content: {-
  (c) The University of Glasgow 2006
  (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
  
  
  TcPat: Typechecking patterns
  -}
  
  {-# LANGUAGE RankNTypes, CPP #-}
  
  module Eta.TypeCheck.TcPat ( tcLetPat, TcSigFun, TcPragFun
               , TcSigInfo(..), TcPatSynInfo(..)
               , findScopedTyVars, isPartialSig
               , LetBndrSpec(..), addInlinePrags, warnPrags
               , tcPat, tcPats, newNoSigLetBndr
               , addDataConStupidTheta, badFieldCon, polyPatSig ) where
  
  import {-# SOURCE #-}   Eta.TypeCheck.TcExpr( tcSyntaxOp, tcInferRho)
  
  import Eta.HsSyn.HsSyn
  import Eta.TypeCheck.TcHsSyn
  import Eta.TypeCheck.TcRnMonad
  import Eta.TypeCheck.Inst
  import Eta.BasicTypes.Id
  import qualified Eta.BasicTypes.Id as Id
  import qualified Eta.LanguageExtensions as LangExt
  import Eta.BasicTypes.Var
  import Eta.BasicTypes.Name
  import Eta.BasicTypes.NameSet
  import Eta.TypeCheck.TcEnv
  import Eta.TypeCheck.TcMType
  import Eta.TypeCheck.TcValidity( arityErr )
  import Eta.TypeCheck.TcType
  import qualified Eta.TypeCheck.TcType as TcType
  import Eta.TypeCheck.TcUnify
  import Eta.TypeCheck.TcHsType
  import Eta.Prelude.TysWiredIn
  import Eta.TypeCheck.TcEvidence
  import Eta.Types.TyCon
  import Eta.BasicTypes.DataCon
  import Eta.BasicTypes.PatSyn
  import Eta.BasicTypes.ConLike
  import Eta.Prelude.PrelNames
  import Eta.BasicTypes.BasicTypes hiding (SuccessFlag(..))
  import Eta.Main.DynFlags
  import Eta.BasicTypes.SrcLoc
  import Eta.Utils.Util
  import Eta.Utils.Outputable
  import qualified Eta.Utils.Outputable as Outputable
  import Eta.Utils.FastString
  import Control.Monad
  
  #include "HsVersions.h"
  
  {-
  ************************************************************************
  *                                                                      *
                  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 NoReason (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 _ (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) }
  
  -- HsSpliced is an annotation produced by 'RnSplice.rnSplicePat'.
  -- Here we get rid of it and add the finalizers to the global environment.
  --
  -- See Note [Delaying modFinalizers in untyped splices] in RnSplice.
  tc_pat penv (SplicePat (HsSpliced mod_finalizers (HsSplicedPat pat)))
              pat_ty thing_inside
    = do addModFinalizersWithLclEnv mod_finalizers
         tc_pat penv pat pat_ty thing_inside
  
  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 LangExt.GADTs
          ; families_on <- xoptM LangExt.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)
  