    {-# LANGUAGE CPP #-}
    {-# LANGUAGE DeriveFunctor #-}
    {-# LANGUAGE DerivingStrategies #-}
    {-# LANGUAGE DuplicateRecordFields #-}
    {-# LANGUAGE GeneralizedNewtypeDeriving #-}
    {-# LANGUAGE MultiWayIf #-}
    {-# LANGUAGE RankNTypes #-}
    {-# LANGUAGE ScopedTypeVariables #-}
    {-# LANGUAGE TypeFamilies #-}
    {-# LANGUAGE ViewPatterns #-}

    {-# OPTIONS_GHC -Wno-orphans #-}

    -- | Monadic definitions for the constraint solver
    module GHC.Tc.Solver.Monad (

        -- The TcS monad
        TcS(..), TcSEnv(..),
        runTcS, runTcSEarlyAbort, runTcSWithEvBinds, runTcSInerts,
        failTcS, warnTcS, addErrTcS, wrapTcS, ctLocWarnTcS,
        runTcSEqualities,
        nestTcS, nestImplicTcS, tryShortCutTcS,
        setEvBindsTcS, setTcLevelTcS,
        emitFunDepWanteds,

        selectNextWorkItem,
        getWorkList,
        updWorkListTcS,
        pushLevelNoWorkList,

        runTcPluginTcS, recordUsedGREs,
        matchGlobalInst, TcM.ClsInstResult(..),

        QCInst(..),

        -- TcSMode
        TcSMode(..), getTcSMode, setTcSMode, vanillaTcSMode,

        -- The pipeline
        StopOrContinue(..), continueWith, stopWith,
        startAgainWith, SolverStage(Stage, runSolverStage), simpleStage,
        stopWithStage, nopStage,

        -- Tracing etc
        panicTcS, traceTcS, tryEarlyAbortTcS,
        traceFireTcS, bumpStepCountTcS, csTraceTcS,
        wrapErrTcS, wrapWarnTcS,
        resetUnificationFlag, setUnificationFlag,

        -- Evidence creation and transformation
        MaybeNew(..), freshGoals, isFresh, getEvExpr,
        CanonicalEvidence(..),

        newTcEvBinds, newNoTcEvBinds,
        newWantedEq, emitNewWantedEq,
        newWanted,
        newWantedNC, newWantedEvVarNC,
        newBoundEvVarId,
        unifyTyVar, reportUnifications,
        setEvBind, setWantedEq,
        setWantedEvTerm, setEvBindIfWanted,
        newEvVar, newGivenEvVar, emitNewGivens,
        checkReductionDepth,

        getInstEnvs, getFamInstEnvs,                -- Getting the environments
        getTopEnv, getGblEnv, getLclEnv, setSrcSpan,
        getTcEvBindsVar, getTcLevel,
        getTcEvTyCoVars, getTcEvBindsMap, setTcEvBindsMap,
        tcLookupClass, tcLookupId, tcLookupTyCon,

        getUnifiedRef,

        -- Inerts
        updInertSet, updInertCans,
        getHasGivenEqs, setInertCans,
        getInertEqs, getInertCans, getInertGivens,
        getInertInsols, getInnermostGivenEqLevel,
        getInertSet, setInertSet,
        getUnsolvedInerts,
        removeInertCts, getPendingGivenScs,
        insertFunEq, addInertQCI,
        updInertDicts, updInertIrreds,
        emitWorkNC, emitWork,
        lookupInertDict,

        -- The Model
        kickOutAfterUnification, kickOutRewritable,

        -- Inert Safe Haskell safe-overlap failures
        insertSafeOverlapFailureTcS,
        getSafeOverlapFailures,

        -- Inert solved dictionaries
        getSolvedDicts, setSolvedDicts,
        updSolvedDicts, lookupSolvedDict,

        -- Irreds
        foldIrreds,

        -- The family application cache
        lookupFamAppInert, lookupFamAppCache, extendFamAppCache,
        pprKicked,

        -- Instantiation
        instDFunType,

        -- Unification
        wrapUnifierX, wrapUnifierTcS, unifyFunDeps, uPairsTcM, unifyForAllBody,

        -- MetaTyVars
        newFlexiTcSTy, instFlexiX,
        cloneMetaTyVar,
        tcInstSkolTyVarsX,

        TcLevel,
        isFilledMetaTyVar_maybe, isFilledMetaTyVar, isUnfilledMetaTyVar,
        zonkTyCoVarsAndFV, zonkTcType, zonkTcTypes, zonkTcTyVar, zonkCo,
        zonkTyCoVarsAndFVList,
        zonkSimples, zonkWC,
        zonkTyCoVarKind,

        -- References
        newTcRef, readTcRef, writeTcRef, updTcRef,

        -- Misc
        getDefaultInfo, getDynFlags, getGlobalRdrEnvTcS,
        matchFam, matchFamTcM,
        checkWellLevelledDFun,
        pprEq,

        -- Enforcing invariants for type equalities
        checkTypeEq
    ) where

    import GHC.Prelude

    import GHC.Driver.Env

    import qualified GHC.Tc.Utils.Instantiate as TcM
    import GHC.Core.InstEnv
    import GHC.Tc.Instance.Family as FamInst
    import GHC.Core.FamInstEnv

    import qualified GHC.Tc.Utils.Monad    as TcM
    import qualified GHC.Tc.Utils.TcMType  as TcM
    import qualified GHC.Tc.Instance.Class as TcM( matchGlobalInst, ClsInstResult(..) )
    import qualified GHC.Tc.Utils.Env      as TcM
           ( tcGetDefaultTys
           , tcLookupClass, tcLookupId, tcLookupTyCon
           )
    import GHC.Tc.Zonk.Monad ( ZonkM )
    import qualified GHC.Tc.Zonk.TcType  as TcM

    import GHC.Driver.DynFlags

    import GHC.Tc.Instance.Class( safeOverlap, instanceReturnsDictCon )
    import GHC.Tc.Instance.FunDeps( FunDepEqn(..) )
    import GHC.Utils.Misc

    import GHC.Tc.Solver.Types
    import GHC.Tc.Solver.InertSet
    import GHC.Tc.Errors.Types

    import GHC.Tc.Utils.TcType
    import GHC.Tc.Utils.Unify

    import GHC.Tc.Types.Evidence
    import GHC.Tc.Types
    import GHC.Tc.Types.Origin
    import GHC.Tc.Types.CtLoc
    import GHC.Tc.Types.Constraint

    import GHC.Builtin.Names ( unsatisfiableClassNameKey, callStackTyConName, exceptionContextTyConName )

    import GHC.Core.Type
    import GHC.Core.TyCo.Rep as Rep
    import GHC.Core.TyCo.Tidy
    import GHC.Core.Coercion
    import GHC.Core.Coercion.Axiom( TypeEqn )
    import GHC.Core.Predicate
    import GHC.Core.Reduction
    import GHC.Core.Class
    import GHC.Core.TyCon
    import GHC.Core.Unify (typesAreApart)

    import GHC.Types.Name
    import GHC.Types.TyThing
    import GHC.Types.Name.Reader
    import GHC.Types.DefaultEnv ( DefaultEnv )
    import GHC.Types.Var
    import GHC.Types.Var.Set
    import GHC.Types.Unique.Supply
    import GHC.Types.Unique.Set( elementOfUniqSet )
    import GHC.Types.Id
    import GHC.Types.Basic (allImportLevels)
    import GHC.Types.ThLevelIndex (thLevelIndexFromImportLevel)
    import GHC.Types.SrcLoc

    import GHC.Unit.Module
    import qualified GHC.Rename.Env as TcM
    import GHC.Rename.Env

    import GHC.Utils.Outputable
    import GHC.Utils.Panic
    import GHC.Utils.Logger

    import GHC.Data.Bag as Bag
    import GHC.Data.Pair

    import GHC.Utils.Monad

    import GHC.Exts (oneShot)
    import Control.Monad
    import Data.Foldable hiding ( foldr1 )
    import Data.IORef
    import Data.Maybe( catMaybes )
    import Data.List ( mapAccumL )
    import Data.List.NonEmpty ( nonEmpty )
    import qualified Data.List.NonEmpty as NE
    import qualified Data.Semigroup as S
    import GHC.LanguageExtensions as LangExt

    #if defined(DEBUG)
    import GHC.Types.Unique.Set (nonDetEltsUniqSet)
    import GHC.Data.Graph.Directed
    #endif

    import qualified Data.Set as Set
    import GHC.Unit.Module.Graph

    {- *********************************************************************
    *                                                                      *
                   SolverStage and StopOrContinue
    *                                                                      *
    ********************************************************************* -}

    {- Note [The SolverStage monad]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The SolverStage monad allows us to write simple code like that in
    GHC.Tc.Solver.solveEquality.   At the time of writing it looked like
    this (may get out of date but the idea is clear):

    solveEquality :: ... -> SolverStage Void
    solveEquality ev eq_rel ty1 ty2
      = do { Pair ty1' ty2' <- zonkEqTypes ev eq_rel ty1 ty2
           ; mb_canon <- canonicaliseEquality ev' eq_rel ty1' ty2'
           ; case mb_canon of {
                Left irred_ct -> do { tryQCsIrredEqCt irred_ct
                                    ; solveIrred irred_ct } ;
                Right eq_ct   -> do { tryInertEqs eq_ct
                                    ; tryFunDeps  eq_ct
                                    ; tryQCsEqCt  eq_ct
                                    ; simpleStage (updInertEqs eq_ct)
                                    ; stopWithStage (eqCtEvidence eq_ct) ".." }}}

    Each sub-stage can elect to
      (a) ContinueWith: continue to the next stasge
      (b) StartAgain:   start again at the beginning of the pipeline
      (c) Stop:         stop altogether; constraint is solved

    These three possiblities are described by the `StopOrContinue` data type.
    The `SolverStage` monad does the plumbing.

    Notes:

    (SM1) Each individual stage pretty quickly drops down into
             TcS (StopOrContinue a)
        because the monadic plumbing of `SolverStage` is relatively ineffienct,
        with that three-way split.

    (SM2) We use `SolverStage Void` to express the idea that ContinueWith is
        impossible; we don't need to pattern match on it as a possible outcome:A
        see GHC.Tc.Solver.Solve.solveOne.   To that end, ContinueWith is strict.
    -}

    data StopOrContinue a
      = StartAgain Ct     -- Constraint is not solved, but some unifications
                          --   happened, so go back to the beginning of the pipeline

      | ContinueWith !a   -- The constraint was not solved, although it may have
                          --   been rewritten.  It is strict so that
                          --   ContinueWith Void can't happen; see (SM2) in
                          --   Note [The SolverStage monad]

      | Stop CtEvidence   -- The (rewritten) constraint was solved
             SDoc         -- Tells how it was solved
                          -- Any new sub-goals have been put on the work list
      deriving ((forall a b. (a -> b) -> StopOrContinue a -> StopOrContinue b)
    -> (forall a b. a -> StopOrContinue b -> StopOrContinue a)
    -> Functor StopOrContinue
    forall a b. a -> StopOrContinue b -> StopOrContinue a
    forall a b. (a -> b) -> StopOrContinue a -> StopOrContinue b
    forall (f :: * -> *).
    (forall a b. (a -> b) -> f a -> f b)
    -> (forall a b. a -> f b -> f a) -> Functor f
    $cfmap :: forall a b. (a -> b) -> StopOrContinue a -> StopOrContinue b
    fmap :: forall a b. (a -> b) -> StopOrContinue a -> StopOrContinue b
    $c<$ :: forall a b. a -> StopOrContinue b -> StopOrContinue a
    <$ :: forall a b. a -> StopOrContinue b -> StopOrContinue a
    Functor)

    instance Outputable a => Outputable (StopOrContinue a) where
      ppr :: StopOrContinue a -> SDoc
    ppr (Stop CtEvidence
    ev SDoc
    s)      = String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "Stop" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc
    parens (SDoc
    s SDoc -> SDoc -> SDoc
    forall doc. IsDoc doc => doc -> doc -> doc
    $$ String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "ev:" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> CtEvidence -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr CtEvidence
    ev)
      ppr (ContinueWith a
    w) = String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "ContinueWith" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> a -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr a
    w
      ppr (StartAgain Ct
    w)   = String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "StartAgain" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> Ct -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Ct
    w

    newtype SolverStage a = Stage { forall a. SolverStage a -> TcS (StopOrContinue a)
    runSolverStage :: TcS (StopOrContinue a) }
      deriving( (forall a b. (a -> b) -> SolverStage a -> SolverStage b)
    -> (forall a b. a -> SolverStage b -> SolverStage a)
    -> Functor SolverStage
    forall a b. a -> SolverStage b -> SolverStage a
    forall a b. (a -> b) -> SolverStage a -> SolverStage b
    forall (f :: * -> *).
    (forall a b. (a -> b) -> f a -> f b)
    -> (forall a b. a -> f b -> f a) -> Functor f
    $cfmap :: forall a b. (a -> b) -> SolverStage a -> SolverStage b
    fmap :: forall a b. (a -> b) -> SolverStage a -> SolverStage b
    $c<$ :: forall a b. a -> SolverStage b -> SolverStage a
    <$ :: forall a b. a -> SolverStage b -> SolverStage a
    Functor )

    instance Applicative SolverStage where
      pure :: forall a. a -> SolverStage a
    pure a
    x = TcS (StopOrContinue a) -> SolverStage a
    forall a. TcS (StopOrContinue a) -> SolverStage a
    Stage (StopOrContinue a -> TcS (StopOrContinue a)
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (a -> StopOrContinue a
    forall a. a -> StopOrContinue a
    ContinueWith a
    x))
      <*> :: forall a b. SolverStage (a -> b) -> SolverStage a -> SolverStage b
    (<*>)  = SolverStage (a -> b) -> SolverStage a -> SolverStage b
    forall (m :: * -> *) a b. Monad m => m (a -> b) -> m a -> m b
    ap

    instance Monad SolverStage where
      return :: forall a. a -> SolverStage a
    return          = a -> SolverStage a
    forall a. a -> SolverStage a
    forall (f :: * -> *) a. Applicative f => a -> f a
    pure
      (Stage TcS (StopOrContinue a)
    m) >>= :: forall a b. SolverStage a -> (a -> SolverStage b) -> SolverStage b
    >>= a -> SolverStage b
    k = TcS (StopOrContinue b) -> SolverStage b
    forall a. TcS (StopOrContinue a) -> SolverStage a
    Stage (TcS (StopOrContinue b) -> SolverStage b)
    -> TcS (StopOrContinue b) -> SolverStage b
    forall a b. (a -> b) -> a -> b
    $
                        do { soc <- TcS (StopOrContinue a)
    m
                           ; case soc of
                               StartAgain Ct
    x   -> StopOrContinue b -> TcS (StopOrContinue b)
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Ct -> StopOrContinue b
    forall a. Ct -> StopOrContinue a
    StartAgain Ct
    x)
                               Stop CtEvidence
    ev SDoc
    d      -> StopOrContinue b -> TcS (StopOrContinue b)
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (CtEvidence -> SDoc -> StopOrContinue b
    forall a. CtEvidence -> SDoc -> StopOrContinue a
    Stop CtEvidence
    ev SDoc
    d)
                               ContinueWith a
    x -> SolverStage b -> TcS (StopOrContinue b)
    forall a. SolverStage a -> TcS (StopOrContinue a)
    runSolverStage (a -> SolverStage b
    k a
    x) }

    nopStage :: a -> SolverStage a
    nopStage :: forall a. a -> SolverStage a
    nopStage a
    res = TcS (StopOrContinue a) -> SolverStage a
    forall a. TcS (StopOrContinue a) -> SolverStage a
    Stage (a -> TcS (StopOrContinue a)
    forall a. a -> TcS (StopOrContinue a)
    continueWith a
    res)

    simpleStage :: TcS a -> SolverStage a
    -- Always does a ContinueWith; no Stop or StartAgain
    simpleStage :: forall a. TcS a -> SolverStage a
    simpleStage TcS a
    thing = TcS (StopOrContinue a) -> SolverStage a
    forall a. TcS (StopOrContinue a) -> SolverStage a
    Stage (do { res <- TcS a
    thing; continueWith res })

    startAgainWith :: Ct -> TcS (StopOrContinue a)
    startAgainWith :: forall a. Ct -> TcS (StopOrContinue a)
    startAgainWith Ct
    ct = StopOrContinue a -> TcS (StopOrContinue a)
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Ct -> StopOrContinue a
    forall a. Ct -> StopOrContinue a
    StartAgain Ct
    ct)

    continueWith :: a -> TcS (StopOrContinue a)
    continueWith :: forall a. a -> TcS (StopOrContinue a)
    continueWith a
    ct = StopOrContinue a -> TcS (StopOrContinue a)
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (a -> StopOrContinue a
    forall a. a -> StopOrContinue a
    ContinueWith a
    ct)

    stopWith :: CtEvidence -> String -> TcS (StopOrContinue a)
    stopWith :: forall a. CtEvidence -> String -> TcS (StopOrContinue a)
    stopWith CtEvidence
    ev String
    s = StopOrContinue a -> TcS (StopOrContinue a)
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (CtEvidence -> SDoc -> StopOrContinue a
    forall a. CtEvidence -> SDoc -> StopOrContinue a
    Stop CtEvidence
    ev (String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    s))

    stopWithStage :: CtEvidence -> String -> SolverStage a
    stopWithStage :: forall a. CtEvidence -> String -> SolverStage a
    stopWithStage CtEvidence
    ev String
    s = TcS (StopOrContinue a) -> SolverStage a
    forall a. TcS (StopOrContinue a) -> SolverStage a
    Stage (CtEvidence -> String -> TcS (StopOrContinue a)
    forall a. CtEvidence -> String -> TcS (StopOrContinue a)
    stopWith CtEvidence
    ev String
    s)

    {- *********************************************************************
    *                                                                      *
                       Inert instances: inert_qcis
    *                                                                      *
    ********************************************************************* -}

    addInertQCI :: QCInst -> TcS ()
    -- Add a local quantified constraint, typically arising from a type signature
    addInertQCI :: QCInst -> TcS ()
    addInertQCI QCInst
    new_qci
      = do { ics  <- TcS InertCans
    getInertCans
           ; ics1 <- add_qci ics

           -- Update given equalities. C.f updateGivenEqs
           ; tclvl <- getTcLevel
           ; let body_pred    = QCInst -> Type
    qci_body QCInst
    new_qci
                 not_equality = Type -> Bool
    isClassPred Type
    body_pred Bool -> Bool -> Bool
    && Bool -> Bool
    not (Type -> Bool
    isEqClassPred Type
    body_pred)
                      -- True <=> definitely not an equality
                      -- A qci_body like (f a) might be an equality

                 ics2 | Bool
    not_equality = InertCans
    ics1
                      | Bool
    otherwise    = InertCans
    ics1 { inert_given_eq_lvl = tclvl
                                            , inert_given_eqs    = True }

           ; setInertCans ics2 }
      where
        add_qci :: InertCans -> TcS InertCans
        -- See Note [Do not add duplicate quantified instances]
        add_qci :: InertCans -> TcS InertCans
    add_qci ics :: InertCans
    ics@(IC { inert_qcis :: InertCans -> [QCInst]
    inert_qcis = [QCInst]
    qcis })
          | (QCInst -> Bool) -> [QCInst] -> Bool
    forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
    any QCInst -> Bool
    same_qci [QCInst]
    qcis
          = do { String -> SDoc -> TcS ()
    traceTcS String
    "skipping duplicate quantified instance" (QCInst -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr QCInst
    new_qci)
               ; InertCans -> TcS InertCans
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return InertCans
    ics }

          | Bool
    otherwise
          = do { String -> SDoc -> TcS ()
    traceTcS String
    "adding new inert quantified instance" (QCInst -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr QCInst
    new_qci)
               ; InertCans -> TcS InertCans
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (InertCans
    ics { inert_qcis = new_qci : qcis }) }

        same_qci :: QCInst -> Bool
    same_qci QCInst
    old_qci = HasDebugCallStack => Type -> Type -> Bool
    Type -> Type -> Bool
    tcEqType (CtEvidence -> Type
    ctEvPred (QCInst -> CtEvidence
    qci_ev QCInst
    old_qci))
                                    (CtEvidence -> Type
    ctEvPred (QCInst -> CtEvidence
    qci_ev QCInst
    new_qci))

    {- Note [Do not add duplicate quantified instances]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    As an optimisation, we avoid adding duplicate quantified instances to the
    inert set; we use a simple duplicate check using tcEqType for simplicity,
    even though it doesn't account for superficial differences, e.g. it will count
    the following two constraints as different (#22223):

      - forall a b. C a b
      - forall b a. C a b

    The main logic that allows us to pick local instances, even in the presence of
    duplicates, is explained in Note [Use only the best matching quantified constraint]
    in GHC.Tc.Solver.Dict.
    -}

    updInertDicts :: DictCt -> TcS ()
    updInertDicts :: DictCt -> TcS ()
    updInertDicts dict_ct :: DictCt
    dict_ct@(DictCt { di_cls :: DictCt -> Class
    di_cls = Class
    cls, di_ev :: DictCt -> CtEvidence
    di_ev = CtEvidence
    ev, di_tys :: DictCt -> [Type]
    di_tys = [Type]
    tys })
      = do { String -> SDoc -> TcS ()
    traceTcS String
    "Adding inert dict" (DictCt -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr DictCt
    dict_ct SDoc -> SDoc -> SDoc
    forall doc. IsDoc doc => doc -> doc -> doc
    $$ Class -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Class
    cls  SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> [Type] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr [Type]
    tys)

           ; if | CtEvidence -> Bool
    isGiven CtEvidence
    ev, Just (Type
    str_ty, Type
    _) <- Class -> [Type] -> Maybe (Type, Type)
    isIPPred_maybe Class
    cls [Type]
    tys
                -> -- For [G] ?x::ty, remove any dicts mentioning ?x,
                  --    from /both/ inert_cans /and/ inert_solved_dicts (#23761)
                   -- See (SIP1) and (SIP2) in Note [Shadowing of implicit parameters]
                   (InertSet -> InertSet) -> TcS ()
    updInertSet ((InertSet -> InertSet) -> TcS ())
    -> (InertSet -> InertSet) -> TcS ()
    forall a b. (a -> b) -> a -> b
    $ \ inerts :: InertSet
    inerts@(IS { inert_cans :: InertSet -> InertCans
    inert_cans = InertCans
    ics, inert_solved_dicts :: InertSet -> DictMap DictCt
    inert_solved_dicts = DictMap DictCt
    solved }) ->
                   InertSet
    inerts { inert_cans         = updDicts (filterDicts (does_not_mention_ip_for str_ty)) ics
                          , inert_solved_dicts = filterDicts (does_not_mention_ip_for str_ty) solved }
                | Bool
    otherwise
                -> () -> TcS ()
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()
           -- Add the new constraint to the inert set
           ; (InertCans -> InertCans) -> TcS ()
    updInertCans ((DictMap DictCt -> DictMap DictCt) -> InertCans -> InertCans
    updDicts (DictCt -> DictMap DictCt -> DictMap DictCt
    addDict DictCt
    dict_ct)) }
      where
        -- Does this class constraint or any of its superclasses mention
        -- an implicit parameter (?str :: ty) for the given 'str' and any type 'ty'?
        does_not_mention_ip_for :: Type -> DictCt -> Bool
        does_not_mention_ip_for :: Type -> DictCt -> Bool
    does_not_mention_ip_for Type
    str_ty (DictCt { di_cls :: DictCt -> Class
    di_cls = Class
    cls, di_tys :: DictCt -> [Type]
    di_tys = [Type]
    tys })
          = Bool -> Bool
    not (Bool -> Bool) -> Bool -> Bool
    forall a b. (a -> b) -> a -> b
    $ (Type -> Bool) -> (Type -> Bool) -> Class -> [Type] -> Bool
    mightMentionIP (Bool -> Bool
    not (Bool -> Bool) -> (Type -> Bool) -> Type -> Bool
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . Type -> Type -> Bool
    typesAreApart Type
    str_ty) (Bool -> Type -> Bool
    forall a b. a -> b -> a
    const Bool
    True) Class
    cls [Type]
    tys
            -- See Note [Using typesAreApart when calling mightMentionIP]
            -- in GHC.Core.Predicate

    updInertIrreds :: IrredCt -> TcS ()
    updInertIrreds :: IrredCt -> TcS ()
    updInertIrreds IrredCt
    irred
      = do { tc_lvl <- TcS TcLevel
    getTcLevel
           ; updInertCans $ addIrredToCans tc_lvl irred }

    {- *********************************************************************
    *                                                                      *
                      Kicking out
    *                                                                      *
    ************************************************************************
    -}

    -----------------------------------------
    kickOutRewritable  :: KickOutSpec -> CtFlavourRole -> TcS ()
    kickOutRewritable :: KickOutSpec -> CtFlavourRole -> TcS ()
    kickOutRewritable KickOutSpec
    ko_spec CtFlavourRole
    new_fr
      = do { ics <- TcS InertCans
    getInertCans
           ; let (kicked_out, ics') = kickOutRewritableLHS ko_spec new_fr ics
                 n_kicked = Cts -> Int
    forall a. Bag a -> Int
    lengthBag Cts
    kicked_out
           ; setInertCans ics'

           ; unless (isEmptyBag kicked_out) $
             do { emitWork kicked_out

                  -- The famapp-cache contains Given evidence from the inert set.
                  -- If we're kicking out Givens, we need to remove this evidence
                  -- from the cache, too.
                ; let kicked_given_ev_vars = (Ct -> VarSet -> VarSet) -> VarSet -> Cts -> VarSet
    forall a b. (a -> b -> b) -> b -> Bag a -> b
    forall (t :: * -> *) a b.
    Foldable t =>
    (a -> b -> b) -> b -> t a -> b
    foldr Ct -> VarSet -> VarSet
    add_one VarSet
    emptyVarSet Cts
    kicked_out
                      add_one :: Ct -> VarSet -> VarSet
                      add_one Ct
    ct VarSet
    vs | CtGiven (GivenCt { ctev_evar :: GivenCtEvidence -> TcTyVar
    ctev_evar = TcTyVar
    ev_var }) <- Ct -> CtEvidence
    ctEvidence Ct
    ct
                                    = VarSet
    vs VarSet -> TcTyVar -> VarSet
    `extendVarSet` TcTyVar
    ev_var
                                    | Bool
    otherwise = VarSet
    vs

                ; when (new_fr `eqCanRewriteFR` (Given, NomEq) &&
                       -- if this isn't true, no use looking through the constraints
                        not (isEmptyVarSet kicked_given_ev_vars)) $
                  do { traceTcS "Given(s) have been kicked out; drop from famapp-cache"
                                (ppr kicked_given_ev_vars)
                     ; dropFromFamAppCache kicked_given_ev_vars }

                ; csTraceTcS $
                  hang (text "Kick out")
                     2 (vcat [ text "n-kicked =" <+> int n_kicked
                             , text "kicked_out =" <+> ppr kicked_out
                             , text "Residual inerts =" <+> ppr ics' ]) } }

    kickOutAfterUnification :: [TcTyVar] -> TcS ()
    kickOutAfterUnification :: [TcTyVar] -> TcS ()
    kickOutAfterUnification [TcTyVar]
    tv_list = case [TcTyVar] -> Maybe (NonEmpty TcTyVar)
    forall a. [a] -> Maybe (NonEmpty a)
    nonEmpty [TcTyVar]
    tv_list of
        Maybe (NonEmpty TcTyVar)
    Nothing -> () -> TcS ()
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()
        Just NonEmpty TcTyVar
    tvs -> do
           { let tv_set :: VarSet
    tv_set = [TcTyVar] -> VarSet
    mkVarSet [TcTyVar]
    tv_list

           ; n_kicked <- KickOutSpec -> CtFlavourRole -> TcS ()
    kickOutRewritable (VarSet -> KickOutSpec
    KOAfterUnify VarSet
    tv_set) (CtFlavour
    Given, EqRel
    NomEq)
                         -- Given because the tv := xi is given; NomEq because
                         -- only nominal equalities are solved by unification

           -- Set the unification flag if we have done outer unifications
           -- that might affect an earlier implication constraint
           ; let min_tv_lvl = (TcLevel -> TcLevel -> TcLevel) -> NonEmpty TcLevel -> TcLevel
    forall (t :: * -> *) a. Foldable1 t => (a -> a -> a) -> t a -> a
    foldr1 TcLevel -> TcLevel -> TcLevel
    minTcLevel ((TcTyVar -> TcLevel) -> NonEmpty TcTyVar -> NonEmpty TcLevel
    forall a b. (a -> b) -> NonEmpty a -> NonEmpty b
    NE.map TcTyVar -> TcLevel
    tcTyVarLevel NonEmpty TcTyVar
    tvs)
           ; ambient_lvl <- getTcLevel
           ; when (ambient_lvl `strictlyDeeperThan` min_tv_lvl) $
             setUnificationFlag min_tv_lvl

           ; traceTcS "kickOutAfterUnification" (ppr tvs $$ text "n_kicked =" <+> ppr n_kicked)
           ; return n_kicked }

    kickOutAfterFillingCoercionHole :: CoercionHole -> TcS ()
    -- See Wrinkle (URW2) in Note [Unify only if the rewriter set is empty]
    -- in GHC.Tc.Solver.Equality
    --
    -- It's possible that this could just go ahead and unify, but could there
    -- be occurs-check problems? Seems simpler just to kick out.
    kickOutAfterFillingCoercionHole :: CoercionHole -> TcS ()
    kickOutAfterFillingCoercionHole CoercionHole
    hole
      = do { ics <- TcS InertCans
    getInertCans
           ; let (kicked_out, ics') = kick_out ics
                 n_kicked           = [EqCt] -> Int
    forall a. [a] -> Int
    forall (t :: * -> *) a. Foldable t => t a -> Int
    length [EqCt]
    kicked_out

           ; unless (n_kicked == 0) $
             do { updWorkListTcS (extendWorkListRewrittenEqs kicked_out)
                ; csTraceTcS $
                  hang (text "Kick out, hole =" <+> ppr hole)
                     2 (vcat [ text "n-kicked =" <+> int n_kicked
                             , text "kicked_out =" <+> ppr kicked_out
                             , text "Residual inerts =" <+> ppr ics' ]) }

           ; setInertCans ics' }
      where
        kick_out :: InertCans -> ([EqCt], InertCans)
        kick_out :: InertCans -> ([EqCt], InertCans)
    kick_out ics :: InertCans
    ics@(IC { inert_eqs :: InertCans -> InertEqs
    inert_eqs = InertEqs
    eqs })
          = ([EqCt]
    eqs_to_kick, InertCans
    ics { inert_eqs = eqs_to_keep })
          where
            ([EqCt]
    eqs_to_kick, InertEqs
    eqs_to_keep) = (EqCt -> Bool) -> InertEqs -> ([EqCt], InertEqs)
    partitionInertEqs EqCt -> Bool
    kick_out_eq InertEqs
    eqs

        kick_out_eq :: EqCt -> Bool    -- True: kick out; False: keep.
        kick_out_eq :: EqCt -> Bool
    kick_out_eq (EqCt { eq_ev :: EqCt -> CtEvidence
    eq_ev = CtEvidence
    ev ,eq_lhs :: EqCt -> CanEqLHS
    eq_lhs = CanEqLHS
    lhs })
          | CtWanted (WantedCt { ctev_rewriters :: WantedCtEvidence -> RewriterSet
    ctev_rewriters = RewriterSet UniqSet CoercionHole
    rewriters }) <- CtEvidence
    ev
          , TyVarLHS TcTyVar
    tv <- CanEqLHS
    lhs
          , TcTyVar -> Bool
    isMetaTyVar TcTyVar
    tv
          = CoercionHole
    hole CoercionHole -> UniqSet CoercionHole -> Bool
    forall a. Uniquable a => a -> UniqSet a -> Bool
    `elementOfUniqSet` UniqSet CoercionHole
    rewriters
          | Bool
    otherwise
          = Bool
    False

    --------------
    insertSafeOverlapFailureTcS :: InstanceWhat -> DictCt -> TcS ()
    -- See Note [Safe Haskell Overlapping Instances Implementation] in GHC.Tc.Solver
    insertSafeOverlapFailureTcS :: InstanceWhat -> DictCt -> TcS ()
    insertSafeOverlapFailureTcS InstanceWhat
    what DictCt
    item
      | InstanceWhat -> Bool
    safeOverlap InstanceWhat
    what
      = () -> TcS ()
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()
      | Bool
    otherwise
      = (InertSet -> InertSet) -> TcS ()
    updInertSet (\InertSet
    is -> InertSet
    is { inert_safehask = addDict item (inert_safehask is) })

    getSafeOverlapFailures :: TcS (Bag DictCt)
    -- See Note [Safe Haskell Overlapping Instances Implementation] in GHC.Tc.Solver
    getSafeOverlapFailures :: TcS (Bag DictCt)
    getSafeOverlapFailures
     = do { IS { inert_safehask = safehask } <- TcS InertSet
    getInertSet
          ; return $ foldDicts consBag safehask emptyBag }

    --------------
    updSolvedDicts :: InstanceWhat -> DictCt -> TcS ()
    -- Conditionally add a new item in the solved set of the monad
    -- See Note [Solved dictionaries] in GHC.Tc.Solver.InertSet
    updSolvedDicts :: InstanceWhat -> DictCt -> TcS ()
    updSolvedDicts InstanceWhat
    what dict_ct :: DictCt
    dict_ct@(DictCt { di_cls :: DictCt -> Class
    di_cls = Class
    cls, di_tys :: DictCt -> [Type]
    di_tys = [Type]
    tys, di_ev :: DictCt -> CtEvidence
    di_ev = CtEvidence
    ev })
      | CtEvidence -> Bool
    isWanted CtEvidence
    ev
      , InstanceWhat -> Bool
    instanceReturnsDictCon InstanceWhat
    what
      = do { is_callstack    <- (Type -> Bool) -> Name -> TcS (Type -> Bool)
    is_tyConTy Type -> Bool
    isCallStackTy        Name
    callStackTyConName
           ; is_exceptionCtx <- is_tyConTy isExceptionContextTy exceptionContextTyConName
           ; let contains_callstack_or_exceptionCtx =
                   (Type -> Bool) -> (Type -> Bool) -> Class -> [Type] -> Bool
    mightMentionIP
                     (Bool -> Type -> Bool
    forall a b. a -> b -> a
    const Bool
    True)
                        -- NB: the name of the call-stack IP is irrelevant
                        -- e.g (?foo :: CallStack) counts!
                     (Type -> Bool
    is_callstack (Type -> Bool) -> (Type -> Bool) -> Type -> Bool
    forall (f :: * -> *). Applicative f => f Bool -> f Bool -> f Bool
    <||> Type -> Bool
    is_exceptionCtx)
                     Class
    cls [Type]
    tys
           -- See Note [Don't add HasCallStack constraints to the solved set]
           ; unless contains_callstack_or_exceptionCtx $
        do { traceTcS "updSolvedDicts:" $ ppr dict_ct
           ; updInertSet $ \ InertSet
    ics ->
               InertSet
    ics { inert_solved_dicts = addSolvedDict dict_ct (inert_solved_dicts ics) }
           } }
      | Bool
    otherwise
      = () -> TcS ()
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()
      where

        -- Return a predicate that decides whether a type is CallStack
        -- or ExceptionContext, accounting for e.g. type family reduction, as
        -- per Note [Using typesAreApart when calling mightMentionIP].
        --
        -- See Note [Using isCallStackTy in mightMentionIP].
        is_tyConTy :: (Type -> Bool) -> Name -> TcS (Type -> Bool)
        is_tyConTy :: (Type -> Bool) -> Name -> TcS (Type -> Bool)
    is_tyConTy Type -> Bool
    is_eq Name
    tc_name
          = do { (mb_tc, _) <- TcM (Maybe TyCon, Messages TcRnMessage)
    -> TcS (Maybe TyCon, Messages TcRnMessage)
    forall a. TcM a -> TcS a
    wrapTcS (TcM (Maybe TyCon, Messages TcRnMessage)
     -> TcS (Maybe TyCon, Messages TcRnMessage))
    -> TcM (Maybe TyCon, Messages TcRnMessage)
    -> TcS (Maybe TyCon, Messages TcRnMessage)
    forall a b. (a -> b) -> a -> b
    $ TcRn TyCon -> TcM (Maybe TyCon, Messages TcRnMessage)
    forall a. TcRn a -> TcRn (Maybe a, Messages TcRnMessage)
    TcM.tryTc (TcRn TyCon -> TcM (Maybe TyCon, Messages TcRnMessage))
    -> TcRn TyCon -> TcM (Maybe TyCon, Messages TcRnMessage)
    forall a b. (a -> b) -> a -> b
    $ Name -> TcRn TyCon
    TcM.tcLookupTyCon Name
    tc_name
               ; case mb_tc of
                  Just TyCon
    tc ->
                    (Type -> Bool) -> TcS (Type -> Bool)
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ((Type -> Bool) -> TcS (Type -> Bool))
    -> (Type -> Bool) -> TcS (Type -> Bool)
    forall a b. (a -> b) -> a -> b
    $ \ Type
    ty -> Bool -> Bool
    not (Type -> Type -> Bool
    typesAreApart Type
    ty (TyCon -> Type
    mkTyConTy TyCon
    tc))
                  Maybe TyCon
    Nothing ->
                    (Type -> Bool) -> TcS (Type -> Bool)
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return Type -> Bool
    is_eq
               }

    {- Note [Don't add HasCallStack constraints to the solved set]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We must not add solved Wanted dictionaries that mention HasCallStack constraints
    to the solved set, or we might fail to accumulate the proper call stack, as was
    reported in #25529.

    Recall that HasCallStack constraints (and the related HasExceptionContext
    constraints) are implicit parameter constraints, and are accumulated as per
    Note [Overview of implicit CallStacks] in GHC.Tc.Types.Evidence.

    When we solve a Wanted that contains a HasCallStack constraint, we don't want
    to cache the result, because re-using that solution means re-using the call-stack
    in a different context!

    See also Note [Shadowing of implicit parameters], which deals with a similar
    problem with Given implicit parameter constraints.

    Note [Using isCallStackTy in mightMentionIP]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    To implement Note [Don't add HasCallStack constraints to the solved set],
    we need to check whether a constraint contains a HasCallStack or HasExceptionContext
    constraint. We do this using the 'mentionsIP' function, but as per
    Note [Using typesAreApart when calling mightMentionIP] we don't want to simply do:

      mightMentionIP
        (const True) -- (ignore the implicit parameter string)
        (isCallStackTy <||> isExceptionContextTy)

    because this does not account for e.g. a type family that reduces to CallStack.
    The predicate we want to use instead is:

        \ ty -> not (typesAreApart ty callStackTy && typesAreApart ty exceptionContextTy)

    However, this is made difficult by the fact that CallStack and ExceptionContext
    are not wired-in types; they are only known-key. This means we must look them
    up using 'tcLookupTyCon'. However, this might fail, e.g. if we are in the middle
    of typechecking ghc-internal and these data-types have not been typechecked yet!

    In that case, we simply fall back to the naive 'isCallStackTy'/'isExceptionContextTy'
    logic.

    Note that it would be somewhat painful to wire-in ExceptionContext: at the time
    of writing (March 2025), this would require wiring in the ExceptionAnnotation
    class, as well as SomeExceptionAnnotation, which is a data type with existentials.
    -}

    getSolvedDicts :: TcS (DictMap DictCt)
    getSolvedDicts :: TcS (DictMap DictCt)
    getSolvedDicts = do { ics <- TcS InertSet
    getInertSet; return (inert_solved_dicts ics) }

    setSolvedDicts :: DictMap DictCt -> TcS ()
    setSolvedDicts :: DictMap DictCt -> TcS ()
    setSolvedDicts DictMap DictCt
    solved_dicts
      = (InertSet -> InertSet) -> TcS ()
    updInertSet ((InertSet -> InertSet) -> TcS ())
    -> (InertSet -> InertSet) -> TcS ()
    forall a b. (a -> b) -> a -> b
    $ \ InertSet
    ics ->
        InertSet
    ics { inert_solved_dicts = solved_dicts }

    {- *********************************************************************
    *                                                                      *
                      Other inert-set operations
    *                                                                      *
    ********************************************************************* -}

    updInertSet :: (InertSet -> InertSet) -> TcS ()
    -- Modify the inert set with the supplied function
    updInertSet :: (InertSet -> InertSet) -> TcS ()
    updInertSet InertSet -> InertSet
    upd_fn
      = do { is_var <- TcS (IORef InertSet)
    getInertSetRef
           ; wrapTcS (do { curr_inert <- TcM.readTcRef is_var
                         ; TcM.writeTcRef is_var (upd_fn curr_inert) }) }

    getInertCans :: TcS InertCans
    getInertCans :: TcS InertCans
    getInertCans = do { inerts <- TcS InertSet
    getInertSet; return (inert_cans inerts) }

    setInertCans :: InertCans -> TcS ()
    setInertCans :: InertCans -> TcS ()
    setInertCans InertCans
    ics = (InertSet -> InertSet) -> TcS ()
    updInertSet ((InertSet -> InertSet) -> TcS ())
    -> (InertSet -> InertSet) -> TcS ()
    forall a b. (a -> b) -> a -> b
    $ \ InertSet
    inerts -> InertSet
    inerts { inert_cans = ics }

    updRetInertCans :: (InertCans -> (a, InertCans)) -> TcS a
    -- Modify the inert set with the supplied function
    updRetInertCans :: forall a. (InertCans -> (a, InertCans)) -> TcS a
    updRetInertCans InertCans -> (a, InertCans)
    upd_fn
      = do { is_var <- TcS (IORef InertSet)
    getInertSetRef
           ; wrapTcS (do { inerts <- TcM.readTcRef is_var
                         ; let (res, cans') = upd_fn (inert_cans inerts)
                         ; TcM.writeTcRef is_var (inerts { inert_cans = cans' })
                         ; return res }) }

    updInertCans :: (InertCans -> InertCans) -> TcS ()
    -- Modify the inert set with the supplied function
    updInertCans :: (InertCans -> InertCans) -> TcS ()
    updInertCans InertCans -> InertCans
    upd_fn
      = (InertSet -> InertSet) -> TcS ()
    updInertSet ((InertSet -> InertSet) -> TcS ())
    -> (InertSet -> InertSet) -> TcS ()
    forall a b. (a -> b) -> a -> b
    $ \ InertSet
    inerts -> InertSet
    inerts { inert_cans = upd_fn (inert_cans inerts) }

    getInertEqs :: TcS InertEqs
    getInertEqs :: TcS InertEqs
    getInertEqs = do { inert <- TcS InertCans
    getInertCans; return (inert_eqs inert) }

    getInnermostGivenEqLevel :: TcS TcLevel
    getInnermostGivenEqLevel :: TcS TcLevel
    getInnermostGivenEqLevel = do { inert <- TcS InertCans
    getInertCans
                                  ; return (inert_given_eq_lvl inert) }

    -- | Retrieves all insoluble constraints from the inert set,
    -- specifically including Given constraints.
    --
    -- This consists of:
    --
    --  - insoluble equalities, such as @Int ~# Bool@;
    --  - constraints that are top-level custom type errors, of the form
    --    @TypeError msg@, but not constraints such as @Eq (TypeError msg)@
    --    in which the type error is nested;
    --  - unsatisfiable constraints, of the form @Unsatisfiable msg@.
    --
    -- The inclusion of Givens is important for pattern match warnings, as we
    -- want to consider a pattern match that introduces insoluble Givens to be
    -- redundant (see Note [Pattern match warnings with insoluble Givens] in GHC.Tc.Solver).
    getInertInsols :: TcS Cts
    getInertInsols :: TcS Cts
    getInertInsols
      = do { inert <- TcS InertCans
    getInertCans
           ; let insols = (IrredCt -> Bool) -> Bag IrredCt -> Bag IrredCt
    forall a. (a -> Bool) -> Bag a -> Bag a
    filterBag IrredCt -> Bool
    insolubleIrredCt (InertCans -> Bag IrredCt
    inert_irreds InertCans
    inert)
                 unsats = DictMap DictCt -> Unique -> Bag DictCt
    forall a. DictMap a -> Unique -> Bag a
    findDictsByTyConKey (InertCans -> DictMap DictCt
    inert_dicts InertCans
    inert) Unique
    unsatisfiableClassNameKey
           ; return $ fmap CDictCan unsats `unionBags` fmap CIrredCan insols }

    getInertGivens :: TcS [Ct]
    -- Returns the Given constraints in the inert set
    getInertGivens :: TcS [Ct]
    getInertGivens
      = do { inerts <- TcS InertCans
    getInertCans
           ; let all_cts = (IrredCt -> [Ct] -> [Ct]) -> Bag IrredCt -> [Ct] -> [Ct]
    forall b. (IrredCt -> b -> b) -> Bag IrredCt -> b -> b
    foldIrreds ((:) (Ct -> [Ct] -> [Ct]) -> (IrredCt -> Ct) -> IrredCt -> [Ct] -> [Ct]
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . IrredCt -> Ct
    CIrredCan) (InertCans -> Bag IrredCt
    inert_irreds InertCans
    inerts)
                         ([Ct] -> [Ct]) -> [Ct] -> [Ct]
    forall a b. (a -> b) -> a -> b
    $ (DictCt -> [Ct] -> [Ct]) -> DictMap DictCt -> [Ct] -> [Ct]
    forall a b. (a -> b -> b) -> DictMap a -> b -> b
    foldDicts  ((:) (Ct -> [Ct] -> [Ct]) -> (DictCt -> Ct) -> DictCt -> [Ct] -> [Ct]
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . DictCt -> Ct
    CDictCan)  (InertCans -> DictMap DictCt
    inert_dicts InertCans
    inerts)
                         ([Ct] -> [Ct]) -> [Ct] -> [Ct]
    forall a b. (a -> b) -> a -> b
    $ (EqCt -> [Ct] -> [Ct]) -> FunEqMap [EqCt] -> [Ct] -> [Ct]
    forall b. (EqCt -> b -> b) -> FunEqMap [EqCt] -> b -> b
    foldFunEqs ((:) (Ct -> [Ct] -> [Ct]) -> (EqCt -> Ct) -> EqCt -> [Ct] -> [Ct]
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . EqCt -> Ct
    CEqCan)    (InertCans -> FunEqMap [EqCt]
    inert_funeqs InertCans
    inerts)
                         ([Ct] -> [Ct]) -> [Ct] -> [Ct]
    forall a b. (a -> b) -> a -> b
    $ (EqCt -> [Ct] -> [Ct]) -> InertEqs -> [Ct] -> [Ct]
    forall b. (EqCt -> b -> b) -> InertEqs -> b -> b
    foldTyEqs  ((:) (Ct -> [Ct] -> [Ct]) -> (EqCt -> Ct) -> EqCt -> [Ct] -> [Ct]
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . EqCt -> Ct
    CEqCan)    (InertCans -> InertEqs
    inert_eqs InertCans
    inerts)
                         ([Ct] -> [Ct]) -> [Ct] -> [Ct]
    forall a b. (a -> b) -> a -> b
    $ []
           ; return (filter isGivenCt all_cts) }

    getPendingGivenScs :: TcS [Ct]
    -- Find all inert Given dictionaries, or quantified constraints, such that
    --     1. cc_pend_sc flag has fuel strictly > 0
    --     2. belongs to the current level
    -- For each such dictionary:
    -- * Return it (with unmodified cc_pend_sc) in sc_pending
    -- * Modify the dict in the inert set to have cc_pend_sc = doNotExpand
    --   to record that we have expanded superclasses for this dict
    getPendingGivenScs :: TcS [Ct]
    getPendingGivenScs = do { lvl <- TcS TcLevel
    getTcLevel
                            ; updRetInertCans (get_sc_pending lvl) }

    get_sc_pending :: TcLevel -> InertCans -> ([Ct], InertCans)
    get_sc_pending :: TcLevel -> InertCans -> ([Ct], InertCans)
    get_sc_pending TcLevel
    this_lvl ic :: InertCans
    ic@(IC { inert_dicts :: InertCans -> DictMap DictCt
    inert_dicts = DictMap DictCt
    dicts, inert_qcis :: InertCans -> [QCInst]
    inert_qcis = [QCInst]
    insts })
      = Bool -> SDoc -> ([Ct], InertCans) -> ([Ct], InertCans)
    forall a. HasCallStack => Bool -> SDoc -> a -> a
    assertPpr ((Ct -> Bool) -> [Ct] -> Bool
    forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
    all Ct -> Bool
    isGivenCt [Ct]
    sc_pending) ([Ct] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr [Ct]
    sc_pending)
           -- When getPendingScDics is called,
           -- there are never any Wanteds in the inert set
        ([Ct]
    sc_pending, InertCans
    ic { inert_dicts = dicts', inert_qcis = insts' })
      where
        sc_pending :: [Ct]
    sc_pending = [Ct]
    sc_pend_insts [Ct] -> [Ct] -> [Ct]
    forall a. [a] -> [a] -> [a]
    ++ (DictCt -> Ct) -> [DictCt] -> [Ct]
    forall a b. (a -> b) -> [a] -> [b]
    map DictCt -> Ct
    CDictCan [DictCt]
    sc_pend_dicts

        sc_pend_dicts :: [DictCt]
        sc_pend_dicts :: [DictCt]
    sc_pend_dicts = (DictCt -> [DictCt] -> [DictCt])
    -> DictMap DictCt -> [DictCt] -> [DictCt]
    forall a b. (a -> b -> b) -> DictMap a -> b -> b
    foldDicts DictCt -> [DictCt] -> [DictCt]
    get_pending DictMap DictCt
    dicts []
        dicts' :: DictMap DictCt
    dicts' = (DictCt -> DictMap DictCt -> DictMap DictCt)
    -> DictMap DictCt -> [DictCt] -> DictMap DictCt
    forall a b. (a -> b -> b) -> b -> [a] -> b
    forall (t :: * -> *) a b.
    Foldable t =>
    (a -> b -> b) -> b -> t a -> b
    foldr DictCt -> DictMap DictCt -> DictMap DictCt
    exhaustAndAdd DictMap DictCt
    dicts [DictCt]
    sc_pend_dicts

        ([Ct]
    sc_pend_insts, [QCInst]
    insts') = ([Ct] -> QCInst -> ([Ct], QCInst))
    -> [Ct] -> [QCInst] -> ([Ct], [QCInst])
    forall (t :: * -> *) s a b.
    Traversable t =>
    (s -> a -> (s, b)) -> s -> t a -> (s, t b)
    mapAccumL [Ct] -> QCInst -> ([Ct], QCInst)
    get_pending_inst [] [QCInst]
    insts

        exhaustAndAdd :: DictCt -> DictMap DictCt -> DictMap DictCt
        exhaustAndAdd :: DictCt -> DictMap DictCt -> DictMap DictCt
    exhaustAndAdd DictCt
    ct DictMap DictCt
    dicts = DictCt -> DictMap DictCt -> DictMap DictCt
    addDict (DictCt
    ct {di_pend_sc = doNotExpand}) DictMap DictCt
    dicts
        -- Exhaust the fuel for this constraint before adding it as
        -- we don't want to expand these constraints again

        get_pending :: DictCt -> [DictCt] -> [DictCt]  -- Get dicts with cc_pend_sc > 0
        get_pending :: DictCt -> [DictCt] -> [DictCt]
    get_pending DictCt
    dict [DictCt]
    dicts
            | DictCt -> Bool
    isPendingScDictCt DictCt
    dict
            , CtEvidence -> Bool
    belongs_to_this_level (DictCt -> CtEvidence
    dictCtEvidence DictCt
    dict)
            = DictCt
    dict DictCt -> [DictCt] -> [DictCt]
    forall a. a -> [a] -> [a]
    : [DictCt]
    dicts
            | Bool
    otherwise
            = [DictCt]
    dicts

        get_pending_inst :: [Ct] -> QCInst -> ([Ct], QCInst)
        get_pending_inst :: [Ct] -> QCInst -> ([Ct], QCInst)
    get_pending_inst [Ct]
    cts qci :: QCInst
    qci@(QCI { qci_ev :: QCInst -> CtEvidence
    qci_ev = CtEvidence
    ev })
           | Just QCInst
    qci' <- QCInst -> Maybe QCInst
    pendingScInst_maybe QCInst
    qci
           , CtEvidence -> Bool
    belongs_to_this_level CtEvidence
    ev
           = (QCInst -> Ct
    CQuantCan QCInst
    qci Ct -> [Ct] -> [Ct]
    forall a. a -> [a] -> [a]
    : [Ct]
    cts, QCInst
    qci')
           -- qci' have their fuel exhausted
           -- we don't want to expand these constraints again
           -- qci is expanded
           | Bool
    otherwise
           = ([Ct]
    cts, QCInst
    qci)

        belongs_to_this_level :: CtEvidence -> Bool
    belongs_to_this_level CtEvidence
    ev = CtLoc -> TcLevel
    ctLocLevel (CtEvidence -> CtLoc
    ctEvLoc CtEvidence
    ev) TcLevel -> TcLevel -> Bool
    `sameDepthAs` TcLevel
    this_lvl
        -- We only want Givens from this level; see (3a) in
        -- Note [The superclass story] in GHC.Tc.Solver.Dict

    getUnsolvedInerts :: TcS Cts    -- All simple constraints
    -- Return all the unsolved [Wanted] constraints
    --
    -- Post-condition: the returned simple constraints are all fully zonked
    --                     (because they come from the inert set)
    --                 the unsolved implics may not be
    getUnsolvedInerts :: TcS Cts
    getUnsolvedInerts
     = do { IC { inert_eqs    = tv_eqs
               , inert_funeqs = fun_eqs
               , inert_irreds = irreds
               , inert_dicts  = idicts
               , inert_qcis   = qcis
               } <- TcS InertCans
    getInertCans

          ; let unsolved_tv_eqs  = (EqCt -> Cts -> Cts) -> InertEqs -> Cts -> Cts
    forall b. (EqCt -> b -> b) -> InertEqs -> b -> b
    foldTyEqs  ((EqCt -> Ct) -> EqCt -> Cts -> Cts
    forall a. (a -> Ct) -> a -> Cts -> Cts
    add_if_unsolved EqCt -> Ct
    CEqCan)    InertEqs
    tv_eqs Cts
    emptyCts
                unsolved_fun_eqs = (EqCt -> Cts -> Cts) -> FunEqMap [EqCt] -> Cts -> Cts
    forall b. (EqCt -> b -> b) -> FunEqMap [EqCt] -> b -> b
    foldFunEqs ((EqCt -> Ct) -> EqCt -> Cts -> Cts
    forall a. (a -> Ct) -> a -> Cts -> Cts
    add_if_unsolved EqCt -> Ct
    CEqCan)    FunEqMap [EqCt]
    fun_eqs Cts
    emptyCts
                unsolved_irreds  = (IrredCt -> Cts -> Cts) -> Cts -> Bag IrredCt -> Cts
    forall a b. (a -> b -> b) -> b -> Bag a -> b
    forall (t :: * -> *) a b.
    Foldable t =>
    (a -> b -> b) -> b -> t a -> b
    foldr      ((IrredCt -> Ct) -> IrredCt -> Cts -> Cts
    forall a. (a -> Ct) -> a -> Cts -> Cts
    add_if_unsolved IrredCt -> Ct
    CIrredCan) Cts
    emptyCts Bag IrredCt
    irreds
                unsolved_dicts   = (DictCt -> Cts -> Cts) -> DictMap DictCt -> Cts -> Cts
    forall a b. (a -> b -> b) -> DictMap a -> b -> b
    foldDicts  ((DictCt -> Ct) -> DictCt -> Cts -> Cts
    forall a. (a -> Ct) -> a -> Cts -> Cts
    add_if_unsolved DictCt -> Ct
    CDictCan)  DictMap DictCt
    idicts Cts
    emptyCts
                unsolved_qcis    = (QCInst -> Cts -> Cts) -> Cts -> [QCInst] -> Cts
    forall a b. (a -> b -> b) -> b -> [a] -> b
    forall (t :: * -> *) a b.
    Foldable t =>
    (a -> b -> b) -> b -> t a -> b
    foldr      ((QCInst -> Ct) -> QCInst -> Cts -> Cts
    forall a. (a -> Ct) -> a -> Cts -> Cts
    add_if_unsolved QCInst -> Ct
    CQuantCan) Cts
    emptyCts [QCInst]
    qcis

          ; traceTcS "getUnsolvedInerts" $
            vcat [ text " tv eqs =" <+> ppr unsolved_tv_eqs
                 , text "fun eqs =" <+> ppr unsolved_fun_eqs
                 , text "dicts =" <+> ppr unsolved_dicts
                 , text "irreds =" <+> ppr unsolved_irreds ]

          ; return ( unsolved_tv_eqs  `unionBags`
                     unsolved_fun_eqs `unionBags`
                     unsolved_irreds  `unionBags`
                     unsolved_dicts   `unionBags`
                     unsolved_qcis ) }
      where
        add_if_unsolved :: (a -> Ct) -> a -> Cts -> Cts
        add_if_unsolved :: forall a. (a -> Ct) -> a -> Cts -> Cts
    add_if_unsolved a -> Ct
    mk_ct a
    thing Cts
    cts
          | Ct -> Bool
    isWantedCt Ct
    ct = Ct
    ct Ct -> Cts -> Cts
    `consCts` Cts
    cts
          | Bool
    otherwise     = Cts
    cts
          where
            ct :: Ct
    ct = a -> Ct
    mk_ct a
    thing

    getHasGivenEqs :: TcLevel             -- TcLevel of this implication
                   -> TcS ( HasGivenEqs   -- are there Given equalities?
                          , InertIrreds ) -- Insoluble equalities arising from givens
    -- See Note [Tracking Given equalities] in GHC.Tc.Solver.InertSet
    getHasGivenEqs :: TcLevel -> TcS (HasGivenEqs, Bag IrredCt)
    getHasGivenEqs TcLevel
    tclvl
      = do { inerts@(IC { inert_irreds       = irreds
                        , inert_given_eqs    = given_eqs
                        , inert_given_eq_lvl = ge_lvl })
                  <- TcS InertCans
    getInertCans
           ; let given_insols = (IrredCt -> Bool) -> Bag IrredCt -> Bag IrredCt
    forall a. (a -> Bool) -> Bag a -> Bag a
    filterBag IrredCt -> Bool
    insoluble_given_equality Bag IrredCt
    irreds
                          -- Specifically includes ones that originated in some
                          -- outer context but were refined to an insoluble by
                          -- a local equality; so no level-check needed

                 -- See Note [HasGivenEqs] in GHC.Tc.Types.Constraint, and
                 -- Note [Tracking Given equalities] in GHC.Tc.Solver.InertSet
                 has_ge | TcLevel
    ge_lvl TcLevel -> TcLevel -> Bool
    `sameDepthAs` TcLevel
    tclvl = HasGivenEqs
    MaybeGivenEqs
                        | Bool
    given_eqs                  = HasGivenEqs
    LocalGivenEqs
                        | Bool
    otherwise                  = HasGivenEqs
    NoGivenEqs

           ; traceTcS "getHasGivenEqs" $
             vcat [ text "given_eqs:" <+> ppr given_eqs
                  , text "ge_lvl:" <+> ppr ge_lvl
                  , text "ambient level:" <+> ppr tclvl
                  , text "Inerts:" <+> ppr inerts
                  , text "Insols:" <+> ppr given_insols]
           ; return (has_ge, given_insols) }
      where
        insoluble_given_equality :: IrredCt -> Bool
        -- Check for unreachability; specifically do not include UserError/Unsatisfiable
        insoluble_given_equality :: IrredCt -> Bool
    insoluble_given_equality (IrredCt { ir_ev :: IrredCt -> CtEvidence
    ir_ev = CtEvidence
    ev, ir_reason :: IrredCt -> CtIrredReason
    ir_reason = CtIrredReason
    reason })
           = CtIrredReason -> Bool
    isInsolubleReason CtIrredReason
    reason Bool -> Bool -> Bool
    && CtEvidence -> Bool
    isGiven CtEvidence
    ev

    removeInertCts :: [Ct] -> InertCans -> InertCans
    -- ^ Remove inert constraints from the 'InertCans', for use when a
    -- typechecker plugin wishes to discard a given.
    removeInertCts :: [Ct] -> InertCans -> InertCans
    removeInertCts [Ct]
    cts InertCans
    icans = (InertCans -> Ct -> InertCans) -> InertCans -> [Ct] -> InertCans
    forall b a. (b -> a -> b) -> b -> [a] -> b
    forall (t :: * -> *) b a.
    Foldable t =>
    (b -> a -> b) -> b -> t a -> b
    foldl' InertCans -> Ct -> InertCans
    removeInertCt InertCans
    icans [Ct]
    cts

    removeInertCt :: InertCans -> Ct -> InertCans
    removeInertCt :: InertCans -> Ct -> InertCans
    removeInertCt InertCans
    is Ct
    ct
      = case Ct
    ct of
          CDictCan DictCt
    dict_ct -> InertCans
    is { inert_dicts = delDict dict_ct (inert_dicts is) }
          CEqCan    EqCt
    eq_ct  -> EqCt -> InertCans -> InertCans
    delEq    EqCt
    eq_ct InertCans
    is
          CIrredCan IrredCt
    ir_ct  -> IrredCt -> InertCans -> InertCans
    delIrred IrredCt
    ir_ct InertCans
    is
          CQuantCan {}     -> String -> InertCans
    forall a. HasCallStack => String -> a
    panic String
    "removeInertCt: CQuantCan"
          CNonCanonical {} -> String -> InertCans
    forall a. HasCallStack => String -> a
    panic String
    "removeInertCt: CNonCanonical"

    -- | Looks up a family application in the inerts.
    lookupFamAppInert :: (CtFlavourRole -> Bool)  -- can it rewrite the target?
                      -> TyCon -> [Type] -> TcS (Maybe EqCt)
    lookupFamAppInert :: (CtFlavourRole -> Bool) -> TyCon -> [Type] -> TcS (Maybe EqCt)
    lookupFamAppInert CtFlavourRole -> Bool
    rewrite_pred TyCon
    fam_tc [Type]
    tys
      = do { IS { inert_cans = IC { inert_funeqs = inert_funeqs } } <- TcS InertSet
    getInertSet
           ; return (lookup_inerts inert_funeqs) }
      where
        lookup_inerts :: FunEqMap [EqCt] -> Maybe EqCt
    lookup_inerts FunEqMap [EqCt]
    inert_funeqs
          = case FunEqMap [EqCt] -> TyCon -> [Type] -> Maybe [EqCt]
    forall a. FunEqMap a -> TyCon -> [Type] -> Maybe a
    findFunEq FunEqMap [EqCt]
    inert_funeqs TyCon
    fam_tc [Type]
    tys of
              Maybe [EqCt]
    Nothing              -> Maybe EqCt
    forall a. Maybe a
    Nothing
              Just ([EqCt]
    ecl :: [EqCt]) -> (EqCt -> Bool) -> [EqCt] -> Maybe EqCt
    forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Maybe a
    find (CtFlavourRole -> Bool
    rewrite_pred (CtFlavourRole -> Bool) -> (EqCt -> CtFlavourRole) -> EqCt -> Bool
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . EqCt -> CtFlavourRole
    eqCtFlavourRole) [EqCt]
    ecl

    ---------------------------
    lookupFamAppCache :: TyCon -> [Type] -> TcS (Maybe Reduction)
    lookupFamAppCache :: TyCon -> [Type] -> TcS (Maybe Reduction)
    lookupFamAppCache TyCon
    fam_tc [Type]
    tys
      = do { IS { inert_famapp_cache = famapp_cache } <- TcS InertSet
    getInertSet
           ; case findFunEq famapp_cache fam_tc tys of
               result :: Maybe Reduction
    result@(Just Reduction
    redn) ->
                 do { String -> SDoc -> TcS ()
    traceTcS String
    "famapp_cache hit" ([SDoc] -> SDoc
    forall doc. IsDoc doc => [doc] -> doc
    vcat [ Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr (TyCon -> [Type] -> Type
    mkTyConApp TyCon
    fam_tc [Type]
    tys)
                                                        , Reduction -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Reduction
    redn ])
                    ; Maybe Reduction -> TcS (Maybe Reduction)
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return Maybe Reduction
    result }
               Maybe Reduction
    Nothing -> Maybe Reduction -> TcS (Maybe Reduction)
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return Maybe Reduction
    forall a. Maybe a
    Nothing }

    extendFamAppCache :: TyCon -> [Type] -> Reduction -> TcS ()
    -- NB: co :: rhs ~ F tys, to match expectations of rewriter
    extendFamAppCache :: TyCon -> [Type] -> Reduction -> TcS ()
    extendFamAppCache TyCon
    tc [Type]
    xi_args stuff :: Reduction
    stuff@(Reduction Coercion
    _ Type
    ty)
      = do { dflags <- TcS DynFlags
    forall (m :: * -> *). HasDynFlags m => m DynFlags
    getDynFlags
           ; when (gopt Opt_FamAppCache dflags) $
        do { traceTcS "extendFamAppCache" (vcat [ ppr tc <+> ppr xi_args
                                                , ppr ty ])
           ; updInertSet $ \ is :: InertSet
    is@(IS { inert_famapp_cache :: InertSet -> FunEqMap Reduction
    inert_famapp_cache = FunEqMap Reduction
    fc }) ->
                InertSet
    is { inert_famapp_cache = insertFunEq fc tc xi_args stuff } } }

    -- Remove entries from the cache whose evidence mentions variables in the
    -- supplied set
    dropFromFamAppCache :: VarSet -> TcS ()
    dropFromFamAppCache :: VarSet -> TcS ()
    dropFromFamAppCache VarSet
    varset
      = (InertSet -> InertSet) -> TcS ()
    updInertSet (\inerts :: InertSet
    inerts@(IS { inert_famapp_cache :: InertSet -> FunEqMap Reduction
    inert_famapp_cache = FunEqMap Reduction
    famapp_cache }) ->
                       InertSet
    inerts { inert_famapp_cache = filterTcAppMap check famapp_cache })
      where
        check :: Reduction -> Bool
        check :: Reduction -> Bool
    check Reduction
    redn
          = Bool -> Bool
    not ((TcTyVar -> Bool) -> Coercion -> Bool
    anyFreeVarsOfCo (TcTyVar -> VarSet -> Bool
    `elemVarSet` VarSet
    varset) (Coercion -> Bool) -> Coercion -> Bool
    forall a b. (a -> b) -> a -> b
    $ Reduction -> Coercion
    reductionCoercion Reduction
    redn)

    {-
    ************************************************************************
    *                                                                      *
    *              The TcS solver monad                                    *
    *                                                                      *
    ************************************************************************

    Note [The TcS monad]
    ~~~~~~~~~~~~~~~~~~~~
    The TcS monad is a weak form of the main Tc monad

    All you can do is
        * fail
        * allocate new variables
        * fill in evidence variables

    Filling in a dictionary evidence variable means to create a binding
    for it, so TcS carries a mutable location where the binding can be
    added.  This is initialised from the innermost implication constraint.
    -}

    -- | The mode for the constraint solving monad.
    data TcSMode
      = TcSMode -- See Note [TcSMode], where each field is documented
                { TcSMode -> Bool
    tcsmResumable        :: Bool
                       -- ^ Do not restore type-equality cycles
                , TcSMode -> Bool
    tcsmEarlyAbort       :: Bool
                       -- ^ Abort early on insoluble constraints
                , TcSMode -> Bool
    tcsmSkipOverlappable :: Bool
                       -- ^ Do not select an OVERLAPPABLE instance
                , TcSMode -> Bool
    tcsmFullySolveQCIs   :: Bool
                       -- ^ Fully solve quantified constraints
                }

    vanillaTcSMode :: TcSMode
    vanillaTcSMode :: TcSMode
    vanillaTcSMode = TcSMode { tcsmResumable :: Bool
    tcsmResumable        = Bool
    False
                             , tcsmEarlyAbort :: Bool
    tcsmEarlyAbort       = Bool
    False
                             , tcsmSkipOverlappable :: Bool
    tcsmSkipOverlappable = Bool
    False
                             , tcsmFullySolveQCIs :: Bool
    tcsmFullySolveQCIs   = Bool
    False }

    instance Outputable TcSMode where
      ppr :: TcSMode -> SDoc
    ppr (TcSMode { tcsmResumable :: TcSMode -> Bool
    tcsmResumable = Bool
    pm, tcsmEarlyAbort :: TcSMode -> Bool
    tcsmEarlyAbort = Bool
    ea
                   , tcsmSkipOverlappable :: TcSMode -> Bool
    tcsmSkipOverlappable = Bool
    so, tcsmFullySolveQCIs :: TcSMode -> Bool
    tcsmFullySolveQCIs = Bool
    fs })
        = String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "TcSMode" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <> (SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc
    braces (SDoc -> SDoc) -> SDoc -> SDoc
    forall a b. (a -> b) -> a -> b
    $ [SDoc] -> SDoc
    cat ([SDoc] -> SDoc) -> [SDoc] -> SDoc
    forall a b. (a -> b) -> a -> b
    $ SDoc -> [SDoc] -> [SDoc]
    forall doc. IsLine doc => doc -> [doc] -> [doc]
    punctuate SDoc
    forall doc. IsLine doc => doc
    comma ([SDoc] -> [SDoc]) -> [SDoc] -> [SDoc]
    forall a b. (a -> b) -> a -> b
    $ [Maybe SDoc] -> [SDoc]
    forall a. [Maybe a] -> [a]
    catMaybes ([Maybe SDoc] -> [SDoc]) -> [Maybe SDoc] -> [SDoc]
    forall a b. (a -> b) -> a -> b
    $
                             [ Bool -> String -> Maybe SDoc
    forall {a}. IsLine a => Bool -> String -> Maybe a
    pp_one Bool
    pm String
    "Resumable", Bool -> String -> Maybe SDoc
    forall {a}. IsLine a => Bool -> String -> Maybe a
    pp_one Bool
    ea String
    "EarlyAbort"
                             , Bool -> String -> Maybe SDoc
    forall {a}. IsLine a => Bool -> String -> Maybe a
    pp_one Bool
    so String
    "SkipOverlappable", Bool -> String -> Maybe SDoc
    forall {a}. IsLine a => Bool -> String -> Maybe a
    pp_one Bool
    fs String
    "FullySolveQCIs" ])
          -- We get something like TcSMode{EarlyAbort,FullySolveQCIs},
          -- mentioning just the flags that are on
        where
          pp_one :: Bool -> String -> Maybe a
    pp_one Bool
    True String
    s  = a -> Maybe a
    forall a. a -> Maybe a
    Just (String -> a
    forall doc. IsLine doc => String -> doc
    text String
    s)
          pp_one Bool
    False String
    _ = Maybe a
    forall a. Maybe a
    Nothing

    {- Note [TcSMode]
    ~~~~~~~~~~~~~~~~~
    The constraint solver can operate in different modes:

    * `tcsmResumable`: Used by the pattern match overlap checker.  The idea is that
      the returned InertSet will later be resumed, so we do not want to restore
      type-equality cycles See also Note [Type equality cycles] in GHC.Tc.Solver.Equality

    * `tcsmEarlyAbort`: Abort (fail in the monad) as soon as we come across an
      insoluble constraint. This is used to fail-fast when checking for hole-fits.
      See Note [Speeding up valid hole-fits].

    * `tcsmSkipOverlappable`: don't use OVERLAPPABLE instances.  Used by the
      short-cut solver.  See Note [Shortcut solving] in GHC.Tc.Solver.Dict

    * `tcsmFullSolveQCIs`: fully solve quantified constraints, or leave them alone.
      Used (only) for SPECIALISE pragmas;
      see (NFS1) in Note [Handling new-form SPECIALISE pragmas] in GHC.Tc.Gen.Sig
    -}

    data TcSEnv
      = TcSEnv {
          TcSEnv -> EvBindsVar
    tcs_ev_binds    :: EvBindsVar,

          TcSEnv -> IORef Int
    tcs_unified     :: IORef Int,
             -- The number of unification variables we have filled
             -- The important thing is whether it is non-zero, so it
             -- could equally well be a Bool instead of an Int.

          TcSEnv -> IORef (Maybe TcLevel)
    tcs_unif_lvl  :: IORef (Maybe TcLevel),
             -- The Unification Level Flag
             -- Outermost level at which we have unified a meta tyvar
             -- Starts at Nothing, then (Just i), then (Just j) where j<i
             -- See Note [The Unification Level Flag]

          TcSEnv -> IORef Int
    tcs_count     :: IORef Int, -- Global step count

          TcSEnv -> IORef InertSet
    tcs_inerts    :: IORef InertSet, -- Current inert set

          -- | The mode of operation for the constraint solver.
          -- See Note [TcSMode]
          TcSEnv -> TcSMode
    tcs_mode :: TcSMode,

          TcSEnv -> IORef WorkList
    tcs_worklist :: IORef WorkList
        }

    ---------------
    newtype TcS a = TcS { forall a. TcS a -> TcSEnv -> TcM a
    unTcS :: TcSEnv -> TcM a }
      deriving ((forall a b. (a -> b) -> TcS a -> TcS b)
    -> (forall a b. a -> TcS b -> TcS a) -> Functor TcS
    forall a b. a -> TcS b -> TcS a
    forall a b. (a -> b) -> TcS a -> TcS b
    forall (f :: * -> *).
    (forall a b. (a -> b) -> f a -> f b)
    -> (forall a b. a -> f b -> f a) -> Functor f
    $cfmap :: forall a b. (a -> b) -> TcS a -> TcS b
    fmap :: forall a b. (a -> b) -> TcS a -> TcS b
    $c<$ :: forall a b. a -> TcS b -> TcS a
    <$ :: forall a b. a -> TcS b -> TcS a
    Functor)

    instance MonadFix TcS where
      mfix :: forall a. (a -> TcS a) -> TcS a
    mfix a -> TcS a
    k = (TcSEnv -> TcM a) -> TcS a
    forall a. (TcSEnv -> TcM a) -> TcS a
    TcS ((TcSEnv -> TcM a) -> TcS a) -> (TcSEnv -> TcM a) -> TcS a
    forall a b. (a -> b) -> a -> b
    $ \TcSEnv
    env -> (a -> TcM a) -> TcM a
    forall a.
    (a -> IOEnv (Env TcGblEnv TcLclEnv) a)
    -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. MonadFix m => (a -> m a) -> m a
    mfix (\a
    x -> TcS a -> TcSEnv -> TcM a
    forall a. TcS a -> TcSEnv -> TcM a
    unTcS (a -> TcS a
    k a
    x) TcSEnv
    env)

    -- | Smart constructor for 'TcS', as describe in Note [The one-shot state
    -- monad trick] in "GHC.Utils.Monad".
    mkTcS :: (TcSEnv -> TcM a) -> TcS a
    mkTcS :: forall a. (TcSEnv -> TcM a) -> TcS a
    mkTcS TcSEnv -> TcM a
    f = (TcSEnv -> TcM a) -> TcS a
    forall a. (TcSEnv -> TcM a) -> TcS a
    TcS ((TcSEnv -> TcM a) -> TcSEnv -> TcM a
    forall a b. (a -> b) -> a -> b
    oneShot TcSEnv -> TcM a
    f)

    instance Applicative TcS where
      pure :: forall a. a -> TcS a
    pure a
    x = (TcSEnv -> TcM a) -> TcS a
    forall a. (TcSEnv -> TcM a) -> TcS a
    mkTcS ((TcSEnv -> TcM a) -> TcS a) -> (TcSEnv -> TcM a) -> TcS a
    forall a b. (a -> b) -> a -> b
    $ \TcSEnv
    _ -> a -> TcM a
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return a
    x
      <*> :: forall a b. TcS (a -> b) -> TcS a -> TcS b
    (<*>) = TcS (a -> b) -> TcS a -> TcS b
    forall (m :: * -> *) a b. Monad m => m (a -> b) -> m a -> m b
    ap

    instance Monad TcS where
      TcS a
    m >>= :: forall a b. TcS a -> (a -> TcS b) -> TcS b
    >>= a -> TcS b
    k   = (TcSEnv -> TcM b) -> TcS b
    forall a. (TcSEnv -> TcM a) -> TcS a
    mkTcS ((TcSEnv -> TcM b) -> TcS b) -> (TcSEnv -> TcM b) -> TcS b
    forall a b. (a -> b) -> a -> b
    $ \TcSEnv
    ebs -> do
        TcS a -> TcSEnv -> TcM a
    forall a. TcS a -> TcSEnv -> TcM a
    unTcS TcS a
    m TcSEnv
    ebs TcM a -> (a -> TcM b) -> TcM b
    forall a b.
    IOEnv (Env TcGblEnv TcLclEnv) a
    -> (a -> IOEnv (Env TcGblEnv TcLclEnv) b)
    -> IOEnv (Env TcGblEnv TcLclEnv) b
    forall (m :: * -> *) a b. Monad m => m a -> (a -> m b) -> m b
    >>= (\a
    r -> TcS b -> TcSEnv -> TcM b
    forall a. TcS a -> TcSEnv -> TcM a
    unTcS (a -> TcS b
    k a
    r) TcSEnv
    ebs)

    instance MonadIO TcS where
      liftIO :: forall a. IO a -> TcS a
    liftIO IO a
    act = (TcSEnv -> TcM a) -> TcS a
    forall a. (TcSEnv -> TcM a) -> TcS a
    TcS ((TcSEnv -> TcM a) -> TcS a) -> (TcSEnv -> TcM a) -> TcS a
    forall a b. (a -> b) -> a -> b
    $ \TcSEnv
    _env -> IO a -> TcM a
    forall a. IO a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. MonadIO m => IO a -> m a
    liftIO IO a
    act

    instance MonadFail TcS where
      fail :: forall a. HasCallStack => String -> TcS a
    fail String
    err  = (TcSEnv -> TcM a) -> TcS a
    forall a. (TcSEnv -> TcM a) -> TcS a
    mkTcS ((TcSEnv -> TcM a) -> TcS a) -> (TcSEnv -> TcM a) -> TcS a
    forall a b. (a -> b) -> a -> b
    $ \TcSEnv
    _ -> String -> TcM a
    forall a. HasCallStack => String -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a.
    (MonadFail m, HasCallStack) =>
    String -> m a
    fail String
    err

    instance MonadUnique TcS where
       getUniqueSupplyM :: TcS UniqSupply
    getUniqueSupplyM = TcM UniqSupply -> TcS UniqSupply
    forall a. TcM a -> TcS a
    wrapTcS TcM UniqSupply
    forall (m :: * -> *). MonadUnique m => m UniqSupply
    getUniqueSupplyM

    instance HasModule TcS where
       getModule :: TcS Module
    getModule = TcM Module -> TcS Module
    forall a. TcM a -> TcS a
    wrapTcS TcM Module
    forall (m :: * -> *). HasModule m => m Module
    getModule

    instance MonadThings TcS where
       lookupThing :: Name -> TcS TyThing
    lookupThing Name
    n = TcM TyThing -> TcS TyThing
    forall a. TcM a -> TcS a
    wrapTcS (Name -> TcM TyThing
    forall (m :: * -> *). MonadThings m => Name -> m TyThing
    lookupThing Name
    n)

    -- Basic functionality
    -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    wrapTcS :: TcM a -> TcS a
    -- Do not export wrapTcS, because it promotes an arbitrary TcM to TcS,
    -- and TcS is supposed to have limited functionality
    wrapTcS :: forall a. TcM a -> TcS a
    wrapTcS TcM a
    action = (TcSEnv -> TcM a) -> TcS a
    forall a. (TcSEnv -> TcM a) -> TcS a
    mkTcS ((TcSEnv -> TcM a) -> TcS a) -> (TcSEnv -> TcM a) -> TcS a
    forall a b. (a -> b) -> a -> b
    $ \TcSEnv
    _env -> TcM a
    action -- a TcM action will not use the TcEvBinds

    liftZonkTcS :: ZonkM a -> TcS a
    liftZonkTcS :: forall a. ZonkM a -> TcS a
    liftZonkTcS = TcM a -> TcS a
    forall a. TcM a -> TcS a
    wrapTcS (TcM a -> TcS a) -> (ZonkM a -> TcM a) -> ZonkM a -> TcS a
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . ZonkM a -> TcM a
    forall a. ZonkM a -> TcM a
    TcM.liftZonkM

    wrap2TcS :: (TcM a -> TcM a) -> TcS a -> TcS a
    wrap2TcS :: forall a. (TcM a -> TcM a) -> TcS a -> TcS a
    wrap2TcS TcM a -> TcM a
    fn (TcS TcSEnv -> TcM a
    thing) = (TcSEnv -> TcM a) -> TcS a
    forall a. (TcSEnv -> TcM a) -> TcS a
    mkTcS ((TcSEnv -> TcM a) -> TcS a) -> (TcSEnv -> TcM a) -> TcS a
    forall a b. (a -> b) -> a -> b
    $ \TcSEnv
    env -> TcM a -> TcM a
    fn (TcSEnv -> TcM a
    thing TcSEnv
    env)

    wrapErrTcS :: TcM a -> TcS a
    -- The thing wrapped should just fail
    -- There's no static check; it's up to the user
    -- Having a variant for each error message is too painful
    wrapErrTcS :: forall a. TcM a -> TcS a
    wrapErrTcS = TcM a -> TcS a
    forall a. TcM a -> TcS a
    wrapTcS

    wrapWarnTcS :: TcM a -> TcS a
    -- The thing wrapped should just add a warning, or no-op
    -- There's no static check; it's up to the user
    wrapWarnTcS :: forall a. TcM a -> TcS a
    wrapWarnTcS = TcM a -> TcS a
    forall a. TcM a -> TcS a
    wrapTcS

    panicTcS  :: SDoc -> TcS a
    failTcS   :: TcRnMessage -> TcS a
    warnTcS, addErrTcS :: TcRnMessage -> TcS ()
    failTcS :: forall a. TcRnMessage -> TcS a
    failTcS      = TcM a -> TcS a
    forall a. TcM a -> TcS a
    wrapTcS (TcM a -> TcS a) -> (TcRnMessage -> TcM a) -> TcRnMessage -> TcS a
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . TcRnMessage -> TcM a
    forall a. TcRnMessage -> TcRn a
    TcM.failWith
    warnTcS :: TcRnMessage -> TcS ()
    warnTcS TcRnMessage
    msg  = TcM () -> TcS ()
    forall a. TcM a -> TcS a
    wrapTcS (TcRnMessage -> TcM ()
    TcM.addDiagnostic TcRnMessage
    msg)
    addErrTcS :: TcRnMessage -> TcS ()
    addErrTcS    = TcM () -> TcS ()
    forall a. TcM a -> TcS a
    wrapTcS (TcM () -> TcS ())
    -> (TcRnMessage -> TcM ()) -> TcRnMessage -> TcS ()
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . TcRnMessage -> TcM ()
    TcM.addErr
    panicTcS :: forall a. SDoc -> TcS a
    panicTcS SDoc
    doc = String -> SDoc -> TcS a
    forall a. HasCallStack => String -> SDoc -> a
    pprPanic String
    "GHC.Tc.Solver.Monad" SDoc
    doc

    tryEarlyAbortTcS :: TcS ()
    -- Abort (fail in the monad) if the mode is TcSEarlyAbort
    tryEarlyAbortTcS :: TcS ()
    tryEarlyAbortTcS
      = (TcSEnv -> TcM ()) -> TcS ()
    forall a. (TcSEnv -> TcM a) -> TcS a
    mkTcS (\TcSEnv
    env -> Bool -> TcM () -> TcM ()
    forall (f :: * -> *). Applicative f => Bool -> f () -> f ()
    when (TcSMode -> Bool
    tcsmEarlyAbort (TcSEnv -> TcSMode
    tcs_mode TcSEnv
    env)) TcM ()
    forall env a. IOEnv env a
    TcM.failM)

    -- | Emit a warning within the 'TcS' monad at the location given by the 'CtLoc'.
    ctLocWarnTcS :: CtLoc -> TcRnMessage -> TcS ()
    ctLocWarnTcS :: CtLoc -> TcRnMessage -> TcS ()
    ctLocWarnTcS CtLoc
    loc TcRnMessage
    msg = TcM () -> TcS ()
    forall a. TcM a -> TcS a
    wrapTcS (TcM () -> TcS ()) -> TcM () -> TcS ()
    forall a b. (a -> b) -> a -> b
    $ CtLoc -> TcM () -> TcM ()
    forall a. CtLoc -> TcM a -> TcM a
    TcM.setCtLocM CtLoc
    loc (TcM () -> TcM ()) -> TcM () -> TcM ()
    forall a b. (a -> b) -> a -> b
    $ TcRnMessage -> TcM ()
    TcM.addDiagnostic TcRnMessage
    msg

    traceTcS :: String -> SDoc -> TcS ()
    traceTcS :: String -> SDoc -> TcS ()
    traceTcS String
    herald SDoc
    doc = TcM () -> TcS ()
    forall a. TcM a -> TcS a
    wrapTcS (String -> SDoc -> TcM ()
    TcM.traceTc String
    herald SDoc
    doc)
    {-# INLINE traceTcS #-}  -- see Note [INLINE conditional tracing utilities]

    runTcPluginTcS :: TcPluginM a -> TcS a
    runTcPluginTcS :: forall a. TcPluginM a -> TcS a
    runTcPluginTcS = TcM a -> TcS a
    forall a. TcM a -> TcS a
    wrapTcS (TcM a -> TcS a) -> (TcPluginM a -> TcM a) -> TcPluginM a -> TcS a
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . TcPluginM a -> TcM a
    forall a. TcPluginM a -> TcM a
    runTcPluginM

    instance HasDynFlags TcS where
        getDynFlags :: TcS DynFlags
    getDynFlags = TcM DynFlags -> TcS DynFlags
    forall a. TcM a -> TcS a
    wrapTcS TcM DynFlags
    forall (m :: * -> *). HasDynFlags m => m DynFlags
    getDynFlags

    getGlobalRdrEnvTcS :: TcS GlobalRdrEnv
    getGlobalRdrEnvTcS :: TcS GlobalRdrEnv
    getGlobalRdrEnvTcS = TcM GlobalRdrEnv -> TcS GlobalRdrEnv
    forall a. TcM a -> TcS a
    wrapTcS TcM GlobalRdrEnv
    TcM.getGlobalRdrEnv

    bumpStepCountTcS :: TcS ()
    bumpStepCountTcS :: TcS ()
    bumpStepCountTcS = (TcSEnv -> TcM ()) -> TcS ()
    forall a. (TcSEnv -> TcM a) -> TcS a
    mkTcS ((TcSEnv -> TcM ()) -> TcS ()) -> (TcSEnv -> TcM ()) -> TcS ()
    forall a b. (a -> b) -> a -> b
    $ \TcSEnv
    env ->
      do { let ref :: IORef Int
    ref = TcSEnv -> IORef Int
    tcs_count TcSEnv
    env
         ; n <- IORef Int -> IOEnv (Env TcGblEnv TcLclEnv) Int
    forall (m :: * -> *) a. MonadIO m => TcRef a -> m a
    TcM.readTcRef IORef Int
    ref
         ; TcM.writeTcRef ref (n+1) }

    csTraceTcS :: SDoc -> TcS ()
    csTraceTcS :: SDoc -> TcS ()
    csTraceTcS SDoc
    doc
      = TcM () -> TcS ()
    forall a. TcM a -> TcS a
    wrapTcS (TcM () -> TcS ()) -> TcM () -> TcS ()
    forall a b. (a -> b) -> a -> b
    $ TcM SDoc -> TcM ()
    csTraceTcM (SDoc -> TcM SDoc
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return SDoc
    doc)
    {-# INLINE csTraceTcS #-}  -- see Note [INLINE conditional tracing utilities]

    traceFireTcS :: CtEvidence -> SDoc -> TcS ()
    -- Dump a rule-firing trace
    traceFireTcS :: CtEvidence -> SDoc -> TcS ()
    traceFireTcS CtEvidence
    ev SDoc
    doc
      = (TcSEnv -> TcM ()) -> TcS ()
    forall a. (TcSEnv -> TcM a) -> TcS a
    mkTcS ((TcSEnv -> TcM ()) -> TcS ()) -> (TcSEnv -> TcM ()) -> TcS ()
    forall a b. (a -> b) -> a -> b
    $ \TcSEnv
    env -> TcM SDoc -> TcM ()
    csTraceTcM (TcM SDoc -> TcM ()) -> TcM SDoc -> TcM ()
    forall a b. (a -> b) -> a -> b
    $
        do { n <- IORef Int -> IOEnv (Env TcGblEnv TcLclEnv) Int
    forall (m :: * -> *) a. MonadIO m => TcRef a -> m a
    TcM.readTcRef (TcSEnv -> IORef Int
    tcs_count TcSEnv
    env)
           ; tclvl <- TcM.getTcLevel
           ; return (hang (text "Step" <+> int n
                           <> brackets (text "l:" <> ppr tclvl <> comma <>
                                        text "d:" <> ppr (ctLocDepth (ctEvLoc ev)))
                           <+> doc <> colon)
                         4 (ppr ev)) }
    {-# INLINE traceFireTcS #-}  -- see Note [INLINE conditional tracing utilities]

    csTraceTcM :: TcM SDoc -> TcM ()
    -- Constraint-solver tracing, -ddump-cs-trace
    csTraceTcM :: TcM SDoc -> TcM ()
    csTraceTcM TcM SDoc
    mk_doc
      = do { logger <- IOEnv (Env TcGblEnv TcLclEnv) Logger
    forall (m :: * -> *). HasLogger m => m Logger
    getLogger
           ; when (  logHasDumpFlag logger Opt_D_dump_cs_trace
                      || logHasDumpFlag logger Opt_D_dump_tc_trace)
                  ( do { msg <- mk_doc
                       ; TcM.dumpTcRn False
                           Opt_D_dump_cs_trace
                           "" FormatText
                           msg }) }
    {-# INLINE csTraceTcM #-}  -- see Note [INLINE conditional tracing utilities]

    runTcS :: TcS a                -- What to run
           -> TcM (a, EvBindMap)
    runTcS :: forall a. TcS a -> TcM (a, EvBindMap)
    runTcS TcS a
    tcs
      = do { ev_binds_var <- TcM EvBindsVar
    TcM.newTcEvBinds
           ; res <- runTcSWithEvBinds ev_binds_var tcs
           ; ev_binds <- TcM.getTcEvBindsMap ev_binds_var
           ; return (res, ev_binds) }

    -- | This variant of 'runTcS' will immediately fail upon encountering an
    -- insoluble ct. See Note [Speeding up valid hole-fits]. Its one usage
    -- site does not need the ev_binds, so we do not return them.
    runTcSEarlyAbort :: TcS a -> TcM a
    runTcSEarlyAbort :: forall a. TcS a -> TcM a
    runTcSEarlyAbort TcS a
    tcs
      = do { ev_binds_var <- TcM EvBindsVar
    TcM.newTcEvBinds
           ; runTcSWithEvBinds' mode ev_binds_var tcs }
      where
        mode :: TcSMode
    mode = TcSMode
    vanillaTcSMode { tcsmEarlyAbort = True }

    -- | This can deal only with equality constraints.
    runTcSEqualities :: TcS a -> TcM a
    runTcSEqualities :: forall a. TcS a -> TcM a
    runTcSEqualities TcS a
    thing_inside
      = do { ev_binds_var <- TcM EvBindsVar
    TcM.newNoTcEvBinds
           ; runTcSWithEvBinds ev_binds_var thing_inside }

    -- | A variant of 'runTcS' that takes and returns an 'InertSet' for
    -- later resumption of the 'TcS' session.
    runTcSInerts :: InertSet -> TcS a -> TcM (a, InertSet)
    runTcSInerts :: forall a. InertSet -> TcS a -> TcM (a, InertSet)
    runTcSInerts InertSet
    inerts TcS a
    tcs
      = do { ev_binds_var <- TcM EvBindsVar
    TcM.newTcEvBinds
           ; runTcSWithEvBinds' (vanillaTcSMode { tcsmResumable = True })
                                 ev_binds_var $
             do { setInertSet inerts
                ; a <- tcs
                ; new_inerts <- getInertSet
                ; return (a, new_inerts) } }

    runTcSWithEvBinds :: EvBindsVar
                      -> TcS a
                      -> TcM a
    runTcSWithEvBinds :: forall a. EvBindsVar -> TcS a -> TcM a
    runTcSWithEvBinds = TcSMode -> EvBindsVar -> TcS a -> TcM a
    forall a. TcSMode -> EvBindsVar -> TcS a -> TcM a
    runTcSWithEvBinds' TcSMode
    vanillaTcSMode

    runTcSWithEvBinds' :: TcSMode
                       -> EvBindsVar
                       -> TcS a
                       -> TcM a
    runTcSWithEvBinds' :: forall a. TcSMode -> EvBindsVar -> TcS a -> TcM a
    runTcSWithEvBinds' TcSMode
    mode EvBindsVar
    ev_binds_var TcS a
    thing_inside
      = do { unified_var <- Int -> IOEnv (Env TcGblEnv TcLclEnv) (IORef Int)
    forall (m :: * -> *) a. MonadIO m => a -> m (TcRef a)
    TcM.newTcRef Int
    0
           ; step_count  <- TcM.newTcRef 0

           -- Make a fresh, empty inert set
           -- Subtle point: see (TGE6) in Note [Tracking Given equalities]
           --               in GHC.Tc.Solver.InertSet
           ; tc_lvl      <- TcM.getTcLevel
           ; inert_var   <- TcM.newTcRef (emptyInertSet tc_lvl)

           ; wl_var      <- TcM.newTcRef emptyWorkList
           ; unif_lvl_var <- TcM.newTcRef Nothing
           ; let env = TcSEnv { tcs_ev_binds :: EvBindsVar
    tcs_ev_binds           = EvBindsVar
    ev_binds_var
                              , tcs_unified :: IORef Int
    tcs_unified            = IORef Int
    unified_var
                              , tcs_unif_lvl :: IORef (Maybe TcLevel)
    tcs_unif_lvl           = IORef (Maybe TcLevel)
    unif_lvl_var
                              , tcs_count :: IORef Int
    tcs_count              = IORef Int
    step_count
                              , tcs_inerts :: IORef InertSet
    tcs_inerts             = IORef InertSet
    inert_var
                              , tcs_mode :: TcSMode
    tcs_mode               = TcSMode
    mode
                              , tcs_worklist :: IORef WorkList
    tcs_worklist           = IORef WorkList
    wl_var }

                 -- Run the computation
           ; res <- unTcS thing_inside env

           ; count <- TcM.readTcRef step_count
           ; when (count > 0) $
             csTraceTcM $ return (text "Constraint solver steps =" <+> int count)

           -- Restore tyvar cycles: see Note [Type equality cycles] in
           --                       GHC.Tc.Solver.Equality
           -- But /not/ when tcsmResumable is set: see Note [TcSMode]
           ; unless (tcsmResumable mode) $
             do { inert_set <- TcM.readTcRef inert_var
                ; restoreTyVarCycles inert_set }

    #if defined(DEBUG)
           ; ev_binds <- TcM.getTcEvBindsMap ev_binds_var
           ; checkForCyclicBinds ev_binds
    #endif

           ; return res }

    ----------------------------
    #if defined(DEBUG)
    checkForCyclicBinds :: EvBindMap -> TcM ()
    checkForCyclicBinds ev_binds_map
      | null cycles
      = return ()
      | null coercion_cycles
      = TcM.traceTc "Cycle in evidence binds" $ ppr cycles
      | otherwise
      = pprPanic "Cycle in coercion bindings" $ ppr coercion_cycles
      where
        ev_binds = evBindMapBinds ev_binds_map

        cycles :: [[EvBind]]
        cycles = [c | CyclicSCC c <- stronglyConnCompFromEdgedVerticesUniq edges]

        coercion_cycles = [c | c <- cycles, any is_co_bind c]
        is_co_bind (EvBind { eb_lhs = b }) = isEqPred (varType b)

        edges :: [ Node EvVar EvBind ]
        edges = [ DigraphNode bind bndr (nonDetEltsUniqSet (nestedEvIdsOfTerm rhs))
                | bind@(EvBind { eb_lhs = bndr, eb_rhs = rhs}) <- bagToList ev_binds ]
                -- It's OK to use nonDetEltsUFM here as
                -- stronglyConnCompFromEdgedVertices is still deterministic even
                -- if the edges are in nondeterministic order as explained in
                -- Note [Deterministic SCC] in GHC.Data.Graph.Directed.
    #endif

    ----------------------------
    setEvBindsTcS :: EvBindsVar -> TcS a -> TcS a
    setEvBindsTcS :: forall a. EvBindsVar -> TcS a -> TcS a
    setEvBindsTcS EvBindsVar
    ref (TcS TcSEnv -> TcM a
    thing_inside)
     = (TcSEnv -> TcM a) -> TcS a
    forall a. (TcSEnv -> TcM a) -> TcS a
    TcS ((TcSEnv -> TcM a) -> TcS a) -> (TcSEnv -> TcM a) -> TcS a
    forall a b. (a -> b) -> a -> b
    $ \ TcSEnv
    env -> TcSEnv -> TcM a
    thing_inside (TcSEnv
    env { tcs_ev_binds = ref })

    setTcLevelTcS :: TcLevel -> TcS a -> TcS a
    setTcLevelTcS :: forall a. TcLevel -> TcS a -> TcS a
    setTcLevelTcS TcLevel
    lvl (TcS TcSEnv -> TcM a
    thing_inside)
     = (TcSEnv -> TcM a) -> TcS a
    forall a. (TcSEnv -> TcM a) -> TcS a
    TcS ((TcSEnv -> TcM a) -> TcS a) -> (TcSEnv -> TcM a) -> TcS a
    forall a b. (a -> b) -> a -> b
    $ \ TcSEnv
    env -> TcLevel -> TcM a -> TcM a
    forall a. TcLevel -> TcM a -> TcM a
    TcM.setTcLevel TcLevel
    lvl (TcSEnv -> TcM a
    thing_inside TcSEnv
    env)

    nestImplicTcS :: EvBindsVar
                  -> TcLevel -> TcS a
                  -> TcS a
    nestImplicTcS :: forall a. EvBindsVar -> TcLevel -> TcS a -> TcS a
    nestImplicTcS EvBindsVar
    ev_binds_var TcLevel
    inner_tclvl (TcS TcSEnv -> TcM a
    thing_inside)
      = (TcSEnv -> TcM a) -> TcS a
    forall a. (TcSEnv -> TcM a) -> TcS a
    TcS ((TcSEnv -> TcM a) -> TcS a) -> (TcSEnv -> TcM a) -> TcS a
    forall a b. (a -> b) -> a -> b
    $ \ env :: TcSEnv
    env@(TcSEnv { tcs_inerts :: TcSEnv -> IORef InertSet
    tcs_inerts = IORef InertSet
    old_inert_var }) ->
        do { inerts <- IORef InertSet -> IOEnv (Env TcGblEnv TcLclEnv) InertSet
    forall (m :: * -> *) a. MonadIO m => TcRef a -> m a
    TcM.readTcRef IORef InertSet
    old_inert_var

           -- Initialise the inert_cans from the inert_givens of the parent
           -- so that the child is not polluted with the parent's inert Wanteds
           -- See Note [trySolveImplication] in GHC.Tc.Solver.Solve
           -- All other InertSet fields are inherited
           ; let nest_inert = InertSet
    inerts { inert_cycle_breakers = pushCycleBreakerVarStack
                                                                (inert_cycle_breakers inerts)
                                     , inert_cans = (inert_givens inerts)
                                                       { inert_given_eqs = False } }
           ; new_inert_var <- TcM.newTcRef nest_inert
           ; new_wl_var    <- TcM.newTcRef emptyWorkList
           ; let nest_env = TcSEnv
    env { tcs_ev_binds = ev_binds_var
                                , tcs_inerts   = new_inert_var
                                , tcs_worklist = new_wl_var }
           ; res <- TcM.setTcLevel inner_tclvl $
                    thing_inside nest_env

           ; out_inert_set <- TcM.readTcRef new_inert_var
           ; restoreTyVarCycles out_inert_set

    #if defined(DEBUG)
           -- Perform a check that the thing_inside did not cause cycles
           ; ev_binds <- TcM.getTcEvBindsMap ev_binds_var
           ; checkForCyclicBinds ev_binds
    #endif
           ; return res }

    nestTcS :: TcS a -> TcS a
    -- Use the current untouchables, augmenting the current
    -- evidence bindings, and solved dictionaries
    -- But have no effect on the InertCans, or on the inert_famapp_cache
    -- (we want to inherit the latter from processing the Givens)
    nestTcS :: forall a. TcS a -> TcS a
    nestTcS (TcS TcSEnv -> TcM a
    thing_inside)
      = (TcSEnv -> TcM a) -> TcS a
    forall a. (TcSEnv -> TcM a) -> TcS a
    TcS ((TcSEnv -> TcM a) -> TcS a) -> (TcSEnv -> TcM a) -> TcS a
    forall a b. (a -> b) -> a -> b
    $ \ env :: TcSEnv
    env@(TcSEnv { tcs_inerts :: TcSEnv -> IORef InertSet
    tcs_inerts = IORef InertSet
    inerts_var }) ->
        do { inerts <- IORef InertSet -> IOEnv (Env TcGblEnv TcLclEnv) InertSet
    forall (m :: * -> *) a. MonadIO m => TcRef a -> m a
    TcM.readTcRef IORef InertSet
    inerts_var
           ; new_inert_var <- TcM.newTcRef inerts
           ; new_wl_var    <- TcM.newTcRef emptyWorkList
           ; let nest_env = TcSEnv
    env { tcs_inerts   = new_inert_var
                                , tcs_worklist = new_wl_var }
                            -- Inherit tcs_ev_binds from caller

           ; res <- thing_inside nest_env

           ; new_inerts <- TcM.readTcRef new_inert_var
           ; TcM.updTcRef inerts_var (`updateInertsWith` new_inerts)

           ; return res }

    tryShortCutTcS :: TcS Bool -> TcS Bool
    -- Like nestTcS, but
    --   (a) be a no-op if the nested computation returns False
    --   (b) if (but only if) success, propagate nested bindings to the caller
    -- Use only by the short-cut solver;
    --   see Note [Shortcut solving] in GHC.Tc.Solver.Dict
    tryShortCutTcS :: TcS Bool -> TcS Bool
    tryShortCutTcS (TcS TcSEnv -> TcM Bool
    thing_inside)
      = (TcSEnv -> TcM Bool) -> TcS Bool
    forall a. (TcSEnv -> TcM a) -> TcS a
    TcS ((TcSEnv -> TcM Bool) -> TcS Bool)
    -> (TcSEnv -> TcM Bool) -> TcS Bool
    forall a b. (a -> b) -> a -> b
    $ \ env :: TcSEnv
    env@(TcSEnv { tcs_mode :: TcSEnv -> TcSMode
    tcs_mode = TcSMode
    mode
                            , tcs_inerts :: TcSEnv -> IORef InertSet
    tcs_inerts = IORef InertSet
    inerts_var
                            , tcs_ev_binds :: TcSEnv -> EvBindsVar
    tcs_ev_binds = EvBindsVar
    old_ev_binds_var }) ->
        do { -- Initialise a fresh inert set, with no Givens and no Wanteds
             --    (i.e. empty `inert_cans`)
             -- But inherit all the InertSet cache fields; in particular
             --  * the given_eq_lvl, so we don't accidentally unify a
             --    unification variable from outside a GADT match
             --  * the `solved_dicts`; see wrinkle (SCS3) of Note [Shortcut solving]
             --  * the `famapp_cache`; similarly
             old_inerts <- IORef InertSet -> IOEnv (Env TcGblEnv TcLclEnv) InertSet
    forall (m :: * -> *) a. MonadIO m => TcRef a -> m a
    TcM.readTcRef IORef InertSet
    inerts_var
           ; let given_eq_lvl = InertCans -> TcLevel
    inert_given_eq_lvl (InertSet -> InertCans
    inert_cans InertSet
    old_inerts)
                 new_inerts   = InertSet
    old_inerts { inert_cans = emptyInertCans given_eq_lvl }
           ; new_inert_var <- TcM.newTcRef new_inerts

           ; new_wl_var       <- TcM.newTcRef emptyWorkList
           ; new_ev_binds_var <- TcM.cloneEvBindsVar old_ev_binds_var
           ; let nest_env = TcSEnv
    env { tcs_mode     = mode { tcsmSkipOverlappable = True }
                                , tcs_ev_binds = new_ev_binds_var
                                , tcs_inerts   = new_inert_var
                                , tcs_worklist = new_wl_var }

           ; TcM.traceTc "tryTcS {" $
             vcat [ text "old_ev_binds:" <+> ppr old_ev_binds_var
                  , text "new_ev_binds:" <+> ppr new_ev_binds_var
                  , ppr old_inerts ]
           ; solved <- thing_inside nest_env
           ; TcM.traceTc "tryTcS }" (ppr solved)

           ; if not solved
             then return False
             else do {  -- Successfully solved
                       -- Add the new bindings to the existing ones
                     ; TcM.updTcEvBinds old_ev_binds_var new_ev_binds_var

                     -- Update the existing inert set
                     ; new_inerts <- TcM.readTcRef new_inert_var
                     ; TcM.updTcRef inerts_var (`updateInertsWith` new_inerts)

                     ; TcM.traceTc "tryTcS update" (ppr (inert_solved_dicts new_inerts))

                     ; return True } }

    updateInertsWith :: InertSet -> InertSet -> InertSet
    -- Update the current inert set with bits from a nested solve,
    -- that finished with a new inert set
    -- In particular, propagate:
    --    - solved dictionaires; see Note [Propagate the solved dictionaries]
    --    - Safe Haskell failures
    updateInertsWith :: InertSet -> InertSet -> InertSet
    updateInertsWith InertSet
    current_inerts
                     (IS { inert_solved_dicts :: InertSet -> DictMap DictCt
    inert_solved_dicts = DictMap DictCt
    new_solved
                         , inert_safehask :: InertSet -> DictMap DictCt
    inert_safehask     = DictMap DictCt
    new_safehask })
      = InertSet
    current_inerts { inert_solved_dicts = new_solved
                       , inert_safehask     = new_safehask }

    {- Note [Propagate the solved dictionaries]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    It's really quite important that nestTcS does not discard the solved
    dictionaries from the thing_inside.
    Consider
       Eq [a]
       forall b. empty =>  Eq [a]
    We solve the simple (Eq [a]), under nestTcS, and then turn our attention to
    the implications.  It's definitely fine to use the solved dictionaries on
    the inner implications, and it can make a significant performance difference
    if you do so.
    -}

    -- Getters and setters of GHC.Tc.Utils.Env fields
    -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    getTcSMode :: TcS TcSMode
    getTcSMode :: TcS TcSMode
    getTcSMode = (TcSEnv -> TcM TcSMode) -> TcS TcSMode
    forall a. (TcSEnv -> TcM a) -> TcS a
    TcS (TcSMode -> TcM TcSMode
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (TcSMode -> TcM TcSMode)
    -> (TcSEnv -> TcSMode) -> TcSEnv -> TcM TcSMode
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . TcSEnv -> TcSMode
    tcs_mode)

    setTcSMode :: TcSMode -> TcS a -> TcS a
    setTcSMode :: forall a. TcSMode -> TcS a -> TcS a
    setTcSMode TcSMode
    mode TcS a
    thing_inside
      = (TcSEnv -> TcM a) -> TcS a
    forall a. (TcSEnv -> TcM a) -> TcS a
    TcS (\TcSEnv
    env -> TcS a -> TcSEnv -> TcM a
    forall a. TcS a -> TcSEnv -> TcM a
    unTcS TcS a
    thing_inside (TcSEnv
    env { tcs_mode = mode }))

    getUnifiedRef :: TcS (IORef Int)
    getUnifiedRef :: TcS (IORef Int)
    getUnifiedRef = (TcSEnv -> IOEnv (Env TcGblEnv TcLclEnv) (IORef Int))
    -> TcS (IORef Int)
    forall a. (TcSEnv -> TcM a) -> TcS a
    TcS (IORef Int -> IOEnv (Env TcGblEnv TcLclEnv) (IORef Int)
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (IORef Int -> IOEnv (Env TcGblEnv TcLclEnv) (IORef Int))
    -> (TcSEnv -> IORef Int)
    -> TcSEnv
    -> IOEnv (Env TcGblEnv TcLclEnv) (IORef Int)
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . TcSEnv -> IORef Int
    tcs_unified)

    -- Getter of inerts and worklist
    getInertSetRef :: TcS (IORef InertSet)
    getInertSetRef :: TcS (IORef InertSet)
    getInertSetRef = (TcSEnv -> IOEnv (Env TcGblEnv TcLclEnv) (IORef InertSet))
    -> TcS (IORef InertSet)
    forall a. (TcSEnv -> TcM a) -> TcS a
    TcS (IORef InertSet -> IOEnv (Env TcGblEnv TcLclEnv) (IORef InertSet)
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (IORef InertSet -> IOEnv (Env TcGblEnv TcLclEnv) (IORef InertSet))
    -> (TcSEnv -> IORef InertSet)
    -> TcSEnv
    -> IOEnv (Env TcGblEnv TcLclEnv) (IORef InertSet)
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . TcSEnv -> IORef InertSet
    tcs_inerts)

    getInertSet :: TcS InertSet
    getInertSet :: TcS InertSet
    getInertSet = TcS (IORef InertSet)
    getInertSetRef TcS (IORef InertSet)
    -> (IORef InertSet -> TcS InertSet) -> TcS InertSet
    forall a b. TcS a -> (a -> TcS b) -> TcS b
    forall (m :: * -> *) a b. Monad m => m a -> (a -> m b) -> m b
    >>= IORef InertSet -> TcS InertSet
    forall a. TcRef a -> TcS a
    readTcRef

    setInertSet :: InertSet -> TcS ()
    setInertSet :: InertSet -> TcS ()
    setInertSet InertSet
    is = do { r <- TcS (IORef InertSet)
    getInertSetRef; writeTcRef r is }

    newTcRef :: a -> TcS (TcRef a)
    newTcRef :: forall a. a -> TcS (TcRef a)
    newTcRef a
    x = TcM (TcRef a) -> TcS (TcRef a)
    forall a. TcM a -> TcS a
    wrapTcS (a -> TcM (TcRef a)
    forall (m :: * -> *) a. MonadIO m => a -> m (TcRef a)
    TcM.newTcRef a
    x)

    readTcRef :: TcRef a -> TcS a
    readTcRef :: forall a. TcRef a -> TcS a
    readTcRef TcRef a
    ref = TcM a -> TcS a
    forall a. TcM a -> TcS a
    wrapTcS (TcRef a -> TcM a
    forall (m :: * -> *) a. MonadIO m => TcRef a -> m a
    TcM.readTcRef TcRef a
    ref)

    writeTcRef :: TcRef a -> a -> TcS ()
    writeTcRef :: forall a. TcRef a -> a -> TcS ()
    writeTcRef TcRef a
    ref a
    val = TcM () -> TcS ()
    forall a. TcM a -> TcS a
    wrapTcS (TcRef a -> a -> TcM ()
    forall (m :: * -> *) a. MonadIO m => TcRef a -> a -> m ()
    TcM.writeTcRef TcRef a
    ref a
    val)

    updTcRef :: TcRef a -> (a->a) -> TcS ()
    updTcRef :: forall a. TcRef a -> (a -> a) -> TcS ()
    updTcRef TcRef a
    ref a -> a
    upd_fn = TcM () -> TcS ()
    forall a. TcM a -> TcS a
    wrapTcS (TcRef a -> (a -> a) -> TcM ()
    forall (m :: * -> *) a. MonadIO m => TcRef a -> (a -> a) -> m ()
    TcM.updTcRef TcRef a
    ref a -> a
    upd_fn)

    getTcEvBindsVar :: TcS EvBindsVar
    getTcEvBindsVar :: TcS EvBindsVar
    getTcEvBindsVar = (TcSEnv -> TcM EvBindsVar) -> TcS EvBindsVar
    forall a. (TcSEnv -> TcM a) -> TcS a
    TcS (EvBindsVar -> TcM EvBindsVar
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (EvBindsVar -> TcM EvBindsVar)
    -> (TcSEnv -> EvBindsVar) -> TcSEnv -> TcM EvBindsVar
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . TcSEnv -> EvBindsVar
    tcs_ev_binds)

    getTcLevel :: TcS TcLevel
    getTcLevel :: TcS TcLevel
    getTcLevel = TcM TcLevel -> TcS TcLevel
    forall a. TcM a -> TcS a
    wrapTcS TcM TcLevel
    TcM.getTcLevel

    getTcEvTyCoVars :: EvBindsVar -> TcS [TcCoercion]
    getTcEvTyCoVars :: EvBindsVar -> TcS [Coercion]
    getTcEvTyCoVars EvBindsVar
    ev_binds_var
      = TcM [Coercion] -> TcS [Coercion]
    forall a. TcM a -> TcS a
    wrapTcS (TcM [Coercion] -> TcS [Coercion])
    -> TcM [Coercion] -> TcS [Coercion]
    forall a b. (a -> b) -> a -> b
    $ EvBindsVar -> TcM [Coercion]
    TcM.getTcEvTyCoVars EvBindsVar
    ev_binds_var

    getTcEvBindsMap :: EvBindsVar -> TcS EvBindMap
    getTcEvBindsMap :: EvBindsVar -> TcS EvBindMap
    getTcEvBindsMap EvBindsVar
    ev_binds_var
      = TcM EvBindMap -> TcS EvBindMap
    forall a. TcM a -> TcS a
    wrapTcS (TcM EvBindMap -> TcS EvBindMap) -> TcM EvBindMap -> TcS EvBindMap
    forall a b. (a -> b) -> a -> b
    $ EvBindsVar -> TcM EvBindMap
    TcM.getTcEvBindsMap EvBindsVar
    ev_binds_var

    setTcEvBindsMap :: EvBindsVar -> EvBindMap -> TcS ()
    setTcEvBindsMap :: EvBindsVar -> EvBindMap -> TcS ()
    setTcEvBindsMap EvBindsVar
    ev_binds_var EvBindMap
    binds
      = TcM () -> TcS ()
    forall a. TcM a -> TcS a
    wrapTcS (TcM () -> TcS ()) -> TcM () -> TcS ()
    forall a b. (a -> b) -> a -> b
    $ EvBindsVar -> EvBindMap -> TcM ()
    TcM.setTcEvBindsMap EvBindsVar
    ev_binds_var EvBindMap
    binds

    unifyTyVar :: TcTyVar -> TcType -> TcS ()
    -- Unify a meta-tyvar with a type
    -- We keep track of how many unifications have happened in tcs_unified,
    --
    -- We should never unify the same variable twice!
    unifyTyVar :: TcTyVar -> Type -> TcS ()
    unifyTyVar TcTyVar
    tv Type
    ty
      = Bool -> SDoc -> TcS () -> TcS ()
    forall a. HasCallStack => Bool -> SDoc -> a -> a
    assertPpr (TcTyVar -> Bool
    isMetaTyVar TcTyVar
    tv) (TcTyVar -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcTyVar
    tv) (TcS () -> TcS ()) -> TcS () -> TcS ()
    forall a b. (a -> b) -> a -> b
    $
        (TcSEnv -> TcM ()) -> TcS ()
    forall a. (TcSEnv -> TcM a) -> TcS a
    TcS ((TcSEnv -> TcM ()) -> TcS ()) -> (TcSEnv -> TcM ()) -> TcS ()
    forall a b. (a -> b) -> a -> b
    $ \ TcSEnv
    env ->
        do { String -> SDoc -> TcM ()
    TcM.traceTc String
    "unifyTyVar" (TcTyVar -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcTyVar
    tv SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    ":=" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    ty)
           ; ZonkM () -> TcM ()
    forall a. ZonkM a -> TcM a
    TcM.liftZonkM (ZonkM () -> TcM ()) -> ZonkM () -> TcM ()
    forall a b. (a -> b) -> a -> b
    $ HasDebugCallStack => TcTyVar -> Type -> ZonkM ()
    TcTyVar -> Type -> ZonkM ()
    TcM.writeMetaTyVar TcTyVar
    tv Type
    ty
           ; IORef Int -> (Int -> Int) -> TcM ()
    forall (m :: * -> *) a. MonadIO m => TcRef a -> (a -> a) -> m ()
    TcM.updTcRef (TcSEnv -> IORef Int
    tcs_unified TcSEnv
    env) (Int -> Int -> Int
    forall a. Num a => a -> a -> a
    +Int
    1) }

    reportUnifications :: TcS a -> TcS (Int, a)
    -- Record how many unifications are done by thing_inside
    -- We could return a Bool instead of an Int;
    -- all that matters is whether it is no-zero
    reportUnifications :: forall a. TcS a -> TcS (Int, a)
    reportUnifications (TcS TcSEnv -> TcM a
    thing_inside)
      = (TcSEnv -> TcM (Int, a)) -> TcS (Int, a)
    forall a. (TcSEnv -> TcM a) -> TcS a
    TcS ((TcSEnv -> TcM (Int, a)) -> TcS (Int, a))
    -> (TcSEnv -> TcM (Int, a)) -> TcS (Int, a)
    forall a b. (a -> b) -> a -> b
    $ \ TcSEnv
    env ->
        do { inner_unified <- Int -> IOEnv (Env TcGblEnv TcLclEnv) (IORef Int)
    forall (m :: * -> *) a. MonadIO m => a -> m (TcRef a)
    TcM.newTcRef Int
    0
           ; res <- thing_inside (env { tcs_unified = inner_unified })
           ; n_unifs <- TcM.readTcRef inner_unified
           ; TcM.updTcRef (tcs_unified env) (+ n_unifs)
           ; return (n_unifs, res) }

    getDefaultInfo ::  TcS (DefaultEnv, Bool)
    getDefaultInfo :: TcS (DefaultEnv, Bool)
    getDefaultInfo = TcM (DefaultEnv, Bool) -> TcS (DefaultEnv, Bool)
    forall a. TcM a -> TcS a
    wrapTcS TcM (DefaultEnv, Bool)
    TcM.tcGetDefaultTys

    -- Just get some environments needed for instance looking up and matching
    -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    getInstEnvs :: TcS InstEnvs
    getInstEnvs :: TcS InstEnvs
    getInstEnvs = TcM InstEnvs -> TcS InstEnvs
    forall a. TcM a -> TcS a
    wrapTcS (TcM InstEnvs -> TcS InstEnvs) -> TcM InstEnvs -> TcS InstEnvs
    forall a b. (a -> b) -> a -> b
    $ TcM InstEnvs
    TcM.tcGetInstEnvs

    getFamInstEnvs :: TcS (FamInstEnv, FamInstEnv)
    getFamInstEnvs :: TcS (FamInstEnv, FamInstEnv)
    getFamInstEnvs = TcM (FamInstEnv, FamInstEnv) -> TcS (FamInstEnv, FamInstEnv)
    forall a. TcM a -> TcS a
    wrapTcS (TcM (FamInstEnv, FamInstEnv) -> TcS (FamInstEnv, FamInstEnv))
    -> TcM (FamInstEnv, FamInstEnv) -> TcS (FamInstEnv, FamInstEnv)
    forall a b. (a -> b) -> a -> b
    $ TcM (FamInstEnv, FamInstEnv)
    FamInst.tcGetFamInstEnvs

    getTopEnv :: TcS HscEnv
    getTopEnv :: TcS HscEnv
    getTopEnv = TcM HscEnv -> TcS HscEnv
    forall a. TcM a -> TcS a
    wrapTcS (TcM HscEnv -> TcS HscEnv) -> TcM HscEnv -> TcS HscEnv
    forall a b. (a -> b) -> a -> b
    $ TcM HscEnv
    forall gbl lcl. TcRnIf gbl lcl HscEnv
    TcM.getTopEnv

    getGblEnv :: TcS TcGblEnv
    getGblEnv :: TcS TcGblEnv
    getGblEnv = TcM TcGblEnv -> TcS TcGblEnv
    forall a. TcM a -> TcS a
    wrapTcS (TcM TcGblEnv -> TcS TcGblEnv) -> TcM TcGblEnv -> TcS TcGblEnv
    forall a b. (a -> b) -> a -> b
    $ TcM TcGblEnv
    forall gbl lcl. TcRnIf gbl lcl gbl
    TcM.getGblEnv

    getLclEnv :: TcS TcLclEnv
    getLclEnv :: TcS TcLclEnv
    getLclEnv = TcM TcLclEnv -> TcS TcLclEnv
    forall a. TcM a -> TcS a
    wrapTcS (TcM TcLclEnv -> TcS TcLclEnv) -> TcM TcLclEnv -> TcS TcLclEnv
    forall a b. (a -> b) -> a -> b
    $ TcM TcLclEnv
    forall gbl lcl. TcRnIf gbl lcl lcl
    TcM.getLclEnv

    setSrcSpan :: RealSrcSpan -> TcS a -> TcS a
    setSrcSpan :: forall a. RealSrcSpan -> TcS a -> TcS a
    setSrcSpan RealSrcSpan
    ss = (TcM a -> TcM a) -> TcS a -> TcS a
    forall a. (TcM a -> TcM a) -> TcS a -> TcS a
    wrap2TcS (SrcSpan -> TcM a -> TcM a
    forall a. SrcSpan -> TcRn a -> TcRn a
    TcM.setSrcSpan (RealSrcSpan -> Maybe BufSpan -> SrcSpan
    RealSrcSpan RealSrcSpan
    ss Maybe BufSpan
    forall a. Monoid a => a
    mempty))

    tcLookupClass :: Name -> TcS Class
    tcLookupClass :: Name -> TcS Class
    tcLookupClass Name
    c = TcM Class -> TcS Class
    forall a. TcM a -> TcS a
    wrapTcS (TcM Class -> TcS Class) -> TcM Class -> TcS Class
    forall a b. (a -> b) -> a -> b
    $ Name -> TcM Class
    TcM.tcLookupClass Name
    c

    tcLookupId :: Name -> TcS Id
    tcLookupId :: Name -> TcS TcTyVar
    tcLookupId Name
    n = TcM TcTyVar -> TcS TcTyVar
    forall a. TcM a -> TcS a
    wrapTcS (TcM TcTyVar -> TcS TcTyVar) -> TcM TcTyVar -> TcS TcTyVar
    forall a b. (a -> b) -> a -> b
    $ Name -> TcM TcTyVar
    TcM.tcLookupId Name
    n

    tcLookupTyCon :: Name -> TcS TyCon
    tcLookupTyCon :: Name -> TcS TyCon
    tcLookupTyCon Name
    n = TcRn TyCon -> TcS TyCon
    forall a. TcM a -> TcS a
    wrapTcS (TcRn TyCon -> TcS TyCon) -> TcRn TyCon -> TcS TyCon
    forall a b. (a -> b) -> a -> b
    $ Name -> TcRn TyCon
    TcM.tcLookupTyCon Name
    n

    -- Any use of this function is a bit suspect, because it violates the
    -- pure veneer of TcS. But it's just about warnings around unused imports
    -- and local constructors (GHC will issue fewer warnings than it otherwise
    -- might), so it's not worth losing sleep over.
    recordUsedGREs :: Bag GlobalRdrElt -> TcS ()
    recordUsedGREs :: Bag GlobalRdrElt -> TcS ()
    recordUsedGREs Bag GlobalRdrElt
    gres
      = do { TcM () -> TcS ()
    forall a. TcM a -> TcS a
    wrapTcS (TcM () -> TcS ()) -> TcM () -> TcS ()
    forall a b. (a -> b) -> a -> b
    $ DeprecationWarnings -> [GlobalRdrElt] -> TcM ()
    TcM.addUsedGREs DeprecationWarnings
    NoDeprecationWarnings [GlobalRdrElt]
    gre_list
             -- If a newtype constructor was imported, don't warn about not
             -- importing it...
           ; TcM () -> TcS ()
    forall a. TcM a -> TcS a
    wrapTcS (TcM () -> TcS ()) -> TcM () -> TcS ()
    forall a b. (a -> b) -> a -> b
    $ (GlobalRdrElt -> TcM ()) -> [GlobalRdrElt] -> TcM ()
    forall (t :: * -> *) (f :: * -> *) a b.
    (Foldable t, Applicative f) =>
    (a -> f b) -> t a -> f ()
    traverse_ (Name -> TcM ()
    TcM.keepAlive (Name -> TcM ())
    -> (GlobalRdrElt -> Name) -> GlobalRdrElt -> TcM ()
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . GlobalRdrElt -> Name
    forall info. GlobalRdrEltX info -> Name
    greName) [GlobalRdrElt]
    gre_list }
             -- ...and similarly, if a newtype constructor was defined in the same
             -- module, don't warn about it being unused.
             -- See Note [Tracking unused binding and imports] in GHC.Tc.Utils.

      where
        gre_list :: [GlobalRdrElt]
    gre_list = Bag GlobalRdrElt -> [GlobalRdrElt]
    forall a. Bag a -> [a]
    bagToList Bag GlobalRdrElt
    gres

    -- Various smaller utilities [TODO, maybe will be absorbed in the instance matcher]
    -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    checkWellLevelledDFun :: CtLoc -> InstanceWhat -> PredType -> TcS ()
    -- Check that we do not try to use an instance before it is available.  E.g.
    --    instance Eq T where ...
    --    f x = $( ... (\(p::T) -> p == p)... )
    -- Here we can't use the equality function from the instance in the splice

    checkWellLevelledDFun :: CtLoc -> InstanceWhat -> Type -> TcS ()
    checkWellLevelledDFun CtLoc
    loc InstanceWhat
    what Type
    pred
      = do
          mbind_lvl <- HasCallStack =>
    InstanceWhat -> TcS (Maybe (Set ThLevelIndex, Bool))
    InstanceWhat -> TcS (Maybe (Set ThLevelIndex, Bool))
    checkWellLevelledInstanceWhat InstanceWhat
    what
          case mbind_lvl of
            Just (Set ThLevelIndex
    bind_lvls, Bool
    is_local) ->
              TcM () -> TcS ()
    forall a. TcM a -> TcS a
    wrapTcS (TcM () -> TcS ()) -> TcM () -> TcS ()
    forall a b. (a -> b) -> a -> b
    $ CtLoc -> TcM () -> TcM ()
    forall a. CtLoc -> TcM a -> TcM a
    TcM.setCtLocM CtLoc
    loc (TcM () -> TcM ()) -> TcM () -> TcM ()
    forall a b. (a -> b) -> a -> b
    $ do
                  { use_lvl <- ThLevel -> ThLevelIndex
    thLevelIndex (ThLevel -> ThLevelIndex)
    -> IOEnv (Env TcGblEnv TcLclEnv) ThLevel
    -> IOEnv (Env TcGblEnv TcLclEnv) ThLevelIndex
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> IOEnv (Env TcGblEnv TcLclEnv) ThLevel
    TcM.getThLevel
                  ; dflags <- getDynFlags
                  ; checkCrossLevelClsInst dflags (LevelCheckInstance what pred) bind_lvls use_lvl is_local  }
            -- If no level information is returned for an InstanceWhat, then it's safe to use
            -- at any level.
            Maybe (Set ThLevelIndex, Bool)
    Nothing -> () -> TcS ()
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()

    -- TODO: Unify this with checkCrossLevelLifting function
    checkCrossLevelClsInst :: DynFlags -> LevelCheckReason
                           -> Set.Set ThLevelIndex -> ThLevelIndex
                           -> Bool -> TcM ()
    checkCrossLevelClsInst :: DynFlags
    -> LevelCheckReason
    -> Set ThLevelIndex
    -> ThLevelIndex
    -> Bool
    -> TcM ()
    checkCrossLevelClsInst DynFlags
    dflags LevelCheckReason
    reason Set ThLevelIndex
    bind_lvls ThLevelIndex
    use_lvl_idx Bool
    is_local
      -- If the Id is imported, then allow with ImplicitStagePersistence
      | Bool -> Bool
    not Bool
    is_local
      , Extension -> DynFlags -> Bool
    xopt Extension
    LangExt.ImplicitStagePersistence DynFlags
    dflags
      = () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()
      -- NB: Do this check after the ImplicitStagePersistence check, because
      -- it will do some computation to work out the levels of instances.
      | ThLevelIndex
    use_lvl_idx ThLevelIndex -> Set ThLevelIndex -> Bool
    forall a. Ord a => a -> Set a -> Bool
    `Set.member` Set ThLevelIndex
    bind_lvls = () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()
      -- With ImplicitStagePersistence, using later than bound is fine
      | Extension -> DynFlags -> Bool
    xopt Extension
    LangExt.ImplicitStagePersistence DynFlags
    dflags
      , (ThLevelIndex -> Bool) -> Set ThLevelIndex -> Bool
    forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
    any (ThLevelIndex
    use_lvl_idx ThLevelIndex -> ThLevelIndex -> Bool
    forall a. Ord a => a -> a -> Bool
    >=) Set ThLevelIndex
    bind_lvls  = () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()
      | Bool
    otherwise = TcRnMessage -> TcM ()
    TcM.addErrTc (LevelCheckReason
    -> Set ThLevelIndex
    -> ThLevelIndex
    -> Maybe ErrorItem
    -> DiagnosticReason
    -> TcRnMessage
    TcRnBadlyLevelled LevelCheckReason
    reason Set ThLevelIndex
    bind_lvls ThLevelIndex
    use_lvl_idx Maybe ErrorItem
    forall a. Maybe a
    Nothing DiagnosticReason
    ErrorWithoutFlag)

    -- | Returns the ThLevel of evidence for the solved constraint (if it has evidence)
    -- See Note [Well-levelled instance evidence]
    checkWellLevelledInstanceWhat :: HasCallStack => InstanceWhat -> TcS (Maybe (Set.Set ThLevelIndex, Bool))
    checkWellLevelledInstanceWhat :: HasCallStack =>
    InstanceWhat -> TcS (Maybe (Set ThLevelIndex, Bool))
    checkWellLevelledInstanceWhat InstanceWhat
    what
      | TopLevInstance { iw_dfun_id :: InstanceWhat -> TcTyVar
    iw_dfun_id = TcTyVar
    dfun_id } <- InstanceWhat
    what
        = (Set ThLevelIndex, Bool) -> Maybe (Set ThLevelIndex, Bool)
    forall a. a -> Maybe a
    Just ((Set ThLevelIndex, Bool) -> Maybe (Set ThLevelIndex, Bool))
    -> TcS (Set ThLevelIndex, Bool)
    -> TcS (Maybe (Set ThLevelIndex, Bool))
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> Name -> TcS (Set ThLevelIndex, Bool)
    checkNameVisibleLevels (TcTyVar -> Name
    idName TcTyVar
    dfun_id)
      | BuiltinTypeableInstance TyCon
    tc <- InstanceWhat
    what
        -- The typeable instance is always defined in the same module as the TyCon.
        = (Set ThLevelIndex, Bool) -> Maybe (Set ThLevelIndex, Bool)
    forall a. a -> Maybe a
    Just ((Set ThLevelIndex, Bool) -> Maybe (Set ThLevelIndex, Bool))
    -> TcS (Set ThLevelIndex, Bool)
    -> TcS (Maybe (Set ThLevelIndex, Bool))
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> Name -> TcS (Set ThLevelIndex, Bool)
    checkNameVisibleLevels (TyCon -> Name
    tyConName TyCon
    tc)
      | Bool
    otherwise = Maybe (Set ThLevelIndex, Bool)
    -> TcS (Maybe (Set ThLevelIndex, Bool))
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return Maybe (Set ThLevelIndex, Bool)
    forall a. Maybe a
    Nothing

    -- | Check the levels at which the given name is visible, including a boolean
    -- indicating if the name is local or not.
    checkNameVisibleLevels :: Name -> TcS (Set.Set ThLevelIndex, Bool)
    checkNameVisibleLevels :: Name -> TcS (Set ThLevelIndex, Bool)
    checkNameVisibleLevels Name
    name = do
      cur_mod <- TcGblEnv -> Module
    forall t. ContainsModule t => t -> Module
    extractModule (TcGblEnv -> Module) -> TcS TcGblEnv -> TcS Module
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> TcS TcGblEnv
    getGblEnv
      if nameIsLocalOrFrom cur_mod name
        then return (Set.singleton topLevelIndex, True)
        else do
          lvls <- checkModuleVisibleLevels (nameModule name)
          return (lvls, False)

    {- Note [Using the module graph to compute TH level visiblities]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When typechecking a module M, in order to implement GHC proposal #682
    (see Note [Explicit Level Imports] in GHC.Tc.Gen.Head), we need to be able to
    compute the Template Haskell levels that typeclass instances are visible at in M.

    To do this, we use the "level 0 imports" module graph, which we query via
    GHC.Unit.Module.Graph.mgQueryZero. For example, if we want all modules that are
    visible at level -1 from M, we do the following:

      1. start with all the direct of M imports at level -1, i.e. the "splice imports"
      2. then look at all modules that are reachable from these using only level 0
         normal imports, using 'mgQueryZero'.

    This works precisely because, as specified in the proposal, with -XNoImplicitStagePersistence,
    modules only export items at level 0. In particular, instances are only exported
    at level 0.

    See the SI36 test for an illustration.
    -}

    -- | Check which TH levels a module is visable at
    --
    -- Used to check visibility of instances (do not use this for normal identifiers).
    checkModuleVisibleLevels :: Module -> TcS (Set.Set ThLevelIndex)
    checkModuleVisibleLevels :: Module -> TcS (Set ThLevelIndex)
    checkModuleVisibleLevels Module
    check_mod = do
      cur_mod <- TcGblEnv -> Module
    forall t. ContainsModule t => t -> Module
    extractModule (TcGblEnv -> Module) -> TcS TcGblEnv -> TcS Module
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> TcS TcGblEnv
    getGblEnv
      hsc_env <- getTopEnv

      -- 0. The keys for the scope of the current module.
      let mkKey ImportLevel
    s Module
    m = (ModNodeKeyWithUid -> ImportLevel -> ZeroScopeKey
    ModuleScope (Module -> IsBootInterface -> ModNodeKeyWithUid
    moduleToMnk Module
    m IsBootInterface
    NotBoot) ImportLevel
    s)
          cur_mod_scope_key ImportLevel
    s = ImportLevel -> Module -> ZeroScopeKey
    mkKey ImportLevel
    s Module
    cur_mod

      -- 1. is_visible checks that a specific key is visible from the given level in the
      -- current module.
      let is_visible :: ImportLevel -> ZeroScopeKey -> Bool
          is_visible ImportLevel
    s ZeroScopeKey
    k = ModuleGraph -> ZeroScopeKey -> ZeroScopeKey -> Bool
    mgQueryZero (HscEnv -> ModuleGraph
    hsc_mod_graph HscEnv
    hsc_env) (ImportLevel -> ZeroScopeKey
    cur_mod_scope_key ImportLevel
    s) ZeroScopeKey
    k

      -- 2. The key we are looking for, either the module itself in the home package or the
      -- module unit id of the module we are checking.
      let instance_key = if Module -> UnitId
    moduleUnitId Module
    check_mod UnitId -> Set UnitId -> Bool
    forall a. Ord a => a -> Set a -> Bool
    `Set.member` HscEnv -> Set UnitId
    hsc_all_home_unit_ids HscEnv
    hsc_env
                           then ImportLevel -> Module -> ZeroScopeKey
    mkKey ImportLevel
    NormalLevel Module
    check_mod
                           else UnitId -> ZeroScopeKey
    UnitScope (Module -> UnitId
    moduleUnitId Module
    check_mod)

      -- 3. For each level, check if the key is visible from that level.
      let lvls = [ ImportLevel -> ThLevelIndex
    thLevelIndexFromImportLevel ImportLevel
    lvl | ImportLevel
    lvl <- [ImportLevel]
    allImportLevels, ImportLevel -> ZeroScopeKey -> Bool
    is_visible ImportLevel
    lvl ZeroScopeKey
    instance_key]
      return $ Set.fromList lvls

    {-
    Note [Well-levelled instance evidence]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    Evidence for instances must obey the same level restrictions as normal bindings.
    In particular, it is forbidden to use an instance in a top-level splice in the
    module which the instance is defined. This is because the evidence is bound at
    the top-level and top-level definitions are forbidden from being using in top-level splices in
    the same module.

    For example, suppose you have a function..  foo :: Show a => Code Q a -> Code Q ()
    then the following program is disallowed,

    ```
    data T a = T a deriving (Show)

    main :: IO ()
    main =
      let x = $$(foo [|| T () ||])
      in return ()
    ```

    because the `foo` function (used in a top-level splice) requires `Show T` evidence,
    which is defined at the top-level and therefore fails with an error that we have violated
    the stage restriction.

    ```
    Main.hs:12:14: error:
        • GHC stage restriction:
            instance for ‘Show
                            (T ())’ is used in a top-level splice, quasi-quote, or annotation,
            and must be imported, not defined locally
        • In the expression: foo [|| T () ||]
          In the Template Haskell splice $$(foo [|| T () ||])
          In the expression: $$(foo [|| T () ||])
       |
    12 |   let x = $$(foo [|| T () ||])
       |
    ```

    Solving a `Typeable (T t1 ...tn)` constraint generates code that relies on
    `$tcT`, the `TypeRep` for `T`; and we must check that this reference to `$tcT`
    is well levelled.  It's easy to know the level of `$tcT`: for imported TyCons it
    will be the level of the imported TyCon Name, and for local TyCons it will be `toplevel`.

    Therefore the `InstanceWhat` type had to be extended with
    a special case for `Typeable`, which recorded the TyCon the evidence was for and
    could them be used to check that we were not attempting to evidence in a level incorrect
    manner.

    -}

    pprEq :: TcType -> TcType -> SDoc
    pprEq :: Type -> Type -> SDoc
    pprEq Type
    ty1 Type
    ty2 = Type -> SDoc
    pprParendType Type
    ty1 SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> Char -> SDoc
    forall doc. IsLine doc => Char -> doc
    char Char
    '~' SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> Type -> SDoc
    pprParendType Type
    ty2

    isFilledMetaTyVar_maybe :: TcTyVar -> TcS (Maybe Type)
    isFilledMetaTyVar_maybe :: TcTyVar -> TcS (Maybe Type)
    isFilledMetaTyVar_maybe TcTyVar
    tv = TcM (Maybe Type) -> TcS (Maybe Type)
    forall a. TcM a -> TcS a
    wrapTcS (TcTyVar -> TcM (Maybe Type)
    TcM.isFilledMetaTyVar_maybe TcTyVar
    tv)

    isFilledMetaTyVar :: TcTyVar -> TcS Bool
    isFilledMetaTyVar :: TcTyVar -> TcS Bool
    isFilledMetaTyVar TcTyVar
    tv = TcM Bool -> TcS Bool
    forall a. TcM a -> TcS a
    wrapTcS (TcTyVar -> TcM Bool
    TcM.isFilledMetaTyVar TcTyVar
    tv)

    isUnfilledMetaTyVar :: TcTyVar -> TcS Bool
    isUnfilledMetaTyVar :: TcTyVar -> TcS Bool
    isUnfilledMetaTyVar TcTyVar
    tv = TcM Bool -> TcS Bool
    forall a. TcM a -> TcS a
    wrapTcS (TcM Bool -> TcS Bool) -> TcM Bool -> TcS Bool
    forall a b. (a -> b) -> a -> b
    $ TcTyVar -> TcM Bool
    TcM.isUnfilledMetaTyVar TcTyVar
    tv

    zonkTyCoVarsAndFV :: TcTyCoVarSet -> TcS TcTyCoVarSet
    zonkTyCoVarsAndFV :: VarSet -> TcS VarSet
    zonkTyCoVarsAndFV VarSet
    tvs = ZonkM VarSet -> TcS VarSet
    forall a. ZonkM a -> TcS a
    liftZonkTcS (VarSet -> ZonkM VarSet
    TcM.zonkTyCoVarsAndFV VarSet
    tvs)

    zonkTyCoVarsAndFVList :: [TcTyCoVar] -> TcS [TcTyCoVar]
    zonkTyCoVarsAndFVList :: [TcTyVar] -> TcS [TcTyVar]
    zonkTyCoVarsAndFVList [TcTyVar]
    tvs = ZonkM [TcTyVar] -> TcS [TcTyVar]
    forall a. ZonkM a -> TcS a
    liftZonkTcS ([TcTyVar] -> ZonkM [TcTyVar]
    TcM.zonkTyCoVarsAndFVList [TcTyVar]
    tvs)

    zonkCo :: Coercion -> TcS Coercion
    zonkCo :: Coercion -> TcS Coercion
    zonkCo = TcM Coercion -> TcS Coercion
    forall a. TcM a -> TcS a
    wrapTcS (TcM Coercion -> TcS Coercion)
    -> (Coercion -> TcM Coercion) -> Coercion -> TcS Coercion
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . (ZonkM Coercion -> TcM Coercion)
    -> (Coercion -> ZonkM Coercion) -> Coercion -> TcM Coercion
    forall a b. (a -> b) -> (Coercion -> a) -> Coercion -> b
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    fmap ZonkM Coercion -> TcM Coercion
    forall a. ZonkM a -> TcM a
    TcM.liftZonkM Coercion -> ZonkM Coercion
    TcM.zonkCo

    zonkTcType :: TcType -> TcS TcType
    zonkTcType :: Type -> TcS Type
    zonkTcType Type
    ty = ZonkM Type -> TcS Type
    forall a. ZonkM a -> TcS a
    liftZonkTcS (Type -> ZonkM Type
    TcM.zonkTcType Type
    ty)

    zonkTcTypes :: [TcType] -> TcS [TcType]
    zonkTcTypes :: [Type] -> TcS [Type]
    zonkTcTypes [Type]
    tys = ZonkM [Type] -> TcS [Type]
    forall a. ZonkM a -> TcS a
    liftZonkTcS ([Type] -> ZonkM [Type]
    TcM.zonkTcTypes [Type]
    tys)

    zonkTcTyVar :: TcTyVar -> TcS TcType
    zonkTcTyVar :: TcTyVar -> TcS Type
    zonkTcTyVar TcTyVar
    tv = ZonkM Type -> TcS Type
    forall a. ZonkM a -> TcS a
    liftZonkTcS (TcTyVar -> ZonkM Type
    TcM.zonkTcTyVar TcTyVar
    tv)

    zonkSimples :: Cts -> TcS Cts
    zonkSimples :: Cts -> TcS Cts
    zonkSimples Cts
    cts = ZonkM Cts -> TcS Cts
    forall a. ZonkM a -> TcS a
    liftZonkTcS (Cts -> ZonkM Cts
    TcM.zonkSimples Cts
    cts)

    zonkWC :: WantedConstraints -> TcS WantedConstraints
    zonkWC :: WantedConstraints -> TcS WantedConstraints
    zonkWC WantedConstraints
    wc = ZonkM WantedConstraints -> TcS WantedConstraints
    forall a. ZonkM a -> TcS a
    liftZonkTcS (WantedConstraints -> ZonkM WantedConstraints
    TcM.zonkWC WantedConstraints
    wc)

    zonkTyCoVarKind :: TcTyCoVar -> TcS TcTyCoVar
    zonkTyCoVarKind :: TcTyVar -> TcS TcTyVar
    zonkTyCoVarKind TcTyVar
    tv = ZonkM TcTyVar -> TcS TcTyVar
    forall a. ZonkM a -> TcS a
    liftZonkTcS (TcTyVar -> ZonkM TcTyVar
    TcM.zonkTyCoVarKind TcTyVar
    tv)

    ----------------------------
    pprKicked :: Int -> SDoc
    pprKicked :: Int -> SDoc
    pprKicked Int
    0 = SDoc
    forall doc. IsOutput doc => doc
    empty
    pprKicked Int
    n = SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc
    parens (Int -> SDoc
    forall doc. IsLine doc => Int -> doc
    int Int
    n SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "kicked out")

    {- *********************************************************************
    *                                                                      *
    *              The work list
    *                                                                      *
    ********************************************************************* -}

    getTcSWorkListRef :: TcS (IORef WorkList)
    getTcSWorkListRef :: TcS (IORef WorkList)
    getTcSWorkListRef = (TcSEnv -> IOEnv (Env TcGblEnv TcLclEnv) (IORef WorkList))
    -> TcS (IORef WorkList)
    forall a. (TcSEnv -> TcM a) -> TcS a
    TcS (IORef WorkList -> IOEnv (Env TcGblEnv TcLclEnv) (IORef WorkList)
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (IORef WorkList -> IOEnv (Env TcGblEnv TcLclEnv) (IORef WorkList))
    -> (TcSEnv -> IORef WorkList)
    -> TcSEnv
    -> IOEnv (Env TcGblEnv TcLclEnv) (IORef WorkList)
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . TcSEnv -> IORef WorkList
    tcs_worklist)

    getWorkList :: TcS WorkList
    getWorkList :: TcS WorkList
    getWorkList = do { wl_var <- TcS (IORef WorkList)
    getTcSWorkListRef
                     ; readTcRef wl_var }

    updWorkListTcS :: (WorkList -> WorkList) -> TcS ()
    updWorkListTcS :: (WorkList -> WorkList) -> TcS ()
    updWorkListTcS WorkList -> WorkList
    f
      = do { wl_var <- TcS (IORef WorkList)
    getTcSWorkListRef
           ; updTcRef wl_var f }

    emitWorkNC :: [CtEvidence] -> TcS ()
    emitWorkNC :: [CtEvidence] -> TcS ()
    emitWorkNC [CtEvidence]
    evs
      | [CtEvidence] -> Bool
    forall a. [a] -> Bool
    forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [CtEvidence]
    evs
      = () -> TcS ()
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()
      | Bool
    otherwise
      = Cts -> TcS ()
    emitWork ([Ct] -> Cts
    forall a. [a] -> Bag a
    listToBag ((CtEvidence -> Ct) -> [CtEvidence] -> [Ct]
    forall a b. (a -> b) -> [a] -> [b]
    map CtEvidence -> Ct
    mkNonCanonical [CtEvidence]
    evs))

    emitWork :: Cts -> TcS ()
    emitWork :: Cts -> TcS ()
    emitWork Cts
    cts
      | Cts -> Bool
    forall a. Bag a -> Bool
    isEmptyBag Cts
    cts    -- Avoid printing, among other work
      = () -> TcS ()
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()
      | Bool
    otherwise
      = do { String -> SDoc -> TcS ()
    traceTcS String
    "Emitting fresh work" (Cts -> SDoc
    forall a. Outputable a => Bag a -> SDoc
    pprBag Cts
    cts)
           ; (WorkList -> WorkList) -> TcS ()
    updWorkListTcS (Cts -> WorkList -> WorkList
    extendWorkListCts Cts
    cts) }

    selectNextWorkItem :: TcS (Maybe Ct)
    -- Pick which work item to do next
    -- See Note [Prioritise equalities]
    --
    -- Postcondition: if the returned item is a Wanted equality,
    --                then its rewriter set is fully zonked.
    --
    -- Suppose a constraint c1 is rewritten by another, c2.  When c2
    -- gets solved, c1 has no rewriters, and can be prioritised; see
    -- Note [Prioritise Wanteds with empty RewriterSet] in
    -- GHC.Tc.Types.Constraint wrinkle (PER1)

    -- ToDo: if wl_rw_eqs is long, we'll re-zonk it each time we pick
    --       a new item from wl_rest.  Bad.
    selectNextWorkItem :: TcS (Maybe Ct)
    selectNextWorkItem
      = do { wl_var <- TcS (IORef WorkList)
    getTcSWorkListRef
           ; wl     <- readTcRef wl_var

           ; case wl of { WL { wl_eqs_N :: WorkList -> [Ct]
    wl_eqs_N = [Ct]
    eqs_N, wl_eqs_X :: WorkList -> [Ct]
    wl_eqs_X = [Ct]
    eqs_X
                             , wl_rw_eqs :: WorkList -> [Ct]
    wl_rw_eqs = [Ct]
    rw_eqs, wl_rest :: WorkList -> [Ct]
    wl_rest = [Ct]
    rest }
               | Ct
    ct:[Ct]
    cts <- [Ct]
    eqs_N  -> Ct -> WorkList -> TcS (Maybe Ct)
    pick_me Ct
    ct (WorkList
    wl { wl_eqs_N  = cts })
               | Ct
    ct:[Ct]
    cts <- [Ct]
    eqs_X  -> Ct -> WorkList -> TcS (Maybe Ct)
    pick_me Ct
    ct (WorkList
    wl { wl_eqs_X  = cts })
               | Bool
    otherwise        -> [Ct] -> [Ct] -> TcS (Maybe Ct)
    try_rws [] [Ct]
    rw_eqs
               where
                 pick_me :: Ct -> WorkList -> TcS (Maybe Ct)
                 pick_me :: Ct -> WorkList -> TcS (Maybe Ct)
    pick_me Ct
    ct WorkList
    new_wl
                   = do { IORef WorkList -> WorkList -> TcS ()
    forall a. TcRef a -> a -> TcS ()
    writeTcRef IORef WorkList
    wl_var WorkList
    new_wl
                        ; Maybe Ct -> TcS (Maybe Ct)
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Ct -> Maybe Ct
    forall a. a -> Maybe a
    Just Ct
    ct) }
                     -- NB: no need for checkReductionDepth (ctLoc ct) (ctPred ct)
                     -- This is done by GHC.Tc.Solver.Dict.chooseInstance

                 -- try_rws looks through rw_eqs to find one that has an empty
                 -- rewriter set, after zonking.  If none such, call try_rest.
                 try_rws :: [Ct] -> [Ct] -> TcS (Maybe Ct)
    try_rws [Ct]
    acc (Ct
    ct:[Ct]
    cts)
                    = do { ct' <- ZonkM Ct -> TcS Ct
    forall a. ZonkM a -> TcS a
    liftZonkTcS (Ct -> ZonkM Ct
    TcM.zonkCtRewriterSet Ct
    ct)
                         ; if ctHasNoRewriters ct'
                           then pick_me ct' (wl { wl_rw_eqs = cts ++ acc })
                           else try_rws (ct':acc) cts }
                 try_rws [Ct]
    acc [] = [Ct] -> TcS (Maybe Ct)
    try_rest [Ct]
    acc

                 try_rest :: [Ct] -> TcS (Maybe Ct)
    try_rest [Ct]
    zonked_rws
                   | Ct
    ct:[Ct]
    cts <- [Ct]
    rest       = Ct -> WorkList -> TcS (Maybe Ct)
    pick_me Ct
    ct (WorkList
    wl { wl_rw_eqs = zonked_rws, wl_rest = cts })
                   | Ct
    ct:[Ct]
    cts <- [Ct]
    zonked_rws = Ct -> WorkList -> TcS (Maybe Ct)
    pick_me Ct
    ct (WorkList
    wl { wl_rw_eqs = cts })
                   | Bool
    otherwise            = Maybe Ct -> TcS (Maybe Ct)
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return Maybe Ct
    forall a. Maybe a
    Nothing
         } }

    pushLevelNoWorkList :: SDoc -> TcS a -> TcS (TcLevel, a)
    -- Push the level and run thing_inside
    -- However, thing_inside should not generate any work items
    #if defined(DEBUG)
    pushLevelNoWorkList err_doc (TcS thing_inside)
      = TcS (\env -> TcM.pushTcLevelM $
                     thing_inside (env { tcs_worklist = wl_panic })
            )
      where
        wl_panic  = pprPanic "GHC.Tc.Solver.Monad.buildImplication" err_doc
                             -- This panic checks that the thing-inside
                             -- does not emit any work-list constraints
    #else
    pushLevelNoWorkList :: forall a. SDoc -> TcS a -> TcS (TcLevel, a)
    pushLevelNoWorkList SDoc
    _ (TcS TcSEnv -> TcM a
    thing_inside)
      = (TcSEnv -> TcM (TcLevel, a)) -> TcS (TcLevel, a)
    forall a. (TcSEnv -> TcM a) -> TcS a
    TcS (\TcSEnv
    env -> TcM a -> TcM (TcLevel, a)
    forall a. TcM a -> TcM (TcLevel, a)
    TcM.pushTcLevelM (TcSEnv -> TcM a
    thing_inside TcSEnv
    env))  -- Don't check
    #endif

    {- *********************************************************************
    *                                                                      *
    *              The Unification Level Flag                              *
    *                                                                      *
    ********************************************************************* -}

    {- Note [The Unification Level Flag]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider a deep tree of implication constraints
       forall[1] a.                              -- Outer-implic
          C alpha[1]                               -- Simple
          forall[2] c. ....(C alpha[1])....        -- Implic-1
          forall[2] b. ....(alpha[1] ~ Int)....    -- Implic-2

    The (C alpha) is insoluble until we know alpha.  We solve alpha
    by unifying alpha:=Int somewhere deep inside Implic-2. But then we
    must try to solve the Outer-implic all over again. This time we can
    solve (C alpha) both in Outer-implic, and nested inside Implic-1.

    When should we iterate solving a level-n implication?
    Answer: if any unification of a tyvar at level n takes place
            in the ic_implics of that implication.

    * What if a unification takes place at level n-1? Then don't iterate
      level n, because we'll iterate level n-1, and that will in turn iterate
      level n.

    * What if a unification takes place at level n, in the ic_simples of
      level n?  No need to track this, because the kick-out mechanism deals
      with it.  (We can't drop kick-out in favour of iteration, because kick-out
      works for skolem-equalities, not just unifications.)

    So the monad-global Unification Level Flag, kept in tcs_unif_lvl keeps
    track of
      - Whether any unifications at all have taken place (Nothing => no unifications)
      - If so, what is the outermost level that has seen a unification (Just lvl)

    The iteration is done in the simplify_loop/maybe_simplify_again loop in GHC.Tc.Solver.

    It helpful not to iterate unless there is a chance of progress.  #8474 is
    an example:

      * There's a deeply-nested chain of implication constraints.
           ?x:alpha => ?y1:beta1 => ... ?yn:betan => [W] ?x:Int

      * From the innermost one we get a [W] alpha[1] ~ Int,
        so we can unify.

      * It's better not to iterate the inner implications, but go all the
        way out to level 1 before iterating -- because iterating level 1
        will iterate the inner levels anyway.

    (In the olden days when we "floated" thse Derived constraints, this was
    much, much more important -- we got exponential behaviour, as each iteration
    produced the same Derived constraint.)
    -}

    resetUnificationFlag :: TcS Bool
    -- We are at ambient level i
    -- If the unification flag = Just i, reset it to Nothing and return True
    -- Otherwise leave it unchanged and return False
    resetUnificationFlag :: TcS Bool
    resetUnificationFlag
      = (TcSEnv -> TcM Bool) -> TcS Bool
    forall a. (TcSEnv -> TcM a) -> TcS a
    TcS ((TcSEnv -> TcM Bool) -> TcS Bool)
    -> (TcSEnv -> TcM Bool) -> TcS Bool
    forall a b. (a -> b) -> a -> b
    $ \TcSEnv
    env ->
        do { let ref :: IORef (Maybe TcLevel)
    ref = TcSEnv -> IORef (Maybe TcLevel)
    tcs_unif_lvl TcSEnv
    env
           ; ambient_lvl <- TcM TcLevel
    TcM.getTcLevel
           ; mb_lvl <- TcM.readTcRef ref
           ; TcM.traceTc "resetUnificationFlag" $
             vcat [ text "ambient:" <+> ppr ambient_lvl
                  , text "unif_lvl:" <+> ppr mb_lvl ]
           ; case mb_lvl of
               Maybe TcLevel
    Nothing       -> Bool -> TcM Bool
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return Bool
    False
               Just TcLevel
    unif_lvl | TcLevel
    ambient_lvl TcLevel -> TcLevel -> Bool
    `strictlyDeeperThan` TcLevel
    unif_lvl
                             -> Bool -> TcM Bool
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return Bool
    False
                             | Bool
    otherwise
                             -> do { IORef (Maybe TcLevel) -> Maybe TcLevel -> TcM ()
    forall (m :: * -> *) a. MonadIO m => TcRef a -> a -> m ()
    TcM.writeTcRef IORef (Maybe TcLevel)
    ref Maybe TcLevel
    forall a. Maybe a
    Nothing
                                   ; Bool -> TcM Bool
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return Bool
    True } }

    setUnificationFlag :: TcLevel -> TcS ()
    -- (setUnificationFlag i) sets the unification level to (Just i)
    -- unless it already is (Just j) where j <= i
    setUnificationFlag :: TcLevel -> TcS ()
    setUnificationFlag TcLevel
    lvl
      = (TcSEnv -> TcM ()) -> TcS ()
    forall a. (TcSEnv -> TcM a) -> TcS a
    TcS ((TcSEnv -> TcM ()) -> TcS ()) -> (TcSEnv -> TcM ()) -> TcS ()
    forall a b. (a -> b) -> a -> b
    $ \TcSEnv
    env ->
        do { let ref :: IORef (Maybe TcLevel)
    ref = TcSEnv -> IORef (Maybe TcLevel)
    tcs_unif_lvl TcSEnv
    env
           ; mb_lvl <- IORef (Maybe TcLevel)
    -> IOEnv (Env TcGblEnv TcLclEnv) (Maybe TcLevel)
    forall (m :: * -> *) a. MonadIO m => TcRef a -> m a
    TcM.readTcRef IORef (Maybe TcLevel)
    ref
           ; case mb_lvl of
               Just TcLevel
    unif_lvl | TcLevel
    lvl TcLevel -> TcLevel -> Bool
    `deeperThanOrSame` TcLevel
    unif_lvl
                             -> () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()
               Maybe TcLevel
    _ -> IORef (Maybe TcLevel) -> Maybe TcLevel -> TcM ()
    forall (m :: * -> *) a. MonadIO m => TcRef a -> a -> m ()
    TcM.writeTcRef IORef (Maybe TcLevel)
    ref (TcLevel -> Maybe TcLevel
    forall a. a -> Maybe a
    Just TcLevel
    lvl) }

    {- *********************************************************************
    *                                                                      *
    *                Instantiation etc.
    *                                                                      *
    ********************************************************************* -}

    -- Instantiations
    -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    instDFunType :: DFunId -> [DFunInstType] -> TcS ([TcType], TcThetaType)
    instDFunType :: TcTyVar -> [Maybe Type] -> TcS ([Type], [Type])
    instDFunType TcTyVar
    dfun_id [Maybe Type]
    inst_tys
      = TcM ([Type], [Type]) -> TcS ([Type], [Type])
    forall a. TcM a -> TcS a
    wrapTcS (TcM ([Type], [Type]) -> TcS ([Type], [Type]))
    -> TcM ([Type], [Type]) -> TcS ([Type], [Type])
    forall a b. (a -> b) -> a -> b
    $ TcTyVar -> [Maybe Type] -> TcM ([Type], [Type])
    TcM.instDFunType TcTyVar
    dfun_id [Maybe Type]
    inst_tys

    newFlexiTcSTy :: Kind -> TcS TcType
    newFlexiTcSTy :: Type -> TcS Type
    newFlexiTcSTy Type
    knd = TcM Type -> TcS Type
    forall a. TcM a -> TcS a
    wrapTcS (Type -> TcM Type
    TcM.newFlexiTyVarTy Type
    knd)

    cloneMetaTyVar :: TcTyVar -> TcS TcTyVar
    cloneMetaTyVar :: TcTyVar -> TcS TcTyVar
    cloneMetaTyVar TcTyVar
    tv = TcM TcTyVar -> TcS TcTyVar
    forall a. TcM a -> TcS a
    wrapTcS (TcTyVar -> TcM TcTyVar
    TcM.cloneMetaTyVar TcTyVar
    tv)

    instFlexiX :: Subst -> [TKVar] -> TcS Subst
    instFlexiX :: Subst -> [TcTyVar] -> TcS Subst
    instFlexiX Subst
    subst [TcTyVar]
    tvs = TcM Subst -> TcS Subst
    forall a. TcM a -> TcS a
    wrapTcS (Subst -> [TcTyVar] -> TcM Subst
    instFlexiXTcM Subst
    subst [TcTyVar]
    tvs)

    instFlexiXTcM :: Subst -> [TKVar] -> TcM Subst
    -- Makes fresh tyvar, extends the substitution, and the in-scope set
    -- Takes account of the case [k::Type, a::k, ...],
    -- where we must substitute for k in a's kind
    instFlexiXTcM :: Subst -> [TcTyVar] -> TcM Subst
    instFlexiXTcM Subst
    subst []
      = Subst -> TcM Subst
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return Subst
    subst
    instFlexiXTcM Subst
    subst (TcTyVar
    tv:[TcTyVar]
    tvs)
      = do { uniq <- TcRnIf TcGblEnv TcLclEnv Unique
    forall gbl lcl. TcRnIf gbl lcl Unique
    TcM.newUnique
           ; details <- TcM.newMetaDetails TauTv
           ; let name   = Name -> Unique -> Name
    setNameUnique (TcTyVar -> Name
    tyVarName TcTyVar
    tv) Unique
    uniq
                 kind   = Subst -> Type -> Type
    substTyUnchecked Subst
    subst (TcTyVar -> Type
    tyVarKind TcTyVar
    tv)
                 tv'    = Name -> Type -> TcTyVarDetails -> TcTyVar
    mkTcTyVar Name
    name Type
    kind TcTyVarDetails
    details
                 subst' = Subst -> TcTyVar -> TcTyVar -> Subst
    extendTvSubstWithClone Subst
    subst TcTyVar
    tv TcTyVar
    tv'
           ; instFlexiXTcM subst' tvs  }

    matchGlobalInst :: DynFlags -> Class -> [Type] -> CtLoc -> TcS TcM.ClsInstResult
    matchGlobalInst :: DynFlags -> Class -> [Type] -> CtLoc -> TcS ClsInstResult
    matchGlobalInst DynFlags
    dflags Class
    cls [Type]
    tys CtLoc
    loc
      = do { mode <- TcS TcSMode
    getTcSMode
           ; let skip_overlappable = TcSMode -> Bool
    tcsmSkipOverlappable TcSMode
    mode
           ; wrapTcS $ TcM.matchGlobalInst dflags skip_overlappable cls tys (Just loc) }

    tcInstSkolTyVarsX :: SkolemInfo -> Subst -> [TyVar] -> TcS (Subst, [TcTyVar])
    tcInstSkolTyVarsX :: SkolemInfo -> Subst -> [TcTyVar] -> TcS (Subst, [TcTyVar])
    tcInstSkolTyVarsX SkolemInfo
    skol_info Subst
    subst [TcTyVar]
    tvs = TcM (Subst, [TcTyVar]) -> TcS (Subst, [TcTyVar])
    forall a. TcM a -> TcS a
    wrapTcS (TcM (Subst, [TcTyVar]) -> TcS (Subst, [TcTyVar]))
    -> TcM (Subst, [TcTyVar]) -> TcS (Subst, [TcTyVar])
    forall a b. (a -> b) -> a -> b
    $ SkolemInfo -> Subst -> [TcTyVar] -> TcM (Subst, [TcTyVar])
    TcM.tcInstSkolTyVarsX SkolemInfo
    skol_info Subst
    subst [TcTyVar]
    tvs

    -- Creating and setting evidence variables and CtFlavors
    -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    data MaybeNew = Fresh WantedCtEvidence | Cached EvExpr

    isFresh :: MaybeNew -> Bool
    isFresh :: MaybeNew -> Bool
    isFresh (Fresh {})  = Bool
    True
    isFresh (Cached {}) = Bool
    False

    freshGoals :: [MaybeNew] -> [WantedCtEvidence]
    freshGoals :: [MaybeNew] -> [WantedCtEvidence]
    freshGoals [MaybeNew]
    mns = [ WantedCtEvidence
    ctev | Fresh WantedCtEvidence
    ctev <- [MaybeNew]
    mns ]

    getEvExpr :: MaybeNew -> EvExpr
    getEvExpr :: MaybeNew -> EvExpr
    getEvExpr (Fresh WantedCtEvidence
    ctev) = HasDebugCallStack => CtEvidence -> EvExpr
    CtEvidence -> EvExpr
    ctEvExpr (WantedCtEvidence -> CtEvidence
    CtWanted WantedCtEvidence
    ctev)
    getEvExpr (Cached EvExpr
    evt) = EvExpr
    evt

    setEvBind :: EvBind -> TcS ()
    setEvBind :: EvBind -> TcS ()
    setEvBind EvBind
    ev_bind
      = do { evb <- TcS EvBindsVar
    getTcEvBindsVar
           ; wrapTcS $ TcM.addTcEvBind evb ev_bind }

    -- | Mark variables as used filling a coercion hole
    addUsedCoercion :: TcCoercion -> TcS ()
    addUsedCoercion :: Coercion -> TcS ()
    addUsedCoercion Coercion
    co
      = do { ev_binds_var <- TcS EvBindsVar
    getTcEvBindsVar
           ; wrapTcS (TcM.updTcRef (ebv_tcvs ev_binds_var) (co :)) }

    -- | Equalities only
    setWantedEq :: HasDebugCallStack => TcEvDest -> TcCoercion -> TcS ()
    setWantedEq :: HasDebugCallStack => TcEvDest -> Coercion -> TcS ()
    setWantedEq (HoleDest CoercionHole
    hole) Coercion
    co
      = do { Coercion -> TcS ()
    addUsedCoercion Coercion
    co
           ; CoercionHole -> Coercion -> TcS ()
    fillCoercionHole CoercionHole
    hole Coercion
    co }
    setWantedEq (EvVarDest TcTyVar
    ev) Coercion
    _ = String -> SDoc -> TcS ()
    forall a. HasCallStack => String -> SDoc -> a
    pprPanic String
    "setWantedEq: EvVarDest" (TcTyVar -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcTyVar
    ev)

    -- | Good for both equalities and non-equalities
    setWantedEvTerm :: TcEvDest -> CanonicalEvidence -> EvTerm -> TcS ()
    setWantedEvTerm :: TcEvDest -> CanonicalEvidence -> EvTerm -> TcS ()
    setWantedEvTerm (HoleDest CoercionHole
    hole) CanonicalEvidence
    _canonical EvTerm
    tm
      | Just Coercion
    co <- EvTerm -> Maybe Coercion
    evTermCoercion_maybe EvTerm
    tm
      = do { Coercion -> TcS ()
    addUsedCoercion Coercion
    co
           ; CoercionHole -> Coercion -> TcS ()
    fillCoercionHole CoercionHole
    hole Coercion
    co }
      | Bool
    otherwise
      = -- See Note [Yukky eq_sel for a HoleDest]
        do { let co_var :: TcTyVar
    co_var = CoercionHole -> TcTyVar
    coHoleCoVar CoercionHole
    hole
           ; EvBind -> TcS ()
    setEvBind (TcTyVar -> CanonicalEvidence -> EvTerm -> EvBind
    mkWantedEvBind TcTyVar
    co_var CanonicalEvidence
    EvCanonical EvTerm
    tm)
           ; CoercionHole -> Coercion -> TcS ()
    fillCoercionHole CoercionHole
    hole (TcTyVar -> Coercion
    mkCoVarCo TcTyVar
    co_var) }

    setWantedEvTerm (EvVarDest TcTyVar
    ev_id) CanonicalEvidence
    canonical EvTerm
    tm
      = EvBind -> TcS ()
    setEvBind (TcTyVar -> CanonicalEvidence -> EvTerm -> EvBind
    mkWantedEvBind TcTyVar
    ev_id CanonicalEvidence
    canonical EvTerm
    tm)

    {- Note [Yukky eq_sel for a HoleDest]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    How can it be that a Wanted with HoleDest gets evidence that isn't
    just a coercion? i.e. evTermCoercion_maybe returns Nothing.

    Consider [G] forall a. blah => a ~ T
             [W] S ~# T

    Then doTopReactEqPred carefully looks up the (boxed) constraint (S ~ T)
    in the quantified constraints, and wraps the (boxed) evidence it
    gets back in an eq_sel to extract the unboxed (S ~# T).  We can't put
    that term into a coercion, so we add a value binding
        h = eq_sel (...)
    and the coercion variable h to fill the coercion hole.
    We even re-use the CoHole's Id for this binding!

    Yuk!
    -}

    fillCoercionHole :: CoercionHole -> Coercion -> TcS ()
    fillCoercionHole :: CoercionHole -> Coercion -> TcS ()
    fillCoercionHole CoercionHole
    hole Coercion
    co
      = do { TcM () -> TcS ()
    forall a. TcM a -> TcS a
    wrapTcS (TcM () -> TcS ()) -> TcM () -> TcS ()
    forall a b. (a -> b) -> a -> b
    $ CoercionHole -> Coercion -> TcM ()
    TcM.fillCoercionHole CoercionHole
    hole Coercion
    co
           ; CoercionHole -> TcS ()
    kickOutAfterFillingCoercionHole CoercionHole
    hole }

    setEvBindIfWanted :: CtEvidence -> CanonicalEvidence -> EvTerm -> TcS ()
    setEvBindIfWanted :: CtEvidence -> CanonicalEvidence -> EvTerm -> TcS ()
    setEvBindIfWanted CtEvidence
    ev CanonicalEvidence
    canonical EvTerm
    tm
      = case CtEvidence
    ev of
          CtWanted (WantedCt { ctev_dest :: WantedCtEvidence -> TcEvDest
    ctev_dest = TcEvDest
    dest }) -> TcEvDest -> CanonicalEvidence -> EvTerm -> TcS ()
    setWantedEvTerm TcEvDest
    dest CanonicalEvidence
    canonical EvTerm
    tm
          CtEvidence
    _                                        -> () -> TcS ()
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()

    newTcEvBinds :: TcS EvBindsVar
    newTcEvBinds :: TcS EvBindsVar
    newTcEvBinds = TcM EvBindsVar -> TcS EvBindsVar
    forall a. TcM a -> TcS a
    wrapTcS TcM EvBindsVar
    TcM.newTcEvBinds

    newNoTcEvBinds :: TcS EvBindsVar
    newNoTcEvBinds :: TcS EvBindsVar
    newNoTcEvBinds = TcM EvBindsVar -> TcS EvBindsVar
    forall a. TcM a -> TcS a
    wrapTcS TcM EvBindsVar
    TcM.newNoTcEvBinds

    newEvVar :: TcPredType -> TcS EvVar
    newEvVar :: Type -> TcS TcTyVar
    newEvVar Type
    pred = TcM TcTyVar -> TcS TcTyVar
    forall a. TcM a -> TcS a
    wrapTcS (Type -> TcM TcTyVar
    forall gbl lcl. Type -> TcRnIf gbl lcl TcTyVar
    TcM.newEvVar Type
    pred)

    newGivenEvVar :: CtLoc -> (TcPredType, EvTerm) -> TcS GivenCtEvidence
    -- Make a new variable of the given PredType,
    -- immediately bind it to the given term
    -- and return its CtEvidence
    -- See Note [Bind new Givens immediately] in GHC.Tc.Types.Constraint
    newGivenEvVar :: CtLoc -> (Type, EvTerm) -> TcS GivenCtEvidence
    newGivenEvVar CtLoc
    loc (Type
    pred, EvTerm
    rhs)
      = do { new_ev <- Type -> EvTerm -> TcS TcTyVar
    newBoundEvVarId Type
    pred EvTerm
    rhs
           ; return $ GivenCt { ctev_pred = pred, ctev_evar = new_ev, ctev_loc = loc } }

    -- | Make a new 'Id' of the given type, bound (in the monad's EvBinds) to the
    -- given term
    newBoundEvVarId :: TcPredType -> EvTerm -> TcS EvVar
    newBoundEvVarId :: Type -> EvTerm -> TcS TcTyVar
    newBoundEvVarId Type
    pred EvTerm
    rhs
      = do { new_ev <- Type -> TcS TcTyVar
    newEvVar Type
    pred
           ; setEvBind (mkGivenEvBind new_ev rhs)
           ; return new_ev }

    emitNewGivens :: CtLoc -> [(Role,TcCoercion)] -> TcS ()
    emitNewGivens :: CtLoc -> [(Role, Coercion)] -> TcS ()
    emitNewGivens CtLoc
    loc [(Role, Coercion)]
    pts
      = do { String -> SDoc -> TcS ()
    traceTcS String
    "emitNewGivens" ([(Role, Coercion)] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr [(Role, Coercion)]
    pts)
           ; gs <- ((Type, EvTerm) -> TcS GivenCtEvidence)
    -> [(Type, EvTerm)] -> TcS [GivenCtEvidence]
    forall (t :: * -> *) (m :: * -> *) a b.
    (Traversable t, Monad m) =>
    (a -> m b) -> t a -> m (t b)
    forall (m :: * -> *) a b. Monad m => (a -> m b) -> [a] -> m [b]
    mapM (CtLoc -> (Type, EvTerm) -> TcS GivenCtEvidence
    newGivenEvVar CtLoc
    loc) ([(Type, EvTerm)] -> TcS [GivenCtEvidence])
    -> [(Type, EvTerm)] -> TcS [GivenCtEvidence]
    forall a b. (a -> b) -> a -> b
    $
                    [ (Role -> Type -> Type -> Type
    mkEqPredRole Role
    role Type
    ty1 Type
    ty2, Coercion -> EvTerm
    evCoercion Coercion
    co)
                    | (Role
    role, Coercion
    co) <- [(Role, Coercion)]
    pts
                    , let Pair Type
    ty1 Type
    ty2 = HasDebugCallStack => Coercion -> TypeEqn
    Coercion -> TypeEqn
    coercionKind Coercion
    co
                    , Bool -> Bool
    not (Type
    ty1 HasDebugCallStack => Type -> Type -> Bool
    Type -> Type -> Bool
    `tcEqType` Type
    ty2) ] -- Kill reflexive Givens at birth
           ; emitWorkNC (map CtGiven gs) }

    emitNewWantedEq :: CtLoc -> RewriterSet -> Role -> TcType -> TcType -> TcS Coercion
    -- | Emit a new Wanted equality into the work-list
    emitNewWantedEq :: CtLoc -> RewriterSet -> Role -> Type -> Type -> TcS Coercion
    emitNewWantedEq CtLoc
    loc RewriterSet
    rewriters Role
    role Type
    ty1 Type
    ty2
      = do { (wtd, co) <- CtLoc
    -> RewriterSet
    -> Role
    -> Type
    -> Type
    -> TcS (WantedCtEvidence, Coercion)
    newWantedEq CtLoc
    loc RewriterSet
    rewriters Role
    role Type
    ty1 Type
    ty2
           ; updWorkListTcS (extendWorkListEq rewriters (mkNonCanonical $ CtWanted wtd))
           ; return co }

    -- | Create a new Wanted constraint holding a coercion hole
    -- for an equality between the two types at the given 'Role'.
    newWantedEq :: CtLoc -> RewriterSet -> Role -> TcType -> TcType
                -> TcS (WantedCtEvidence, Coercion)
    newWantedEq :: CtLoc
    -> RewriterSet
    -> Role
    -> Type
    -> Type
    -> TcS (WantedCtEvidence, Coercion)
    newWantedEq CtLoc
    loc RewriterSet
    rewriters Role
    role Type
    ty1 Type
    ty2
      = do { hole <- TcM CoercionHole -> TcS CoercionHole
    forall a. TcM a -> TcS a
    wrapTcS (TcM CoercionHole -> TcS CoercionHole)
    -> TcM CoercionHole -> TcS CoercionHole
    forall a b. (a -> b) -> a -> b
    $ Type -> TcM CoercionHole
    TcM.newCoercionHole Type
    pty
           ; let wtd = WantedCt { ctev_pred :: Type
    ctev_pred      = Type
    pty
                                , ctev_dest :: TcEvDest
    ctev_dest      = CoercionHole -> TcEvDest
    HoleDest CoercionHole
    hole
                                , ctev_loc :: CtLoc
    ctev_loc       = CtLoc
    loc
                                , ctev_rewriters :: RewriterSet
    ctev_rewriters = RewriterSet
    rewriters }
           ; return (wtd, mkHoleCo hole) }
      where
        pty :: Type
    pty = Role -> Type -> Type -> Type
    mkEqPredRole Role
    role Type
    ty1 Type
    ty2

    -- | Create a new Wanted constraint holding an evidence variable.
    --
    -- Don't use this for equality constraints: use 'newWantedEq' instead.
    newWantedEvVarNC :: CtLoc -> RewriterSet
                     -> TcPredType -> TcS WantedCtEvidence
    -- Don't look up in the solved/inerts; we know it's not there
    newWantedEvVarNC :: CtLoc -> RewriterSet -> Type -> TcS WantedCtEvidence
    newWantedEvVarNC CtLoc
    loc RewriterSet
    rewriters Type
    pty
      = Bool -> SDoc -> TcS WantedCtEvidence -> TcS WantedCtEvidence
    forall a. HasCallStack => Bool -> SDoc -> a -> a
    assertPpr (Bool -> Bool
    not (Type -> Bool
    isEqPred Type
    pty)) (Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    pty) (TcS WantedCtEvidence -> TcS WantedCtEvidence)
    -> TcS WantedCtEvidence -> TcS WantedCtEvidence
    forall a b. (a -> b) -> a -> b
    $
        do { new_ev <- Type -> TcS TcTyVar
    newEvVar Type
    pty
           ; traceTcS "Emitting new wanted" (ppr new_ev <+> dcolon <+> ppr pty $$
                                             pprCtLoc loc)
           ; return $ WantedCt { ctev_pred      = pty
                               , ctev_dest      = EvVarDest new_ev
                               , ctev_loc       = loc
                               , ctev_rewriters = rewriters }
           }

    -- | `newWantedEvVar` makes up a fresh evidence variable for the given predicate.
    -- For ClassPred, check if this exact predicate type is in the solved dicts
    --      But do /not/ look in the inert_cans, because doing so bypasses all
    --      all the careful tests in tryInertDicts, leading to
    --      #20666 (prohibitedSuperClassSolve) and #26117 (short-cut solve)
    -- We do no such caching forIPs, holes, or errors; so for anything
    -- except ClassPred, this is the same as newWantedEvVarNC
    --
    -- Don't use this for equality constraints: this function is only for
    -- constraints with 'EvVarDest'.
    newWantedEvVar :: CtLoc -> RewriterSet
                   -> TcPredType -> TcS MaybeNew
    newWantedEvVar :: CtLoc -> RewriterSet -> Type -> TcS MaybeNew
    newWantedEvVar CtLoc
    loc RewriterSet
    rewriters Type
    pty
      = case HasDebugCallStack => Type -> Pred
    Type -> Pred
    classifyPredType Type
    pty of
          EqPred {} -> String -> SDoc -> TcS MaybeNew
    forall a. HasCallStack => String -> SDoc -> a
    pprPanic String
    "newWantedEvVar: HoleDestPred" (Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    pty)
          ClassPred Class
    cls [Type]
    tys
            -> do { inerts <- TcS InertSet
    getInertSet
                  ; case lookupSolvedDict inerts cls tys of
                     Just CtEvidence
    ev -> do { String -> SDoc -> TcS ()
    traceTcS String
    "newWantedEvVar/cache hit" (SDoc -> TcS ()) -> SDoc -> TcS ()
    forall a b. (a -> b) -> a -> b
    $ CtEvidence -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr CtEvidence
    ev
                                   ; MaybeNew -> TcS MaybeNew
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (MaybeNew -> TcS MaybeNew) -> MaybeNew -> TcS MaybeNew
    forall a b. (a -> b) -> a -> b
    $ EvExpr -> MaybeNew
    Cached (HasDebugCallStack => CtEvidence -> EvExpr
    CtEvidence -> EvExpr
    ctEvExpr CtEvidence
    ev) }
                     Maybe CtEvidence
    Nothing -> do { ev <- CtLoc -> RewriterSet -> Type -> TcS WantedCtEvidence
    newWantedEvVarNC CtLoc
    loc RewriterSet
    rewriters Type
    pty
                                   ; return (Fresh ev) } }
          Pred
    _other -> do { ev <- CtLoc -> RewriterSet -> Type -> TcS WantedCtEvidence
    newWantedEvVarNC CtLoc
    loc RewriterSet
    rewriters Type
    pty
                       ; return (Fresh ev) }

    -- | Create a new Wanted constraint, potentially looking up
    -- non-equality constraints in the cache instead of creating
    -- a new one from scratch.
    --
    -- Deals with both equality and non-equality constraints.
    newWanted :: CtLoc -> RewriterSet -> PredType -> TcS MaybeNew
    newWanted :: CtLoc -> RewriterSet -> Type -> TcS MaybeNew
    newWanted CtLoc
    loc RewriterSet
    rewriters Type
    pty
      | Just (Role
    role, Type
    ty1, Type
    ty2) <- Type -> Maybe (Role, Type, Type)
    getEqPredTys_maybe Type
    pty
      = WantedCtEvidence -> MaybeNew
    Fresh (WantedCtEvidence -> MaybeNew)
    -> ((WantedCtEvidence, Coercion) -> WantedCtEvidence)
    -> (WantedCtEvidence, Coercion)
    -> MaybeNew
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . (WantedCtEvidence, Coercion) -> WantedCtEvidence
    forall a b. (a, b) -> a
    fst ((WantedCtEvidence, Coercion) -> MaybeNew)
    -> TcS (WantedCtEvidence, Coercion) -> TcS MaybeNew
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> CtLoc
    -> RewriterSet
    -> Role
    -> Type
    -> Type
    -> TcS (WantedCtEvidence, Coercion)
    newWantedEq CtLoc
    loc RewriterSet
    rewriters Role
    role Type
    ty1 Type
    ty2
      | Bool
    otherwise
      = CtLoc -> RewriterSet -> Type -> TcS MaybeNew
    newWantedEvVar CtLoc
    loc RewriterSet
    rewriters Type
    pty

    -- | Create a new Wanted constraint.
    --
    -- Deals with both equality and non-equality constraints.
    --
    -- Does not attempt to re-use non-equality constraints that already
    -- exist in the inert set.
    newWantedNC :: CtLoc -> RewriterSet -> PredType -> TcS WantedCtEvidence
    newWantedNC :: CtLoc -> RewriterSet -> Type -> TcS WantedCtEvidence
    newWantedNC CtLoc
    loc RewriterSet
    rewriters Type
    pty
      | Just (Role
    role, Type
    ty1, Type
    ty2) <- Type -> Maybe (Role, Type, Type)
    getEqPredTys_maybe Type
    pty
      = (WantedCtEvidence, Coercion) -> WantedCtEvidence
    forall a b. (a, b) -> a
    fst ((WantedCtEvidence, Coercion) -> WantedCtEvidence)
    -> TcS (WantedCtEvidence, Coercion) -> TcS WantedCtEvidence
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> CtLoc
    -> RewriterSet
    -> Role
    -> Type
    -> Type
    -> TcS (WantedCtEvidence, Coercion)
    newWantedEq CtLoc
    loc RewriterSet
    rewriters Role
    role Type
    ty1 Type
    ty2
      | Bool
    otherwise
      = CtLoc -> RewriterSet -> Type -> TcS WantedCtEvidence
    newWantedEvVarNC CtLoc
    loc RewriterSet
    rewriters Type
    pty

    -- | Checks if the depth of the given location is too much. Fails if
    -- it's too big, with an appropriate error message.
    checkReductionDepth :: CtLoc -> TcType   -- ^ type being reduced
                        -> TcS ()
    checkReductionDepth :: CtLoc -> Type -> TcS ()
    checkReductionDepth CtLoc
    loc Type
    ty
      = do { dflags <- TcS DynFlags
    forall (m :: * -> *). HasDynFlags m => m DynFlags
    getDynFlags
           ; when (subGoalDepthExceeded (reductionDepth dflags) (ctLocDepth loc)) $
             wrapErrTcS $ solverDepthError loc ty }

    matchFam :: TyCon -> [Type] -> TcS (Maybe ReductionN)
    matchFam :: TyCon -> [Type] -> TcS (Maybe Reduction)
    matchFam TyCon
    tycon [Type]
    args = TcM (Maybe Reduction) -> TcS (Maybe Reduction)
    forall a. TcM a -> TcS a
    wrapTcS (TcM (Maybe Reduction) -> TcS (Maybe Reduction))
    -> TcM (Maybe Reduction) -> TcS (Maybe Reduction)
    forall a b. (a -> b) -> a -> b
    $ TyCon -> [Type] -> TcM (Maybe Reduction)
    matchFamTcM TyCon
    tycon [Type]
    args

    matchFamTcM :: TyCon -> [Type] -> TcM (Maybe ReductionN)
    -- Given (F tys) return (ty, co), where co :: F tys ~N ty
    matchFamTcM :: TyCon -> [Type] -> TcM (Maybe Reduction)
    matchFamTcM TyCon
    tycon [Type]
    args
      = do { fam_envs <- TcM (FamInstEnv, FamInstEnv)
    FamInst.tcGetFamInstEnvs
           ; let match_fam_result
                  = (FamInstEnv, FamInstEnv)
    -> Role -> TyCon -> [Type] -> Maybe Reduction
    reduceTyFamApp_maybe (FamInstEnv, FamInstEnv)
    fam_envs Role
    Nominal TyCon
    tycon [Type]
    args
           ; TcM.traceTc "matchFamTcM" $
             vcat [ text "Matching:" <+> ppr (mkTyConApp tycon args)
                  , ppr_res match_fam_result ]
           ; return match_fam_result }
      where
        ppr_res :: Maybe Reduction -> SDoc
    ppr_res Maybe Reduction
    Nothing = String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "Match failed"
        ppr_res (Just (Reduction Coercion
    co Type
    ty))
          = SDoc -> Int -> SDoc -> SDoc
    hang (String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "Match succeeded:")
              Int
    2 ([SDoc] -> SDoc
    forall doc. IsDoc doc => [doc] -> doc
    vcat [ String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "Rewrites to:" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    ty
                      , String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "Coercion:" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> Coercion -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Coercion
    co ])

    solverDepthError :: CtLoc -> TcType -> TcM a
    solverDepthError :: forall a. CtLoc -> Type -> TcM a
    solverDepthError CtLoc
    loc Type
    ty
      = CtLoc -> TcM a -> TcM a
    forall a. CtLoc -> TcM a -> TcM a
    TcM.setCtLocM CtLoc
    loc (TcM a -> TcM a) -> TcM a -> TcM a
    forall a b. (a -> b) -> a -> b
    $
        do { (ty, env0) <- ZonkM (Type, TidyEnv) -> TcM (Type, TidyEnv)
    forall a. ZonkM a -> TcM a
    TcM.liftZonkM (ZonkM (Type, TidyEnv) -> TcM (Type, TidyEnv))
    -> ZonkM (Type, TidyEnv) -> TcM (Type, TidyEnv)
    forall a b. (a -> b) -> a -> b
    $
               do { ty   <- Type -> ZonkM Type
    TcM.zonkTcType Type
    ty
                  ; env0 <- TcM.tcInitTidyEnv
                  ; return (ty, env0) }
           ; let (tidy_env, tidy_ty)  = tidyOpenTypeX env0 ty
                 msg = Type -> SubGoalDepth -> TcRnMessage
    TcRnSolverDepthError Type
    tidy_ty SubGoalDepth
    depth
           ; TcM.failWithTcM (tidy_env, msg) }
      where
        depth :: SubGoalDepth
    depth = CtLoc -> SubGoalDepth
    ctLocDepth CtLoc
    loc

    {-
    ************************************************************************
    *                                                                      *
                  Emitting equalities arising from fundeps
    *                                                                      *
    ************************************************************************
    -}

    emitFunDepWanteds :: CtEvidence  -- The work item
                      -> [FunDepEqn (CtLoc, RewriterSet)]
                      -> TcS Bool  -- True <=> some unification happened

    emitFunDepWanteds :: CtEvidence -> [FunDepEqn (CtLoc, RewriterSet)] -> TcS Bool
    emitFunDepWanteds CtEvidence
    _ [] = Bool -> TcS Bool
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return Bool
    False -- common case noop
    -- See Note [FunDep and implicit parameter reactions] in GHC.Tc.Solver.Dict

    emitFunDepWanteds CtEvidence
    ev [FunDepEqn (CtLoc, RewriterSet)]
    fd_eqns
      = CtEvidence -> Role -> (UnifyEnv -> TcM ()) -> TcS Bool
    unifyFunDeps CtEvidence
    ev Role
    Nominal UnifyEnv -> TcM ()
    do_fundeps
      where
        do_fundeps :: UnifyEnv -> TcM ()
        do_fundeps :: UnifyEnv -> TcM ()
    do_fundeps UnifyEnv
    env = (FunDepEqn (CtLoc, RewriterSet) -> TcM ())
    -> [FunDepEqn (CtLoc, RewriterSet)] -> TcM ()
    forall (t :: * -> *) (m :: * -> *) a b.
    (Foldable t, Monad m) =>
    (a -> m b) -> t a -> m ()
    mapM_ (UnifyEnv -> FunDepEqn (CtLoc, RewriterSet) -> TcM ()
    do_one UnifyEnv
    env) [FunDepEqn (CtLoc, RewriterSet)]
    fd_eqns

        do_one :: UnifyEnv -> FunDepEqn (CtLoc, RewriterSet) -> TcM ()
        do_one :: UnifyEnv -> FunDepEqn (CtLoc, RewriterSet) -> TcM ()
    do_one UnifyEnv
    uenv (FDEqn { fd_qtvs :: forall loc. FunDepEqn loc -> [TcTyVar]
    fd_qtvs = [TcTyVar]
    tvs, fd_eqs :: forall loc. FunDepEqn loc -> [TypeEqn]
    fd_eqs = [TypeEqn]
    eqs, fd_loc :: forall loc. FunDepEqn loc -> loc
    fd_loc = (CtLoc
    loc, RewriterSet
    rewriters) })
          = do { eqs' <- [TcTyVar] -> [TypeEqn] -> TcM [TypeEqn]
    instantiate_eqs [TcTyVar]
    tvs ([TypeEqn] -> [TypeEqn]
    forall a. [a] -> [a]
    reverse [TypeEqn]
    eqs)
                         -- (reverse eqs): See Note [Reverse order of fundep equations]
               ; uPairsTcM env_one eqs' }
          where
            env_one :: UnifyEnv
    env_one = UnifyEnv
    uenv { u_rewriters = u_rewriters uenv S.<> rewriters
                           , u_loc       = loc }

        instantiate_eqs :: [TyVar] -> [TypeEqn] -> TcM [TypeEqn]
        instantiate_eqs :: [TcTyVar] -> [TypeEqn] -> TcM [TypeEqn]
    instantiate_eqs [TcTyVar]
    tvs [TypeEqn]
    eqs
          | [TcTyVar] -> Bool
    forall a. [a] -> Bool
    forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [TcTyVar]
    tvs
          = [TypeEqn] -> TcM [TypeEqn]
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return [TypeEqn]
    eqs
          | Bool
    otherwise
          = do { String -> SDoc -> TcM ()
    TcM.traceTc String
    "emitFunDepWanteds 2" ([TcTyVar] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr [TcTyVar]
    tvs SDoc -> SDoc -> SDoc
    forall doc. IsDoc doc => doc -> doc -> doc
    $$ [TypeEqn] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr [TypeEqn]
    eqs)
               ; subst <- Subst -> [TcTyVar] -> TcM Subst
    instFlexiXTcM Subst
    emptySubst [TcTyVar]
    tvs  -- Takes account of kind substitution
               ; return [ Pair (substTyUnchecked subst' ty1) ty2
                               -- ty2 does not mention fd_qtvs, so no need to subst it.
                               -- See GHC.Tc.Instance.Fundeps Note [Improving against instances]
                               --     Wrinkle (1)
                        | Pair ty1 ty2 <- eqs
                        , let subst' = Subst -> VarSet -> Subst
    extendSubstInScopeSet Subst
    subst (Type -> VarSet
    tyCoVarsOfType Type
    ty1) ]
                              -- The free vars of ty1 aren't just fd_qtvs: ty1 is the result
                              -- of matching with the [W] constraint. So we add its free
                              -- vars to InScopeSet, to satisfy substTy's invariants, even
                              -- though ty1 will never (currently) be a poytype, so this
                              -- InScopeSet will never be looked at.
               }

    {-
    ************************************************************************
    *                                                                      *
                  Unification
    *                                                                      *
    ************************************************************************

    Note [wrapUnifierTcS]
    ~~~~~~~~~~~~~~~~~~~
    When decomposing equalities we often create new wanted constraints for
    (s ~ t).  But what if s=t?  Then it'd be faster to return Refl right away.

    Rather than making an equality test (which traverses the structure of the type,
    perhaps fruitlessly), we call uType (via wrapUnifierTcS) to traverse the common
    structure, and bales out when it finds a difference by creating a new deferred
    Wanted constraint.  But where it succeeds in finding common structure, it just
    builds a coercion to reflect it.

    This is all much faster than creating a new constraint, putting it in the
    work list, picking it out, canonicalising it, etc etc.

    Note [unifyFunDeps]
    ~~~~~~~~~~~~~~~~~~~
    The Bool returned by `unifyFunDeps` is True if we have unified a variable
    that occurs in the constraint we are trying to solve; it is not in the
    inert set so `wrapUnifierTcS` won't kick it out.  Instead we want to send it
    back to the start of the pipeline.  Hence the Bool.

    It's vital that we don't return (not (null unified)) because the fundeps
    may create fresh variables; unifying them (alone) should not make us send
    the constraint back to the start, or we'll get an infinite loop.  See
    Note [Fundeps with instances, and equality orientation] in GHC.Tc.Solver.Dict
    and Note [Improvement orientation] in GHC.Tc.Solver.Equality.
    -}

    uPairsTcM :: UnifyEnv -> [TypeEqn] -> TcM ()
    uPairsTcM :: UnifyEnv -> [TypeEqn] -> TcM ()
    uPairsTcM UnifyEnv
    uenv [TypeEqn]
    eqns = (TypeEqn -> TcM Coercion) -> [TypeEqn] -> TcM ()
    forall (t :: * -> *) (m :: * -> *) a b.
    (Foldable t, Monad m) =>
    (a -> m b) -> t a -> m ()
    mapM_ (\(Pair Type
    ty1 Type
    ty2) -> UnifyEnv -> Type -> Type -> TcM Coercion
    uType UnifyEnv
    uenv Type
    ty1 Type
    ty2) [TypeEqn]
    eqns

    unifyFunDeps :: CtEvidence -> Role
                 -> (UnifyEnv -> TcM ())
                 -> TcS Bool
    unifyFunDeps :: CtEvidence -> Role -> (UnifyEnv -> TcM ()) -> TcS Bool
    unifyFunDeps CtEvidence
    ev Role
    role UnifyEnv -> TcM ()
    do_unifications
      = do { (_, _, unified) <- CtEvidence
    -> Role -> (UnifyEnv -> TcM ()) -> TcS ((), Cts, [TcTyVar])
    forall a.
    CtEvidence
    -> Role -> (UnifyEnv -> TcM a) -> TcS (a, Cts, [TcTyVar])
    wrapUnifierTcS CtEvidence
    ev Role
    role UnifyEnv -> TcM ()
    do_unifications
           ; return (any (`elemVarSet` fvs) unified) }
             -- See Note [unifyFunDeps]
      where
        fvs :: VarSet
    fvs = Type -> VarSet
    tyCoVarsOfType (CtEvidence -> Type
    ctEvPred CtEvidence
    ev)

    unifyForAllBody :: CtEvidence -> Role -> (UnifyEnv -> TcM a)
                    -> TcS (a, Cts)
    -- We /return/ the equality constraints we generate,
    -- rather than emitting them into the monad.
    -- See See (SF5) in Note [Solving forall equalities] in GHC.Tc.Solver.Equality
    unifyForAllBody :: forall a. CtEvidence -> Role -> (UnifyEnv -> TcM a) -> TcS (a, Cts)
    unifyForAllBody CtEvidence
    ev Role
    role UnifyEnv -> TcM a
    unify_body
      = do { (res, cts, unified) <- CtEvidence
    -> Role -> (UnifyEnv -> TcM a) -> TcS (a, Cts, [TcTyVar])
    forall a.
    CtEvidence
    -> Role -> (UnifyEnv -> TcM a) -> TcS (a, Cts, [TcTyVar])
    wrapUnifierX CtEvidence
    ev Role
    role UnifyEnv -> TcM a
    unify_body

           -- Kick out any inert constraint that we have unified
           ; _ <- kickOutAfterUnification unified

           ; return (res, cts) }

    wrapUnifierTcS :: CtEvidence -> Role
                   -> (UnifyEnv -> TcM a)  -- Some calls to uType
                   -> TcS (a, Bag Ct, [TcTyVar])
    -- Invokes the do_unifications argument, with a suitable UnifyEnv.
    -- Emit deferred equalities and kick-out from the inert set as a
    -- result of any unifications.
    -- Very good short-cut when the two types are equal, or nearly so
    -- See Note [wrapUnifierTcS]
    --
    -- The [TcTyVar] is the list of unification variables that were
    -- unified the process; the (Bag Ct) are the deferred constraints.

    wrapUnifierTcS :: forall a.
    CtEvidence
    -> Role -> (UnifyEnv -> TcM a) -> TcS (a, Cts, [TcTyVar])
    wrapUnifierTcS CtEvidence
    ev Role
    role UnifyEnv -> TcM a
    do_unifications
      = do { (res, cts, unified) <- CtEvidence
    -> Role -> (UnifyEnv -> TcM a) -> TcS (a, Cts, [TcTyVar])
    forall a.
    CtEvidence
    -> Role -> (UnifyEnv -> TcM a) -> TcS (a, Cts, [TcTyVar])
    wrapUnifierX CtEvidence
    ev Role
    role UnifyEnv -> TcM a
    do_unifications

           -- Emit the deferred constraints
           -- See Note [Work-list ordering] in GHC.Tc.Solved.Equality
           --
           -- All the constraints in `cts` share the same rewriter set so,
           -- rather than looking at it one by one, we pass it to
           -- extendWorkListChildEqs; just a small optimisation.
           ; unless (isEmptyBag cts) $
             updWorkListTcS (extendWorkListChildEqs ev cts)

           -- And kick out any inert constraint that we have unified
           ; _ <- kickOutAfterUnification unified

           ; return (res, cts, unified) }

    wrapUnifierX :: CtEvidence -> Role
                 -> (UnifyEnv -> TcM a)  -- Some calls to uType
                 -> TcS (a, Bag Ct, [TcTyVar])
    wrapUnifierX :: forall a.
    CtEvidence
    -> Role -> (UnifyEnv -> TcM a) -> TcS (a, Cts, [TcTyVar])
    wrapUnifierX CtEvidence
    ev Role
    role UnifyEnv -> TcM a
    do_unifications
      = do { unif_count_ref <- TcS (IORef Int)
    getUnifiedRef
           ; wrapTcS $
             do { defer_ref   <- TcM.newTcRef emptyBag
                ; unified_ref <- TcM.newTcRef []
                ; let env = UE { u_role :: Role
    u_role      = Role
    role
                               , u_rewriters :: RewriterSet
    u_rewriters = CtEvidence -> RewriterSet
    ctEvRewriters CtEvidence
    ev
                               , u_loc :: CtLoc
    u_loc       = CtEvidence -> CtLoc
    ctEvLoc CtEvidence
    ev
                               , u_defer :: TcRef Cts
    u_defer     = TcRef Cts
    defer_ref
                               , u_unified :: Maybe (TcRef [TcTyVar])
    u_unified   = TcRef [TcTyVar] -> Maybe (TcRef [TcTyVar])
    forall a. a -> Maybe a
    Just TcRef [TcTyVar]
    unified_ref}
                  -- u_rewriters: the rewriter set and location from
                  -- the parent constraint `ev` are inherited in any
                  -- new constraints spat out by the unifier

                ; res <- do_unifications env

                ; cts     <- TcM.readTcRef defer_ref
                ; unified <- TcM.readTcRef unified_ref

                -- Don't forget to update the count of variables
                -- unified, lest we forget to iterate (#24146)
                ; unless (null unified) $
                  TcM.updTcRef unif_count_ref (+ (length unified))

                ; return (res, cts, unified) } }

    {-
    ************************************************************************
    *                                                                      *
                  Breaking type variable cycles
    *                                                                      *
    ************************************************************************
    -}

    checkTypeEq :: CtEvidence -> EqRel -> CanEqLHS -> TcType
                -> TcS (PuResult () Reduction)
    -- Used for general CanEqLHSs, ones that do
    -- not have a touchable type variable on the LHS (i.e. not unifying)
    checkTypeEq :: CtEvidence
    -> EqRel -> CanEqLHS -> Type -> TcS (PuResult () Reduction)
    checkTypeEq CtEvidence
    ev EqRel
    eq_rel CanEqLHS
    lhs Type
    rhs =
      case CtEvidence
    ev of
        CtGiven {} ->
          do { String -> SDoc -> TcS ()
    traceTcS String
    "checkTypeEq {" ([SDoc] -> SDoc
    forall doc. IsDoc doc => [doc] -> doc
    vcat [ String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "lhs:" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> CanEqLHS -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr CanEqLHS
    lhs
                                              , String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "rhs:" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    rhs ])
             ; check_result <- TcM (PuResult (TcTyVar, Type) Reduction)
    -> TcS (PuResult (TcTyVar, Type) Reduction)
    forall a. TcM a -> TcS a
    wrapTcS (TyEqFlags (IOEnv (Env TcGblEnv TcLclEnv)) (TcTyVar, Type)
    -> Type -> TcM (PuResult (TcTyVar, Type) Reduction)
    forall (m :: * -> *) a.
    Monad m =>
    TyEqFlags m a -> Type -> m (PuResult a Reduction)
    checkTyEqRhs TyEqFlags (IOEnv (Env TcGblEnv TcLclEnv)) (TcTyVar, Type)
    given_flags Type
    rhs)
             ; traceTcS "checkTypeEq }" (ppr check_result)
             ; case check_result of
                  PuFail CheckTyEqResult
    reason -> PuResult () Reduction -> TcS (PuResult () Reduction)
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (CheckTyEqResult -> PuResult () Reduction
    forall a b. CheckTyEqResult -> PuResult a b
    PuFail CheckTyEqResult
    reason)
                  PuOK Bag (TcTyVar, Type)
    prs Reduction
    redn -> do { new_givens <- ((TcTyVar, Type) -> TcS Ct) -> Bag (TcTyVar, Type) -> TcS Cts
    forall (m :: * -> *) a b.
    Monad m =>
    (a -> m b) -> Bag a -> m (Bag b)
    mapBagM (TcTyVar, Type) -> TcS Ct
    mk_new_given Bag (TcTyVar, Type)
    prs
                                      ; emitWork new_givens
                                      ; updInertSet (addCycleBreakerBindings prs)
                                      ; return (pure redn) } }
        CtWanted {} ->
          do { check_result <- TcM (PuResult Ct Reduction) -> TcS (PuResult Ct Reduction)
    forall a. TcM a -> TcS a
    wrapTcS (TyEqFlags (IOEnv (Env TcGblEnv TcLclEnv)) Ct
    -> Type -> TcM (PuResult Ct Reduction)
    forall (m :: * -> *) a.
    Monad m =>
    TyEqFlags m a -> Type -> m (PuResult a Reduction)
    checkTyEqRhs TyEqFlags (IOEnv (Env TcGblEnv TcLclEnv)) Ct
    wanted_flags Type
    rhs)
             ; case check_result of
                  PuFail CheckTyEqResult
    reason -> PuResult () Reduction -> TcS (PuResult () Reduction)
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (CheckTyEqResult -> PuResult () Reduction
    forall a b. CheckTyEqResult -> PuResult a b
    PuFail CheckTyEqResult
    reason)
                  PuOK Cts
    cts Reduction
    redn -> do { Cts -> TcS ()
    emitWork Cts
    cts
                                      ; PuResult () Reduction -> TcS (PuResult () Reduction)
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Reduction -> PuResult () Reduction
    forall a. a -> PuResult () a
    forall (f :: * -> *) a. Applicative f => a -> f a
    pure Reduction
    redn) } }
      where
        wanted_flags :: TyEqFlags TcM Ct
        wanted_flags :: TyEqFlags (IOEnv (Env TcGblEnv TcLclEnv)) Ct
    wanted_flags = CheckTyEqProblem
    -> CanEqLHS -> TyEqFlags (IOEnv (Env TcGblEnv TcLclEnv)) Ct
    forall (m :: * -> *) a.
    CheckTyEqProblem -> CanEqLHS -> TyEqFlags m a
    notUnifying_TEFTask CheckTyEqProblem
    occ_prob CanEqLHS
    lhs
                       -- checkTypeEq deals only with the non-unifying case

        given_flags :: TyEqFlags TcM (TcTyVar, TcType)
        given_flags :: TyEqFlags (IOEnv (Env TcGblEnv TcLclEnv)) (TcTyVar, Type)
    given_flags = TyEqFlags (IOEnv (Env TcGblEnv TcLclEnv)) Ct
    wanted_flags { tef_fam_app = mkTEFA_Break ev eq_rel BreakGiven }
            -- TEFA_Break used for: [G] a ~ Maybe (F a)
            --                   or [W] F a ~ Maybe (F a)

        -- occ_prob: see Note [Occurs check and representational equality]
        occ_prob :: CheckTyEqProblem
    occ_prob = case EqRel
    eq_rel of
                     EqRel
    NomEq  -> CheckTyEqProblem
    cteInsolubleOccurs
                     EqRel
    ReprEq -> CheckTyEqProblem
    cteSolubleOccurs

        ---------------------------
        mk_new_given :: (TcTyVar, TcType) -> TcS Ct
        mk_new_given :: (TcTyVar, Type) -> TcS Ct
    mk_new_given (TcTyVar
    new_tv, Type
    fam_app)
          = CtEvidence -> Ct
    mkNonCanonical (CtEvidence -> Ct)
    -> (GivenCtEvidence -> CtEvidence) -> GivenCtEvidence -> Ct
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . GivenCtEvidence -> CtEvidence
    CtGiven (GivenCtEvidence -> Ct) -> TcS GivenCtEvidence -> TcS Ct
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> CtLoc -> (Type, EvTerm) -> TcS GivenCtEvidence
    newGivenEvVar CtLoc
    cb_loc (Type
    given_pred, EvTerm
    given_term)
          where
            new_ty :: Type
    new_ty     = TcTyVar -> Type
    mkTyVarTy TcTyVar
    new_tv
            given_pred :: Type
    given_pred = Type -> Type -> Type
    mkNomEqPred Type
    fam_app Type
    new_ty
            given_term :: EvTerm
    given_term = Coercion -> EvTerm
    evCoercion (Coercion -> EvTerm) -> Coercion -> EvTerm
    forall a b. (a -> b) -> a -> b
    $ Type -> Coercion
    mkNomReflCo Type
    new_ty  -- See Detail (4) of Note

        -- See Detail (7) of the Note
        cb_loc :: CtLoc
    cb_loc = CtLoc -> (CtOrigin -> CtOrigin) -> CtLoc
    updateCtLocOrigin (CtEvidence -> CtLoc
    ctEvLoc CtEvidence
    ev) CtOrigin -> CtOrigin
    CycleBreakerOrigin

    -------------------------
    -- | Fill in CycleBreakerTvs with the variables they stand for.
    -- See Note [Type equality cycles] in GHC.Tc.Solver.Equality
    restoreTyVarCycles :: InertSet -> TcM ()
    restoreTyVarCycles :: InertSet -> TcM ()
    restoreTyVarCycles InertSet
    is
      = ZonkM () -> TcM ()
    forall a. ZonkM a -> TcM a
    TcM.liftZonkM
      (ZonkM () -> TcM ()) -> ZonkM () -> TcM ()
    forall a b. (a -> b) -> a -> b
    $ CycleBreakerVarStack -> (TcTyVar -> Type -> ZonkM ()) -> ZonkM ()
    forall (m :: * -> *).
    Monad m =>
    CycleBreakerVarStack -> (TcTyVar -> Type -> m ()) -> m ()
    forAllCycleBreakerBindings_ (InertSet -> CycleBreakerVarStack
    inert_cycle_breakers InertSet
    is) HasDebugCallStack => TcTyVar -> Type -> ZonkM ()
    TcTyVar -> Type -> ZonkM ()
    TcM.writeMetaTyVar
    {-# SPECIALISE forAllCycleBreakerBindings_ ::
          CycleBreakerVarStack -> (TcTyVar -> TcType -> ZonkM ()) -> ZonkM () #-}

    {- Note [Occurs check and representational equality]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    (a ~R# b a) is soluble if b later turns out to be Identity
    So we treat this as a "soluble occurs check".

    Note [Don't cycle-break Wanteds when not unifying]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consdier
      [W] a[2] ~ Maybe (F a[2])

    Should we cycle-break this Wanted, thus?

      [W] a[2] ~ Maybe delta[2]
      [W] delta[2] ~ F a[2]

    For a start, this is dodgy because we might just unify delta, thus undoing
    what we have done, and getting an infinite loop in the solver.  Even if we
    somehow prevented ourselves from doing so, is there any merit in the split?
    Maybe: perhaps we can use that equality on `a` to unlock other constraints?
    Consider
      type instance F (Maybe _) = Bool

      [G] g1: a ~ Maybe Bool
      [W] w1: a ~ Maybe (F a)

    If we loop-break w1 to get
      [W] w1': a ~ Maybe gamma
      [W] w3:  gamma ~ F a
    Now rewrite w3 with w1'
      [W] w3':  gamma ~ F (Maybe gamma)
    Now use the type instance to get
      gamma := Bool
    Now we are left with
      [W] w1': a ~ Maybe Bool
    which we can solve from the Given.

    BUT in this situation we could have rewritten the
    /original/ Wanted from the Given, like this:
      [W] w1': Maybe Bool ~ Maybe (F (Maybe Bool))
    and that is readily soluble.

    In short: loop-breaking Wanteds, when we aren't unifying,
    seems of no merit.  Hence TEFA_Recurse, rather than TEFA_Break,
    in `wanted_flags` in `checkTypeEq`.
    -}