :: forall theta. DerivSpec theta -> theta
ds_theta = [ThetaOrigin]
deriv_rhs })
      = SrcSpan
-> IOEnv (Env TcGblEnv TcLclEnv) [TcType]
-> IOEnv (Env TcGblEnv TcLclEnv) [TcType]
forall a. SrcSpan -> TcRn a -> TcRn a
setSrcSpan SrcSpan
loc  (IOEnv (Env TcGblEnv TcLclEnv) [TcType]
 -> IOEnv (Env TcGblEnv TcLclEnv) [TcType])
-> IOEnv (Env TcGblEnv TcLclEnv) [TcType]
-> IOEnv (Env TcGblEnv TcLclEnv) [TcType]
forall a b. (a -> b) -> a -> b
$
        SDoc
-> IOEnv (Env TcGblEnv TcLclEnv) [TcType]
-> IOEnv (Env TcGblEnv TcLclEnv) [TcType]
forall a. SDoc -> TcM a -> TcM a
addErrCtxt (TcType -> SDoc
derivInstCtxt TcType
the_pred) (IOEnv (Env TcGblEnv TcLclEnv) [TcType]
 -> IOEnv (Env TcGblEnv TcLclEnv) [TcType])
-> IOEnv (Env TcGblEnv TcLclEnv) [TcType]
-> IOEnv (Env TcGblEnv TcLclEnv) [TcType]
forall a b. (a -> b) -> a -> b
$
        do { [TcType]
theta <- TcType
-> [TyVar]
-> [ThetaOrigin]
-> IOEnv (Env TcGblEnv TcLclEnv) [TcType]
simplifyDeriv TcType
the_pred [TyVar]
tyvars [ThetaOrigin]
deriv_rhs
                -- checkValidInstance tyvars theta clas inst_tys
                -- Not necessary; see Note [Exotic derived instance contexts]

           ; String -> SDoc -> IOEnv (Env TcGblEnv TcLclEnv) ()
traceTc "TcDeriv" ([ThetaOrigin] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [ThetaOrigin]
deriv_rhs SDoc -> SDoc -> SDoc
$$ [TcType] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [TcType]
theta)
                -- Claim: the result instance declaration is guaranteed valid
                -- Hence no need to call:
                --   checkValidInstance tyvars theta clas inst_tys
           ; [TcType] -> IOEnv (Env TcGblEnv TcLclEnv) [TcType]
forall (m :: * -> *) a. Monad m => a -> m a
return [TcType]
theta }
      where
        the_pred :: TcType
the_pred = Class -> [TcType] -> TcType
mkClassPred Class
clas [TcType]
inst_tys

derivInstCtxt :: PredType -> MsgDoc
derivInstCtxt :: TcType -> SDoc
derivInstCtxt pred :: TcType
pred
  = String -> SDoc
text "When deriving the instance for" SDoc -> SDoc -> SDoc
<+> SDoc -> SDoc
parens (TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcType
pred)

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

-- | Given @instance (wanted) => C inst_ty@, simplify 'wanted' as much
-- as possible. Fail if not possible.
simplifyDeriv :: PredType -- ^ @C inst_ty@, head of the instance we are
                          -- deriving.  Only used for SkolemInfo.
              -> [TyVar]  -- ^ The tyvars bound by @inst_ty@.
              -> [ThetaOrigin] -- ^ Given and wanted constraints
              -> TcM ThetaType -- ^ Needed constraints (after simplification),
                               -- i.e. @['PredType']@.
simplifyDeriv :: TcType
-> [TyVar]
-> [ThetaOrigin]
-> IOEnv (Env TcGblEnv TcLclEnv) [TcType]
simplifyDeriv pred :: TcType
pred tvs :: [TyVar]
tvs thetas :: [ThetaOrigin]
thetas
  = do { (skol_subst :: TCvSubst
skol_subst, tvs_skols :: [TyVar]
tvs_skols) <- [TyVar] -> TcM (TCvSubst, [TyVar])
tcInstSkolTyVars [TyVar]
tvs -- Skolemize
                -- The constraint solving machinery
                -- expects *TcTyVars* not TyVars.
                -- We use *non-overlappable* (vanilla) skolems
                -- See Note [Overlap and deriving]

       ; let skol_set :: VarSet
skol_set  = [TyVar] -> VarSet
mkVarSet [TyVar]
tvs_skols
             skol_info :: SkolemInfo
skol_info = TcType -> SkolemInfo
DerivSkol TcType
pred
             doc :: SDoc
doc = String -> SDoc
text "deriving" SDoc -> SDoc -> SDoc
<+> SDoc -> SDoc
parens (TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcType
pred)

             mk_given_ev :: PredType -> TcM EvVar
             mk_given_ev :: TcType -> TcM TyVar
mk_given_ev given :: TcType
given =
               let given_pred :: TcType
given_pred = HasCallStack => TCvSubst -> TcType -> TcType
TCvSubst -> TcType -> TcType
substTy TCvSubst
skol_subst TcType
given
               in TcType -> TcM TyVar
forall gbl lcl. TcType -> TcRnIf gbl lcl TyVar
newEvVar TcType
given_pred

             emit_wanted_constraints :: [TyVar] -> [PredOrigin] -> TcM ()
             emit_wanted_constraints :: [TyVar] -> [PredOrigin] -> IOEnv (Env TcGblEnv TcLclEnv) ()
emit_wanted_constraints metas_to_be :: [TyVar]
metas_to_be preds :: [PredOrigin]
preds
               = do { -- We instantiate metas_to_be with fresh meta type
                      -- variables. Currently, these can only be type variables
                      -- quantified in generic default type signatures.
                      -- See Note [Gathering and simplifying constraints for
                      -- DeriveAnyClass]
                      (meta_subst :: TCvSubst
meta_subst, _meta_tvs :: [TyVar]
_meta_tvs) <- [TyVar] -> TcM (TCvSubst, [TyVar])
newMetaTyVars [TyVar]
metas_to_be

                    -- Now make a constraint for each of the instantiated predicates
                    ; let wanted_subst :: TCvSubst
wanted_subst = TCvSubst
skol_subst TCvSubst -> TCvSubst -> TCvSubst
`unionTCvSubst` TCvSubst
meta_subst
                          mk_wanted_ct :: PredOrigin -> IOEnv (Env TcGblEnv TcLclEnv) Ct
mk_wanted_ct (PredOrigin wanted :: TcType
wanted orig :: CtOrigin
orig t_or_k :: TypeOrKind
t_or_k)
                            = do { CtEvidence
ev <- CtOrigin -> Maybe TypeOrKind -> TcType -> TcM CtEvidence
newWanted CtOrigin
orig (TypeOrKind -> Maybe TypeOrKind
forall a. a -> Maybe a
Just TypeOrKind
t_or_k) (TcType -> TcM CtEvidence) -> TcType -> TcM CtEvidence
forall a b. (a -> b) -> a -> b
$
                                         TCvSubst -> TcType -> TcType
substTyUnchecked TCvSubst
wanted_subst TcType
wanted
                                 ; Ct -> IOEnv (Env TcGblEnv TcLclEnv) Ct
forall (m :: * -> *) a. Monad m => a -> m a
return (CtEvidence -> Ct
mkNonCanonical CtEvidence
ev) }
                    ; [Ct]
cts <- (PredOrigin -> IOEnv (Env TcGblEnv TcLclEnv) Ct)
-> [PredOrigin] -> IOEnv (Env TcGblEnv TcLclEnv) [Ct]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
mapM PredOrigin -> IOEnv (Env TcGblEnv TcLclEnv) Ct
mk_wanted_ct [PredOrigin]
preds

                    -- And emit them into the monad
                    ; Cts -> IOEnv (Env TcGblEnv TcLclEnv) ()
emitSimples ([Ct] -> Cts
listToCts [Ct]
cts) }

             -- Create the implications we need to solve. For stock and newtype
             -- deriving, these implication constraints will be simple class
             -- constraints like (C a, Ord b).
             -- But with DeriveAnyClass, we make an implication constraint.
             -- See Note [Gathering and simplifying constraints for DeriveAnyClass]
             mk_wanteds :: ThetaOrigin -> TcM WantedConstraints
             mk_wanteds :: ThetaOrigin -> TcM WantedConstraints
mk_wanteds (ThetaOrigin { to_anyclass_skols :: ThetaOrigin -> [TyVar]
to_anyclass_skols  = [TyVar]
ac_skols
                                     , to_anyclass_metas :: ThetaOrigin -> [TyVar]
to_anyclass_metas  = [TyVar]
ac_metas
                                     , to_anyclass_givens :: ThetaOrigin -> [TcType]
to_anyclass_givens = [TcType]
ac_givens
                                     , to_wanted_origins :: ThetaOrigin -> [PredOrigin]
to_wanted_origins  = [PredOrigin]
preds })
               = do { [TyVar]
ac_given_evs <- (TcType -> TcM TyVar)
-> [TcType] -> IOEnv (Env TcGblEnv TcLclEnv) [TyVar]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
mapM TcType -> TcM TyVar
mk_given_ev [TcType]
ac_givens
                    ; (_, wanteds :: WantedConstraints
wanteds)
                        <- TcM (TcEvBinds, ()) -> TcM ((TcEvBinds, ()), WantedConstraints)
forall a. TcM a -> TcM (a, WantedConstraints)
captureConstraints (TcM (TcEvBinds, ()) -> TcM ((TcEvBinds, ()), WantedConstraints))
-> TcM (TcEvBinds, ()) -> TcM ((TcEvBinds, ()), WantedConstraints)
forall a b. (a -> b) -> a -> b
$
                           SkolemInfo
-> [TyVar]
-> [TyVar]
-> IOEnv (Env TcGblEnv TcLclEnv) ()
-> TcM (TcEvBinds, ())
forall result.
SkolemInfo
-> [TyVar] -> [TyVar] -> TcM result -> TcM (TcEvBinds, result)
checkConstraints SkolemInfo
skol_info [TyVar]
ac_skols [TyVar]
ac_given_evs (IOEnv (Env TcGblEnv TcLclEnv) () -> TcM (TcEvBinds, ()))
-> IOEnv (Env TcGblEnv TcLclEnv) () -> TcM (TcEvBinds, ())
forall a b. (a -> b) -> a -> b
$
                              -- The checkConstraints bumps the TcLevel, and
                              -- wraps the wanted constraints in an implication,
                              -- when (but only when) necessary
                           [TyVar] -> [PredOrigin] -> IOEnv (Env TcGblEnv TcLclEnv) ()
emit_wanted_constraints [TyVar]
ac_metas [PredOrigin]
preds
                    ; WantedConstraints -> TcM WantedConstraints
forall (f :: * -> *) a. Applicative f => a -> f a
pure WantedConstraints
wanteds }

       -- See [STEP DAC BUILD]
       -- Generate the implication constraints, one for each method, to solve
       -- with the skolemized variables.  Start "one level down" because
       -- we are going to wrap the result in an implication with tvs_skols,
       -- in step [DAC RESIDUAL]
       ; (tc_lvl :: TcLevel
tc_lvl, wanteds :: [WantedConstraints]
wanteds) <- TcM [WantedConstraints] -> TcM (TcLevel, [WantedConstraints])
forall a. TcM a -> TcM (TcLevel, a)
pushTcLevelM (TcM [WantedConstraints] -> TcM (TcLevel, [WantedConstraints]))
-> TcM [WantedConstraints] -> TcM (TcLevel, [WantedConstraints])
forall a b. (a -> b) -> a -> b
$
                              (ThetaOrigin -> TcM WantedConstraints)
-> [ThetaOrigin] -> TcM [WantedConstraints]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
mapM ThetaOrigin -> TcM WantedConstraints
mk_wanteds [ThetaOrigin]
thetas

       ; String -> SDoc -> IOEnv (Env TcGblEnv TcLclEnv) ()
traceTc "simplifyDeriv inputs" (SDoc -> IOEnv (Env TcGblEnv TcLclEnv) ())
-> SDoc -> IOEnv (Env TcGblEnv TcLclEnv) ()
forall a b. (a -> b) -> a -> b
$
         [SDoc] -> SDoc
vcat [ [TyVar] -> SDoc
pprTyVars [TyVar]
tvs SDoc -> SDoc -> SDoc
$$ [ThetaOrigin] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [ThetaOrigin]
thetas SDoc -> SDoc -> SDoc
$$ [WantedConstraints] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [WantedConstraints]
wanteds, SDoc
doc ]

       -- See [STEP DAC SOLVE]
       -- Simplify the constraints, starting at the same level at which
       -- they are generated (c.f. the call to runTcSWithEvBinds in
       -- simplifyInfer)
       ; WantedConstraints
solved_wanteds <- TcLevel -> TcM WantedConstraints -> TcM WantedConstraints
forall a. TcLevel -> TcM a -> TcM a
setTcLevel TcLevel
tc_lvl   (TcM WantedConstraints -> TcM WantedConstraints)
-> TcM WantedConstraints -> TcM WantedConstraints
forall a b. (a -> b) -> a -> b
$
                           TcS WantedConstraints -> TcM WantedConstraints
forall a. TcS a -> TcM a
runTcSDeriveds      (TcS WantedConstraints -> TcM WantedConstraints)
-> TcS WantedConstraints -> TcM WantedConstraints
forall a b. (a -> b) -> a -> b
$
                           WantedConstraints -> TcS WantedConstraints
solveWantedsAndDrop (WantedConstraints -> TcS WantedConstraints)
-> WantedConstraints -> TcS WantedConstraints
forall a b. (a -> b) -> a -> b
$
                           [WantedConstraints] -> WantedConstraints
unionsWC [WantedConstraints]
wanteds

       -- It's not yet zonked!  Obviously zonk it before peering at it
       ; WantedConstraints
solved_wanteds <- WantedConstraints -> TcM WantedConstraints
zonkWC WantedConstraints
solved_wanteds

       -- See [STEP DAC HOIST]
       -- Split the resulting constraints into bad and good constraints,
       -- building an @unsolved :: WantedConstraints@ representing all
       -- the constraints we can't just shunt to the predicates.
       -- See Note [Exotic derived instance contexts]
       ; let residual_simple :: Cts
residual_simple = Bool -> WantedConstraints -> Cts
approximateWC Bool
True WantedConstraints
solved_wanteds
             (bad :: Cts
bad, good :: Bag TcType
good) = (Ct -> Either Ct TcType) -> Cts -> (Cts, Bag TcType)
forall a b c. (a -> Either b c) -> Bag a -> (Bag b, Bag c)
partitionBagWith Ct -> Either Ct TcType
get_good Cts
residual_simple

             get_good :: Ct -> Either Ct PredType
             get_good :: Ct -> Either Ct TcType
get_good ct :: Ct
ct | VarSet -> TcType -> Bool
validDerivPred VarSet
skol_set TcType
p
                         , Ct -> Bool
isWantedCt Ct
ct
                         = TcType -> Either Ct TcType
forall a b. b -> Either a b
Right TcType
p
                          -- TODO: This is wrong
                          -- NB re 'isWantedCt': residual_wanted may contain
                          -- unsolved CtDerived and we stick them into the
                          -- bad set so that reportUnsolved may decide what
                          -- to do with them
                         | Bool
otherwise
                         = Ct -> Either Ct TcType
forall a b. a -> Either a b
Left Ct
ct
                           where p :: TcType
p = Ct -> TcType
ctPred Ct
ct

       ; String -> SDoc -> IOEnv (Env TcGblEnv TcLclEnv) ()
traceTc "simplifyDeriv outputs" (SDoc -> IOEnv (Env TcGblEnv TcLclEnv) ())
-> SDoc -> IOEnv (Env TcGblEnv TcLclEnv) ()
forall a b. (a -> b) -> a -> b
$
         [SDoc] -> SDoc
vcat [ [TyVar] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [TyVar]
tvs_skols, Cts -> SDoc
forall a. Outputable a => a -> SDoc
ppr Cts
residual_simple, Bag TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr Bag TcType
good, Cts -> SDoc
forall a. Outputable a => a -> SDoc
ppr Cts
bad ]

       -- Return the good unsolved constraints (unskolemizing on the way out.)
       ; let min_theta :: [TcType]
min_theta = (TcType -> TcType) -> [TcType] -> [TcType]
forall a. (a -> TcType) -> [a] -> [a]
mkMinimalBySCs TcType -> TcType
forall a. a -> a
id (Bag TcType -> [TcType]
forall a. Bag a -> [a]
bagToList Bag TcType
good)
             -- An important property of mkMinimalBySCs (used above) is that in
             -- addition to removing constraints that are made redundant by
             -- superclass relationships, it also removes _duplicate_
             -- constraints.
             -- See Note [Gathering and simplifying constraints for
             --           DeriveAnyClass]
             subst_skol :: TCvSubst
subst_skol = [TyVar] -> [TcType] -> TCvSubst
HasDebugCallStack => [TyVar] -> [TcType] -> TCvSubst
zipTvSubst [TyVar]
tvs_skols ([TcType] -> TCvSubst) -> [TcType] -> TCvSubst
forall a b. (a -> b) -> a -> b
$ [TyVar] -> [TcType]
mkTyVarTys [TyVar]
tvs
                          -- The reverse substitution (sigh)

       -- See [STEP DAC RESIDUAL]
       ; [TyVar]
min_theta_vars <- (TcType -> TcM TyVar)
-> [TcType] -> IOEnv (Env TcGblEnv TcLclEnv) [TyVar]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
mapM TcType -> TcM TyVar
forall gbl lcl. TcType -> TcRnIf gbl lcl TyVar
newEvVar [TcType]
min_theta
       ; (leftover_implic :: Bag Implication
leftover_implic, _)
           <- TcLevel
-> SkolemInfo
-> [TyVar]
-> [TyVar]
-> WantedConstraints
-> TcM (Bag Implication, TcEvBinds)
buildImplicationFor TcLevel
tc_lvl SkolemInfo
skol_info [TyVar]
tvs_skols
                                  [TyVar]
min_theta_vars WantedConstraints
solved_wanteds
       -- This call to simplifyTop is purely for error reporting
       -- See Note [Error reporting for deriving clauses]
       -- See also Note [Exotic derived instance contexts], which are caught
       -- in this line of code.
       ; Bag Implication -> IOEnv (Env TcGblEnv TcLclEnv) ()
simplifyTopImplic Bag Implication
leftover_implic

       ; [TcType] -> IOEnv (Env TcGblEnv TcLclEnv) [TcType]
forall (m :: * -> *) a. Monad m => a -> m a
return (HasCallStack => TCvSubst -> [TcType] -> [TcType]
TCvSubst -> [TcType] -> [TcType]
substTheta TCvSubst
subst_skol [TcType]
min_theta) }

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

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

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

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

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

Note [Gathering and simplifying constraints for DeriveAnyClass]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
DeriveAnyClass works quite differently from stock and newtype deriving in
the way it gathers and simplifies constraints to be used in a derived
instance's context. Stock and newtype deriving gather constraints by looking
at the data constructors of the data type for which we are deriving an
instance. But DeriveAnyClass doesn't need to know about a data type's
definition at all!

To see why, consider this example of DeriveAnyClass:

  class Foo a where
    bar :: forall b. Ix b => a -> b -> String
    default bar :: (Show a, Ix c) => a -> c -> String
    bar x y = show x ++ show (range (y,y))

    baz :: Eq a => a -> a -> Bool
    default baz :: (Ord a, Show a) => a -> a -> Bool
    baz x y = compare x y == EQ

Because 'bar' and 'baz' have default signatures, this generates a top-level
definition for these generic default methods

  $gdm_bar :: forall a. Foo a
           => forall c. (Show a, Ix c)
           => a -> c -> String
  $gdm_bar x y = show x ++ show (range (y,y))

(and similarly for baz).  Now consider a 'deriving' clause
  data Maybe s = ... deriving Foo

This derives an instance of the form:
  instance (CX) => Foo (Maybe s) where
    bar = $gdm_bar
    baz = $gdm_baz

Now it is GHC's job to fill in a suitable instance context (CX).  If
GHC were typechecking the binding
   bar = $gdm bar
it would
   * skolemise the expected type of bar
   * instantiate the type of $gdm_bar with meta-type variables
   * build an implication constraint

[STEP DAC BUILD]
So that's what we do.  We build the constraint (call it C1)

   forall[2] b. Ix b => (Show (Maybe s), Ix cc,
                        Maybe s -> b -> String
                            ~ Maybe s -> cc -> String)

Here:
* The level of this forall constraint is forall[2], because we are later
  going to wrap it in a forall[1] in [STEP DAC RESIDUAL]

* The 'b' comes from the quantified type variable in the expected type
  of bar (i.e., 'to_anyclass_skols' in 'ThetaOrigin'). The 'cc' is a unification
  variable that comes from instantiating the quantified type variable 'c' in
  $gdm_bar's type (i.e., 'to_anyclass_metas' in 'ThetaOrigin).

* The (Ix b) constraint comes from the context of bar's type
  (i.e., 'to_wanted_givens' in 'ThetaOrigin'). The (Show (Maybe s)) and (Ix cc)
  constraints come from the context of $gdm_bar's type
  (i.e., 'to_anyclass_givens' in 'ThetaOrigin').

* The equality constraint (Maybe s -> b -> String) ~ (Maybe s -> cc -> String)
  comes from marrying up the instantiated type of $gdm_bar with the specified
  type of bar. Notice that the type variables from the instance, 's' in this
  case, are global to this constraint.

Note that it is vital that we instantiate the `c` in $gdm_bar's type with a new
unification variable for each iteration of simplifyDeriv. If we re-use the same
unification variable across multiple iterations, then bad things can happen,
such as Trac #14933.

Similarly for 'baz', givng the constraint C2

   forall[2]. Eq (Maybe s) => (Ord a, Show a,
                              Maybe s -> Maybe s -> Bool
                                ~ Maybe s -> Maybe s -> Bool)

In this case baz has no local quantification, so the implication
constraint has no local skolems and there are no unification
variables.

[STEP DAC SOLVE]
We can combine these two implication constraints into a single
constraint (C1, C2), and simplify, unifying cc:=b, to get:

   forall[2] b. Ix b => Show a
   /\
   forall[2]. Eq (Maybe s) => (Ord a, Show a)

[STEP DAC HOIST]
Let's call that (C1', C2').  Now we need to hoist the unsolved
constraints out of the implications to become our candidate for
(CX). That is done by approximateWC, which will return:

  (Show a, Ord a, Show a)

Now we can use mkMinimalBySCs to remove superclasses and duplicates, giving

  (Show a, Ord a)

And that's what GHC uses for CX.

[STEP DAC RESIDUAL]
In this case we have solved all the leftover constraints, but what if
we don't?  Simple!  We just form the final residual constraint

   forall[1] s. CX => (C1',C2')

and simplify that. In simple cases it'll succeed easily, because CX
literally contains the constraints in C1', C2', but if there is anything
more complicated it will be reported in a civilised way.

Note [Error reporting for deriving clauses]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A suprisingly tricky aspect of deriving to get right is reporting sensible
error messages. In particular, if simplifyDeriv reaches a constraint that it
cannot solve, which might include:

1. Insoluble constraints
2. "Exotic" constraints (See Note [Exotic derived instance contexts])

Then we report an error immediately in simplifyDeriv.

Another possible choice is to punt and let another part of the typechecker
(e.g., simplifyInstanceContexts) catch the errors. But this tends to lead
to worse error messages, so we do it directly in simplifyDeriv.

simplifyDeriv checks for errors in a clever way. If the deriving machinery
infers the context (Foo a)--that is, if this instance is to be generated:

  instance Foo a => ...

Then we form an implication of the form:

  forall a. Foo a => <residual_wanted_constraints>

And pass it to the simplifier. If the context (Foo a) is enough to discharge
all the constraints in <residual_wanted_constraints>, then everything is
hunky-dory. But if <residual_wanted_constraints> contains, say, an insoluble
constraint, then (Foo a) won't be able to solve it, causing GHC to error.

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

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

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

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

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

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

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

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

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

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