GHC/Tc/TyCl/PatSyn.hs

    {-# LANGUAGE CPP #-}
    {-# LANGUAGE FlexibleContexts #-}
    {-# LANGUAGE TypeFamilies #-}

    {-# OPTIONS_GHC -Wno-incomplete-record-updates #-}

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

    -}

    -- | Typechecking pattern synonym declarations
    module GHC.Tc.TyCl.PatSyn
       ( tcPatSynDecl
       , tcPatSynBuilderBind
       , patSynBuilderOcc
       )
    where

    import GHC.Prelude

    import GHC.Hs
    import GHC.Tc.Gen.Pat
    import GHC.Core.Multiplicity
    import GHC.Core.Type ( tidyTyCoVarBinders, tidyTypes, tidyType )
    import GHC.Core.TyCo.Subst( extendTvSubstWithClone )
    import GHC.Tc.Utils.Monad
    import GHC.Tc.Gen.Sig ( TcPragEnv, emptyPragEnv, completeSigFromId, lookupPragEnv
                          , addInlinePrags, addInlinePragArity )
    import GHC.Tc.Utils.Env
    import GHC.Tc.Utils.TcMType
    import GHC.Tc.Utils.Zonk
    import GHC.Builtin.Types.Prim
    import GHC.Types.Name
    import GHC.Types.Name.Set
    import GHC.Types.SrcLoc
    import GHC.Core.PatSyn
    import GHC.Utils.Panic
    import GHC.Utils.Outputable
    import GHC.Data.FastString
    import GHC.Types.Var
    import GHC.Types.Var.Env( emptyTidyEnv, mkInScopeSet )
    import GHC.Types.Id
    import GHC.Types.Id.Info( RecSelParent(..), setLevityInfoWithType )
    import GHC.Tc.Gen.Bind
    import GHC.Types.Basic
    import GHC.Tc.Solver
    import GHC.Tc.Utils.Unify
    import GHC.Core.Predicate
    import GHC.Builtin.Types
    import GHC.Tc.Utils.TcType
    import GHC.Tc.Types.Evidence
    import GHC.Tc.Types.Origin
    import GHC.Tc.TyCl.Build
    import GHC.Types.Var.Set
    import GHC.Types.Id.Make
    import GHC.Tc.TyCl.Utils
    import GHC.Core.ConLike
    import GHC.Types.FieldLabel
    import GHC.Rename.Env
    import GHC.Data.Bag
    import GHC.Utils.Misc
    import GHC.Driver.Session ( getDynFlags, xopt_FieldSelectors )
    import Data.Maybe( mapMaybe )
    import Control.Monad ( zipWithM )
    import Data.List( partition, mapAccumL )

    #include "HsVersions.h"

    {-
    ************************************************************************
    *                                                                      *
                        Type checking a pattern synonym
    *                                                                      *
    ************************************************************************
    -}

    tcPatSynDecl :: LocatedA (PatSynBind GhcRn GhcRn)
                 -> TcSigFun
                 -> TcPragEnv -- See Note [Pragmas for pattern synonyms]
                 -> TcM (LHsBinds GhcTc, TcGblEnv)
    tcPatSynDecl (L loc psb@(PSB { psb_id = L _ name })) sig_fn prag_fn
      = setSrcSpanA loc $
        addErrCtxt (text "In the declaration for pattern synonym"
                    <+> quotes (ppr name)) $
        recoverM (recoverPSB psb) $
        case (sig_fn name) of
          Nothing                 -> tcInferPatSynDecl psb prag_fn
          Just (TcPatSynSig tpsi) -> tcCheckPatSynDecl psb tpsi prag_fn
          _                       -> panic "tcPatSynDecl"

    recoverPSB :: PatSynBind GhcRn GhcRn
               -> TcM (LHsBinds GhcTc, TcGblEnv)
    -- See Note [Pattern synonym error recovery]
    recoverPSB (PSB { psb_id = L _ name
                    , psb_args = details })
     = do { matcher_name <- newImplicitBinder name mkMatcherOcc
          ; let placeholder = AConLike $ PatSynCon $
                              mk_placeholder matcher_name
          ; gbl_env <- tcExtendGlobalEnv [placeholder] getGblEnv
          ; return (emptyBag, gbl_env) }
      where
        (_arg_names, is_infix) = collectPatSynArgInfo details
        mk_placeholder matcher_name
          = mkPatSyn name is_infix
                            ([mkTyVarBinder SpecifiedSpec alphaTyVar], []) ([], [])
                            [] -- Arg tys
                            alphaTy
                            (matcher_name, matcher_ty, True) Nothing
                            []  -- Field labels
           where
             -- The matcher_id is used only by the desugarer, so actually
             -- and error-thunk would probably do just as well here.
             matcher_ty = mkSpecForAllTys [alphaTyVar] alphaTy

    {- Note [Pattern synonym error recovery]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If type inference for a pattern synonym fails, we can't continue with
    the rest of tc_patsyn_finish, because we may get knock-on errors, or
    even a crash.  E.g. from
       pattern What = True :: Maybe
    we get a kind error; and we must stop right away (#15289).

    We stop if there are /any/ unsolved constraints, not just insoluble
    ones; because pattern synonyms are top-level things, we will never
    solve them later if we can't solve them now.  And if we were to carry
    on, tc_patsyn_finish does zonkTcTypeToType, which defaults any
    unsolved unificatdion variables to Any, which confuses the error
    reporting no end (#15685).

    So we use simplifyTop to completely solve the constraint, report
    any errors, throw an exception.

    Even in the event of such an error we can recover and carry on, just
    as we do for value bindings, provided we plug in placeholder for the
    pattern synonym: see recoverPSB.  The goal of the placeholder is not
    to cause a raft of follow-on errors.  I've used the simplest thing for
    now, but we might need to elaborate it a bit later.  (e.g.  I've given
    it zero args, which may cause knock-on errors if it is used in a
    pattern.) But it'll do for now.

    -}

    tcInferPatSynDecl :: PatSynBind GhcRn GhcRn
                      -> TcPragEnv
                      -> TcM (LHsBinds GhcTc, TcGblEnv)
    tcInferPatSynDecl (PSB { psb_id = lname@(L _ name), psb_args = details
                           , psb_def = lpat, psb_dir = dir })
                      prag_fn
      = do { traceTc "tcInferPatSynDecl {" $ ppr name

           ; let (arg_names, is_infix) = collectPatSynArgInfo details
           ; (tclvl, wanted, ((lpat', args), pat_ty))
                <- pushLevelAndCaptureConstraints  $
                   tcInferPat PatSyn lpat          $
                   mapM tcLookupId arg_names

           ; let (ex_tvs, prov_dicts) = tcCollectEx lpat'

                 named_taus = (name, pat_ty) : map mk_named_tau args
                 mk_named_tau arg
                   = (getName arg, mkSpecForAllTys ex_tvs (varType arg))
                   -- The mkSpecForAllTys is important (#14552), albeit
                   -- slightly artificial (there is no variable with this funny type).
                   -- We do not want to quantify over variable (alpha::k)
                   -- that mention the existentially-bound type variables
                   -- ex_tvs in its kind k.
                   -- See Note [Type variables whose kind is captured]

           ; ((univ_tvs, req_dicts, ev_binds, _), residual)
                   <- captureConstraints $
                      simplifyInfer tclvl NoRestrictions [] named_taus wanted
           ; top_ev_binds <- checkNoErrs (simplifyTop residual)
           ; addTopEvBinds top_ev_binds $

        do { prov_dicts <- mapM zonkId prov_dicts
           ; let filtered_prov_dicts = mkMinimalBySCs evVarPred prov_dicts
                 -- Filtering: see Note [Remove redundant provided dicts]
                 (prov_theta, prov_evs)
                     = unzip (mapMaybe mkProvEvidence filtered_prov_dicts)
                 req_theta = map evVarPred req_dicts

           -- Report coercions that escape
           -- See Note [Coercions that escape]
           ; args <- mapM zonkId args
           ; let bad_args = [ (arg, bad_cos) | arg <- args ++ prov_dicts
                                  , let bad_cos = filterDVarSet isId $
                                                  (tyCoVarsOfTypeDSet (idType arg))
                                  , not (isEmptyDVarSet bad_cos) ]
           ; mapM_ dependentArgErr bad_args

           -- Report un-quantifiable type variables:
           -- see Note [Unquantified tyvars in a pattern synonym]
           ; dvs <- candidateQTyVarsOfTypes prov_theta
           ; let mk_doc tidy_env
                   = do { (tidy_env2, theta) <- zonkTidyTcTypes tidy_env prov_theta
                        ; return ( tidy_env2
                                 , sep [ text "the provided context:"
                                       , pprTheta theta ] ) }
           ; doNotQuantifyTyVars dvs mk_doc

           ; traceTc "tcInferPatSynDecl }" $ (ppr name $$ ppr ex_tvs)
           ; rec_fields <- lookupConstructorFields name
           ; tc_patsyn_finish lname dir is_infix lpat' prag_fn
                              (mkTyVarBinders InferredSpec univ_tvs
                                , req_theta,  ev_binds, req_dicts)
                              (mkTyVarBinders InferredSpec ex_tvs
                                , mkTyVarTys ex_tvs, prov_theta, prov_evs)
                              (map nlHsVar args, map idType args)
                              pat_ty rec_fields } }

    mkProvEvidence :: EvId -> Maybe (PredType, EvTerm)
    -- See Note [Equality evidence in pattern synonyms]
    mkProvEvidence ev_id
      | EqPred r ty1 ty2 <- classifyPredType pred
      , let k1 = tcTypeKind ty1
            k2 = tcTypeKind ty2
            is_homo = k1 `tcEqType` k2
            homo_tys   = [k1, ty1, ty2]
            hetero_tys = [k1, k2, ty1, ty2]
      = case r of
          ReprEq | is_homo
                 -> Just ( mkClassPred coercibleClass    homo_tys
                         , evDataConApp coercibleDataCon homo_tys eq_con_args )
                 | otherwise -> Nothing
          NomEq  | is_homo
                 -> Just ( mkClassPred eqClass    homo_tys
                         , evDataConApp eqDataCon homo_tys eq_con_args )
                 | otherwise
                 -> Just ( mkClassPred heqClass    hetero_tys
                         , evDataConApp heqDataCon hetero_tys eq_con_args )

      | otherwise
      = Just (pred, EvExpr (evId ev_id))
      where
        pred = evVarPred ev_id
        eq_con_args = [evId ev_id]

    dependentArgErr :: (Id, DTyCoVarSet) -> TcM ()
    -- See Note [Coercions that escape]
    dependentArgErr (arg, bad_cos)
      = failWithTc $  -- fail here: otherwise we get downstream errors
        vcat [ text "Iceland Jack!  Iceland Jack! Stop torturing me!"
             , hang (text "Pattern-bound variable")
                  2 (ppr arg <+> dcolon <+> ppr (idType arg))
             , nest 2 $
               hang (text "has a type that mentions pattern-bound coercion"
                     <> plural bad_co_list <> colon)
                  2 (pprWithCommas ppr bad_co_list)
             , text "Hint: use -fprint-explicit-coercions to see the coercions"
             , text "Probable fix: add a pattern signature" ]
      where
        bad_co_list = dVarSetElems bad_cos

    {- Note [Type variables whose kind is captured]
    ~~-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
      data AST a = Sym [a]
      class Prj s where { prj :: [a] -> Maybe (s a) }
      pattern P x <= Sym (prj -> Just x)

    Here we get a matcher with this type
      $mP :: forall s a. Prj s => AST a -> (s a -> r) -> r -> r

    No problem.  But note that 's' is not fixed by the type of the
    pattern (AST a), nor is it existentially bound.  It's really only
    fixed by the type of the continuation.

    #14552 showed that this can go wrong if the kind of 's' mentions
    existentially bound variables.  We obviously can't make a type like
      $mP :: forall (s::k->*) a. Prj s => AST a -> (forall k. s a -> r)
                                       -> r -> r
    But neither is 's' itself existentially bound, so the forall (s::k->*)
    can't go in the inner forall either.  (What would the matcher apply
    the continuation to?)

    Solution: do not quantiify over any unification variable whose kind
    mentions the existentials.  We can conveniently do that by making the
    "taus" passed to simplifyInfer look like
       forall ex_tvs. arg_ty

    After that, Note [Naughty quantification candidates] in GHC.Tc.Utils.TcMType takes
    over and errors.

    Note [Remove redundant provided dicts]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Recall that
       HRefl :: forall k1 k2 (a1:k1) (a2:k2). (k1 ~ k2, a1 ~ a2)
                                           => a1 :~~: a2
    (NB: technically the (k1~k2) existential dictionary is not necessary,
    but it's there at the moment.)

    Now consider (#14394):
       pattern Foo = HRefl
    in a non-poly-kinded module.  We don't want to get
        pattern Foo :: () => (* ~ *, b ~ a) => a :~~: b
    with that redundant (* ~ *).  We'd like to remove it; hence the call to
    mkMinimalWithSCs.

    Similarly consider
      data S a where { MkS :: Ord a => a -> S a }
      pattern Bam x y <- (MkS (x::a), MkS (y::a)))

    The pattern (Bam x y) binds two (Ord a) dictionaries, but we only
    need one.  Again mkMimimalWithSCs removes the redundant one.

    Note [Equality evidence in pattern synonyms]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
      data X a where
         MkX :: Eq a => [a] -> X (Maybe a)
      pattern P x = MkG x

    Then there is a danger that GHC will infer
      P :: forall a.  () =>
           forall b. (a ~# Maybe b, Eq b) => [b] -> X a

    The 'builder' for P, which is called in user-code, will then
    have type
      $bP :: forall a b. (a ~# Maybe b, Eq b) => [b] -> X a

    and that is bad because (a ~# Maybe b) is not a predicate type
    (see Note [Types for coercions, predicates, and evidence] in GHC.Core.TyCo.Rep
    and is not implicitly instantiated.

    So in mkProvEvidence we lift (a ~# b) to (a ~ b).  Tiresome, and
    marginally less efficient, if the builder/martcher are not inlined.

    See also Note [Lift equality constraints when quantifying] in GHC.Tc.Utils.TcType

    Note [Coercions that escape]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    #14507 showed an example where the inferred type of the matcher
    for the pattern synonym was something like
       $mSO :: forall (r :: TYPE rep) kk (a :: k).
               TypeRep k a
               -> ((Bool ~ k) => TypeRep Bool (a |> co_a2sv) -> r)
               -> (Void# -> r)
               -> r

    What is that co_a2sv :: Bool ~# *??  It was bound (via a superclass
    selection) by the pattern being matched; and indeed it is implicit in
    the context (Bool ~ k).  You could imagine trying to extract it like
    this:
       $mSO :: forall (r :: TYPE rep) kk (a :: k).
               TypeRep k a
               -> ( co :: ((Bool :: *) ~ (k :: *)) =>
                      let co_a2sv = sc_sel co
                      in TypeRep Bool (a |> co_a2sv) -> r)
               -> (Void# -> r)
               -> r

    But we simply don't allow that in types.  Maybe one day but not now.

    How to detect this situation?  We just look for free coercion variables
    in the types of any of the arguments to the matcher.  The error message
    is not very helpful, but at least we don't get a Lint error.

    Note [Unquantified tyvars in a pattern synonym]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider (#21479)

       data T a where MkT :: Int -> T Char   -- A GADT
       foo :: forall b. Bool -> T b          -- Somewhat strange type

       pattern T1 <- (foo -> MkT)

    In the view pattern, foo is instantiated, let's say b :-> b0
    where b0 is a unification variable.  Then matching the GADT
    MkT will add the "provided" constraint b0~Char, so we might infer
       pattern T1 :: () => (b0~Char) => Int -> Bool

    Nothing constrains that `b0`. We don't want to quantify over it.
    We don't want to to zonk to Any (we don't like Any showing up in
    user-visible types).  So we want to error here. See
    Note [Error on unconstrained meta-variables] in GHC.Tc.Utils.TcMType

    Hence the call to doNotQuantifyTyVars here.
    -}

    tcCheckPatSynDecl :: PatSynBind GhcRn GhcRn
                      -> TcPatSynInfo
                      -> TcPragEnv
                      -> TcM (LHsBinds GhcTc, TcGblEnv)
    tcCheckPatSynDecl psb@PSB{ psb_id = lname@(L _ name), psb_args = details
                             , psb_def = lpat, psb_dir = dir }
                      TPSI{ patsig_implicit_bndrs = implicit_bndrs
                          , patsig_univ_bndrs = explicit_univ_bndrs, patsig_req  = req_theta
                          , patsig_ex_bndrs   = explicit_ex_bndrs,   patsig_prov = prov_theta
                          , patsig_body_ty    = sig_body_ty }
                      prag_fn
      = do { traceTc "tcCheckPatSynDecl" $
             vcat [ ppr implicit_bndrs, ppr explicit_univ_bndrs, ppr req_theta
                  , ppr explicit_ex_bndrs, ppr prov_theta, ppr sig_body_ty ]

           ; let decl_arity = length arg_names
                 (arg_names, is_infix) = collectPatSynArgInfo details

           ; (arg_tys, pat_ty) <- case tcSplitFunTysN decl_arity sig_body_ty of
                                     Right stuff  -> return stuff
                                     Left missing -> wrongNumberOfParmsErr name decl_arity missing

           -- Complain about:  pattern P :: () => forall x. x -> P x
           -- The existential 'x' should not appear in the result type
           -- Can't check this until we know P's arity (decl_arity above)
           ; let bad_tvs = filter (`elemVarSet` tyCoVarsOfType pat_ty) $ binderVars explicit_ex_bndrs
           ; checkTc (null bad_tvs) $
             hang (sep [ text "The result type of the signature for" <+> quotes (ppr name) <> comma
                       , text "namely" <+> quotes (ppr pat_ty) ])
                2 (text "mentions existential type variable" <> plural bad_tvs
                   <+> pprQuotedList bad_tvs)

             -- See Note [The pattern-synonym signature splitting rule] in GHC.Tc.Gen.Sig
           ; let univ_fvs = closeOverKinds $
                            (tyCoVarsOfTypes (pat_ty : req_theta) `extendVarSetList` (binderVars explicit_univ_bndrs))
                 (extra_univ, extra_ex) = partition ((`elemVarSet` univ_fvs) . binderVar) implicit_bndrs
                 univ_bndrs = extra_univ ++ explicit_univ_bndrs
                 ex_bndrs   = extra_ex   ++ explicit_ex_bndrs
                 univ_tvs   = binderVars univ_bndrs
                 ex_tvs     = binderVars ex_bndrs

             -- Skolemise the quantified type variables. This is necessary
             -- in order to check the actual pattern type against the
             -- expected type. Even though the tyvars in the type are
             -- already skolems, this step changes their TcLevels,
             -- avoiding level-check errors when unifying.
           ; (skol_subst0, skol_univ_bndrs) <- skolemiseTvBndrsX emptyTCvSubst univ_bndrs
           ; (skol_subst, skol_ex_bndrs)    <- skolemiseTvBndrsX skol_subst0   ex_bndrs
           ; let skol_univ_tvs   = binderVars skol_univ_bndrs
                 skol_ex_tvs     = binderVars skol_ex_bndrs
                 skol_req_theta  = substTheta skol_subst0 req_theta
                 skol_prov_theta = substTheta skol_subst  prov_theta
                 skol_arg_tys    = substTys   skol_subst  (map scaledThing arg_tys)
                 skol_pat_ty     = substTy    skol_subst  pat_ty

                 univ_tv_prs     = [ (getName orig_univ_tv, skol_univ_tv)
                                   | (orig_univ_tv, skol_univ_tv) <- univ_tvs `zip` skol_univ_tvs ]

           -- Right!  Let's check the pattern against the signature
           -- See Note [Checking against a pattern signature]
           ; req_dicts <- newEvVars skol_req_theta
           ; (tclvl, wanted, (lpat', (ex_tvs', prov_dicts, args'))) <-
               ASSERT2( equalLength arg_names arg_tys, ppr name $$ ppr arg_names $$ ppr arg_tys )
               pushLevelAndCaptureConstraints   $
               tcExtendNameTyVarEnv univ_tv_prs $
               tcCheckPat PatSyn lpat (unrestricted skol_pat_ty)   $
               do { let in_scope    = mkInScopeSet (mkVarSet skol_univ_tvs)
                        empty_subst = mkEmptyTCvSubst in_scope
                  ; (inst_subst, ex_tvs') <- mapAccumLM newMetaTyVarX empty_subst skol_ex_tvs
                        -- newMetaTyVarX: see the "Existential type variables"
                        -- part of Note [Checking against a pattern signature]
                  ; traceTc "tcpatsyn1" (vcat [ ppr v <+> dcolon <+> ppr (tyVarKind v) | v <- ex_tvs])
                  ; traceTc "tcpatsyn2" (vcat [ ppr v <+> dcolon <+> ppr (tyVarKind v) | v <- ex_tvs'])
                  ; let prov_theta' = substTheta inst_subst skol_prov_theta
                      -- Add univ_tvs to the in_scope set to
                      -- satisfy the substitution invariant. There's no need to
                      -- add 'ex_tvs' as they are already in the domain of the
                      -- substitution.
                      -- See also Note [The substitution invariant] in GHC.Core.TyCo.Subst.
                  ; prov_dicts <- mapM (emitWanted (ProvCtxtOrigin psb)) prov_theta'
                  ; args'      <- zipWithM (tc_arg inst_subst) arg_names
                                           skol_arg_tys
                  ; return (ex_tvs', prov_dicts, args') }

           ; let skol_info = SigSkol (PatSynCtxt name) pat_ty []
                             -- The type here is a bit bogus, but we do not print
                             -- the type for PatSynCtxt, so it doesn't matter
                             -- See Note [Skolem info for pattern synonyms] in "GHC.Tc.Types.Origin"
           ; (implics, ev_binds) <- buildImplicationFor tclvl skol_info skol_univ_tvs
                                                        req_dicts wanted

           -- Solve the constraints now, because we are about to make a PatSyn,
           -- which should not contain unification variables and the like (#10997)
           ; simplifyTopImplic implics

           -- ToDo: in the bidirectional case, check that the ex_tvs' are all distinct
           -- Otherwise we may get a type error when typechecking the builder,
           -- when that should be impossible

           ; traceTc "tcCheckPatSynDecl }" $ ppr name

           ; rec_fields <- lookupConstructorFields name
           ; tc_patsyn_finish lname dir is_infix lpat' prag_fn
                              (skol_univ_bndrs, skol_req_theta, ev_binds, req_dicts)
                              (skol_ex_bndrs, mkTyVarTys ex_tvs', skol_prov_theta, prov_dicts)
                              (args', skol_arg_tys)
                              skol_pat_ty rec_fields }
      where
        tc_arg :: TCvSubst -> Name -> Type -> TcM (LHsExpr GhcTc)
         -- Look up the variable actually bound by lpat
         -- and check that it has the expected type
        tc_arg subst arg_name arg_ty
          = setSrcSpan (nameSrcSpan arg_name) $
               -- Set the SrcSpan to be the binding site of the Id (#18856)
               -- e.g.  pattern P :: Int -> Maybe (Int,Bool)
               --       pattern P x = Just (x,True)
               -- Before unifying x's actual type with its expected type, in tc_arg, set
               -- location to x's binding site in lpat, namely the 'x' in Just (x,True).
               -- Else the error message location is wherever tcCheckPat finished,
               -- namely the right-hand corner of the pattern
            do { arg_id <- tcLookupId arg_name
               ; wrap <- tcSubTypeSigma (OccurrenceOf (idName arg_id))
                                        GenSigCtxt
                                        (idType arg_id)
                                        (substTy subst arg_ty)
                    -- Why do we need tcSubType here?
                    -- See Note [Pattern synonyms and higher rank types]
               ; return (mkLHsWrap wrap $ nlHsVar arg_id) }

    skolemiseTvBndrsX :: TCvSubst -> [VarBndr TyVar flag]
                      -> TcM (TCvSubst, [VarBndr TcTyVar flag])
    -- Make new TcTyVars, all skolems with levels, but do not clone
    -- The level is one level deeper than the current level
    -- See Note [Skolemising when checking a pattern synonym]
    skolemiseTvBndrsX orig_subst tvs
      = do { tc_lvl <- getTcLevel
           ; let pushed_lvl = pushTcLevel tc_lvl
                 details    = SkolemTv pushed_lvl False

                 mk_skol_tv_x :: TCvSubst -> VarBndr TyVar flag
                              -> (TCvSubst, VarBndr TcTyVar flag)
                 mk_skol_tv_x subst (Bndr tv flag)
                   = (subst', Bndr new_tv flag)
                   where
                     new_kind = substTyUnchecked subst (tyVarKind tv)
                     new_tv   = mkTcTyVar (tyVarName tv) new_kind details
                     subst'   = extendTvSubstWithClone subst tv new_tv

           ; return (mapAccumL mk_skol_tv_x orig_subst tvs) }

    {- Note [Skolemising when checking a pattern synonym]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
       pattern P1 :: forall a. a -> Maybe a
       pattern P1 x <- Just x where
          P1 x = Just (x :: a)

    The scoped type variable 'a' scopes over the builder RHS, Just (x::a).
    But the builder RHS is typechecked much later in tcPatSynBuilderBind,
    and gets its scoped type variables from the type of the builder_id.
    The easiest way to achieve this is not to clone when skolemising.

    Hence a special-purpose skolemiseTvBndrX here, similar to
    GHC.Tc.Utils.Instantiate.tcInstSkolTyVarsX except that the latter
    does cloning.

    [Pattern synonyms and higher rank types]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
      data T = MkT (forall a. a->a)

      pattern P :: (Int -> Int) -> T
      pattern P x <- MkT x

    This should work.  But in the matcher we must match against MkT, and then
    instantiate its argument 'x', to get a function of type (Int -> Int).
    Equality is not enough!  #13752 was an example.

    Note [The pattern-synonym signature splitting rule]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Given a pattern signature, we must split
         the kind-generalised variables, and
         the implicitly-bound variables
    into universal and existential.  The rule is this
    (see discussion on #11224):

         The universal tyvars are the ones mentioned in
              - univ_tvs: the user-specified (forall'd) universals
              - req_theta
              - res_ty
         The existential tyvars are all the rest

    For example

       pattern P :: () => b -> T a
       pattern P x = ...

    Here 'a' is universal, and 'b' is existential.  But there is a wrinkle:
    how do we split the arg_tys from req_ty?  Consider

       pattern Q :: () => b -> S c -> T a
       pattern Q x = ...

    This is an odd example because Q has only one syntactic argument, and
    so presumably is defined by a view pattern matching a function.  But
    it can happen (#11977, #12108).

    We don't know Q's arity from the pattern signature, so we have to wait
    until we see the pattern declaration itself before deciding res_ty is,
    and hence which variables are existential and which are universal.

    And that in turn is why TcPatSynInfo has a separate field,
    patsig_implicit_bndrs, to capture the implicitly bound type variables,
    because we don't yet know how to split them up.

    It's a slight compromise, because it means we don't really know the
    pattern synonym's real signature until we see its declaration.  So,
    for example, in hs-boot file, we may need to think what to do...
    (eg don't have any implicitly-bound variables).

    Note [Checking against a pattern signature]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When checking the actual supplied pattern against the pattern synonym
    signature, we need to be quite careful.

    ----- Provided constraints
    Example

        data T a where
          MkT :: Ord a => a -> T a

        pattern P :: () => Eq a => a -> [T a]
        pattern P x = [MkT x]

    We must check that the (Eq a) that P claims to bind (and to
    make available to matches against P), is derivable from the
    actual pattern.  For example:
        f (P (x::a)) = ...here (Eq a) should be available...
    And yes, (Eq a) is derivable from the (Ord a) bound by P's rhs.

    ----- Existential type variables
    Unusually, we instantiate the existential tyvars of the pattern with
    *meta* type variables.  For example

        data S where
          MkS :: Eq a => [a] -> S

        pattern P :: () => Eq x => x -> S
        pattern P x <- MkS x

    The pattern synonym conceals from its client the fact that MkS has a
    list inside it.  The client just thinks it's a type 'x'.  So we must
    unify x := [a] during type checking, and then use the instantiating type
    [a] (called ex_tys) when building the matcher.  In this case we'll get

       $mP :: S -> (forall x. Ex x => x -> r) -> r -> r
       $mP x k = case x of
                   MkS a (d:Eq a) (ys:[a]) -> let dl :: Eq [a]
                                                  dl = $dfunEqList d
                                              in k [a] dl ys

    All this applies when type-checking the /matching/ side of
    a pattern synonym.  What about the /building/ side?

    * For Unidirectional, there is no builder

    * For ExplicitBidirectional, the builder is completely separate
      code, typechecked in tcPatSynBuilderBind

    * For ImplicitBidirectional, the builder is still typechecked in
      tcPatSynBuilderBind, by converting the pattern to an expression and
      typechecking it.

      At one point, for ImplicitBidirectional I used TyVarTvs (instead of
      TauTvs) in tcCheckPatSynDecl.  But (a) strengthening the check here
      is redundant since tcPatSynBuilderBind does the job, (b) it was
      still incomplete (TyVarTvs can unify with each other), and (c) it
      didn't even work (#13441 was accepted with
      ExplicitBidirectional, but rejected if expressed in
      ImplicitBidirectional form.  Conclusion: trying to be too clever is
      a bad idea.
    -}

    collectPatSynArgInfo :: HsPatSynDetails GhcRn
                         -> ([Name], Bool)
    collectPatSynArgInfo details =
      case details of
        PrefixCon _ names    -> (map unLoc names, False)
        InfixCon name1 name2 -> (map unLoc [name1, name2], True)
        RecCon names         -> (map (unLoc . recordPatSynPatVar) names, False)

    wrongNumberOfParmsErr :: Name -> Arity -> Arity -> TcM a
    wrongNumberOfParmsErr name decl_arity missing
      = failWithTc $
        hang (text "Pattern synonym" <+> quotes (ppr name) <+> ptext (sLit "has")
              <+> speakNOf decl_arity (text "argument"))
           2 (text "but its type signature has" <+> int missing <+> text "fewer arrows")

    -------------------------
    -- Shared by both tcInferPatSyn and tcCheckPatSyn
    tc_patsyn_finish :: LocatedN Name   -- ^ PatSyn Name
                     -> HsPatSynDir GhcRn -- ^ PatSyn type (Uni/Bidir/ExplicitBidir)
                     -> Bool              -- ^ Whether infix
                     -> LPat GhcTc        -- ^ Pattern of the PatSyn
                     -> TcPragEnv
                     -> ([TcInvisTVBinder], [PredType], TcEvBinds, [EvVar])
                     -> ([TcInvisTVBinder], [TcType], [PredType], [EvTerm])
                     -> ([LHsExpr GhcTc], [TcType])  -- ^ Pattern arguments and types
                     -> TcType            -- ^ Pattern type
                     -> [FieldLabel]      -- ^ Selector names
                     -- ^ Whether fields, empty if not record PatSyn
                     -> TcM (LHsBinds GhcTc, TcGblEnv)
    tc_patsyn_finish lname dir is_infix lpat' prag_fn
                     (univ_tvs, req_theta, req_ev_binds, req_dicts)
                     (ex_tvs,   ex_tys,    prov_theta,   prov_dicts)
                     (args, arg_tys)
                     pat_ty field_labels
      = do { -- Zonk everything.  We are about to build a final PatSyn
             -- so there had better be no unification variables in there

           ; ze              <- mkEmptyZonkEnv NoFlexi
           ; (ze, univ_tvs') <- zonkTyVarBindersX   ze univ_tvs
           ; req_theta'      <- zonkTcTypesToTypesX ze req_theta
           ; (ze, ex_tvs')   <- zonkTyVarBindersX   ze ex_tvs
           ; prov_theta'     <- zonkTcTypesToTypesX ze prov_theta
           ; pat_ty'         <- zonkTcTypeToTypeX   ze pat_ty
           ; arg_tys'        <- zonkTcTypesToTypesX ze arg_tys

           ; let (env1, univ_tvs) = tidyTyCoVarBinders emptyTidyEnv univ_tvs'
                 (env2, ex_tvs)   = tidyTyCoVarBinders env1 ex_tvs'
                 req_theta  = tidyTypes env2 req_theta'
                 prov_theta = tidyTypes env2 prov_theta'
                 arg_tys    = tidyTypes env2 arg_tys'
                 pat_ty     = tidyType  env2 pat_ty'

           ; traceTc "tc_patsyn_finish {" $
               ppr (unLoc lname) $$ ppr (unLoc lpat') $$
               ppr (univ_tvs, req_theta, req_ev_binds, req_dicts) $$
               ppr (ex_tvs, prov_theta, prov_dicts) $$
               ppr args $$
               ppr arg_tys $$
               ppr pat_ty

           -- Make the 'matcher'
           ; (matcher, matcher_bind) <- tcPatSynMatcher lname lpat' prag_fn
                                             (binderVars univ_tvs, req_theta, req_ev_binds, req_dicts)
                                             (binderVars ex_tvs, ex_tys, prov_theta, prov_dicts)
                                             (args, arg_tys)
                                             pat_ty

           -- Make the 'builder'
           ; builder <- mkPatSynBuilder dir lname
                                        univ_tvs req_theta
                                        ex_tvs   prov_theta
                                        arg_tys pat_ty

           -- Make the PatSyn itself
           ; let patSyn = mkPatSyn (unLoc lname) is_infix
                            (univ_tvs, req_theta)
                            (ex_tvs, prov_theta)
                            arg_tys
                            pat_ty
                            matcher builder
                            field_labels

           -- Selectors
           ; has_sel <- xopt_FieldSelectors <$> getDynFlags
           ; let rn_rec_sel_binds = mkPatSynRecSelBinds patSyn (patSynFieldLabels patSyn) has_sel
                 tything = AConLike (PatSynCon patSyn)
           ; tcg_env <- tcExtendGlobalEnv [tything] $
                        tcRecSelBinds rn_rec_sel_binds

           ; traceTc "tc_patsyn_finish }" empty
           ; return (matcher_bind, tcg_env) }

    {-
    ************************************************************************
    *                                                                      *
             Constructing the "matcher" Id and its binding
    *                                                                      *
    ************************************************************************
    -}

    tcPatSynMatcher :: LocatedN Name
                    -> LPat GhcTc
                    -> TcPragEnv
                    -> ([TcTyVar], ThetaType, TcEvBinds, [EvVar])
                    -> ([TcTyVar], [TcType], ThetaType, [EvTerm])
                    -> ([LHsExpr GhcTc], [TcType])
                    -> TcType
                    -> TcM (PatSynMatcher, LHsBinds GhcTc)
    -- See Note [Matchers and builders for pattern synonyms] in GHC.Core.PatSyn
    tcPatSynMatcher (L loc ps_name) lpat prag_fn
                    (univ_tvs, req_theta, req_ev_binds, req_dicts)
                    (ex_tvs, ex_tys, prov_theta, prov_dicts)
                    (args, arg_tys) pat_ty
      = do { let loc' = locA loc
           ; rr_name <- newNameAt (mkTyVarOcc "rep") loc'
           ; tv_name <- newNameAt (mkTyVarOcc "r")   loc'
           ; let rr_tv  = mkTyVar rr_name runtimeRepTy
                 rr     = mkTyVarTy rr_tv
                 res_tv = mkTyVar tv_name (tYPE rr)
                 res_ty = mkTyVarTy res_tv
                 is_unlifted = null args && null prov_dicts
                 (cont_args, cont_arg_tys)
                   | is_unlifted = ([nlHsVar voidPrimId], [unboxedUnitTy])
                   | otherwise   = (args,                 arg_tys)
                 cont_ty = mkInfSigmaTy ex_tvs prov_theta $
                           mkVisFunTysMany cont_arg_tys res_ty

                 fail_ty  = mkVisFunTyMany unboxedUnitTy res_ty

           ; matcher_name <- newImplicitBinder ps_name mkMatcherOcc
           ; scrutinee    <- newSysLocalId (fsLit "scrut") Many pat_ty
           ; cont         <- newSysLocalId (fsLit "cont")  Many cont_ty
           ; fail         <- newSysLocalId (fsLit "fail")  Many fail_ty

           ; dflags       <- getDynFlags
           ; let matcher_tau   = mkVisFunTysMany [pat_ty, cont_ty, fail_ty] res_ty
                 matcher_sigma = mkInfSigmaTy (rr_tv:res_tv:univ_tvs) req_theta matcher_tau
                 matcher_id    = mkExportedVanillaId matcher_name matcher_sigma
                                 -- See Note [Exported LocalIds] in GHC.Types.Id

                 inst_wrap = mkWpEvApps prov_dicts <.> mkWpTyApps ex_tys
                 cont' = foldl' nlHsApp (mkLHsWrap inst_wrap (nlHsVar cont)) cont_args

                 fail' = nlHsApps fail [nlHsVar voidPrimId]

                 args = map nlVarPat [scrutinee, cont, fail]
                 lwpat = noLocA $ WildPat pat_ty
                 cases = if isIrrefutableHsPat dflags lpat
                         then [mkHsCaseAlt lpat  cont']
                         else [mkHsCaseAlt lpat  cont',
                               mkHsCaseAlt lwpat fail']
                 body = mkLHsWrap (mkWpLet req_ev_binds) $
                        L (getLoc lpat) $
                        HsCase noExtField (nlHsVar scrutinee) $
                        MG{ mg_alts = L (l2l $ getLoc lpat) cases
                          , mg_ext = MatchGroupTc [unrestricted pat_ty] res_ty
                          , mg_origin = Generated
                          }
                 body' = noLocA $
                         HsLam noExtField $
                         MG{ mg_alts = noLocA [mkSimpleMatch LambdaExpr
                                                             args body]
                           , mg_ext = MatchGroupTc (map unrestricted [pat_ty, cont_ty, fail_ty]) res_ty
                           , mg_origin = Generated
                           }
                 match = mkMatch (mkPrefixFunRhs (L loc ps_name)) []
                                 (mkHsLams (rr_tv:res_tv:univ_tvs)
                                           req_dicts body')
                                 (EmptyLocalBinds noExtField)
                 mg :: MatchGroup GhcTc (LHsExpr GhcTc)
                 mg = MG{ mg_alts = L (l2l $ getLoc match) [match]
                        , mg_ext = MatchGroupTc [] res_ty
                        , mg_origin = Generated
                        }
                 matcher_arity = length req_theta + 3
                 -- See Note [Pragmas for pattern synonyms]

           -- Add INLINE pragmas; see Note [Pragmas for pattern synonyms]
           -- NB: prag_fn is keyed by the PatSyn Name, not the (internal) matcher name
           ; matcher_prag_id <- addInlinePrags matcher_id              $
                                map (addInlinePragArity matcher_arity) $
                                lookupPragEnv prag_fn ps_name

           ; let bind = FunBind{ fun_id = L loc matcher_prag_id
                               , fun_matches = mg
                               , fun_ext = idHsWrapper
                               , fun_tick = [] }
                 matcher_bind = unitBag (noLocA bind)
           ; traceTc "tcPatSynMatcher" (ppr ps_name $$ ppr (idType matcher_id))
           ; traceTc "tcPatSynMatcher" (ppr matcher_bind)

           ; return ((matcher_name, matcher_sigma, is_unlifted), matcher_bind) }

    mkPatSynRecSelBinds :: PatSyn
                        -> [FieldLabel]  -- ^ Visible field labels
                        -> FieldSelectors
                        -> [(Id, LHsBind GhcRn)]
    mkPatSynRecSelBinds ps fields has_sel
      = [ mkOneRecordSelector [PatSynCon ps] (RecSelPatSyn ps) fld_lbl has_sel
        | fld_lbl <- fields ]

    isUnidirectional :: HsPatSynDir a -> Bool
    isUnidirectional Unidirectional          = True
    isUnidirectional ImplicitBidirectional   = False
    isUnidirectional ExplicitBidirectional{} = False

    {-
    ************************************************************************
    *                                                                      *
             Constructing the "builder" Id
    *                                                                      *
    ************************************************************************
    -}

    mkPatSynBuilder :: HsPatSynDir a -> LocatedN Name
                    -> [InvisTVBinder] -> ThetaType
                    -> [InvisTVBinder] -> ThetaType
                    -> [Type] -> Type
                    -> TcM PatSynBuilder
    mkPatSynBuilder dir (L _ name)
                      univ_bndrs req_theta ex_bndrs prov_theta
                      arg_tys pat_ty
      | isUnidirectional dir
      = return Nothing
      | otherwise
      = do { builder_name <- newImplicitBinder name mkBuilderOcc
           ; let theta          = req_theta ++ prov_theta
                 need_dummy_arg = isUnliftedType pat_ty && null arg_tys && null theta
                 builder_sigma  = add_void need_dummy_arg $
                                  mkInvisForAllTys univ_bndrs $
                                  mkInvisForAllTys ex_bndrs $
                                  mkPhiTy theta $
                                  mkVisFunTysMany arg_tys $
                                  pat_ty

           ; return (Just (builder_name, builder_sigma, need_dummy_arg)) }

    tcPatSynBuilderBind :: TcPragEnv
                        -> PatSynBind GhcRn GhcRn
                        -> TcM (LHsBinds GhcTc)
    -- See Note [Matchers and builders for pattern synonyms] in GHC.Core.PatSyn
    tcPatSynBuilderBind prag_fn (PSB { psb_id = ps_lname@(L loc ps_name)
                                     , psb_def = lpat
                                     , psb_dir = dir
                                     , psb_args = details })
      | isUnidirectional dir
      = return emptyBag

      | Left why <- mb_match_group       -- Can't invert the pattern
      = setSrcSpan (getLocA lpat) $ failWithTc $
        vcat [ hang (text "Invalid right-hand side of bidirectional pattern synonym"
                     <+> quotes (ppr ps_name) <> colon)
                  2 why
             , text "RHS pattern:" <+> ppr lpat ]

      | Right match_group <- mb_match_group  -- Bidirectional
      = do { patsyn <- tcLookupPatSyn ps_name
           ; case patSynBuilder patsyn of {
               Nothing -> return emptyBag ;
                 -- This case happens if we found a type error in the
                 -- pattern synonym, recovered, and put a placeholder
                 -- with patSynBuilder=Nothing in the environment

               Just (builder_name, builder_ty, need_dummy_arg) ->  -- Normal case
        do { -- Bidirectional, so patSynBuilder returns Just
             let pat_ty = patSynResultType patsyn
                 builder_id = modifyIdInfo (`setLevityInfoWithType` pat_ty) $
                              mkExportedVanillaId builder_name builder_ty
                             -- See Note [Exported LocalIds] in GHC.Types.Id

                 (_, req_theta, _, prov_theta, arg_tys, _) = patSynSigBndr patsyn
                 builder_arity = length req_theta + length prov_theta
                                 + length arg_tys
                                 + (if need_dummy_arg then 1 else 0)

           -- Add INLINE pragmas; see Note [Pragmas for pattern synonyms]
           -- NB: prag_fn is keyed by the PatSyn Name, not the (internal) builder name
           ; builder_id <- addInlinePrags builder_id              $
                           map (addInlinePragArity builder_arity) $
                           lookupPragEnv prag_fn ps_name

           ; let match_group' | need_dummy_arg = add_dummy_arg match_group
                              | otherwise      = match_group

                 bind = FunBind { fun_id      = L loc (idName builder_id)
                                , fun_matches = match_group'
                                , fun_ext     = emptyNameSet
                                , fun_tick    = [] }

                 sig = completeSigFromId (PatSynCtxt ps_name) builder_id

           ; traceTc "tcPatSynBuilderBind {" $
             vcat [ ppr patsyn
                  , ppr builder_id <+> dcolon <+> ppr (idType builder_id) ]
           ; (builder_binds, _) <- tcPolyCheck emptyPragEnv sig (noLocA bind)
           ; traceTc "tcPatSynBuilderBind }" $ ppr builder_binds
           ; return builder_binds } } }

    #if __GLASGOW_HASKELL__ <= 810
      | otherwise = panic "tcPatSynBuilderBind"  -- Both cases dealt with
    #endif
      where
        mb_match_group
           = case dir of
               ExplicitBidirectional explicit_mg -> Right explicit_mg
               ImplicitBidirectional -> fmap mk_mg (tcPatToExpr ps_name args lpat)
               Unidirectional -> panic "tcPatSynBuilderBind"

        mk_mg :: LHsExpr GhcRn -> MatchGroup GhcRn (LHsExpr GhcRn)
        mk_mg body = mkMatchGroup Generated (noLocA [builder_match])
              where
                builder_args  = [L (na2la loc) (VarPat noExtField (L loc n))
                                | L loc n <- args]
                builder_match = mkMatch (mkPrefixFunRhs ps_lname)
                                        builder_args body
                                        (EmptyLocalBinds noExtField)

        args = case details of
                  PrefixCon _ args   -> args
                  InfixCon arg1 arg2 -> [arg1, arg2]
                  RecCon args        -> map recordPatSynPatVar args

        add_dummy_arg :: MatchGroup GhcRn (LHsExpr GhcRn)
                      -> MatchGroup GhcRn (LHsExpr GhcRn)
        add_dummy_arg mg@(MG { mg_alts =
                               (L l [L loc match@(Match { m_pats = pats })]) })
          = mg { mg_alts = L l [L loc (match { m_pats = nlWildPatName : pats })] }
        add_dummy_arg other_mg = pprPanic "add_dummy_arg" $
                                 pprMatches other_mg

    patSynBuilderOcc :: PatSyn -> Maybe (HsExpr GhcTc, TcSigmaType)
    patSynBuilderOcc ps
      | Just (_, builder_ty, add_void_arg) <- patSynBuilder ps
      , let builder_expr = HsConLikeOut noExtField (PatSynCon ps)
      = Just $
        if add_void_arg
        then ( builder_expr   -- still just return builder_expr; the void# arg
                              -- is added by dsConLike in the desugarer
             , tcFunResultTy builder_ty )
        else (builder_expr, builder_ty)

      | otherwise  -- Unidirectional
      = Nothing

    add_void :: Bool -> Type -> Type
    add_void need_dummy_arg ty
      | need_dummy_arg = mkVisFunTyMany unboxedUnitTy ty
      | otherwise      = ty

    tcPatToExpr :: Name -> [LocatedN Name] -> LPat GhcRn
                -> Either SDoc (LHsExpr GhcRn)
    -- Given a /pattern/, return an /expression/ that builds a value
    -- that matches the pattern.  E.g. if the pattern is (Just [x]),
    -- the expression is (Just [x]).  They look the same, but the
    -- input uses constructors from HsPat and the output uses constructors
    -- from HsExpr.
    --
    -- Returns (Left r) if the pattern is not invertible, for reason r.
    -- See Note [Builder for a bidirectional pattern synonym]
    tcPatToExpr name args pat = go pat
      where
        lhsVars = mkNameSet (map unLoc args)

        -- Make a prefix con for prefix and infix patterns for simplicity
        mkPrefixConExpr :: LocatedN Name -> [LPat GhcRn]
                        -> Either SDoc (HsExpr GhcRn)
        mkPrefixConExpr lcon@(L loc _) pats
          = do { exprs <- mapM go pats
               ; let con = L (l2l loc) (HsVar noExtField lcon)
               ; return (unLoc $ mkHsApps con exprs)
               }

        mkRecordConExpr :: LocatedN Name -> HsRecFields GhcRn (LPat GhcRn)
                        -> Either SDoc (HsExpr GhcRn)
        mkRecordConExpr con (HsRecFields fields dd)
          = do { exprFields <- mapM go' fields
               ; return (RecordCon noExtField con (HsRecFields exprFields dd)) }

        go' :: LHsRecField GhcRn (LPat GhcRn) -> Either SDoc (LHsRecField GhcRn (LHsExpr GhcRn))
        go' (L l rf) = L l <$> traverse go rf

        go :: LPat GhcRn -> Either SDoc (LHsExpr GhcRn)
        go (L loc p) = L loc <$> go1 p

        go1 :: Pat GhcRn -> Either SDoc (HsExpr GhcRn)
        go1 (ConPat NoExtField con info)
          = case info of
              PrefixCon _ ps -> mkPrefixConExpr con ps
              InfixCon l r   -> mkPrefixConExpr con [l,r]
              RecCon fields  -> mkRecordConExpr con fields

        go1 (SigPat _ pat _) = go1 (unLoc pat)
            -- See Note [Type signatures and the builder expression]

        go1 (VarPat _ (L l var))
            | var `elemNameSet` lhsVars
            = return $ HsVar noExtField (L l var)
            | otherwise
            = Left (quotes (ppr var) <+> text "is not bound by the LHS of the pattern synonym")
        go1 (ParPat _ pat)          = fmap (HsPar noAnn) $ go pat
        go1 p@(ListPat reb pats)
          | Nothing <- reb = do { exprs <- mapM go pats
                                ; return $ ExplicitList noExtField exprs }
          | otherwise                   = notInvertibleListPat p
        go1 (TuplePat _ pats box)       = do { exprs <- mapM go pats
                                             ; return $ ExplicitTuple noExtField
                                               (map (Present noAnn) exprs) box }
        go1 (SumPat _ pat alt arity)    = do { expr <- go1 (unLoc pat)
                                             ; return $ ExplicitSum noExtField alt arity
                                                                       (noLocA expr)
                                             }
        go1 (LitPat _ lit)              = return $ HsLit noComments lit
        go1 (NPat _ (L _ n) mb_neg _)
            | Just (SyntaxExprRn neg) <- mb_neg
                                        = return $ unLoc $ foldl' nlHsApp (noLocA neg)
                                                           [noLocA (HsOverLit noAnn n)]
            | otherwise                 = return $ HsOverLit noAnn n
        go1 (SplicePat _ (HsSpliced _ _ (HsSplicedPat pat)))
                                        = go1 pat
        go1 (SplicePat _ (HsSpliced{})) = panic "Invalid splice variety"

        -- The following patterns are not invertible.
        go1 p@(BangPat {})                       = notInvertible p -- #14112
        go1 p@(LazyPat {})                       = notInvertible p
        go1 p@(WildPat {})                       = notInvertible p
        go1 p@(AsPat {})                         = notInvertible p
        go1 p@(ViewPat {})                       = notInvertible p
        go1 p@(NPlusKPat {})                     = notInvertible p
        go1 p@(SplicePat _ (HsTypedSplice {}))   = notInvertible p
        go1 p@(SplicePat _ (HsUntypedSplice {})) = notInvertible p
        go1 p@(SplicePat _ (HsQuasiQuote {}))    = notInvertible p

        notInvertible p = Left (not_invertible_msg p)

        not_invertible_msg p
          =   text "Pattern" <+> quotes (ppr p) <+> text "is not invertible"
          $+$ hang (text "Suggestion: instead use an explicitly bidirectional"
                    <+> text "pattern synonym, e.g.")
                 2 (hang (text "pattern" <+> pp_name <+> pp_args <+> larrow
                          <+> ppr pat <+> text "where")
                       2 (pp_name <+> pp_args <+> equals <+> text "..."))
          where
            pp_name = ppr name
            pp_args = hsep (map ppr args)

        -- We should really be able to invert list patterns, even when
        -- rebindable syntax is on, but doing so involves a bit of
        -- refactoring; see #14380.  Until then we reject with a
        -- helpful error message.
        notInvertibleListPat p
          = Left (vcat [ not_invertible_msg p
                       , text "Reason: rebindable syntax is on."
                       , text "This is fixable: add use-case to #14380" ])

    {- Note [Builder for a bidirectional pattern synonym]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    For a bidirectional pattern synonym we need to produce an /expression/
    that matches the supplied /pattern/, given values for the arguments
    of the pattern synonym.  For example
      pattern F x y = (Just x, [y])
    The 'builder' for F looks like
      $builderF x y = (Just x, [y])

    We can't always do this:
     * Some patterns aren't invertible; e.g. view patterns
          pattern F x = (reverse -> x:_)

     * The RHS pattern might bind more variables than the pattern
       synonym, so again we can't invert it
          pattern F x = (x,y)

     * Ditto wildcards
          pattern F x = (x,_)

    Note [Redundant constraints for builder]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The builder can have redundant constraints, which are awkward to eliminate.
    Consider
       pattern P = Just 34
    To match against this pattern we need (Eq a, Num a).  But to build
    (Just 34) we need only (Num a).  Fortunately instTcSigFromId sets
    sig_warn_redundant to False.

    ************************************************************************
    *                                                                      *
             Helper functions
    *                                                                      *
    ************************************************************************

    Note [As-patterns in pattern synonym definitions]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The rationale for rejecting as-patterns in pattern synonym definitions
    is that an as-pattern would introduce nonindependent pattern synonym
    arguments, e.g. given a pattern synonym like:

            pattern K x y = x@(Just y)

    one could write a nonsensical function like

            f (K Nothing x) = ...

    or
            g (K (Just True) False) = ...

    Note [Type signatures and the builder expression]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
       pattern L x = Left x :: Either [a] [b]

    In tc{Infer/Check}PatSynDecl we will check that the pattern has the
    specified type.  We check the pattern *as a pattern*, so the type
    signature is a pattern signature, and so brings 'a' and 'b' into
    scope.  But we don't have a way to bind 'a, b' in the LHS, as we do
    'x', say.  Nevertheless, the signature may be useful to constrain
    the type.

    When making the binding for the *builder*, though, we don't want
      $buildL x = Left x :: Either [a] [b]
    because that wil either mean (forall a b. Either [a] [b]), or we'll
    get a complaint that 'a' and 'b' are out of scope. (Actually the
    latter; #9867.)  No, the job of the signature is done, so when
    converting the pattern to an expression (for the builder RHS) we
    simply discard the signature.

    Note [Record PatSyn Desugaring]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    It is important that prov_theta comes before req_theta as this ordering is used
    when desugaring record pattern synonym updates.

    Any change to this ordering should make sure to change GHC.HsToCore.Expr if you
    want to avoid difficult to decipher core lint errors!

    Note [Pragmas for pattern synonyms]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    INLINE and NOINLINE pragmas are supported for pattern synonyms.
    They affect both the matcher and the builder.
    (See Note [Matchers and builders for pattern synonyms] in PatSyn)

    For example:
        pattern InlinedPattern x = [x]
        {-# INLINE InlinedPattern #-}

        pattern NonInlinedPattern x = [x]
        {-# NOINLINE NonInlinedPattern #-}

    For pattern synonyms with explicit builders, only a pragma for the
    entire pattern synonym is supported. For example:
        pattern HeadC x <- x:xs where
          HeadC x = [x]
          -- This wouldn't compile: {-# INLINE HeadC #-}
        {-# INLINE HeadC #-} -- But this works

    When no pragma is provided for a pattern, the inlining decision might change
    between different versions of GHC.

    Implementation notes.  The prag_fn passed in to tcPatSynDecl will have a binding
    for the /pattern synonym/ Name, thus
          InlinedPattern :-> INLINE
    From this we cook up an INLINE pragma for the matcher (in tcPatSynMatcher)
    and builder (in tcPatSynBuilderBind), by looking up the /pattern synonym/
    Name in the prag_fn, and then using addInlinePragArity to add the right
    inl_sat field to that INLINE pragma for the matcher or builder respectively.
     -}

    -- Walk the whole pattern and for all ConPatOuts, collect the
    -- existentially-bound type variables and evidence binding variables.
    --
    -- These are used in computing the type of a pattern synonym and also
    -- in generating matcher functions, since success continuations need
    -- to be passed these pattern-bound evidences.
    tcCollectEx
      :: LPat GhcTc
      -> ( [TyVar]        -- Existentially-bound type variables
                          -- in correctly-scoped order; e.g. [ k:*, x:k ]
         , [EvVar] )      -- and evidence variables

    tcCollectEx pat = go pat
      where
        go :: LPat GhcTc -> ([TyVar], [EvVar])
        go = go1 . unLoc

        go1 :: Pat GhcTc -> ([TyVar], [EvVar])
        go1 (LazyPat _ p)      = go p
        go1 (AsPat _ _ p)      = go p
        go1 (ParPat _ p)       = go p
        go1 (BangPat _ p)      = go p
        go1 (ListPat _ ps)     = mergeMany . map go $ ps
        go1 (TuplePat _ ps _)  = mergeMany . map go $ ps
        go1 (SumPat _ p _ _)   = go p
        go1 (ViewPat _ _ p)    = go p
        go1 con@ConPat{ pat_con_ext = con' }
                               = merge (cpt_tvs con', cpt_dicts con') $
                                  goConDetails $ pat_args con
        go1 (SigPat _ p _)     = go p
        go1 (XPat (CoPat _ p _)) = go1 p
        go1 (NPlusKPat _ n k _ geq subtract)
          = pprPanic "TODO: NPlusKPat" $ ppr n $$ ppr k $$ ppr geq $$ ppr subtract
        go1 _                   = empty

        goConDetails :: HsConPatDetails GhcTc -> ([TyVar], [EvVar])
        goConDetails (PrefixCon _ ps) = mergeMany . map go $ ps
        goConDetails (InfixCon p1 p2) = go p1 `merge` go p2
        goConDetails (RecCon HsRecFields{ rec_flds = flds })
          = mergeMany . map goRecFd $ flds

        goRecFd :: LHsRecField GhcTc (LPat GhcTc) -> ([TyVar], [EvVar])
        goRecFd (L _ HsRecField{ hsRecFieldArg = p }) = go p

        merge (vs1, evs1) (vs2, evs2) = (vs1 ++ vs2, evs1 ++ evs2)
        mergeMany = foldr merge empty
        empty     = ([], [])