typecheck/TcPat.hs

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

    TcPat: Typechecking patterns
    -}

    {-# LANGUAGE CPP, RankNTypes #-}

    module TcPat ( tcLetPat, TcSigFun, TcPragFun
                 , TcSigInfo(..), TcPatSynInfo(..)
                 , findScopedTyVars, isPartialSig
                 , LetBndrSpec(..), addInlinePrags, warnPrags
                 , tcPat, tcPats, newNoSigLetBndr
                 , addDataConStupidTheta, badFieldCon, polyPatSig ) where

    #include "HsVersions.h"

    import {-# SOURCE #-}   TcExpr( tcSyntaxOp, tcInferRho)

    import HsSyn
    import TcHsSyn
    import TcRnMonad
    import Inst
    import Id
    import Var
    import Name
    import NameSet
    import TcEnv
    --import TcExpr
    import TcMType
    import TcValidity( arityErr )
    import TcType
    import TcUnify
    import TcHsType
    import TysWiredIn
    import TcEvidence
    import TyCon
    import DataCon
    import PatSyn
    import ConLike
    import PrelNames
    import BasicTypes hiding (SuccessFlag(..))
    import DynFlags
    import SrcLoc
    import Util
    import Outputable
    import FastString
    import Control.Monad

    {-
    ************************************************************************
    *                                                                      *
                    External interface
    *                                                                      *
    ************************************************************************
    -}

    tcLetPat :: TcSigFun -> LetBndrSpec
             -> LPat Name -> TcSigmaType
             -> TcM a
             -> TcM (LPat TcId, a)
    tcLetPat sig_fn no_gen pat pat_ty thing_inside
      = tc_lpat pat pat_ty penv thing_inside
      where
        penv = PE { pe_lazy = True
                  , pe_ctxt = LetPat sig_fn no_gen }

    -----------------
    tcPats :: HsMatchContext Name
           -> [LPat Name]            -- Patterns,
           -> [TcSigmaType]          --   and their types
           -> TcM a                  --   and the checker for the body
           -> TcM ([LPat TcId], a)

    -- This is the externally-callable wrapper function
    -- Typecheck the patterns, extend the environment to bind the variables,
    -- do the thing inside, use any existentially-bound dictionaries to
    -- discharge parts of the returning LIE, and deal with pattern type
    -- signatures

    --   1. Initialise the PatState
    --   2. Check the patterns
    --   3. Check the body
    --   4. Check that no existentials escape

    tcPats ctxt pats pat_tys thing_inside
      = tc_lpats penv pats pat_tys thing_inside
      where
        penv = PE { pe_lazy = False, pe_ctxt = LamPat ctxt }

    tcPat :: HsMatchContext Name
          -> LPat Name -> TcSigmaType
          -> TcM a                 -- Checker for body, given
                                   -- its result type
          -> TcM (LPat TcId, a)
    tcPat ctxt pat pat_ty thing_inside
      = tc_lpat pat pat_ty penv thing_inside
      where
        penv = PE { pe_lazy = False, pe_ctxt = LamPat ctxt }

    -----------------
    data PatEnv
      = PE { pe_lazy :: Bool        -- True <=> lazy context, so no existentials allowed
           , pe_ctxt :: PatCtxt     -- Context in which the whole pattern appears
           }

    data PatCtxt
      = LamPat   -- Used for lambdas, case etc
           (HsMatchContext Name)

      | LetPat   -- Used only for let(rec) pattern bindings
                 -- See Note [Typing patterns in pattern bindings]
           TcSigFun        -- Tells type sig if any
           LetBndrSpec     -- True <=> no generalisation of this let

    data LetBndrSpec
      = LetLclBndr            -- The binder is just a local one;
                              -- an AbsBinds will provide the global version

      | LetGblBndr TcPragFun  -- Genrealisation plan is NoGen, so there isn't going
                              -- to be an AbsBinds; So we must bind the global version
                              -- of the binder right away.
                              -- Oh, and dhhere is the inline-pragma information

    makeLazy :: PatEnv -> PatEnv
    makeLazy penv = penv { pe_lazy = True }

    inPatBind :: PatEnv -> Bool
    inPatBind (PE { pe_ctxt = LetPat {} }) = True
    inPatBind (PE { pe_ctxt = LamPat {} }) = False

    ---------------
    type TcPragFun = Name -> [LSig Name]
    type TcSigFun  = Name -> Maybe TcSigInfo

    data TcSigInfo
      = TcSigInfo {
            sig_id     :: TcId,         --  *Polymorphic* binder for this value...

            sig_tvs    :: [(Maybe Name, TcTyVar)],
                               -- Instantiated type and kind variables
                               -- Just n <=> this skolem is lexically in scope with name n
                               -- See Note [Binding scoped type variables]

            sig_nwcs   :: [(Name, TcTyVar)],
                               -- Instantiated wildcard variables

            sig_theta  :: TcThetaType,  -- Instantiated theta

            sig_extra_cts :: Maybe SrcSpan, -- Just loc <=> An extra-constraints
                                            -- wildcard was present. Any extra
                                            -- constraints inferred during
                                            -- type-checking will be added to the
                                            -- partial type signature. Stores the
                                            -- location of the wildcard.

            sig_tau    :: TcSigmaType,  -- Instantiated tau
                                        -- See Note [sig_tau may be polymorphic]

            sig_loc    :: SrcSpan,      -- The location of the signature

            sig_partial :: Bool         -- True <=> a partial type signature
                                        -- containing wildcards
        }
      | TcPatSynInfo TcPatSynInfo

    data TcPatSynInfo
      = TPSI {
            patsig_name  :: Name,
            patsig_tau   :: TcSigmaType,
            patsig_ex    :: [TcTyVar],
            patsig_prov  :: TcThetaType,
            patsig_univ  :: [TcTyVar],
            patsig_req   :: TcThetaType
        }

    findScopedTyVars  -- See Note [Binding scoped type variables]
      :: LHsType Name             -- The HsType
      -> TcType                   -- The corresponding Type:
                                  --   uses same Names as the HsType
      -> [TcTyVar]                -- The instantiated forall variables of the Type
      -> [(Maybe Name, TcTyVar)]  -- In 1-1 correspondence with the instantiated vars
    findScopedTyVars hs_ty sig_ty inst_tvs
      = zipWith find sig_tvs inst_tvs
      where
        find sig_tv inst_tv
          | tv_name `elemNameSet` scoped_names = (Just tv_name, inst_tv)
          | otherwise                          = (Nothing,      inst_tv)
          where
            tv_name = tyVarName sig_tv

        scoped_names = mkNameSet (hsExplicitTvs hs_ty)
        (sig_tvs,_)  = tcSplitForAllTys sig_ty

    instance NamedThing TcSigInfo where
        getName TcSigInfo{ sig_id = id } = idName id
        getName (TcPatSynInfo tpsi) = patsig_name tpsi

    instance Outputable TcSigInfo where
        ppr (TcSigInfo { sig_id = id, sig_tvs = tyvars, sig_theta = theta, sig_tau = tau })
            = ppr id <+> dcolon <+> vcat [ pprSigmaType (mkSigmaTy (map snd tyvars) theta tau)
                                         , ppr (map fst tyvars) ]
        ppr (TcPatSynInfo tpsi) = text "TcPatSynInfo" <+> ppr tpsi

    instance Outputable TcPatSynInfo where
        ppr (TPSI{ patsig_name = name}) = ppr name

    isPartialSig :: TcSigInfo -> Bool
    isPartialSig = sig_partial

    {-
    Note [Binding scoped type variables]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The type variables *brought into lexical scope* by a type signature may
    be a subset of the *quantified type variables* of the signatures, for two reasons:

    * With kind polymorphism a signature like
        f :: forall f a. f a -> f a
      may actually give rise to
        f :: forall k. forall (f::k -> *) (a:k). f a -> f a
      So the sig_tvs will be [k,f,a], but only f,a are scoped.
      NB: the scoped ones are not necessarily the *inital* ones!

    * Even aside from kind polymorphism, tere may be more instantiated
      type variables than lexically-scoped ones.  For example:
            type T a = forall b. b -> (a,b)
            f :: forall c. T c
      Here, the signature for f will have one scoped type variable, c,
      but two instantiated type variables, c' and b'.

    The function findScopedTyVars takes
      * hs_ty:    the original HsForAllTy
      * sig_ty:   the corresponding Type (which is guaranteed to use the same Names
                  as the HsForAllTy)
      * inst_tvs: the skolems instantiated from the forall's in sig_ty
    It returns a [(Maybe Name, TcTyVar)], in 1-1 correspondence with inst_tvs
    but with a (Just n) for the lexically scoped name of each in-scope tyvar.

    Note [sig_tau may be polymorphic]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Note that "sig_tau" might actually be a polymorphic type,
    if the original function had a signature like
       forall a. Eq a => forall b. Ord b => ....
    But that's ok: tcMatchesFun (called by tcRhs) can deal with that
    It happens, too!  See Note [Polymorphic methods] in TcClassDcl.

    Note [Existential check]
    ~~~~~~~~~~~~~~~~~~~~~~~~
    Lazy patterns can't bind existentials.  They arise in two ways:
      * Let bindings      let { C a b = e } in b
      * Twiddle patterns  f ~(C a b) = e
    The pe_lazy field of PatEnv says whether we are inside a lazy
    pattern (perhaps deeply)

    If we aren't inside a lazy pattern then we can bind existentials,
    but we need to be careful about "extra" tyvars. Consider
        (\C x -> d) : pat_ty -> res_ty
    When looking for existential escape we must check that the existential
    bound by C don't unify with the free variables of pat_ty, OR res_ty
    (or of course the environment).   Hence we need to keep track of the
    res_ty free vars.

    ************************************************************************
    *                                                                      *
                    Binders
    *                                                                      *
    ************************************************************************
    -}

    tcPatBndr :: PatEnv -> Name -> TcSigmaType -> TcM (TcCoercion, TcId)
    -- (coi, xp) = tcPatBndr penv x pat_ty
    -- Then coi : pat_ty ~ typeof(xp)
    --
    tcPatBndr (PE { pe_ctxt = LetPat lookup_sig no_gen}) bndr_name pat_ty
              -- See Note [Typing patterns in pattern bindings]
      | LetGblBndr prags <- no_gen
      , Just sig <- lookup_sig bndr_name
      = do { bndr_id <- addInlinePrags (sig_id sig) (prags bndr_name)
           ; traceTc "tcPatBndr(gbl,sig)" (ppr bndr_id $$ ppr (idType bndr_id))
           ; co <- unifyPatType (idType bndr_id) pat_ty
           ; return (co, bndr_id) }

      | otherwise
      = do { bndr_id <- newNoSigLetBndr no_gen bndr_name pat_ty
           ; traceTc "tcPatBndr(no-sig)" (ppr bndr_id $$ ppr (idType bndr_id))
           ; return (mkTcNomReflCo pat_ty, bndr_id) }

    tcPatBndr (PE { pe_ctxt = _lam_or_proc }) bndr_name pat_ty
      = do { bndr <- mkLocalBinder bndr_name pat_ty
           ; return (mkTcNomReflCo pat_ty, bndr) }

    ------------
    newNoSigLetBndr :: LetBndrSpec -> Name -> TcType -> TcM TcId
    -- In the polymorphic case (no_gen = LetLclBndr), generate a "monomorphic version"
    --    of the Id; the original name will be bound to the polymorphic version
    --    by the AbsBinds
    -- In the monomorphic case (no_gen = LetBglBndr) there is no AbsBinds, and we
    --    use the original name directly
    newNoSigLetBndr LetLclBndr name ty
      =do  { mono_name <- newLocalName name
           ; mkLocalBinder mono_name ty }
    newNoSigLetBndr (LetGblBndr prags) name ty
      = do { id <- mkLocalBinder name ty
           ; addInlinePrags id (prags name) }

    ----------
    addInlinePrags :: TcId -> [LSig Name] -> TcM TcId
    addInlinePrags poly_id prags
      = do { traceTc "addInlinePrags" (ppr poly_id $$ ppr prags)
           ; tc_inl inl_sigs }
      where
        inl_sigs = filter isInlineLSig prags
        tc_inl [] = return poly_id
        tc_inl (L loc (InlineSig _ prag) : other_inls)
           = do { unless (null other_inls) (setSrcSpan loc warn_dup_inline)
                ; traceTc "addInlinePrag" (ppr poly_id $$ ppr prag)
                ; return (poly_id `setInlinePragma` prag) }
        tc_inl _ = panic "tc_inl"

        warn_dup_inline = warnPrags poly_id inl_sigs $
                          ptext (sLit "Duplicate INLINE pragmas for")

    warnPrags :: Id -> [LSig Name] -> SDoc -> TcM ()
    warnPrags id bad_sigs herald
      = addWarnTc (hang (herald <+> quotes (ppr id))
                      2 (ppr_sigs bad_sigs))
      where
        ppr_sigs sigs = vcat (map (ppr . getLoc) sigs)

    -----------------
    mkLocalBinder :: Name -> TcType -> TcM TcId
    mkLocalBinder name ty
      = return (Id.mkLocalId name ty)

    {-
    Note [Typing patterns in pattern bindings]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Suppose we are typing a pattern binding
        pat = rhs
    Then the PatCtxt will be (LetPat sig_fn let_bndr_spec).

    There can still be signatures for the binders:
         data T = MkT (forall a. a->a) Int
         x :: forall a. a->a
         y :: Int
         MkT x y = <rhs>

    Two cases, dealt with by the LetPat case of tcPatBndr

     * If we are generalising (generalisation plan is InferGen or
       CheckGen), then the let_bndr_spec will be LetLclBndr.  In that case
       we want to bind a cloned, local version of the variable, with the
       type given by the pattern context, *not* by the signature (even if
       there is one; see Trac #7268). The mkExport part of the
       generalisation step will do the checking and impedence matching
       against the signature.

     * If for some some reason we are not generalising (plan = NoGen), the
       LetBndrSpec will be LetGblBndr.  In that case we must bind the
       global version of the Id, and do so with precisely the type given
       in the signature.  (Then we unify with the type from the pattern
       context type.

    ************************************************************************
    *                                                                      *
                    The main worker functions
    *                                                                      *
    ************************************************************************

    Note [Nesting]
    ~~~~~~~~~~~~~~
    tcPat takes a "thing inside" over which the pattern scopes.  This is partly
    so that tcPat can extend the environment for the thing_inside, but also
    so that constraints arising in the thing_inside can be discharged by the
    pattern.

    This does not work so well for the ErrCtxt carried by the monad: we don't
    want the error-context for the pattern to scope over the RHS.
    Hence the getErrCtxt/setErrCtxt stuff in tcMultiple
    -}

    --------------------
    type Checker inp out =  forall r.
                              inp
                           -> PatEnv
                           -> TcM r
                           -> TcM (out, r)

    tcMultiple :: Checker inp out -> Checker [inp] [out]
    tcMultiple tc_pat args penv thing_inside
      = do  { err_ctxt <- getErrCtxt
            ; let loop _ []
                    = do { res <- thing_inside
                         ; return ([], res) }

                  loop penv (arg:args)
                    = do { (p', (ps', res))
                                    <- tc_pat arg penv $
                                       setErrCtxt err_ctxt $
                                       loop penv args
                    -- setErrCtxt: restore context before doing the next pattern
                    -- See note [Nesting] above

                         ; return (p':ps', res) }

            ; loop penv args }

    --------------------
    tc_lpat :: LPat Name
            -> TcSigmaType
            -> PatEnv
            -> TcM a
            -> TcM (LPat TcId, a)
    tc_lpat (L span pat) pat_ty penv thing_inside
      = setSrcSpan span $
        do  { (pat', res) <- maybeWrapPatCtxt pat (tc_pat penv pat pat_ty)
                                              thing_inside
            ; return (L span pat', res) }

    tc_lpats :: PatEnv
             -> [LPat Name] -> [TcSigmaType]
             -> TcM a
             -> TcM ([LPat TcId], a)
    tc_lpats penv pats tys thing_inside
      = ASSERT2( equalLength pats tys, ppr pats $$ ppr tys )
        tcMultiple (\(p,t) -> tc_lpat p t)
                    (zipEqual "tc_lpats" pats tys)
                    penv thing_inside

    --------------------
    tc_pat  :: PatEnv
            -> Pat Name
            -> TcSigmaType  -- Fully refined result type
            -> TcM a                -- Thing inside
            -> TcM (Pat TcId,       -- Translated pattern
                    a)              -- Result of thing inside

    tc_pat penv (VarPat name) pat_ty thing_inside
      = do  { (co, id) <- tcPatBndr penv name pat_ty
            ; res <- tcExtendIdEnv1 name id thing_inside
            ; return (mkHsWrapPatCo co (VarPat id) pat_ty, res) }

    tc_pat penv (ParPat pat) pat_ty thing_inside
      = do  { (pat', res) <- tc_lpat pat pat_ty penv thing_inside
            ; return (ParPat pat', res) }

    tc_pat penv (BangPat pat) pat_ty thing_inside
      = do  { (pat', res) <- tc_lpat pat pat_ty penv thing_inside
            ; return (BangPat pat', res) }

    tc_pat penv lpat@(LazyPat pat) pat_ty thing_inside
      = do  { (pat', (res, pat_ct))
                    <- tc_lpat pat pat_ty (makeLazy penv) $
                       captureConstraints thing_inside
                    -- Ignore refined penv', revert to penv

            ; emitConstraints pat_ct
            -- captureConstraints/extendConstraints:
            --   see Note [Hopping the LIE in lazy patterns]

            -- Check there are no unlifted types under the lazy pattern
            ; when (any (isUnLiftedType . idType) $ collectPatBinders pat') $
                   lazyUnliftedPatErr lpat

            -- Check that the expected pattern type is itself lifted
            ; pat_ty' <- newFlexiTyVarTy liftedTypeKind
            ; _ <- unifyType pat_ty pat_ty'

            ; return (LazyPat pat', res) }

    tc_pat _ p@(QuasiQuotePat _) _ _
      = pprPanic "Should never see QuasiQuotePat in type checker" (ppr p)

    tc_pat _ (WildPat _) pat_ty thing_inside
      = do  { res <- thing_inside
            ; return (WildPat pat_ty, res) }

    tc_pat penv (AsPat (L nm_loc name) pat) pat_ty thing_inside
      = do  { (co, bndr_id) <- setSrcSpan nm_loc (tcPatBndr penv name pat_ty)
            ; (pat', res) <- tcExtendIdEnv1 name bndr_id $
                             tc_lpat pat (idType bndr_id) penv thing_inside
                -- NB: if we do inference on:
                --          \ (y@(x::forall a. a->a)) = e
                -- we'll fail.  The as-pattern infers a monotype for 'y', which then
                -- fails to unify with the polymorphic type for 'x'.  This could
                -- perhaps be fixed, but only with a bit more work.
                --
                -- If you fix it, don't forget the bindInstsOfPatIds!
            ; return (mkHsWrapPatCo co (AsPat (L nm_loc bndr_id) pat') pat_ty, res) }

    tc_pat penv (ViewPat expr pat _) overall_pat_ty thing_inside
      = do  {
             -- Morally, expr must have type `forall a1...aN. OPT' -> B`
             -- where overall_pat_ty is an instance of OPT'.
             -- Here, we infer a rho type for it,
             -- which replaces the leading foralls and constraints
             -- with fresh unification variables.
            ; (expr',expr'_inferred) <- tcInferRho expr

             -- next, we check that expr is coercible to `overall_pat_ty -> pat_ty`
             -- NOTE: this forces pat_ty to be a monotype (because we use a unification
             -- variable to find it).  this means that in an example like
             -- (view -> f)    where view :: _ -> forall b. b
             -- we will only be able to use view at one instantation in the
             -- rest of the view
            ; (expr_co, pat_ty) <- tcInfer $ \ pat_ty ->
                    unifyType expr'_inferred (mkFunTy overall_pat_ty pat_ty)

             -- pattern must have pat_ty
            ; (pat', res) <- tc_lpat pat pat_ty penv thing_inside

            ; return (ViewPat (mkLHsWrapCo expr_co expr') pat' overall_pat_ty, res) }

    -- Type signatures in patterns
    -- See Note [Pattern coercions] below
    tc_pat penv (SigPatIn pat sig_ty) pat_ty thing_inside
      = do  { (inner_ty, tv_binds, nwc_binds, wrap) <- tcPatSig (inPatBind penv)
                                                                sig_ty pat_ty
            ; (pat', res) <- tcExtendTyVarEnv2 (tv_binds ++ nwc_binds) $
                             tc_lpat pat inner_ty penv thing_inside
            ; return (mkHsWrapPat wrap (SigPatOut pat' inner_ty) pat_ty, res) }

    ------------------------
    -- Lists, tuples, arrays
    tc_pat penv (ListPat pats _ Nothing) pat_ty thing_inside
      = do  { (coi, elt_ty) <- matchExpectedPatTy matchExpectedListTy pat_ty
            ; (pats', res) <- tcMultiple (\p -> tc_lpat p elt_ty)
                                         pats penv thing_inside
            ; return (mkHsWrapPat coi (ListPat pats' elt_ty Nothing) pat_ty, res)
            }

    tc_pat penv (ListPat pats _ (Just (_,e))) pat_ty thing_inside
      = do  { list_pat_ty <- newFlexiTyVarTy liftedTypeKind
            ; e' <- tcSyntaxOp ListOrigin e (mkFunTy pat_ty list_pat_ty)
            ; (coi, elt_ty) <- matchExpectedPatTy matchExpectedListTy list_pat_ty
            ; (pats', res) <- tcMultiple (\p -> tc_lpat p elt_ty)
                                         pats penv thing_inside
            ; return (mkHsWrapPat coi (ListPat pats' elt_ty (Just (pat_ty,e'))) list_pat_ty, res)
            }

    tc_pat penv (PArrPat pats _) pat_ty thing_inside
      = do  { (coi, elt_ty) <- matchExpectedPatTy matchExpectedPArrTy pat_ty
            ; (pats', res) <- tcMultiple (\p -> tc_lpat p elt_ty)
                                         pats penv thing_inside
            ; return (mkHsWrapPat coi (PArrPat pats' elt_ty) pat_ty, res)
            }

    tc_pat penv (TuplePat pats boxity _) pat_ty thing_inside
      = do  { let tc = tupleTyCon (boxityNormalTupleSort boxity) (length pats)
            ; (coi, arg_tys) <- matchExpectedPatTy (matchExpectedTyConApp tc) pat_ty
            ; (pats', res) <- tc_lpats penv pats arg_tys thing_inside

            ; dflags <- getDynFlags

            -- Under flag control turn a pattern (x,y,z) into ~(x,y,z)
            -- so that we can experiment with lazy tuple-matching.
            -- This is a pretty odd place to make the switch, but
            -- it was easy to do.
            ; let
                  unmangled_result = TuplePat pats' boxity arg_tys
                                     -- pat_ty /= pat_ty iff coi /= IdCo
                  possibly_mangled_result
                    | gopt Opt_IrrefutableTuples dflags &&
                      isBoxed boxity            = LazyPat (noLoc unmangled_result)
                    | otherwise                 = unmangled_result

            ; ASSERT( length arg_tys == length pats )      -- Syntactically enforced
              return (mkHsWrapPat coi possibly_mangled_result pat_ty, res)
            }

    ------------------------
    -- Data constructors
    tc_pat penv (ConPatIn con arg_pats) pat_ty thing_inside
      = tcConPat penv con pat_ty arg_pats thing_inside

    ------------------------
    -- Literal patterns
    tc_pat _ (LitPat simple_lit) pat_ty thing_inside
      = do  { let lit_ty = hsLitType simple_lit
            ; co <- unifyPatType lit_ty pat_ty
                    -- coi is of kind: pat_ty ~ lit_ty
            ; res <- thing_inside
            ; return ( mkHsWrapPatCo co (LitPat simple_lit) pat_ty
                     , res) }

    ------------------------
    -- Overloaded patterns: n, and n+k
    tc_pat _ (NPat (L l over_lit) mb_neg eq) pat_ty thing_inside
      = do  { let orig = LiteralOrigin over_lit
            ; lit'    <- newOverloadedLit orig over_lit pat_ty
            ; eq'     <- tcSyntaxOp orig eq (mkFunTys [pat_ty, pat_ty] boolTy)
            ; mb_neg' <- case mb_neg of
                            Nothing  -> return Nothing      -- Positive literal
                            Just neg ->     -- Negative literal
                                            -- The 'negate' is re-mappable syntax
                                do { neg' <- tcSyntaxOp orig neg (mkFunTy pat_ty pat_ty)
                                   ; return (Just neg') }
            ; res <- thing_inside
            ; return (NPat (L l lit') mb_neg' eq', res) }

    tc_pat penv (NPlusKPat (L nm_loc name) (L loc lit) ge minus) pat_ty thing_inside
      = do  { (co, bndr_id) <- setSrcSpan nm_loc (tcPatBndr penv name pat_ty)
            ; let pat_ty' = idType bndr_id
                  orig    = LiteralOrigin lit
            ; lit' <- newOverloadedLit orig lit pat_ty'

            -- The '>=' and '-' parts are re-mappable syntax
            ; ge'    <- tcSyntaxOp orig ge    (mkFunTys [pat_ty', pat_ty'] boolTy)
            ; minus' <- tcSyntaxOp orig minus (mkFunTys [pat_ty', pat_ty'] pat_ty')
            ; let pat' = NPlusKPat (L nm_loc bndr_id) (L loc lit') ge' minus'

            -- The Report says that n+k patterns must be in Integral
            -- We may not want this when using re-mappable syntax, though (ToDo?)
            ; icls <- tcLookupClass integralClassName
            ; instStupidTheta orig [mkClassPred icls [pat_ty']]

            ; res <- tcExtendIdEnv1 name bndr_id thing_inside
            ; return (mkHsWrapPatCo co pat' pat_ty, res) }

    tc_pat _ _other_pat _ _ = panic "tc_pat"        -- ConPatOut, SigPatOut

    ----------------
    unifyPatType :: TcType -> TcType -> TcM TcCoercion
    -- In patterns we want a coercion from the
    -- context type (expected) to the actual pattern type
    -- But we don't want to reverse the args to unifyType because
    -- that controls the actual/expected stuff in error messages
    unifyPatType actual_ty expected_ty
      = do { coi <- unifyType actual_ty expected_ty
           ; return (mkTcSymCo coi) }

    {-
    Note [Hopping the LIE in lazy patterns]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In a lazy pattern, we must *not* discharge constraints from the RHS
    from dictionaries bound in the pattern.  E.g.
            f ~(C x) = 3
    We can't discharge the Num constraint from dictionaries bound by
    the pattern C!

    So we have to make the constraints from thing_inside "hop around"
    the pattern.  Hence the captureConstraints and emitConstraints.

    The same thing ensures that equality constraints in a lazy match
    are not made available in the RHS of the match. For example
            data T a where { T1 :: Int -> T Int; ... }
            f :: T a -> Int -> a
            f ~(T1 i) y = y
    It's obviously not sound to refine a to Int in the right
    hand side, because the arugment might not match T1 at all!

    Finally, a lazy pattern should not bind any existential type variables
    because they won't be in scope when we do the desugaring

    ************************************************************************
    *                                                                      *
            Most of the work for constructors is here
            (the rest is in the ConPatIn case of tc_pat)
    *                                                                      *
    ************************************************************************

    [Pattern matching indexed data types]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider the following declarations:

      data family Map k :: * -> *
      data instance Map (a, b) v = MapPair (Map a (Pair b v))

    and a case expression

      case x :: Map (Int, c) w of MapPair m -> ...

    As explained by [Wrappers for data instance tycons] in MkIds.lhs, the
    worker/wrapper types for MapPair are

      $WMapPair :: forall a b v. Map a (Map a b v) -> Map (a, b) v
      $wMapPair :: forall a b v. Map a (Map a b v) -> :R123Map a b v

    So, the type of the scrutinee is Map (Int, c) w, but the tycon of MapPair is
    :R123Map, which means the straight use of boxySplitTyConApp would give a type
    error.  Hence, the smart wrapper function boxySplitTyConAppWithFamily calls
    boxySplitTyConApp with the family tycon Map instead, which gives us the family
    type list {(Int, c), w}.  To get the correct split for :R123Map, we need to
    unify the family type list {(Int, c), w} with the instance types {(a, b), v}
    (provided by tyConFamInst_maybe together with the family tycon).  This
    unification yields the substitution [a -> Int, b -> c, v -> w], which gives us
    the split arguments for the representation tycon :R123Map as {Int, c, w}

    In other words, boxySplitTyConAppWithFamily implicitly takes the coercion

      Co123Map a b v :: {Map (a, b) v ~ :R123Map a b v}

    moving between representation and family type into account.  To produce type
    correct Core, this coercion needs to be used to case the type of the scrutinee
    from the family to the representation type.  This is achieved by
    unwrapFamInstScrutinee using a CoPat around the result pattern.

    Now it might appear seem as if we could have used the previous GADT type
    refinement infrastructure of refineAlt and friends instead of the explicit
    unification and CoPat generation.  However, that would be wrong.  Why?  The
    whole point of GADT refinement is that the refinement is local to the case
    alternative.  In contrast, the substitution generated by the unification of
    the family type list and instance types needs to be propagated to the outside.
    Imagine that in the above example, the type of the scrutinee would have been
    (Map x w), then we would have unified {x, w} with {(a, b), v}, yielding the
    substitution [x -> (a, b), v -> w].  In contrast to GADT matching, the
    instantiation of x with (a, b) must be global; ie, it must be valid in *all*
    alternatives of the case expression, whereas in the GADT case it might vary
    between alternatives.

    RIP GADT refinement: refinements have been replaced by the use of explicit
    equality constraints that are used in conjunction with implication constraints
    to express the local scope of GADT refinements.
    -}

    --      Running example:
    -- MkT :: forall a b c. (a~[b]) => b -> c -> T a
    --       with scrutinee of type (T ty)

    tcConPat :: PatEnv -> Located Name
             -> TcRhoType           -- Type of the pattern
             -> HsConPatDetails Name -> TcM a
             -> TcM (Pat TcId, a)
    tcConPat penv con_lname@(L _ con_name) pat_ty arg_pats thing_inside
      = do  { con_like <- tcLookupConLike con_name
            ; case con_like of
                RealDataCon data_con -> tcDataConPat penv con_lname data_con
                                                     pat_ty arg_pats thing_inside
                PatSynCon pat_syn -> tcPatSynPat penv con_lname pat_syn
                                                 pat_ty arg_pats thing_inside
            }

    tcDataConPat :: PatEnv -> Located Name -> DataCon
                 -> TcRhoType               -- Type of the pattern
                 -> HsConPatDetails Name -> TcM a
                 -> TcM (Pat TcId, a)
    tcDataConPat penv (L con_span con_name) data_con pat_ty arg_pats thing_inside
      = do  { let tycon = dataConTyCon data_con
                      -- For data families this is the representation tycon
                  (univ_tvs, ex_tvs, eq_spec, theta, arg_tys, _)
                    = dataConFullSig data_con
                  header = L con_span (RealDataCon data_con)

              -- Instantiate the constructor type variables [a->ty]
              -- This may involve doing a family-instance coercion,
              -- and building a wrapper
            ; (wrap, ctxt_res_tys) <- matchExpectedPatTy (matchExpectedConTy tycon) pat_ty

              -- Add the stupid theta
            ; setSrcSpan con_span $ addDataConStupidTheta data_con ctxt_res_tys

            ; checkExistentials ex_tvs penv
            ; (tenv, ex_tvs') <- tcInstSuperSkolTyVarsX
                                   (zipTopTvSubst univ_tvs ctxt_res_tys) ex_tvs
                         -- Get location from monad, not from ex_tvs

            ; let -- pat_ty' = mkTyConApp tycon ctxt_res_tys
                  -- pat_ty' is type of the actual constructor application
                  -- pat_ty' /= pat_ty iff coi /= IdCo

                  arg_tys' = substTys tenv arg_tys

            ; traceTc "tcConPat" (vcat [ ppr con_name, ppr univ_tvs, ppr ex_tvs, ppr eq_spec
                                       , ppr ex_tvs', ppr ctxt_res_tys, ppr arg_tys' ])
            ; if null ex_tvs && null eq_spec && null theta
              then do { -- The common case; no class bindings etc
                        -- (see Note [Arrows and patterns])
                        (arg_pats', res) <- tcConArgs (RealDataCon data_con) arg_tys'
                                                      arg_pats penv thing_inside
                      ; let res_pat = ConPatOut { pat_con = header,
                                                  pat_tvs = [], pat_dicts = [],
                                                  pat_binds = emptyTcEvBinds,
                                                  pat_args = arg_pats',
                                                  pat_arg_tys = ctxt_res_tys,
                                                  pat_wrap = idHsWrapper }

                      ; return (mkHsWrapPat wrap res_pat pat_ty, res) }

              else do   -- The general case, with existential,
                        -- and local equality constraints
            { let theta'   = substTheta tenv (eqSpecPreds eq_spec ++ theta)
                               -- order is *important* as we generate the list of
                               -- dictionary binders from theta'
                  no_equalities = not (any isEqPred theta')
                  skol_info = case pe_ctxt penv of
                                LamPat mc -> PatSkol (RealDataCon data_con) mc
                                LetPat {} -> UnkSkol -- Doesn't matter

            ; gadts_on    <- xoptM Opt_GADTs
            ; families_on <- xoptM Opt_TypeFamilies
            ; checkTc (no_equalities || gadts_on || families_on)
                      (text "A pattern match on a GADT requires the" <+>
                       text "GADTs or TypeFamilies language extension")
                      -- Trac #2905 decided that a *pattern-match* of a GADT
                      -- should require the GADT language flag.
                      -- Re TypeFamilies see also #7156

            ; given <- newEvVars theta'
            ; (ev_binds, (arg_pats', res))
                 <- checkConstraints skol_info ex_tvs' given $
                    tcConArgs (RealDataCon data_con) arg_tys' arg_pats penv thing_inside

            ; let res_pat = ConPatOut { pat_con   = header,
                                        pat_tvs   = ex_tvs',
                                        pat_dicts = given,
                                        pat_binds = ev_binds,
                                        pat_args  = arg_pats',
                                        pat_arg_tys = ctxt_res_tys,
                                        pat_wrap  = idHsWrapper }
            ; return (mkHsWrapPat wrap res_pat pat_ty, res)
            } }

    tcPatSynPat :: PatEnv -> Located Name -> PatSyn
                -> TcRhoType                -- Type of the pattern
                -> HsConPatDetails Name -> TcM a
                -> TcM (Pat TcId, a)
    tcPatSynPat penv (L con_span _) pat_syn pat_ty arg_pats thing_inside
      = do  { let (univ_tvs, ex_tvs, prov_theta, req_theta, arg_tys, ty) = patSynSig pat_syn

            ; (subst, univ_tvs') <- tcInstTyVars univ_tvs

            ; checkExistentials ex_tvs penv
            ; (tenv, ex_tvs') <- tcInstSuperSkolTyVarsX subst ex_tvs
            ; let ty' = substTy tenv ty
                  arg_tys' = substTys tenv arg_tys
                  prov_theta' = substTheta tenv prov_theta
                  req_theta' = substTheta tenv req_theta

            ; wrap <- coToHsWrapper <$> unifyType ty' pat_ty
            ; traceTc "tcPatSynPat" (ppr pat_syn $$
                                     ppr pat_ty $$
                                     ppr ty' $$
                                     ppr ex_tvs' $$
                                     ppr prov_theta' $$
                                     ppr req_theta' $$
                                     ppr arg_tys')

            ; prov_dicts' <- newEvVars prov_theta'

            ; let skol_info = case pe_ctxt penv of
                                LamPat mc -> PatSkol (PatSynCon pat_syn) mc
                                LetPat {} -> UnkSkol -- Doesn't matter

            ; req_wrap <- instCall PatOrigin (mkTyVarTys univ_tvs') req_theta'
            ; traceTc "instCall" (ppr req_wrap)

            ; traceTc "checkConstraints {" Outputable.empty
            ; (ev_binds, (arg_pats', res))
                 <- checkConstraints skol_info ex_tvs' prov_dicts' $
                    tcConArgs (PatSynCon pat_syn) arg_tys' arg_pats penv thing_inside

            ; traceTc "checkConstraints }" (ppr ev_binds)
            ; let res_pat = ConPatOut { pat_con   = L con_span $ PatSynCon pat_syn,
                                        pat_tvs   = ex_tvs',
                                        pat_dicts = prov_dicts',
                                        pat_binds = ev_binds,
                                        pat_args  = arg_pats',
                                        pat_arg_tys = mkTyVarTys univ_tvs',
                                        pat_wrap  = req_wrap }
            ; return (mkHsWrapPat wrap res_pat pat_ty, res) }

    ----------------------------
    matchExpectedPatTy :: (TcRhoType -> TcM (TcCoercion, a))
                        -> TcRhoType -> TcM (HsWrapper, a)
    -- See Note [Matching polytyped patterns]
    -- Returns a wrapper : pat_ty ~ inner_ty
    matchExpectedPatTy inner_match pat_ty
      | null tvs && null theta
      = do { (co, res) <- inner_match pat_ty
           ; return (coToHsWrapper (mkTcSymCo co), res) }
             -- The Sym is because the inner_match returns a coercion
             -- that is the other way round to matchExpectedPatTy

      | otherwise
      = do { (subst, tvs') <- tcInstTyVars tvs
           ; wrap1 <- instCall PatOrigin (mkTyVarTys tvs') (substTheta subst theta)
           ; (wrap2, arg_tys) <- matchExpectedPatTy inner_match (TcType.substTy subst tau)
           ; return (wrap2 <.> wrap1, arg_tys) }
      where
        (tvs, theta, tau) = tcSplitSigmaTy pat_ty

    ----------------------------
    matchExpectedConTy :: TyCon      -- The TyCon that this data
                                     -- constructor actually returns
                       -> TcRhoType  -- The type of the pattern
                       -> TcM (TcCoercion, [TcSigmaType])
    -- See Note [Matching constructor patterns]
    -- Returns a coercion : T ty1 ... tyn ~ pat_ty
    -- This is the same way round as matchExpectedListTy etc
    -- but the other way round to matchExpectedPatTy
    matchExpectedConTy data_tc pat_ty
      | Just (fam_tc, fam_args, co_tc) <- tyConFamInstSig_maybe data_tc
             -- Comments refer to Note [Matching constructor patterns]
             -- co_tc :: forall a. T [a] ~ T7 a
      = do { (subst, tvs') <- tcInstTyVars (tyConTyVars data_tc)
                 -- tys = [ty1,ty2]

           ; traceTc "matchExpectedConTy" (vcat [ppr data_tc,
                                                 ppr (tyConTyVars data_tc),
                                                 ppr fam_tc, ppr fam_args])
           ; co1 <- unifyType (mkTyConApp fam_tc (substTys subst fam_args)) pat_ty
                 -- co1 : T (ty1,ty2) ~ pat_ty

           ; let tys' = mkTyVarTys tvs'
                 co2 = mkTcUnbranchedAxInstCo Nominal co_tc tys'
                 -- co2 : T (ty1,ty2) ~ T7 ty1 ty2

           ; return (mkTcSymCo co2 `mkTcTransCo` co1, tys') }

      | otherwise
      = matchExpectedTyConApp data_tc pat_ty
                 -- coi : T tys ~ pat_ty

    {-
    Note [Matching constructor patterns]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Suppose (coi, tys) = matchExpectedConType data_tc pat_ty

     * In the simple case, pat_ty = tc tys

     * If pat_ty is a polytype, we want to instantiate it
       This is like part of a subsumption check.  Eg
          f :: (forall a. [a]) -> blah
          f [] = blah

     * In a type family case, suppose we have
              data family T a
              data instance T (p,q) = A p | B q
           Then we'll have internally generated
                  data T7 p q = A p | B q
                  axiom coT7 p q :: T (p,q) ~ T7 p q

           So if pat_ty = T (ty1,ty2), we return (coi, [ty1,ty2]) such that
               coi = coi2 . coi1 : T7 t ~ pat_ty
               coi1 : T (ty1,ty2) ~ pat_ty
               coi2 : T7 ty1 ty2 ~ T (ty1,ty2)

       For families we do all this matching here, not in the unifier,
       because we never want a whisper of the data_tycon to appear in
       error messages; it's a purely internal thing
    -}

    tcConArgs :: ConLike -> [TcSigmaType]
              -> Checker (HsConPatDetails Name) (HsConPatDetails Id)

    tcConArgs con_like arg_tys (PrefixCon arg_pats) penv thing_inside
      = do  { checkTc (con_arity == no_of_args)     -- Check correct arity
                      (arityErr "Constructor" con_like con_arity no_of_args)
            ; let pats_w_tys = zipEqual "tcConArgs" arg_pats arg_tys
            ; (arg_pats', res) <- tcMultiple tcConArg pats_w_tys
                                                  penv thing_inside
            ; return (PrefixCon arg_pats', res) }
      where
        con_arity  = conLikeArity con_like
        no_of_args = length arg_pats

    tcConArgs con_like arg_tys (InfixCon p1 p2) penv thing_inside
      = do  { checkTc (con_arity == 2)      -- Check correct arity
                      (arityErr "Constructor" con_like con_arity 2)
            ; let [arg_ty1,arg_ty2] = arg_tys       -- This can't fail after the arity check
            ; ([p1',p2'], res) <- tcMultiple tcConArg [(p1,arg_ty1),(p2,arg_ty2)]
                                                  penv thing_inside
            ; return (InfixCon p1' p2', res) }
      where
        con_arity  = conLikeArity con_like

    tcConArgs con_like arg_tys (RecCon (HsRecFields rpats dd)) penv thing_inside
      = do  { (rpats', res) <- tcMultiple tc_field rpats penv thing_inside
            ; return (RecCon (HsRecFields rpats' dd), res) }
      where
        tc_field :: Checker (LHsRecField FieldLabel (LPat Name))
                            (LHsRecField TcId (LPat TcId))
        tc_field (L l (HsRecField field_lbl pat pun)) penv thing_inside
          = do { (sel_id, pat_ty) <- wrapLocFstM find_field_ty field_lbl
               ; (pat', res) <- tcConArg (pat, pat_ty) penv thing_inside
               ; return (L l (HsRecField sel_id pat' pun), res) }

        find_field_ty :: FieldLabel -> TcM (Id, TcType)
        find_field_ty field_lbl
            = case [ty | (f,ty) <- field_tys, f == field_lbl] of

                    -- No matching field; chances are this field label comes from some
                    -- other record type (or maybe none).  If this happens, just fail,
                    -- otherwise we get crashes later (Trac #8570), and similar:
                    --      f (R { foo = (a,b) }) = a+b
                    -- If foo isn't one of R's fields, we don't want to crash when
                    -- typechecking the "a+b".
               [] -> failWith (badFieldCon con_like field_lbl)

                    -- The normal case, when the field comes from the right constructor
               (pat_ty : extras) ->
                    ASSERT( null extras )
                    do { sel_id <- tcLookupField field_lbl
                       ; return (sel_id, pat_ty) }

        field_tys :: [(FieldLabel, TcType)]
        field_tys = case con_like of
            RealDataCon data_con -> zip (dataConFieldLabels data_con) arg_tys
              -- Don't use zipEqual! If the constructor isn't really a record, then
              -- dataConFieldLabels will be empty (and each field in the pattern
              -- will generate an error below).
            PatSynCon{} -> []

    conLikeArity :: ConLike -> Arity
    conLikeArity (RealDataCon data_con) = dataConSourceArity data_con
    conLikeArity (PatSynCon   pat_syn)  = patSynArity pat_syn

    tcConArg :: Checker (LPat Name, TcSigmaType) (LPat Id)
    tcConArg (arg_pat, arg_ty) penv thing_inside
      = tc_lpat arg_pat arg_ty penv thing_inside

    addDataConStupidTheta :: DataCon -> [TcType] -> TcM ()
    -- Instantiate the "stupid theta" of the data con, and throw
    -- the constraints into the constraint set
    addDataConStupidTheta data_con inst_tys
      | null stupid_theta = return ()
      | otherwise         = instStupidTheta origin inst_theta
      where
        origin = OccurrenceOf (dataConName data_con)
            -- The origin should always report "occurrence of C"
            -- even when C occurs in a pattern
        stupid_theta = dataConStupidTheta data_con
        tenv = mkTopTvSubst (dataConUnivTyVars data_con `zip` inst_tys)
             -- NB: inst_tys can be longer than the univ tyvars
             --     because the constructor might have existentials
        inst_theta = substTheta tenv stupid_theta

    {-
    Note [Arrows and patterns]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    (Oct 07) Arrow noation has the odd property that it involves
    "holes in the scope". For example:
      expr :: Arrow a => a () Int
      expr = proc (y,z) -> do
              x <- term -< y
              expr' -< x

    Here the 'proc (y,z)' binding scopes over the arrow tails but not the
    arrow body (e.g 'term').  As things stand (bogusly) all the
    constraints from the proc body are gathered together, so constraints
    from 'term' will be seen by the tcPat for (y,z).  But we must *not*
    bind constraints from 'term' here, because the desugarer will not make
    these bindings scope over 'term'.

    The Right Thing is not to confuse these constraints together. But for
    now the Easy Thing is to ensure that we do not have existential or
    GADT constraints in a 'proc', and to short-cut the constraint
    simplification for such vanilla patterns so that it binds no
    constraints. Hence the 'fast path' in tcConPat; but it's also a good
    plan for ordinary vanilla patterns to bypass the constraint
    simplification step.

    ************************************************************************
    *                                                                      *
                    Note [Pattern coercions]
    *                                                                      *
    ************************************************************************

    In principle, these program would be reasonable:

            f :: (forall a. a->a) -> Int
            f (x :: Int->Int) = x 3

            g :: (forall a. [a]) -> Bool
            g [] = True

    In both cases, the function type signature restricts what arguments can be passed
    in a call (to polymorphic ones).  The pattern type signature then instantiates this
    type.  For example, in the first case,  (forall a. a->a) <= Int -> Int, and we
    generate the translated term
            f = \x' :: (forall a. a->a).  let x = x' Int in x 3

    From a type-system point of view, this is perfectly fine, but it's *very* seldom useful.
    And it requires a significant amount of code to implement, because we need to decorate
    the translated pattern with coercion functions (generated from the subsumption check
    by tcSub).

    So for now I'm just insisting on type *equality* in patterns.  No subsumption.

    Old notes about desugaring, at a time when pattern coercions were handled:

    A SigPat is a type coercion and must be handled one at at time.  We can't
    combine them unless the type of the pattern inside is identical, and we don't
    bother to check for that.  For example:

            data T = T1 Int | T2 Bool
            f :: (forall a. a -> a) -> T -> t
            f (g::Int->Int)   (T1 i) = T1 (g i)
            f (g::Bool->Bool) (T2 b) = T2 (g b)

    We desugar this as follows:

            f = \ g::(forall a. a->a) t::T ->
                let gi = g Int
                in case t of { T1 i -> T1 (gi i)
                               other ->
                let gb = g Bool
                in case t of { T2 b -> T2 (gb b)
                               other -> fail }}

    Note that we do not treat the first column of patterns as a
    column of variables, because the coerced variables (gi, gb)
    would be of different types.  So we get rather grotty code.
    But I don't think this is a common case, and if it was we could
    doubtless improve it.

    Meanwhile, the strategy is:
            * treat each SigPat coercion (always non-identity coercions)
                    as a separate block
            * deal with the stuff inside, and then wrap a binding round
                    the result to bind the new variable (gi, gb, etc)

    ************************************************************************
    *                                                                      *
    \subsection{Errors and contexts}
    *                                                                      *
    ************************************************************************
    -}

    maybeWrapPatCtxt :: Pat Name -> (TcM a -> TcM b) -> TcM a -> TcM b
    -- Not all patterns are worth pushing a context
    maybeWrapPatCtxt pat tcm thing_inside
      | not (worth_wrapping pat) = tcm thing_inside
      | otherwise                = addErrCtxt msg $ tcm $ popErrCtxt thing_inside
                                   -- Remember to pop before doing thing_inside
      where
       worth_wrapping (VarPat {}) = False
       worth_wrapping (ParPat {}) = False
       worth_wrapping (AsPat {})  = False
       worth_wrapping _           = True
       msg = hang (ptext (sLit "In the pattern:")) 2 (ppr pat)

    -----------------------------------------------
    checkExistentials :: [TyVar] -> PatEnv -> TcM ()
              -- See Note [Arrows and patterns]
    checkExistentials [] _                                 = return ()
    checkExistentials _ (PE { pe_ctxt = LetPat {}})        = failWithTc existentialLetPat
    checkExistentials _ (PE { pe_ctxt = LamPat ProcExpr }) = failWithTc existentialProcPat
    checkExistentials _ (PE { pe_lazy = True })            = failWithTc existentialLazyPat
    checkExistentials _ _                                  = return ()

    existentialLazyPat :: SDoc
    existentialLazyPat
      = hang (ptext (sLit "An existential or GADT data constructor cannot be used"))
           2 (ptext (sLit "inside a lazy (~) pattern"))

    existentialProcPat :: SDoc
    existentialProcPat
      = ptext (sLit "Proc patterns cannot use existential or GADT data constructors")

    existentialLetPat :: SDoc
    existentialLetPat
      = vcat [text "My brain just exploded",
              text "I can't handle pattern bindings for existential or GADT data constructors.",
              text "Instead, use a case-expression, or do-notation, to unpack the constructor."]

    badFieldCon :: ConLike -> Name -> SDoc
    badFieldCon con field
      = hsep [ptext (sLit "Constructor") <+> quotes (ppr con),
              ptext (sLit "does not have field"), quotes (ppr field)]

    polyPatSig :: TcType -> SDoc
    polyPatSig sig_ty
      = hang (ptext (sLit "Illegal polymorphic type signature in pattern:"))
           2 (ppr sig_ty)

    lazyUnliftedPatErr :: OutputableBndr name => Pat name -> TcM ()
    lazyUnliftedPatErr pat
      = failWithTc $
        hang (ptext (sLit "A lazy (~) pattern cannot contain unlifted types:"))
           2 (ppr pat)