GHC/Tc/Gen/Splice.hs

    {-# LANGUAGE CPP                    #-}
    {-# LANGUAGE FlexibleInstances      #-}
    {-# LANGUAGE FunctionalDependencies #-}
    {-# LANGUAGE GADTs                  #-}
    {-# LANGUAGE InstanceSigs           #-}
    {-# LANGUAGE MultiWayIf             #-}
    {-# LANGUAGE ScopedTypeVariables    #-}
    {-# LANGUAGE TupleSections          #-}
    {-# LANGUAGE TypeFamilies           #-}

    {-# OPTIONS_GHC -fno-warn-orphans #-}
    {-# OPTIONS_GHC -Wno-incomplete-uni-patterns #-}

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

    -}

    -- | Template Haskell splices
    module GHC.Tc.Gen.Splice(
         tcSpliceExpr, tcTypedBracket, tcUntypedBracket,
    --     runQuasiQuoteExpr, runQuasiQuotePat,
    --     runQuasiQuoteDecl, runQuasiQuoteType,
         runAnnotation,

         runMetaE, runMetaP, runMetaT, runMetaD, runQuasi,
         tcTopSpliceExpr, lookupThName_maybe,
         defaultRunMeta, runMeta', runRemoteModFinalizers,
         finishTH, runTopSplice
          ) where

    #include "HsVersions.h"

    import GHC.Prelude

    import GHC.Driver.Plugins
    import GHC.Driver.Main
    import GHC.Driver.Session
    import GHC.Driver.Env
    import GHC.Driver.Hooks

    import GHC.Hs

    import GHC.Tc.Utils.Monad
    import GHC.Tc.Utils.TcType
    import GHC.Tc.Gen.Expr
    import GHC.Tc.Utils.Unify
    import GHC.Tc.Utils.Env
    import GHC.Tc.Types.Origin
    import GHC.Tc.Types.Evidence
    import GHC.Tc.Utils.Zonk
    import GHC.Tc.Solver
    import GHC.Tc.Utils.TcMType
    import GHC.Tc.Gen.HsType
    import GHC.Tc.Instance.Family
    import GHC.Tc.Utils.Instantiate

    import GHC.Core.Multiplicity
    import GHC.Core.Coercion( etaExpandCoAxBranch )
    import GHC.Core.Type as Type
    import GHC.Core.TyCo.Rep as TyCoRep
    import GHC.Core.FamInstEnv
    import GHC.Core.InstEnv as InstEnv

    import GHC.Builtin.Names.TH
    import GHC.Builtin.Names
    import GHC.Builtin.Types

    import GHC.ThToHs
    import GHC.HsToCore.Docs
    import GHC.HsToCore.Expr
    import GHC.HsToCore.Monad
    import GHC.IfaceToCore
    import GHC.Iface.Load

    import GHCi.Message
    import GHCi.RemoteTypes
    import GHC.Runtime.Interpreter

    import GHC.Rename.Splice( traceSplice, SpliceInfo(..))
    import GHC.Rename.Expr
    import GHC.Rename.Env
    import GHC.Rename.Utils  ( HsDocContext(..) )
    import GHC.Rename.Fixity ( lookupFixityRn_help )
    import GHC.Rename.HsType

    import GHC.Core.Class
    import GHC.Core.TyCon
    import GHC.Core.Coercion.Axiom
    import GHC.Core.PatSyn
    import GHC.Core.ConLike
    import GHC.Core.DataCon as DataCon

    import GHC.Types.FieldLabel
    import GHC.Types.SrcLoc
    import GHC.Types.Name.Env
    import GHC.Types.Name.Set
    import GHC.Types.Name.Reader
    import GHC.Types.Name.Occurrence as OccName
    import GHC.Types.Var
    import GHC.Types.Id
    import GHC.Types.Id.Info
    import GHC.Types.Unique
    import GHC.Types.Var.Set
    import GHC.Types.Meta
    import GHC.Types.Basic hiding( SuccessFlag(..) )
    import GHC.Types.Error
    import GHC.Types.Fixity as Hs
    import GHC.Types.Annotations
    import GHC.Types.Name
    import GHC.Serialized

    import GHC.Unit.Finder
    import GHC.Unit.Module
    import GHC.Unit.Module.ModIface
    import GHC.Unit.Module.Deps

    import GHC.Utils.Misc
    import GHC.Utils.Panic as Panic
    import GHC.Utils.Lexeme
    import GHC.Utils.Outputable
    import GHC.Utils.Logger

    import GHC.Utils.TmpFs ( newTempName, TempFileLifetime(..) )

    import GHC.Data.FastString
    import GHC.Data.Maybe( MaybeErr(..) )
    import qualified GHC.Data.EnumSet as EnumSet

    import qualified Language.Haskell.TH as TH
    -- THSyntax gives access to internal functions and data types
    import qualified Language.Haskell.TH.Syntax as TH

    #if defined(HAVE_INTERNAL_INTERPRETER)
    -- Because GHC.Desugar might not be in the base library of the bootstrapping compiler
    import GHC.Desugar      ( AnnotationWrapper(..) )
    import Unsafe.Coerce    ( unsafeCoerce )
    #endif

    import Control.Monad
    import Control.Exception
    import Data.Binary
    import Data.Binary.Get
    import Data.List        ( find )
    import Data.Maybe
    import qualified Data.ByteString as B
    import qualified Data.ByteString.Lazy as LB
    import Data.Dynamic  ( fromDynamic, toDyn )
    import qualified Data.IntMap as IntMap
    import qualified Data.Map as Map
    import Data.Typeable ( typeOf, Typeable, TypeRep, typeRep )
    import Data.Data (Data)
    import Data.Proxy    ( Proxy (..) )

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Main interface + stubs for the non-GHCI case
    *                                                                      *
    ************************************************************************
    -}

    tcTypedBracket   :: HsExpr GhcRn -> HsBracket GhcRn -> ExpRhoType -> TcM (HsExpr GhcTc)
    tcUntypedBracket :: HsExpr GhcRn -> HsBracket GhcRn -> [PendingRnSplice] -> ExpRhoType
                     -> TcM (HsExpr GhcTc)
    tcSpliceExpr     :: HsSplice GhcRn  -> ExpRhoType -> TcM (HsExpr GhcTc)
            -- None of these functions add constraints to the LIE

    -- runQuasiQuoteExpr :: HsQuasiQuote RdrName -> RnM (LHsExpr RdrName)
    -- runQuasiQuotePat  :: HsQuasiQuote RdrName -> RnM (LPat RdrName)
    -- runQuasiQuoteType :: HsQuasiQuote RdrName -> RnM (LHsType RdrName)
    -- runQuasiQuoteDecl :: HsQuasiQuote RdrName -> RnM [LHsDecl RdrName]

    runAnnotation     :: CoreAnnTarget -> LHsExpr GhcRn -> TcM Annotation
    {-
    ************************************************************************
    *                                                                      *
    \subsection{Quoting an expression}
    *                                                                      *
    ************************************************************************
    -}

    -- See Note [How brackets and nested splices are handled]
    -- tcTypedBracket :: HsBracket Name -> TcRhoType -> TcM (HsExpr TcId)
    tcTypedBracket rn_expr brack@(TExpBr _ expr) res_ty
      = addErrCtxt (quotationCtxtDoc brack) $
        do { cur_stage <- getStage
           ; ps_ref <- newMutVar []
           ; lie_var <- getConstraintVar   -- Any constraints arising from nested splices
                                           -- should get thrown into the constraint set
                                           -- from outside the bracket

           -- Make a new type variable for the type of the overall quote
           ; m_var <- mkTyVarTy <$> mkMetaTyVar
           -- Make sure the type variable satisfies Quote
           ; ev_var <- emitQuoteWanted m_var
           -- Bundle them together so they can be used in GHC.HsToCore.Quote for desugaring
           -- brackets.
           ; let wrapper = QuoteWrapper ev_var m_var
           -- Typecheck expr to make sure it is valid,
           -- Throw away the typechecked expression but return its type.
           -- We'll typecheck it again when we splice it in somewhere
           ; (_tc_expr, expr_ty) <- setStage (Brack cur_stage (TcPending ps_ref lie_var wrapper)) $
                                    tcScalingUsage Many $
                                    -- Scale by Many, TH lifting is currently nonlinear (#18465)
                                    tcInferRhoNC expr
                                    -- NC for no context; tcBracket does that
           ; let rep = getRuntimeRep expr_ty
           ; meta_ty <- tcTExpTy m_var expr_ty
           ; ps' <- readMutVar ps_ref
           ; texpco <- tcLookupId unsafeCodeCoerceName
           ; tcWrapResultO (Shouldn'tHappenOrigin "TExpBr")
                           rn_expr
                           (unLoc (mkHsApp (mkLHsWrap (applyQuoteWrapper wrapper)
                                                      (nlHsTyApp texpco [rep, expr_ty]))
                                          (noLocA (HsTcBracketOut noExtField (Just wrapper) brack ps'))))
                           meta_ty res_ty }
    tcTypedBracket _ other_brack _
      = pprPanic "tcTypedBracket" (ppr other_brack)

    -- tcUntypedBracket :: HsBracket Name -> [PendingRnSplice] -> ExpRhoType -> TcM (HsExpr TcId)
    -- See Note [Typechecking Overloaded Quotes]
    tcUntypedBracket rn_expr brack ps res_ty
      = do { traceTc "tc_bracket untyped" (ppr brack $$ ppr ps)

           -- Create the type m Exp for expression bracket, m Type for a type
           -- bracket and so on. The brack_info is a Maybe because the
           -- VarBracket ('a) isn't overloaded, but also shouldn't contain any
           -- splices.
           ; (brack_info, expected_type) <- brackTy brack

           -- Match the expected type with the type of all the internal
           -- splices. They might have further constrained types and if they do
           -- we want to reflect that in the overall type of the bracket.
           ; ps' <- case quoteWrapperTyVarTy <$> brack_info of
                      Just m_var -> mapM (tcPendingSplice m_var) ps
                      Nothing -> ASSERT(null ps) return []

           ; traceTc "tc_bracket done untyped" (ppr expected_type)

           -- Unify the overall type of the bracket with the expected result
           -- type
           ; tcWrapResultO BracketOrigin rn_expr
                (HsTcBracketOut noExtField brack_info brack ps')
                expected_type res_ty

           }

    -- | A type variable with kind * -> * named "m"
    mkMetaTyVar :: TcM TyVar
    mkMetaTyVar =
      newNamedFlexiTyVar (fsLit "m") (mkVisFunTyMany liftedTypeKind liftedTypeKind)

    -- | For a type 'm', emit the constraint 'Quote m'.
    emitQuoteWanted :: Type -> TcM EvVar
    emitQuoteWanted m_var =  do
            quote_con <- tcLookupTyCon quoteClassName
            emitWantedEvVar BracketOrigin $
              mkTyConApp quote_con [m_var]

    ---------------
    -- | Compute the expected type of a quotation, and also the QuoteWrapper in
    -- the case where it is an overloaded quotation. All quotation forms are
    -- overloaded aprt from Variable quotations ('foo)
    brackTy :: HsBracket GhcRn -> TcM (Maybe QuoteWrapper, Type)
    brackTy b =
      let mkTy n = do
            -- New polymorphic type variable for the bracket
            m_var <- mkTyVarTy <$> mkMetaTyVar
            -- Emit a Quote constraint for the bracket
            ev_var <- emitQuoteWanted m_var
            -- Construct the final expected type of the quote, for example
            -- m Exp or m Type
            final_ty <- mkAppTy m_var <$> tcMetaTy n
            -- Return the evidence variable and metavariable to be used during
            -- desugaring.
            let wrapper = QuoteWrapper ev_var m_var
            return (Just wrapper, final_ty)
      in
      case b of
        (VarBr {}) -> (Nothing,) <$> tcMetaTy nameTyConName
                                               -- Result type is Var (not Quote-monadic)
        (ExpBr {})  -> mkTy expTyConName  -- Result type is m Exp
        (TypBr {})  -> mkTy typeTyConName -- Result type is m Type
        (DecBrG {}) -> mkTy decsTyConName -- Result type is m [Dec]
        (PatBr {})  -> mkTy patTyConName  -- Result type is m Pat
        (DecBrL {}) -> panic "tcBrackTy: Unexpected DecBrL"
        (TExpBr {}) -> panic "tcUntypedBracket: Unexpected TExpBr"

    ---------------
    -- | Typechecking a pending splice from a untyped bracket
    tcPendingSplice :: TcType -- Metavariable for the expected overall type of the
                              -- quotation.
                    -> PendingRnSplice
                    -> TcM PendingTcSplice
    tcPendingSplice m_var (PendingRnSplice flavour splice_name expr)
      -- See Note [Typechecking Overloaded Quotes]
      = do { meta_ty <- tcMetaTy meta_ty_name
             -- Expected type of splice, e.g. m Exp
           ; let expected_type = mkAppTy m_var meta_ty
           ; expr' <- tcScalingUsage Many $ tcCheckPolyExpr expr expected_type
                      -- Scale by Many, TH lifting is currently nonlinear (#18465)
           ; return (PendingTcSplice splice_name expr') }
      where
         meta_ty_name = case flavour of
                           UntypedExpSplice  -> expTyConName
                           UntypedPatSplice  -> patTyConName
                           UntypedTypeSplice -> typeTyConName
                           UntypedDeclSplice -> decsTyConName

    ---------------
    -- Takes a m and tau and returns the type m (TExp tau)
    tcTExpTy :: TcType -> TcType -> TcM TcType
    tcTExpTy m_ty exp_ty
      = do { unless (isTauTy exp_ty) $ addErr (err_msg exp_ty)
           ; codeCon <- tcLookupTyCon codeTyConName
           ; let rep = getRuntimeRep exp_ty
           ; return (mkTyConApp codeCon [rep, m_ty, exp_ty]) }
      where
        err_msg ty
          = vcat [ text "Illegal polytype:" <+> ppr ty
                 , text "The type of a Typed Template Haskell expression must" <+>
                   text "not have any quantification." ]

    quotationCtxtDoc :: HsBracket GhcRn -> SDoc
    quotationCtxtDoc br_body
      = hang (text "In the Template Haskell quotation")
             2 (ppr br_body)

      -- The whole of the rest of the file is the else-branch (ie stage2 only)

    {-
    Note [How top-level splices are handled]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Top-level splices (those not inside a [| .. |] quotation bracket) are handled
    very straightforwardly:

      1. tcTopSpliceExpr: typecheck the body e of the splice $(e)

      2. runMetaT: desugar, compile, run it, and convert result back to
         GHC.Hs syntax RdrName (of the appropriate flavour, eg HsType RdrName,
         HsExpr RdrName etc)

      3. treat the result as if that's what you saw in the first place
         e.g for HsType, rename and kind-check
             for HsExpr, rename and type-check

         (The last step is different for decls, because they can *only* be
          top-level: we return the result of step 2.)

    Note [How brackets and nested splices are handled]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Nested splices (those inside a [| .. |] quotation bracket),
    are treated quite differently.

    Remember, there are two forms of bracket
             typed   [|| e ||]
       and untyped   [|  e  |]

    The life cycle of a typed bracket:
       * Starts as HsBracket

       * When renaming:
            * Set the ThStage to (Brack s RnPendingTyped)
            * Rename the body
            * Result is still a HsBracket

       * When typechecking:
            * Set the ThStage to (Brack s (TcPending ps_var lie_var))
            * Typecheck the body, and throw away the elaborated result
            * Nested splices (which must be typed) are typechecked, and
              the results accumulated in ps_var; their constraints
              accumulate in lie_var
            * Result is a HsTcBracketOut rn_brack pending_splices
              where rn_brack is the incoming renamed bracket

    The life cycle of a un-typed bracket:
       * Starts as HsBracket

       * When renaming:
            * Set the ThStage to (Brack s (RnPendingUntyped ps_var))
            * Rename the body
            * Nested splices (which must be untyped) are renamed, and the
              results accumulated in ps_var
            * Result is still (HsRnBracketOut rn_body pending_splices)

       * When typechecking a HsRnBracketOut
            * Typecheck the pending_splices individually
            * Ignore the body of the bracket; just check that the context
              expects a bracket of that type (e.g. a [p| pat |] bracket should
              be in a context needing a (Q Pat)
            * Result is a HsTcBracketOut rn_brack pending_splices
              where rn_brack is the incoming renamed bracket

    In both cases, desugaring happens like this:
      * HsTcBracketOut is desugared by GHC.HsToCore.Quote.dsBracket.  It

          a) Extends the ds_meta environment with the PendingSplices
             attached to the bracket

          b) Converts the quoted (HsExpr Name) to a CoreExpr that, when
             run, will produce a suitable TH expression/type/decl.  This
             is why we leave the *renamed* expression attached to the bracket:
             the quoted expression should not be decorated with all the goop
             added by the type checker

      * Each splice carries a unique Name, called a "splice point", thus
        ${n}(e).  The name is initialised to an (Unqual "splice") when the
        splice is created; the renamer gives it a unique.

      * When GHC.HsToCore.Quote (used to desugar the body of the bracket) comes across
        a splice, it looks up the splice's Name, n, in the ds_meta envt,
        to find an (HsExpr Id) that should be substituted for the splice;
        it just desugars it to get a CoreExpr (GHC.HsToCore.Quote.repSplice).

    Example:
        Source:       f = [| Just $(g 3) |]
          The [| |] part is a HsBracket

        Typechecked:  f = [| Just ${s7}(g 3) |]{s7 = g Int 3}
          The [| |] part is a HsBracketOut, containing *renamed*
            (not typechecked) expression
          The "s7" is the "splice point"; the (g Int 3) part
            is a typechecked expression

        Desugared:    f = do { s7 <- g Int 3
                             ; return (ConE "Data.Maybe.Just" s7) }

    Note [Template Haskell state diagram]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Here are the ThStages, s, their corresponding level numbers
    (the result of (thLevel s)), and their state transitions.
    The top level of the program is stage Comp:

         Start here
             |
             V
          -----------     $      ------------   $
          |  Comp   | ---------> |  Splice  | -----|
          |   1     |            |    0     | <----|
          -----------            ------------
            ^     |                ^      |
          $ |     | [||]         $ |      | [||]
            |     v                |      v
       --------------          ----------------
       | Brack Comp |          | Brack Splice |
       |     2      |          |      1       |
       --------------          ----------------

    * Normal top-level declarations start in state Comp
           (which has level 1).
      Annotations start in state Splice, since they are
           treated very like a splice (only without a '$')

    * Code compiled in state Splice (and only such code)
      will be *run at compile time*, with the result replacing
      the splice

    * The original paper used level -1 instead of 0, etc.

    * The original paper did not allow a splice within a
      splice, but there is no reason not to. This is the
      $ transition in the top right.

    Note [Template Haskell levels]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    * Imported things are impLevel (= 0)

    * However things at level 0 are not *necessarily* imported.
          eg  $( \b -> ... )   here b is bound at level 0

    * In GHCi, variables bound by a previous command are treated
      as impLevel, because we have bytecode for them.

    * Variables are bound at the "current level"

    * The current level starts off at outerLevel (= 1)

    * The level is decremented by splicing $(..)
                   incremented by brackets [| |]
                   incremented by name-quoting 'f

    * When a variable is used, checkWellStaged compares
            bind:  binding level, and
            use:   current level at usage site

      Generally
            bind > use      Always error (bound later than used)
                            [| \x -> $(f x) |]

            bind = use      Always OK (bound same stage as used)
                            [| \x -> $(f [| x |]) |]

            bind < use      Inside brackets, it depends
                            Inside splice, OK
                            Inside neither, OK

      For (bind < use) inside brackets, there are three cases:
        - Imported things   OK      f = [| map |]
        - Top-level things  OK      g = [| f |]
        - Non-top-level     Only if there is a liftable instance
                                    h = \(x:Int) -> [| x |]

      To track top-level-ness we use the ThBindEnv in TcLclEnv

      For example:
               f = ...
               g1 = $(map ...)         is OK
               g2 = $(f ...)           is not OK; because we haven't compiled f yet

    Note [Typechecking Overloaded Quotes]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    The main function for typechecking untyped quotations is `tcUntypedBracket`.

    Consider an expression quote, `[| e |]`, its type is `forall m . Quote m => m Exp`.
    When we typecheck it we therefore create a template of a metavariable `m` applied to `Exp` and
    emit a constraint `Quote m`. All this is done in the `brackTy` function.
    `brackTy` also selects the correct contents type for the quotation (Exp, Type, Decs etc).

    The meta variable and the constraint evidence variable are
    returned together in a `QuoteWrapper` and then passed along to two further places
    during compilation:

    1. Typechecking nested splices (immediately in tcPendingSplice)
    2. Desugaring quotations (see GHC.HsToCore.Quote)

    `tcPendingSplice` takes the `m` type variable as an argument and checks
    each nested splice against this variable `m`. During this
    process the variable `m` can either be fixed to a specific value or further constrained by the
    nested splices.

    Once we have checked all the nested splices, the quote type is checked against
    the expected return type.

    The process is very simple and like typechecking a list where the quotation is
    like the container and the splices are the elements of the list which must have
    a specific type.

    After the typechecking process is completed, the evidence variable for `Quote m`
    and the type `m` is stored in a `QuoteWrapper` which is passed through the pipeline
    and used when desugaring quotations.

    Typechecking typed quotations is a similar idea but the `QuoteWrapper` is stored
    in the `PendingStuff` as the nested splices are gathered up in a different way
    to untyped splices. Untyped splices are found in the renamer but typed splices are
    not typechecked and extracted until during typechecking.

    -}

    -- | We only want to produce warnings for TH-splices if the user requests so.
    -- See Note [Warnings for TH splices].
    getThSpliceOrigin :: TcM Origin
    getThSpliceOrigin = do
      warn <- goptM Opt_EnableThSpliceWarnings
      if warn then return FromSource else return Generated

    {- Note [Warnings for TH splices]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We only produce warnings for TH splices when the user requests so
    (-fenable-th-splice-warnings). There are multiple reasons:

      * It's not clear that the user that compiles a splice is the author of the code
        that produces the warning. Think of the situation where they just splice in
        code from a third-party library that produces incomplete pattern matches.
        In this scenario, the user isn't even able to fix that warning.
      * Gathering information for producing the warnings (pattern-match check
        warnings in particular) is costly. There's no point in doing so if the user
        is not interested in those warnings.

    That's why we store Origin flags in the Haskell AST. The functions from ThToHs
    take such a flag and depending on whether TH splice warnings were enabled or
    not, we pass FromSource (if the user requests warnings) or Generated
    (otherwise). This is implemented in getThSpliceOrigin.

    For correct pattern-match warnings it's crucial that we annotate the Origin
    consistently (#17270). In the future we could offer the Origin as part of the
    TH AST. That would enable us to give quotes from the current module get
    FromSource origin, and/or third library authors to tag certain parts of
    generated code as FromSource to enable warnings.
    That effort is tracked in #14838.
    -}

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Splicing an expression}
    *                                                                      *
    ************************************************************************
    -}

    tcSpliceExpr splice@(HsTypedSplice _ _ name expr) res_ty
      = addErrCtxt (spliceCtxtDoc splice) $
        setSrcSpan (getLocA expr)    $ do
        { stage <- getStage
        ; case stage of
              Splice {}            -> tcTopSplice expr res_ty
              Brack pop_stage pend -> tcNestedSplice pop_stage pend name expr res_ty
              RunSplice _          ->
                -- See Note [RunSplice ThLevel] in "GHC.Tc.Types".
                pprPanic ("tcSpliceExpr: attempted to typecheck a splice when " ++
                          "running another splice") (ppr splice)
              Comp                 -> tcTopSplice expr res_ty
        }
    tcSpliceExpr splice _
      = pprPanic "tcSpliceExpr" (ppr splice)

    {- Note [Collecting modFinalizers in typed splices]
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    'qAddModFinalizer' of the @Quasi TcM@ instance adds finalizers in the local
    environment (see Note [Delaying modFinalizers in untyped splices] in
    GHC.Rename.Splice). Thus after executing the splice, we move the finalizers to the
    finalizer list in the global environment and set them to use the current local
    environment (with 'addModFinalizersWithLclEnv').

    -}

    tcNestedSplice :: ThStage -> PendingStuff -> Name
                    -> LHsExpr GhcRn -> ExpRhoType -> TcM (HsExpr GhcTc)
        -- See Note [How brackets and nested splices are handled]
        -- A splice inside brackets
    tcNestedSplice pop_stage (TcPending ps_var lie_var q@(QuoteWrapper _ m_var)) splice_name expr res_ty
      = do { res_ty <- expTypeToType res_ty
           ; let rep = getRuntimeRep res_ty
           ; meta_exp_ty <- tcTExpTy m_var res_ty
           ; expr' <- setStage pop_stage $
                      setConstraintVar lie_var $
                      tcCheckMonoExpr expr meta_exp_ty
           ; untype_code <- tcLookupId unTypeCodeName
           ; let expr'' = mkHsApp
                            (mkLHsWrap (applyQuoteWrapper q)
                              (nlHsTyApp untype_code [rep, res_ty])) expr'
           ; ps <- readMutVar ps_var
           ; writeMutVar ps_var (PendingTcSplice splice_name expr'' : ps)

           -- The returned expression is ignored; it's in the pending splices
           -- But we still return a plausible expression
           --   (a) in case we print it in debug messages, and
           --   (b) because we test whether it is tagToEnum in Tc.Gen.Expr.tcApp
           ; return (HsSpliceE noAnn $
                     HsSpliced noExtField (ThModFinalizers []) $
                     HsSplicedExpr (unLoc expr'')) }

    tcNestedSplice _ _ splice_name _ _
      = pprPanic "tcNestedSplice: rename stage found" (ppr splice_name)

    tcTopSplice :: LHsExpr GhcRn -> ExpRhoType -> TcM (HsExpr GhcTc)
    tcTopSplice expr res_ty
      = do { -- Typecheck the expression,
             -- making sure it has type Q (T res_ty)
             res_ty <- expTypeToType res_ty
           ; q_type <- tcMetaTy qTyConName
           -- Top level splices must still be of type Q (TExp a)
           ; meta_exp_ty <- tcTExpTy q_type res_ty
           ; q_expr <- tcTopSpliceExpr Typed $
                       tcCheckMonoExpr expr meta_exp_ty
           ; lcl_env <- getLclEnv
           ; let delayed_splice
                  = DelayedSplice lcl_env expr res_ty q_expr
           ; return (HsSpliceE noAnn (XSplice (HsSplicedT delayed_splice)))

           }

    -- This is called in the zonker
    -- See Note [Running typed splices in the zonker]
    runTopSplice :: DelayedSplice -> TcM (HsExpr GhcTc)
    runTopSplice (DelayedSplice lcl_env orig_expr res_ty q_expr)
      = setLclEnv lcl_env $ do {
             zonked_ty <- zonkTcType res_ty
           ; zonked_q_expr <- zonkTopLExpr q_expr
            -- See Note [Collecting modFinalizers in typed splices].
           ; modfinalizers_ref <- newTcRef []
             -- Run the expression
           ; expr2 <- setStage (RunSplice modfinalizers_ref) $
                        runMetaE zonked_q_expr
           ; mod_finalizers <- readTcRef modfinalizers_ref
           ; addModFinalizersWithLclEnv $ ThModFinalizers mod_finalizers
           -- We use orig_expr here and not q_expr when tracing as a call to
           -- unsafeTExpCoerce is added to the original expression by the
           -- typechecker when typed quotes are type checked.
           ; traceSplice (SpliceInfo { spliceDescription = "expression"
                                     , spliceIsDecl      = False
                                     , spliceSource      = Just orig_expr
                                     , spliceGenerated   = ppr expr2 })
            -- Rename and typecheck the spliced-in expression,
            -- making sure it has type res_ty
            -- These steps should never fail; this is a *typed* splice
           ; (res, wcs) <-
                captureConstraints $
                  addErrCtxt (spliceResultDoc zonked_q_expr) $ do
                    { (exp3, _fvs) <- rnLExpr expr2
                    ; tcCheckMonoExpr exp3 zonked_ty }
           ; ev <- simplifyTop wcs
           ; return $ unLoc (mkHsDictLet (EvBinds ev) res)
           }

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

    spliceCtxtDoc :: HsSplice GhcRn -> SDoc
    spliceCtxtDoc splice
      = hang (text "In the Template Haskell splice")
             2 (pprSplice splice)

    spliceResultDoc :: LHsExpr GhcTc -> SDoc
    spliceResultDoc expr
      = sep [ text "In the result of the splice:"
            , nest 2 (char '$' <> ppr expr)
            , text "To see what the splice expanded to, use -ddump-splices"]

    -------------------
    tcTopSpliceExpr :: SpliceType -> TcM (LHsExpr GhcTc) -> TcM (LHsExpr GhcTc)
    -- Note [How top-level splices are handled]
    -- Type check an expression that is the body of a top-level splice
    --   (the caller will compile and run it)
    -- Note that set the level to Splice, regardless of the original level,
    -- before typechecking the expression.  For example:
    --      f x = $( ...$(g 3) ... )
    -- The recursive call to tcCheckPolyExpr will simply expand the
    -- inner escape before dealing with the outer one

    tcTopSpliceExpr isTypedSplice tc_action
      = checkNoErrs $  -- checkNoErrs: must not try to run the thing
                       -- if the type checker fails!
        unsetGOptM Opt_DeferTypeErrors $
                       -- Don't defer type errors.  Not only are we
                       -- going to run this code, but we do an unsafe
                       -- coerce, so we get a seg-fault if, say we
                       -- splice a type into a place where an expression
                       -- is expected (#7276)
        setStage (Splice isTypedSplice) $
        do {    -- Typecheck the expression
             (expr', wanted) <- captureConstraints tc_action
           ; const_binds     <- simplifyTop wanted

              -- Zonk it and tie the knot of dictionary bindings
           ; return $ mkHsDictLet (EvBinds const_binds) expr' }

    {-
    ************************************************************************
    *                                                                      *
            Annotations
    *                                                                      *
    ************************************************************************
    -}

    runAnnotation target expr = do
        -- Find the classes we want instances for in order to call toAnnotationWrapper
        loc <- getSrcSpanM
        data_class <- tcLookupClass dataClassName
        to_annotation_wrapper_id <- tcLookupId toAnnotationWrapperName

        -- Check the instances we require live in another module (we want to execute it..)
        -- and check identifiers live in other modules using TH stage checks. tcSimplifyStagedExpr
        -- also resolves the LIE constraints to detect e.g. instance ambiguity
        zonked_wrapped_expr' <- zonkTopLExpr =<< tcTopSpliceExpr Untyped (
               do { (expr', expr_ty) <- tcInferRhoNC expr
                    -- We manually wrap the typechecked expression in a call to toAnnotationWrapper
                    -- By instantiating the call >here< it gets registered in the
                    -- LIE consulted by tcTopSpliceExpr
                    -- and hence ensures the appropriate dictionary is bound by const_binds
                  ; wrapper <- instCall AnnOrigin [expr_ty] [mkClassPred data_class [expr_ty]]
                  ; let loc' = noAnnSrcSpan loc
                  ; let specialised_to_annotation_wrapper_expr
                          = L loc' (mkHsWrap wrapper
                                     (HsVar noExtField (L (noAnnSrcSpan loc) to_annotation_wrapper_id)))
                  ; return (L loc' (HsApp noComments
                                    specialised_to_annotation_wrapper_expr expr'))
                                    })

        -- Run the appropriately wrapped expression to get the value of
        -- the annotation and its dictionaries. The return value is of
        -- type AnnotationWrapper by construction, so this conversion is
        -- safe
        serialized <- runMetaAW zonked_wrapped_expr'
        return Annotation {
                   ann_target = target,
                   ann_value = serialized
               }

    convertAnnotationWrapper :: ForeignHValue -> TcM (Either SDoc Serialized)
    convertAnnotationWrapper fhv = do
      interp <- tcGetInterp
      case interpInstance interp of
        ExternalInterp {} -> Right <$> runTH THAnnWrapper fhv
    #if defined(HAVE_INTERNAL_INTERPRETER)
        InternalInterp    -> do
          annotation_wrapper <- liftIO $ wormhole interp fhv
          return $ Right $
            case unsafeCoerce annotation_wrapper of
               AnnotationWrapper value | let serialized = toSerialized serializeWithData value ->
                   -- Got the value and dictionaries: build the serialized value and
                   -- call it a day. We ensure that we seq the entire serialized value
                   -- in order that any errors in the user-written code for the
                   -- annotation are exposed at this point.  This is also why we are
                   -- doing all this stuff inside the context of runMeta: it has the
                   -- facilities to deal with user error in a meta-level expression
                   seqSerialized serialized `seq` serialized

    -- | Force the contents of the Serialized value so weknow it doesn't contain any bottoms
    seqSerialized :: Serialized -> ()
    seqSerialized (Serialized the_type bytes) = the_type `seq` bytes `seqList` ()

    #endif

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Running an expression}
    *                                                                      *
    ************************************************************************
    -}

    runQuasi :: TH.Q a -> TcM a
    runQuasi act = TH.runQ act

    runRemoteModFinalizers :: ThModFinalizers -> TcM ()
    runRemoteModFinalizers (ThModFinalizers finRefs) = do
      let withForeignRefs [] f = f []
          withForeignRefs (x : xs) f = withForeignRef x $ \r ->
            withForeignRefs xs $ \rs -> f (r : rs)
      interp <- tcGetInterp
      case interpInstance interp of
    #if defined(HAVE_INTERNAL_INTERPRETER)
        InternalInterp -> do
          qs <- liftIO (withForeignRefs finRefs $ mapM localRef)
          runQuasi $ sequence_ qs
    #endif

        ExternalInterp conf iserv -> withIServ_ conf iserv $ \i -> do
          tcg <- getGblEnv
          th_state <- readTcRef (tcg_th_remote_state tcg)
          case th_state of
            Nothing -> return () -- TH was not started, nothing to do
            Just fhv -> do
              liftIO $ withForeignRef fhv $ \st ->
                withForeignRefs finRefs $ \qrefs ->
                  writeIServ i (putMessage (RunModFinalizers st qrefs))
              () <- runRemoteTH i []
              readQResult i

    runQResult
      :: (a -> String)
      -> (Origin -> SrcSpan -> a -> b)
      -> (ForeignHValue -> TcM a)
      -> SrcSpan
      -> ForeignHValue {- TH.Q a -}
      -> TcM b
    runQResult show_th f runQ expr_span hval
      = do { th_result <- runQ hval
           ; th_origin <- getThSpliceOrigin
           ; traceTc "Got TH result:" (text (show_th th_result))
           ; return (f th_origin expr_span th_result) }

    -----------------
    runMeta :: (MetaHook TcM -> LHsExpr GhcTc -> TcM hs_syn)
            -> LHsExpr GhcTc
            -> TcM hs_syn
    runMeta unwrap e = do
        hooks <- getHooks
        case runMetaHook hooks of
            Nothing -> unwrap defaultRunMeta e
            Just h  -> unwrap h e

    defaultRunMeta :: MetaHook TcM
    defaultRunMeta (MetaE r)
      = fmap r . runMeta' True ppr (runQResult TH.pprint convertToHsExpr runTHExp)
    defaultRunMeta (MetaP r)
      = fmap r . runMeta' True ppr (runQResult TH.pprint convertToPat runTHPat)
    defaultRunMeta (MetaT r)
      = fmap r . runMeta' True ppr (runQResult TH.pprint convertToHsType runTHType)
    defaultRunMeta (MetaD r)
      = fmap r . runMeta' True ppr (runQResult TH.pprint convertToHsDecls runTHDec)
    defaultRunMeta (MetaAW r)
      = fmap r . runMeta' False (const empty) (const convertAnnotationWrapper)
        -- We turn off showing the code in meta-level exceptions because doing so exposes
        -- the toAnnotationWrapper function that we slap around the user's code

    ----------------
    runMetaAW :: LHsExpr GhcTc         -- Of type AnnotationWrapper
              -> TcM Serialized
    runMetaAW = runMeta metaRequestAW

    runMetaE :: LHsExpr GhcTc          -- Of type (Q Exp)
             -> TcM (LHsExpr GhcPs)
    runMetaE = runMeta metaRequestE

    runMetaP :: LHsExpr GhcTc          -- Of type (Q Pat)
             -> TcM (LPat GhcPs)
    runMetaP = runMeta metaRequestP

    runMetaT :: LHsExpr GhcTc          -- Of type (Q Type)
             -> TcM (LHsType GhcPs)
    runMetaT = runMeta metaRequestT

    runMetaD :: LHsExpr GhcTc          -- Of type Q [Dec]
             -> TcM [LHsDecl GhcPs]
    runMetaD = runMeta metaRequestD

    ---------------
    runMeta' :: Bool                 -- Whether code should be printed in the exception message
             -> (hs_syn -> SDoc)                                    -- how to print the code
             -> (SrcSpan -> ForeignHValue -> TcM (Either SDoc hs_syn))        -- How to run x
             -> LHsExpr GhcTc        -- Of type x; typically x = Q TH.Exp, or
                                     --    something like that
             -> TcM hs_syn           -- Of type t
    runMeta' show_code ppr_hs run_and_convert expr
      = do  { traceTc "About to run" (ppr expr)
            ; recordThSpliceUse -- seems to be the best place to do this,
                                -- we catch all kinds of splices and annotations.

            -- Check that we've had no errors of any sort so far.
            -- For example, if we found an error in an earlier defn f, but
            -- recovered giving it type f :: forall a.a, it'd be very dodgy
            -- to carry ont.  Mind you, the staging restrictions mean we won't
            -- actually run f, but it still seems wrong. And, more concretely,
            -- see #5358 for an example that fell over when trying to
            -- reify a function with a "?" kind in it.  (These don't occur
            -- in type-correct programs.
            ; failIfErrsM

            -- run plugins
            ; hsc_env <- getTopEnv
            ; expr' <- withPlugins hsc_env spliceRunAction expr

            -- Desugar
            ; ds_expr <- initDsTc (dsLExpr expr')
            -- Compile and link it; might fail if linking fails
            ; src_span <- getSrcSpanM
            ; traceTc "About to run (desugared)" (ppr ds_expr)
            ; either_hval <- tryM $ liftIO $
                             GHC.Driver.Main.hscCompileCoreExpr hsc_env src_span ds_expr
            ; case either_hval of {
                Left exn   -> fail_with_exn "compile and link" exn ;
                Right hval -> do

            {       -- Coerce it to Q t, and run it

                    -- Running might fail if it throws an exception of any kind (hence tryAllM)
                    -- including, say, a pattern-match exception in the code we are running
                    --
                    -- We also do the TH -> HS syntax conversion inside the same
                    -- exception-catching thing so that if there are any lurking
                    -- exceptions in the data structure returned by hval, we'll
                    -- encounter them inside the try
                    --
                    -- See Note [Exceptions in TH]
              let expr_span = getLocA expr
            ; either_tval <- tryAllM $
                             setSrcSpan expr_span $ -- Set the span so that qLocation can
                                                    -- see where this splice is
                 do { mb_result <- run_and_convert expr_span hval
                    ; case mb_result of
                        Left err     -> failWithTc err
                        Right result -> do { traceTc "Got HsSyn result:" (ppr_hs result)
                                           ; return $! result } }

            ; case either_tval of
                Right v -> return v
                Left se -> case fromException se of
                             Just IOEnvFailure -> failM -- Error already in Tc monad
                             _ -> fail_with_exn "run" se -- Exception
            }}}
      where
        -- see Note [Concealed TH exceptions]
        fail_with_exn :: Exception e => String -> e -> TcM a
        fail_with_exn phase exn = do
            exn_msg <- liftIO $ Panic.safeShowException exn
            let msg = vcat [text "Exception when trying to" <+> text phase <+> text "compile-time code:",
                            nest 2 (text exn_msg),
                            if show_code then text "Code:" <+> ppr expr else empty]
            failWithTc msg

    {-
    Note [Running typed splices in the zonker]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    See #15471 for the full discussion.

    For many years typed splices were run immediately after they were type checked
    however, this is too early as it means to zonk some type variables before
    they can be unified with type variables in the surrounding context.

    For example,

    ```
    module A where

    test_foo :: forall a . Q (TExp (a -> a))
    test_foo = [|| id ||]

    module B where

    import A

    qux = $$(test_foo)
    ```

    We would expect `qux` to have inferred type `forall a . a -> a` but if
    we run the splices too early the unified variables are zonked to `Any`. The
    inferred type is the unusable `Any -> Any`.

    To run the splice, we must compile `test_foo` all the way to byte code.
    But at the moment when the type checker is looking at the splice, test_foo
    has type `Q (TExp (alpha -> alpha))` and we
    certainly can't compile code involving unification variables!

    We could default `alpha` to `Any` but then we infer `qux :: Any -> Any`
    which definitely is not what we want.  Moreover, if we had
      qux = [$$(test_foo), (\x -> x +1::Int)]
    then `alpha` would have to be `Int`.

    Conclusion: we must defer taking decisions about `alpha` until the
    typechecker is done; and *then* we can run the splice.  It's fine to do it
    later, because we know it'll produce type-correct code.

    Deferring running the splice until later, in the zonker, means that the
    unification variables propagate upwards from the splice into the surrounding
    context and are unified correctly.

    This is implemented by storing the arguments we need for running the splice
    in a `DelayedSplice`. In the zonker, the arguments are passed to
    `GHC.Tc.Gen.Splice.runTopSplice` and the expression inserted into the AST as normal.

    Note [Exceptions in TH]
    ~~~~~~~~~~~~~~~~~~~~~~~
    Suppose we have something like this
            $( f 4 )
    where
            f :: Int -> Q [Dec]
            f n | n>3       = fail "Too many declarations"
                | otherwise = ...

    The 'fail' is a user-generated failure, and should be displayed as a
    perfectly ordinary compiler error message, not a panic or anything
    like that.  Here's how it's processed:

      * 'fail' is the monad fail.  The monad instance for Q in TH.Syntax
        effectively transforms (fail s) to
            qReport True s >> fail
        where 'qReport' comes from the Quasi class and fail from its monad
        superclass.

      * The TcM monad is an instance of Quasi (see GHC.Tc.Gen.Splice), and it implements
        (qReport True s) by using addErr to add an error message to the bag of errors.
        The 'fail' in TcM raises an IOEnvFailure exception

     * 'qReport' forces the message to ensure any exception hidden in unevaluated
       thunk doesn't get into the bag of errors. Otherwise the following splice
       will trigger panic (#8987):
            $(fail undefined)
       See also Note [Concealed TH exceptions]

      * So, when running a splice, we catch all exceptions; then for
            - an IOEnvFailure exception, we assume the error is already
                    in the error-bag (above)
            - other errors, we add an error to the bag
        and then fail

    Note [Concealed TH exceptions]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When displaying the error message contained in an exception originated from TH
    code, we need to make sure that the error message itself does not contain an
    exception.  For example, when executing the following splice:

        $( error ("foo " ++ error "bar") )

    the message for the outer exception is a thunk which will throw the inner
    exception when evaluated.

    For this reason, we display the message of a TH exception using the
    'safeShowException' function, which recursively catches any exception thrown
    when showing an error message.

    To call runQ in the Tc monad, we need to make TcM an instance of Quasi:
    -}

    instance TH.Quasi TcM where
      qNewName s = do { u <- newUnique
                      ; let i = toInteger (getKey u)
                      ; return (TH.mkNameU s i) }

      -- 'msg' is forced to ensure exceptions don't escape,
      -- see Note [Exceptions in TH]
      qReport True msg  = seqList msg $ addErr  (text msg)
      qReport False msg = seqList msg $ addWarn NoReason (text msg)

      qLocation = do { m <- getModule
                     ; l <- getSrcSpanM
                     ; r <- case l of
                            UnhelpfulSpan _ -> pprPanic "qLocation: Unhelpful location"
                                                        (ppr l)
                            RealSrcSpan s _ -> return s
                     ; return (TH.Loc { TH.loc_filename = unpackFS (srcSpanFile r)
                                      , TH.loc_module   = moduleNameString (moduleName m)
                                      , TH.loc_package  = unitString (moduleUnit m)
                                      , TH.loc_start = (srcSpanStartLine r, srcSpanStartCol r)
                                      , TH.loc_end = (srcSpanEndLine   r, srcSpanEndCol   r) }) }

      qLookupName       = lookupName
      qReify            = reify
      qReifyFixity nm   = lookupThName nm >>= reifyFixity
      qReifyType        = reifyTypeOfThing
      qReifyInstances   = reifyInstances
      qReifyRoles       = reifyRoles
      qReifyAnnotations = reifyAnnotations
      qReifyModule      = reifyModule
      qReifyConStrictness nm = do { nm' <- lookupThName nm
                                  ; dc  <- tcLookupDataCon nm'
                                  ; let bangs = dataConImplBangs dc
                                  ; return (map reifyDecidedStrictness bangs) }

            -- For qRecover, discard error messages if
            -- the recovery action is chosen.  Otherwise
            -- we'll only fail higher up.
      qRecover recover main = tryTcDiscardingErrs recover main

      qAddDependentFile fp = do
        ref <- fmap tcg_dependent_files getGblEnv
        dep_files <- readTcRef ref
        writeTcRef ref (fp:dep_files)

      qAddTempFile suffix = do
        dflags <- getDynFlags
        logger <- getLogger
        tmpfs  <- hsc_tmpfs <$> getTopEnv
        liftIO $ newTempName logger tmpfs dflags TFL_GhcSession suffix

      qAddTopDecls thds = do
          l <- getSrcSpanM
          th_origin <- getThSpliceOrigin
          let either_hval = convertToHsDecls th_origin l thds
          ds <- case either_hval of
                  Left exn -> failWithTc $
                    hang (text "Error in a declaration passed to addTopDecls:")
                       2 exn
                  Right ds -> return ds
          mapM_ (checkTopDecl . unLoc) ds
          th_topdecls_var <- fmap tcg_th_topdecls getGblEnv
          updTcRef th_topdecls_var (\topds -> ds ++ topds)
        where
          checkTopDecl :: HsDecl GhcPs -> TcM ()
          checkTopDecl (ValD _ binds)
            = mapM_ bindName (collectHsBindBinders CollNoDictBinders binds)
          checkTopDecl (SigD _ _)
            = return ()
          checkTopDecl (AnnD _ _)
            = return ()
          checkTopDecl (ForD _ (ForeignImport { fd_name = L _ name }))
            = bindName name
          checkTopDecl _
            = addErr $ text "Only function, value, annotation, and foreign import declarations may be added with addTopDecl"

          bindName :: RdrName -> TcM ()
          bindName (Exact n)
            = do { th_topnames_var <- fmap tcg_th_topnames getGblEnv
                 ; updTcRef th_topnames_var (\ns -> extendNameSet ns n)
                 }

          bindName name =
              addErr $
              hang (text "The binder" <+> quotes (ppr name) <+> ptext (sLit "is not a NameU."))
                 2 (text "Probable cause: you used mkName instead of newName to generate a binding.")

      qAddForeignFilePath lang fp = do
        var <- fmap tcg_th_foreign_files getGblEnv
        updTcRef var ((lang, fp) :)

      qAddModFinalizer fin = do
          r <- liftIO $ mkRemoteRef fin
          fref <- liftIO $ mkForeignRef r (freeRemoteRef r)
          addModFinalizerRef fref

      qAddCorePlugin plugin = do
          hsc_env <- getTopEnv
          r <- liftIO $ findHomeModule hsc_env (mkModuleName plugin)
          let err = hang
                (text "addCorePlugin: invalid plugin module "
                   <+> text (show plugin)
                )
                2
                (text "Plugins in the current package can't be specified.")
          case r of
            Found {} -> addErr err
            FoundMultiple {} -> addErr err
            _ -> return ()
          th_coreplugins_var <- tcg_th_coreplugins <$> getGblEnv
          updTcRef th_coreplugins_var (plugin:)

      qGetQ :: forall a. Typeable a => TcM (Maybe a)
      qGetQ = do
          th_state_var <- fmap tcg_th_state getGblEnv
          th_state <- readTcRef th_state_var
          -- See #10596 for why we use a scoped type variable here.
          return (Map.lookup (typeRep (Proxy :: Proxy a)) th_state >>= fromDynamic)

      qPutQ x = do
          th_state_var <- fmap tcg_th_state getGblEnv
          updTcRef th_state_var (\m -> Map.insert (typeOf x) (toDyn x) m)

      qIsExtEnabled = xoptM

      qExtsEnabled =
        EnumSet.toList . extensionFlags . hsc_dflags <$> getTopEnv

      qPutDoc doc_loc s = do
        th_doc_var <- tcg_th_docs <$> getGblEnv
        resolved_doc_loc <- resolve_loc doc_loc
        is_local <- checkLocalName resolved_doc_loc
        unless is_local $ failWithTc $ text
          "Can't add documentation to" <+> ppr_loc doc_loc <+>
          text "as it isn't inside the current module"
        updTcRef th_doc_var (Map.insert resolved_doc_loc s)
        where
          resolve_loc (TH.DeclDoc n) = DeclDoc <$> lookupThName n
          resolve_loc (TH.ArgDoc n i) = ArgDoc <$> lookupThName n <*> pure i
          resolve_loc (TH.InstDoc t) = InstDoc <$> fmap getName (lookupThInstName t)
          resolve_loc TH.ModuleDoc = pure ModuleDoc

          ppr_loc (TH.DeclDoc n) = ppr_th n
          ppr_loc (TH.ArgDoc n _) = ppr_th n
          ppr_loc (TH.InstDoc t) = ppr_th t
          ppr_loc TH.ModuleDoc = text "the module header"

          -- It doesn't make sense to add documentation to something not inside
          -- the current module. So check for it!
          checkLocalName (DeclDoc n) = nameIsLocalOrFrom <$> getModule <*> pure n
          checkLocalName (ArgDoc n _) = nameIsLocalOrFrom <$> getModule <*> pure n
          checkLocalName (InstDoc n) = nameIsLocalOrFrom <$> getModule <*> pure n
          checkLocalName ModuleDoc = pure True

      qGetDoc (TH.DeclDoc n) = lookupThName n >>= lookupDeclDoc
      qGetDoc (TH.InstDoc t) = lookupThInstName t >>= lookupDeclDoc
      qGetDoc (TH.ArgDoc n i) = lookupThName n >>= lookupArgDoc i
      qGetDoc TH.ModuleDoc = do
        (moduleDoc, _, _) <- getGblEnv >>= extractDocs
        return (fmap unpackHDS moduleDoc)

    -- | Looks up documentation for a declaration in first the current module,
    -- otherwise tries to find it in another module via 'hscGetModuleInterface'.
    lookupDeclDoc :: Name -> TcM (Maybe String)
    lookupDeclDoc nm = do
      (_, DeclDocMap declDocs, _) <- getGblEnv >>= extractDocs
      fam_insts <- tcg_fam_insts <$> getGblEnv
      traceTc "lookupDeclDoc" (ppr nm <+> ppr declDocs <+> ppr fam_insts)
      case Map.lookup nm declDocs of
        Just doc -> pure $ Just (unpackHDS doc)
        Nothing -> do
          -- Wasn't in the current module. Try searching other external ones!
          mIface <- getExternalModIface nm
          case mIface of
            Nothing -> pure Nothing
            Just ModIface { mi_decl_docs = DeclDocMap dmap } ->
              pure $ unpackHDS <$> Map.lookup nm dmap

    -- | Like 'lookupDeclDoc', looks up documentation for a function argument. If
    -- it can't find any documentation for a function in this module, it tries to
    -- find it in another module.
    lookupArgDoc :: Int -> Name -> TcM (Maybe String)
    lookupArgDoc i nm = do
      (_, _, ArgDocMap argDocs) <- getGblEnv >>= extractDocs
      case Map.lookup nm argDocs of
        Just m -> pure $ unpackHDS <$> IntMap.lookup i m
        Nothing -> do
          mIface <- getExternalModIface nm
          case mIface of
            Nothing -> pure Nothing
            Just ModIface { mi_arg_docs = ArgDocMap amap } ->
              pure $ unpackHDS <$> (Map.lookup nm amap >>= IntMap.lookup i)

    -- | Returns the module a Name belongs to, if it is isn't local.
    getExternalModIface :: Name -> TcM (Maybe ModIface)
    getExternalModIface nm = do
      isLocal <- nameIsLocalOrFrom <$> getModule <*> pure nm
      if isLocal
        then pure Nothing
        else case nameModule_maybe nm of
              Nothing -> pure Nothing
              Just modNm -> do
                hsc_env <- getTopEnv
                iface <- liftIO $ hscGetModuleInterface hsc_env modNm
                pure (Just iface)

    -- | Find the GHC name of the first instance that matches the TH type
    lookupThInstName :: TH.Type -> TcM Name
    lookupThInstName th_type = do
      cls_name <- inst_cls_name th_type
      insts <- reifyInstances' cls_name (inst_arg_types th_type)
      case insts of   -- This expands any type synonyms
        Left  (_, (inst:_)) -> return $ getName inst
        Left  (_, [])       -> noMatches
        Right (_, (inst:_)) -> return $ getName inst
        Right (_, [])       -> noMatches
      where
        noMatches = failWithTc $
          text "Couldn't find any instances of"
            <+> ppr_th th_type
            <+> text "to add documentation to"

        -- | Get the name of the class for the instance we are documenting
        -- > inst_cls_name (Monad Maybe) == Monad
        -- > inst_cls_name C = C
        inst_cls_name :: TH.Type -> TcM TH.Name
        inst_cls_name (TH.AppT t _)           = inst_cls_name t
        inst_cls_name (TH.SigT n _)           = inst_cls_name n
        inst_cls_name (TH.VarT n)             = pure n
        inst_cls_name (TH.ConT n)             = pure n
        inst_cls_name (TH.PromotedT n)        = pure n
        inst_cls_name (TH.InfixT _ n _)       = pure n
        inst_cls_name (TH.UInfixT _ n _)      = pure n
        inst_cls_name (TH.ParensT t)          = inst_cls_name t

        inst_cls_name (TH.ForallT _ _ _)      = inst_cls_name_err
        inst_cls_name (TH.ForallVisT _ _)     = inst_cls_name_err
        inst_cls_name (TH.AppKindT _ _)       = inst_cls_name_err
        inst_cls_name (TH.TupleT _)           = inst_cls_name_err
        inst_cls_name (TH.UnboxedTupleT _)    = inst_cls_name_err
        inst_cls_name (TH.UnboxedSumT _)      = inst_cls_name_err
        inst_cls_name TH.ArrowT               = inst_cls_name_err
        inst_cls_name TH.MulArrowT            = inst_cls_name_err
        inst_cls_name TH.EqualityT            = inst_cls_name_err
        inst_cls_name TH.ListT                = inst_cls_name_err
        inst_cls_name (TH.PromotedTupleT _)   = inst_cls_name_err
        inst_cls_name TH.PromotedNilT         = inst_cls_name_err
        inst_cls_name TH.PromotedConsT        = inst_cls_name_err
        inst_cls_name TH.StarT                = inst_cls_name_err
        inst_cls_name TH.ConstraintT          = inst_cls_name_err
        inst_cls_name (TH.LitT _)             = inst_cls_name_err
        inst_cls_name TH.WildCardT            = inst_cls_name_err
        inst_cls_name (TH.ImplicitParamT _ _) = inst_cls_name_err

        inst_cls_name_err = failWithTc $
          text "Couldn't work out what instance"
            <+> ppr_th th_type
            <+> text "is supposed to be"

        -- | Basically does the opposite of 'mkThAppTs'
        -- > inst_arg_types (Monad Maybe) == [Maybe]
        -- > inst_arg_types C == []
        inst_arg_types :: TH.Type -> [TH.Type]
        inst_arg_types (TH.AppT _ args) =
          let go (TH.AppT t ts) = t:go ts
              go t = [t]
            in go args
        inst_arg_types _ = []

    -- | Adds a mod finalizer reference to the local environment.
    addModFinalizerRef :: ForeignRef (TH.Q ()) -> TcM ()
    addModFinalizerRef finRef = do
        th_stage <- getStage
        case th_stage of
          RunSplice th_modfinalizers_var -> updTcRef th_modfinalizers_var (finRef :)
          -- This case happens only if a splice is executed and the caller does
          -- not set the 'ThStage' to 'RunSplice' to collect finalizers.
          -- See Note [Delaying modFinalizers in untyped splices] in GHC.Rename.Splice.
          _ ->
            pprPanic "addModFinalizer was called when no finalizers were collected"
                     (ppr th_stage)

    -- | Releases the external interpreter state.
    finishTH :: TcM ()
    finishTH = do
      hsc_env <- getTopEnv
      case interpInstance <$> hsc_interp hsc_env of
        Nothing                  -> pure ()
    #if defined(HAVE_INTERNAL_INTERPRETER)
        Just InternalInterp      -> pure ()
    #endif
        Just (ExternalInterp {}) -> do
          tcg <- getGblEnv
          writeTcRef (tcg_th_remote_state tcg) Nothing

    runTHExp :: ForeignHValue -> TcM TH.Exp
    runTHExp = runTH THExp

    runTHPat :: ForeignHValue -> TcM TH.Pat
    runTHPat = runTH THPat

    runTHType :: ForeignHValue -> TcM TH.Type
    runTHType = runTH THType

    runTHDec :: ForeignHValue -> TcM [TH.Dec]
    runTHDec = runTH THDec

    runTH :: Binary a => THResultType -> ForeignHValue -> TcM a
    runTH ty fhv = do
      interp <- tcGetInterp
      case interpInstance interp of
    #if defined(HAVE_INTERNAL_INTERPRETER)
        InternalInterp -> do
           -- Run it in the local TcM
          hv <- liftIO $ wormhole interp fhv
          r <- runQuasi (unsafeCoerce hv :: TH.Q a)
          return r
    #endif

        ExternalInterp conf iserv ->
          -- Run it on the server.  For an overview of how TH works with
          -- Remote GHCi, see Note [Remote Template Haskell] in
          -- libraries/ghci/GHCi/TH.hs.
          withIServ_ conf iserv $ \i -> do
            rstate <- getTHState i
            loc <- TH.qLocation
            liftIO $
              withForeignRef rstate $ \state_hv ->
              withForeignRef fhv $ \q_hv ->
                writeIServ i (putMessage (RunTH state_hv q_hv ty (Just loc)))
            runRemoteTH i []
            bs <- readQResult i
            return $! runGet get (LB.fromStrict bs)

    -- | communicate with a remotely-running TH computation until it finishes.
    -- See Note [Remote Template Haskell] in libraries/ghci/GHCi/TH.hs.
    runRemoteTH
      :: IServInstance
      -> [Messages DecoratedSDoc]   --  saved from nested calls to qRecover
      -> TcM ()
    runRemoteTH iserv recovers = do
      THMsg msg <- liftIO $ readIServ iserv getTHMessage
      case msg of
        RunTHDone -> return ()
        StartRecover -> do -- Note [TH recover with -fexternal-interpreter]
          v <- getErrsVar
          msgs <- readTcRef v
          writeTcRef v emptyMessages
          runRemoteTH iserv (msgs : recovers)
        EndRecover caught_error -> do
          let (prev_msgs, rest) = case recovers of
                 [] -> panic "EndRecover"
                 a : b -> (a,b)
          v <- getErrsVar
          warn_msgs <- getWarningMessages <$> readTcRef v
          -- keep the warnings only if there were no errors
          writeTcRef v $ if caught_error
            then prev_msgs
            else mkMessages warn_msgs `unionMessages` prev_msgs
          runRemoteTH iserv rest
        _other -> do
          r <- handleTHMessage msg
          liftIO $ writeIServ iserv (put r)
          runRemoteTH iserv recovers

    -- | Read a value of type QResult from the iserv
    readQResult :: Binary a => IServInstance -> TcM a
    readQResult i = do
      qr <- liftIO $ readIServ i get
      case qr of
        QDone a -> return a
        QException str -> liftIO $ throwIO (ErrorCall str)
        QFail str -> fail str

    {- Note [TH recover with -fexternal-interpreter]

    Recover is slightly tricky to implement.

    The meaning of "recover a b" is
     - Do a
       - If it finished with no errors, then keep the warnings it generated
       - If it failed, discard any messages it generated, and do b

    Note that "failed" here can mean either
      (1) threw an exception (failTc)
      (2) generated an error message (addErrTcM)

    The messages are managed by GHC in the TcM monad, whereas the
    exception-handling is done in the ghc-iserv process, so we have to
    coordinate between the two.

    On the server:
      - emit a StartRecover message
      - run "a; FailIfErrs" inside a try
      - emit an (EndRecover x) message, where x = True if "a; FailIfErrs" failed
      - if "a; FailIfErrs" failed, run "b"

    Back in GHC, when we receive:

      FailIfErrrs
        failTc if there are any error messages (= failIfErrsM)
      StartRecover
        save the current messages and start with an empty set.
      EndRecover caught_error
        Restore the previous messages,
        and merge in the new messages if caught_error is false.
    -}

    -- | Retrieve (or create, if it hasn't been created already), the
    -- remote TH state.  The TH state is a remote reference to an IORef
    -- QState living on the server, and we have to pass this to each RunTH
    -- call we make.
    --
    -- The TH state is stored in tcg_th_remote_state in the TcGblEnv.
    --
    getTHState :: IServInstance -> TcM (ForeignRef (IORef QState))
    getTHState i = do
      tcg <- getGblEnv
      th_state <- readTcRef (tcg_th_remote_state tcg)
      case th_state of
        Just rhv -> return rhv
        Nothing -> do
          interp <- tcGetInterp
          fhv <- liftIO $ mkFinalizedHValue interp =<< iservCall i StartTH
          writeTcRef (tcg_th_remote_state tcg) (Just fhv)
          return fhv

    wrapTHResult :: TcM a -> TcM (THResult a)
    wrapTHResult tcm = do
      e <- tryM tcm   -- only catch 'fail', treat everything else as catastrophic
      case e of
        Left e -> return (THException (show e))
        Right a -> return (THComplete a)

    handleTHMessage :: THMessage a -> TcM a
    handleTHMessage msg = case msg of
      NewName a -> wrapTHResult $ TH.qNewName a
      Report b str -> wrapTHResult $ TH.qReport b str
      LookupName b str -> wrapTHResult $ TH.qLookupName b str
      Reify n -> wrapTHResult $ TH.qReify n
      ReifyFixity n -> wrapTHResult $ TH.qReifyFixity n
      ReifyType n -> wrapTHResult $ TH.qReifyType n
      ReifyInstances n ts -> wrapTHResult $ TH.qReifyInstances n ts
      ReifyRoles n -> wrapTHResult $ TH.qReifyRoles n
      ReifyAnnotations lookup tyrep ->
        wrapTHResult $ (map B.pack <$> getAnnotationsByTypeRep lookup tyrep)
      ReifyModule m -> wrapTHResult $ TH.qReifyModule m
      ReifyConStrictness nm -> wrapTHResult $ TH.qReifyConStrictness nm
      AddDependentFile f -> wrapTHResult $ TH.qAddDependentFile f
      AddTempFile s -> wrapTHResult $ TH.qAddTempFile s
      AddModFinalizer r -> do
        interp <- hscInterp <$> getTopEnv
        wrapTHResult $ liftIO (mkFinalizedHValue interp r) >>= addModFinalizerRef
      AddCorePlugin str -> wrapTHResult $ TH.qAddCorePlugin str
      AddTopDecls decs -> wrapTHResult $ TH.qAddTopDecls decs
      AddForeignFilePath lang str -> wrapTHResult $ TH.qAddForeignFilePath lang str
      IsExtEnabled ext -> wrapTHResult $ TH.qIsExtEnabled ext
      ExtsEnabled -> wrapTHResult $ TH.qExtsEnabled
      PutDoc l s -> wrapTHResult $ TH.qPutDoc l s
      GetDoc l -> wrapTHResult $ TH.qGetDoc l
      FailIfErrs -> wrapTHResult failIfErrsM
      _ -> panic ("handleTHMessage: unexpected message " ++ show msg)

    getAnnotationsByTypeRep :: TH.AnnLookup -> TypeRep -> TcM [[Word8]]
    getAnnotationsByTypeRep th_name tyrep
      = do { name <- lookupThAnnLookup th_name
           ; topEnv <- getTopEnv
           ; epsHptAnns <- liftIO $ prepareAnnotations topEnv Nothing
           ; tcg <- getGblEnv
           ; let selectedEpsHptAnns = findAnnsByTypeRep epsHptAnns name tyrep
           ; let selectedTcgAnns = findAnnsByTypeRep (tcg_ann_env tcg) name tyrep
           ; return (selectedEpsHptAnns ++ selectedTcgAnns) }

    {-
    ************************************************************************
    *                                                                      *
                Instance Testing
    *                                                                      *
    ************************************************************************
    -}

    reifyInstances :: TH.Name -> [TH.Type] -> TcM [TH.Dec]
    reifyInstances th_nm th_tys
      = do { insts <- reifyInstances' th_nm th_tys
           ; case insts of
               Left (cls, cls_insts) ->
                 reifyClassInstances cls cls_insts
               Right (tc, fam_insts) ->
                 reifyFamilyInstances tc fam_insts }

    reifyInstances' :: TH.Name
                    -> [TH.Type]
                    -> TcM (Either (Class, [ClsInst]) (TyCon, [FamInst]))
                    -- ^ Returns 'Left' in the case that the instances were found to
                    -- be class instances, or 'Right' if they are family instances.
    reifyInstances' th_nm th_tys
       = addErrCtxt (text "In the argument of reifyInstances:"
                     <+> ppr_th th_nm <+> sep (map ppr_th th_tys)) $
         do { loc <- getSrcSpanM
            ; th_origin <- getThSpliceOrigin
            ; rdr_ty <- cvt th_origin loc (mkThAppTs (TH.ConT th_nm) th_tys)
              -- #9262 says to bring vars into scope, like in HsForAllTy case
              -- of rnHsTyKi
            ; let tv_rdrs = extractHsTyRdrTyVars rdr_ty
              -- Rename  to HsType Name
            ; ((tv_names, rn_ty), _fvs)
                <- checkNoErrs $ -- If there are out-of-scope Names here, then we
                                 -- must error before proceeding to typecheck the
                                 -- renamed type, as that will result in GHC
                                 -- internal errors (#13837).
                   rnImplicitTvOccs Nothing tv_rdrs $ \ tv_names ->
                   do { (rn_ty, fvs) <- rnLHsType doc rdr_ty
                      ; return ((tv_names, rn_ty), fvs) }

            ; (tclvl, wanted, (tvs, ty))
                <- pushLevelAndSolveEqualitiesX "reifyInstances"  $
                   bindImplicitTKBndrs_Skol tv_names              $
                   tcInferLHsType rn_ty

            ; tvs <- zonkAndScopedSort tvs

            -- Avoid error cascade if there are unsolved
            ; reportUnsolvedEqualities ReifySkol tvs tclvl wanted

            ; ty <- zonkTcTypeToType ty
                    -- Substitute out the meta type variables
                    -- In particular, the type might have kind
                    -- variables inside it (#7477)

            ; traceTc "reifyInstances'" (ppr ty $$ ppr (tcTypeKind ty))
            ; case splitTyConApp_maybe ty of   -- This expands any type synonyms
                Just (tc, tys)                 -- See #7910
                   | Just cls <- tyConClass_maybe tc
                   -> do { inst_envs <- tcGetInstEnvs
                         ; let (matches, unifies, _) = lookupInstEnv False inst_envs cls tys
                         ; traceTc "reifyInstances'1" (ppr matches)
                         ; return $ Left (cls, map fst matches ++ unifies) }
                   | isOpenFamilyTyCon tc
                   -> do { inst_envs <- tcGetFamInstEnvs
                         ; let matches = lookupFamInstEnv inst_envs tc tys
                         ; traceTc "reifyInstances'2" (ppr matches)
                         ; return $ Right (tc, map fim_instance matches) }
                _  -> bale_out (hang (text "reifyInstances:" <+> quotes (ppr ty))
                                   2 (text "is not a class constraint or type family application")) }
      where
        doc = ClassInstanceCtx
        bale_out msg = failWithTc msg

        cvt :: Origin -> SrcSpan -> TH.Type -> TcM (LHsType GhcPs)
        cvt origin loc th_ty = case convertToHsType origin loc th_ty of
          Left msg -> failWithTc msg
          Right ty -> return ty

    {-
    ************************************************************************
    *                                                                      *
                            Reification
    *                                                                      *
    ************************************************************************
    -}

    lookupName :: Bool      -- True  <=> type namespace
                            -- False <=> value namespace
               -> String -> TcM (Maybe TH.Name)
    lookupName is_type_name s
      = do { mb_nm <- lookupOccRn_maybe rdr_name
           ; return (fmap reifyName mb_nm) }
      where
        th_name = TH.mkName s       -- Parses M.x into a base of 'x' and a module of 'M'

        occ_fs :: FastString
        occ_fs = mkFastString (TH.nameBase th_name)

        occ :: OccName
        occ | is_type_name
            = if isLexVarSym occ_fs || isLexCon occ_fs
                                 then mkTcOccFS    occ_fs
                                 else mkTyVarOccFS occ_fs
            | otherwise
            = if isLexCon occ_fs then mkDataOccFS occ_fs
                                 else mkVarOccFS  occ_fs

        rdr_name = case TH.nameModule th_name of
                     Nothing  -> mkRdrUnqual occ
                     Just mod -> mkRdrQual (mkModuleName mod) occ

    getThing :: TH.Name -> TcM TcTyThing
    getThing th_name
      = do  { name <- lookupThName th_name
            ; traceIf (text "reify" <+> text (show th_name) <+> brackets (ppr_ns th_name) <+> ppr name)
            ; tcLookupTh name }
            -- ToDo: this tcLookup could fail, which would give a
            --       rather unhelpful error message
      where
        ppr_ns (TH.Name _ (TH.NameG TH.DataName  _pkg _mod)) = text "data"
        ppr_ns (TH.Name _ (TH.NameG TH.TcClsName _pkg _mod)) = text "tc"
        ppr_ns (TH.Name _ (TH.NameG TH.VarName   _pkg _mod)) = text "var"
        ppr_ns _ = panic "reify/ppr_ns"

    reify :: TH.Name -> TcM TH.Info
    reify th_name
      = do  { traceTc "reify 1" (text (TH.showName th_name))
            ; thing <- getThing th_name
            ; traceTc "reify 2" (ppr thing)
            ; reifyThing thing }

    lookupThName :: TH.Name -> TcM Name
    lookupThName th_name = do
        mb_name <- lookupThName_maybe th_name
        case mb_name of
            Nothing   -> failWithTc (notInScope th_name)
            Just name -> return name

    lookupThName_maybe :: TH.Name -> TcM (Maybe Name)
    lookupThName_maybe th_name
      =  do { names <- mapMaybeM lookupOccRn_maybe (thRdrNameGuesses th_name)
              -- Pick the first that works
              -- E.g. reify (mkName "A") will pick the class A in preference to the data constructor A
            ; return (listToMaybe names) }

    tcLookupTh :: Name -> TcM TcTyThing
    -- This is a specialised version of GHC.Tc.Utils.Env.tcLookup; specialised mainly in that
    -- it gives a reify-related error message on failure, whereas in the normal
    -- tcLookup, failure is a bug.
    tcLookupTh name
      = do  { (gbl_env, lcl_env) <- getEnvs
            ; case lookupNameEnv (tcl_env lcl_env) name of {
                    Just thing -> return thing;
                    Nothing    ->

              case lookupNameEnv (tcg_type_env gbl_env) name of {
                    Just thing -> return (AGlobal thing);
                    Nothing    ->

              -- EZY: I don't think this choice matters, no TH in signatures!
              if nameIsLocalOrFrom (tcg_semantic_mod gbl_env) name
              then  -- It's defined in this module
                    failWithTc (notInEnv name)

              else
         do { mb_thing <- tcLookupImported_maybe name
            ; case mb_thing of
                Succeeded thing -> return (AGlobal thing)
                Failed msg      -> failWithTc msg
        }}}}

    notInScope :: TH.Name -> SDoc
    notInScope th_name = quotes (text (TH.pprint th_name)) <+>
                         text "is not in scope at a reify"
            -- Ugh! Rather an indirect way to display the name

    notInEnv :: Name -> SDoc
    notInEnv name = quotes (ppr name) <+>
                         text "is not in the type environment at a reify"

    ------------------------------
    reifyRoles :: TH.Name -> TcM [TH.Role]
    reifyRoles th_name
      = do { thing <- getThing th_name
           ; case thing of
               AGlobal (ATyCon tc) -> return (map reify_role (tyConRoles tc))
               _ -> failWithTc (text "No roles associated with" <+> (ppr thing))
           }
      where
        reify_role Nominal          = TH.NominalR
        reify_role Representational = TH.RepresentationalR
        reify_role Phantom          = TH.PhantomR

    ------------------------------
    reifyThing :: TcTyThing -> TcM TH.Info
    -- The only reason this is monadic is for error reporting,
    -- which in turn is mainly for the case when TH can't express
    -- some random GHC extension

    reifyThing (AGlobal (AnId id))
      = do  { ty <- reifyType (idType id)
            ; let v = reifyName id
            ; case idDetails id of
                ClassOpId cls -> return (TH.ClassOpI v ty (reifyName cls))
                RecSelId{sel_tycon=RecSelData tc}
                              -> return (TH.VarI (reifySelector id tc) ty Nothing)
                _             -> return (TH.VarI     v ty Nothing)
        }

    reifyThing (AGlobal (ATyCon tc))   = reifyTyCon tc
    reifyThing (AGlobal (AConLike (RealDataCon dc)))
      = do  { let name = dataConName dc
            ; ty <- reifyType (idType (dataConWrapId dc))
            ; return (TH.DataConI (reifyName name) ty
                                  (reifyName (dataConOrigTyCon dc)))
            }

    reifyThing (AGlobal (AConLike (PatSynCon ps)))
      = do { let name = reifyName ps
           ; ty <- reifyPatSynType (patSynSigBndr ps)
           ; return (TH.PatSynI name ty) }

    reifyThing (ATcId {tct_id = id})
      = do  { ty1 <- zonkTcType (idType id) -- Make use of all the info we have, even
                                            -- though it may be incomplete
            ; ty2 <- reifyType ty1
            ; return (TH.VarI (reifyName id) ty2 Nothing) }

    reifyThing (ATyVar tv tv1)
      = do { ty1 <- zonkTcTyVar tv1
           ; ty2 <- reifyType ty1
           ; return (TH.TyVarI (reifyName tv) ty2) }

    reifyThing thing = pprPanic "reifyThing" (pprTcTyThingCategory thing)

    -------------------------------------------
    reifyAxBranch :: TyCon -> CoAxBranch -> TcM TH.TySynEqn
    reifyAxBranch fam_tc (CoAxBranch { cab_tvs = tvs
                                     , cab_lhs = lhs
                                     , cab_rhs = rhs })
                -- remove kind patterns (#8884)
      = do { tvs' <- reifyTyVarsToMaybe tvs
           ; let lhs_types_only = filterOutInvisibleTypes fam_tc lhs
           ; lhs' <- reifyTypes lhs_types_only
           ; annot_th_lhs <- zipWith3M annotThType (tyConArgsPolyKinded fam_tc)
                                       lhs_types_only lhs'
           ; let lhs_type = mkThAppTs (TH.ConT $ reifyName fam_tc) annot_th_lhs
           ; rhs'  <- reifyType rhs
           ; return (TH.TySynEqn tvs' lhs_type rhs') }

    reifyTyCon :: TyCon -> TcM TH.Info
    reifyTyCon tc
      | Just cls <- tyConClass_maybe tc
      = reifyClass cls

      | isFunTyCon tc
      = return (TH.PrimTyConI (reifyName tc) 2                False)

      | isPrimTyCon tc
      = return (TH.PrimTyConI (reifyName tc) (length (tyConVisibleTyVars tc))
                              (isUnliftedTyCon tc))

      | isTypeFamilyTyCon tc
      = do { let tvs      = tyConTyVars tc
                 res_kind = tyConResKind tc
                 resVar   = famTcResVar tc

           ; kind' <- reifyKind res_kind
           ; let (resultSig, injectivity) =
                     case resVar of
                       Nothing   -> (TH.KindSig kind', Nothing)
                       Just name ->
                         let thName   = reifyName name
                             injAnnot = tyConInjectivityInfo tc
                             sig = TH.TyVarSig (TH.KindedTV thName () kind')
                             inj = case injAnnot of
                                     NotInjective -> Nothing
                                     Injective ms ->
                                         Just (TH.InjectivityAnn thName injRHS)
                                       where
                                         injRHS = map (reifyName . tyVarName)
                                                      (filterByList ms tvs)
                         in (sig, inj)
           ; tvs' <- reifyTyVars (tyConVisibleTyVars tc)
           ; let tfHead =
                   TH.TypeFamilyHead (reifyName tc) tvs' resultSig injectivity
           ; if isOpenTypeFamilyTyCon tc
             then do { fam_envs <- tcGetFamInstEnvs
                     ; instances <- reifyFamilyInstances tc
                                      (familyInstances fam_envs tc)
                     ; return (TH.FamilyI (TH.OpenTypeFamilyD tfHead) instances) }
             else do { eqns <-
                         case isClosedSynFamilyTyConWithAxiom_maybe tc of
                           Just ax -> mapM (reifyAxBranch tc) $
                                      fromBranches $ coAxiomBranches ax
                           Nothing -> return []
                     ; return (TH.FamilyI (TH.ClosedTypeFamilyD tfHead eqns)
                          []) } }

      | isDataFamilyTyCon tc
      = do { let res_kind = tyConResKind tc

           ; kind' <- fmap Just (reifyKind res_kind)

           ; tvs' <- reifyTyVars (tyConVisibleTyVars tc)
           ; fam_envs <- tcGetFamInstEnvs
           ; instances <- reifyFamilyInstances tc (familyInstances fam_envs tc)
           ; return (TH.FamilyI
                           (TH.DataFamilyD (reifyName tc) tvs' kind') instances) }

      | Just (_, rhs) <- synTyConDefn_maybe tc  -- Vanilla type synonym
      = do { rhs' <- reifyType rhs
           ; tvs' <- reifyTyVars (tyConVisibleTyVars tc)
           ; return (TH.TyConI
                       (TH.TySynD (reifyName tc) tvs' rhs'))
           }

      | otherwise
      = do  { cxt <- reifyCxt (tyConStupidTheta tc)
            ; let tvs      = tyConTyVars tc
                  dataCons = tyConDataCons tc
                  isGadt   = isGadtSyntaxTyCon tc
            ; cons <- mapM (reifyDataCon isGadt (mkTyVarTys tvs)) dataCons
            ; r_tvs <- reifyTyVars (tyConVisibleTyVars tc)
            ; let name = reifyName tc
                  deriv = []        -- Don't know about deriving
                  decl | isNewTyCon tc =
                           TH.NewtypeD cxt name r_tvs Nothing (head cons) deriv
                       | otherwise     =
                           TH.DataD    cxt name r_tvs Nothing       cons  deriv
            ; return (TH.TyConI decl) }

    reifyDataCon :: Bool -> [Type] -> DataCon -> TcM TH.Con
    reifyDataCon isGadtDataCon tys dc
      = do { let -- used for H98 data constructors
                 (ex_tvs, theta, arg_tys)
                     = dataConInstSig dc tys
                 -- used for GADTs data constructors
                 g_user_tvs' = dataConUserTyVarBinders dc
                 (g_univ_tvs, _, g_eq_spec, g_theta', g_arg_tys', g_res_ty')
                     = dataConFullSig dc
                 (srcUnpks, srcStricts)
                     = mapAndUnzip reifySourceBang (dataConSrcBangs dc)
                 dcdBangs  = zipWith TH.Bang srcUnpks srcStricts
                 fields    = dataConFieldLabels dc
                 name      = reifyName dc
                 -- Universal tvs present in eq_spec need to be filtered out, as
                 -- they will not appear anywhere in the type.
                 eq_spec_tvs = mkVarSet (map eqSpecTyVar g_eq_spec)

           ; (univ_subst, _)
                  -- See Note [Freshen reified GADT constructors' universal tyvars]
               <- freshenTyVarBndrs $
                  filterOut (`elemVarSet` eq_spec_tvs) g_univ_tvs
           ; let (tvb_subst, g_user_tvs) = subst_tv_binders univ_subst g_user_tvs'
                 g_theta   = substTys tvb_subst g_theta'
                 g_arg_tys = substTys tvb_subst (map scaledThing g_arg_tys')
                 g_res_ty  = substTy  tvb_subst g_res_ty'

           ; r_arg_tys <- reifyTypes (if isGadtDataCon then g_arg_tys else arg_tys)

           ; main_con <-
               if | not (null fields) && not isGadtDataCon ->
                      return $ TH.RecC name (zip3 (map reifyFieldLabel fields)
                                             dcdBangs r_arg_tys)
                  | not (null fields) -> do
                      { res_ty <- reifyType g_res_ty
                      ; return $ TH.RecGadtC [name]
                                         (zip3 (map (reifyName . flSelector) fields)
                                          dcdBangs r_arg_tys) res_ty }
                    -- We need to check not isGadtDataCon here because GADT
                    -- constructors can be declared infix.
                    -- See Note [Infix GADT constructors] in GHC.Tc.TyCl.
                  | dataConIsInfix dc && not isGadtDataCon ->
                      ASSERT( r_arg_tys `lengthIs` 2 ) do
                      { let [r_a1, r_a2] = r_arg_tys
                            [s1,   s2]   = dcdBangs
                      ; return $ TH.InfixC (s1,r_a1) name (s2,r_a2) }
                  | isGadtDataCon -> do
                      { res_ty <- reifyType g_res_ty
                      ; return $ TH.GadtC [name] (dcdBangs `zip` r_arg_tys) res_ty }
                  | otherwise ->
                      return $ TH.NormalC name (dcdBangs `zip` r_arg_tys)

           ; let (ex_tvs', theta') | isGadtDataCon = (g_user_tvs, g_theta)
                                   | otherwise     = ASSERT( all isTyVar ex_tvs )
                                                     -- no covars for haskell syntax
                                                     (map mk_specified ex_tvs, theta)
                 ret_con | null ex_tvs' && null theta' = return main_con
                         | otherwise                   = do
                             { cxt <- reifyCxt theta'
                             ; ex_tvs'' <- reifyTyVarBndrs ex_tvs'
                             ; return (TH.ForallC ex_tvs'' cxt main_con) }
           ; ASSERT( r_arg_tys `equalLength` dcdBangs )
             ret_con }
      where
        mk_specified tv = Bndr tv SpecifiedSpec

        subst_tv_binders subst tv_bndrs =
          let tvs            = binderVars tv_bndrs
              flags          = map binderArgFlag tv_bndrs
              (subst', tvs') = substTyVarBndrs subst tvs
              tv_bndrs'      = map (\(tv,fl) -> Bndr tv fl) (zip tvs' flags)
          in (subst', tv_bndrs')

    {-
    Note [Freshen reified GADT constructors' universal tyvars]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Suppose one were to reify this GADT:

      data a :~: b where
        Refl :: forall a b. (a ~ b) => a :~: b

    We ought to be careful here about the uniques we give to the occurrences of `a`
    and `b` in this definition. That is because in the original DataCon, all uses
    of `a` and `b` have the same unique, since `a` and `b` are both universally
    quantified type variables--that is, they are used in both the (:~:) tycon as
    well as in the constructor type signature. But when we turn the DataCon
    definition into the reified one, the `a` and `b` in the constructor type
    signature becomes differently scoped than the `a` and `b` in `data a :~: b`.

    While it wouldn't technically be *wrong* per se to re-use the same uniques for
    `a` and `b` across these two different scopes, it's somewhat annoying for end
    users of Template Haskell, since they wouldn't be able to rely on the
    assumption that all TH names have globally distinct uniques (#13885). For this
    reason, we freshen the universally quantified tyvars that go into the reified
    GADT constructor type signature to give them distinct uniques from their
    counterparts in the tycon.
    -}

    ------------------------------
    reifyClass :: Class -> TcM TH.Info
    reifyClass cls
      = do  { cxt <- reifyCxt theta
            ; inst_envs <- tcGetInstEnvs
            ; insts <- reifyClassInstances cls (InstEnv.classInstances inst_envs cls)
            ; assocTys <- concatMapM reifyAT ats
            ; ops <- concatMapM reify_op op_stuff
            ; tvs' <- reifyTyVars (tyConVisibleTyVars (classTyCon cls))
            ; let dec = TH.ClassD cxt (reifyName cls) tvs' fds' (assocTys ++ ops)
            ; return (TH.ClassI dec insts) }
      where
        (_, fds, theta, _, ats, op_stuff) = classExtraBigSig cls
        fds' = map reifyFunDep fds
        reify_op (op, def_meth)
          = do { let (_, _, ty) = tcSplitMethodTy (idType op)
                   -- Use tcSplitMethodTy to get rid of the extraneous class
                   -- variables and predicates at the beginning of op's type
                   -- (see #15551).
               ; ty' <- reifyType ty
               ; let nm' = reifyName op
               ; case def_meth of
                    Just (_, GenericDM gdm_ty) ->
                      do { gdm_ty' <- reifyType gdm_ty
                         ; return [TH.SigD nm' ty', TH.DefaultSigD nm' gdm_ty'] }
                    _ -> return [TH.SigD nm' ty'] }

        reifyAT :: ClassATItem -> TcM [TH.Dec]
        reifyAT (ATI tycon def) = do
          tycon' <- reifyTyCon tycon
          case tycon' of
            TH.FamilyI dec _ -> do
              let (tyName, tyArgs) = tfNames dec
              (dec :) <$> maybe (return [])
                                (fmap (:[]) . reifyDefImpl tyName tyArgs . fst)
                                def
            _ -> pprPanic "reifyAT" (text (show tycon'))

        reifyDefImpl :: TH.Name -> [TH.Name] -> Type -> TcM TH.Dec
        reifyDefImpl n args ty =
          TH.TySynInstD . TH.TySynEqn Nothing (mkThAppTs (TH.ConT n) (map TH.VarT args))
                                      <$> reifyType ty

        tfNames :: TH.Dec -> (TH.Name, [TH.Name])
        tfNames (TH.OpenTypeFamilyD (TH.TypeFamilyHead n args _ _))
          = (n, map bndrName args)
        tfNames d = pprPanic "tfNames" (text (show d))

        bndrName :: TH.TyVarBndr flag -> TH.Name
        bndrName (TH.PlainTV n _)    = n
        bndrName (TH.KindedTV n _ _) = n

    ------------------------------
    -- | Annotate (with TH.SigT) a type if the first parameter is True
    -- and if the type contains a free variable.
    -- This is used to annotate type patterns for poly-kinded tyvars in
    -- reifying class and type instances.
    -- See @Note [Reified instances and explicit kind signatures]@.
    annotThType :: Bool   -- True <=> annotate
                -> TyCoRep.Type -> TH.Type -> TcM TH.Type
      -- tiny optimization: if the type is annotated, don't annotate again.
    annotThType _    _  th_ty@(TH.SigT {}) = return th_ty
    annotThType True ty th_ty
      | not $ isEmptyVarSet $ filterVarSet isTyVar $ tyCoVarsOfType ty
      = do { let ki = tcTypeKind ty
           ; th_ki <- reifyKind ki
           ; return (TH.SigT th_ty th_ki) }
    annotThType _    _ th_ty = return th_ty

    -- | For every argument type that a type constructor accepts,
    -- report whether or not the argument is poly-kinded. This is used to
    -- eventually feed into 'annotThType'.
    -- See @Note [Reified instances and explicit kind signatures]@.
    tyConArgsPolyKinded :: TyCon -> [Bool]
    tyConArgsPolyKinded tc =
         map (is_poly_ty . tyVarKind)      tc_vis_tvs
         -- See "Wrinkle: Oversaturated data family instances" in
         -- @Note [Reified instances and explicit kind signatures]@
      ++ map (is_poly_ty . tyCoBinderType) tc_res_kind_vis_bndrs -- (1) in Wrinkle
      ++ repeat True                                             -- (2) in Wrinkle
      where
        is_poly_ty :: Type -> Bool
        is_poly_ty ty = not $
                        isEmptyVarSet $
                        filterVarSet isTyVar $
                        tyCoVarsOfType ty

        tc_vis_tvs :: [TyVar]
        tc_vis_tvs = tyConVisibleTyVars tc

        tc_res_kind_vis_bndrs :: [TyCoBinder]
        tc_res_kind_vis_bndrs = filter isVisibleBinder $ fst $ splitPiTys $ tyConResKind tc

    {-
    Note [Reified instances and explicit kind signatures]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Reified class instances and type family instances often include extra kind
    information to disambiguate instances. Here is one such example that
    illustrates this (#8953):

        type family Poly (a :: k) :: Type
        type instance Poly (x :: Bool)    = Int
        type instance Poly (x :: Maybe k) = Double

    If you're not careful, reifying these instances might yield this:

        type instance Poly x = Int
        type instance Poly x = Double

    To avoid this, we go through some care to annotate things with extra kind
    information. Some functions which accomplish this feat include:

    * annotThType: This annotates a type with a kind signature if the type contains
      a free variable.
    * tyConArgsPolyKinded: This checks every argument that a type constructor can
      accept and reports if the type of the argument is poly-kinded. This
      information is ultimately fed into annotThType.

    -----
    -- Wrinkle: Oversaturated data family instances
    -----

    What constitutes an argument to a type constructor in the definition of
    tyConArgsPolyKinded? For most type constructors, it's simply the visible
    type variable binders (i.e., tyConVisibleTyVars). There is one corner case
    we must keep in mind, however: data family instances can appear oversaturated
    (#17296). For instance:

        data family   Foo :: Type -> Type
        data instance Foo x

        data family Bar :: k
        data family Bar x

    For these sorts of data family instances, tyConVisibleTyVars isn't enough,
    as they won't give you the kinds of the oversaturated arguments. We must
    also consult:

    1. The kinds of the arguments in the result kind (i.e., the tyConResKind).
       This will tell us, e.g., the kind of `x` in `Foo x` above.
    2. If we go beyond the number of arguments in the result kind (like the
       `x` in `Bar x`), then we conservatively assume that the argument's
       kind is poly-kinded.

    -----
    -- Wrinkle: data family instances with return kinds
    -----

    Another squirrelly corner case is this:

        data family Foo (a :: k)
        data instance Foo :: Bool -> Type
        data instance Foo :: Char -> Type

    If you're not careful, reifying these instances might yield this:

        data instance Foo
        data instance Foo

    We can fix this ambiguity by reifying the instances' explicit return kinds. We
    should only do this if necessary (see
    Note [When does a tycon application need an explicit kind signature?] in GHC.Core.Type),
    but more importantly, we *only* do this if either of the following are true:

    1. The data family instance has no constructors.
    2. The data family instance is declared with GADT syntax.

    If neither of these are true, then reifying the return kind would yield
    something like this:

        data instance (Bar a :: Type) = MkBar a

    Which is not valid syntax.
    -}

    ------------------------------
    reifyClassInstances :: Class -> [ClsInst] -> TcM [TH.Dec]
    reifyClassInstances cls insts
      = mapM (reifyClassInstance (tyConArgsPolyKinded (classTyCon cls))) insts

    reifyClassInstance :: [Bool]  -- True <=> the corresponding tv is poly-kinded
                                  -- includes only *visible* tvs
                       -> ClsInst -> TcM TH.Dec
    reifyClassInstance is_poly_tvs i
      = do { cxt <- reifyCxt theta
           ; let vis_types = filterOutInvisibleTypes cls_tc types
           ; thtypes <- reifyTypes vis_types
           ; annot_thtypes <- zipWith3M annotThType is_poly_tvs vis_types thtypes
           ; let head_ty = mkThAppTs (TH.ConT (reifyName cls)) annot_thtypes
           ; return $ (TH.InstanceD over cxt head_ty []) }
      where
         (_tvs, theta, cls, types) = tcSplitDFunTy (idType dfun)
         cls_tc   = classTyCon cls
         dfun     = instanceDFunId i
         over     = case overlapMode (is_flag i) of
                      NoOverlap _     -> Nothing
                      Overlappable _  -> Just TH.Overlappable
                      Overlapping _   -> Just TH.Overlapping
                      Overlaps _      -> Just TH.Overlaps
                      Incoherent _    -> Just TH.Incoherent

    ------------------------------
    reifyFamilyInstances :: TyCon -> [FamInst] -> TcM [TH.Dec]
    reifyFamilyInstances fam_tc fam_insts
      = mapM (reifyFamilyInstance (tyConArgsPolyKinded fam_tc)) fam_insts

    reifyFamilyInstance :: [Bool] -- True <=> the corresponding tv is poly-kinded
                                  -- includes only *visible* tvs
                        -> FamInst -> TcM TH.Dec
    reifyFamilyInstance is_poly_tvs (FamInst { fi_flavor = flavor
                                             , fi_axiom = ax
                                             , fi_fam = fam })
      | let fam_tc = coAxiomTyCon ax
            branch = coAxiomSingleBranch ax
      , CoAxBranch { cab_tvs = tvs, cab_lhs = lhs, cab_rhs = rhs } <- branch
      = case flavor of
          SynFamilyInst ->
                   -- remove kind patterns (#8884)
            do { th_tvs <- reifyTyVarsToMaybe tvs
               ; let lhs_types_only = filterOutInvisibleTypes fam_tc lhs
               ; th_lhs <- reifyTypes lhs_types_only
               ; annot_th_lhs <- zipWith3M annotThType is_poly_tvs lhs_types_only
                                                       th_lhs
               ; let lhs_type = mkThAppTs (TH.ConT $ reifyName fam) annot_th_lhs
               ; th_rhs <- reifyType rhs
               ; return (TH.TySynInstD (TH.TySynEqn th_tvs lhs_type th_rhs)) }

          DataFamilyInst rep_tc ->
            do { let -- eta-expand lhs types, because sometimes data/newtype
                     -- instances are eta-reduced; See #9692
                     -- See Note [Eta reduction for data families] in GHC.Core.Coercion.Axiom
                     (ee_tvs, ee_lhs, _) = etaExpandCoAxBranch branch
                     fam'     = reifyName fam
                     dataCons = tyConDataCons rep_tc
                     isGadt   = isGadtSyntaxTyCon rep_tc
               ; th_tvs <- reifyTyVarsToMaybe ee_tvs
               ; cons <- mapM (reifyDataCon isGadt (mkTyVarTys ee_tvs)) dataCons
               ; let types_only = filterOutInvisibleTypes fam_tc ee_lhs
               ; th_tys <- reifyTypes types_only
               ; annot_th_tys <- zipWith3M annotThType is_poly_tvs types_only th_tys
               ; let lhs_type = mkThAppTs (TH.ConT fam') annot_th_tys
               ; mb_sig <-
                   -- See "Wrinkle: data family instances with return kinds" in
                   -- Note [Reified instances and explicit kind signatures]
                   if (null cons || isGadtSyntaxTyCon rep_tc)
                         && tyConAppNeedsKindSig False fam_tc (length ee_lhs)
                   then do { let full_kind = tcTypeKind (mkTyConApp fam_tc ee_lhs)
                           ; th_full_kind <- reifyKind full_kind
                           ; pure $ Just th_full_kind }
                   else pure Nothing
               ; return $
                   if isNewTyCon rep_tc
                   then TH.NewtypeInstD [] th_tvs lhs_type mb_sig (head cons) []
                   else TH.DataInstD    [] th_tvs lhs_type mb_sig       cons  []
               }

    ------------------------------
    reifyType :: TyCoRep.Type -> TcM TH.Type
    -- Monadic only because of failure
    reifyType ty                | tcIsLiftedTypeKind ty = return TH.StarT
      -- Make sure to use tcIsLiftedTypeKind here, since we don't want to confuse it
      -- with Constraint (#14869).
    reifyType ty@(ForAllTy (Bndr _ argf) _)
                                = reify_for_all argf ty
    reifyType (LitTy t)         = do { r <- reifyTyLit t; return (TH.LitT r) }
    reifyType (TyVarTy tv)      = return (TH.VarT (reifyName tv))
    reifyType (TyConApp tc tys) = reify_tc_app tc tys   -- Do not expand type synonyms here
    reifyType ty@(AppTy {})     = do
      let (ty_head, ty_args) = splitAppTys ty
      ty_head' <- reifyType ty_head
      ty_args' <- reifyTypes (filter_out_invisible_args ty_head ty_args)
      pure $ mkThAppTs ty_head' ty_args'
      where
        -- Make sure to filter out any invisible arguments. For instance, if you
        -- reify the following:
        --
        --   newtype T (f :: forall a. a -> Type) = MkT (f Bool)
        --
        -- Then you should receive back `f Bool`, not `f Type Bool`, since the
        -- `Type` argument is invisible (#15792).
        filter_out_invisible_args :: Type -> [Type] -> [Type]
        filter_out_invisible_args ty_head ty_args =
          filterByList (map isVisibleArgFlag $ appTyArgFlags ty_head ty_args)
                       ty_args
    reifyType ty@(FunTy { ft_af = af, ft_mult = Many, ft_arg = t1, ft_res = t2 })
      | InvisArg <- af = reify_for_all Inferred ty  -- Types like ((?x::Int) => Char -> Char)
      | otherwise      = do { [r1,r2] <- reifyTypes [t1,t2]
                            ; return (TH.ArrowT `TH.AppT` r1 `TH.AppT` r2) }
    reifyType ty@(FunTy { ft_af = af, ft_mult = tm, ft_arg = t1, ft_res = t2 })
      | InvisArg <- af = noTH (sLit "linear invisible argument") (ppr ty)
      | otherwise      = do { [rm,r1,r2] <- reifyTypes [tm,t1,t2]
                            ; return (TH.MulArrowT `TH.AppT` rm `TH.AppT` r1 `TH.AppT` r2) }
    reifyType (CastTy t _)      = reifyType t -- Casts are ignored in TH
    reifyType ty@(CoercionTy {})= noTH (sLit "coercions in types") (ppr ty)

    reify_for_all :: TyCoRep.ArgFlag -> TyCoRep.Type -> TcM TH.Type
    -- Arg of reify_for_all is always ForAllTy or a predicate FunTy
    reify_for_all argf ty
      | isVisibleArgFlag argf
      = do let (req_bndrs, phi) = tcSplitForAllReqTVBinders ty
           tvbndrs' <- reifyTyVarBndrs req_bndrs
           phi' <- reifyType phi
           pure $ TH.ForallVisT tvbndrs' phi'
      | otherwise
      = do let (inv_bndrs, phi) = tcSplitForAllInvisTVBinders ty
           tvbndrs' <- reifyTyVarBndrs inv_bndrs
           let (cxt, tau) = tcSplitPhiTy phi
           cxt' <- reifyCxt cxt
           tau' <- reifyType tau
           pure $ TH.ForallT tvbndrs' cxt' tau'

    reifyTyLit :: TyCoRep.TyLit -> TcM TH.TyLit
    reifyTyLit (NumTyLit n) = return (TH.NumTyLit n)
    reifyTyLit (StrTyLit s) = return (TH.StrTyLit (unpackFS s))
    reifyTyLit (CharTyLit c) = return (TH.CharTyLit c)

    reifyTypes :: [Type] -> TcM [TH.Type]
    reifyTypes = mapM reifyType

    reifyPatSynType
      :: ([InvisTVBinder], ThetaType, [InvisTVBinder], ThetaType, [Scaled Type], Type) -> TcM TH.Type
    -- reifies a pattern synonym's type and returns its *complete* type
    -- signature; see NOTE [Pattern synonym signatures and Template
    -- Haskell]
    reifyPatSynType (univTyVars, req, exTyVars, prov, argTys, resTy)
      = do { univTyVars' <- reifyTyVarBndrs univTyVars
           ; req'        <- reifyCxt req
           ; exTyVars'   <- reifyTyVarBndrs exTyVars
           ; prov'       <- reifyCxt prov
           ; tau'        <- reifyType (mkVisFunTys argTys resTy)
           ; return $ TH.ForallT univTyVars' req'
                    $ TH.ForallT exTyVars' prov' tau' }

    reifyKind :: Kind -> TcM TH.Kind
    reifyKind = reifyType

    reifyCxt :: [PredType] -> TcM [TH.Pred]
    reifyCxt   = mapM reifyType

    reifyFunDep :: ([TyVar], [TyVar]) -> TH.FunDep
    reifyFunDep (xs, ys) = TH.FunDep (map reifyName xs) (map reifyName ys)

    class ReifyFlag flag flag' | flag -> flag' where
        reifyFlag :: flag -> flag'

    instance ReifyFlag () () where
        reifyFlag () = ()

    instance ReifyFlag Specificity TH.Specificity where
        reifyFlag SpecifiedSpec = TH.SpecifiedSpec
        reifyFlag InferredSpec  = TH.InferredSpec

    reifyTyVars :: [TyVar] -> TcM [TH.TyVarBndr ()]
    reifyTyVars = reifyTyVarBndrs . map mk_bndr
      where
        mk_bndr tv = Bndr tv ()

    reifyTyVarBndrs :: ReifyFlag flag flag'
                    => [VarBndr TyVar flag] -> TcM [TH.TyVarBndr flag']
    reifyTyVarBndrs = mapM reify_tvbndr
      where
        -- even if the kind is *, we need to include a kind annotation,
        -- in case a poly-kind would be inferred without the annotation.
        -- See #8953 or test th/T8953
        reify_tvbndr (Bndr tv fl) = TH.KindedTV (reifyName tv)
                                                (reifyFlag fl)
                                                <$> reifyKind (tyVarKind tv)

    reifyTyVarsToMaybe :: [TyVar] -> TcM (Maybe [TH.TyVarBndr ()])
    reifyTyVarsToMaybe []  = pure Nothing
    reifyTyVarsToMaybe tys = Just <$> reifyTyVars tys

    reify_tc_app :: TyCon -> [Type.Type] -> TcM TH.Type
    reify_tc_app tc tys
      = do { tys' <- reifyTypes (filterOutInvisibleTypes tc tys)
           ; maybe_sig_t (mkThAppTs r_tc tys') }
      where
        arity       = tyConArity tc

        r_tc | isUnboxedSumTyCon tc           = TH.UnboxedSumT (arity `div` 2)
             | isUnboxedTupleTyCon tc         = TH.UnboxedTupleT (arity `div` 2)
             | isPromotedTupleTyCon tc        = TH.PromotedTupleT (arity `div` 2)
                 -- See Note [Unboxed tuple RuntimeRep vars] in GHC.Core.TyCon
             | isTupleTyCon tc                = if isPromotedDataCon tc
                                                then TH.PromotedTupleT arity
                                                else TH.TupleT arity
             | tc `hasKey` constraintKindTyConKey
                                              = TH.ConstraintT
             | tc `hasKey` unrestrictedFunTyConKey = TH.ArrowT
             | tc `hasKey` listTyConKey       = TH.ListT
             | tc `hasKey` nilDataConKey      = TH.PromotedNilT
             | tc `hasKey` consDataConKey     = TH.PromotedConsT
             | tc `hasKey` heqTyConKey        = TH.EqualityT
             | tc `hasKey` eqPrimTyConKey     = TH.EqualityT
             | tc `hasKey` eqReprPrimTyConKey = TH.ConT (reifyName coercibleTyCon)
             | isPromotedDataCon tc           = TH.PromotedT (reifyName tc)
             | otherwise                      = TH.ConT (reifyName tc)

        -- See Note [When does a tycon application need an explicit kind
        -- signature?] in GHC.Core.TyCo.Rep
        maybe_sig_t th_type
          | tyConAppNeedsKindSig
              False -- We don't reify types using visible kind applications, so
                    -- don't count specified binders as contributing towards
                    -- injective positions in the kind of the tycon.
              tc (length tys)
          = do { let full_kind = tcTypeKind (mkTyConApp tc tys)
               ; th_full_kind <- reifyKind full_kind
               ; return (TH.SigT th_type th_full_kind) }
          | otherwise
          = return th_type

    ------------------------------
    reifyName :: NamedThing n => n -> TH.Name
    reifyName thing
      | isExternalName name
                  = mk_varg pkg_str mod_str occ_str
      | otherwise = TH.mkNameU occ_str (toInteger $ getKey (getUnique name))
            -- Many of the things we reify have local bindings, and
            -- NameL's aren't supposed to appear in binding positions, so
            -- we use NameU.  When/if we start to reify nested things, that
            -- have free variables, we may need to generate NameL's for them.
      where
        name    = getName thing
        mod     = ASSERT( isExternalName name ) nameModule name
        pkg_str = unitString (moduleUnit mod)
        mod_str = moduleNameString (moduleName mod)
        occ_str = occNameString occ
        occ     = nameOccName name
        mk_varg | OccName.isDataOcc occ = TH.mkNameG_d
                | OccName.isVarOcc  occ = TH.mkNameG_v
                | OccName.isTcOcc   occ = TH.mkNameG_tc
                | otherwise             = pprPanic "reifyName" (ppr name)

    -- See Note [Reifying field labels]
    reifyFieldLabel :: FieldLabel -> TH.Name
    reifyFieldLabel fl
      | flIsOverloaded fl
                  = TH.Name (TH.mkOccName occ_str) (TH.NameQ (TH.mkModName mod_str))
      | otherwise = TH.mkNameG_v pkg_str mod_str occ_str
      where
        name    = flSelector fl
        mod     = ASSERT( isExternalName name ) nameModule name
        pkg_str = unitString (moduleUnit mod)
        mod_str = moduleNameString (moduleName mod)
        occ_str = unpackFS (flLabel fl)

    reifySelector :: Id -> TyCon -> TH.Name
    reifySelector id tc
      = case find ((idName id ==) . flSelector) (tyConFieldLabels tc) of
          Just fl -> reifyFieldLabel fl
          Nothing -> pprPanic "reifySelector: missing field" (ppr id $$ ppr tc)

    ------------------------------
    reifyFixity :: Name -> TcM (Maybe TH.Fixity)
    reifyFixity name
      = do { (found, fix) <- lookupFixityRn_help name
           ; return (if found then Just (conv_fix fix) else Nothing) }
        where
          conv_fix (Hs.Fixity _ i d) = TH.Fixity i (conv_dir d)
          conv_dir Hs.InfixR = TH.InfixR
          conv_dir Hs.InfixL = TH.InfixL
          conv_dir Hs.InfixN = TH.InfixN

    reifyUnpackedness :: DataCon.SrcUnpackedness -> TH.SourceUnpackedness
    reifyUnpackedness NoSrcUnpack = TH.NoSourceUnpackedness
    reifyUnpackedness SrcNoUnpack = TH.SourceNoUnpack
    reifyUnpackedness SrcUnpack   = TH.SourceUnpack

    reifyStrictness :: DataCon.SrcStrictness -> TH.SourceStrictness
    reifyStrictness NoSrcStrict = TH.NoSourceStrictness
    reifyStrictness SrcStrict   = TH.SourceStrict
    reifyStrictness SrcLazy     = TH.SourceLazy

    reifySourceBang :: DataCon.HsSrcBang
                    -> (TH.SourceUnpackedness, TH.SourceStrictness)
    reifySourceBang (HsSrcBang _ u s) = (reifyUnpackedness u, reifyStrictness s)

    reifyDecidedStrictness :: DataCon.HsImplBang -> TH.DecidedStrictness
    reifyDecidedStrictness HsLazy     = TH.DecidedLazy
    reifyDecidedStrictness HsStrict   = TH.DecidedStrict
    reifyDecidedStrictness HsUnpack{} = TH.DecidedUnpack

    reifyTypeOfThing :: TH.Name -> TcM TH.Type
    reifyTypeOfThing th_name = do
      thing <- getThing th_name
      case thing of
        AGlobal (AnId id) -> reifyType (idType id)
        AGlobal (ATyCon tc) -> reifyKind (tyConKind tc)
        AGlobal (AConLike (RealDataCon dc)) ->
          reifyType (idType (dataConWrapId dc))
        AGlobal (AConLike (PatSynCon ps)) ->
          reifyPatSynType (patSynSigBndr ps)
        ATcId{tct_id = id} -> zonkTcType (idType id) >>= reifyType
        ATyVar _ tctv -> zonkTcTyVar tctv >>= reifyType
        -- Impossible cases, supposedly:
        AGlobal (ACoAxiom _) -> panic "reifyTypeOfThing: ACoAxiom"
        ATcTyCon _ -> panic "reifyTypeOfThing: ATcTyCon"
        APromotionErr _ -> panic "reifyTypeOfThing: APromotionErr"

    ------------------------------
    lookupThAnnLookup :: TH.AnnLookup -> TcM CoreAnnTarget
    lookupThAnnLookup (TH.AnnLookupName th_nm) = fmap NamedTarget (lookupThName th_nm)
    lookupThAnnLookup (TH.AnnLookupModule (TH.Module pn mn))
      = return $ ModuleTarget $
        mkModule (stringToUnit $ TH.pkgString pn) (mkModuleName $ TH.modString mn)

    reifyAnnotations :: Data a => TH.AnnLookup -> TcM [a]
    reifyAnnotations th_name
      = do { name <- lookupThAnnLookup th_name
           ; topEnv <- getTopEnv
           ; epsHptAnns <- liftIO $ prepareAnnotations topEnv Nothing
           ; tcg <- getGblEnv
           ; let selectedEpsHptAnns = findAnns deserializeWithData epsHptAnns name
           ; let selectedTcgAnns = findAnns deserializeWithData (tcg_ann_env tcg) name
           ; return (selectedEpsHptAnns ++ selectedTcgAnns) }

    ------------------------------
    modToTHMod :: Module -> TH.Module
    modToTHMod m = TH.Module (TH.PkgName $ unitString  $ moduleUnit m)
                             (TH.ModName $ moduleNameString $ moduleName m)

    reifyModule :: TH.Module -> TcM TH.ModuleInfo
    reifyModule (TH.Module (TH.PkgName pkgString) (TH.ModName mString)) = do
      this_mod <- getModule
      let reifMod = mkModule (stringToUnit pkgString) (mkModuleName mString)
      if (reifMod == this_mod) then reifyThisModule else reifyFromIface reifMod
        where
          reifyThisModule = do
            usages <- fmap (map modToTHMod . moduleEnvKeys . imp_mods) getImports
            return $ TH.ModuleInfo usages

          reifyFromIface reifMod = do
            iface <- loadInterfaceForModule (text "reifying module from TH for" <+> ppr reifMod) reifMod
            let usages = [modToTHMod m | usage <- mi_usages iface,
                                         Just m <- [usageToModule (moduleUnit reifMod) usage] ]
            return $ TH.ModuleInfo usages

          usageToModule :: Unit -> Usage -> Maybe Module
          usageToModule _ (UsageFile {}) = Nothing
          usageToModule this_pkg (UsageHomeModule { usg_mod_name = mn }) = Just $ mkModule this_pkg mn
          usageToModule _ (UsagePackageModule { usg_mod = m }) = Just m
          usageToModule _ (UsageMergedRequirement { usg_mod = m }) = Just m

    ------------------------------
    mkThAppTs :: TH.Type -> [TH.Type] -> TH.Type
    mkThAppTs fun_ty arg_tys = foldl' TH.AppT fun_ty arg_tys

    noTH :: PtrString -> SDoc -> TcM a
    noTH s d = failWithTc (hsep [text "Can't represent" <+> ptext s <+>
                                    text "in Template Haskell:",
                                 nest 2 d])

    ppr_th :: TH.Ppr a => a -> SDoc
    ppr_th x = text (TH.pprint x)

    {-
    Note [Reifying field labels]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When reifying a datatype declared with DuplicateRecordFields enabled, we want
    the reified names of the fields to be labels rather than selector functions.
    That is, we want (reify ''T) and (reify 'foo) to produce

        data T = MkT { foo :: Int }
        foo :: T -> Int

    rather than

        data T = MkT { $sel:foo:MkT :: Int }
        $sel:foo:MkT :: T -> Int

    because otherwise TH code that uses the field names as strings will silently do
    the wrong thing.  Thus we use the field label (e.g. foo) as the OccName, rather
    than the selector (e.g. $sel:foo:MkT).  Since the Orig name M.foo isn't in the
    environment, NameG can't be used to represent such fields.  Instead,
    reifyFieldLabel uses NameQ.

    However, this means that extracting the field name from the output of reify, and
    trying to reify it again, may fail with an ambiguity error if there are multiple
    such fields defined in the module (see the test case
    overloadedrecflds/should_fail/T11103.hs).  The "proper" fix requires changes to
    the TH AST to make it able to represent duplicate record fields.
    -}

    tcGetInterp :: TcM Interp
    tcGetInterp = do
       hsc_env <- getTopEnv
       case hsc_interp hsc_env of
          Nothing -> liftIO $ throwIO (InstallationError "Template haskell requires a target code interpreter")
          Just i  -> pure i