typecheck/TcInstDcls.hs

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

    TcInstDecls: Typechecking instance declarations
    -}

    {-# LANGUAGE CPP #-}

    module TcInstDcls ( tcInstDecls1, tcInstDeclsDeriv, tcInstDecls2 ) where

    #include "HsVersions.h"

    import HsSyn
    import TcBinds
    import TcTyClsDecls
    import TcClassDcl( tcClassDecl2, tcATDefault,
                       HsSigFun, mkHsSigFun,
                       findMethodBind, instantiateMethod )
    import TcSigs
    import TcRnMonad
    import TcValidity
    import TcHsSyn    ( zonkTyBndrsX, emptyZonkEnv
                      , zonkTcTypeToTypes, zonkTcTypeToType )
    import TcMType
    import TcType
    import BuildTyCl
    import Inst
    import InstEnv
    import FamInst
    import FamInstEnv
    import TcDeriv
    import TcEnv
    import TcHsType
    import TcUnify
    import CoreSyn    ( Expr(..), mkApps, mkVarApps, mkLams )
    import MkCore     ( nO_METHOD_BINDING_ERROR_ID )
    import CoreUnfold ( mkInlineUnfoldingWithArity, mkDFunUnfolding )
    import Type
    import TcEvidence
    import TyCon
    import CoAxiom
    import DataCon
    import ConLike
    import Class
    import Var
    import VarEnv
    import VarSet
    import PrelNames  ( typeableClassName, genericClassNames
                      , knownNatClassName, knownSymbolClassName )
    import Bag
    import BasicTypes
    import DynFlags
    import ErrUtils
    import FastString
    import Id
    import MkId
    import Name
    import NameSet
    import Outputable
    import SrcLoc
    import Util
    import BooleanFormula ( isUnsatisfied, pprBooleanFormulaNice )
    import qualified GHC.LanguageExtensions as LangExt

    import Control.Monad
    import Maybes

    {-
    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 InlineRule
      unfoldings. See Note [RULEs enabled in SimplGently] in SimplUtils

    Note [ClassOp/DFun selection]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    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 CoreUnfold.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 MkId mkDictSelId

     * We make 'df' CONLIKE, so that shared uses still match; eg
          let d = df d1 d2
          in ...(op2 d)...(op1 d)...

    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
    Trac #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 (Trac #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 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 [Binding when looking up instances].  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 Name]          -- Source code instance decls
       -> TcM (TcGblEnv,            -- The full inst env
               [InstInfo Name],     -- Source-code instance decls to process;
                                    -- contains all dfuns for this module
               [DerivInfo])         -- From data family instances

    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 <- addClsInsts local_infos $
                        addFamInsts fam_insts   $
                        getGblEnv

           ; return ( gbl_env
                    , local_infos
                    , concat datafam_deriv_infos ) }

    -- | 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]
      -> [LTyClDecl Name]
      -> [LDerivDecl Name]
      -> TcM (TcGblEnv, [InstInfo Name], HsValBinds Name)
    tcInstDeclsDeriv datafam_deriv_infos tyclds derivds
      = do th_stage <- getStage -- See Note [Deriving inside TH brackets]
           if isBrackStage th_stage
           then do { gbl_env <- getGblEnv
                   ; return (gbl_env, bagToList emptyBag, emptyValBindsOut) }
           else do { data_deriv_infos <- mkDerivInfos tyclds
                   ; let deriv_infos = datafam_deriv_infos ++ data_deriv_infos
                   ; (tcg_env, info_bag, valbinds) <- tcDeriving deriv_infos derivds
                   ; return (tcg_env, bagToList info_bag, valbinds) }

    addClsInsts :: [InstInfo Name] -> TcM a -> TcM a
    addClsInsts infos thing_inside
      = tcExtendLocalInstEnv (map iSpec infos) thing_inside

    addFamInsts :: [FamInst] -> TcM a -> TcM a
    -- Extend (a) the family instance envt
    --        (b) the type envt with stuff from data type decls
    addFamInsts fam_insts thing_inside
      = tcExtendLocalFamInstEnv fam_insts $
        tcExtendGlobalEnv axioms $
        tcExtendTyConEnv data_rep_tycons  $
        do { traceTc "addFamInsts" (pprFamInsts fam_insts)
           ; tcg_env <- tcAddImplicits data_rep_tycons
                        -- Does not add its axiom; that comes from
                        -- adding the 'axioms' above
           ; setGblEnv tcg_env thing_inside }
      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 (Trac #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 Name
                    -> TcM ([InstInfo Name], [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 Nothing (L loc decl)
           ; return ([], [fam_inst], []) }

    tcLocalInstDecl (L loc (DataFamInstD { dfid_inst = decl }))
      = do { (fam_inst, m_deriv_info) <- tcDataFamInstDecl Nothing (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 Name
                  -> TcM ([InstInfo Name], [FamInst], [DerivInfo])
    -- The returned DerivInfos are for any associated data families
    tcClsInstDecl (L loc (ClsInstDecl { cid_poly_ty = poly_ty, cid_binds = binds
                                      , cid_sigs = uprags, cid_tyfam_insts = ats
                                      , cid_overlap_mode = overlap_mode
                                      , cid_datafam_insts = adts }))
      = setSrcSpan loc                      $
        addErrCtxt (instDeclCtxt1 poly_ty)  $
        do  { (tyvars, theta, clas, inst_tys) <- tcHsClsInstType InstDeclCtxt poly_ty
            ; let mini_env   = mkVarEnv (classTyVars clas `zip` inst_tys)
                  mini_subst = mkTvSubst (mkInScopeSet (mkVarSet tyvars)) mini_env
                  mb_info    = Just (clas, tyvars, mini_env)

            -- Next, process any associated types.
            ; traceTc "tcLocalInstDecl" (ppr poly_ty)
            ; tyfam_insts0  <- tcExtendTyVarEnv tyvars $
                               mapAndRecoverM (tcTyFamInstDecl mb_info) ats
            ; datafam_stuff <- tcExtendTyVarEnv tyvars $
                               mapAndRecoverM (tcDataFamInstDecl mb_info) adts
            ; let (datafam_insts, m_deriv_infos) = unzip datafam_stuff
                  deriv_infos                    = catMaybes m_deriv_infos

            -- Check for missing associated types and build them
            -- from their defaults (if available)
            ; let defined_ats = mkNameSet (map (tyFamInstDeclName . unLoc) ats)
                                `unionNameSet`
                                mkNameSet (map (unLoc . dfid_tycon . unLoc) adts)
            ; tyfam_insts1 <- mapM (tcATDefault True loc mini_subst defined_ats)
                                   (classATItems clas)

            -- Finally, construct the Core representation of the instance.
            -- (This no longer includes the associated types.)
            ; dfun_name <- newDFunName clas inst_tys (getLoc (hsSigType poly_ty))
                    -- Dfun location is that of instance *header*

            ; ispec <- newClsInst (fmap unLoc overlap_mode) dfun_name tyvars theta
                                  clas inst_tys

            ; let inst_info = InstInfo { iSpec  = ispec
                                       , iBinds = InstBindings
                                         { ib_binds = binds
                                         , ib_tyvars = map Var.varName tyvars -- Scope over bindings
                                         , ib_pragmas = uprags
                                         , ib_extensions = []
                                         , ib_derived = False } }

            ; doClsInstErrorChecks inst_info

            ; return ( [inst_info], tyfam_insts0 ++ concat tyfam_insts1 ++ datafam_insts
                     , deriv_infos ) }

    doClsInstErrorChecks :: InstInfo Name -> TcM ()
    doClsInstErrorChecks inst_info
     = do { traceTc "doClsInstErrorChecks" (ppr ispec)
          ; dflags <- getDynFlags
          ; is_boot <- tcIsHsBootOrSig

             -- In hs-boot files there should be no bindings
          ; failIfTc (is_boot && not no_binds) badBootDeclErr

             -- If not in an hs-boot file, abstract classes cannot have
             -- instances declared
          ; failIfTc (not is_boot && isAbstractClass clas) abstractClassInstErr

             -- Handwritten instances of any rejected
             -- class is always forbidden
             -- #12837
          ; failIfTc (clas_nm `elem` rejectedClassNames) clas_err

             -- Check for hand-written Generic instances (disallowed in Safe Haskell)
          ; when (clas_nm `elem` genericClassNames) $
            do { failIfTc (safeLanguageOn dflags) gen_inst_err
               ; when (safeInferOn dflags) (recordUnsafeInfer emptyBag) }
      }
      where
        ispec    = iSpec inst_info
        binds    = iBinds inst_info
        no_binds = isEmptyLHsBinds (ib_binds binds) && null (ib_pragmas binds)
        clas_nm  = is_cls_nm ispec
        clas     = is_cls ispec

        gen_inst_err = hang (text ("Generic instances can only be "
                                ++ "derived in Safe Haskell.") $+$
                             text "Replace the following instance:")
                          2 (pprInstanceHdr ispec)

        abstractClassInstErr =
            text "Cannot define instance for abstract class" <+> quotes (ppr clas_nm)

        -- Report an error or a warning for certain class instances.
        -- If we are working on an .hs-boot file, we just report a warning,
        -- and ignore the instance.  We do this, to give users a chance to fix
        -- their code.
        rejectedClassNames = [ typeableClassName
                             , knownNatClassName
                             , knownSymbolClassName ]
        clas_err = text "Class" <+> quotes (ppr clas_nm)
                        <+> text "does not support user-specified instances"

    {-
    ************************************************************************
    *                                                                      *
                   Type checking 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).
    -}

    tcFamInstDeclCombined :: Maybe ClsInstInfo
                          -> Located Name -> TcM TyCon
    tcFamInstDeclCombined mb_clsinfo fam_tc_lname
      = do { -- Type family instances require -XTypeFamilies
             -- and can't (currently) be in an hs-boot file
           ; traceTc "tcFamInstDecl" (ppr fam_tc_lname)
           ; type_families <- xoptM LangExt.TypeFamilies
           ; is_boot <- tcIsHsBootOrSig   -- Are we compiling an hs-boot file?
           ; checkTc type_families $ badFamInstDecl fam_tc_lname
           ; checkTc (not is_boot) $ badBootFamInstDeclErr

           -- Look up the family TyCon and check for validity including
           -- check that toplevel type instances are not for associated types.
           ; fam_tc <- tcLookupLocatedTyCon fam_tc_lname
           ; when (isNothing mb_clsinfo &&   -- Not in a class decl
                   isTyConAssoc fam_tc)      -- but an associated type
                  (addErr $ assocInClassErr fam_tc_lname)

           ; return fam_tc }

    tcTyFamInstDecl :: Maybe ClsInstInfo
                    -> LTyFamInstDecl Name -> TcM FamInst
      -- "type instance"
    tcTyFamInstDecl mb_clsinfo (L loc decl@(TyFamInstDecl { tfid_eqn = eqn }))
      = setSrcSpan loc           $
        tcAddTyFamInstCtxt decl  $
        do { let fam_lname = tfe_tycon (unLoc eqn)
           ; fam_tc <- tcFamInstDeclCombined mb_clsinfo fam_lname

             -- (0) Check it's an open type family
           ; checkTc (isFamilyTyCon fam_tc)         (notFamily fam_tc)
           ; checkTc (isTypeFamilyTyCon fam_tc)     (wrongKindOfFamily fam_tc)
           ; checkTc (isOpenTypeFamilyTyCon fam_tc) (notOpenFamily fam_tc)

             -- (1) do the work of verifying the synonym group
           ; co_ax_branch <- tcTyFamInstEqn (famTyConShape fam_tc) mb_clsinfo eqn

             -- (2) check for validity
           ; checkValidCoAxBranch mb_clsinfo 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 }

    tcDataFamInstDecl :: Maybe ClsInstInfo
                      -> LDataFamInstDecl Name -> TcM (FamInst, Maybe DerivInfo)
      -- "newtype instance" and "data instance"
    tcDataFamInstDecl mb_clsinfo
        (L loc decl@(DataFamInstDecl
           { dfid_pats = pats
           , dfid_tycon = fam_tc_name
           , dfid_defn = defn@HsDataDefn { dd_ND = new_or_data, dd_cType = cType
                                         , dd_ctxt = ctxt, dd_cons = cons
                                         , dd_derivs = derivs } }))
      = setSrcSpan loc             $
        tcAddDataFamInstCtxt decl  $
        do { fam_tc <- tcFamInstDeclCombined mb_clsinfo fam_tc_name

             -- Check that the family declaration is for the right kind
           ; checkTc (isFamilyTyCon fam_tc) (notFamily fam_tc)
           ; checkTc (isDataFamilyTyCon fam_tc) (wrongKindOfFamily fam_tc)

             -- Kind check type patterns
           ; tcFamTyPats (famTyConShape fam_tc) mb_clsinfo pats
                         (kcDataDefn (unLoc fam_tc_name) pats defn) $
                 \tvs pats res_kind ->
        do { stupid_theta <- solveEqualities $ tcHsContext ctxt

                -- Zonk the patterns etc into the Type world
           ; (ze, tvs')    <- zonkTyBndrsX emptyZonkEnv tvs
           ; pats'         <- zonkTcTypeToTypes ze pats
           ; res_kind'     <- zonkTcTypeToType  ze res_kind
           ; stupid_theta' <- zonkTcTypeToTypes ze stupid_theta

           ; gadt_syntax <- dataDeclChecks (tyConName fam_tc) new_or_data stupid_theta' cons

             -- Construct representation tycon
           ; rep_tc_name <- newFamInstTyConName fam_tc_name pats'
           ; axiom_name  <- newFamInstAxiomName fam_tc_name [pats']

           ; let (eta_pats, etad_tvs) = eta_reduce pats'
                 eta_tvs              = filterOut (`elem` etad_tvs) tvs'
                 full_tvs             = eta_tvs ++ etad_tvs
                     -- Put the eta-removed tyvars at the end
                     -- Remember, tvs' is in arbitrary order (except kind vars are
                     -- first, so there is no reason to suppose that the etad_tvs
                     -- (obtained from the pats) are at the end (Trac #11148)
                 orig_res_ty          = mkTyConApp fam_tc pats'

           ; (rep_tc, axiom) <- fixM $ \ ~(rec_rep_tc, _) ->
               do { let ty_binders = mkTyConBindersPreferAnon full_tvs liftedTypeKind
                  ; data_cons <- tcConDecls rec_rep_tc
                                            (ty_binders, orig_res_ty) cons
                  ; tc_rhs <- case new_or_data of
                         DataType -> return (mkDataTyConRhs data_cons)
                         NewType  -> ASSERT( not (null data_cons) )
                                     mkNewTyConRhs rep_tc_name rec_rep_tc (head data_cons)
                  -- freshen tyvars
                  ; let axiom  = mkSingleCoAxiom Representational
                                                 axiom_name eta_tvs [] fam_tc eta_pats
                                                 (mkTyConApp rep_tc (mkTyVarTys eta_tvs))
                        parent = DataFamInstTyCon axiom fam_tc pats'

                          -- NB: Use the full_tvs from the pats. See bullet toward
                          -- the end of Note [Data type families] in TyCon
                        rep_tc   = mkAlgTyCon rep_tc_name
                                              ty_binders liftedTypeKind
                                              (map (const Nominal) full_tvs)
                                              (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) }

             -- 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)
           ; checkValidFamPats mb_clsinfo fam_tc tvs' [] pats'

             -- Result kind must be '*' (otherwise, we have too few patterns)
           ; checkTc (isLiftedTypeKind res_kind') $
             tooFewParmsErr (tyConArity fam_tc)

           ; checkValidTyCon rep_tc

           ; let m_deriv_info = case derivs of
                   L _ []    -> Nothing
                   L _ preds ->
                     Just $ DerivInfo { di_rep_tc  = rep_tc
                                      , di_clauses = preds
                                      , di_ctxt    = tcMkDataFamInstCtxt decl }

           ; fam_inst <- newFamInst (DataFamilyInst rep_tc) axiom
           ; return (fam_inst, m_deriv_info) } }
      where
        eta_reduce :: [Type] -> ([Type], [TyVar])
        -- See Note [Eta reduction for data families] in FamInstEnv
        -- 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 pats
          = go (reverse pats) []
        go (pat:pats) etad_tvs
          | Just tv <- getTyVar_maybe pat
          , not (tv `elemVarSet` tyCoVarsOfTypes pats)
          = go pats (tv : etad_tvs)
        go pats etad_tvs = (reverse pats, etad_tvs)

    {- *********************************************************************
    *                                                                      *
          Type-checking instance declarations, pass 2
    *                                                                      *
    ********************************************************************* -}

    tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo Name]
                 -> TcM (LHsBinds Id)
    -- (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 = unionManyBags dm_binds_s

              -- (b) instance declarations
            ; let dm_ids = collectHsBindsBinders dm_binds
                  -- Add the default method Ids (again)
                  -- (they were arready added in TcTyDecls.tcAddImplicits)
                  -- See Note [Default methods in the type environment]
            ; inst_binds_s <- tcExtendGlobalValEnv dm_ids $
                              mapM tcInstDecl2 inst_decls

              -- Done
            ; return (dm_binds `unionBags` unionManyBags 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 Name -> TcM (LHsBinds Id)
                -- Returns a binding for the dfun
    tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = ibinds })
      = recoverM (return emptyLHsBinds)             $
        setSrcSpan loc                              $
        addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
        do {  -- Instantiate the instance decl with skolem constants
           ; (inst_tyvars, dfun_theta, inst_head) <- tcSkolDFunType dfun_id
           ; dfun_ev_vars <- newEvVars dfun_theta
                         -- We instantiate the dfun_id with superSkolems.
                         -- See Note [Subtle interaction of recursion and overlap]
                         -- and Note [Binding when looking up instances]

           ; let (clas, inst_tys) = tcSplitDFunHead inst_head
                 (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
           ; ((sc_meth_ids, sc_meth_binds, sc_meth_implics), tclvl)
                 <- pushTcLevelM $
                    do { (sc_ids, sc_binds, sc_implics)
                            <- tcSuperClasses dfun_id clas inst_tyvars dfun_ev_vars
                                              inst_tys dfun_ev_binds
                                              sc_theta'

                          -- Typecheck the methods
                       ; (meth_ids, meth_binds, meth_implics)
                            <- tcMethods 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   `unionBags` meth_binds
                                , sc_implics `unionBags` meth_implics ) }

           ; env <- getLclEnv
           ; emitImplication $ Implic { ic_tclvl  = tclvl
                                      , ic_skols  = inst_tyvars
                                      , ic_no_eqs = False
                                      , ic_given  = dfun_ev_vars
                                      , ic_wanted = mkImplicWC sc_meth_implics
                                      , ic_status = IC_Unsolved
                                      , ic_binds  = dfun_ev_binds_var
                                      , ic_needed = emptyVarSet
                                      , ic_env    = env
                                      , ic_info   = InstSkol }

           -- Create the result bindings
           ; self_dict <- newDict clas inst_tys
           ; let class_tc      = classTyCon clas
                 [dict_constr] = tyConDataCons 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)
                                         (HsConLikeOut (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 Id -> Id -> HsExpr Id
                 app_to_meth fun meth_id = L loc fun `HsApp` L loc (wrapId arg_wrapper meth_id)

                 inst_tv_tys = mkTyVarTys inst_tyvars
                 arg_wrapper = mkWpEvVarApps dfun_ev_vars <.> mkWpTyApps inst_tv_tys

                 is_newtype = isNewTyCon class_tc
                 dfun_id_w_prags = addDFunPrags dfun_id sc_meth_ids
                 dfun_spec_prags
                    | is_newtype = SpecPrags []
                    | otherwise  = SpecPrags spec_inst_prags
                        -- Newtype dfuns just inline unconditionally,
                        -- so don't attempt to specialise them

                 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 = AbsBinds { abs_tvs = inst_tyvars
                                      , abs_ev_vars = dfun_ev_vars
                                      , abs_exports = [export]
                                      , abs_ev_binds = []
                                      , abs_binds = unitBag dict_bind }

           ; return (unitBag (L loc main_bind) `unionBags` sc_meth_binds)
           }
     where
       dfun_id = instanceDFunId ispec
       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
     | is_newtype
      = dfun_id `setIdUnfolding`  mkInlineUnfoldingWithArity 0 con_app
                `setInlinePragma` alwaysInlinePragma { inl_sat = Just 0 }
     | otherwise
     = dfun_id `setIdUnfolding`  mkDFunUnfolding dfun_bndrs dict_con dict_args
               `setInlinePragma` dfunInlinePragma
     where
       con_app    = mkLams dfun_bndrs $
                    mkApps (Var (dataConWrapId dict_con)) dict_args
                     -- mkApps is OK because of the checkForLevPoly call in checkValidClass
                     -- See Note [Levity polymorphism checking] in DsMonad
       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]  = tyConDataCons clas_tc
       is_newtype  = isNewTyCon clas_tc

    wrapId :: HsWrapper -> id -> HsExpr id
    wrapId wrapper id = mkHsWrap wrapper (HsVar (noLoc 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 declaratation, 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 TcSimplify 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 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 :: DFunId -> Class -> [TcTyVar] -> [EvVar] -> [TcType]
                   -> TcEvBinds
                   -> TcThetaType
                   -> TcM ([EvVar], LHsBinds Id, 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 dfun_id cls tyvars dfun_evs inst_tys dfun_ev_binds sc_theta
      = do { (ids, binds, implics) <- mapAndUnzip3M tc_super (zip sc_theta [fIRST_TAG..])
           ; return (ids, listToBag binds, listToBag implics) }
      where
        loc = getSrcSpan dfun_id
        size = sizeTypes inst_tys
        tc_super (sc_pred, n)
          = do { (sc_implic, ev_binds_var, sc_ev_tm)
                    <- checkInstConstraints $ emitWanted (ScOrigin size) sc_pred

               ; sc_top_name  <- newName (mkSuperDictAuxOcc n (getOccName cls))
               ; sc_ev_id     <- newEvVar sc_pred
               ; addTcEvBind ev_binds_var $ mkWantedEvBind sc_ev_id sc_ev_tm
               ; let sc_top_ty = mkInvForAllTys tyvars (mkLamTypes dfun_evs sc_pred)
                     sc_top_id = mkLocalId sc_top_name 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 = AbsBinds { abs_tvs      = tyvars
                                     , abs_ev_vars  = dfun_evs
                                     , abs_exports  = [export]
                                     , abs_ev_binds = [dfun_ev_binds, local_ev_binds]
                                     , abs_binds    = emptyBag }
               ; return (sc_top_id, L loc bind, sc_implic) }

    -------------------
    checkInstConstraints :: TcM result
                         -> TcM (Implication, EvBindsVar, result)
    -- See Note [Typechecking plan for instance declarations]
    checkInstConstraints thing_inside
      = do { (tclvl, wanted, result) <- pushLevelAndCaptureConstraints  $
                                        thing_inside

           ; ev_binds_var <- newTcEvBinds
           ; env <- getLclEnv
           ; let implic = Implic { ic_tclvl  = tclvl
                                 , ic_skols  = []
                                 , ic_no_eqs = False
                                 , ic_given  = []
                                 , ic_wanted = wanted
                                 , ic_status = IC_Unsolved
                                 , ic_binds  = ev_binds_var
                                 , ic_needed = emptyVarSet
                                 , ic_env    = env
                                 , ic_info   = InstSkol }

           ; return (implic, ev_binds_var, result) }

    {-
    Note [Recursive superclasses]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    See Trac #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 Trac #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 smaller dictionary
      (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 TcValidity.sizeTypes.
    Let's suppose we are building a dictionary of size 3, 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.

    What does "we have a smaller dictionary" mean?  It might be
    one of the arguments of the instance, or one of its superclasses.
    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 is smaller than the thing we are building (UserOfRegs r (Maybe a).

    But for (i2) that isn't the case, so we must add an explicit, and
    perhaps surprising, (Ord r) argument to the instance declaration.

    Here's another example from Trac #6161:

           class       Super a => Duper a  where ...
           class Duper (Fam a) => Foo a    where ...
    (i3)   instance Foo a => Duper (Fam a) where ...
    (i4)   instance              Foo Float where ...

    It would be horribly wrong to define
       dfDuperFam :: Foo a -> Duper (Fam a)  -- from (i3)
       dfDuperFam d = MkDuper (sc_sel1 (sc_sel2 d)) ...

       dfFooFloat :: Foo Float               -- from (i4)
       dfFooFloat = MkFoo (dfDuperFam dfFooFloat) ...

    Now the Super superclass of Duper is definitely bottom!

    This won't happen because when processing (i3) we can use the
    superclasses of (Foo a), which is smaller, namely Duper (Fam a).  But
    that is *not* smaller than the target so we can't take *its*
    superclasses.  As a result the program is rightly rejected, unless you
    add (Super (Fam a)) to the context of (i3).

    Note [Solving superclass constraints]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    How do we ensure that every superclass witness is generated by
    one of (sc1) (sc2) or (sc3) in Note [Recursive superclasses].
    Answer:

      * Superclass "wanted" constraints have CtOrigin of (ScOrigin size)
        where 'size' is the size of the instance declaration. e.g.
              class C a => D a where...
              instance blah => D [a] where ...
        The wanted superclass constraint for C [a] has origin
        ScOrigin size, where size = size( D [a] ).

      * (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 infinity), thus lifting any restrictions by
        making prohibitedSuperClassSolve return False.

      * (sc2) ScOrigin wanted constraints can't be solved from a
        superclass selection, except at a smaller type.  This test is
        implemented by TcInteract.prohibitedSuperClassSolve

      * The "given" constraints of an instance decl have CtOrigin
        GivenOrigin InstSkol.

      * When we make a superclass selection from InstSkol we use
        a SkolemInfo of (InstSC size), where 'size' is the size of
        the constraint whose superclass we are taking.  An similarly
        when taking the superclass of an InstSC.  This is implemented
        in TcCanonical.newSCWorkFromFlavored

    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 neede
         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 :: DFunId -> Class
              -> [TcTyVar] -> [EvVar]
              -> [TcType]
              -> TcEvBinds
              -> ([Located TcSpecPrag], TcPragEnv)
              -> [ClassOpItem]
              -> InstBindings Name
              -> TcM ([Id], LHsBinds Id, Bag Implication)
            -- The returned inst_meth_ids all have types starting
            --      forall tvs. theta => ...
    tcMethods 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 })
      = tcExtendTyVarEnv2 (lexical_tvs `zip` tyvars) $
           -- The lexical_tvs scope over the 'where' part
        do { traceTc "tcInstMeth" (ppr sigs $$ ppr binds)
           ; checkMinimalDefinition
           ; (ids, binds, mb_implics) <- set_exts exts $
                                         mapAndUnzip3M tc_item op_items
           ; return (ids, listToBag binds, listToBag (catMaybes mb_implics)) }
      where
        set_exts :: [LangExt.Extension] -> TcM a -> TcM a
        set_exts es thing = foldr setXOptM thing es

        hs_sig_fn = mkHsSigFun sigs
        inst_loc  = getSrcSpan dfun_id

        ----------------------
        tc_item :: ClassOpItem -> TcM (Id, LHsBind Id, Maybe Implication)
        tc_item (sel_id, dm_info)
          | Just (user_bind, bndr_loc, prags) <- findMethodBind (idName sel_id) binds prag_fn
          = tcMethodBody 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 Id, Maybe Implication)

        tc_default sel_id (Just (dm_name, _))
          = do { (meth_bind, inline_prags) <- mkDefMethBind clas inst_tys sel_id dm_name
               ; tcMethodBody 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 }

        tc_default sel_id Nothing     -- No default method at all
          = do { traceTc "tc_def: warn" (ppr sel_id)
               ; (meth_id, _) <- mkMethIds clas tyvars dfun_ev_vars
                                           inst_tys sel_id
               ; dflags <- getDynFlags
               ; let meth_bind = mkVarBind meth_id $
                                 mkLHsWrap lam_wrapper (error_rhs dflags)
               ; return (meth_id, meth_bind, Nothing) }
          where
            error_rhs dflags = L inst_loc $ HsApp 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 (HsStringPrim NoSourceText
                                                  (unsafeMkByteString (error_string dflags))))
            meth_tau     = funResultTy (piResultTys (idType sel_id) inst_tys)
            error_string dflags = showSDoc dflags
                                  (hcat [ppr inst_loc, vbar, ppr sel_id ])
            lam_wrapper  = mkWpTyLams tyvars <.> mkWpLams dfun_ev_vars

        ----------------------
        -- Check if one of the minimal complete definitions is satisfied
        checkMinimalDefinition
          = whenIsJust (isUnsatisfied methodExists (classMinimalDef clas)) $
            warnUnsatisfiedMinimalDefinition

        methodExists meth = isJust (findMethodBind meth binds prag_fn)

    ------------------------
    tcMethodBody :: Class -> [TcTyVar] -> [EvVar] -> [TcType]
                 -> TcEvBinds -> Bool
                 -> HsSigFun
                 -> [LTcSpecPrag] -> [LSig Name]
                 -> Id -> LHsBind Name -> SrcSpan
                 -> TcM (TcId, LHsBind Id, Maybe Implication)
    tcMethodBody 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)
           ; (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 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 $
                    tcMethodBodyHelp sig_fn sel_id local_meth_id (L bind_loc lm_bind)

           ; global_meth_id <- addInlinePrags global_meth_id prags
           ; spec_prags     <- 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 = 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 }

            ; return (global_meth_id, L bind_loc full_bind, Just meth_implic) }
      where
            -- 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 Name -> TcM (LHsBinds TcId)
    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 { let ctxt = FunSigCtxt sel_name True
           ; (sig_ty, hs_wrap)
                 <- setSrcSpan (getLoc (hsSigType hs_sig_ty)) $
                    do { inst_sigs <- xoptM LangExt.InstanceSigs
                       ; checkTc inst_sigs (misplacedInstSig sel_name hs_sig_ty)
                       ; sig_ty  <- tcHsSigType (FunSigCtxt sel_name False) hs_sig_ty
                       ; let local_meth_ty = idType local_meth_id
                       ; hs_wrap <- addErrCtxtM (methSigCtxt sel_name sig_ty local_meth_ty) $
                                    tcSubType_NC ctxt sig_ty local_meth_ty
                       ; return (sig_ty, hs_wrap) }

           ; inner_meth_name <- newName (nameOccName sel_name)
           ; let inner_meth_id  = mkLocalId inner_meth_name sig_ty
                 inner_meth_sig = CompleteSig { sig_bndr = inner_meth_id
                                              , sig_ctxt = ctxt
                                              , sig_loc  = getLoc (hsSigType hs_sig_ty) }

           ; (tc_bind, [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 (unitBag $ L (getLoc meth_bind) $
                     AbsBinds { abs_tvs = [], abs_ev_vars = []
                              , abs_exports = [export]
                              , abs_binds = tc_bind, abs_ev_binds = [] }) }

      | otherwise  -- No instance signature
      = do { let ctxt = FunSigCtxt sel_name False
                        -- False <=> 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  poly_meth_ty
                  local_meth_id = mkLocalId local_meth_name local_meth_ty

            ; return (poly_meth_id, local_meth_id) }
      where
        sel_name      = idName sel_id
        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 -> TcM (TidyEnv, MsgDoc)
    methSigCtxt sel_name sig_ty meth_ty env0
      = do { (env1, sig_ty)  <- zonkTidyTcType env0 sig_ty
           ; (env2, meth_ty) <- zonkTidyTcType env1 meth_ty
           ; let msg = hang (text "When checking that instance signature for" <+> quotes (ppr sel_name))
                          2 (vcat [ text "is more general than its signature in the class"
                                  , text "Instance sig:" <+> ppr sig_ty
                                  , text "   Class sig:" <+> ppr meth_ty ])
           ; return (env2, msg) }

    misplacedInstSig :: Name -> LHsSigType Name -> SDoc
    misplacedInstSig name hs_ty
      = vcat [ hang (text "Illegal type signature in instance declaration:")
                  2 (hang (pprPrefixName name)
                        2 (dcolon <+> ppr hs_ty))
             , text "(Use InstanceSigs to allow this)" ]

    {- 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
    singature 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 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 SPECIALISE pragmas] in TcBinds
    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 :: Class -> [Type] -> Id -> Name -> TcM (LHsBind Name, [LSig Name])
    -- The is a default method (vanailla or generic) defined in the class
    -- So make a binding   op = $dmop @t1 @t2
    -- where $dmop is the name of the default method in the class,
    -- and t1,t2 are the instance types.
    -- See Note [Default methods in instances] for why we use
    -- visible type application here
    mkDefMethBind clas inst_tys sel_id dm_name
      = do  { dflags <- getDynFlags
            ; dm_id <- tcLookupId dm_name
            ; let inline_prag = idInlinePragma dm_id
                  inline_prags | isAnyInlinePragma inline_prag
                               = [noLoc (InlineSig fn inline_prag)]
                               | otherwise
                               = []
                     -- Copy the inline pragma (if any) from the default method
                     -- to this version. Note [INLINE and default methods]

                  fn   = noLoc (idName sel_id)
                  visible_inst_tys = [ ty | (tcb, ty) <- tyConBinders (classTyCon clas) `zip` inst_tys
                                          , tyConBinderArgFlag tcb /= Inferred ]
                  rhs  = foldl mk_vta (nlHsVar dm_name) visible_inst_tys
                  bind = noLoc $ mkTopFunBind Generated fn $
                                 [mkSimpleMatch (mkPrefixFunRhs fn) [] rhs]

            ; liftIO (dumpIfSet_dyn dflags Opt_D_dump_deriv "Filling in method body"
                       (vcat [ppr clas <+> ppr inst_tys,
                              nest 2 (ppr sel_id <+> equals <+> ppr rhs)]))

           ; return (bind, inline_prags) }
      where
        mk_vta :: LHsExpr Name -> Type -> LHsExpr Name
        mk_vta fun ty = noLoc (HsAppType fun (mkEmptyWildCardBndrs $ noLoc $ HsCoreTy ty))
           -- NB: use visible type application
           -- See Note [Default methods in instances]

    ----------------------
    derivBindCtxt :: Id -> Class -> [Type ] -> SDoc
    derivBindCtxt sel_id clas tys
       = vcat [ text "When typechecking the code for" <+> quotes (ppr sel_id)
              , nest 2 (text "in a derived instance for"
                        <+> quotes (pprClassPred clas tys) <> colon)
              , nest 2 $ text "To see the code I am typechecking, use -ddump-deriv" ]

    warnUnsatisfiedMinimalDefinition :: ClassMinimalDef -> TcM ()
    warnUnsatisfiedMinimalDefinition mindef
      = do { warn <- woptM Opt_WarnMissingMethods
           ; warnTc (Reason Opt_WarnMissingMethods) warn message
           }
      where
        message = vcat [text "No explicit implementation for"
                       ,nest 2 $ pprBooleanFormulaNice mindef
                       ]

    {-
    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 is ambiguous.  So we use Visible Type Application
    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 (Trac #1061) and generic default methods
    (Trac #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.

    Note [INLINE and default methods]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Default methods need special case.  They are supposed to behave rather like
    macros.  For exmample

      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 Trac #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 Name
                    -> TcM ([Located TcSpecPrag], 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 Name -> TcM TcSpecPrag
    tcSpecInst dfun_id prag@(SpecInstSig _ hs_ty)
      = addErrCtxt (spec_ctxt prag) $
        do  { (tyvars, theta, clas, tys) <- tcHsClsInstType SpecInstCtxt hs_ty
            ; let spec_dfun_ty = mkDictFunTy tyvars theta clas tys
            ; co_fn <- tcSpecWrapper SpecInstCtxt (idType dfun_id) spec_dfun_ty
            ; return (SpecPrag dfun_id co_fn defaultInlinePragma) }
      where
        spec_ctxt prag = hang (text "In the SPECIALISE pragma") 2 (ppr prag)

    tcSpecInst _  _ = panic "tcSpecInst"

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Error messages}
    *                                                                      *
    ************************************************************************
    -}

    instDeclCtxt1 :: LHsSigType Name -> SDoc
    instDeclCtxt1 hs_inst_ty
      = inst_decl_ctxt (ppr (getLHsInstDeclHead hs_inst_ty))

    instDeclCtxt2 :: Type -> SDoc
    instDeclCtxt2 dfun_ty
      = inst_decl_ctxt (ppr (mkClassPred cls tys))
      where
        (_,_,cls,tys) = tcSplitDFunTy dfun_ty

    inst_decl_ctxt :: SDoc -> SDoc
    inst_decl_ctxt doc = hang (text "In the instance declaration for")
                            2 (quotes doc)

    badBootFamInstDeclErr :: SDoc
    badBootFamInstDeclErr
      = text "Illegal family instance in hs-boot file"

    notFamily :: TyCon -> SDoc
    notFamily tycon
      = vcat [ text "Illegal family instance for" <+> quotes (ppr tycon)
             , nest 2 $ parens (ppr tycon <+> text "is not an indexed type family")]

    tooFewParmsErr :: Arity -> SDoc
    tooFewParmsErr arity
      = text "Family instance has too few parameters; expected" <+>
        ppr arity

    assocInClassErr :: Located Name -> SDoc
    assocInClassErr name
     = text "Associated type" <+> quotes (ppr name) <+>
       text "must be inside a class instance"

    badFamInstDecl :: Located Name -> SDoc
    badFamInstDecl tc_name
      = vcat [ text "Illegal family instance for" <+>
               quotes (ppr tc_name)
             , nest 2 (parens $ text "Use TypeFamilies to allow indexed type families") ]

    notOpenFamily :: TyCon -> SDoc
    notOpenFamily tc
      = text "Illegal instance for closed family" <+> quotes (ppr tc)