GHC/Tc/Gen/Expr.hs

    {-# LANGUAGE CPP                 #-}
    {-# LANGUAGE DataKinds           #-}
    {-# LANGUAGE FlexibleContexts    #-}
    {-# LANGUAGE ScopedTypeVariables #-}
    {-# LANGUAGE TupleSections       #-}
    {-# LANGUAGE TypeFamilies        #-}
    {-# LANGUAGE UndecidableInstances #-} -- Wrinkle in Note [Trees That Grow]
                                          -- in module Language.Haskell.Syntax.Extension
    {-# OPTIONS_GHC -Wno-incomplete-uni-patterns   #-}

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

    -}

    module GHC.Tc.Gen.Expr
           ( tcCheckPolyExpr, tcCheckPolyExprNC,
             tcCheckMonoExpr, tcCheckMonoExprNC,
             tcMonoExpr, tcMonoExprNC,
             tcInferRho, tcInferRhoNC,
             tcPolyExpr, tcExpr,
             tcSyntaxOp, tcSyntaxOpGen, SyntaxOpType(..), synKnownType,
             tcCheckId,
             addAmbiguousNameErr,
             getFixedTyVars ) where

    #include "HsVersions.h"

    import GHC.Prelude

    import {-# SOURCE #-}   GHC.Tc.Gen.Splice( tcSpliceExpr, tcTypedBracket, tcUntypedBracket )

    import GHC.Hs
    import GHC.Rename.Utils
    import GHC.Tc.Utils.Zonk
    import GHC.Tc.Utils.Monad
    import GHC.Tc.Utils.Unify
    import GHC.Types.Basic
    import GHC.Core.Multiplicity
    import GHC.Core.UsageEnv
    import GHC.Tc.Utils.Instantiate
    import GHC.Tc.Gen.App
    import GHC.Tc.Gen.Head
    import GHC.Tc.Gen.Bind        ( tcLocalBinds )
    import GHC.Tc.Instance.Family ( tcGetFamInstEnvs )
    import GHC.Core.FamInstEnv    ( FamInstEnvs )
    import GHC.Rename.Env         ( addUsedGRE )
    import GHC.Tc.Utils.Env
    import GHC.Tc.Gen.Arrow
    import GHC.Tc.Gen.Match
    import GHC.Tc.Gen.HsType
    import GHC.Tc.Gen.Pat
    import GHC.Tc.Utils.TcMType
    import GHC.Tc.Types.Origin
    import GHC.Tc.Utils.TcType as TcType
    import GHC.Types.Id
    import GHC.Types.Id.Info
    import GHC.Core.ConLike
    import GHC.Core.DataCon
    import GHC.Core.PatSyn
    import GHC.Types.Name
    import GHC.Types.Name.Env
    import GHC.Types.Name.Set
    import GHC.Types.Name.Reader
    import GHC.Core.TyCon
    import GHC.Core.Type
    import GHC.Tc.Types.Evidence
    import GHC.Types.Var.Set
    import GHC.Builtin.Types
    import GHC.Builtin.Names
    import GHC.Driver.Session
    import GHC.Types.SrcLoc
    import GHC.Utils.Misc
    import GHC.Data.List.SetOps
    import GHC.Data.Maybe
    import GHC.Utils.Outputable as Outputable
    import GHC.Utils.Panic
    import GHC.Data.FastString
    import Control.Monad
    import GHC.Core.Class(classTyCon)
    import GHC.Types.Unique.Set ( UniqSet, mkUniqSet, elementOfUniqSet, nonDetEltsUniqSet )

    import Data.Function
    import Data.List (partition, sortBy, groupBy, intersect)

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Main wrappers}
    *                                                                      *
    ************************************************************************
    -}

    tcCheckPolyExpr, tcCheckPolyExprNC
      :: LHsExpr GhcRn         -- Expression to type check
      -> TcSigmaType           -- Expected type (could be a polytype)
      -> TcM (LHsExpr GhcTc) -- Generalised expr with expected type

    -- tcCheckPolyExpr is a convenient place (frequent but not too frequent)
    -- place to add context information.
    -- The NC version does not do so, usually because the caller wants
    -- to do so themselves.

    tcCheckPolyExpr   expr res_ty = tcPolyLExpr   expr (mkCheckExpType res_ty)
    tcCheckPolyExprNC expr res_ty = tcPolyLExprNC expr (mkCheckExpType res_ty)

    -- These versions take an ExpType
    tcPolyLExpr, tcPolyLExprNC :: LHsExpr GhcRn -> ExpSigmaType
                               -> TcM (LHsExpr GhcTc)

    tcPolyLExpr (L loc expr) res_ty
      = setSrcSpanA loc   $  -- Set location /first/; see GHC.Tc.Utils.Monad
        addExprCtxt expr $  -- Note [Error contexts in generated code]
        do { expr' <- tcPolyExpr expr res_ty
           ; return (L loc expr') }

    tcPolyLExprNC (L loc expr) res_ty
      = setSrcSpanA loc    $
        do { expr' <- tcPolyExpr expr res_ty
           ; return (L loc expr') }

    ---------------
    tcCheckMonoExpr, tcCheckMonoExprNC
        :: LHsExpr GhcRn     -- Expression to type check
        -> TcRhoType         -- Expected type
                             -- Definitely no foralls at the top
        -> TcM (LHsExpr GhcTc)
    tcCheckMonoExpr   expr res_ty = tcMonoExpr   expr (mkCheckExpType res_ty)
    tcCheckMonoExprNC expr res_ty = tcMonoExprNC expr (mkCheckExpType res_ty)

    tcMonoExpr, tcMonoExprNC
        :: LHsExpr GhcRn     -- Expression to type check
        -> ExpRhoType        -- Expected type
                             -- Definitely no foralls at the top
        -> TcM (LHsExpr GhcTc)

    tcMonoExpr (L loc expr) res_ty
      = setSrcSpanA loc   $  -- Set location /first/; see GHC.Tc.Utils.Monad
        addExprCtxt expr $  -- Note [Error contexts in generated code]
        do  { expr' <- tcExpr expr res_ty
            ; return (L loc expr') }

    tcMonoExprNC (L loc expr) res_ty
      = setSrcSpanA loc $
        do  { expr' <- tcExpr expr res_ty
            ; return (L loc expr') }

    ---------------
    tcInferRho, tcInferRhoNC :: LHsExpr GhcRn -> TcM (LHsExpr GhcTc, TcRhoType)
    -- Infer a *rho*-type. The return type is always instantiated.
    tcInferRho (L loc expr)
      = setSrcSpanA loc   $  -- Set location /first/; see GHC.Tc.Utils.Monad
        addExprCtxt expr $  -- Note [Error contexts in generated code]
        do { (expr', rho) <- tcInfer (tcExpr expr)
           ; return (L loc expr', rho) }

    tcInferRhoNC (L loc expr)
      = setSrcSpanA loc $
        do { (expr', rho) <- tcInfer (tcExpr expr)
           ; return (L loc expr', rho) }

    {- *********************************************************************
    *                                                                      *
            tcExpr: the main expression typechecker
    *                                                                      *
    ********************************************************************* -}

    tcPolyExpr :: HsExpr GhcRn -> ExpSigmaType -> TcM (HsExpr GhcTc)
    tcPolyExpr expr res_ty
      = do { traceTc "tcPolyExpr" (ppr res_ty)
           ; (wrap, expr') <- tcSkolemiseExpType GenSigCtxt res_ty $ \ res_ty ->
                              tcExpr expr res_ty
           ; return $ mkHsWrap wrap expr' }

    tcExpr :: HsExpr GhcRn -> ExpRhoType -> TcM (HsExpr GhcTc)

    -- Use tcApp to typecheck appplications, which are treated specially
    -- by Quick Look.  Specifically:
    --   - HsVar         lone variables, to ensure that they can get an
    --                     impredicative instantiation (via Quick Look
    --                     driven by res_ty (in checking mode)).
    --   - HsApp         value applications
    --   - HsAppType     type applications
    --   - ExprWithTySig (e :: type)
    --   - HsRecFld      overloaded record fields
    --   - HsExpanded    renamer expansions
    --   - HsOpApp       operator applications
    --   - HsOverLit     overloaded literals
    -- These constructors are the union of
    --   - ones taken apart by GHC.Tc.Gen.Head.splitHsApps
    --   - ones understood by GHC.Tc.Gen.Head.tcInferAppHead_maybe
    -- See Note [Application chains and heads] in GHC.Tc.Gen.App
    tcExpr e@(HsVar {})              res_ty = tcApp e res_ty
    tcExpr e@(HsApp {})              res_ty = tcApp e res_ty
    tcExpr e@(OpApp {})              res_ty = tcApp e res_ty
    tcExpr e@(HsAppType {})          res_ty = tcApp e res_ty
    tcExpr e@(ExprWithTySig {})      res_ty = tcApp e res_ty
    tcExpr e@(HsRecFld {})           res_ty = tcApp e res_ty
    tcExpr e@(XExpr (HsExpanded {})) res_ty = tcApp e res_ty

    tcExpr e@(HsOverLit _ lit) res_ty
      = do { mb_res <- tcShortCutLit lit res_ty
             -- See Note [Short cut for overloaded literals] in GHC.Tc.Utils.Zonk
           ; case mb_res of
               Just lit' -> return (HsOverLit noAnn lit')
               Nothing   -> tcApp e res_ty }

    -- Typecheck an occurrence of an unbound Id
    --
    -- Some of these started life as a true expression hole "_".
    -- Others might simply be variables that accidentally have no binding site
    tcExpr (HsUnboundVar _ occ) res_ty
      = do { ty <- expTypeToType res_ty    -- Allow Int# etc (#12531)
           ; her <- emitNewExprHole occ ty
           ; tcEmitBindingUsage bottomUE   -- Holes fit any usage environment
                                           -- (#18491)
           ; return (HsUnboundVar her occ) }

    tcExpr e@(HsLit x lit) res_ty
      = do { let lit_ty = hsLitType lit
           ; tcWrapResult e (HsLit x (convertLit lit)) lit_ty res_ty }

    tcExpr (HsPar x expr) res_ty
      = do { expr' <- tcMonoExprNC expr res_ty
           ; return (HsPar x expr') }

    tcExpr (HsPragE x prag expr) res_ty
      = do { expr' <- tcMonoExpr expr res_ty
           ; return (HsPragE x (tcExprPrag prag) expr') }

    tcExpr (NegApp x expr neg_expr) res_ty
      = do  { (expr', neg_expr')
                <- tcSyntaxOp NegateOrigin neg_expr [SynAny] res_ty $
                   \[arg_ty] [arg_mult] ->
                   tcScalingUsage arg_mult $ tcCheckMonoExpr expr arg_ty
            ; return (NegApp x expr' neg_expr') }

    tcExpr e@(HsIPVar _ x) res_ty
      = do {   {- Implicit parameters must have a *tau-type* not a
                  type scheme.  We enforce this by creating a fresh
                  type variable as its type.  (Because res_ty may not
                  be a tau-type.) -}
             ip_ty <- newOpenFlexiTyVarTy
           ; let ip_name = mkStrLitTy (hsIPNameFS x)
           ; ipClass <- tcLookupClass ipClassName
           ; ip_var <- emitWantedEvVar origin (mkClassPred ipClass [ip_name, ip_ty])
           ; tcWrapResult e
                       (fromDict ipClass ip_name ip_ty (HsVar noExtField (noLocA ip_var)))
                       ip_ty res_ty }
      where
      -- Coerces a dictionary for `IP "x" t` into `t`.
      fromDict ipClass x ty = mkHsWrap $ mkWpCastR $
                              unwrapIP $ mkClassPred ipClass [x,ty]
      origin = IPOccOrigin x

    tcExpr (HsLam _ match) res_ty
      = do  { (wrap, match') <- tcMatchLambda herald match_ctxt match res_ty
            ; return (mkHsWrap wrap (HsLam noExtField match')) }
      where
        match_ctxt = MC { mc_what = LambdaExpr, mc_body = tcBody }
        herald = sep [ text "The lambda expression" <+>
                       quotes (pprSetDepth (PartWay 1) $
                               pprMatches match),
                            -- The pprSetDepth makes the abstraction print briefly
                       text "has"]

    tcExpr e@(HsLamCase x matches) res_ty
      = do { (wrap, matches')
               <- tcMatchLambda msg match_ctxt matches res_ty
               -- The laziness annotation is because we don't want to fail here
               -- if there are multiple arguments
           ; return (mkHsWrap wrap $ HsLamCase x matches') }
      where
        msg = sep [ text "The function" <+> quotes (ppr e)
                  , text "requires"]
        match_ctxt = MC { mc_what = CaseAlt, mc_body = tcBody }

    {-
    ************************************************************************
    *                                                                      *
                    Explicit lists
    *                                                                      *
    ************************************************************************
    -}

    -- Explict lists [e1,e2,e3] have been expanded already in the renamer
    -- The expansion includes an ExplicitList, but it is always the built-in
    -- list type, so that's all we need concern ourselves with here.  See
    -- GHC.Rename.Expr. Note [Handling overloaded and rebindable constructs]
    tcExpr (ExplicitList _ exprs) res_ty
      = do  { res_ty <- expTypeToType res_ty
            ; (coi, elt_ty) <- matchExpectedListTy res_ty
            ; let tc_elt expr = tcCheckPolyExpr expr elt_ty
            ; exprs' <- mapM tc_elt exprs
            ; return $ mkHsWrapCo coi $ ExplicitList elt_ty exprs' }

    tcExpr expr@(ExplicitTuple x tup_args boxity) res_ty
      | all tupArgPresent tup_args
      = do { let arity  = length tup_args
                 tup_tc = tupleTyCon boxity arity
                   -- NB: tupleTyCon doesn't flatten 1-tuples
                   -- See Note [Don't flatten tuples from HsSyn] in GHC.Core.Make
           ; res_ty <- expTypeToType res_ty
           ; (coi, arg_tys) <- matchExpectedTyConApp tup_tc res_ty
                               -- Unboxed tuples have RuntimeRep vars, which we
                               -- don't care about here
                               -- See Note [Unboxed tuple RuntimeRep vars] in GHC.Core.TyCon
           ; let arg_tys' = case boxity of Unboxed -> drop arity arg_tys
                                           Boxed   -> arg_tys
           ; tup_args1 <- tcTupArgs tup_args arg_tys'
           ; return $ mkHsWrapCo coi (ExplicitTuple x tup_args1 boxity) }

      | otherwise
      = -- The tup_args are a mixture of Present and Missing (for tuple sections)
        do { let arity = length tup_args

           ; arg_tys <- case boxity of
               { Boxed   -> newFlexiTyVarTys arity liftedTypeKind
               ; Unboxed -> replicateM arity newOpenFlexiTyVarTy }

           -- Handle tuple sections where
           ; tup_args1 <- tcTupArgs tup_args arg_tys

           ; let expr'       = ExplicitTuple x tup_args1 boxity
                 missing_tys = [Scaled mult ty | (Missing (Scaled mult _), ty) <- zip tup_args1 arg_tys]

                 -- See Note [Linear fields generalization] in GHC.Tc.Gen.App
                 act_res_ty
                     = mkVisFunTys missing_tys (mkTupleTy1 boxity arg_tys)
                       -- See Note [Don't flatten tuples from HsSyn] in GHC.Core.Make

           ; traceTc "ExplicitTuple" (ppr act_res_ty $$ ppr res_ty)

           ; tcWrapResultMono expr expr' act_res_ty res_ty }

    tcExpr (ExplicitSum _ alt arity expr) res_ty
      = do { let sum_tc = sumTyCon arity
           ; res_ty <- expTypeToType res_ty
           ; (coi, arg_tys) <- matchExpectedTyConApp sum_tc res_ty
           ; -- Drop levity vars, we don't care about them here
             let arg_tys' = drop arity arg_tys
           ; expr' <- tcCheckPolyExpr expr (arg_tys' `getNth` (alt - 1))
           ; return $ mkHsWrapCo coi (ExplicitSum arg_tys' alt arity expr' ) }

    {-
    ************************************************************************
    *                                                                      *
                    Let, case, if, do
    *                                                                      *
    ************************************************************************
    -}

    tcExpr (HsLet x binds expr) res_ty
      = do  { (binds', expr') <- tcLocalBinds binds $
                                 tcMonoExpr expr res_ty
            ; return (HsLet x binds' expr') }

    tcExpr (HsCase x scrut matches) res_ty
      = do  {  -- We used to typecheck the case alternatives first.
               -- The case patterns tend to give good type info to use
               -- when typechecking the scrutinee.  For example
               --   case (map f) of
               --     (x:xs) -> ...
               -- will report that map is applied to too few arguments
               --
               -- But now, in the GADT world, we need to typecheck the scrutinee
               -- first, to get type info that may be refined in the case alternatives
              mult <- newFlexiTyVarTy multiplicityTy

              -- Typecheck the scrutinee.  We use tcInferRho but tcInferSigma
              -- would also be possible (tcMatchesCase accepts sigma-types)
              -- Interesting litmus test: do these two behave the same?
              --     case id        of {..}
              --     case (\v -> v) of {..}
              -- This design choice is discussed in #17790
            ; (scrut', scrut_ty) <- tcScalingUsage mult $ tcInferRho scrut

            ; traceTc "HsCase" (ppr scrut_ty)
            ; matches' <- tcMatchesCase match_ctxt (Scaled mult scrut_ty) matches res_ty
            ; return (HsCase x scrut' matches') }
     where
        match_ctxt = MC { mc_what = CaseAlt,
                          mc_body = tcBody }

    tcExpr (HsIf x pred b1 b2) res_ty
      = do { pred'    <- tcCheckMonoExpr pred boolTy
           ; (u1,b1') <- tcCollectingUsage $ tcMonoExpr b1 res_ty
           ; (u2,b2') <- tcCollectingUsage $ tcMonoExpr b2 res_ty
           ; tcEmitBindingUsage (supUE u1 u2)
           ; return (HsIf x pred' b1' b2') }

    tcExpr (HsMultiIf _ alts) res_ty
      = do { alts' <- mapM (wrapLocM $ tcGRHS match_ctxt res_ty) alts
           ; res_ty <- readExpType res_ty
           ; return (HsMultiIf res_ty alts') }
      where match_ctxt = MC { mc_what = IfAlt, mc_body = tcBody }

    tcExpr (HsDo _ do_or_lc stmts) res_ty
      = tcDoStmts do_or_lc stmts res_ty

    tcExpr (HsProc x pat cmd) res_ty
      = do  { (pat', cmd', coi) <- tcProc pat cmd res_ty
            ; return $ mkHsWrapCo coi (HsProc x pat' cmd') }

    -- Typechecks the static form and wraps it with a call to 'fromStaticPtr'.
    -- See Note [Grand plan for static forms] in GHC.Iface.Tidy.StaticPtrTable for an overview.
    -- To type check
    --      (static e) :: p a
    -- we want to check (e :: a),
    -- and wrap (static e) in a call to
    --    fromStaticPtr :: IsStatic p => StaticPtr a -> p a

    tcExpr (HsStatic fvs expr) res_ty
      = do  { res_ty          <- expTypeToType res_ty
            ; (co, (p_ty, expr_ty)) <- matchExpectedAppTy res_ty
            ; (expr', lie)    <- captureConstraints $
                addErrCtxt (hang (text "In the body of a static form:")
                                 2 (ppr expr)
                           ) $
                tcCheckPolyExprNC expr expr_ty

            -- Check that the free variables of the static form are closed.
            -- It's OK to use nonDetEltsUniqSet here as the only side effects of
            -- checkClosedInStaticForm are error messages.
            ; mapM_ checkClosedInStaticForm $ nonDetEltsUniqSet fvs

            -- Require the type of the argument to be Typeable.
            -- The evidence is not used, but asking the constraint ensures that
            -- the current implementation is as restrictive as future versions
            -- of the StaticPointers extension.
            ; typeableClass <- tcLookupClass typeableClassName
            ; _ <- emitWantedEvVar StaticOrigin $
                      mkTyConApp (classTyCon typeableClass)
                                 [liftedTypeKind, expr_ty]

            -- Insert the constraints of the static form in a global list for later
            -- validation.
            ; emitStaticConstraints lie

            -- Wrap the static form with the 'fromStaticPtr' call.
            ; fromStaticPtr <- newMethodFromName StaticOrigin fromStaticPtrName
                                                 [p_ty]
            ; let wrap = mkWpTyApps [expr_ty]
            ; loc <- getSrcSpanM
            ; return $ mkHsWrapCo co $ HsApp noComments
                                (L (noAnnSrcSpan loc) $ mkHsWrap wrap fromStaticPtr)
                                (L (noAnnSrcSpan loc) (HsStatic fvs expr'))
            }

    {-
    ************************************************************************
    *                                                                      *
                    Record construction and update
    *                                                                      *
    ************************************************************************
    -}

    tcExpr expr@(RecordCon { rcon_con = L loc con_name
                           , rcon_flds = rbinds }) res_ty
      = do  { con_like <- tcLookupConLike con_name

            ; (con_expr, con_sigma) <- tcInferId con_name
            ; (con_wrap, con_tau)   <- topInstantiate orig con_sigma
                  -- a shallow instantiation should really be enough for
                  -- a data constructor.
            ; let arity = conLikeArity con_like
                  Right (arg_tys, actual_res_ty) = tcSplitFunTysN arity con_tau

            ; checkTc (conLikeHasBuilder con_like) $
              nonBidirectionalErr (conLikeName con_like)

            ; rbinds' <- tcRecordBinds con_like (map scaledThing arg_tys) rbinds
                       -- It is currently not possible for a record to have
                       -- multiplicities. When they do, `tcRecordBinds` will take
                       -- scaled types instead. Meanwhile, it's safe to take
                       -- `scaledThing` above, as we know all the multiplicities are
                       -- Many.

            ; let rcon_tc = mkHsWrap con_wrap con_expr
                  expr' = RecordCon { rcon_ext = rcon_tc
                                    , rcon_con = L loc con_like
                                    , rcon_flds = rbinds' }

            ; ret <- tcWrapResultMono expr expr' actual_res_ty res_ty

            -- Check for missing fields.  We do this after type-checking to get
            -- better types in error messages (cf #18869).  For example:
            --     data T a = MkT { x :: a, y :: a }
            --     r = MkT { y = True }
            -- Then we'd like to warn about a missing field `x :: True`, rather than `x :: a0`.
            --
            -- NB: to do this really properly we should delay reporting until typechecking is complete,
            -- via a new `HoleSort`.  But that seems too much work.
            ; checkMissingFields con_like rbinds arg_tys

            ; return ret }
      where
        orig = OccurrenceOf con_name

    {-
    Note [Type of a record update]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The main complication with RecordUpd is that we need to explicitly
    handle the *non-updated* fields.  Consider:

            data T a b c = MkT1 { fa :: a, fb :: (b,c) }
                         | MkT2 { fa :: a, fb :: (b,c), fc :: c -> c }
                         | MkT3 { fd :: a }

            upd :: T a b c -> (b',c) -> T a b' c
            upd t x = t { fb = x}

    The result type should be (T a b' c)
    not (T a b c),   because 'b' *is not* mentioned in a non-updated field
    not (T a b' c'), because 'c' *is*     mentioned in a non-updated field
    NB that it's not good enough to look at just one constructor; we must
    look at them all; cf #3219

    After all, upd should be equivalent to:
            upd t x = case t of
                            MkT1 p q -> MkT1 p x
                            MkT2 a b -> MkT2 p b
                            MkT3 d   -> error ...

    So we need to give a completely fresh type to the result record,
    and then constrain it by the fields that are *not* updated ("p" above).
    We call these the "fixed" type variables, and compute them in getFixedTyVars.

    Note that because MkT3 doesn't contain all the fields being updated,
    its RHS is simply an error, so it doesn't impose any type constraints.
    Hence the use of 'relevant_cont'.

    Note [Implicit type sharing]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We also take into account any "implicit" non-update fields.  For example
            data T a b where { MkT { f::a } :: T a a; ... }
    So the "real" type of MkT is: forall ab. (a~b) => a -> T a b

    Then consider
            upd t x = t { f=x }
    We infer the type
            upd :: T a b -> a -> T a b
            upd (t::T a b) (x::a)
               = case t of { MkT (co:a~b) (_:a) -> MkT co x }
    We can't give it the more general type
            upd :: T a b -> c -> T c b

    Note [Criteria for update]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    We want to allow update for existentials etc, provided the updated
    field isn't part of the existential. For example, this should be ok.
      data T a where { MkT { f1::a, f2::b->b } :: T a }
      f :: T a -> b -> T b
      f t b = t { f1=b }

    The criterion we use is this:

      The types of the updated fields
      mention only the universally-quantified type variables
      of the data constructor

    NB: this is not (quite) the same as being a "naughty" record selector
    (See Note [Naughty record selectors]) in GHC.Tc.TyCl), at least
    in the case of GADTs. Consider
       data T a where { MkT :: { f :: a } :: T [a] }
    Then f is not "naughty" because it has a well-typed record selector.
    But we don't allow updates for 'f'.  (One could consider trying to
    allow this, but it makes my head hurt.  Badly.  And no one has asked
    for it.)

    In principle one could go further, and allow
      g :: T a -> T a
      g t = t { f2 = \x -> x }
    because the expression is polymorphic...but that seems a bridge too far.

    Note [Data family example]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
        data instance T (a,b) = MkT { x::a, y::b }
      --->
        data :TP a b = MkT { a::a, y::b }
        coTP a b :: T (a,b) ~ :TP a b

    Suppose r :: T (t1,t2), e :: t3
    Then  r { x=e } :: T (t3,t1)
      --->
          case r |> co1 of
            MkT x y -> MkT e y |> co2
          where co1 :: T (t1,t2) ~ :TP t1 t2
                co2 :: :TP t3 t2 ~ T (t3,t2)
    The wrapping with co2 is done by the constructor wrapper for MkT

    Outgoing invariants
    ~~~~~~~~~~~~~~~~~~~
    In the outgoing (HsRecordUpd scrut binds cons in_inst_tys out_inst_tys):

      * cons are the data constructors to be updated

      * in_inst_tys, out_inst_tys have same length, and instantiate the
            *representation* tycon of the data cons.  In Note [Data
            family example], in_inst_tys = [t1,t2], out_inst_tys = [t3,t2]

    Note [Mixed Record Field Updates]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider the following pattern synonym.

      data MyRec = MyRec { foo :: Int, qux :: String }

      pattern HisRec{f1, f2} = MyRec{foo = f1, qux=f2}

    This allows updates such as the following

      updater :: MyRec -> MyRec
      updater a = a {f1 = 1 }

    It would also make sense to allow the following update (which we reject).

      updater a = a {f1 = 1, qux = "two" } ==? MyRec 1 "two"

    This leads to confusing behaviour when the selectors in fact refer the same
    field.

      updater a = a {f1 = 1, foo = 2} ==? ???

    For this reason, we reject a mixture of pattern synonym and normal record
    selectors in the same update block. Although of course we still allow the
    following.

      updater a = (a {f1 = 1}) {foo = 2}

      > updater (MyRec 0 "str")
      MyRec 2 "str"

    -}

    -- Record updates via dot syntax are replaced by desugared expressions
    -- in the renamer. See Note [Overview of record dot syntax] in
    -- GHC.Hs.Expr. This is why we match on 'rupd_flds = Left rbnds' here
    -- and panic otherwise.
    tcExpr expr@(RecordUpd { rupd_expr = record_expr, rupd_flds = Left rbnds }) res_ty
      = ASSERT( notNull rbnds )
        do  { -- STEP -2: typecheck the record_expr, the record to be updated
              (record_expr', record_rho) <- tcScalingUsage Many $ tcInferRho record_expr
                -- Record update drops some of the content of the record (namely the
                -- content of the field being updated). As a consequence, unless the
                -- field being updated is unrestricted in the record, or we need an
                -- unrestricted record. Currently, we simply always require an
                -- unrestricted record.
                --
                -- Consider the following example:
                --
                -- data R a = R { self :: a }
                -- bad :: a ⊸ ()
                -- bad x = let r = R x in case r { self = () } of { R x' -> x' }
                --
                -- This should definitely *not* typecheck.

            -- STEP -1  See Note [Disambiguating record fields] in GHC.Tc.Gen.Head
            -- After this we know that rbinds is unambiguous
            ; rbinds <- disambiguateRecordBinds record_expr record_rho rbnds res_ty
            ; let upd_flds = map (unLoc . hsRecFieldLbl . unLoc) rbinds
                  upd_fld_occs = map (occNameFS . rdrNameOcc . rdrNameAmbiguousFieldOcc) upd_flds
                  sel_ids      = map selectorAmbiguousFieldOcc upd_flds
            -- STEP 0
            -- Check that the field names are really field names
            -- and they are all field names for proper records or
            -- all field names for pattern synonyms.
            ; let bad_guys = [ setSrcSpan loc $ addErrTc (notSelector fld_name)
                             | fld <- rbinds,
                               -- Excludes class ops
                               let L loc sel_id = hsRecUpdFieldId (unLoc fld),
                               not (isRecordSelector sel_id),
                               let fld_name = idName sel_id ]
            ; unless (null bad_guys) (sequence bad_guys >> failM)
            -- See note [Mixed Record Selectors]
            ; let (data_sels, pat_syn_sels) =
                    partition isDataConRecordSelector sel_ids
            ; MASSERT( all isPatSynRecordSelector pat_syn_sels )
            ; checkTc ( null data_sels || null pat_syn_sels )
                      ( mixedSelectors data_sels pat_syn_sels )

            -- STEP 1
            -- Figure out the tycon and data cons from the first field name
            ; let   -- It's OK to use the non-tc splitters here (for a selector)
                  sel_id : _  = sel_ids

                  mtycon :: Maybe TyCon
                  mtycon = case idDetails sel_id of
                              RecSelId (RecSelData tycon) _ -> Just tycon
                              _ -> Nothing

                  con_likes :: [ConLike]
                  con_likes = case idDetails sel_id of
                                 RecSelId (RecSelData tc) _
                                    -> map RealDataCon (tyConDataCons tc)
                                 RecSelId (RecSelPatSyn ps) _
                                    -> [PatSynCon ps]
                                 _  -> panic "tcRecordUpd"
                    -- NB: for a data type family, the tycon is the instance tycon

                  relevant_cons = conLikesWithFields con_likes upd_fld_occs
                    -- A constructor is only relevant to this process if
                    -- it contains *all* the fields that are being updated
                    -- Other ones will cause a runtime error if they occur

            -- Step 2
            -- Check that at least one constructor has all the named fields
            -- i.e. has an empty set of bad fields returned by badFields
            ; checkTc (not (null relevant_cons)) (badFieldsUpd rbinds con_likes)

            -- Take apart a representative constructor
            ; let con1 = ASSERT( not (null relevant_cons) ) head relevant_cons
                  (con1_tvs, _, _, _prov_theta, req_theta, scaled_con1_arg_tys, _)
                     = conLikeFullSig con1
                  con1_arg_tys = map scaledThing scaled_con1_arg_tys
                    -- We can safely drop the fields' multiplicities because
                    -- they are currently always 1: there is no syntax for record
                    -- fields with other multiplicities yet. This way we don't need
                    -- to handle it in the rest of the function
                  con1_flds   = map flLabel $ conLikeFieldLabels con1
                  con1_tv_tys = mkTyVarTys con1_tvs
                  con1_res_ty = case mtycon of
                                  Just tc -> mkFamilyTyConApp tc con1_tv_tys
                                  Nothing -> conLikeResTy con1 con1_tv_tys

            -- Check that we're not dealing with a unidirectional pattern
            -- synonym
            ; checkTc (conLikeHasBuilder con1) $
              nonBidirectionalErr (conLikeName con1)

            -- STEP 3    Note [Criteria for update]
            -- Check that each updated field is polymorphic; that is, its type
            -- mentions only the universally-quantified variables of the data con
            ; let flds1_w_tys  = zipEqual "tcExpr:RecConUpd" con1_flds con1_arg_tys
                  bad_upd_flds = filter bad_fld flds1_w_tys
                  con1_tv_set  = mkVarSet con1_tvs
                  bad_fld (fld, ty) = fld `elem` upd_fld_occs &&
                                          not (tyCoVarsOfType ty `subVarSet` con1_tv_set)
            ; checkTc (null bad_upd_flds) (badFieldTypes bad_upd_flds)

            -- STEP 4  Note [Type of a record update]
            -- Figure out types for the scrutinee and result
            -- Both are of form (T a b c), with fresh type variables, but with
            -- common variables where the scrutinee and result must have the same type
            -- These are variables that appear in *any* arg of *any* of the
            -- relevant constructors *except* in the updated fields
            --
            ; let fixed_tvs = getFixedTyVars upd_fld_occs con1_tvs relevant_cons
                  is_fixed_tv tv = tv `elemVarSet` fixed_tvs

                  mk_inst_ty :: TCvSubst -> (TyVar, TcType) -> TcM (TCvSubst, TcType)
                  -- Deals with instantiation of kind variables
                  --   c.f. GHC.Tc.Utils.TcMType.newMetaTyVars
                  mk_inst_ty subst (tv, result_inst_ty)
                    | is_fixed_tv tv   -- Same as result type
                    = return (extendTvSubst subst tv result_inst_ty, result_inst_ty)
                    | otherwise        -- Fresh type, of correct kind
                    = do { (subst', new_tv) <- newMetaTyVarX subst tv
                         ; return (subst', mkTyVarTy new_tv) }

            ; (result_subst, con1_tvs') <- newMetaTyVars con1_tvs
            ; let result_inst_tys = mkTyVarTys con1_tvs'
                  init_subst = mkEmptyTCvSubst (getTCvInScope result_subst)

            ; (scrut_subst, scrut_inst_tys) <- mapAccumLM mk_inst_ty init_subst
                                                          (con1_tvs `zip` result_inst_tys)

            ; let rec_res_ty    = TcType.substTy result_subst con1_res_ty
                  scrut_ty      = TcType.substTy scrut_subst  con1_res_ty
                  con1_arg_tys' = map (TcType.substTy result_subst) con1_arg_tys

            ; co_scrut <- unifyType (Just (ppr record_expr)) record_rho scrut_ty
                    -- NB: normal unification is OK here (as opposed to subsumption),
                    -- because for this to work out, both record_rho and scrut_ty have
                    -- to be normal datatypes -- no contravariant stuff can go on

            -- STEP 5
            -- Typecheck the bindings
            ; rbinds'      <- tcRecordUpd con1 con1_arg_tys' rbinds

            -- STEP 6: Deal with the stupid theta
            ; let theta' = substThetaUnchecked scrut_subst (conLikeStupidTheta con1)
            ; instStupidTheta RecordUpdOrigin theta'

            -- Step 7: make a cast for the scrutinee, in the
            --         case that it's from a data family
            ; let fam_co :: HsWrapper   -- RepT t1 .. tn ~R scrut_ty
                  fam_co | Just tycon <- mtycon
                         , Just co_con <- tyConFamilyCoercion_maybe tycon
                         = mkWpCastR (mkTcUnbranchedAxInstCo co_con scrut_inst_tys [])
                         | otherwise
                         = idHsWrapper

            -- Step 8: Check that the req constraints are satisfied
            -- For normal data constructors req_theta is empty but we must do
            -- this check for pattern synonyms.
            ; let req_theta' = substThetaUnchecked scrut_subst req_theta
            ; req_wrap <- instCallConstraints RecordUpdOrigin req_theta'

            -- Phew!
            ; let upd_tc = RecordUpdTc { rupd_cons = relevant_cons
                                       , rupd_in_tys = scrut_inst_tys
                                       , rupd_out_tys = result_inst_tys
                                       , rupd_wrap = req_wrap }
                  expr' = RecordUpd { rupd_expr = mkLHsWrap fam_co $
                                                    mkLHsWrapCo co_scrut record_expr'
                                    , rupd_flds = Left rbinds'
                                    , rupd_ext = upd_tc }

            ; tcWrapResult expr expr' rec_res_ty res_ty }
    tcExpr (RecordUpd {}) _ = panic "GHC.Tc.Gen.Expr: tcExpr: The impossible happened!"

    {-
    ************************************************************************
    *                                                                      *
            Arithmetic sequences                    e.g. [a,b..]
            and their parallel-array counterparts   e.g. [: a,b.. :]

    *                                                                      *
    ************************************************************************
    -}

    tcExpr (ArithSeq _ witness seq) res_ty
      = tcArithSeq witness seq res_ty

    {-
    ************************************************************************
    *                                                                      *
                    Record dot syntax
    *                                                                      *
    ************************************************************************
    -}

    -- These terms have been replaced by desugaring in the renamer. See
    -- Note [Overview of record dot syntax].
    tcExpr (HsGetField _ _ _) _ = panic "GHC.Tc.Gen.Expr: tcExpr: HsGetField: Not implemented"
    tcExpr (HsProjection _ _) _ = panic "GHC.Tc.Gen.Expr: tcExpr: HsProjection: Not implemented"

    {-
    ************************************************************************
    *                                                                      *
                    Template Haskell
    *                                                                      *
    ************************************************************************
    -}

    -- HsSpliced is an annotation produced by 'GHC.Rename.Splice.rnSpliceExpr'.
    -- Here we get rid of it and add the finalizers to the global environment.
    --
    -- See Note [Delaying modFinalizers in untyped splices] in GHC.Rename.Splice.
    tcExpr (HsSpliceE _ (HsSpliced _ mod_finalizers (HsSplicedExpr expr)))
           res_ty
      = do addModFinalizersWithLclEnv mod_finalizers
           tcExpr expr res_ty
    tcExpr (HsSpliceE _ splice)          res_ty = tcSpliceExpr splice res_ty
    tcExpr e@(HsBracket _ brack)         res_ty = tcTypedBracket e brack res_ty
    tcExpr e@(HsRnBracketOut _ brack ps) res_ty = tcUntypedBracket e brack ps res_ty

    {-
    ************************************************************************
    *                                                                      *
                    Catch-all
    *                                                                      *
    ************************************************************************
    -}

    tcExpr (HsConLikeOut {})   ty = pprPanic "tcExpr:HsConLikeOut" (ppr ty)
    tcExpr (HsOverLabel {})    ty = pprPanic "tcExpr:HsOverLabel"  (ppr ty)
    tcExpr (SectionL {})       ty = pprPanic "tcExpr:SectionL"    (ppr ty)
    tcExpr (SectionR {})       ty = pprPanic "tcExpr:SectionR"    (ppr ty)
    tcExpr (HsTcBracketOut {}) ty = pprPanic "tcExpr:HsTcBracketOut"    (ppr ty)
    tcExpr (HsTick {})         ty = pprPanic "tcExpr:HsTick"    (ppr ty)
    tcExpr (HsBinTick {})      ty = pprPanic "tcExpr:HsBinTick"    (ppr ty)

    {-
    ************************************************************************
    *                                                                      *
                    Arithmetic sequences [a..b] etc
    *                                                                      *
    ************************************************************************
    -}

    tcArithSeq :: Maybe (SyntaxExpr GhcRn) -> ArithSeqInfo GhcRn -> ExpRhoType
               -> TcM (HsExpr GhcTc)

    tcArithSeq witness seq@(From expr) res_ty
      = do { (wrap, elt_mult, elt_ty, wit') <- arithSeqEltType witness res_ty
           ; expr' <-tcScalingUsage elt_mult $ tcCheckPolyExpr expr elt_ty
           ; enum_from <- newMethodFromName (ArithSeqOrigin seq)
                                  enumFromName [elt_ty]
           ; return $ mkHsWrap wrap $
             ArithSeq enum_from wit' (From expr') }

    tcArithSeq witness seq@(FromThen expr1 expr2) res_ty
      = do { (wrap, elt_mult, elt_ty, wit') <- arithSeqEltType witness res_ty
           ; expr1' <- tcScalingUsage elt_mult $ tcCheckPolyExpr expr1 elt_ty
           ; expr2' <- tcScalingUsage elt_mult $ tcCheckPolyExpr expr2 elt_ty
           ; enum_from_then <- newMethodFromName (ArithSeqOrigin seq)
                                  enumFromThenName [elt_ty]
           ; return $ mkHsWrap wrap $
             ArithSeq enum_from_then wit' (FromThen expr1' expr2') }

    tcArithSeq witness seq@(FromTo expr1 expr2) res_ty
      = do { (wrap, elt_mult, elt_ty, wit') <- arithSeqEltType witness res_ty
           ; expr1' <- tcScalingUsage elt_mult $ tcCheckPolyExpr expr1 elt_ty
           ; expr2' <- tcScalingUsage elt_mult $ tcCheckPolyExpr expr2 elt_ty
           ; enum_from_to <- newMethodFromName (ArithSeqOrigin seq)
                                  enumFromToName [elt_ty]
           ; return $ mkHsWrap wrap $
             ArithSeq enum_from_to wit' (FromTo expr1' expr2') }

    tcArithSeq witness seq@(FromThenTo expr1 expr2 expr3) res_ty
      = do { (wrap, elt_mult, elt_ty, wit') <- arithSeqEltType witness res_ty
            ; expr1' <- tcScalingUsage elt_mult $ tcCheckPolyExpr expr1 elt_ty
            ; expr2' <- tcScalingUsage elt_mult $ tcCheckPolyExpr expr2 elt_ty
            ; expr3' <- tcScalingUsage elt_mult $ tcCheckPolyExpr expr3 elt_ty
            ; eft <- newMethodFromName (ArithSeqOrigin seq)
                                  enumFromThenToName [elt_ty]
            ; return $ mkHsWrap wrap $
              ArithSeq eft wit' (FromThenTo expr1' expr2' expr3') }

    -----------------
    arithSeqEltType :: Maybe (SyntaxExpr GhcRn) -> ExpRhoType
                    -> TcM (HsWrapper, Mult, TcType, Maybe (SyntaxExpr GhcTc))
    arithSeqEltType Nothing res_ty
      = do { res_ty <- expTypeToType res_ty
           ; (coi, elt_ty) <- matchExpectedListTy res_ty
           ; return (mkWpCastN coi, One, elt_ty, Nothing) }
    arithSeqEltType (Just fl) res_ty
      = do { ((elt_mult, elt_ty), fl')
               <- tcSyntaxOp ListOrigin fl [SynList] res_ty $
                  \ [elt_ty] [elt_mult] -> return (elt_mult, elt_ty)
           ; return (idHsWrapper, elt_mult, elt_ty, Just fl') }

    ----------------
    tcTupArgs :: [HsTupArg GhcRn] -> [TcSigmaType] -> TcM [HsTupArg GhcTc]
    tcTupArgs args tys
      = do MASSERT( equalLength args tys )
           checkTupSize (length args)
           mapM go (args `zip` tys)
      where
        go (Missing {},     arg_ty) = do { mult <- newFlexiTyVarTy multiplicityTy
                                         ; return (Missing (Scaled mult arg_ty)) }
        go (Present x expr, arg_ty) = do { expr' <- tcCheckPolyExpr expr arg_ty
                                         ; return (Present x expr') }

    ---------------------------
    -- See TcType.SyntaxOpType also for commentary
    tcSyntaxOp :: CtOrigin
               -> SyntaxExprRn
               -> [SyntaxOpType]           -- ^ shape of syntax operator arguments
               -> ExpRhoType               -- ^ overall result type
               -> ([TcSigmaType] -> [Mult] -> TcM a) -- ^ Type check any arguments,
                                                     -- takes a type per hole and a
                                                     -- multiplicity per arrow in
                                                     -- the shape.
               -> TcM (a, SyntaxExprTc)
    -- ^ Typecheck a syntax operator
    -- The operator is a variable or a lambda at this stage (i.e. renamer
    -- output)t
    tcSyntaxOp orig expr arg_tys res_ty
      = tcSyntaxOpGen orig expr arg_tys (SynType res_ty)

    -- | Slightly more general version of 'tcSyntaxOp' that allows the caller
    -- to specify the shape of the result of the syntax operator
    tcSyntaxOpGen :: CtOrigin
                  -> SyntaxExprRn
                  -> [SyntaxOpType]
                  -> SyntaxOpType
                  -> ([TcSigmaType] -> [Mult] -> TcM a)
                  -> TcM (a, SyntaxExprTc)
    tcSyntaxOpGen orig (SyntaxExprRn op) arg_tys res_ty thing_inside
      = do { (expr, sigma) <- tcInferAppHead (op, VACall op 0 noSrcSpan) [] Nothing
                 -- Ugh!! But all this code is scheduled for demolition anyway
           ; traceTc "tcSyntaxOpGen" (ppr op $$ ppr expr $$ ppr sigma)
           ; (result, expr_wrap, arg_wraps, res_wrap)
               <- tcSynArgA orig sigma arg_tys res_ty $
                  thing_inside
           ; traceTc "tcSyntaxOpGen" (ppr op $$ ppr expr $$ ppr sigma )
           ; return (result, SyntaxExprTc { syn_expr = mkHsWrap expr_wrap expr
                                          , syn_arg_wraps = arg_wraps
                                          , syn_res_wrap  = res_wrap }) }
    tcSyntaxOpGen _ NoSyntaxExprRn _ _ _ = panic "tcSyntaxOpGen"

    {-
    Note [tcSynArg]
    ~~~~~~~~~~~~~~~
    Because of the rich structure of SyntaxOpType, we must do the
    contra-/covariant thing when working down arrows, to get the
    instantiation vs. skolemisation decisions correct (and, more
    obviously, the orientation of the HsWrappers). We thus have
    two tcSynArgs.
    -}

    -- works on "expected" types, skolemising where necessary
    -- See Note [tcSynArg]
    tcSynArgE :: CtOrigin
              -> TcSigmaType
              -> SyntaxOpType                -- ^ shape it is expected to have
              -> ([TcSigmaType] -> [Mult] -> TcM a) -- ^ check the arguments
              -> TcM (a, HsWrapper)
               -- ^ returns a wrapper :: (type of right shape) "->" (type passed in)
    tcSynArgE orig sigma_ty syn_ty thing_inside
      = do { (skol_wrap, (result, ty_wrapper))
               <- tcTopSkolemise GenSigCtxt sigma_ty
                    (\ rho_ty -> go rho_ty syn_ty)
           ; return (result, skol_wrap <.> ty_wrapper) }
        where
        go rho_ty SynAny
          = do { result <- thing_inside [rho_ty] []
               ; return (result, idHsWrapper) }

        go rho_ty SynRho   -- same as SynAny, because we skolemise eagerly
          = do { result <- thing_inside [rho_ty] []
               ; return (result, idHsWrapper) }

        go rho_ty SynList
          = do { (list_co, elt_ty) <- matchExpectedListTy rho_ty
               ; result <- thing_inside [elt_ty] []
               ; return (result, mkWpCastN list_co) }

        go rho_ty (SynFun arg_shape res_shape)
          = do { ( match_wrapper                         -- :: (arg_ty -> res_ty) "->" rho_ty
                 , ( ( (result, arg_ty, res_ty, op_mult)
                     , res_wrapper )                     -- :: res_ty_out "->" res_ty
                   , arg_wrapper1, [], arg_wrapper2 ) )  -- :: arg_ty "->" arg_ty_out
                   <- matchExpectedFunTys herald GenSigCtxt 1 (mkCheckExpType rho_ty) $
                      \ [arg_ty] res_ty ->
                      do { arg_tc_ty <- expTypeToType (scaledThing arg_ty)
                         ; res_tc_ty <- expTypeToType res_ty

                             -- another nested arrow is too much for now,
                             -- but I bet we'll never need this
                         ; MASSERT2( case arg_shape of
                                       SynFun {} -> False;
                                       _         -> True
                                   , text "Too many nested arrows in SyntaxOpType" $$
                                     pprCtOrigin orig )

                         ; let arg_mult = scaledMult arg_ty
                         ; tcSynArgA orig arg_tc_ty [] arg_shape $
                           \ arg_results arg_res_mults ->
                           tcSynArgE orig res_tc_ty res_shape $
                           \ res_results res_res_mults ->
                           do { result <- thing_inside (arg_results ++ res_results) ([arg_mult] ++ arg_res_mults ++ res_res_mults)
                              ; return (result, arg_tc_ty, res_tc_ty, arg_mult) }}

               ; return ( result
                        , match_wrapper <.>
                          mkWpFun (arg_wrapper2 <.> arg_wrapper1) res_wrapper
                                  (Scaled op_mult arg_ty) res_ty doc ) }
          where
            herald = text "This rebindable syntax expects a function with"
            doc = text "When checking a rebindable syntax operator arising from" <+> ppr orig

        go rho_ty (SynType the_ty)
          = do { wrap   <- tcSubTypePat orig GenSigCtxt the_ty rho_ty
               ; result <- thing_inside [] []
               ; return (result, wrap) }

    -- works on "actual" types, instantiating where necessary
    -- See Note [tcSynArg]
    tcSynArgA :: CtOrigin
              -> TcSigmaType
              -> [SyntaxOpType]              -- ^ argument shapes
              -> SyntaxOpType                -- ^ result shape
              -> ([TcSigmaType] -> [Mult] -> TcM a) -- ^ check the arguments
              -> TcM (a, HsWrapper, [HsWrapper], HsWrapper)
                -- ^ returns a wrapper to be applied to the original function,
                -- wrappers to be applied to arguments
                -- and a wrapper to be applied to the overall expression
    tcSynArgA orig sigma_ty arg_shapes res_shape thing_inside
      = do { (match_wrapper, arg_tys, res_ty)
               <- matchActualFunTysRho herald orig Nothing
                                       (length arg_shapes) sigma_ty
                  -- match_wrapper :: sigma_ty "->" (arg_tys -> res_ty)
           ; ((result, res_wrapper), arg_wrappers)
               <- tc_syn_args_e (map scaledThing arg_tys) arg_shapes $ \ arg_results arg_res_mults ->
                  tc_syn_arg    res_ty  res_shape  $ \ res_results ->
                  thing_inside (arg_results ++ res_results) (map scaledMult arg_tys ++ arg_res_mults)
           ; return (result, match_wrapper, arg_wrappers, res_wrapper) }
      where
        herald = text "This rebindable syntax expects a function with"

        tc_syn_args_e :: [TcSigmaType] -> [SyntaxOpType]
                      -> ([TcSigmaType] -> [Mult] -> TcM a)
                      -> TcM (a, [HsWrapper])
                        -- the wrappers are for arguments
        tc_syn_args_e (arg_ty : arg_tys) (arg_shape : arg_shapes) thing_inside
          = do { ((result, arg_wraps), arg_wrap)
                   <- tcSynArgE     orig arg_ty  arg_shape  $ \ arg1_results arg1_mults ->
                      tc_syn_args_e      arg_tys arg_shapes $ \ args_results args_mults ->
                      thing_inside (arg1_results ++ args_results) (arg1_mults ++ args_mults)
               ; return (result, arg_wrap : arg_wraps) }
        tc_syn_args_e _ _ thing_inside = (, []) <$> thing_inside [] []

        tc_syn_arg :: TcSigmaType -> SyntaxOpType
                   -> ([TcSigmaType] -> TcM a)
                   -> TcM (a, HsWrapper)
                      -- the wrapper applies to the overall result
        tc_syn_arg res_ty SynAny thing_inside
          = do { result <- thing_inside [res_ty]
               ; return (result, idHsWrapper) }
        tc_syn_arg res_ty SynRho thing_inside
          = do { (inst_wrap, rho_ty) <- topInstantiate orig res_ty
                   -- inst_wrap :: res_ty "->" rho_ty
               ; result <- thing_inside [rho_ty]
               ; return (result, inst_wrap) }
        tc_syn_arg res_ty SynList thing_inside
          = do { (inst_wrap, rho_ty) <- topInstantiate orig res_ty
                   -- inst_wrap :: res_ty "->" rho_ty
               ; (list_co, elt_ty)   <- matchExpectedListTy rho_ty
                   -- list_co :: [elt_ty] ~N rho_ty
               ; result <- thing_inside [elt_ty]
               ; return (result, mkWpCastN (mkTcSymCo list_co) <.> inst_wrap) }
        tc_syn_arg _ (SynFun {}) _
          = pprPanic "tcSynArgA hits a SynFun" (ppr orig)
        tc_syn_arg res_ty (SynType the_ty) thing_inside
          = do { wrap   <- tcSubType orig GenSigCtxt res_ty the_ty
               ; result <- thing_inside []
               ; return (result, wrap) }

    {-
    Note [Push result type in]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    Unify with expected result before type-checking the args so that the
    info from res_ty percolates to args.  This is when we might detect a
    too-few args situation.  (One can think of cases when the opposite
    order would give a better error message.)
    experimenting with putting this first.

    Here's an example where it actually makes a real difference

       class C t a b | t a -> b
       instance C Char a Bool

       data P t a = forall b. (C t a b) => MkP b
       data Q t   = MkQ (forall a. P t a)

       f1, f2 :: Q Char;
       f1 = MkQ (MkP True)
       f2 = MkQ (MkP True :: forall a. P Char a)

    With the change, f1 will type-check, because the 'Char' info from
    the signature is propagated into MkQ's argument. With the check
    in the other order, the extra signature in f2 is reqd.
    -}

    {- *********************************************************************
    *                                                                      *
                     Record bindings
    *                                                                      *
    ********************************************************************* -}

    getFixedTyVars :: [FieldLabelString] -> [TyVar] -> [ConLike] -> TyVarSet
    -- These tyvars must not change across the updates
    getFixedTyVars upd_fld_occs univ_tvs cons
          = mkVarSet [tv1 | con <- cons
                          , let (u_tvs, _, eqspec, prov_theta
                                 , req_theta, arg_tys, _)
                                  = conLikeFullSig con
                                theta = eqSpecPreds eqspec
                                         ++ prov_theta
                                         ++ req_theta
                                flds = conLikeFieldLabels con
                                fixed_tvs = exactTyCoVarsOfTypes (map scaledThing fixed_tys)
                                        -- fixed_tys: See Note [Type of a record update]
                                            `unionVarSet` tyCoVarsOfTypes theta
                                        -- Universally-quantified tyvars that
                                        -- appear in any of the *implicit*
                                        -- arguments to the constructor are fixed
                                        -- See Note [Implicit type sharing]

                                fixed_tys = [ty | (fl, ty) <- zip flds arg_tys
                                                , not (flLabel fl `elem` upd_fld_occs)]
                          , (tv1,tv) <- univ_tvs `zip` u_tvs
                          , tv `elemVarSet` fixed_tvs ]

    -- Disambiguate the fields in a record update.
    -- See Note [Disambiguating record fields] in GHC.Tc.Gen.Head
    disambiguateRecordBinds :: LHsExpr GhcRn -> TcRhoType
                     -> [LHsRecUpdField GhcRn] -> ExpRhoType
                     -> TcM [LHsRecField' GhcTc (AmbiguousFieldOcc GhcTc) (LHsExpr GhcRn)]
    disambiguateRecordBinds record_expr record_rho rbnds res_ty
        -- Are all the fields unambiguous?
      = case mapM isUnambiguous rbnds of
                         -- If so, just skip to looking up the Ids
                         -- Always the case if DuplicateRecordFields is off
          Just rbnds' -> mapM lookupSelector rbnds'
          Nothing     -> -- If not, try to identify a single parent
            do { fam_inst_envs <- tcGetFamInstEnvs
                 -- Look up the possible parents for each field
               ; rbnds_with_parents <- getUpdFieldsParents
               ; let possible_parents = map (map fst . snd) rbnds_with_parents
                 -- Identify a single parent
               ; p <- identifyParent fam_inst_envs possible_parents
                 -- Pick the right selector with that parent for each field
               ; checkNoErrs $ mapM (pickParent p) rbnds_with_parents }
      where
        -- Extract the selector name of a field update if it is unambiguous
        isUnambiguous :: LHsRecUpdField GhcRn -> Maybe (LHsRecUpdField GhcRn,Name)
        isUnambiguous x = case unLoc (hsRecFieldLbl (unLoc x)) of
                            Unambiguous sel_name _ -> Just (x, sel_name)
                            Ambiguous{}            -> Nothing

        -- Look up the possible parents and selector GREs for each field
        getUpdFieldsParents :: TcM [(LHsRecUpdField GhcRn
                                    , [(RecSelParent, GlobalRdrElt)])]
        getUpdFieldsParents
          = fmap (zip rbnds) $ mapM
              (lookupParents False . unLoc . hsRecUpdFieldRdr . unLoc)
              rbnds

        -- Given a the lists of possible parents for each field,
        -- identify a single parent
        identifyParent :: FamInstEnvs -> [[RecSelParent]] -> TcM RecSelParent
        identifyParent fam_inst_envs possible_parents
          = case foldr1 intersect possible_parents of
            -- No parents for all fields: record update is ill-typed
            []  -> failWithTc (noPossibleParents rbnds)

            -- Exactly one datatype with all the fields: use that
            [p] -> return p

            -- Multiple possible parents: try harder to disambiguate
            -- Can we get a parent TyCon from the pushed-in type?
            _:_ | Just p <- tyConOfET fam_inst_envs res_ty ->
                  do { reportAmbiguousField p
                     ; return (RecSelData p) }

            -- Does the expression being updated have a type signature?
            -- If so, try to extract a parent TyCon from it
                | Just {} <- obviousSig (unLoc record_expr)
                , Just tc <- tyConOf fam_inst_envs record_rho
                -> do { reportAmbiguousField tc
                      ; return (RecSelData tc) }

            -- Nothing else we can try...
            _ -> failWithTc badOverloadedUpdate

        -- Make a field unambiguous by choosing the given parent.
        -- Emits an error if the field cannot have that parent,
        -- e.g. if the user writes
        --     r { x = e } :: T
        -- where T does not have field x.
        pickParent :: RecSelParent
                   -> (LHsRecUpdField GhcRn, [(RecSelParent, GlobalRdrElt)])
                   -> TcM (LHsRecField' GhcTc (AmbiguousFieldOcc GhcTc) (LHsExpr GhcRn))
        pickParent p (upd, xs)
          = case lookup p xs of
                          -- Phew! The parent is valid for this field.
                          -- Previously ambiguous fields must be marked as
                          -- used now that we know which one is meant, but
                          -- unambiguous ones shouldn't be recorded again
                          -- (giving duplicate deprecation warnings).
              Just gre -> do { unless (null (tail xs)) $ do
                                 let L loc _ = hsRecFieldLbl (unLoc upd)
                                 setSrcSpan loc $ addUsedGRE True gre
                             ; lookupSelector (upd, greMangledName gre) }
                          -- The field doesn't belong to this parent, so report
                          -- an error but keep going through all the fields
              Nothing  -> do { addErrTc (fieldNotInType p
                                          (unLoc (hsRecUpdFieldRdr (unLoc upd))))
                             ; lookupSelector (upd, greMangledName (snd (head xs))) }

        -- Given a (field update, selector name) pair, look up the
        -- selector to give a field update with an unambiguous Id
        lookupSelector :: (LHsRecUpdField GhcRn, Name)
                     -> TcM (LHsRecField' GhcRn (AmbiguousFieldOcc GhcTc) (LHsExpr GhcRn))
        lookupSelector (L l upd, n)
          = do { i <- tcLookupId n
               ; let L loc af = hsRecFieldLbl upd
                     lbl      = rdrNameAmbiguousFieldOcc af
               -- ; return $ L l upd { hsRecFieldLbl
               --                = L loc (Unambiguous i (L (noAnnSrcSpan loc) lbl)) }
               ; return $ L l HsRecField
                   { hsRecFieldAnn = hsRecFieldAnn upd
                   , hsRecFieldLbl
                           = L loc (Unambiguous i (L (noAnnSrcSpan loc) lbl))
                   , hsRecFieldArg = hsRecFieldArg upd
                   , hsRecPun = hsRecPun upd
                   }
               }

        -- See Note [Deprecating ambiguous fields] in GHC.Tc.Gen.Head
        reportAmbiguousField :: TyCon -> TcM ()
        reportAmbiguousField parent_type =
            setSrcSpan loc $ warnIfFlag Opt_WarnAmbiguousFields True $
              vcat [ text "The record update" <+> ppr rupd
                       <+> text "with type" <+> ppr parent_type
                       <+> text "is ambiguous."
                   , text "This will not be supported by -XDuplicateRecordFields in future releases of GHC."
                   ]
          where
            rupd = RecordUpd { rupd_expr = record_expr, rupd_flds = Left rbnds, rupd_ext = noExtField }
            loc  = getLocA (head rbnds)

    {-
    Game plan for record bindings
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    1. Find the TyCon for the bindings, from the first field label.

    2. Instantiate its tyvars and unify (T a1 .. an) with expected_ty.

    For each binding field = value

    3. Instantiate the field type (from the field label) using the type
       envt from step 2.

    4  Type check the value using tcCheckPolyExprNC (in tcRecordField),
       passing the field type as the expected argument type.

    This extends OK when the field types are universally quantified.
    -}

    tcRecordBinds
            :: ConLike
            -> [TcType]     -- Expected type for each field
            -> HsRecordBinds GhcRn
            -> TcM (HsRecordBinds GhcTc)

    tcRecordBinds con_like arg_tys (HsRecFields rbinds dd)
      = do  { mb_binds <- mapM do_bind rbinds
            ; return (HsRecFields (catMaybes mb_binds) dd) }
      where
        fields = map flSelector $ conLikeFieldLabels con_like
        flds_w_tys = zipEqual "tcRecordBinds" fields arg_tys

        do_bind :: LHsRecField GhcRn (LHsExpr GhcRn)
                -> TcM (Maybe (LHsRecField GhcTc (LHsExpr GhcTc)))
        do_bind (L l fld@(HsRecField { hsRecFieldLbl = f
                                     , hsRecFieldArg = rhs }))

          = do { mb <- tcRecordField con_like flds_w_tys f rhs
               ; case mb of
                   Nothing         -> return Nothing
                   -- Just (f', rhs') -> return (Just (L l (fld { hsRecFieldLbl = f'
                   --                                            , hsRecFieldArg = rhs' }))) }
                   Just (f', rhs') -> return (Just (L l (HsRecField
                                                         { hsRecFieldAnn = hsRecFieldAnn fld
                                                         , hsRecFieldLbl = f'
                                                         , hsRecFieldArg = rhs'
                                                         , hsRecPun = hsRecPun fld}))) }

    tcRecordUpd
            :: ConLike
            -> [TcType]     -- Expected type for each field
            -> [LHsRecField' GhcTc (AmbiguousFieldOcc GhcTc) (LHsExpr GhcRn)]
            -> TcM [LHsRecUpdField GhcTc]

    tcRecordUpd con_like arg_tys rbinds = fmap catMaybes $ mapM do_bind rbinds
      where
        fields = map flSelector $ conLikeFieldLabels con_like
        flds_w_tys = zipEqual "tcRecordUpd" fields arg_tys

        do_bind :: LHsRecField' GhcTc (AmbiguousFieldOcc GhcTc) (LHsExpr GhcRn)
                -> TcM (Maybe (LHsRecUpdField GhcTc))
        do_bind (L l fld@(HsRecField { hsRecFieldLbl = L loc af
                                     , hsRecFieldArg = rhs }))
          = do { let lbl = rdrNameAmbiguousFieldOcc af
                     sel_id = selectorAmbiguousFieldOcc af
                     f = L loc (FieldOcc (idName sel_id) (L (noAnnSrcSpan loc) lbl))
               ; mb <- tcRecordField con_like flds_w_tys f rhs
               ; case mb of
                   Nothing         -> return Nothing
                   Just (f', rhs') ->
                     return (Just
                             (L l (fld { hsRecFieldLbl
                                          = L loc (Unambiguous
                                                   (extFieldOcc (unLoc f'))
                                                   (L (noAnnSrcSpan loc) lbl))
                                       , hsRecFieldArg = rhs' }))) }

    tcRecordField :: ConLike -> Assoc Name Type
                  -> LFieldOcc GhcRn -> LHsExpr GhcRn
                  -> TcM (Maybe (LFieldOcc GhcTc, LHsExpr GhcTc))
    tcRecordField con_like flds_w_tys (L loc (FieldOcc sel_name lbl)) rhs
      | Just field_ty <- assocMaybe flds_w_tys sel_name
          = addErrCtxt (fieldCtxt field_lbl) $
            do { rhs' <- tcCheckPolyExprNC rhs field_ty
               ; let field_id = mkUserLocal (nameOccName sel_name)
                                            (nameUnique sel_name)
                                            Many field_ty loc
                    -- Yuk: the field_id has the *unique* of the selector Id
                    --          (so we can find it easily)
                    --      but is a LocalId with the appropriate type of the RHS
                    --          (so the desugarer knows the type of local binder to make)
               ; return (Just (L loc (FieldOcc field_id lbl), rhs')) }
          | otherwise
          = do { addErrTc (badFieldCon con_like field_lbl)
               ; return Nothing }
      where
            field_lbl = occNameFS $ rdrNameOcc (unLoc lbl)

    checkMissingFields ::  ConLike -> HsRecordBinds GhcRn -> [Scaled TcType] -> TcM ()
    checkMissingFields con_like rbinds arg_tys
      | null field_labels   -- Not declared as a record;
                            -- But C{} is still valid if no strict fields
      = if any isBanged field_strs then
            -- Illegal if any arg is strict
            addErrTc (missingStrictFields con_like [])
        else do
            warn <- woptM Opt_WarnMissingFields
            when (warn && notNull field_strs && null field_labels)
                 (warnTc (Reason Opt_WarnMissingFields) True
                     (missingFields con_like []))

      | otherwise = do              -- A record
        unless (null missing_s_fields) $ do
            fs <- zonk_fields missing_s_fields
            -- It is an error to omit a strict field, because
            -- we can't substitute it with (error "Missing field f")
            addErrTc (missingStrictFields con_like fs)

        warn <- woptM Opt_WarnMissingFields
        when (warn && notNull missing_ns_fields) $ do
            fs <- zonk_fields missing_ns_fields
            -- It is not an error (though we may want) to omit a
            -- lazy field, because we can always use
            -- (error "Missing field f") instead.
            warnTc (Reason Opt_WarnMissingFields) True
                 (missingFields con_like fs)

      where
        -- we zonk the fields to get better types in error messages (#18869)
        zonk_fields fs = forM fs $ \(str,ty) -> do
            ty' <- zonkTcType ty
            return (str,ty')
        missing_s_fields
            = [ (flLabel fl, scaledThing ty) | (fl,str,ty) <- field_info,
                     isBanged str,
                     not (fl `elemField` field_names_used)
              ]
        missing_ns_fields
            = [ (flLabel fl, scaledThing ty) | (fl,str,ty) <- field_info,
                     not (isBanged str),
                     not (fl `elemField` field_names_used)
              ]

        field_names_used = hsRecFields rbinds
        field_labels     = conLikeFieldLabels con_like

        field_info = zip3 field_labels field_strs arg_tys

        field_strs = conLikeImplBangs con_like

        fl `elemField` flds = any (\ fl' -> flSelector fl == fl') flds

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

    Boring and alphabetical:
    -}

    fieldCtxt :: FieldLabelString -> SDoc
    fieldCtxt field_name
      = text "In the" <+> quotes (ppr field_name) <+> ptext (sLit "field of a record")

    badFieldTypes :: [(FieldLabelString,TcType)] -> SDoc
    badFieldTypes prs
      = hang (text "Record update for insufficiently polymorphic field"
                             <> plural prs <> colon)
           2 (vcat [ ppr f <+> dcolon <+> ppr ty | (f,ty) <- prs ])

    badFieldsUpd
      :: [LHsRecField' GhcTc (AmbiguousFieldOcc GhcTc) (LHsExpr GhcRn)]
                   -- Field names that don't belong to a single datacon
      -> [ConLike] -- Data cons of the type which the first field name belongs to
      -> SDoc
    badFieldsUpd rbinds data_cons
      = hang (text "No constructor has all these fields:")
           2 (pprQuotedList conflictingFields)
              -- See Note [Finding the conflicting fields]
      where
        -- A (preferably small) set of fields such that no constructor contains
        -- all of them.  See Note [Finding the conflicting fields]
        conflictingFields = case nonMembers of
            -- nonMember belongs to a different type.
            (nonMember, _) : _ -> [aMember, nonMember]
            [] -> let
                -- All of rbinds belong to one type. In this case, repeatedly add
                -- a field to the set until no constructor contains the set.

                -- Each field, together with a list indicating which constructors
                -- have all the fields so far.
                growingSets :: [(FieldLabelString, [Bool])]
                growingSets = scanl1 combine membership
                combine (_, setMem) (field, fldMem)
                  = (field, zipWith (&&) setMem fldMem)
                in
                -- Fields that don't change the membership status of the set
                -- are redundant and can be dropped.
                map (fst . head) $ groupBy ((==) `on` snd) growingSets

        aMember = ASSERT( not (null members) ) fst (head members)
        (members, nonMembers) = partition (or . snd) membership

        -- For each field, which constructors contain the field?
        membership :: [(FieldLabelString, [Bool])]
        membership = sortMembership $
            map (\fld -> (fld, map (fld `elementOfUniqSet`) fieldLabelSets)) $
              map (occNameFS . rdrNameOcc . rdrNameAmbiguousFieldOcc . unLoc . hsRecFieldLbl . unLoc) rbinds

        fieldLabelSets :: [UniqSet FieldLabelString]
        fieldLabelSets = map (mkUniqSet . map flLabel . conLikeFieldLabels) data_cons

        -- Sort in order of increasing number of True, so that a smaller
        -- conflicting set can be found.
        sortMembership =
          map snd .
          sortBy (compare `on` fst) .
          map (\ item@(_, membershipRow) -> (countTrue membershipRow, item))

        countTrue = count id

    {-
    Note [Finding the conflicting fields]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Suppose we have
      data A = A {a0, a1 :: Int}
             | B {b0, b1 :: Int}
    and we see a record update
      x { a0 = 3, a1 = 2, b0 = 4, b1 = 5 }
    Then we'd like to find the smallest subset of fields that no
    constructor has all of.  Here, say, {a0,b0}, or {a0,b1}, etc.
    We don't really want to report that no constructor has all of
    {a0,a1,b0,b1}, because when there are hundreds of fields it's
    hard to see what was really wrong.

    We may need more than two fields, though; eg
      data T = A { x,y :: Int, v::Int }
              | B { y,z :: Int, v::Int }
              | C { z,x :: Int, v::Int }
    with update
       r { x=e1, y=e2, z=e3 }, we

    Finding the smallest subset is hard, so the code here makes
    a decent stab, no more.  See #7989.
    -}

    mixedSelectors :: [Id] -> [Id] -> SDoc
    mixedSelectors data_sels@(dc_rep_id:_) pat_syn_sels@(ps_rep_id:_)
      = ptext
          (sLit "Cannot use a mixture of pattern synonym and record selectors") $$
        text "Record selectors defined by"
          <+> quotes (ppr (tyConName rep_dc))
          <> text ":"
          <+> pprWithCommas ppr data_sels $$
        text "Pattern synonym selectors defined by"
          <+> quotes (ppr (patSynName rep_ps))
          <> text ":"
          <+> pprWithCommas ppr pat_syn_sels
      where
        RecSelPatSyn rep_ps = recordSelectorTyCon ps_rep_id
        RecSelData rep_dc = recordSelectorTyCon dc_rep_id
    mixedSelectors _ _ = panic "GHC.Tc.Gen.Expr: mixedSelectors emptylists"

    missingStrictFields :: ConLike -> [(FieldLabelString, TcType)] -> SDoc
    missingStrictFields con fields
      = vcat [header, nest 2 rest]
      where
        pprField (f,ty) = ppr f <+> dcolon <+> ppr ty
        rest | null fields = Outputable.empty  -- Happens for non-record constructors
                                               -- with strict fields
             | otherwise   = vcat (fmap pprField fields)

        header = text "Constructor" <+> quotes (ppr con) <+>
                 text "does not have the required strict field(s)" <>
                 if null fields then Outputable.empty else colon

    missingFields :: ConLike -> [(FieldLabelString, TcType)] -> SDoc
    missingFields con fields
      = vcat [header, nest 2 rest]
      where
        pprField (f,ty) = ppr f <+> text "::" <+> ppr ty
        rest | null fields = Outputable.empty
             | otherwise   = vcat (fmap pprField fields)
        header = text "Fields of" <+> quotes (ppr con) <+>
                 text "not initialised" <>
                 if null fields then Outputable.empty else colon

    -- callCtxt fun args = text "In the call" <+> parens (ppr (foldl' mkHsApp fun args))

    noPossibleParents :: [LHsRecUpdField GhcRn] -> SDoc
    noPossibleParents rbinds
      = hang (text "No type has all these fields:")
           2 (pprQuotedList fields)
      where
        fields = map (hsRecFieldLbl . unLoc) rbinds

    badOverloadedUpdate :: SDoc
    badOverloadedUpdate = text "Record update is ambiguous, and requires a type signature"

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Static Pointers}
    *                                                                      *
    ************************************************************************
    -}

    -- | A data type to describe why a variable is not closed.
    data NotClosedReason = NotLetBoundReason
                         | NotTypeClosed VarSet
                         | NotClosed Name NotClosedReason

    -- | Checks if the given name is closed and emits an error if not.
    --
    -- See Note [Not-closed error messages].
    checkClosedInStaticForm :: Name -> TcM ()
    checkClosedInStaticForm name = do
        type_env <- getLclTypeEnv
        case checkClosed type_env name of
          Nothing -> return ()
          Just reason -> addErrTc $ explain name reason
      where
        -- See Note [Checking closedness].
        checkClosed :: TcTypeEnv -> Name -> Maybe NotClosedReason
        checkClosed type_env n = checkLoop type_env (unitNameSet n) n

        checkLoop :: TcTypeEnv -> NameSet -> Name -> Maybe NotClosedReason
        checkLoop type_env visited n =
          -- The @visited@ set is an accumulating parameter that contains the set of
          -- visited nodes, so we avoid repeating cycles in the traversal.
          case lookupNameEnv type_env n of
            Just (ATcId { tct_id = tcid, tct_info = info }) -> case info of
              ClosedLet   -> Nothing
              NotLetBound -> Just NotLetBoundReason
              NonClosedLet fvs type_closed -> listToMaybe $
                -- Look for a non-closed variable in fvs
                [ NotClosed n' reason
                | n' <- nameSetElemsStable fvs
                , not (elemNameSet n' visited)
                , Just reason <- [checkLoop type_env (extendNameSet visited n') n']
                ] ++
                if type_closed then
                  []
                else
                  -- We consider non-let-bound variables easier to figure out than
                  -- non-closed types, so we report non-closed types to the user
                  -- only if we cannot spot the former.
                  [ NotTypeClosed $ tyCoVarsOfType (idType tcid) ]
            -- The binding is closed.
            _ -> Nothing

        -- Converts a reason into a human-readable sentence.
        --
        -- @explain name reason@ starts with
        --
        -- "<name> is used in a static form but it is not closed because it"
        --
        -- and then follows a list of causes. For each id in the path, the text
        --
        -- "uses <id> which"
        --
        -- is appended, yielding something like
        --
        -- "uses <id> which uses <id1> which uses <id2> which"
        --
        -- until the end of the path is reached, which is reported as either
        --
        -- "is not let-bound"
        --
        -- when the final node is not let-bound, or
        --
        -- "has a non-closed type because it contains the type variables:
        -- v1, v2, v3"
        --
        -- when the final node has a non-closed type.
        --
        explain :: Name -> NotClosedReason -> SDoc
        explain name reason =
          quotes (ppr name) <+> text "is used in a static form but it is not closed"
                            <+> text "because it"
                            $$
                            sep (causes reason)

        causes :: NotClosedReason -> [SDoc]
        causes NotLetBoundReason = [text "is not let-bound."]
        causes (NotTypeClosed vs) =
          [ text "has a non-closed type because it contains the"
          , text "type variables:" <+>
            pprVarSet vs (hsep . punctuate comma . map (quotes . ppr))
          ]
        causes (NotClosed n reason) =
          let msg = text "uses" <+> quotes (ppr n) <+> text "which"
           in case reason of
                NotClosed _ _ -> msg : causes reason
                _   -> let (xs0, xs1) = splitAt 1 $ causes reason
                        in fmap (msg <+>) xs0 ++ xs1

    -- Note [Not-closed error messages]
    -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    --
    -- When variables in a static form are not closed, we go through the trouble
    -- of explaining why they aren't.
    --
    -- Thus, the following program
    --
    -- > {-# LANGUAGE StaticPointers #-}
    -- > module M where
    -- >
    -- > f x = static g
    -- >   where
    -- >     g = h
    -- >     h = x
    --
    -- produces the error
    --
    --    'g' is used in a static form but it is not closed because it
    --    uses 'h' which uses 'x' which is not let-bound.
    --
    -- And a program like
    --
    -- > {-# LANGUAGE StaticPointers #-}
    -- > module M where
    -- >
    -- > import Data.Typeable
    -- > import GHC.StaticPtr
    -- >
    -- > f :: Typeable a => a -> StaticPtr TypeRep
    -- > f x = const (static (g undefined)) (h x)
    -- >   where
    -- >     g = h
    -- >     h = typeOf
    --
    -- produces the error
    --
    --    'g' is used in a static form but it is not closed because it
    --    uses 'h' which has a non-closed type because it contains the
    --    type variables: 'a'
    --

    -- Note [Checking closedness]
    -- ~~~~~~~~~~~~~~~~~~~~~~~~~~
    --
    -- @checkClosed@ checks if a binding is closed and returns a reason if it is
    -- not.
    --
    -- The bindings define a graph where the nodes are ids, and there is an edge
    -- from @id1@ to @id2@ if the rhs of @id1@ contains @id2@ among its free
    -- variables.
    --
    -- When @n@ is not closed, it has to exist in the graph some node reachable
    -- from @n@ that it is not a let-bound variable or that it has a non-closed
    -- type. Thus, the "reason" is a path from @n@ to this offending node.
    --
    -- When @n@ is not closed, we traverse the graph reachable from @n@ to build
    -- the reason.
    --