content: {-
  (c) The University of Glasgow 2006
  (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
  
  -}
  
  
  {-# LANGUAGE FlexibleContexts #-}
  {-# LANGUAGE TupleSections #-}
  {-# LANGUAGE TypeFamilies #-}
  {-# LANGUAGE LambdaCase #-}
  
  {-# OPTIONS_GHC -Wno-incomplete-record-updates #-}
  
  -- | Typechecking instance declarations
  module GHC.Tc.TyCl.Instance
     ( tcInstDecls1
     , tcInstDeclsDeriv
     , tcInstDecls2
     )
  where
  
  import GHC.Prelude
  
  import GHC.Hs
  import GHC.Rename.Bind ( rejectBootDecls )
  import GHC.Tc.Errors.Types
  import GHC.Tc.Gen.Bind
  import GHC.Tc.TyCl
  import GHC.Tc.TyCl.Utils ( addTyConsToGblEnv )
  import GHC.Tc.TyCl.Class ( tcClassDecl2, tcATDefault,
                             HsSigFun, mkHsSigFun, findMethodBind,
                             instantiateMethod )
  import GHC.Tc.Solver( pushLevelAndSolveEqualitiesX, reportUnsolvedEqualities )
  import GHC.Tc.Gen.Sig
  import GHC.Tc.Utils.Monad
  import GHC.Tc.Validity
  import GHC.Tc.Zonk.Type
  import GHC.Tc.Zonk.TcType
  import GHC.Tc.Utils.TcMType
  import GHC.Tc.Utils.TcType
  import GHC.Tc.Types.Constraint
  import GHC.Tc.Types.Origin
  import GHC.Tc.TyCl.Build
  import GHC.Tc.Utils.Instantiate
  import GHC.Tc.Instance.Class( AssocInstInfo(..), isNotAssociated )
  import GHC.Tc.Instance.Family
  
  import GHC.Tc.Deriv
  import GHC.Tc.Utils.Env
  import GHC.Tc.Gen.HsType
  import GHC.Tc.Utils.Unify
  import GHC.Tc.Types.Evidence
  
  import GHC.Builtin.Names ( unsatisfiableIdName )
  
  import GHC.Core        ( Expr(..), mkVarApps )
  import GHC.Core.Make   ( nO_METHOD_BINDING_ERROR_ID )
  import GHC.Core.Unfold.Make ( mkDFunUnfolding )
  import GHC.Core.FamInstEnv
  import GHC.Core.Type
  import GHC.Core.Multiplicity
  import GHC.Core.InstEnv
  import GHC.Core.Predicate( classMethodInstTy )
  import GHC.Core.TyCon
  import GHC.Core.Coercion.Axiom
  import GHC.Core.DataCon
  import GHC.Core.ConLike
  import GHC.Core.Class
  
  import GHC.Types.Var as Var
  import GHC.Types.Var.Env
  import GHC.Types.Var.Set
  import GHC.Types.Basic
  import GHC.Types.Fixity
  import GHC.Types.Id
  import GHC.Types.SourceFile
  import GHC.Types.SourceText
  import GHC.Types.Name
  import GHC.Types.Name.Set
  import GHC.Types.SrcLoc
  
  import GHC.Driver.DynFlags
  import GHC.Driver.Ppr
  
  import GHC.Utils.Logger
  import GHC.Utils.Outputable
  import GHC.Utils.Panic
  import GHC.Utils.Misc
  
  import GHC.Data.FastString
  import GHC.Data.List.SetOps
  import GHC.Data.Bag
  import GHC.Data.BooleanFormula ( isUnsatisfied )
  import qualified GHC.LanguageExtensions as LangExt
  
  import Control.Monad
  import Data.Tuple
  import GHC.Data.Maybe
  import Data.List( mapAccumL )
  
  
  {-
  Typechecking instance declarations is done in two passes. The first
  pass, made by @tcInstDecls1@, collects information to be used in the
  second pass.
  
  This pre-processed info includes the as-yet-unprocessed bindings
  inside the instance declaration.  These are type-checked in the second
  pass, when the class-instance envs and GVE contain all the info from
  all the instance and value decls.  Indeed that's the reason we need
  two passes over the instance decls.
  
  
  Note [How instance declarations are translated]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Here is how we translate instance declarations into Core
  
  Running example:
          class C a where
             op1, op2 :: Ix b => a -> b -> b
             op2 = <dm-rhs>
  
          instance C a => C [a]
             {-# INLINE [2] op1 #-}
             op1 = <rhs>
  ===>
          -- Method selectors
          op1,op2 :: forall a. C a => forall b. Ix b => a -> b -> b
          op1 = ...
          op2 = ...
  
          -- Default methods get the 'self' dictionary as argument
          -- so they can call other methods at the same type
          -- Default methods get the same type as their method selector
          $dmop2 :: forall a. C a => forall b. Ix b => a -> b -> b
          $dmop2 = /\a. \(d:C a). /\b. \(d2: Ix b). <dm-rhs>
                 -- NB: type variables 'a' and 'b' are *both* in scope in <dm-rhs>
                 -- Note [Tricky type variable scoping]
  
          -- A top-level definition for each instance method
          -- Here op1_i, op2_i are the "instance method Ids"
          -- The INLINE pragma comes from the user pragma
          {-# INLINE [2] op1_i #-}  -- From the instance decl bindings
          op1_i, op2_i :: forall a. C a => forall b. Ix b => [a] -> b -> b
          op1_i = /\a. \(d:C a).
                 let this :: C [a]
                     this = df_i a d
                       -- Note [Subtle interaction of recursion and overlap]
  
                     local_op1 :: forall b. Ix b => [a] -> b -> b
                     local_op1 = <rhs>
                       -- Source code; run the type checker on this
                       -- NB: Type variable 'a' (but not 'b') is in scope in <rhs>
                       -- Note [Tricky type variable scoping]
  
                 in local_op1 a d
  
          op2_i = /\a \d:C a. $dmop2 [a] (df_i a d)
  
          -- The dictionary function itself
          {-# NOINLINE CONLIKE df_i #-}   -- Never inline dictionary functions
          df_i :: forall a. C a -> C [a]
          df_i = /\a. \d:C a. MkC (op1_i a d) (op2_i a d)
                  -- But see Note [Default methods in instances]
                  -- We can't apply the type checker to the default-method call
  
          -- Use a RULE to short-circuit applications of the class ops
          {-# RULE "op1@C[a]" forall a, d:C a.
                              op1 [a] (df_i d) = op1_i a d #-}
  
  Note [Instances and loop breakers]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  * Note that df_i may be mutually recursive with both op1_i and op2_i.
    It's crucial that df_i is not chosen as the loop breaker, even
    though op1_i has a (user-specified) INLINE pragma.
  
  * Instead the idea is to inline df_i into op1_i, which may then select
    methods from the MkC record, and thereby break the recursion with
    df_i, leaving a *self*-recursive op1_i.  (If op1_i doesn't call op at
    the same type, it won't mention df_i, so there won't be recursion in
    the first place.)
  
  * If op1_i is marked INLINE by the user there's a danger that we won't
    inline df_i in it, and that in turn means that (since it'll be a
    loop-breaker because df_i isn't), op1_i will ironically never be
    inlined.  But this is OK: the recursion breaking happens by way of
    a RULE (the magic ClassOp rule above), and RULES work inside stable
    unfoldings. See Note [RULEs enabled in InitialPhase] in GHC.Core.Opt.Simplify.Utils
  
  Note [ClassOp/DFun selection]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  This important Note explains how DFunIds and ClassOps work.
  
  One thing we see a lot is stuff like
      op2 (df d1 d2)
  where 'op2' is a ClassOp and 'df' is DFun.  Now, we could inline *both*
  'op2' and 'df' to get
       case (MkD ($cop1 d1 d2) ($cop2 d1 d2) ... of
         MkD _ op2 _ _ _ -> op2
  And that will reduce to ($cop2 d1 d2) which is what we wanted.
  
  But it's tricky to make this work in practice, because it requires us to
  inline both 'op2' and 'df'.  But neither is keen to inline without having
  seen the other's result; and it's very easy to get code bloat (from the
  big intermediate) if you inline a bit too much.
  
  Instead we use a cunning trick.
   * We arrange that 'df' and 'op2' NEVER inline.
  
   * We arrange that 'df' is ALWAYS defined in the sylised form
        df d1 d2 = MkD ($cop1 d1 d2) ($cop2 d1 d2) ...
  
   * We give 'df' a magical unfolding (DFunUnfolding [$cop1, $cop2, ..])
     that lists its methods.
  
   * We make GHC.Core.Unfold.exprIsConApp_maybe spot a DFunUnfolding and return
     a suitable constructor application -- inlining df "on the fly" as it
     were.
  
   * ClassOp rules: We give the ClassOp 'op2' a BuiltinRule that
     extracts the right piece iff its argument satisfies
     exprIsConApp_maybe.  This is done in GHC.Types.Id.Make.mkDictSelId
  
   * We make 'df' CONLIKE, so that shared uses still match; eg
        let d = df d1 d2
        in ...(op2 d)...(op1 d)...
  
   * ClassOps have no unfolding; they work /only/ through their RULE.
     But a ClassOp might be called with no arguments, or with an argument that
     the rule doesn't fire on.   So each ClassOp does get an executable top-level
     definition, injected by GHC.Iface.Tidy.getClassImplicitBinds, which uses
     `GHC.Types.Id.Make.mkDictSelRhs` to make the code.
  
  Note [Single-method classes]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  If the class has just one method (or, more accurately, just one element
  of {superclasses + methods}), then we use a different strategy.
  
     class C a where op :: a -> a
     instance C a => C [a] where op = <blah>
  
  We translate the class decl into a newtype, which just gives a
  top-level axiom. The "constructor" MkC expands to a cast, as does the
  class-op selector.
  
     axiom Co:C a :: C a ~ (a->a)
  
     op :: forall a. C a -> (a -> a)
     op a d = d |> (Co:C a)
  
     MkC :: forall a. (a->a) -> C a
     MkC = /\a.\op. op |> (sym Co:C a)
  
  The clever RULE stuff doesn't work now, because ($df a d) isn't
  a constructor application, so exprIsConApp_maybe won't return
  Just <blah>.
  
  Instead, we simply rely on the fact that casts are cheap:
  
     $df :: forall a. C a => C [a]
     {-# INLINE df #-}  -- NB: INLINE this
     $df = /\a. \d. MkC [a] ($cop_list a d)
         = $cop_list |> forall a. C a -> (sym (Co:C [a]))
  
     $cop_list :: forall a. C a => [a] -> [a]
     $cop_list = <blah>
  
  So if we see
     (op ($df a d))
  we'll inline 'op' and '$df', since both are simply casts, and
  good things happen.
  
  Why do we use this different strategy?  Because otherwise we
  end up with non-inlined dictionaries that look like
      $df = $cop |> blah
  which adds an extra indirection to every use, which seems stupid.  See
  #4138 for an example (although the regression reported there
  wasn't due to the indirection).
  
  There is an awkward wrinkle though: we want to be very
  careful when we have
      instance C a => C [a] where
        {-# INLINE op #-}
        op = ...
  then we'll get an INLINE pragma on $cop_list but it's important that
  $cop_list only inlines when it's applied to *two* arguments (the
  dictionary and the list argument).  So we must not eta-expand $df
  above.  We ensure that this doesn't happen by putting an INLINE
  pragma on the dfun itself; after all, it ends up being just a cast.
  
  There is one more dark corner to the INLINE story, even more deeply
  buried.  Consider this (#3772):
  
      class DeepSeq a => C a where
        gen :: Int -> a
  
      instance C a => C [a] where
        gen n = ...
  
      class DeepSeq a where
        deepSeq :: a -> b -> b
  
      instance DeepSeq a => DeepSeq [a] where
        {-# INLINE deepSeq #-}
        deepSeq xs b = foldr deepSeq b xs
  
  That gives rise to these defns:
  
      $cdeepSeq :: DeepSeq a -> [a] -> b -> b
      -- User INLINE( 3 args )!
      $cdeepSeq a (d:DS a) b (x:[a]) (y:b) = ...
  
      $fDeepSeq[] :: DeepSeq a -> DeepSeq [a]
      -- DFun (with auto INLINE pragma)
      $fDeepSeq[] a d = $cdeepSeq a d |> blah
  
      $cp1 a d :: C a => DeepSep [a]
      -- We don't want to eta-expand this, lest
      -- $cdeepSeq gets inlined in it!
      $cp1 a d = $fDeepSep[] a (scsel a d)
  
      $fC[] :: C a => C [a]
      -- Ordinary DFun
      $fC[] a d = MkC ($cp1 a d) ($cgen a d)
  
  Here $cp1 is the code that generates the superclass for C [a].  The
  issue is this: we must not eta-expand $cp1 either, or else $fDeepSeq[]
  and then $cdeepSeq will inline there, which is definitely wrong.  Like
  on the dfun, we solve this by adding an INLINE pragma to $cp1.
  
  Note [Subtle interaction of recursion and overlap]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Consider this
    class C a where { op1,op2 :: a -> a }
    instance C a => C [a] where
      op1 x = op2 x ++ op2 x
      op2 x = ...
    instance C [Int] where
      ...
  
  When type-checking the C [a] instance, we need a C [a] dictionary (for
  the call of op2).  If we look up in the instance environment, we find
  an overlap.  And in *general* the right thing is to complain (see Note
  [Overlapping instances] in GHC.Core.InstEnv).  But in *this* case it's wrong to
  complain, because we just want to delegate to the op2 of this same
  instance.
  
  Why is this justified?  Because we generate a (C [a]) constraint in
  a context in which 'a' cannot be instantiated to anything that matches
  other overlapping instances, or else we would not be executing this
  version of op1 in the first place.
  
  It might even be a bit disguised:
  
    nullFail :: C [a] => [a] -> [a]
    nullFail x = op2 x ++ op2 x
  
    instance C a => C [a] where
      op1 x = nullFail x
  
  Precisely this is used in package 'regex-base', module Context.hs.
  See the overlapping instances for RegexContext, and the fact that they
  call 'nullFail' just like the example above.  The DoCon package also
  does the same thing; it shows up in module Fraction.hs.
  
  Conclusion: when typechecking the methods in a C [a] instance, we want to
  treat the 'a' as an *existential* type variable, in the sense described
  by Note [Super skolems: binding when looking up instances] in GHC.Core.InstEnv
  That is why isOverlappableTyVar responds True to an InstSkol, which is the kind
  of skolem we use in tcInstDecl2.
  
  Note [Tricky type variable scoping]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  In our example
          class C a where
             op1, op2 :: Ix b => a -> b -> b
             op2 = <dm-rhs>
  
          instance C a => C [a]
             {-# INLINE [2] op1 #-}
             op1 = <rhs>
  
  note that 'a' and 'b' are *both* in scope in <dm-rhs>, but only 'a' is
  in scope in <rhs>.  In particular, we must make sure that 'b' is in
  scope when typechecking <dm-rhs>.  This is achieved by subFunTys,
  which brings appropriate tyvars into scope. This happens for both
  <dm-rhs> and for <rhs>, but that doesn't matter: the *renamer* will have
  complained if 'b' is mentioned in <rhs>.
  
  
  
  ************************************************************************
  *                                                                      *
  \subsection{Extracting instance decls}
  *                                                                      *
  ************************************************************************
  
  Gather up the instance declarations from their various sources
  -}
  
  tcInstDecls1    -- Deal with both source-code and imported instance decls
     :: [LInstDecl GhcRn]         -- Source code instance decls
     -> TcM (TcGblEnv,            -- The full inst env
             [InstInfo GhcRn],    -- Source-code instance decls to process;
                                  -- contains all dfuns for this module
             [DerivInfo],         -- From data family instances
             ThBindEnv)           -- TH binding levels
  
  tcInstDecls1 inst_decls
    = do {    -- Do class and family instance declarations
         ; stuff <- mapAndRecoverM tcLocalInstDecl inst_decls
  
         ; let (local_infos_s, fam_insts_s, datafam_deriv_infos) = unzip3 stuff
               fam_insts   = concat fam_insts_s
               local_infos = concat local_infos_s
  
         ; (gbl_env, th_bndrs) <-
             addClsInsts local_infos $
             addFamInsts fam_insts
  
         ; return ( gbl_env
                  , local_infos
                  , concat datafam_deriv_infos
                  , th_bndrs ) }
  
  -- | Use DerivInfo for data family instances (produced by tcInstDecls1),
  --   datatype declarations (TyClDecl), and standalone deriving declarations
  --   (DerivDecl) to check and process all derived class instances.
  tcInstDeclsDeriv
    :: [DerivInfo]
    -> [LDerivDecl GhcRn]
    -> TcM (TcGblEnv, [InstInfo GhcRn], HsValBinds GhcRn)
  tcInstDeclsDeriv deriv_infos derivds
    = do th_lvl <- getThLevel -- See Note [Deriving inside TH brackets]
         if isBrackLevel th_lvl
         then do { gbl_env <- getGblEnv
                 ; return (gbl_env, bagToList emptyBag, emptyValBindsOut) }
         else do { (tcg_env, info_bag, valbinds) <- tcDeriving deriv_infos derivds
                 ; return (tcg_env, bagToList info_bag, valbinds) }
  
  addClsInsts :: [InstInfo GhcRn] -> TcM a -> TcM a
  addClsInsts infos thing_inside
    = tcExtendLocalInstEnv (map iSpec infos) thing_inside
  
  addFamInsts :: [FamInst] -> TcM (TcGblEnv, ThBindEnv)
  -- Extend (a) the family instance envt
  --        (b) the type envt with stuff from data type decls
  addFamInsts fam_insts
    = tcExtendLocalFamInstEnv fam_insts $
      tcExtendGlobalEnv axioms          $
      do { traceTc "addFamInsts" (pprFamInsts fam_insts)
         ; (gbl_env, th_bndrs) <- addTyConsToGblEnv data_rep_tycons
                      -- Does not add its axiom; that comes
                      -- from adding the 'axioms' above
         ; return (gbl_env, th_bndrs)
         }
    where
      axioms = map (ACoAxiom . toBranchedAxiom . famInstAxiom) fam_insts
      data_rep_tycons = famInstsRepTyCons fam_insts
        -- The representation tycons for 'data instances' declarations
  
  {-
  Note [Deriving inside TH brackets]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Given a declaration bracket
    [d| data T = A | B deriving( Show ) |]
  
  there is really no point in generating the derived code for deriving(
  Show) and then type-checking it. This will happen at the call site
  anyway, and the type check should never fail!  Moreover (#6005)
  the scoping of the generated code inside the bracket does not seem to
  work out.
  
  The easy solution is simply not to generate the derived instances at
  all.  (A less brutal solution would be to generate them with no
  bindings.)  This will become moot when we shift to the new TH plan, so
  the brutal solution will do.
  -}
  
  tcLocalInstDecl :: LInstDecl GhcRn
                  -> TcM ([InstInfo GhcRn], [FamInst], [DerivInfo])
          -- A source-file instance declaration
          -- Type-check all the stuff before the "where"
          --
          -- We check for respectable instance type, and context
  tcLocalInstDecl (L loc (TyFamInstD { tfid_inst = decl }))
    = do { fam_inst <- tcTyFamInstDecl NotAssociated (L loc decl)
         ; return ([], [fam_inst], []) }
  
  tcLocalInstDecl (L loc (DataFamInstD { dfid_inst = decl }))
    = do { (fam_inst, m_deriv_info) <- tcDataFamInstDecl NotAssociated emptyVarEnv (L loc decl)
         ; return ([], [fam_inst], maybeToList m_deriv_info) }
  
  tcLocalInstDecl (L loc (ClsInstD { cid_inst = decl }))
    = do { (insts, fam_insts, deriv_infos) <- tcClsInstDecl (L loc decl)
         ; return (insts, fam_insts, deriv_infos) }
  
  tcClsInstDecl :: LClsInstDecl GhcRn
                -> TcM ([InstInfo GhcRn], [FamInst], [DerivInfo])
  -- The returned DerivInfos are for any associated data families
  tcClsInstDecl (L loc (ClsInstDecl { cid_ext = lwarn
                                    , cid_poly_ty = hs_ty, cid_binds = binds
                                    , cid_sigs = uprags, cid_tyfam_insts = ats
                                    , cid_overlap_mode = overlap_mode
                                    , cid_datafam_insts = adts }))
    = setSrcSpanA loc                   $
      addErrCtxt (instDeclCtxt1 hs_ty)  $
      do  { dfun_ty <- tcHsClsInstType (InstDeclCtxt False) hs_ty
          ; let (tyvars, theta, clas, inst_tys) = tcSplitDFunTy dfun_ty
               -- NB: tcHsClsInstType does checkValidInstance
          ; skol_info <- mkSkolemInfo (InstSkol IsClsInst pSizeZero)
                         -- pSizeZero: here the size part of InstSkol is irrelevant
          ; (subst, skol_tvs) <- tcInstSkolTyVars skol_info tyvars
          ; let tv_skol_prs = [ (tyVarName tv, skol_tv)
                              | (tv, skol_tv) <- tyvars `zip` skol_tvs ]
                -- Map from the skolemized Names to the original Names.
                -- See Note [Associated data family instances and di_scoped_tvs].
                tv_skol_env = mkVarEnv $ map swap tv_skol_prs
                n_inferred = countWhile ((== Inferred) . binderFlag) $
                             fst $ splitForAllForAllTyBinders dfun_ty
                visible_skol_tvs = drop n_inferred skol_tvs
  
          ; traceTc "tcLocalInstDecl 1" (ppr dfun_ty $$ ppr (invisibleBndrCount dfun_ty) $$ ppr skol_tvs)
  
          -- Next, process any associated types.
          ; (datafam_stuff, tyfam_insts)
               <- tcExtendNameTyVarEnv tv_skol_prs $
                  do  { let mini_env   = mkVarEnv (classTyVars clas `zip` substTys subst inst_tys)
                            mini_subst = mkTvSubst (mkInScopeSet (mkVarSet skol_tvs)) mini_env
                            mb_info    = InClsInst { ai_class = clas
                                                   , ai_tyvars = visible_skol_tvs
                                                   , ai_inst_env = mini_env }
                      ; df_stuff  <- mapAndRecoverM (tcDataFamInstDecl mb_info tv_skol_env) adts
                      ; tf_insts1 <- mapAndRecoverM (tcTyFamInstDecl mb_info)   ats
  
                        -- Check for missing associated types and build them
                        -- from their defaults (if available)
                      ; is_boot <- tcIsHsBootOrSig
                      ; let atItems = classATItems clas
                      ; tf_insts2 <- mapM (tcATDefault (locA loc) mini_subst defined_ats)
                                          (if is_boot then [] else atItems)
                        -- Don't default type family instances, but rather omit, in hsig/hs-boot.
                        -- Since hsig/hs-boot files are essentially large binders we want omission
                        -- of the definition to result in no restriction, rather than for example
                        -- attempting to "pattern match" with the invisible defaults and generate
                        -- equalities. Without further handling, this would just result in a panic
                        -- anyway.
                        -- See https://github.com/ghc-proposals/ghc-proposals/pull/320 for
                        -- additional discussion.
                      ; return (df_stuff, tf_insts1 ++ concat tf_insts2) }
  
  
          -- Finally, construct the Core representation of the instance.
          -- (This no longer includes the associated types.)
          ; dfun_name <- newDFunName clas inst_tys (getLocA hs_ty)
                  -- Dfun location is that of instance *header*
  
          ; let warn = fmap unLoc lwarn
          ; ispec <- newClsInst (fmap unLoc overlap_mode) dfun_name
                                tyvars theta clas inst_tys warn
  
          ; let inst_binds = InstBindings
                               { ib_binds = binds
                               , ib_tyvars = map Var.varName tyvars -- Scope over bindings
                               , ib_pragmas = uprags
                               , ib_extensions = []
                               , ib_derived = False }
                inst_info = InstInfo { iSpec  = ispec, iBinds = inst_binds }
  
                (datafam_insts, m_deriv_infos) = unzip datafam_stuff
                deriv_infos                    = catMaybes m_deriv_infos
                all_insts                      = tyfam_insts ++ datafam_insts
  
           -- In hs-boot files there should be no bindings
          ; gbl_env <- getGblEnv;
          ; case tcg_src gbl_env of
            { HsSrcFile -> return ()
            ; HsBootOrSig boot_or_sig ->
               do { rejectBootDecls boot_or_sig BootBindsRn binds
                  ; rejectBootDecls boot_or_sig BootInstanceSigs uprags } }
          ; return ([inst_info], all_insts, deriv_infos) }
    where
      defined_ats = mkNameSet (map (tyFamInstDeclName . unLoc) ats)
                    `unionNameSet`
                    mkNameSet (map (unLoc . feqn_tycon
                                          . dfid_eqn
                                          . unLoc) adts)
  
  {-
  ************************************************************************
  *                                                                      *
                 Type family instances
  *                                                                      *
  ************************************************************************
  
  Family instances are somewhat of a hybrid.  They are processed together with
  class instance heads, but can contain data constructors and hence they share a
  lot of kinding and type checking code with ordinary algebraic data types (and
  GADTs).
  -}
  
  tcTyFamInstDecl :: AssocInstInfo
                  -> LTyFamInstDecl GhcRn -> TcM FamInst
    -- "type instance"; open type families only
    -- See Note [Associated type instances]
  tcTyFamInstDecl mb_clsinfo (L loc decl@(TyFamInstDecl { tfid_eqn = eqn }))
    = setSrcSpanA loc                        $
      tcAddOpenTyFamInstCtxt mb_clsinfo decl $
      do { let fam_lname = feqn_tycon eqn
         ; fam_tc <- tcLookupLocatedTyCon fam_lname
         ; tcFamInstDeclChecks mb_clsinfo IAmType fam_tc
  
           -- (0) Check it's an open type family
         ; checkTc (isTypeFamilyTyCon fam_tc) $
             TcRnIllegalInstance $ IllegalFamilyInstance $
               FamilyCategoryMismatch fam_tc
         ; checkTc (isOpenTypeFamilyTyCon fam_tc) $
             TcRnIllegalInstance $ IllegalFamilyInstance $
               NotAnOpenFamilyTyCon fam_tc
  
           -- (1) do the work of verifying the synonym group
           -- For some reason we don't have a location for the equation
           -- itself, so we make do with the location of family name
         ; (co_ax_branch, co_ax_validity_info)
            <- tcTyFamInstEqn fam_tc mb_clsinfo
                  (L (l2l $ getLoc fam_lname) eqn)
  
           -- (2) check for validity
         ; checkConsistentFamInst mb_clsinfo fam_tc co_ax_branch
         ; checkTyFamEqnValidityInfo fam_tc co_ax_validity_info
         ; checkValidCoAxBranch fam_tc co_ax_branch
  
           -- (3) construct coercion axiom
         ; rep_tc_name <- newFamInstAxiomName fam_lname [coAxBranchLHS co_ax_branch]
         ; let axiom = mkUnbranchedCoAxiom rep_tc_name fam_tc co_ax_branch
         ; newFamInst SynFamilyInst axiom }
  
  
  ---------------------
  tcFamInstDeclChecks :: AssocInstInfo -> TypeOrData -> TyCon -> TcM ()
  -- Used for both type and data families
  tcFamInstDeclChecks mb_clsinfo ty_or_data fam_tc
    = do { -- Type family instances require -XTypeFamilies
           -- and can't (currently) be in an hs-boot file
         ; traceTc "tcFamInstDecl" (ppr fam_tc)
         ; type_families <- xoptM LangExt.TypeFamilies
         ; hs_src        <- tcHscSource   -- Are we compiling an hs-boot file?
         ; checkTc type_families (TcRnTyFamsDisabled (TyFamsDisabledInstance fam_tc))
         ; case hs_src of
             HsBootOrSig boot_or_sig ->
               addErrTc $ TcRnIllegalHsBootOrSigDecl boot_or_sig (BootFamInst fam_tc)
             HsSrcFile               ->
               return ()
  
         -- Check that it is a family TyCon
         ; checkTc (isFamilyTyCon fam_tc) $
            TcRnIllegalInstance $ IllegalFamilyInstance $
              NotAFamilyTyCon ty_or_data fam_tc
  
         -- Check that top-level type instances are not for associated types.
         ; when (isNotAssociated mb_clsinfo &&   -- Not in a class decl
                 isTyConAssoc fam_tc) $          -- but an associated type
            addErr $ TcRnIllegalInstance $ IllegalFamilyInstance
                   $ InvalidAssoc $ InvalidAssocInstance
                   $ AssocInstanceNotInAClass fam_tc
         }
  
  {- Note [Associated type instances]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  We allow this:
    class C a where
      type T x a
    instance C Int where
      type T (S y) Int = y
      type T Z     Int = Char
  
  Note that
    a) The variable 'x' is not bound by the class decl
    b) 'x' is instantiated to a non-type-variable in the instance
    c) There are several type instance decls for T in the instance
  
  All this is fine.  Of course, you can't give any *more* instances
  for (T ty Int) elsewhere, because it's an *associated* type.
  
  
  ************************************************************************
  *                                                                      *
                 Data family instances
  *                                                                      *
  ************************************************************************
  
  For some reason data family instances are a lot more complicated
  than type family instances
  -}
  
  tcDataFamInstDecl ::
       AssocInstInfo
    -> TyVarEnv Name -- If this is an associated data family instance, maps the
                     -- parent class's skolemized type variables to their
                     -- original Names. If this is a non-associated instance,
                     -- this will be empty.
                     -- See Note [Associated data family instances and di_scoped_tvs].
    -> LDataFamInstDecl GhcRn -> TcM (FamInst, Maybe DerivInfo)
    -- "newtype instance" and "data instance"
  tcDataFamInstDecl mb_clsinfo tv_skol_env
      (L loc decl@(DataFamInstDecl { dfid_eqn =
        FamEqn { feqn_bndrs  = outer_bndrs
               , feqn_pats   = hs_pats
               , feqn_tycon  = lfam_name@(L _ fam_name)
               , feqn_fixity = fixity
               , feqn_rhs    = HsDataDefn { dd_cType   = cType
                                          , dd_ctxt    = hs_ctxt
                                          , dd_cons    = hs_cons
                                          , dd_kindSig = m_ksig
                                          , dd_derivs  = derivs } }}))
    = setSrcSpanA loc                      $
      tcAddDataFamInstCtxt mb_clsinfo new_or_data decl $
      do { fam_tc <- tcLookupLocatedTyCon lfam_name
  
         ; tcFamInstDeclChecks mb_clsinfo (IAmData new_or_data) fam_tc
  
         -- Check that the family declaration is for the right kind
         ; checkTc (isDataFamilyTyCon fam_tc) $
            TcRnIllegalInstance $ IllegalFamilyInstance $
              FamilyCategoryMismatch fam_tc
         ; gadt_syntax <- dataDeclChecks fam_name hs_ctxt hs_cons
            -- Do /not/ check that the number of patterns = tyConArity fam_tc
            -- See [Arity of data families] in GHC.Core.FamInstEnv
         ; skol_info <- mkSkolemInfo FamInstSkol
         ; (qtvs, non_user_tvs, pats, tc_res_kind, stupid_theta)
               <- tcDataFamInstHeader mb_clsinfo skol_info fam_tc outer_bndrs fixity
                                      hs_ctxt hs_pats m_ksig hs_cons
  
         -- Eta-reduce the axiom if possible
         -- Quite tricky: see Note [Implementing eta reduction for data families]
         ; let (eta_pats, eta_tcbs) = eta_reduce fam_tc pats
               eta_tvs       = map binderVar eta_tcbs
               post_eta_qtvs = filterOut (`elem` eta_tvs) qtvs
  
               full_tcbs = mkTyConBindersPreferAnon post_eta_qtvs
                              (tyCoVarsOfType (mkSpecForAllTys eta_tvs tc_res_kind))
                           ++ eta_tcbs
                   -- Put the eta-removed tyvars at the end
                   -- Remember, qtvs is in arbitrary order, except kind vars are
                   -- first, so there is no reason to suppose that the eta_tvs
                   -- (obtained from the pats) are at the end (#11148)
  
         -- Eta-expand the representation tycon until it has result
         -- kind `TYPE r`, for some `r`. If UnliftedNewtypes is not enabled, we
         -- go one step further and ensure that it has kind `TYPE 'LiftedRep`.
         --
         -- See also Note [Arity of data families] in GHC.Core.FamInstEnv
         -- NB: we can do this after eta-reducing the axiom, because if
         --     we did it before the "extra" tvs from etaExpandAlgTyCon
         --     would always be eta-reduced
         --
         ; (extra_tcbs, tc_res_kind) <- etaExpandAlgTyCon skol_info full_tcbs tc_res_kind
  
         -- Check the result kind; it may come from a user-written signature.
         -- See Note [Datatype return kinds] in GHC.Tc.TyCl point 4(a)
         ; let extra_pats    = map (mkTyVarTy . binderVar) extra_tcbs
               all_pats      = pats `chkAppend` extra_pats
               orig_res_ty   = mkTyConApp fam_tc all_pats
               tc_ty_binders = full_tcbs `chkAppend` extra_tcbs
  
         ; traceTc "tcDataFamInstDecl 1" $
           vcat [ text "Fam tycon:" <+> ppr fam_tc
                , text "Pats:" <+> ppr pats
                , text "visibilities:" <+> ppr (tcbVisibilities fam_tc pats)
                , text "all_pats:" <+> ppr all_pats
                , text "tc_ty_binders" <+> ppr tc_ty_binders
                , text "fam_tc_binders:" <+> ppr (tyConBinders fam_tc)
                , text "tc_res_kind:" <+> ppr tc_res_kind
                , text "eta_pats" <+> ppr eta_pats
                , text "eta_tcbs" <+> ppr eta_tcbs ]
  
         -- Zonk the patterns etc into the Type world
         ; (ty_binders, res_kind, all_pats, eta_pats, stupid_theta,
             zonked_post_eta_qtvs, zonked_eta_tvs) <-
           initZonkEnv NoFlexi $
           runZonkBndrT (zonkTyVarBindersX tc_ty_binders) $ \ ty_binders ->
             do { res_kind             <- zonkTcTypeToTypeX   tc_res_kind
                ; all_pats             <- zonkTcTypesToTypesX all_pats
                ; eta_pats             <- zonkTcTypesToTypesX eta_pats
                ; stupid_theta         <- zonkTcTypesToTypesX stupid_theta
                ; zonked_post_eta_qtvs <- mapM lookupTyVarX   post_eta_qtvs
                ; zonked_eta_tvs       <- mapM lookupTyVarX   eta_tvs
                      -- All these qtvs are in ty_binders, and hence will be in
                      -- the ZonkEnv, ze.  We need the zonked (TyVar) versions to
                      -- put in the CoAxiom that we are about to build.
                ; return (ty_binders, res_kind, all_pats, eta_pats, stupid_theta,
                           zonked_post_eta_qtvs, zonked_eta_tvs) }
  
         ; traceTc "tcDataFamInstDecl" $
           vcat [ text "Fam tycon:" <+> ppr fam_tc
                , text "Pats:" <+> ppr pats
                , text "visibilities:" <+> ppr (tcbVisibilities fam_tc pats)
                , text "all_pats:" <+> ppr all_pats
                , text "ty_binders" <+> ppr ty_binders
                , text "fam_tc_binders:" <+> ppr (tyConBinders fam_tc)
                , text "res_kind:" <+> ppr res_kind
                , text "eta_pats" <+> ppr eta_pats
                , text "eta_tcbs" <+> ppr eta_tcbs ]
         ; (rep_tc, (axiom, ax_rhs)) <- fixM $ \ ~(rec_rep_tc, _) ->
             do { data_cons <- tcExtendTyVarEnv (binderVars tc_ty_binders) $
                    -- For H98 decls, the tyvars scope
                    -- over the data constructors
                    tcConDecls (DDataInstance orig_res_ty) rec_rep_tc tc_ty_binders tc_res_kind
                        hs_cons
  
                ; rep_tc_name <- newFamInstTyConName lfam_name pats
                ; axiom_name  <- newFamInstAxiomName lfam_name [pats]
                ; tc_rhs <- case data_cons of
                       DataTypeCons type_data data_cons -> return $
                          mkLevPolyDataTyConRhs
                            (isFixedRuntimeRepKind res_kind)
                            type_data
                            data_cons
                       NewTypeCon data_con -> mkNewTyConRhs rep_tc_name rec_rep_tc data_con
  
                ; let ax_rhs = mkTyConApp rep_tc (mkTyVarTys zonked_post_eta_qtvs)
                      axiom  = mkSingleCoAxiom Representational axiom_name
                                   zonked_post_eta_qtvs zonked_eta_tvs
                                   [] fam_tc eta_pats ax_rhs
                      parent = DataFamInstTyCon axiom fam_tc all_pats
  
                        -- NB: Use the full ty_binders from the pats. See bullet toward
                        -- the end of Note [Data type families] in GHC.Core.TyCon
                      rep_tc   = mkAlgTyCon rep_tc_name
                                            ty_binders res_kind
                                            (map (const Nominal) ty_binders)
                                            (fmap unLoc cType) stupid_theta
                                            tc_rhs parent
                                            gadt_syntax
                   -- We always assume that indexed types are recursive.  Why?
                   -- (1) Due to their open nature, we can never be sure that a
                   -- further instance might not introduce a new recursive
                   -- dependency.  (2) They are always valid loop breakers as
                   -- they involve a coercion.
  
                ; return (rep_tc, (axiom, ax_rhs)) }
  
         -- Remember to check validity; no recursion to worry about here
         -- Check that left-hand sides are ok (mono-types, no type families,
         -- consistent instantiations, etc)
         ; let ax_branch = coAxiomSingleBranch axiom
         ; checkConsistentFamInst mb_clsinfo fam_tc ax_branch
         ; checkFamPatBinders fam_tc zonked_post_eta_qtvs non_user_tvs eta_pats ax_rhs
         ; checkValidCoAxBranch fam_tc ax_branch
         ; checkValidTyCon rep_tc
  
         ; let scoped_tvs = map mk_deriv_info_scoped_tv_pr (tyConTyVars rep_tc)
               m_deriv_info = case derivs of
                 []    -> Nothing
                 preds ->
                   Just $ DerivInfo { di_rep_tc     = rep_tc
                                    , di_scoped_tvs = scoped_tvs
                                    , di_clauses    = preds
                                    , di_ctxt       = tcMkDataFamInstCtxt mb_clsinfo new_or_data decl
                                    }
  
         ; fam_inst <- newFamInst (DataFamilyInst rep_tc) axiom
         ; return (fam_inst, m_deriv_info) }
    where
  
      new_or_data = dataDefnConsNewOrData hs_cons
  
      eta_reduce :: TyCon -> [Type] -> ([Type], [TyConBinder])
      -- See Note [Eta reduction for data families] in GHC.Core.Coercion.Axiom
      -- Splits the incoming patterns into two: the [TyVar]
      -- are the patterns that can be eta-reduced away.
      -- e.g.     T [a] Int a d c   ==>  (T [a] Int a, [d,c])
      --
      -- NB: quadratic algorithm, but types are small here
      eta_reduce fam_tc pats
          = go (reverse (zip3 pats fvs_s vis_s)) []
          where
            vis_s :: [TyConBndrVis]
            vis_s = tcbVisibilities fam_tc pats
  
            fvs_s :: [TyCoVarSet]  -- 1-1 correspondence with pats
                                   -- Each elt is the free vars of all /earlier/ pats
            (_, fvs_s) = mapAccumL add_fvs emptyVarSet pats
            add_fvs fvs pat = (fvs `unionVarSet` tyCoVarsOfType pat, fvs)
  
      go ((pat, fvs_to_the_left, tcb_vis):pats) etad_tvs
        | Just tv <- getTyVar_maybe pat
        , not (tv `elemVarSet` fvs_to_the_left)
        = go pats (Bndr tv tcb_vis : etad_tvs)
      go pats etad_tvs = (reverse (map fstOf3 pats), etad_tvs)
  
      -- Create a Name-TyVar mapping to bring into scope when typechecking any
      -- deriving clauses this data family instance may have.
      -- See Note [Associated data family instances and di_scoped_tvs].
      mk_deriv_info_scoped_tv_pr :: TyVar -> (Name, TyVar)
      mk_deriv_info_scoped_tv_pr tv =
        let n = lookupWithDefaultVarEnv tv_skol_env (tyVarName tv) tv
        in (n, tv)
  
  {-
  Note [Associated data family instances and di_scoped_tvs]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Some care is required to implement `deriving` correctly for associated data
  family instances. Consider this example from #18055:
  
    class C a where
      data D a
  
    class X a b
  
    instance C (Maybe a) where
      data D (Maybe a) deriving (X a)
  
  When typechecking the `X a` in `deriving (X a)`, we must ensure that the `a`
  from the instance header is brought into scope. This is the role of
  di_scoped_tvs, which maps from the original, renamed `a` to the skolemized,
  typechecked `a`. When typechecking the `deriving` clause, this mapping will be
  consulted when looking up the `a` in `X a`.
  
  A naïve attempt at creating the di_scoped_tvs is to simply reuse the
  tyConTyVars of the representation TyCon for `data D (Maybe a)`. This is only
  half correct, however. We do want the typechecked `a`'s Name in the /range/
  of the mapping, but we do not want it in the /domain/ of the mapping.
  To ensure that the original `a`'s Name ends up in the domain, we consult a
  TyVarEnv (passed as an argument to tcDataFamInstDecl) that maps from the
  typechecked `a`'s Name to the original `a`'s Name. In the even that
  tcDataFamInstDecl is processing a non-associated data family instance, this
  TyVarEnv will simply be empty, and there is nothing to worry about.
  -}
  
  -----------------------
  tcDataFamInstHeader
      :: AssocInstInfo -> SkolemInfo -> TyCon -> HsOuterFamEqnTyVarBndrs GhcRn
      -> LexicalFixity -> Maybe (LHsContext GhcRn)
      -> HsFamEqnPats GhcRn -> Maybe (LHsKind GhcRn) -> DataDefnCons (LConDecl GhcRn)
      -> TcM ([TcTyVar], TyVarSet, [TcType], TcKind, TcThetaType)
           -- All skolem TcTyVars, all zonked so it's clear what the free vars are
  -- The "header" of a data family instance is the part other than
  -- the data constructors themselves
  --    e.g.  data instance D [a] :: * -> * where ...
  -- Here the "header" is the bit before the "where"
  tcDataFamInstHeader mb_clsinfo skol_info fam_tc hs_outer_bndrs fixity
                      hs_ctxt hs_pats m_ksig hs_cons
    = do { traceTc "tcDataFamInstHeader {" (ppr fam_tc <+> ppr hs_pats)
         ; (tclvl, wanted, (outer_bndrs, (stupid_theta, lhs_ty, master_res_kind, instance_res_kind)))
              <- pushLevelAndSolveEqualitiesX "tcDataFamInstHeader" $
                 bindOuterFamEqnTKBndrs skol_info hs_outer_bndrs    $  -- Binds skolem TcTyVars
                 do { stupid_theta <- tcHsContext hs_ctxt
                    ; (lhs_ty, lhs_kind) <- tcFamTyPats fam_tc hs_pats
                    ; (lhs_applied_ty, lhs_applied_kind)
                        <- tcInstInvisibleTyBinders lhs_ty lhs_kind
                        -- See Note [Data family/instance return kinds]
                        -- in GHC.Tc.TyCl point (DF3)
  
                    -- Ensure that the instance is consistent
                    -- with its parent class
                    ; addConsistencyConstraints mb_clsinfo lhs_ty
  
                    -- Add constraints from the data constructors
                    -- Fix #25611
                    -- See DESIGN CHOICE in Note [Kind inference for data family instances]
                    ; when is_H98_or_newtype $ kcConDecls lhs_applied_kind hs_cons
  
                    -- Check that the result kind of the TyCon applied to its args
                    -- is compatible with the explicit signature (or Type, if there
                    -- is none)
                    ; let hs_lhs = nlHsTyConApp NotPromoted fixity (noUserRdr $ getName fam_tc) hs_pats
                    -- Add constraints from the result signature
                    ; res_kind <- tc_kind_sig m_ksig
                    ; _ <- unifyKind (Just . HsTypeRnThing $ unLoc hs_lhs) lhs_applied_kind res_kind
  
                    ; traceTc "tcDataFamInstHeader" $
                      vcat [ ppr fam_tc, ppr m_ksig, ppr lhs_applied_kind, ppr res_kind, ppr m_ksig]
                    ; return ( stupid_theta
                             , lhs_applied_ty
                             , lhs_applied_kind
                             , res_kind ) }
  
         ; outer_bndrs <- scopedSortOuter outer_bndrs
         ; let outer_tvs = outerTyVars outer_bndrs
         ; checkFamTelescope tclvl hs_outer_bndrs outer_tvs
  
         -- This code (and the stuff immediately above) is very similar
         -- to that in tcTyFamInstEqnGuts.  Maybe we should abstract the
         -- common code; but for the moment I concluded that it's
         -- clearer to duplicate it.  Still, if you fix a bug here,
         -- check there too!
  
         -- See GHC.Tc.TyCl Note [Generalising in tcTyFamInstEqnGuts]
         ; dvs  <- candidateQTyVarsWithBinders outer_tvs lhs_ty
         ; qtvs <- quantifyTyVars skol_info TryNotToDefaultNonStandardTyVars dvs
         ; let final_tvs = scopedSort (qtvs ++ outer_tvs)
               -- This scopedSort is important: the qtvs may be /interleaved/ with
               -- the outer_tvs.  See Note [Generalising in tcTyFamInstEqnGuts]
         ; reportUnsolvedEqualities skol_info final_tvs tclvl wanted
  
         ; (final_tvs, non_user_tvs, lhs_ty, master_res_kind, instance_res_kind, stupid_theta) <-
            liftZonkM $ do
              { final_tvs         <- mapM zonkTcTyVarToTcTyVar final_tvs
              ; non_user_tvs      <- mapM zonkTcTyVarToTcTyVar qtvs
              ; lhs_ty            <- zonkTcType                lhs_ty
              ; master_res_kind   <- zonkTcType                master_res_kind
              ; instance_res_kind <- zonkTcType                instance_res_kind
              ; stupid_theta      <- zonkTcTypes               stupid_theta
              ; return (final_tvs, non_user_tvs, lhs_ty, master_res_kind, instance_res_kind, stupid_theta) }
  
         -- Check that res_kind is OK with checkDataKindSig.  We need to
         -- check that it's ok because res_kind can come from a user-written
         -- kind signature.  See Note [Datatype return kinds], point (4a)
         ; checkDataKindSig (DataInstanceSort new_or_data) master_res_kind
         ; checkDataKindSig (DataInstanceSort new_or_data) instance_res_kind
  
         -- Split up the LHS type to get the type patterns
         -- For the scopedSort see Note [Generalising in tcTyFamInstEqnGuts]
         ; let pats      = unravelFamInstPats lhs_ty
  
         ; return (final_tvs, mkVarSet non_user_tvs, pats, master_res_kind, stupid_theta) }
    where
      fam_name  = tyConName fam_tc
      data_ctxt = DataKindCtxt fam_name
      new_or_data = dataDefnConsNewOrData hs_cons
      is_H98_or_newtype = case hs_cons of
        NewTypeCon{} -> True
        DataTypeCons _ cons -> all isH98 cons
      isH98 (L _ (ConDeclH98 {})) = True
      isH98 _ = False
  
      -- See Note [Implementation of UnliftedNewtypes] in GHC.Tc.TyCl, families (2),
      -- Note [Implementation of UnliftedDatatypes]
      -- and Note [Defaulting result kind of newtype/data family instance].
      tc_kind_sig Nothing
        = do { unlifted_newtypes  <- xoptM LangExt.UnliftedNewtypes
             ; unlifted_datatypes <- xoptM LangExt.UnliftedDatatypes
             ; case new_or_data of
                 NewType  | unlifted_newtypes  -> newOpenTypeKind
                 DataType | unlifted_datatypes -> newOpenTypeKind
                 _                             -> pure liftedTypeKind
             }
  
      -- See Note [Result kind signature for a data family instance]
      tc_kind_sig (Just hs_kind)
        = do { sig_kind <- tcLHsKindSig data_ctxt hs_kind
             ; (_tvs', inner_kind') <- tcSkolemiseInvisibleBndrs (SigTypeSkol data_ctxt) sig_kind
                     -- Perhaps surprisingly, we don't need the skolemised tvs themselves
             ; return inner_kind' }
  
  {- Note [Result kind signature for a data family instance]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  The expected type might have a forall at the type. Normally, we
  can't skolemise in kinds because we don't have type-level lambda.
  But here, we're at the top-level of an instance declaration, so
  we actually have a place to put the regeneralised variables.
  Thus: skolemise away. cf. GHC.Tc.Utils.Unify.tcTopSkolemise
  Examples in indexed-types/should_compile/T12369
  
  Note [Defaulting result kind of newtype/data family instance]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  It is tempting to let `tc_kind_sig` just return `newOpenTypeKind`
  even without `-XUnliftedNewtypes`, but we rely on `tc_kind_sig` to
  constrain the result kind of a newtype instance to `Type`.
  Consider the following example:
  
    -- no UnliftedNewtypes
    data family D :: k -> k
    newtype instance D a = MkIntD a
  
  `tc_kind_sig` defaulting to `newOpenTypeKind` would result in `D a`
  having kind `forall r. TYPE r` instead of `Type`, which would be
  rejected validity checking. The same applies to Data Instances.
  
  Note [Implementing eta reduction for data families]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Consider
     data D :: * -> * -> * -> * -> *
  
     data instance D [(a,b)] p q :: * -> * where
        D1 :: blah1
        D2 :: blah2
  
  Then we'll generate a representation data type
    data Drep a b p q z where
        D1 :: blah1
        D2 :: blah2
  
  and an axiom to connect them
    axiom AxDrep forall a b p q z. D [(a,b]] p q z = Drep a b p q z
  
  except that we'll eta-reduce the axiom to
    axiom AxDrep forall a b. D [(a,b]] = Drep a b
  
  This is described at some length in Note [Eta reduction for data families]
  in GHC.Core.Coercion.Axiom. There are several fiddly subtleties lurking here,
  however, so this Note aims to describe these subtleties:
  
  * The representation tycon Drep is parameterised over the free
    variables of the pattern, in no particular order. So there is no
    guarantee that 'p' and 'q' will come last in Drep's parameters, and
    in the right order.  So, if the /patterns/ of the family instance
    are eta-reducible, we re-order Drep's parameters to put the
    eta-reduced type variables last.
  
  * Although we eta-reduce the axiom, we eta-/expand/ the representation
    tycon Drep.  The kind of D says it takes four arguments, but the
    data instance header only supplies three.  But the AlgTyCon for Drep
    itself must have enough TyConBinders so that its result kind is Type.
    So, with etaExpandAlgTyCon we make up some extra TyConBinders.
    See point (3) in Note [Datatype return kinds] in GHC.Tc.TyCl.
  
  * The result kind in the instance might be a polykind, like this:
       data family DP a :: forall k. k -> *
       data instance DP [b] :: forall k1 k2. (k1,k2) -> *
  
    So in type-checking the LHS (DP Int) we need to check that it is
    more polymorphic than the signature.  To do that we must skolemise
    the signature and instantiate the call of DP.  So we end up with
       data instance DP [b] @(k1,k2) (z :: (k1,k2)) where
  
    Note that we must parameterise the representation tycon DPrep over
    'k1' and 'k2', as well as 'b'.
  
    The skolemise bit is done in tc_kind_sig, while the instantiate bit
    is done by tcFamTyPats.
  
  * Very fiddly point.  When we eta-reduce to
       axiom AxDrep forall a b. D [(a,b]] = Drep a b
  
    we want the kind of (D [(a,b)]) to be the same as the kind of
    (Drep a b).  This ensures that applying the axiom doesn't change the
    kind.  Why is that hard?  Because the kind of (Drep a b) depends on
    the TyConBndrVis on Drep's arguments. In particular do we have
      (forall (k::*). blah) or (* -> blah)?
  
    We must match whatever D does!  In #15817 we had
        data family X a :: forall k. * -> *   -- Note: a forall that is not used
        data instance X Int b = MkX
  
    So the data instance is really
        data istance X Int @k b = MkX
  
    The axiom will look like
        axiom    X Int = Xrep
  
    and it's important that XRep :: forall k * -> *, following X.
  
    To achieve this we get the TyConBndrVis flags from tcbVisibilities,
    and use those flags for any eta-reduced arguments.  Sigh.
  
  * The final turn of the knife is that tcbVisibilities is itself
    tricky to sort out.  Consider
        data family D k :: k
    Then consider D (forall k2. k2 -> k2) Type Type
    The visibility flags on an application of D may affected by the arguments
    themselves.  Heavy sigh.  But not truly hard; that's what tcbVisibilities
    does.
  
  * Happily, we don't need to worry about the possibility of
    building an inhomogeneous axiom, described in GHC.Tc.TyCl.Build
    Note [Newtype eta and homogeneous axioms].   For example
       type F :: Type -> forall (b :: Type) -> Type
       data family F a b
       newtype instance F Int b = MkF (Proxy b)
    we get a newtype, and a eta-reduced axiom connecting the data family
    with the newtype:
       type R:FIntb :: forall (b :: Type) -> Type
       newtype R:FIntb b = MkF (Proxy b)
       axiom Foo.D:R:FIntb0 :: F Int = Foo.R:FIntb
    Now the subtleties of Note [Newtype eta and homogeneous axioms] are
    dealt with by the newtype (via mkNewTyConRhs called in tcDataFamInstDecl)
    while the axiom connecting F Int ~ R:FIntb is eta-reduced, but the
    quantifier 'b' is derived from the original data family F, and so the
    kinds will always match.
  
  Note [Kind inference for data family instances]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Consider this GADT-style data type declaration, where I have used
  fresh variables in the data constructor's type, to stress that c,d are
  quite distinct from a,b.
     data T a b where
       MkT :: forall c d. c d -> T c d
  
  Following Note [Inferring kinds for type declarations] in GHC.Tc.TyCl,
  to infer T's kind, we initially give T :: kappa, a monomorpic kind,
  gather constraints from the header and data constructors, and conclude
     T :: (kappa1 -> type) -> kappa1 -> Type
  Then we generalise, giving
     T :: forall k. (k->Type) -> k -> Type
  
  Now what about a data /instance/ decl
     data family T :: forall k. (k->Type) -> k -> Type
  
     data instance T p Int where ...
  
  No doubt here! The poly-kinded T is instantiated with k=Type, so the
  header really looks like
     data instance T @Type (p :: Type->Type) Int where ...
  
  But what about this?
     data instance T p q where
        MkT :: forall r. r Int -> T r Int
  
  So what kind do 'p' and 'q' have?  No clues from the header, but from
  the data constructor we can clearly see that (r :: Type->Type).  Does
  that mean that the /entire data instance/ is instantiated at Type,
  like this?
     data instance T @Type (p :: Type->Type) (q :: Type) where
        ...
  
  Not at all! This is a /GADT/-style decl, so the kind argument might
  be specialised in this particular data constructor, thus:
     data instance T @k (p :: k->Type) (q :: k) where
       MkT :: forall (r :: Type -> Type).
              r Int -> T @Type r Int
  (and perhaps specialised differently in some other data
  constructor MkT2).
  
  The key difference in this case and 'data T' at the top of this Note
  is that we have no known kind for 'data T'. We thus forbid different
  specialisations of T in its constructors, in an attempt to avoid
  inferring polymorphic recursion. In data family T, however, there is
  no problem with polymorphic recursion: we already /fully know/ T's
  kind -- that came from the family declaration, and is not influenced
  by the data instances -- and hence we /can/ specialise T's kind
  differently in different GADT data constructors.
  
  SHORT SUMMARY: In a data instance decl, it's not clear whether kind
  constraints arising from the data constructors should be considered
  local to the (GADT) data /constructor/ or should apply to the entire
  data instance.
  
  DESIGN CHOICE: In a data/newtype family instance declaration:
  * We take account of the data constructors (via `kcConDecls`) for:
    * Haskell-98 style data instance declarations
    * All newtype instance declarations
    For Haskell-98 style declarations, there is no GADT refinement. And for
    GADT-style newtype declarations, no GADT matching is allowed anyway,
    so it's just a syntactic difference from Haskell-98.
  
  * We /ignore/ the data constructors for:
    * GADT-style data instance declarations
    Here, the instance kinds are influenced only by the header.
  
  This choice is implemented by the guarded call to `kcConDecls` in
  `tcDataFamInstHeader`.
  
  Observations:
  * With `UnliftedNewtypes` or `UnliftedDatatypes`, looking at the data
    constructors is necessary to infer the kind of the result type for
    certain cases. Otherwise, additional kind signatures are required.
    Consider the following example in #25611:
  
      data family Fix :: (k -> Type) -> k
      newtype instance Fix f = In { out :: f (Fix f) }
  
    If we are not looking at the data constructors:
    * Without `UnliftedNewtypes`, it is accepted since `Fix f` is defaulted
      to `Type`.
    * But with `UnliftedNewtypes`, `Fix f` is defaulted to `TYPE r` where
      `r` is not scoped over the data constructor. Then the header `Fix f :: TYPE r`
      will fail to kind unify with `f (Fix f) :: Type`.
  
    Hence, we need to look at the data constructor to infer `Fix f :: Type`
    for this newtype instance.
  
  This DESIGN CHOICE strikes a balance between well-rounded kind inference
  and implementation simplicity. See #25611, #18891, and !4419 for more
  discussion of this issue.
  
  Kind inference for data types (Xie et al) https://arxiv.org/abs/1911.06153
  takes a slightly different approach.
  -}
  
  
  {- *********************************************************************
  *                                                                      *
        Class instance declarations, pass 2
  *                                                                      *
  ********************************************************************* -}
  
  tcInstDecls2 :: [LTyClDecl GhcRn] -> [InstInfo GhcRn]
               -> TcM (LHsBinds GhcTc)
  -- (a) From each class declaration,
  --      generate any default-method bindings
  -- (b) From each instance decl
  --      generate the dfun binding
  
  tcInstDecls2 tycl_decls inst_decls
    = do  { -- (a) Default methods from class decls
            let class_decls = filter (isClassDecl . unLoc) tycl_decls
          ; dm_binds_s <- mapM tcClassDecl2 class_decls
          ; let dm_binds = concat dm_binds_s
  
            -- (b) instance declarations
          ; let dm_ids = collectHsBindsBinders CollNoDictBinders dm_binds
                -- Add the default method Ids (again)
                -- (they were already added in GHC.Tc.TyCl.Utils.tcAddImplicits)
                -- See Note [Default methods in the type environment]
          ; inst_binds_s <- tcExtendGlobalValEnv dm_ids $
                            mapM tcInstDecl2 inst_decls
  
            -- Done
          ; return (dm_binds ++ concat inst_binds_s) }
  
  {- Note [Default methods in the type environment]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  The default method Ids are already in the type environment (see Note
  [Default method Ids and Template Haskell] in TcTyDcls), BUT they
  don't have their InlinePragmas yet.  Usually that would not matter,
  because the simplifier propagates information from binding site to
  use.  But, unusually, when compiling instance decls we *copy* the
  INLINE pragma from the default method to the method for that
  particular operation (see Note [INLINE and default methods] below).
  
  So right here in tcInstDecls2 we must re-extend the type envt with
  the default method Ids replete with their INLINE pragmas.  Urk.
  -}
  
  tcInstDecl2 :: InstInfo GhcRn -> TcM (LHsBinds GhcTc)
              -- Returns a binding for the dfun
  tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = ibinds })
    = recoverM (return emptyLHsBinds)    $
      setSrcSpan loc                     $
      addErrCtxt (instDeclCtxt2 dfun_ty) $
      do {  -- Instantiate the instance decl with skolem constants
           (skol_info, inst_tyvars, dfun_theta, clas, inst_tys) <- tcSkolDFunType dfun_ty
         ; dfun_ev_vars <- newEvVars dfun_theta
  
         ; let (class_tyvars, sc_theta, _, op_items) = classBigSig clas
               sc_theta' = substTheta (zipTvSubst class_tyvars inst_tys) sc_theta
  
         ; traceTc "tcInstDecl2" (vcat [ppr inst_tyvars, ppr inst_tys, ppr dfun_theta, ppr sc_theta'])
  
                        -- Deal with 'SPECIALISE instance' pragmas
                        -- See Note [SPECIALISE instance pragmas]
         ; spec_inst_info@(spec_inst_prags,_) <- tcSpecInstPrags dfun_id ibinds
  
           -- Typecheck superclasses and methods
           -- See Note [Typechecking plan for instance declarations]
         ; dfun_ev_binds_var <- newTcEvBinds
         ; let dfun_ev_binds = TcEvBinds dfun_ev_binds_var
         ; (tclvl, (sc_meth_ids, sc_meth_binds, sc_meth_implics))
               <- pushTcLevelM $
                  do { (sc_ids, sc_binds, sc_implics)
                          <- tcSuperClasses skol_info dfun_id clas inst_tyvars
                                            dfun_ev_vars dfun_ev_binds sc_theta'
  
                        -- Typecheck the methods
                     ; (meth_ids, meth_binds, meth_implics)
                          <- tcMethods skol_info dfun_id clas inst_tyvars dfun_ev_vars
                                       inst_tys dfun_ev_binds spec_inst_info
                                       op_items ibinds
  
                     ; return ( sc_ids     ++          meth_ids
                              , sc_binds   ++ meth_binds
                              , sc_implics `unionBags` meth_implics ) }
  
         ; imp <- newImplication
         ; emitImplication $
           imp { ic_tclvl  = tclvl
               , ic_skols  = inst_tyvars
               , ic_given  = dfun_ev_vars
               , ic_wanted = mkImplicWC sc_meth_implics
               , ic_binds  = dfun_ev_binds_var
               , ic_info   = skol_info }
  
         -- Create the result bindings
         ; self_dict <- newDict clas inst_tys
         ; let class_tc      = classTyCon clas
               loc'          = noAnnSrcSpan loc
               dict_constr   = tyConSingleDataCon class_tc
               dict_bind = mkVarBind self_dict (L loc' con_app_args)
  
                       -- We don't produce a binding for the dict_constr; instead we
                       -- rely on the simplifier to unfold this saturated application
                       -- We do this rather than generate an HsCon directly, because
                       -- it means that the special cases (e.g. dictionary with only one
                       -- member) are dealt with by the common MkId.mkDataConWrapId
                       -- code rather than needing to be repeated here.
                       --    con_app_tys  = MkD ty1 ty2
                       --    con_app_scs  = MkD ty1 ty2 sc1 sc2
                       --    con_app_args = MkD ty1 ty2 sc1 sc2 op1 op2
               con_app_tys  = mkHsWrap (mkWpTyApps inst_tys) $
                              mkConLikeTc (RealDataCon dict_constr)
                         -- NB: We *can* have covars in inst_tys, in the case of
                         -- promoted GADT constructors.
  
               con_app_args = foldl' app_to_meth con_app_tys sc_meth_ids
  
               app_to_meth :: HsExpr GhcTc -> Id -> HsExpr GhcTc
               app_to_meth fun meth_id = HsApp noExtField (L loc' fun)
                                              (L loc' (wrapId arg_wrapper meth_id))
  
               inst_tv_tys = mkTyVarTys inst_tyvars
               arg_wrapper = mkWpEvVarApps dfun_ev_vars <.> mkWpTyApps inst_tv_tys
  
               dfun_id_w_prags = addDFunPrags dfun_id sc_meth_ids
               dfun_spec_prags = SpecPrags spec_inst_prags
  
               export = ABE { abe_wrap = idHsWrapper
                            , abe_poly = dfun_id_w_prags
                            , abe_mono = self_dict
                            , abe_prags = dfun_spec_prags }
                            -- NB: see Note [SPECIALISE instance pragmas]
               main_bind = XHsBindsLR $
                           AbsBinds { abs_tvs = inst_tyvars
                                    , abs_ev_vars = dfun_ev_vars
                                    , abs_exports = [export]
                                    , abs_ev_binds = []
                                    , abs_binds = [dict_bind]
                                    , abs_sig = True }
  
         ; return (L loc' main_bind : sc_meth_binds)
         }
   where
     dfun_id = instanceDFunId ispec
     dfun_ty = idType dfun_id
     loc     = getSrcSpan dfun_id
  
  addDFunPrags :: DFunId -> [Id] -> DFunId
  -- DFuns need a special Unfolding and InlinePrag
  --    See Note [ClassOp/DFun selection]
  --    and Note [Single-method classes]
  -- It's easiest to create those unfoldings right here, where
  -- have all the pieces in hand, even though we are messing with
  -- Core at this point, which the typechecker doesn't usually do
  -- However we take care to build the unfolding using the TyVars from
  -- the DFunId rather than from the skolem pieces that the typechecker
  -- is messing with.
  addDFunPrags dfun_id sc_meth_ids
   = dfun_id `setIdUnfolding`  mkDFunUnfolding dfun_bndrs dict_con dict_args
             `setInlinePragma` dfunInlinePragma
             -- NB: mkDFunUnfolding takes care of unary classes
   where
     dict_args  = map Type inst_tys ++
                  [mkVarApps (Var id) dfun_bndrs | id <- sc_meth_ids]
  
     (dfun_tvs, dfun_theta, clas, inst_tys) = tcSplitDFunTy (idType dfun_id)
     ev_ids      = mkTemplateLocalsNum 1                    dfun_theta
     dfun_bndrs  = dfun_tvs ++ ev_ids
     clas_tc     = classTyCon clas
     dict_con    = tyConSingleDataCon clas_tc
  
  wrapId :: HsWrapper -> Id -> HsExpr GhcTc
  wrapId wrapper id = mkHsWrap wrapper (mkHsVar (noLocA id))
  
  {- Note [Typechecking plan for instance declarations]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  For instance declarations we generate the following bindings and implication
  constraints.  Example:
  
     instance Ord a => Ord [a] where compare = <compare-rhs>
  
  generates this:
  
     Bindings:
        -- Method bindings
        $ccompare :: forall a. Ord a => a -> a -> Ordering
        $ccompare = /\a \(d:Ord a). let <meth-ev-binds> in ...
  
        -- Superclass bindings
        $cp1Ord :: forall a. Ord a => Eq [a]
        $cp1Ord = /\a \(d:Ord a). let <sc-ev-binds>
                 in dfEqList (dw :: Eq a)
  
     Constraints:
        forall a. Ord a =>
                  -- Method constraint
               (forall. (empty) => <constraints from compare-rhs>)
                  -- Superclass constraint
            /\ (forall. (empty) => dw :: Eq a)
  
  Notice that
  
   * Per-meth/sc implication.  There is one inner implication per
     superclass or method, with no skolem variables or givens.  The only
     reason for this one is to gather the evidence bindings privately
     for this superclass or method.  This implication is generated
     by checkInstConstraints.
  
   * Overall instance implication. There is an overall enclosing
     implication for the whole instance declaration, with the expected
     skolems and givens.  We need this to get the correct "redundant
     constraint" warnings, gathering all the uses from all the methods
     and superclasses.  See GHC.Tc.Solver Note [Tracking redundant
     constraints]
  
   * The given constraints in the outer implication may generate
     evidence, notably by superclass selection.  Since the method and
     superclass bindings are top-level, we want that evidence copied
     into *every* method or superclass definition.  (Some of it will
     be usused in some, but dead-code elimination will drop it.)
  
     We achieve this by putting the evidence variable for the overall
     instance implication into the AbsBinds for each method/superclass.
     Hence the 'dfun_ev_binds' passed into tcMethods and tcSuperClasses.
     (And that in turn is why the abs_ev_binds field of AbBinds is a
     [TcEvBinds] rather than simply TcEvBinds.
  
     This is a bit of a hack, but works very nicely in practice.
  
   * Note that if a method has a locally-polymorphic binding, there will
     be yet another implication for that, generated by tcPolyCheck
     in tcMethodBody. E.g.
            class C a where
              foo :: forall b. Ord b => blah
  
  
  ************************************************************************
  *                                                                      *
        Type-checking superclasses
  *                                                                      *
  ************************************************************************
  -}
  
  tcSuperClasses :: SkolemInfoAnon -> DFunId -> Class -> [TcTyVar]
                 -> [EvVar]
                 -> TcEvBinds
                 -> TcThetaType
                 -> TcM ([EvVar], LHsBinds GhcTc, Bag Implication)
  -- Make a new top-level function binding for each superclass,
  -- something like
  --    $Ordp1 :: forall a. Ord a => Eq [a]
  --    $Ordp1 = /\a \(d:Ord a). dfunEqList a (sc_sel d)
  --
  -- See Note [Recursive superclasses] for why this is so hard!
  -- In effect, we build a special-purpose solver for the first step
  -- of solving each superclass constraint
  tcSuperClasses skol_info dfun_id cls tyvars dfun_evs dfun_ev_binds sc_theta
    = do { (ids, binds, implics) <- mapAndUnzip3M tc_super (zip sc_theta [fIRST_TAG..])
         ; return (ids, binds, listToBag implics) }
    where
      loc = getSrcSpan dfun_id
      tc_super (sc_pred, n)
        = do { (sc_implic, ev_binds_var, sc_ev_tm)
                  <- checkInstConstraints skol_info $
                     emitWanted (ScOrigin IsClsInst NakedSc) sc_pred
                     -- ScOrigin IsClsInst True: see Note [Solving superclass constraints]
  
             ; sc_top_name  <- newName (mkSuperDictAuxOcc n (getOccName cls))
             ; sc_ev_id     <- newEvVar sc_pred
             ; addTcEvBind ev_binds_var $ mkWantedEvBind sc_ev_id EvCanonical sc_ev_tm
             ; let sc_top_ty = tcMkDFunSigmaTy tyvars (map idType dfun_evs) sc_pred
                   sc_top_id = mkLocalId sc_top_name ManyTy sc_top_ty
                   export = ABE { abe_wrap = idHsWrapper
                                , abe_poly = sc_top_id
                                , abe_mono = sc_ev_id
                                , abe_prags = noSpecPrags }
                   local_ev_binds = TcEvBinds ev_binds_var
                   bind = XHsBindsLR $
                          AbsBinds { abs_tvs      = tyvars
                                   , abs_ev_vars  = dfun_evs
                                   , abs_exports  = [export]
                                   , abs_ev_binds = [dfun_ev_binds, local_ev_binds]
                                   , abs_binds    = []
                                   , abs_sig      = False }
             ; return (sc_top_id, L (noAnnSrcSpan loc) bind, sc_implic) }
  
  -------------------
  checkInstConstraints :: SkolemInfoAnon -> TcM result
                       -> TcM (Implication, EvBindsVar, result)
  -- See Note [Typechecking plan for instance declarations]
  checkInstConstraints skol_info thing_inside
    = do { (tclvl, wanted, result) <- pushLevelAndCaptureConstraints  $
                                      thing_inside
  
         ; ev_binds_var <- newTcEvBinds
         ; implic <- newImplication
         ; let implic' = implic { ic_tclvl  = tclvl
                                , ic_wanted = wanted
                                , ic_binds  = ev_binds_var
                                , ic_info   = skol_info }
  
         ; return (implic', ev_binds_var, result) }
  
  {-
  Note [Recursive superclasses]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  See #3731, #4809, #5751, #5913, #6117, #6161, which all
  describe somewhat more complicated situations, but ones
  encountered in practice.
  
  See also tests tcrun020, tcrun021, tcrun033, and #11427.
  
  ----- THE PROBLEM --------
  The problem is that it is all too easy to create a class whose
  superclass is bottom when it should not be.
  
  Consider the following (extreme) situation:
          class C a => D a where ...
          instance D [a] => D [a] where ...   (dfunD)
          instance C [a] => C [a] where ...   (dfunC)
  Although this looks wrong (assume D [a] to prove D [a]), it is only a
  more extreme case of what happens with recursive dictionaries, and it
  can, just about, make sense because the methods do some work before
  recursing.
  
  To implement the dfunD we must generate code for the superclass C [a],
  which we had better not get by superclass selection from the supplied
  argument:
         dfunD :: forall a. D [a] -> D [a]
         dfunD = \d::D [a] -> MkD (scsel d) ..
  
  Otherwise if we later encounter a situation where
  we have a [Wanted] dw::D [a] we might solve it thus:
       dw := dfunD dw
  Which is all fine except that now ** the superclass C is bottom **!
  
  The instance we want is:
         dfunD :: forall a. D [a] -> D [a]
         dfunD = \d::D [a] -> MkD (dfunC (scsel d)) ...
  
  ----- THE SOLUTION --------
  The basic solution is simple: be very careful about using superclass
  selection to generate a superclass witness in a dictionary function
  definition.  More precisely:
  
    Superclass Invariant: in every class dictionary,
                          every superclass dictionary field
                          is non-bottom
  
  To achieve the Superclass Invariant, in a dfun definition we can
  generate a guaranteed-non-bottom superclass witness from:
    (sc1) one of the dictionary arguments itself (all non-bottom)
    (sc2) an immediate superclass of a non-bottom dictionary that is
          /Paterson-smaller/ than the instance head
          See Note [The PatersonSize of a type] in GHC.Tc.Utils.TcType
    (sc3) a call of a dfun (always returns a dictionary constructor)
  
  The tricky case is (sc2).  We proceed by induction on the size of the
  (type of) the dictionary, defined by GHC.Tc.Utils.TcType.pSizeType.  Let's
  suppose we are building a dictionary of size 3 (the "head"), and suppose
  the Superclass Invariant holds of smaller dictionaries.  Then if we have a
  smaller dictionary, its immediate superclasses will be non-bottom by
  induction.
  
  Why "Paterson-smaller"? See Note [Paterson conditions] in GHC.Tc.Validity.
  We want to be sure that the superclass dictionary is smaller /for any
  ground instatiation/ of the instance, so we need to account for type
  variables that occur more than once, and for type families (#20666).  And
  that's exactly what the Paterson conditions check!
  
  Here is an example, taken from CmmExpr:
         class Ord r => UserOfRegs r a where ...
  (i1)   instance UserOfRegs r a => UserOfRegs r (Maybe a) where
  (i2)   instance (Ord r, UserOfRegs r CmmReg) => UserOfRegs r CmmExpr where
  
  For (i1) we can get the (Ord r) superclass by selection from
  (UserOfRegs r a), since it (i.e. UserOfRegs r a) is smaller than the
  thing we are building, namely (UserOfRegs r (Maybe a)).
  
  But for (i2) that isn't the case: (UserOfRegs r CmmReg) is not smaller
  than the thing we are building (UserOfRegs r CmmExpr), so we can't use
  the superclasses of the former.  Hence we must instead add an explicit,
  and perhaps surprising, (Ord r) argument to the instance declaration.
  
  Here's another example from #6161:
  
         class         Super a => Duper a  where ...
         class Duper (Maybe a) => Foo a    where ...
  (i3)   instance Foo a => Duper (Maybe a) where ...
  (i4)   instance                Foo Float where ...
  
  It would be horribly wrong to define
     dfDuperMaybe :: Foo a -> Duper (Maybe a)  -- from (i3)
     dfDuperMaybe d = MkDuper (sc_sel1 (sc_sel2 d)) ...
  
     dfFooFloat :: Foo Float               -- from (i4)
     dfFooFloat = MkFoo (dfDuperMaybe dfFooFloat) ...
  
  Let's expand the RHS of dfFooFloat:
     dfFooFloat = MkFoo (MkDuper (sc_sel1 (sc_sel2 dfFooFloat)) ...) ...
  That superclass argument to MkDuper is bottom!
  
  This program gets rejected because:
  * When processing (i3) we need to construct a dictionary for Super
    (Maybe a), to put in the superclass field of (Duper (Maybe a)).
  * We /can/ use the superclasses of (Foo a), because the latter is
    smaller than the head of the instance, namely Duper (Maybe a).
  * So we know (by (sc2)) that this Duper (Maybe a) dictionary is
    non-bottom.  But because (Duper (Maybe a)) is not smaller than the
    instance head (Duper (Maybe a)), we can't take *its* superclasses.
  As a result the program is rightly rejected, unless you add
  (Super (Maybe a)) to the context of (i3).
  
  Wrinkle (W1):
      (sc2) says we only get a non-bottom dict if the dict we are
      selecting from is itself non-bottom.  So in a superclass chain,
      all the dictionaries in the chain must be non-bottom.
          class C a => D3 a
          class D2 a [[Maybe b]] => D1 a b
          class D3 a             => D2 a b
          class C a => E a b
          instance D1 a b => E a [b]
      The instance needs the wanted superclass (C a).  We can get it
      by superclass selection from
         D1 a b --> D2 a [[Maybe b]] --> D3 a --> C a
      But on the way we go through the too-big (D2 a [[Maybe b]]), and
      we don't know that is non-bottom.
  
  Note [Solving superclass constraints]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  How do we ensure that every superclass witness in an instance declaration
  is generated by one of (sc1) (sc2) or (sc3) in Note [Recursive superclasses]?
  Answer:
  
    * The "given" constraints of an instance decl have CtOrigin of
      (GivenOrigin (InstSkol head_size)), where head_size is the
      PatersonSize of the head of the instance declaration.  E.g. in
          instance D a => C [a]
      the `[G] D a` constraint has a CtOrigin whose head_size is the
      PatersonSize of (C [a]).
  
    * When we make a superclass selection from a Given (transitively)
      we give it a CtOrigin of (GivenSCOrigin skol_info sc_depth blocked).
  
      The 'blocked :: Bool' flag says if the superclass can be used to
      solve a superclass Wanted. The new superclass is blocked unless:
  
         it is the superclass of an unblocked dictionary (wrinkle (W1)),
         that is Paterson-smaller than the instance head.
  
      This is implemented in GHC.Tc.Solver.Dict.mk_strict_superclasses
      (in the mk_given_loc helper function).
  
    * Superclass "Wanted" constraints have CtOrigin of (ScOrigin NakedSc)
      The 'NakedSc' says that this is a naked superclass Wanted; we must
      be careful when solving it.
  
    * (sc1) When we rewrite such a wanted constraint, it retains its
      origin.  But if we apply an instance declaration, we can set the
      origin to (ScOrigin NotNakedSc), thus lifting any restrictions by
      making prohibitedSuperClassSolve return False. This happens
      in GHC.Tc.Solver.Dict.checkInstanceOK.
  
    * (sc2) ScOrigin wanted constraints can't be solved from a
      superclass selection, except at a smaller type.  This test is
      implemented by GHC.Tc.Solver.InertSet.prohibitedSuperClassSolve
  
  Note [Silent superclass arguments] (historical interest only)
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  NB1: this note describes our *old* solution to the
       recursive-superclass problem. I'm keeping the Note
       for now, just as institutional memory.
       However, the code for silent superclass arguments
       was removed in late Dec 2014
  
  NB2: the silent-superclass solution introduced new problems
       of its own, in the form of instance overlap.  Tests
       SilentParametersOverlapping, T5051, and T7862 are examples
  
  NB3: the silent-superclass solution also generated tons of
       extra dictionaries.  For example, in monad-transformer
       code, when constructing a Monad dictionary you had to pass
       an Applicative dictionary; and to construct that you need
       a Functor dictionary. Yet these extra dictionaries were
       often never used.  Test T3064 compiled *far* faster after
       silent superclasses were eliminated.
  
  Our solution to this problem "silent superclass arguments".  We pass
  to each dfun some ``silent superclass arguments’’, which are the
  immediate superclasses of the dictionary we are trying to
  construct. In our example:
         dfun :: forall a. C [a] -> D [a] -> D [a]
         dfun = \(dc::C [a]) (dd::D [a]) -> DOrd dc ...
  Notice the extra (dc :: C [a]) argument compared to the previous version.
  
  This gives us:
  
       -----------------------------------------------------------
       DFun Superclass Invariant
       ~~~~~~~~~~~~~~~~~~~~~~~~
       In the body of a DFun, every superclass argument to the
       returned dictionary is
         either   * one of the arguments of the DFun,
         or       * constant, bound at top level
       -----------------------------------------------------------
  
  This net effect is that it is safe to treat a dfun application as
  wrapping a dictionary constructor around its arguments (in particular,
  a dfun never picks superclasses from the arguments under the
  dictionary constructor). No superclass is hidden inside a dfun
  application.
  
  The extra arguments required to satisfy the DFun Superclass Invariant
  always come first, and are called the "silent" arguments.  You can
  find out how many silent arguments there are using Id.dfunNSilent;
  and then you can just drop that number of arguments to see the ones
  that were in the original instance declaration.
  
  DFun types are built (only) by MkId.mkDictFunId, so that is where we
  decide what silent arguments are to be added.
  -}
  
  {-
  ************************************************************************
  *                                                                      *
        Type-checking an instance method
  *                                                                      *
  ************************************************************************
  
  tcMethod
  - Make the method bindings, as a [(NonRec, HsBinds)], one per method
  - Remembering to use fresh Name (the instance method Name) as the binder
  - Bring the instance method Ids into scope, for the benefit of tcInstSig
  - Use sig_fn mapping instance method Name -> instance tyvars
  - Ditto prag_fn
  - Use tcValBinds to do the checking
  -}
  
  tcMethods :: SkolemInfoAnon -> DFunId -> Class
            -> [TcTyVar] -> [EvVar]
            -> [TcType]
            -> TcEvBinds
            -> ([LTcSpecPrag], TcPragEnv)
            -> [ClassOpItem]
            -> InstBindings GhcRn
            -> TcM ([Id], LHsBinds GhcTc, Bag Implication)
          -- The returned inst_meth_ids all have types starting
          --      forall tvs. theta => ...
  tcMethods _skol_info dfun_id clas tyvars dfun_ev_vars inst_tys
                    dfun_ev_binds (spec_inst_prags, prag_fn) op_items
                    (InstBindings { ib_binds      = binds
                                  , ib_tyvars     = lexical_tvs
                                  , ib_pragmas    = sigs
                                  , ib_extensions = exts
                                  , ib_derived    = is_derived })
    = tcExtendNameTyVarEnv (lexical_tvs `zip` tyvars) $
         -- The lexical_tvs scope over the 'where' part
      do { traceTc "tcInstMeth" (ppr sigs $$ ppr binds)
         ; checkMinimalDefinition
         ; checkMethBindMembership
         ; (ids, binds, mb_implics) <- set_exts exts $
                                       unset_warnings_deriving $
                                       mapAndUnzip3M tc_item op_items
         ; return (ids, binds, listToBag (catMaybes mb_implics)) }
    where
      set_exts :: [LangExt.Extension] -> TcM a -> TcM a
      set_exts es thing = foldr setXOptM thing es
  
      -- See Note [Avoid -Winaccessible-code when deriving]
      unset_warnings_deriving :: TcM a -> TcM a
      unset_warnings_deriving
        | is_derived = unsetWOptM Opt_WarnInaccessibleCode
        | otherwise  = id
  
      hs_sig_fn = mkHsSigFun sigs
      inst_loc  = getSrcSpan dfun_id
  
      unsat_thetas =
        mapMaybe (\ id -> (id,) <$> isUnsatisfiableCt_maybe (idType id)) dfun_ev_vars
  
      ----------------------
      tc_item :: ClassOpItem -> TcM (Id, LHsBind GhcTc, Maybe Implication)
      tc_item (sel_id, dm_info)
        | Just (user_bind, bndr_loc, prags) <- findMethodBind (idName sel_id) binds prag_fn
        = tcMethodBody False clas tyvars dfun_ev_vars inst_tys
                       dfun_ev_binds is_derived hs_sig_fn
                       spec_inst_prags prags
                       sel_id user_bind bndr_loc
        | otherwise
        = do { traceTc "tc_def" (ppr sel_id)
             ; tc_default sel_id dm_info }
  
      ----------------------
      tc_default :: Id -> DefMethInfo
                 -> TcM (TcId, LHsBind GhcTc, Maybe Implication)
  
      tc_default sel_id mb_dm = case mb_dm of
  
        -- If the instance has an "Unsatisfiable msg" context,
        -- add method bindings that use "unsatisfiable".
        --
        -- See Note [Implementation of Unsatisfiable constraints],
        -- in GHC.Tc.Errors, point (D).
        _ | (theta_id,unsat_msg) : _ <- unsat_thetas
          -> do { (meth_id, _) <- mkMethIds clas tyvars dfun_ev_vars
                                           inst_tys sel_id
               ; unsat_id <- tcLookupId unsatisfiableIdName
               -- Recall that unsatisfiable :: forall {rep} (msg :: ErrorMessage) (a :: TYPE rep). Unsatisfiable msg => a
               --
               -- So we need to instantiate the forall and pass the dictionary evidence.
               ; let meth_rhs = L inst_loc' $
                       wrapId
                       (   mkWpEvApps [EvExpr $ Var theta_id]
                       <.> mkWpTyApps [getRuntimeRep meth_tau, unsat_msg, meth_tau])
                       unsat_id
                     meth_bind = mkVarBind meth_id $ mkLHsWrap lam_wrapper meth_rhs
               ; return (meth_id, meth_bind, Nothing) }
  
        Just (dm_name, dm_spec) ->
          do { (meth_bind, inline_prags) <- mkDefMethBind inst_loc dfun_id clas sel_id dm_name dm_spec
             ; tcMethodBody (is_vanilla_dm dm_spec)
                            clas tyvars dfun_ev_vars inst_tys
                            dfun_ev_binds is_derived hs_sig_fn
                            spec_inst_prags inline_prags
                            sel_id meth_bind inst_loc }
  
        -- No default method
        Nothing ->
          do { traceTc "tc_def: warn" (ppr sel_id)
             ; (meth_id, _) <- mkMethIds clas tyvars dfun_ev_vars
                                         inst_tys sel_id
             ; dflags <- getDynFlags
              -- Add a binding whose RHS is an error
              -- "No explicit nor default method for class operation 'meth'".
             ; let meth_rhs  = error_rhs dflags
                   meth_bind = mkVarBind meth_id $ mkLHsWrap lam_wrapper meth_rhs
             ; return (meth_id, meth_bind, Nothing) }
  
        where
          inst_loc' = noAnnSrcSpan inst_loc
          error_rhs dflags = L inst_loc'
                           $ HsApp noExtField error_fun (error_msg dflags)
          error_fun    = L inst_loc' $
                         wrapId (mkWpTyApps
                                  [ getRuntimeRep meth_tau, meth_tau])
                                nO_METHOD_BINDING_ERROR_ID
          error_msg dflags = L inst_loc'
                                      (HsLit noExtField (HsStringPrim NoSourceText
                                                (unsafeMkByteString (error_string dflags))))
          meth_tau     = classMethodInstTy sel_id inst_tys
          error_string dflags = showSDoc dflags
                                (hcat [ppr inst_loc, vbar, quotes (ppr sel_id) ])
          lam_wrapper  = mkWpTyLams tyvars <.> mkWpEvLams dfun_ev_vars
  
      ----------------------
      -- Check if one of the minimal complete definitions is satisfied
      checkMinimalDefinition
        = when (null unsat_thetas) $
          -- Don't warn if there is an "Unsatisfiable" constraint in the context.
          --
          -- See Note [Implementation of Unsatisfiable constraints] in GHC.Tc.Errors,
          -- point (D).
          whenIsJust (isUnsatisfied (methodExists . unLoc) (classMinimalDef clas)) $
          warnUnsatisfiedMinimalDefinition
  
      methodExists meth = isJust (findMethodBind meth binds prag_fn)
  
      ----------------------
      -- Check if any method bindings do not correspond to the class.
      -- See Note [Mismatched class methods and associated type families].
      checkMethBindMembership
        = mapM_ (addErrTc . TcRnBadMethodErr (className clas)) mismatched_meths
        where
          bind_nms         = map unLoc $ collectMethodBinders binds
          cls_meth_nms     = map (idName . fst) op_items
          mismatched_meths = bind_nms `minusList` cls_meth_nms
  
      is_vanilla_dm :: DefMethSpec ty -> Bool
      -- See (TRC5) in Note [Tracking redundant constraints]
      --            in GHC.Tc.Solver.Solve
      is_vanilla_dm VanillaDM      = True
      is_vanilla_dm (GenericDM {}) = False
  
  {-
  Note [Mismatched class methods and associated type families]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  It's entirely possible for someone to put methods or associated type family
  instances inside of a class in which it doesn't belong. For instance, we'd
  want to fail if someone wrote this:
  
    instance Eq () where
      type Rep () = Maybe
      compare = undefined
  
  Since neither the type family `Rep` nor the method `compare` belong to the
  class `Eq`. Normally, this is caught in the renamer when resolving RdrNames,
  since that would discover that the parent class `Eq` is incorrect.
  
  However, there is a scenario in which the renamer could fail to catch this:
  if the instance was generated through Template Haskell, as in #12387. In that
  case, Template Haskell will provide fully resolved names (e.g.,
  `GHC.Classes.compare`), so the renamer won't notice the sleight-of-hand going
  on. For this reason, we also put an extra validity check for this in the
  typechecker as a last resort.
  
  Note [Avoid -Winaccessible-code when deriving]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  -Winaccessible-code can be particularly noisy when deriving instances for
  GADTs. Consider the following example (adapted from #8128):
  
    data T a where
      MkT1 :: Int -> T Int
      MkT2 :: T Bool
      MkT3 :: T Bool
    deriving instance Eq (T a)
    deriving instance Ord (T a)
  
  In the derived Ord instance, GHC will generate the following code:
  
    instance Ord (T a) where
      compare x y
        = case x of
            MkT2
              -> case y of
                   MkT1 {} -> GT
                   MkT2    -> EQ
                   _       -> LT
            ...
  
  However, that MkT1 is unreachable, since the type indices for MkT1 and MkT2
  differ, so if -Winaccessible-code is enabled, then deriving this instance will
  result in unwelcome warnings.
  
  One conceivable approach to fixing this issue would be to change `deriving Ord`
  such that it becomes smarter about not generating unreachable cases. This,
  however, would be a highly nontrivial refactor, as we'd have to propagate
  through typing information everywhere in the algorithm that generates Ord
  instances in order to determine which cases were unreachable. This seems like
  a lot of work for minimal gain, so we have opted not to go for this approach.
  
  Instead, we take the following approach:
  
  1. In tcMethods (which typechecks method bindings), use 'setInGeneratedCode'.
  2. When creating Implications during typechecking, record this flag
     (in ic_warn_inaccessible) at the time of creation.
  3. After typechecking comes error reporting, where GHC must decide how to
     report inaccessible code to the user, on an Implication-by-Implication
     basis. If the ic_warn_inaccessible field of the Implication is False, then
     we don't bother reporting it. That's it!
  -}
  
  ------------------------
  tcMethodBody :: Bool   -- True <=> This is a vanilla default method
                         -- See (TRC5) in Note [Tracking redundant constraints]
                         --            in GHC.Tc.Solver.Solve
               -> Class -> [TcTyVar] -> [EvVar] -> [TcType]
               -> TcEvBinds -> Bool
               -> HsSigFun
               -> [LTcSpecPrag] -> [LSig GhcRn]
               -> Id -> LHsBind GhcRn -> SrcSpan
               -> TcM (TcId, LHsBind GhcTc, Maybe Implication)
  tcMethodBody is_vanilla_dm clas tyvars dfun_ev_vars inst_tys
               dfun_ev_binds is_derived
               sig_fn spec_inst_prags prags
               sel_id (L bind_loc meth_bind) bndr_loc
    = add_meth_ctxt $
      do { traceTc "tcMethodBody" (ppr sel_id <+> ppr (idType sel_id) $$ ppr bndr_loc)
         ; let skol_info = MethSkol meth_name is_vanilla_dm
         ; (global_meth_id, local_meth_id) <- setSrcSpan bndr_loc $
                                              mkMethIds clas tyvars dfun_ev_vars
                                                        inst_tys sel_id
  
         ; let lm_bind = meth_bind { fun_id = L (noAnnSrcSpan bndr_loc)
                                                (idName local_meth_id) }
                         -- Substitute the local_meth_name for the binder
                         -- NB: the binding is always a FunBind
  
              -- taking instance signature into account might change the type of
              -- the local_meth_id
         ; (meth_implic, ev_binds_var, tc_bind)
               <- checkInstConstraints skol_info $
                  tcMethodBodyHelp sig_fn sel_id local_meth_id (L bind_loc lm_bind)
  
         ; global_meth_id <- addInlinePrags global_meth_id prags
         ; spec_prags     <- tcExtendIdEnv1 meth_name global_meth_id $
                             -- tcExtendIdEnv1: tricky point: a SPECIALISE pragma in prags
                             -- mentions sel_name but the pragma is really for global_meth_id.
                             -- So we bind sel_name to global_meth_id, just in the pragmas.
                             -- Example:
                             --    instance C [a] where
                             --       op :: forall b. Ord b => b -> a -> a
                             --       {-# SPECIALISE op @Int #-}
                             -- The specialisation is for the `op` for this instance decl, not
                             -- for the global selector-id, of course.
                             tcSpecPrags global_meth_id prags
  
          ; let specs  = mk_meth_spec_prags global_meth_id spec_inst_prags spec_prags
                export = ABE { abe_poly  = global_meth_id
                             , abe_mono  = local_meth_id
                             , abe_wrap  = idHsWrapper
                             , abe_prags = specs }
  
                local_ev_binds = TcEvBinds ev_binds_var
                full_bind = XHsBindsLR $
                            AbsBinds { abs_tvs      = tyvars
                                     , abs_ev_vars  = dfun_ev_vars
                                     , abs_exports  = [export]
                                     , abs_ev_binds = [dfun_ev_binds, local_ev_binds]
                                     , abs_binds    = tc_bind
                                     , abs_sig      = True }
  
          ; return (global_meth_id, L bind_loc full_bind, Just meth_implic) }
    where
      meth_name = idName sel_id
  
          -- For instance decls that come from deriving clauses
          -- we want to print out the full source code if there's an error
          -- because otherwise the user won't see the code at all
      add_meth_ctxt thing
        | is_derived = addLandmarkErrCtxt (DerivBindCtxt sel_id clas inst_tys) thing
        | otherwise  = thing
  
  tcMethodBodyHelp :: HsSigFun -> Id -> TcId
                   -> LHsBind GhcRn -> TcM (LHsBinds GhcTc)
  tcMethodBodyHelp hs_sig_fn sel_id local_meth_id meth_bind
    | Just hs_sig_ty <- hs_sig_fn sel_name
                -- There is a signature in the instance
                -- See Note [Instance method signatures]
    = do { (sig_ty, hs_wrap)
               <- setSrcSpan (getLocA hs_sig_ty) $
                  do { inst_sigs <- xoptM LangExt.InstanceSigs
                     ; checkTc inst_sigs (TcRnMisplacedInstSig sel_name hs_sig_ty)
                     ; let ctxt = FunSigCtxt sel_name NoRRC
                     ; sig_ty  <- tcHsSigType ctxt hs_sig_ty
                     ; let local_meth_ty = idType local_meth_id
                                  -- False <=> do not report redundant constraints when
                                  --           checking instance-sig <= class-meth-sig
                                  -- The instance-sig is the focus here; the class-meth-sig
                                  -- is fixed (#18036)
                     ; let orig = InstanceSigOrigin sel_name sig_ty local_meth_ty
                     ; hs_wrap <- addErrCtxtM (methSigCtxt sel_name sig_ty local_meth_ty) $
                                  tcSubTypeSigma orig ctxt sig_ty local_meth_ty
                     ; return (sig_ty, hs_wrap) }
  
         ; inner_meth_name <- newName (nameOccName sel_name)
         ; let ctxt = FunSigCtxt sel_name (lhsSigTypeContextSpan hs_sig_ty)
                      -- WantRCC <=> check for redundant constraints in the
                      --          user-specified instance signature
               inner_meth_id  = mkLocalId inner_meth_name ManyTy sig_ty
               inner_meth_sig = CSig { sig_bndr = inner_meth_id
                                     , sig_ctxt = ctxt
                                     , sig_loc  = getLocA hs_sig_ty }
  
         ; (tc_bind, [Scaled _ inner_id]) <- tcPolyCheck no_prag_fn inner_meth_sig meth_bind
  
         ; let export = ABE { abe_poly  = local_meth_id
                            , abe_mono  = inner_id
                            , abe_wrap  = hs_wrap
                            , abe_prags = noSpecPrags }
  
         ; return (singleton $ L (getLoc meth_bind) $ XHsBindsLR $
                   AbsBinds { abs_tvs = [], abs_ev_vars = []
                            , abs_exports = [export]
                            , abs_binds = tc_bind, abs_ev_binds = []
                            , abs_sig = True }) }
  
    | otherwise  -- No instance signature
    = do { let ctxt = FunSigCtxt sel_name NoRRC
                      -- NoRRC <=> don't report redundant constraints
                      -- The signature is not under the users control!
               tc_sig = completeSigFromId ctxt local_meth_id
                -- Absent a type sig, there are no new scoped type variables here
                -- Only the ones from the instance decl itself, which are already
                -- in scope.  Example:
                --      class C a where { op :: forall b. Eq b => ... }
                --      instance C [c] where { op = <rhs> }
                -- In <rhs>, 'c' is scope but 'b' is not!
  
         ; (tc_bind, _) <- tcPolyCheck no_prag_fn tc_sig meth_bind
         ; return tc_bind }
  
    where
      sel_name   = idName sel_id
      no_prag_fn = emptyPragEnv   -- No pragmas for local_meth_id;
                                  -- they are all for meth_id
  
  ------------------------
  mkMethIds :: Class -> [TcTyVar] -> [EvVar]
            -> [TcType] -> Id -> TcM (TcId, TcId)
               -- returns (poly_id, local_id), but ignoring any instance signature
               -- See Note [Instance method signatures]
  mkMethIds clas tyvars dfun_ev_vars inst_tys sel_id
    = do  { poly_meth_name  <- newName (mkClassOpAuxOcc sel_occ)
          ; local_meth_name <- newName sel_occ
                    -- Base the local_meth_name on the selector name, because
                    -- type errors from tcMethodBody come from here
          ; let poly_meth_id  = mkLocalId poly_meth_name  ManyTy poly_meth_ty
                local_meth_id = mkLocalId local_meth_name ManyTy local_meth_ty
  
          ; return (poly_meth_id, local_meth_id) }
    where
      sel_name      = idName sel_id
      -- Force so that a thunk doesn't end up in a Name (#19619)
      !sel_occ      = nameOccName sel_name
      local_meth_ty = instantiateMethod clas sel_id inst_tys
      poly_meth_ty  = mkSpecSigmaTy tyvars theta local_meth_ty
      theta         = map idType dfun_ev_vars
  
  methSigCtxt :: Name -> TcType -> TcType -> TidyEnv -> ZonkM (TidyEnv, ErrCtxtMsg)
  methSigCtxt sel_name sig_ty meth_ty env0
    = do { (env1, sig_ty)  <- zonkTidyTcType env0 sig_ty
         ; (env2, meth_ty) <- zonkTidyTcType env1 meth_ty
         ; return (env2, MethSigCtxt sel_name sig_ty meth_ty) }
  
  {- Note [Instance method signatures]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  With -XInstanceSigs we allow the user to supply a signature for the
  method in an instance declaration.  Here is an artificial example:
  
         data T a = MkT a
         instance Ord a => Ord (T a) where
           (>) :: forall b. b -> b -> Bool
           (>) = error "You can't compare Ts"
  
  The instance signature can be *more* polymorphic than the instantiated
  class method (in this case: Age -> Age -> Bool), but it cannot be less
  polymorphic.  Moreover, if a signature is given, the implementation
  code should match the signature, and type variables bound in the
  signature should scope over the method body.
  
  We achieve this by building a TcSigInfo for the method, whether or not
  there is an instance method signature, and using that to typecheck
  the declaration (in tcMethodBody).  That means, conveniently,
  that the type variables bound in the signature will scope over the body.
  
  What about the check that the instance method signature is more
  polymorphic than the instantiated class method type?  We just do a
  tcSubType call in tcMethodBodyHelp, and generate a nested AbsBind, like
  this (for the example above
  
   AbsBind { abs_tvs = [a], abs_ev_vars = [d:Ord a]
           , abs_exports
               = ABExport { (>) :: forall a. Ord a => T a -> T a -> Bool
                          , gr_lcl :: T a -> T a -> Bool }
           , abs_binds
               = AbsBind { abs_tvs = [], abs_ev_vars = []
                         , abs_exports = ABExport { gr_lcl :: T a -> T a -> Bool
                                                  , gr_inner :: forall b. b -> b -> Bool }
                         , abs_binds = AbsBind { abs_tvs = [b], abs_ev_vars = []
                                               , ..etc.. }
                 } }
  
  Wow!  Three nested AbsBinds!
   * The outer one abstracts over the tyvars and dicts for the instance
   * The middle one is only present if there is an instance signature,
     and does the impedance matching for that signature
   * The inner one is for the method binding itself against either the
     signature from the class, or the instance signature.
  -}
  
  ----------------------
  mk_meth_spec_prags :: Id -> [LTcSpecPrag] -> [LTcSpecPrag] -> TcSpecPrags
          -- Adapt the 'SPECIALISE instance' pragmas to work for this method Id
          -- There are two sources:
          --   * spec_prags_for_me: {-# SPECIALISE op :: <blah> #-}
          --   * spec_prags_from_inst: derived from {-# SPECIALISE instance :: <blah> #-}
          --     These ones have the dfun inside, but [perhaps surprisingly]
          --     the correct wrapper.
          -- See Note [Handling old-form SPECIALISE pragmas] in GHC.Tc.Gen.Bind
  mk_meth_spec_prags meth_id spec_inst_prags spec_prags_for_me
    = SpecPrags (spec_prags_for_me ++ spec_prags_from_inst)
    where
      spec_prags_from_inst
         | isInlinePragma (idInlinePragma meth_id)
         = []  -- Do not inherit SPECIALISE from the instance if the
               -- method is marked INLINE, because then it'll be inlined
               -- and the specialisation would do nothing. (Indeed it'll provoke
               -- a warning from the desugarer
         | otherwise
         = [ L inst_loc (SpecPrag meth_id wrap inl)
           | L inst_loc (SpecPrag _       wrap inl) <- spec_inst_prags]
  
  
  mkDefMethBind :: SrcSpan -> DFunId -> Class -> Id -> Name
                -> DefMethSpec Type
                -> TcM (LHsBind GhcRn, [LSig GhcRn])
  -- The is a default method (vanailla or generic) defined in the class
  -- So make a binding   op @m1 @m2 @m3 = $dmop @i1 @i2 @m1 @m2 @m3
  -- where $dmop is the name of the default method in the class;
  -- i1 and t2 are the instance types; and m1, m2, and m3 are the type variables
  -- from the method's type signature. See Note [Default methods in instances] for
  -- why we use visible type application here.
  mkDefMethBind loc dfun_id clas sel_id dm_name dm_spec
    = do  { logger <- getLogger
          ; dm_id <- tcLookupId dm_name
          ; let inline_prag = idInlinePragma dm_id
                inline_prags | isAnyInlinePragma inline_prag
                             = [noLocA (InlineSig noAnn fn inline_prag)]
                             | otherwise
                             = []
                   -- Copy the inline pragma (if any) from the default method
                   -- to this version. Note [INLINE and default methods]
  
          ; liftIO (putDumpFileMaybe logger Opt_D_dump_deriv "Filling in method body"
                     FormatHaskell
                     (vcat [ppr clas <+> ppr inst_tys,
                            nest 2 (ppr bind)]))
  
         ; return (bind, inline_prags) }
    where
      (_, _, _, inst_tys) = tcSplitDFunTy (idType dfun_id)
      (_, _, sel_tau) = tcSplitMethodTy (idType sel_id)
      (sel_tvbs, _) = tcSplitForAllInvisTVBinders sel_tau
  
      -- Compute the instance types to use in the visible type application. See
      -- Note [Default methods in instances].
      visible_inst_tys =
        [ ty | (tcb, ty) <- tyConBinders (classTyCon clas) `zip` inst_tys
             , tyConBinderForAllTyFlag tcb /= Inferred ]
  
      visible_sel_tvbs =
        case dm_spec of
          -- When dealing with a vanilla default method, compute the type
          -- variables from the method's type signature. That way, we can bind
          -- them with TypeAbstractions (visible_sel_pats) and use them in the
          -- visible type application (visible_sel_tys). See Note [Default methods
          -- in instances] (Wrinkle: Ambiguous types from vanilla method type
          -- signatures).
          VanillaDM -> filter (\tvb -> binderFlag tvb /= InferredSpec) sel_tvbs
          -- If we are dealing with a generic default method, on the other hand,
          -- don't bother doing any of this. See Note [Default methods
          -- in instances] (Wrinkle: Ambiguous types from generic default method
          -- type signatures).
          GenericDM {} -> []
      visible_sel_pats = map mk_ty_pat visible_sel_tvbs
      visible_sel_tys = map (mkTyVarTy . binderVar) visible_sel_tvbs
  
      fn   = noLocA (idName sel_id)
      rhs  = foldl' mk_vta (nlHsVar dm_name) $
             visible_inst_tys ++ visible_sel_tys
      bind = L (noAnnSrcSpan loc)
            $ mkTopFunBind (Generated OtherExpansion SkipPmc) fn
                [mkSimpleMatch (mkPrefixFunRhs fn noAnn) (noLocA visible_sel_pats) rhs]
  
      mk_ty_pat :: VarBndr TyVar Specificity -> LPat GhcRn
      mk_ty_pat (Bndr tv spec) =
        noLocA $
        InvisPat spec $
        HsTP (HsTPRn [] [tyVarName tv] []) $
        nlHsTyVar NotPromoted $
        tyVarName tv
  
      mk_vta :: LHsExpr GhcRn -> Type -> LHsExpr GhcRn
      mk_vta fun ty = noLocA (HsAppType noExtField fun
          (mkEmptyWildCardBndrs $ type_to_hs_type ty))
         -- NB: use visible type application
         -- See Note [Default methods in instances]
  
      type_to_hs_type :: Type -> LHsType GhcRn
      type_to_hs_type = parenthesizeHsType appPrec . noLocA . XHsType
  
  ----------------------
  
  warnUnsatisfiedMinimalDefinition :: ClassMinimalDef -> TcM ()
  warnUnsatisfiedMinimalDefinition mindef
    = do { warn <- woptM Opt_WarnMissingMethods
         ; let msg = TcRnUnsatisfiedMinimalDef mindef
         ; diagnosticTc warn msg
         }
  
  {-
  Note [Export helper functions]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  We arrange to export the "helper functions" of an instance declaration,
  so that they are not subject to preInlineUnconditionally, even if their
  RHS is trivial.  Reason: they are mentioned in the DFunUnfolding of
  the dict fun as Ids, not as CoreExprs, so we can't substitute a
  non-variable for them.
  
  We could change this by making DFunUnfoldings have CoreExprs, but it
  seems a bit simpler this way.
  
  Note [Default methods in instances]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Consider this
  
     class Baz v x where
        foo :: x -> x
        foo y = <blah>
  
     instance Baz Int Int
  
  From the class decl we get
  
     $dmfoo :: forall v x. Baz v x => x -> x
     $dmfoo y = <blah>
  
  Notice that the type of `v` is ambiguous.  So we use Visible Type Application
  (VTA) to disambiguate:
  
     $dBazIntInt = MkBaz fooIntInt
     fooIntInt = $dmfoo @Int @Int
  
  Lacking VTA we'd get ambiguity errors involving the default method.  This applies
  equally to vanilla default methods (#1061) and generic default methods
  (#12220).
  
  Historical note: before we had VTA we had to generate
  post-type-checked code, which took a lot more code, and didn't work for
  generic default methods.
  
  -----
  -- Wrinkle: Ambiguous types from vanilla method type signatures
  -----
  
  In the Bar example above, the ambiguity arises from `v`, a type variable
  arising from the class header. It is also possible for the ambiguity to arise
  from a type variable bound by the method's type signature itself (see #14266
  and #25148). For example:
  
     class A t where
        f :: forall x m. Monoid x => t m -> m
        f = <blah>
  
     instance A []
  
  The class declaration gives rise to the following default function:
  
    $dmf :: forall t. A t => forall x m. Monoid x => t m -> m
    $dmf = <blah>
  
  And the instance declaration gives rise to generated code that looks roughly
  like this:
  
     instance A [] where
        f = $dmf @[] ...
  
  In this example, it is not enough to use VTA to specify the type of `t`, since
  the type of `x` (bound by `f`'s type signature) is also ambiguous. We need to
  generate code that looks more like this:
  
     instance A [] where
        f = $dmf @[] @x @m
  
  But where should `x` and `m` be bound? It's tempting to use ScopedTypeVariables
  and InstanceSigs to accomplish this:
  
     instance A [] where
        f :: forall x m. Monoid x => [m] -> m
        f = $dmf @[] @x @m
  
  GHC will reject this code, however, as the type signature for `f` will fail the
  subtype check for InstanceSigs:
  
      • Could not deduce (Monoid x0)
        from the context: Monoid x
          bound by the type signature for:
                     f :: forall x m. Monoid x => [m] -> m
        The type variable ‘x0’ is ambiguous
      • When checking that instance signature for ‘f’
          is more general than its signature in the class
          Instance sig: forall x m. Monoid x => [m] -> m
             Class sig: forall x m. Monoid x => [m] -> m
        In the instance declaration for ‘A []’
  
  See #17898. To avoid this problem, we instead bind `x` and `m` using
  TypeAbstractions:
  
     instance A [] where
        f @x @m = $dmf @[] @x @m
  
  This resolves the ambiguity and avoids the need for a subtype check. (We also
  use a similar trick for resolving ambiguity in GeneralizedNewtypeDeriving: see
  also Note [GND and ambiguity] in GHC.Tc.Deriv.Generate.)
  
  -----
  -- Wrinkle: Ambiguous types from generic default method type signatures
  -----
  
  Note that the approach described above (in Wrinkle: Ambiguous types from
  vanilla method type signatures) will only work for vanilla default methods and
  /not/ for generic default methods (i.e., methods using DefaultSignatures). This
  is because for vanilla default methods, the type of the generated $dm* function
  will always quantify the same type variables as the method's original type
  signature, in the same order and with the same specificities. For example, the
  type of the $dmf function will be:
  
     $dmf :: forall t. A t => forall x m. Monoid x => t m -> m
  
  As such, it is guaranteed that the type variables from the method's original
  type signature will line up exactly with the type variables from the $dm*
  function (after instantiating all of the class variables):
  
     instance A [] where
        f @x @m = $dmf @[] @x @m
  
  We cannot guarantee this property for generic default methods, however. As
  such, we must be more conservative and generate code without instantiating any
  of the type variables bound by the method's type signature (only the type
  variables bound by the class header):
  
     instance A [] where
        f = $dmf @[]
  
  There are a number of reasons why we cannot reliably instantiate the type
  variables bound by a generic default method's type signature:
  
  * Default methods can quantify type variables in a different order, e.g.,
  
      class A t where
         f :: forall x m. Monoid x => t m -> m
         default f :: forall m x. Monoid x => t m -> m
         f = <blah>
  
    Note that the default signature quantifies the type variables in the opposite
    order from the method's original type signature. As such, the type of $dmf
    will be:
  
      $dmf :: forall t. A t => forall m x. Monoid x => t m -> m
  
    Therefore, `f @x @m = $dmf @[] @x @m` would be incorrect. Nor would it be
    straightforward to infer what the correct order of type variables should be.
  
  * Default methods can quantify a different number of type variables, e.g.,
  
      class A t where
         f :: forall x m. Monoid x => t m -> m
         default f :: forall p q r m. C a t p q r => t m -> m
         f = <blah>
  
    This gives rise to:
  
      $dmf :: forall t. A t => forall p q r m. C a t p q r => t m -> m
  
    And thus generating `f @x @m = $dmf @[] @x @m` would be incorrect, for
    similar reasons as in the example above.
  
  * Default methods can use different type variable specificities, e.g.,
  
      class A t where
         f :: forall x m. Monoid x => t m -> m
         default f :: forall {x} m. Monoid x => t m -> m
         f = <blah>
  
    This gives rise to:
  
      $dmf :: forall t. A t => forall {x} m. Monoid x => t m -> m
  
    Therefore, generating `f @x @m = $dmf @[] @x @m` would be incorrect because
    the `x` in the type of $dmf is inferred, so it is not eligible for visible
    type application.
  
  As such, we do not bother trying to resolve the ambiguity of any method-bound
  type variables when dealing with generic defaults. This means that GHC won't be
  able to typecheck the default method examples above, but so be it.
  
  Note [INLINE and default methods]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Default methods need special case.  They are supposed to behave rather like
  macros.  For example
  
    class Foo a where
      op1, op2 :: Bool -> a -> a
  
      {-# INLINE op1 #-}
      op1 b x = op2 (not b) x
  
    instance Foo Int where
      -- op1 via default method
      op2 b x = <blah>
  
  The instance declaration should behave
  
     just as if 'op1' had been defined with the
     code, and INLINE pragma, from its original
     definition.
  
  That is, just as if you'd written
  
    instance Foo Int where
      op2 b x = <blah>
  
      {-# INLINE op1 #-}
      op1 b x = op2 (not b) x
  
  So for the above example we generate:
  
    {-# INLINE $dmop1 #-}
    -- $dmop1 has an InlineCompulsory unfolding
    $dmop1 d b x = op2 d (not b) x
  
    $fFooInt = MkD $cop1 $cop2
  
    {-# INLINE $cop1 #-}
    $cop1 = $dmop1 $fFooInt
  
    $cop2 = <blah>
  
  Note carefully:
  
  * We *copy* any INLINE pragma from the default method $dmop1 to the
    instance $cop1.  Otherwise we'll just inline the former in the
    latter and stop, which isn't what the user expected
  
  * Regardless of its pragma, we give the default method an
    unfolding with an InlineCompulsory source. That means
    that it'll be inlined at every use site, notably in
    each instance declaration, such as $cop1.  This inlining
    must happen even though
      a) $dmop1 is not saturated in $cop1
      b) $cop1 itself has an INLINE pragma
  
    It's vital that $dmop1 *is* inlined in this way, to allow the mutual
    recursion between $fooInt and $cop1 to be broken
  
  * To communicate the need for an InlineCompulsory to the desugarer
    (which makes the Unfoldings), we use the IsDefaultMethod constructor
    in TcSpecPrags.
  
  
  ************************************************************************
  *                                                                      *
          Specialise instance pragmas
  *                                                                      *
  ************************************************************************
  
  Note [SPECIALISE instance pragmas]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Consider
  
     instance (Ix a, Ix b) => Ix (a,b) where
       {-# SPECIALISE instance Ix (Int,Int) #-}
       range (x,y) = ...
  
  We make a specialised version of the dictionary function, AND
  specialised versions of each *method*.  Thus we should generate
  something like this:
  
    $dfIxPair :: (Ix a, Ix b) => Ix (a,b)
    {-# DFUN [$crangePair, ...] #-}
    {-# SPECIALISE $dfIxPair :: Ix (Int,Int) #-}
    $dfIxPair da db = Ix ($crangePair da db) (...other methods...)
  
    $crange :: (Ix a, Ix b) -> ((a,b),(a,b)) -> [(a,b)]
    {-# SPECIALISE $crange :: ((Int,Int),(Int,Int)) -> [(Int,Int)] #-}
    $crange da db = <blah>
  
  The SPECIALISE pragmas are acted upon by the desugarer, which generate
  
    dii :: Ix Int
    dii = ...
  
    $s$dfIxPair :: Ix ((Int,Int),(Int,Int))
    {-# DFUN [$crangePair di di, ...] #-}
    $s$dfIxPair = Ix ($crangePair di di) (...)
  
    {-# RULE forall (d1,d2:Ix Int). $dfIxPair Int Int d1 d2 = $s$dfIxPair #-}
  
    $s$crangePair :: ((Int,Int),(Int,Int)) -> [(Int,Int)]
    $c$crangePair = ...specialised RHS of $crangePair...
  
    {-# RULE forall (d1,d2:Ix Int). $crangePair Int Int d1 d2 = $s$crangePair #-}
  
  Note that
  
    * The specialised dictionary $s$dfIxPair is very much needed, in case we
      call a function that takes a dictionary, but in a context where the
      specialised dictionary can be used.  See #7797.
  
    * The ClassOp rule for 'range' works equally well on $s$dfIxPair, because
      it still has a DFunUnfolding.  See Note [ClassOp/DFun selection]
  
    * A call (range ($dfIxPair Int Int d1 d2)) might simplify two ways:
         --> {ClassOp rule for range}     $crangePair Int Int d1 d2
         --> {SPEC rule for $crangePair}  $s$crangePair
      or thus:
         --> {SPEC rule for $dfIxPair}    range $s$dfIxPair
         --> {ClassOpRule for range}      $s$crangePair
      It doesn't matter which way.
  
    * We want to specialise the RHS of both $dfIxPair and $crangePair,
      but the SAME HsWrapper will do for both!  We can call tcSpecPrag
      just once, and pass the result (in spec_inst_info) to tcMethods.
  -}
  
  tcSpecInstPrags :: DFunId -> InstBindings GhcRn
                  -> TcM ([LTcSpecPrag], TcPragEnv)
  tcSpecInstPrags dfun_id (InstBindings { ib_binds = binds, ib_pragmas = uprags })
    = do { spec_inst_prags <- mapM (wrapLocM (tcSpecInst dfun_id)) $
                              filter isSpecInstLSig uprags
               -- The filter removes the pragmas for methods
         ; return (spec_inst_prags, mkPragEnv uprags binds) }
  
  ------------------------------
  tcSpecInst :: Id -> Sig GhcRn -> TcM TcSpecPrag
  tcSpecInst dfun_id prag@(SpecInstSig _ hs_ty)
    = addErrCtxt (SpecPragmaCtxt prag) $
      do  { spec_dfun_ty <- tcHsClsInstType SpecInstCtxt hs_ty
          ; co_fn <- tcSpecWrapper SpecInstCtxt (idType dfun_id) spec_dfun_ty
          ; return (SpecPrag dfun_id co_fn defaultInlinePragma) }
  
  tcSpecInst _  _ = panic "tcSpecInst"
  
  {-
  ************************************************************************
  *                                                                      *
  \subsection{Error messages}
  *                                                                      *
  ************************************************************************
  -}
  
  instDeclCtxt1 :: LHsSigType GhcRn -> ErrCtxtMsg
  instDeclCtxt1 hs_inst_ty
    = InstDeclErrCtxt $ Left $ getLHsInstDeclHead hs_inst_ty
  
  instDeclCtxt2 :: Type -> ErrCtxtMsg
  instDeclCtxt2 dfun_ty
    = InstDeclErrCtxt $ Right $ head_ty
    where
      (_,_,head_ty) = tcSplitQuantPredTy dfun_ty
  