    {-# LANGUAGE CPP #-}
    {-# LANGUAGE GeneralizedNewtypeDeriving #-}

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

    -- | This module defines types and simple operations over constraints, as used
    -- in the type-checker and constraint solver.
    module GHC.Tc.Types.Constraint (
            -- QCInst
            QCInst(..), isPendingScInst,

            -- Canonical constraints
            Xi, Ct(..), Cts,
            emptyCts, andCts, andManyCts, pprCts,
            singleCt, listToCts, ctsElts, consCts, snocCts, extendCtsList,
            isEmptyCts,
            isPendingScDict, superClassesMightHelp, getPendingWantedScs,
            isWantedCt, isDerivedCt, isGivenCt,
            isUserTypeErrorCt, getUserTypeErrorMsg,
            ctEvidence, ctLoc, setCtLoc, ctPred, ctFlavour, ctEqRel, ctOrigin,
            ctEvId, mkTcEqPredLikeEv,
            mkNonCanonical, mkNonCanonicalCt, mkGivens,
            mkIrredCt,
            ctEvPred, ctEvLoc, ctEvOrigin, ctEvEqRel,
            ctEvExpr, ctEvTerm, ctEvCoercion, ctEvEvId,
            tyCoVarsOfCt, tyCoVarsOfCts,
            tyCoVarsOfCtList, tyCoVarsOfCtsList,

            CtIrredReason(..), HoleSet, isInsolubleReason,

            CheckTyEqResult, CheckTyEqProblem, cteProblem, cterClearOccursCheck,
            cteOK, cteImpredicative, cteTypeFamily, cteHoleBlocker,
            cteInsolubleOccurs, cteSolubleOccurs, cterSetOccursCheckSoluble,
            cterHasNoProblem, cterHasProblem, cterHasOnlyProblem,
            cterRemoveProblem, cterHasOccursCheck, cterFromKind,

            CanEqLHS(..), canEqLHS_maybe, canEqLHSKind, canEqLHSType,
            eqCanEqLHS,

            Hole(..), HoleSort(..), isOutOfScopeHole,

            WantedConstraints(..), insolubleWC, emptyWC, isEmptyWC,
            isSolvedWC, andWC, unionsWC, mkSimpleWC, mkImplicWC,
            addInsols, dropMisleading, addSimples, addImplics, addHoles,
            tyCoVarsOfWC, dropDerivedWC, dropDerivedSimples,
            tyCoVarsOfWCList, insolubleCt, insolubleEqCt,
            isDroppableCt, insolubleImplic,
            arisesFromGivens,

            Implication(..), implicationPrototype, checkTelescopeSkol,
            ImplicStatus(..), isInsolubleStatus, isSolvedStatus,
            HasGivenEqs(..),
            SubGoalDepth, initialSubGoalDepth, maxSubGoalDepth,
            bumpSubGoalDepth, subGoalDepthExceeded,
            CtLoc(..), ctLocSpan, ctLocEnv, ctLocLevel, ctLocOrigin,
            ctLocTypeOrKind_maybe,
            ctLocDepth, bumpCtLocDepth, isGivenLoc,
            setCtLocOrigin, updateCtLocOrigin, setCtLocEnv, setCtLocSpan,
            pprCtLoc,

            -- CtEvidence
            CtEvidence(..), TcEvDest(..),
            mkKindLoc, toKindLoc, mkGivenLoc,
            isWanted, isGiven, isDerived,
            ctEvRole,

            wrapType,

            CtFlavour(..), ShadowInfo(..), ctFlavourContainsDerived, ctEvFlavour,
            CtFlavourRole, ctEvFlavourRole, ctFlavourRole,
            eqCanRewrite, eqCanRewriteFR, eqMayRewriteFR,
            eqCanDischargeFR,

            -- Pretty printing
            pprEvVarTheta,
            pprEvVars, pprEvVarWithType,

      )
      where

    #include "HsVersions.h"

    import GHC.Prelude

    import {-# SOURCE #-} GHC.Tc.Types ( TcLclEnv, setLclEnvTcLevel, getLclEnvTcLevel
                                       , setLclEnvLoc, getLclEnvLoc )

    import GHC.Core.Predicate
    import GHC.Core.Type
    import GHC.Core.Coercion
    import GHC.Core.Class
    import GHC.Core.TyCon
    import GHC.Types.Var

    import GHC.Tc.Utils.TcType
    import GHC.Tc.Types.Evidence
    import GHC.Tc.Types.Origin

    import GHC.Core

    import GHC.Core.TyCo.Ppr
    import GHC.Types.Name.Occurrence
    import GHC.Utils.FV
    import GHC.Types.Var.Set
    import GHC.Driver.Session
    import GHC.Types.Basic

    import GHC.Utils.Outputable
    import GHC.Types.SrcLoc
    import GHC.Data.Bag
    import GHC.Utils.Misc
    import GHC.Utils.Panic

    import Control.Monad ( msum )
    import qualified Data.Semigroup ( (<>) )

    -- these are for CheckTyEqResult
    import Data.Word  ( Word8 )
    import Data.List  ( intersperse )

    {-
    ************************************************************************
    *                                                                      *
    *                       Canonical constraints                          *
    *                                                                      *
    *   These are the constraints the low-level simplifier works with      *
    *                                                                      *
    ************************************************************************

    Note [CEqCan occurs check]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    A CEqCan relates a CanEqLHS (a type variable or type family applications) on
    its left to an arbitrary type on its right. It is used for rewriting.
    Because it is used for rewriting, it would be disastrous if the RHS
    were to mention the LHS: this would cause a loop in rewriting.

    We thus perform an occurs-check. There is, of course, some subtlety:

    * For type variables, the occurs-check looks deeply. This is because
      a CEqCan over a meta-variable is also used to inform unification,
      in GHC.Tc.Solver.Interact.solveByUnification. If the LHS appears
      anywhere, at all, in the RHS, unification will create an infinite
      structure, which is bad.

    * For type family applications, the occurs-check is shallow; it looks
      only in places where we might rewrite. (Specifically, it does not
      look in kinds or coercions.) An occurrence of the LHS in, say, an
      RHS coercion is OK, as we do not rewrite in coercions. No loop to
      be found.

      You might also worry about the possibility that a type family
      application LHS doesn't exactly appear in the RHS, but something
      that reduces to the LHS does. Yet that can't happen: the RHS is
      already inert, with all type family redexes reduced. So a simple
      syntactic check is just fine.

    The occurs check is performed in GHC.Tc.Utils.Unify.checkTypeEq
    and forms condition T3 in Note [Extending the inert equalities]
    in GHC.Tc.Solver.Monad.

    -}

    -- | A 'Xi'-type is one that has been fully rewritten with respect
    -- to the inert set; that is, it has been rewritten by the algorithm
    -- in GHC.Tc.Solver.Rewrite. (Historical note: 'Xi', for years and years,
    -- meant that a type was type-family-free. It does *not* mean this
    -- any more.)
    type Xi = TcType

    type Cts = Bag Ct

    data Ct
      -- Atomic canonical constraints
      = CDictCan {  -- e.g.  Num ty
          Ct -> CtEvidence
    cc_ev     :: CtEvidence, -- See Note [Ct/evidence invariant]

          Ct -> Class
    cc_class  :: Class,
          Ct -> [Xi]
    cc_tyargs :: [Xi],   -- cc_tyargs are rewritten w.r.t. inerts, so Xi

          Ct -> Bool
    cc_pend_sc :: Bool   -- See Note [The superclass story] in GHC.Tc.Solver.Canonical
                               -- True <=> (a) cc_class has superclasses
                               --          (b) we have not (yet) added those
                               --              superclasses as Givens
        }

      | CIrredCan {  -- These stand for yet-unusable predicates
          cc_ev     :: CtEvidence,   -- See Note [Ct/evidence invariant]
          Ct -> CtIrredReason
    cc_reason :: CtIrredReason

            -- For the might-be-soluble case, the ctev_pred of the evidence is
            -- of form   (tv xi1 xi2 ... xin)   with a tyvar at the head
            --      or   (lhs1 ~ ty2)  where the CEqCan    kind invariant (TyEq:K) fails
            -- See Note [CIrredCan constraints]

            -- The definitely-insoluble case is for things like
            --    Int ~ Bool      tycons don't match
            --    a ~ [a]         occurs check
        }

      | CEqCan {  -- CanEqLHS ~ rhs
           -- Invariants:
           --   * See Note [inert_eqs: the inert equalities] in GHC.Tc.Solver.Monad
           --   * Many are checked in checkTypeEq in GHC.Tc.Utils.Unify
           --   * (TyEq:OC) lhs does not occur in rhs (occurs check)
           --               Note [CEqCan occurs check]
           --   * (TyEq:F) rhs has no foralls
           --       (this avoids substituting a forall for the tyvar in other types)
           --   * (TyEq:K) tcTypeKind lhs `tcEqKind` tcTypeKind rhs; Note [Ct kind invariant]
           --   * (TyEq:N) If the equality is representational, rhs has no top-level newtype
           --     See Note [No top-level newtypes on RHS of representational equalities]
           --     in GHC.Tc.Solver.Canonical. (Applies only when constructor of newtype is
           --     in scope.)
           --   * (TyEq:TV) If rhs (perhaps under a cast) is also CanEqLHS, then it is oriented
           --     to give best chance of
           --     unification happening; eg if rhs is touchable then lhs is too
           --     Note [TyVar/TyVar orientation] in GHC.Tc.Utils.Unify
           --   * (TyEq:H) The RHS has no blocking coercion holes. See GHC.Tc.Solver.Canonical
           --     Note [Equalities with incompatible kinds], wrinkle (2)
          cc_ev     :: CtEvidence, -- See Note [Ct/evidence invariant]
          Ct -> CanEqLHS
    cc_lhs    :: CanEqLHS,
          Ct -> Xi
    cc_rhs    :: Xi,         -- See invariants above

          Ct -> EqRel
    cc_eq_rel :: EqRel       -- INVARIANT: cc_eq_rel = ctEvEqRel cc_ev
        }

      | CNonCanonical {        -- See Note [NonCanonical Semantics] in GHC.Tc.Solver.Monad
          cc_ev  :: CtEvidence
        }

      | CQuantCan QCInst       -- A quantified constraint
          -- NB: I expect to make more of the cases in Ct
          --     look like this, with the payload in an
          --     auxiliary type

    ------------
    -- | A 'CanEqLHS' is a type that can appear on the left of a canonical
    -- equality: a type variable or exactly-saturated type family application.
    data CanEqLHS
      = TyVarLHS TcTyVar
      | TyFamLHS TyCon  -- ^ of the family
                 [Xi]   -- ^ exactly saturating the family

    instance Outputable CanEqLHS where
      ppr :: CanEqLHS -> SDoc
    ppr (TyVarLHS EvVar
    tv)              = forall a. Outputable a => a -> SDoc
    ppr EvVar
    tv
      ppr (TyFamLHS TyCon
    fam_tc [Xi]
    fam_args) = forall a. Outputable a => a -> SDoc
    ppr (TyCon -> [Xi] -> Xi
    mkTyConApp TyCon
    fam_tc [Xi]
    fam_args)

    ------------
    data QCInst  -- A much simplified version of ClsInst
                 -- See Note [Quantified constraints] in GHC.Tc.Solver.Canonical
      = QCI { QCInst -> CtEvidence
    qci_ev   :: CtEvidence -- Always of type forall tvs. context => ty
                                     -- Always Given
            , QCInst -> [EvVar]
    qci_tvs  :: [TcTyVar]  -- The tvs
            , QCInst -> Xi
    qci_pred :: TcPredType -- The ty
            , QCInst -> Bool
    qci_pend_sc :: Bool    -- Same as cc_pend_sc flag in CDictCan
                                     -- Invariant: True => qci_pred is a ClassPred
        }

    instance Outputable QCInst where
      ppr :: QCInst -> SDoc
    ppr (QCI { qci_ev :: QCInst -> CtEvidence
    qci_ev = CtEvidence
    ev }) = forall a. Outputable a => a -> SDoc
    ppr CtEvidence
    ev

    ------------
    -- | A hole stores the information needed to report diagnostics
    -- about holes in terms (unbound identifiers or underscores) or
    -- in types (also called wildcards, as used in partial type
    -- signatures). See Note [Holes].
    data Hole
      = Hole { Hole -> HoleSort
    hole_sort :: HoleSort -- ^ What flavour of hole is this?
             , Hole -> OccName
    hole_occ  :: OccName  -- ^ The name of this hole
             , Hole -> Xi
    hole_ty   :: TcType   -- ^ Type to be printed to the user
                                     -- For expression holes: type of expr
                                     -- For type holes: the missing type
             , Hole -> CtLoc
    hole_loc  :: CtLoc    -- ^ Where hole was written
             }
               -- For the hole_loc, we usually only want the TcLclEnv stored within.
               -- Except when we rewrite, where we need a whole location. And this
               -- might get reported to the user if reducing type families in a
               -- hole type loops.

    -- | Used to indicate which sort of hole we have.
    data HoleSort = ExprHole HoleExprRef
                     -- ^ Either an out-of-scope variable or a "true" hole in an
                     -- expression (TypedHoles).
                     -- The HoleExprRef says where to write the
                     -- the erroring expression for -fdefer-type-errors.
                  | TypeHole
                     -- ^ A hole in a type (PartialTypeSignatures)
                  | ConstraintHole
                     -- ^ A hole in a constraint, like @f :: (_, Eq a) => ...
                     -- Differentiated from TypeHole because a ConstraintHole
                     -- is simplified differently. See
                     -- Note [Do not simplify ConstraintHoles] in GHC.Tc.Solver.

    instance Outputable Hole where
      ppr :: Hole -> SDoc
    ppr (Hole { hole_sort :: Hole -> HoleSort
    hole_sort = ExprHole HoleExprRef
    ref
                , hole_occ :: Hole -> OccName
    hole_occ  = OccName
    occ
                , hole_ty :: Hole -> Xi
    hole_ty   = Xi
    ty })
        = SDoc -> SDoc
    parens forall a b. (a -> b) -> a -> b
    $ (SDoc -> SDoc
    braces forall a b. (a -> b) -> a -> b
    $ forall a. Outputable a => a -> SDoc
    ppr OccName
    occ SDoc -> SDoc -> SDoc
    <> SDoc
    colon SDoc -> SDoc -> SDoc
    <> forall a. Outputable a => a -> SDoc
    ppr HoleExprRef
    ref) SDoc -> SDoc -> SDoc
    <+> SDoc
    dcolon SDoc -> SDoc -> SDoc
    <+> forall a. Outputable a => a -> SDoc
    ppr Xi
    ty
      ppr (Hole { hole_sort :: Hole -> HoleSort
    hole_sort = HoleSort
    _other
                , hole_occ :: Hole -> OccName
    hole_occ  = OccName
    occ
                , hole_ty :: Hole -> Xi
    hole_ty   = Xi
    ty })
        = SDoc -> SDoc
    braces forall a b. (a -> b) -> a -> b
    $ forall a. Outputable a => a -> SDoc
    ppr OccName
    occ SDoc -> SDoc -> SDoc
    <> SDoc
    colon SDoc -> SDoc -> SDoc
    <> forall a. Outputable a => a -> SDoc
    ppr Xi
    ty

    instance Outputable HoleSort where
      ppr :: HoleSort -> SDoc
    ppr (ExprHole HoleExprRef
    ref) = String -> SDoc
    text String
    "ExprHole:" SDoc -> SDoc -> SDoc
    <+> forall a. Outputable a => a -> SDoc
    ppr HoleExprRef
    ref
      ppr HoleSort
    TypeHole       = String -> SDoc
    text String
    "TypeHole"
      ppr HoleSort
    ConstraintHole = String -> SDoc
    text String
    "ConstraintHole"

    ------------
    -- | Used to indicate extra information about why a CIrredCan is irreducible
    data CtIrredReason
      = IrredShapeReason
          -- ^ this constraint has a non-canonical shape (e.g. @c Int@, for a variable @c@)

      | HoleBlockerReason HoleSet
         -- ^ this constraint is blocked on the coercion hole(s) listed
         -- See Note [Equalities with incompatible kinds] in GHC.Tc.Solver.Canonical
         -- Wrinkle (4a). Why store the HoleSet? See Wrinkle (2) of that
         -- same Note.
         -- INVARIANT: A HoleBlockerReason constraint is a homogeneous equality whose
         --   left hand side can fit in a CanEqLHS.

      | NonCanonicalReason CheckTyEqResult
       -- ^ an equality where some invariant other than (TyEq:H) of 'CEqCan' is not satisfied;
       -- the 'CheckTyEqResult' states exactly why
       -- INVARIANT: the 'CheckTyEqResult' has some bit set other than cteHoleBlocker

      | ReprEqReason
        -- ^ an equality that cannot be decomposed because it is representational.
        -- Example: @a b ~R# Int@.
        -- These might still be solved later.
        -- INVARIANT: The constraint is a representational equality constraint

      | ShapeMismatchReason
        -- ^ a nominal equality that relates two wholly different types,
        -- like @Int ~# Bool@ or @a b ~# 3@.
        -- INVARIANT: The constraint is a nominal equality constraint

      | AbstractTyConReason
        -- ^ an equality like @T a b c ~ Q d e@ where either @T@ or @Q@
        -- is an abstract type constructor. See Note [Skolem abstract data]
        -- in GHC.Core.TyCon.
        -- INVARIANT: The constraint is an equality constraint between two TyConApps

    instance Outputable CtIrredReason where
      ppr :: CtIrredReason -> SDoc
    ppr CtIrredReason
    IrredShapeReason          = String -> SDoc
    text String
    "(irred)"
      ppr (HoleBlockerReason HoleSet
    holes) = SDoc -> SDoc
    parens (String -> SDoc
    text String
    "blocked on" SDoc -> SDoc -> SDoc
    <+> forall a. Outputable a => a -> SDoc
    ppr HoleSet
    holes)
      ppr (NonCanonicalReason CheckTyEqResult
    cter) = forall a. Outputable a => a -> SDoc
    ppr CheckTyEqResult
    cter
      ppr CtIrredReason
    ReprEqReason              = String -> SDoc
    text String
    "(repr)"
      ppr CtIrredReason
    ShapeMismatchReason       = String -> SDoc
    text String
    "(shape)"
      ppr CtIrredReason
    AbstractTyConReason       = String -> SDoc
    text String
    "(abstc)"

    -- | Are we sure that more solving will never solve this constraint?
    isInsolubleReason :: CtIrredReason -> Bool
    isInsolubleReason :: CtIrredReason -> Bool
    isInsolubleReason CtIrredReason
    IrredShapeReason          = Bool
    False
    isInsolubleReason (HoleBlockerReason {})    = Bool
    False
    isInsolubleReason (NonCanonicalReason CheckTyEqResult
    cter) = CheckTyEqResult -> Bool
    cterIsInsoluble CheckTyEqResult
    cter
    isInsolubleReason CtIrredReason
    ReprEqReason              = Bool
    False
    isInsolubleReason CtIrredReason
    ShapeMismatchReason       = Bool
    True
    isInsolubleReason CtIrredReason
    AbstractTyConReason       = Bool
    True

    ------------------------------------------------------------------------------
    --
    -- CheckTyEqResult, defined here because it is stored in a CtIrredReason
    --
    ------------------------------------------------------------------------------

    -- | A set of problems in checking the validity of a type equality.
    -- See 'checkTypeEq'.
    newtype CheckTyEqResult = CTER Word8

    -- | No problems in checking the validity of a type equality.
    cteOK :: CheckTyEqResult
    cteOK :: CheckTyEqResult
    cteOK = Word8 -> CheckTyEqResult
    CTER forall a. Bits a => a
    zeroBits

    -- | Check whether a 'CheckTyEqResult' is marked successful.
    cterHasNoProblem :: CheckTyEqResult -> Bool
    cterHasNoProblem :: CheckTyEqResult -> Bool
    cterHasNoProblem (CTER Word8
    0) = Bool
    True
    cterHasNoProblem CheckTyEqResult
    _        = Bool
    False

    -- | An individual problem that might be logged in a 'CheckTyEqResult'
    newtype CheckTyEqProblem = CTEP Word8

    cteImpredicative, cteTypeFamily, cteHoleBlocker, cteInsolubleOccurs,
      cteSolubleOccurs :: CheckTyEqProblem
    cteImpredicative :: CheckTyEqProblem
    cteImpredicative   = Word8 -> CheckTyEqProblem
    CTEP (forall a. Bits a => Int -> a
    bit Int
    0)   -- forall or (=>) encountered
    cteTypeFamily :: CheckTyEqProblem
    cteTypeFamily      = Word8 -> CheckTyEqProblem
    CTEP (forall a. Bits a => Int -> a
    bit Int
    1)   -- type family encountered
    cteHoleBlocker :: CheckTyEqProblem
    cteHoleBlocker     = Word8 -> CheckTyEqProblem
    CTEP (forall a. Bits a => Int -> a
    bit Int
    2)   -- blocking coercion hole
          -- See Note [Equalities with incompatible kinds] in GHC.Tc.Solver.Canonical
    cteInsolubleOccurs :: CheckTyEqProblem
    cteInsolubleOccurs = Word8 -> CheckTyEqProblem
    CTEP (forall a. Bits a => Int -> a
    bit Int
    3)   -- occurs-check
    cteSolubleOccurs :: CheckTyEqProblem
    cteSolubleOccurs   = Word8 -> CheckTyEqProblem
    CTEP (forall a. Bits a => Int -> a
    bit Int
    4)   -- occurs-check under a type function or in a coercion
                                        -- must be one bit to the left of cteInsolubleOccurs
    -- See also Note [Insoluble occurs check] in GHC.Tc.Errors

    cteProblem :: CheckTyEqProblem -> CheckTyEqResult
    cteProblem :: CheckTyEqProblem -> CheckTyEqResult
    cteProblem (CTEP Word8
    mask) = Word8 -> CheckTyEqResult
    CTER Word8
    mask

    occurs_mask :: Word8
    occurs_mask :: Word8
    occurs_mask = Word8
    insoluble_mask forall a. Bits a => a -> a -> a
    .|. Word8
    soluble_mask
      where
        CTEP Word8
    insoluble_mask = CheckTyEqProblem
    cteInsolubleOccurs
        CTEP Word8
    soluble_mask   = CheckTyEqProblem
    cteSolubleOccurs

    -- | Check whether a 'CheckTyEqResult' has a 'CheckTyEqProblem'
    cterHasProblem :: CheckTyEqResult -> CheckTyEqProblem -> Bool
    CTER Word8
    bits cterHasProblem :: CheckTyEqResult -> CheckTyEqProblem -> Bool
    `cterHasProblem` CTEP Word8
    mask = (Word8
    bits forall a. Bits a => a -> a -> a
    .&. Word8
    mask) forall a. Eq a => a -> a -> Bool
    /= Word8
    0

    -- | Check whether a 'CheckTyEqResult' has one 'CheckTyEqProblem' and no other
    cterHasOnlyProblem :: CheckTyEqResult -> CheckTyEqProblem -> Bool
    CTER Word8
    bits cterHasOnlyProblem :: CheckTyEqResult -> CheckTyEqProblem -> Bool
    `cterHasOnlyProblem` CTEP Word8
    mask = Word8
    bits forall a. Eq a => a -> a -> Bool
    == Word8
    mask

    cterRemoveProblem :: CheckTyEqResult -> CheckTyEqProblem -> CheckTyEqResult
    cterRemoveProblem :: CheckTyEqResult -> CheckTyEqProblem -> CheckTyEqResult
    cterRemoveProblem (CTER Word8
    bits) (CTEP Word8
    mask) = Word8 -> CheckTyEqResult
    CTER (Word8
    bits forall a. Bits a => a -> a -> a
    .&. forall a. Bits a => a -> a
    complement Word8
    mask)

    cterHasOccursCheck :: CheckTyEqResult -> Bool
    cterHasOccursCheck :: CheckTyEqResult -> Bool
    cterHasOccursCheck (CTER Word8
    bits) = (Word8
    bits forall a. Bits a => a -> a -> a
    .&. Word8
    occurs_mask) forall a. Eq a => a -> a -> Bool
    /= Word8
    0

    cterClearOccursCheck :: CheckTyEqResult -> CheckTyEqResult
    cterClearOccursCheck :: CheckTyEqResult -> CheckTyEqResult
    cterClearOccursCheck (CTER Word8
    bits) = Word8 -> CheckTyEqResult
    CTER (Word8
    bits forall a. Bits a => a -> a -> a
    .&. forall a. Bits a => a -> a
    complement Word8
    occurs_mask)

    -- | Mark a 'CheckTyEqResult' as not having an insoluble occurs-check: any occurs
    -- check under a type family or in a representation equality is soluble.
    cterSetOccursCheckSoluble :: CheckTyEqResult -> CheckTyEqResult
    cterSetOccursCheckSoluble :: CheckTyEqResult -> CheckTyEqResult
    cterSetOccursCheckSoluble (CTER Word8
    bits)
      = Word8 -> CheckTyEqResult
    CTER forall a b. (a -> b) -> a -> b
    $ ((Word8
    bits forall a. Bits a => a -> a -> a
    .&. Word8
    insoluble_mask) forall a. Bits a => a -> Int -> a
    `shift` Int
    1) forall a. Bits a => a -> a -> a
    .|. (Word8
    bits forall a. Bits a => a -> a -> a
    .&. forall a. Bits a => a -> a
    complement Word8
    insoluble_mask)
      where
        CTEP Word8
    insoluble_mask = CheckTyEqProblem
    cteInsolubleOccurs

    -- | Retain only information about occurs-check failures, because only that
    -- matters after recurring into a kind.
    cterFromKind :: CheckTyEqResult -> CheckTyEqResult
    cterFromKind :: CheckTyEqResult -> CheckTyEqResult
    cterFromKind (CTER Word8
    bits)
      = Word8 -> CheckTyEqResult
    CTER (Word8
    bits forall a. Bits a => a -> a -> a
    .&. Word8
    occurs_mask)

    cterIsInsoluble :: CheckTyEqResult -> Bool
    cterIsInsoluble :: CheckTyEqResult -> Bool
    cterIsInsoluble (CTER Word8
    bits) = (Word8
    bits forall a. Bits a => a -> a -> a
    .&. Word8
    mask) forall a. Eq a => a -> a -> Bool
    /= Word8
    0
      where
        mask :: Word8
    mask = Word8
    impredicative_mask forall a. Bits a => a -> a -> a
    .|. Word8
    insoluble_occurs_mask

        CTEP Word8
    impredicative_mask    = CheckTyEqProblem
    cteImpredicative
        CTEP Word8
    insoluble_occurs_mask = CheckTyEqProblem
    cteInsolubleOccurs

    instance Semigroup CheckTyEqResult where
      CTER Word8
    bits1 <> :: CheckTyEqResult -> CheckTyEqResult -> CheckTyEqResult
    <> CTER Word8
    bits2 = Word8 -> CheckTyEqResult
    CTER (Word8
    bits1 forall a. Bits a => a -> a -> a
    .|. Word8
    bits2)
    instance Monoid CheckTyEqResult where
      mempty :: CheckTyEqResult
    mempty = CheckTyEqResult
    cteOK

    instance Outputable CheckTyEqResult where
      ppr :: CheckTyEqResult -> SDoc
    ppr CheckTyEqResult
    cter | CheckTyEqResult -> Bool
    cterHasNoProblem CheckTyEqResult
    cter = String -> SDoc
    text String
    "cteOK"
               | Bool
    otherwise
               = SDoc -> SDoc
    parens forall a b. (a -> b) -> a -> b
    $ [SDoc] -> SDoc
    fcat forall a b. (a -> b) -> a -> b
    $ forall a. a -> [a] -> [a]
    intersperse SDoc
    vbar forall a b. (a -> b) -> a -> b
    $ [SDoc]
    set_bits
        where
          all_bits :: [(CheckTyEqProblem, String)]
    all_bits = [ (CheckTyEqProblem
    cteImpredicative,   String
    "cteImpredicative")
                     , (CheckTyEqProblem
    cteTypeFamily,      String
    "cteTypeFamily")
                     , (CheckTyEqProblem
    cteHoleBlocker,     String
    "cteHoleBlocker")
                     , (CheckTyEqProblem
    cteInsolubleOccurs, String
    "cteInsolubleOccurs")
                     , (CheckTyEqProblem
    cteSolubleOccurs,   String
    "cteSolubleOccurs") ]
          set_bits :: [SDoc]
    set_bits = [ String -> SDoc
    text String
    str
                     | (CheckTyEqProblem
    bitmask, String
    str) <- [(CheckTyEqProblem, String)]
    all_bits
                     , CheckTyEqResult
    cter CheckTyEqResult -> CheckTyEqProblem -> Bool
    `cterHasProblem` CheckTyEqProblem
    bitmask ]

    {- Note [CIrredCan constraints]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    CIrredCan constraints are used for constraints that are "stuck"
       - we can't solve them (yet)
       - we can't use them to solve other constraints
       - but they may become soluble if we substitute for some
         of the type variables in the constraint

    Example 1:  (c Int), where c :: * -> Constraint.  We can't do anything
                with this yet, but if later c := Num, *then* we can solve it

    Example 2:  a ~ b, where a :: *, b :: k, where k is a kind variable
                We don't want to use this to substitute 'b' for 'a', in case
                'k' is subsequently unified with (say) *->*, because then
                we'd have ill-kinded types floating about.  Rather we want
                to defer using the equality altogether until 'k' get resolved.

    Note [Ct/evidence invariant]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If  ct :: Ct, then extra fields of 'ct' cache precisely the ctev_pred field
    of (cc_ev ct), and is fully rewritten wrt the substitution.   Eg for CDictCan,
       ctev_pred (cc_ev ct) = (cc_class ct) (cc_tyargs ct)
    This holds by construction; look at the unique place where CDictCan is
    built (in GHC.Tc.Solver.Canonical).

    In contrast, the type of the evidence *term* (ctev_dest / ctev_evar) in
    the evidence may *not* be fully zonked; we are careful not to look at it
    during constraint solving. See Note [Evidence field of CtEvidence].

    Note [Ct kind invariant]
    ~~~~~~~~~~~~~~~~~~~~~~~~
    CEqCan requires that the kind of the lhs matches the kind
    of the rhs. This is necessary because these constraints are used for substitutions
    during solving. If the kinds differed, then the substitution would take a well-kinded
    type to an ill-kinded one.

    Note [Holes]
    ~~~~~~~~~~~~
    This Note explains how GHC tracks *holes*.

    A hole represents one of two conditions:
     - A missing bit of an expression. Example: foo x = x + _
     - A missing bit of a type. Example: bar :: Int -> _

    What these have in common is that both cause GHC to emit a diagnostic to the
    user describing the bit that is left out.

    When a hole is encountered, a new entry of type Hole is added to the ambient
    WantedConstraints. The type (hole_ty) of the hole is then simplified during
    solving (with respect to any Givens in surrounding implications). It is
    reported with all the other errors in GHC.Tc.Errors.

    For expression holes, the user has the option of deferring errors until runtime
    with -fdefer-type-errors. In this case, the hole actually has evidence: this
    evidence is an erroring expression that prints an error and crashes at runtime.
    The ExprHole variant of holes stores an IORef EvTerm that will contain this evidence;
    during constraint generation, this IORef was stored in the HsUnboundVar extension
    field by the type checker. The desugarer simply dereferences to get the CoreExpr.

    Prior to fixing #17812, we used to invent an Id to hold the erroring
    expression, and then bind it during type-checking. But this does not support
    levity-polymorphic out-of-scope identifiers. See
    typecheck/should_compile/T17812. We thus use the mutable-CoreExpr approach
    described above.

    You might think that the type in the HoleExprRef is the same as the type of the
    hole. However, because the hole type (hole_ty) is rewritten with respect to
    givens, this might not be the case. That is, the hole_ty is always (~) to the
    type of the HoleExprRef, but they might not be `eqType`. We need the type of the generated
    evidence to match what is expected in the context of the hole, and so we must
    store these types separately.

    Type-level holes have no evidence at all.
    -}

    mkNonCanonical :: CtEvidence -> Ct
    mkNonCanonical :: CtEvidence -> Ct
    mkNonCanonical CtEvidence
    ev = CNonCanonical { cc_ev :: CtEvidence
    cc_ev = CtEvidence
    ev }

    mkNonCanonicalCt :: Ct -> Ct
    mkNonCanonicalCt :: Ct -> Ct
    mkNonCanonicalCt Ct
    ct = CNonCanonical { cc_ev :: CtEvidence
    cc_ev = Ct -> CtEvidence
    cc_ev Ct
    ct }

    mkIrredCt :: CtIrredReason -> CtEvidence -> Ct
    mkIrredCt :: CtIrredReason -> CtEvidence -> Ct
    mkIrredCt CtIrredReason
    reason CtEvidence
    ev = CIrredCan { cc_ev :: CtEvidence
    cc_ev = CtEvidence
    ev, cc_reason :: CtIrredReason
    cc_reason = CtIrredReason
    reason }

    mkGivens :: CtLoc -> [EvId] -> [Ct]
    mkGivens :: CtLoc -> [EvVar] -> [Ct]
    mkGivens CtLoc
    loc [EvVar]
    ev_ids
      = forall a b. (a -> b) -> [a] -> [b]
    map EvVar -> Ct
    mk [EvVar]
    ev_ids
      where
        mk :: EvVar -> Ct
    mk EvVar
    ev_id = CtEvidence -> Ct
    mkNonCanonical (CtGiven { ctev_evar :: EvVar
    ctev_evar = EvVar
    ev_id
                                           , ctev_pred :: Xi
    ctev_pred = EvVar -> Xi
    evVarPred EvVar
    ev_id
                                           , ctev_loc :: CtLoc
    ctev_loc = CtLoc
    loc })

    ctEvidence :: Ct -> CtEvidence
    ctEvidence :: Ct -> CtEvidence
    ctEvidence (CQuantCan (QCI { qci_ev :: QCInst -> CtEvidence
    qci_ev = CtEvidence
    ev })) = CtEvidence
    ev
    ctEvidence Ct
    ct = Ct -> CtEvidence
    cc_ev Ct
    ct

    ctLoc :: Ct -> CtLoc
    ctLoc :: Ct -> CtLoc
    ctLoc = CtEvidence -> CtLoc
    ctEvLoc forall b c a. (b -> c) -> (a -> b) -> a -> c
    . Ct -> CtEvidence
    ctEvidence

    setCtLoc :: Ct -> CtLoc -> Ct
    setCtLoc :: Ct -> CtLoc -> Ct
    setCtLoc Ct
    ct CtLoc
    loc = Ct
    ct { cc_ev :: CtEvidence
    cc_ev = (Ct -> CtEvidence
    cc_ev Ct
    ct) { ctev_loc :: CtLoc
    ctev_loc = CtLoc
    loc } }

    ctOrigin :: Ct -> CtOrigin
    ctOrigin :: Ct -> CtOrigin
    ctOrigin = CtLoc -> CtOrigin
    ctLocOrigin forall b c a. (b -> c) -> (a -> b) -> a -> c
    . Ct -> CtLoc
    ctLoc

    ctPred :: Ct -> PredType
    -- See Note [Ct/evidence invariant]
    ctPred :: Ct -> Xi
    ctPred Ct
    ct = CtEvidence -> Xi
    ctEvPred (Ct -> CtEvidence
    ctEvidence Ct
    ct)

    ctEvId :: Ct -> EvVar
    -- The evidence Id for this Ct
    ctEvId :: Ct -> EvVar
    ctEvId Ct
    ct = CtEvidence -> EvVar
    ctEvEvId (Ct -> CtEvidence
    ctEvidence Ct
    ct)

    -- | Makes a new equality predicate with the same role as the given
    -- evidence.
    mkTcEqPredLikeEv :: CtEvidence -> TcType -> TcType -> TcType
    mkTcEqPredLikeEv :: CtEvidence -> Xi -> Xi -> Xi
    mkTcEqPredLikeEv CtEvidence
    ev
      = case Xi -> EqRel
    predTypeEqRel Xi
    pred of
          EqRel
    NomEq  -> Xi -> Xi -> Xi
    mkPrimEqPred
          EqRel
    ReprEq -> Xi -> Xi -> Xi
    mkReprPrimEqPred
      where
        pred :: Xi
    pred = CtEvidence -> Xi
    ctEvPred CtEvidence
    ev

    -- | Get the flavour of the given 'Ct'
    ctFlavour :: Ct -> CtFlavour
    ctFlavour :: Ct -> CtFlavour
    ctFlavour = CtEvidence -> CtFlavour
    ctEvFlavour forall b c a. (b -> c) -> (a -> b) -> a -> c
    . Ct -> CtEvidence
    ctEvidence

    -- | Get the equality relation for the given 'Ct'
    ctEqRel :: Ct -> EqRel
    ctEqRel :: Ct -> EqRel
    ctEqRel = CtEvidence -> EqRel
    ctEvEqRel forall b c a. (b -> c) -> (a -> b) -> a -> c
    . Ct -> CtEvidence
    ctEvidence

    instance Outputable Ct where
      ppr :: Ct -> SDoc
    ppr Ct
    ct = forall a. Outputable a => a -> SDoc
    ppr (Ct -> CtEvidence
    ctEvidence Ct
    ct) SDoc -> SDoc -> SDoc
    <+> SDoc -> SDoc
    parens SDoc
    pp_sort
        where
          pp_sort :: SDoc
    pp_sort = case Ct
    ct of
             CEqCan {}        -> String -> SDoc
    text String
    "CEqCan"
             CNonCanonical {} -> String -> SDoc
    text String
    "CNonCanonical"
             CDictCan { cc_pend_sc :: Ct -> Bool
    cc_pend_sc = Bool
    pend_sc }
                | Bool
    pend_sc   -> String -> SDoc
    text String
    "CDictCan(psc)"
                | Bool
    otherwise -> String -> SDoc
    text String
    "CDictCan"
             CIrredCan { cc_reason :: Ct -> CtIrredReason
    cc_reason = CtIrredReason
    reason } -> String -> SDoc
    text String
    "CIrredCan" SDoc -> SDoc -> SDoc
    <> forall a. Outputable a => a -> SDoc
    ppr CtIrredReason
    reason
             CQuantCan (QCI { qci_pend_sc :: QCInst -> Bool
    qci_pend_sc = Bool
    pend_sc })
                | Bool
    pend_sc   -> String -> SDoc
    text String
    "CQuantCan(psc)"
                | Bool
    otherwise -> String -> SDoc
    text String
    "CQuantCan"

    -----------------------------------
    -- | Is a type a canonical LHS? That is, is it a tyvar or an exactly-saturated
    -- type family application?
    -- Does not look through type synonyms.
    canEqLHS_maybe :: Xi -> Maybe CanEqLHS
    canEqLHS_maybe :: Xi -> Maybe CanEqLHS
    canEqLHS_maybe Xi
    xi
      | Just EvVar
    tv <- Xi -> Maybe EvVar
    tcGetTyVar_maybe Xi
    xi
      = forall a. a -> Maybe a
    Just forall a b. (a -> b) -> a -> b
    $ EvVar -> CanEqLHS
    TyVarLHS EvVar
    tv

      | Just (TyCon
    tc, [Xi]
    args) <- HasCallStack => Xi -> Maybe (TyCon, [Xi])
    tcSplitTyConApp_maybe Xi
    xi
      , TyCon -> Bool
    isTypeFamilyTyCon TyCon
    tc
      , [Xi]
    args forall a. [a] -> Int -> Bool
    `lengthIs` TyCon -> Int
    tyConArity TyCon
    tc
      = forall a. a -> Maybe a
    Just forall a b. (a -> b) -> a -> b
    $ TyCon -> [Xi] -> CanEqLHS
    TyFamLHS TyCon
    tc [Xi]
    args

      | Bool
    otherwise
      = forall a. Maybe a
    Nothing

    -- | Convert a 'CanEqLHS' back into a 'Type'
    canEqLHSType :: CanEqLHS -> TcType
    canEqLHSType :: CanEqLHS -> Xi
    canEqLHSType (TyVarLHS EvVar
    tv) = EvVar -> Xi
    mkTyVarTy EvVar
    tv
    canEqLHSType (TyFamLHS TyCon
    fam_tc [Xi]
    fam_args) = TyCon -> [Xi] -> Xi
    mkTyConApp TyCon
    fam_tc [Xi]
    fam_args

    -- | Retrieve the kind of a 'CanEqLHS'
    canEqLHSKind :: CanEqLHS -> TcKind
    canEqLHSKind :: CanEqLHS -> Xi
    canEqLHSKind (TyVarLHS EvVar
    tv) = EvVar -> Xi
    tyVarKind EvVar
    tv
    canEqLHSKind (TyFamLHS TyCon
    fam_tc [Xi]
    fam_args) = HasDebugCallStack => Xi -> [Xi] -> Xi
    piResultTys (TyCon -> Xi
    tyConKind TyCon
    fam_tc) [Xi]
    fam_args

    -- | Are two 'CanEqLHS's equal?
    eqCanEqLHS :: CanEqLHS -> CanEqLHS -> Bool
    eqCanEqLHS :: CanEqLHS -> CanEqLHS -> Bool
    eqCanEqLHS (TyVarLHS EvVar
    tv1) (TyVarLHS EvVar
    tv2) = EvVar
    tv1 forall a. Eq a => a -> a -> Bool
    == EvVar
    tv2
    eqCanEqLHS (TyFamLHS TyCon
    fam_tc1 [Xi]
    fam_args1) (TyFamLHS TyCon
    fam_tc2 [Xi]
    fam_args2)
      = TyCon -> [Xi] -> TyCon -> [Xi] -> Bool
    tcEqTyConApps TyCon
    fam_tc1 [Xi]
    fam_args1 TyCon
    fam_tc2 [Xi]
    fam_args2
    eqCanEqLHS CanEqLHS
    _ CanEqLHS
    _ = Bool
    False

    {-
    ************************************************************************
    *                                                                      *
            Simple functions over evidence variables
    *                                                                      *
    ************************************************************************
    -}

    ---------------- Getting free tyvars -------------------------

    -- | Returns free variables of constraints as a non-deterministic set
    tyCoVarsOfCt :: Ct -> TcTyCoVarSet
    tyCoVarsOfCt :: Ct -> TcTyCoVarSet
    tyCoVarsOfCt = FV -> TcTyCoVarSet
    fvVarSet forall b c a. (b -> c) -> (a -> b) -> a -> c
    . Ct -> FV
    tyCoFVsOfCt

    -- | Returns free variables of constraints as a deterministically ordered.
    -- list. See Note [Deterministic FV] in "GHC.Utils.FV".
    tyCoVarsOfCtList :: Ct -> [TcTyCoVar]
    tyCoVarsOfCtList :: Ct -> [EvVar]
    tyCoVarsOfCtList = FV -> [EvVar]
    fvVarList forall b c a. (b -> c) -> (a -> b) -> a -> c
    . Ct -> FV
    tyCoFVsOfCt

    -- | Returns free variables of constraints as a composable FV computation.
    -- See Note [Deterministic FV] in "GHC.Utils.FV".
    tyCoFVsOfCt :: Ct -> FV
    tyCoFVsOfCt :: Ct -> FV
    tyCoFVsOfCt Ct
    ct = Xi -> FV
    tyCoFVsOfType (Ct -> Xi
    ctPred Ct
    ct)
      -- This must consult only the ctPred, so that it gets *tidied* fvs if the
      -- constraint has been tidied. Tidying a constraint does not tidy the
      -- fields of the Ct, only the predicate in the CtEvidence.

    -- | Returns free variables of a bag of constraints as a non-deterministic
    -- set. See Note [Deterministic FV] in "GHC.Utils.FV".
    tyCoVarsOfCts :: Cts -> TcTyCoVarSet
    tyCoVarsOfCts :: Cts -> TcTyCoVarSet
    tyCoVarsOfCts = FV -> TcTyCoVarSet
    fvVarSet forall b c a. (b -> c) -> (a -> b) -> a -> c
    . Cts -> FV
    tyCoFVsOfCts

    -- | Returns free variables of a bag of constraints as a deterministically
    -- ordered list. See Note [Deterministic FV] in "GHC.Utils.FV".
    tyCoVarsOfCtsList :: Cts -> [TcTyCoVar]
    tyCoVarsOfCtsList :: Cts -> [EvVar]
    tyCoVarsOfCtsList = FV -> [EvVar]
    fvVarList forall b c a. (b -> c) -> (a -> b) -> a -> c
    . Cts -> FV
    tyCoFVsOfCts

    -- | Returns free variables of a bag of constraints as a composable FV
    -- computation. See Note [Deterministic FV] in "GHC.Utils.FV".
    tyCoFVsOfCts :: Cts -> FV
    tyCoFVsOfCts :: Cts -> FV
    tyCoFVsOfCts = forall (t :: * -> *) a b.
    Foldable t =>
    (a -> b -> b) -> b -> t a -> b
    foldr (FV -> FV -> FV
    unionFV forall b c a. (b -> c) -> (a -> b) -> a -> c
    . Ct -> FV
    tyCoFVsOfCt) FV
    emptyFV

    -- | Returns free variables of WantedConstraints as a non-deterministic
    -- set. See Note [Deterministic FV] in "GHC.Utils.FV".
    tyCoVarsOfWC :: WantedConstraints -> TyCoVarSet
    -- Only called on *zonked* things
    tyCoVarsOfWC :: WantedConstraints -> TcTyCoVarSet
    tyCoVarsOfWC = FV -> TcTyCoVarSet
    fvVarSet forall b c a. (b -> c) -> (a -> b) -> a -> c
    . WantedConstraints -> FV
    tyCoFVsOfWC

    -- | Returns free variables of WantedConstraints as a deterministically
    -- ordered list. See Note [Deterministic FV] in "GHC.Utils.FV".
    tyCoVarsOfWCList :: WantedConstraints -> [TyCoVar]
    -- Only called on *zonked* things
    tyCoVarsOfWCList :: WantedConstraints -> [EvVar]
    tyCoVarsOfWCList = FV -> [EvVar]
    fvVarList forall b c a. (b -> c) -> (a -> b) -> a -> c
    . WantedConstraints -> FV
    tyCoFVsOfWC

    -- | Returns free variables of WantedConstraints as a composable FV
    -- computation. See Note [Deterministic FV] in "GHC.Utils.FV".
    tyCoFVsOfWC :: WantedConstraints -> FV
    -- Only called on *zonked* things
    tyCoFVsOfWC :: WantedConstraints -> FV
    tyCoFVsOfWC (WC { wc_simple :: WantedConstraints -> Cts
    wc_simple = Cts
    simple, wc_impl :: WantedConstraints -> Bag Implication
    wc_impl = Bag Implication
    implic, wc_holes :: WantedConstraints -> Bag Hole
    wc_holes = Bag Hole
    holes })
      = Cts -> FV
    tyCoFVsOfCts Cts
    simple FV -> FV -> FV
    `unionFV`
        forall a. (a -> FV) -> Bag a -> FV
    tyCoFVsOfBag Implication -> FV
    tyCoFVsOfImplic Bag Implication
    implic FV -> FV -> FV
    `unionFV`
        forall a. (a -> FV) -> Bag a -> FV
    tyCoFVsOfBag Hole -> FV
    tyCoFVsOfHole Bag Hole
    holes

    -- | Returns free variables of Implication as a composable FV computation.
    -- See Note [Deterministic FV] in "GHC.Utils.FV".
    tyCoFVsOfImplic :: Implication -> FV
    -- Only called on *zonked* things
    tyCoFVsOfImplic :: Implication -> FV
    tyCoFVsOfImplic (Implic { ic_skols :: Implication -> [EvVar]
    ic_skols = [EvVar]
    skols
                            , ic_given :: Implication -> [EvVar]
    ic_given = [EvVar]
    givens
                            , ic_wanted :: Implication -> WantedConstraints
    ic_wanted = WantedConstraints
    wanted })
      | WantedConstraints -> Bool
    isEmptyWC WantedConstraints
    wanted
      = FV
    emptyFV
      | Bool
    otherwise
      = [EvVar] -> FV -> FV
    tyCoFVsVarBndrs [EvVar]
    skols  forall a b. (a -> b) -> a -> b
    $
        [EvVar] -> FV -> FV
    tyCoFVsVarBndrs [EvVar]
    givens forall a b. (a -> b) -> a -> b
    $
        WantedConstraints -> FV
    tyCoFVsOfWC WantedConstraints
    wanted

    tyCoFVsOfHole :: Hole -> FV
    tyCoFVsOfHole :: Hole -> FV
    tyCoFVsOfHole (Hole { hole_ty :: Hole -> Xi
    hole_ty = Xi
    ty }) = Xi -> FV
    tyCoFVsOfType Xi
    ty

    tyCoFVsOfBag :: (a -> FV) -> Bag a -> FV
    tyCoFVsOfBag :: forall a. (a -> FV) -> Bag a -> FV
    tyCoFVsOfBag a -> FV
    tvs_of = forall (t :: * -> *) a b.
    Foldable t =>
    (a -> b -> b) -> b -> t a -> b
    foldr (FV -> FV -> FV
    unionFV forall b c a. (b -> c) -> (a -> b) -> a -> c
    . a -> FV
    tvs_of) FV
    emptyFV

    ---------------------------
    dropDerivedWC :: WantedConstraints -> WantedConstraints
    -- See Note [Dropping derived constraints]
    dropDerivedWC :: WantedConstraints -> WantedConstraints
    dropDerivedWC wc :: WantedConstraints
    wc@(WC { wc_simple :: WantedConstraints -> Cts
    wc_simple = Cts
    simples })
      = WantedConstraints
    wc { wc_simple :: Cts
    wc_simple = Cts -> Cts
    dropDerivedSimples Cts
    simples }
        -- The wc_impl implications are already (recursively) filtered

    --------------------------
    dropDerivedSimples :: Cts -> Cts
    -- Drop all Derived constraints, but make [W] back into [WD],
    -- so that if we re-simplify these constraints we will get all
    -- the right derived constraints re-generated.  Forgetting this
    -- step led to #12936
    dropDerivedSimples :: Cts -> Cts
    dropDerivedSimples Cts
    simples = forall a b. (a -> Maybe b) -> Bag a -> Bag b
    mapMaybeBag Ct -> Maybe Ct
    dropDerivedCt Cts
    simples

    dropDerivedCt :: Ct -> Maybe Ct
    dropDerivedCt :: Ct -> Maybe Ct
    dropDerivedCt Ct
    ct
      = case CtEvidence -> CtFlavour
    ctEvFlavour CtEvidence
    ev of
          Wanted ShadowInfo
    WOnly -> forall a. a -> Maybe a
    Just (Ct
    ct' { cc_ev :: CtEvidence
    cc_ev = CtEvidence
    ev_wd })
          Wanted ShadowInfo
    _     -> forall a. a -> Maybe a
    Just Ct
    ct'
          CtFlavour
    _ | Ct -> Bool
    isDroppableCt Ct
    ct -> forall a. Maybe a
    Nothing
            | Bool
    otherwise        -> forall a. a -> Maybe a
    Just Ct
    ct
      where
        ev :: CtEvidence
    ev    = Ct -> CtEvidence
    ctEvidence Ct
    ct
        ev_wd :: CtEvidence
    ev_wd = CtEvidence
    ev { ctev_nosh :: ShadowInfo
    ctev_nosh = ShadowInfo
    WDeriv }
        ct' :: Ct
    ct'   = Ct -> Ct
    setPendingScDict Ct
    ct -- See Note [Resetting cc_pend_sc]

    {- Note [Resetting cc_pend_sc]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When we discard Derived constraints, in dropDerivedSimples, we must
    set the cc_pend_sc flag to True, so that if we re-process this
    CDictCan we will re-generate its derived superclasses. Otherwise
    we might miss some fundeps.  #13662 showed this up.

    See Note [The superclass story] in GHC.Tc.Solver.Canonical.
    -}

    isDroppableCt :: Ct -> Bool
    isDroppableCt :: Ct -> Bool
    isDroppableCt Ct
    ct
      = CtEvidence -> Bool
    isDerived CtEvidence
    ev Bool -> Bool -> Bool
    && Bool -> Bool
    not Bool
    keep_deriv
        -- Drop only derived constraints, and then only if they
        -- obey Note [Dropping derived constraints]
      where
        ev :: CtEvidence
    ev   = Ct -> CtEvidence
    ctEvidence Ct
    ct
        loc :: CtLoc
    loc  = CtEvidence -> CtLoc
    ctEvLoc CtEvidence
    ev
        orig :: CtOrigin
    orig = CtLoc -> CtOrigin
    ctLocOrigin CtLoc
    loc

        keep_deriv :: Bool
    keep_deriv
          = case Ct
    ct of
              CIrredCan { cc_reason :: Ct -> CtIrredReason
    cc_reason = CtIrredReason
    reason } | CtIrredReason -> Bool
    isInsolubleReason CtIrredReason
    reason -> Bool -> Bool
    keep_eq Bool
    True
              Ct
    _                                                           -> Bool -> Bool
    keep_eq Bool
    False

        keep_eq :: Bool -> Bool
    keep_eq Bool
    definitely_insoluble
           | CtOrigin -> Bool
    isGivenOrigin CtOrigin
    orig    -- Arising only from givens
           = Bool
    definitely_insoluble  -- Keep only definitely insoluble
           | Bool
    otherwise
           = case CtOrigin
    orig of
               -- See Note [Dropping derived constraints]
               -- For fundeps, drop wanted/wanted interactions
               FunDepOrigin2 {} -> Bool
    True   -- Top-level/Wanted
               FunDepOrigin1 Xi
    _ CtOrigin
    orig1 RealSrcSpan
    _ Xi
    _ CtOrigin
    orig2 RealSrcSpan
    _
                 | Bool
    g1 Bool -> Bool -> Bool
    || Bool
    g2  -> Bool
    True  -- Given/Wanted errors: keep all
                 | Bool
    otherwise -> Bool
    False -- Wanted/Wanted errors: discard
                 where
                   g1 :: Bool
    g1 = CtOrigin -> Bool
    isGivenOrigin CtOrigin
    orig1
                   g2 :: Bool
    g2 = CtOrigin -> Bool
    isGivenOrigin CtOrigin
    orig2

               CtOrigin
    _ -> Bool
    False

    arisesFromGivens :: Ct -> Bool
    arisesFromGivens :: Ct -> Bool
    arisesFromGivens Ct
    ct
      = case Ct -> CtEvidence
    ctEvidence Ct
    ct of
          CtGiven {}                   -> Bool
    True
          CtWanted {}                  -> Bool
    False
          CtDerived { ctev_loc :: CtEvidence -> CtLoc
    ctev_loc = CtLoc
    loc } -> CtLoc -> Bool
    isGivenLoc CtLoc
    loc

    isGivenLoc :: CtLoc -> Bool
    isGivenLoc :: CtLoc -> Bool
    isGivenLoc CtLoc
    loc = CtOrigin -> Bool
    isGivenOrigin (CtLoc -> CtOrigin
    ctLocOrigin CtLoc
    loc)

    {- Note [Dropping derived constraints]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In general we discard derived constraints at the end of constraint solving;
    see dropDerivedWC.  For example

     * Superclasses: if we have an unsolved [W] (Ord a), we don't want to
       complain about an unsolved [D] (Eq a) as well.

     * If we have [W] a ~ Int, [W] a ~ Bool, improvement will generate
       [D] Int ~ Bool, and we don't want to report that because it's
       incomprehensible. That is why we don't rewrite wanteds with wanteds!

     * We might float out some Wanteds from an implication, leaving behind
       their insoluble Deriveds. For example:

       forall a[2]. [W] alpha[1] ~ Int
                    [W] alpha[1] ~ Bool
                    [D] Int ~ Bool

       The Derived is insoluble, but we very much want to drop it when floating
       out.

    But (tiresomely) we do keep *some* Derived constraints:

     * Type holes are derived constraints, because they have no evidence
       and we want to keep them, so we get the error report

     * We keep most derived equalities arising from functional dependencies
          - Given/Given interactions (subset of FunDepOrigin1):
            The definitely-insoluble ones reflect unreachable code.

            Others not-definitely-insoluble ones like [D] a ~ Int do not
            reflect unreachable code; indeed if fundeps generated proofs, it'd
            be a useful equality.  See #14763.   So we discard them.

          - Given/Wanted interacGiven or Wanted interacting with an
            instance declaration (FunDepOrigin2)

          - Given/Wanted interactions (FunDepOrigin1); see #9612

          - But for Wanted/Wanted interactions we do /not/ want to report an
            error (#13506).  Consider [W] C Int Int, [W] C Int Bool, with
            a fundep on class C.  We don't want to report an insoluble Int~Bool;
            c.f. "wanteds do not rewrite wanteds".

    To distinguish these cases we use the CtOrigin.

    NB: we keep *all* derived insolubles under some circumstances:

      * They are looked at by simplifyInfer, to decide whether to
        generalise.  Example: [W] a ~ Int, [W] a ~ Bool
        We get [D] Int ~ Bool, and indeed the constraints are insoluble,
        and we want simplifyInfer to see that, even though we don't
        ultimately want to generate an (inexplicable) error message from it

    ************************************************************************
    *                                                                      *
                        CtEvidence
             The "flavor" of a canonical constraint
    *                                                                      *
    ************************************************************************
    -}

    isWantedCt :: Ct -> Bool
    isWantedCt :: Ct -> Bool
    isWantedCt = CtEvidence -> Bool
    isWanted forall b c a. (b -> c) -> (a -> b) -> a -> c
    . Ct -> CtEvidence
    ctEvidence

    isGivenCt :: Ct -> Bool
    isGivenCt :: Ct -> Bool
    isGivenCt = CtEvidence -> Bool
    isGiven forall b c a. (b -> c) -> (a -> b) -> a -> c
    . Ct -> CtEvidence
    ctEvidence

    isDerivedCt :: Ct -> Bool
    isDerivedCt :: Ct -> Bool
    isDerivedCt = CtEvidence -> Bool
    isDerived forall b c a. (b -> c) -> (a -> b) -> a -> c
    . Ct -> CtEvidence
    ctEvidence

    {- Note [Custom type errors in constraints]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    When GHC reports a type-error about an unsolved-constraint, we check
    to see if the constraint contains any custom-type errors, and if so
    we report them.  Here are some examples of constraints containing type
    errors:

    TypeError msg           -- The actual constraint is a type error

    TypError msg ~ Int      -- Some type was supposed to be Int, but ended up
                            -- being a type error instead

    Eq (TypeError msg)      -- A class constraint is stuck due to a type error

    F (TypeError msg) ~ a   -- A type function failed to evaluate due to a type err

    It is also possible to have constraints where the type error is nested deeper,
    for example see #11990, and also:

    Eq (F (TypeError msg))  -- Here the type error is nested under a type-function
                            -- call, which failed to evaluate because of it,
                            -- and so the `Eq` constraint was unsolved.
                            -- This may happen when one function calls another
                            -- and the called function produced a custom type error.
    -}

    -- | A constraint is considered to be a custom type error, if it contains
    -- custom type errors anywhere in it.
    -- See Note [Custom type errors in constraints]
    getUserTypeErrorMsg :: Ct -> Maybe Type
    getUserTypeErrorMsg :: Ct -> Maybe Xi
    getUserTypeErrorMsg Ct
    ct = Xi -> Maybe Xi
    findUserTypeError (Ct -> Xi
    ctPred Ct
    ct)
      where
      findUserTypeError :: Xi -> Maybe Xi
    findUserTypeError Xi
    t = forall (t :: * -> *) (m :: * -> *) a.
    (Foldable t, MonadPlus m) =>
    t (m a) -> m a
    msum ( Xi -> Maybe Xi
    userTypeError_maybe Xi
    t
                                 forall a. a -> [a] -> [a]
    : forall a b. (a -> b) -> [a] -> [b]
    map Xi -> Maybe Xi
    findUserTypeError (Xi -> [Xi]
    subTys Xi
    t)
                                 )

      subTys :: Xi -> [Xi]
    subTys Xi
    t            = case Xi -> (Xi, [Xi])
    splitAppTys Xi
    t of
                              (Xi
    t,[]) ->
                                case HasDebugCallStack => Xi -> Maybe (TyCon, [Xi])
    splitTyConApp_maybe Xi
    t of
                                  Maybe (TyCon, [Xi])
    Nothing     -> []
                                  Just (TyCon
    _,[Xi]
    ts) -> [Xi]
    ts
                              (Xi
    t,[Xi]
    ts) -> Xi
    t forall a. a -> [a] -> [a]
    : [Xi]
    ts

    isUserTypeErrorCt :: Ct -> Bool
    isUserTypeErrorCt :: Ct -> Bool
    isUserTypeErrorCt Ct
    ct = case Ct -> Maybe Xi
    getUserTypeErrorMsg Ct
    ct of
                             Just Xi
    _ -> Bool
    True
                             Maybe Xi
    _      -> Bool
    False

    isPendingScDict :: Ct -> Maybe Ct
    -- Says whether this is a CDictCan with cc_pend_sc is True,
    -- AND if so flips the flag
    isPendingScDict :: Ct -> Maybe Ct
    isPendingScDict ct :: Ct
    ct@(CDictCan { cc_pend_sc :: Ct -> Bool
    cc_pend_sc = Bool
    True })
                      = forall a. a -> Maybe a
    Just (Ct
    ct { cc_pend_sc :: Bool
    cc_pend_sc = Bool
    False })
    isPendingScDict Ct
    _ = forall a. Maybe a
    Nothing

    isPendingScInst :: QCInst -> Maybe QCInst
    -- Same as isPendingScDict, but for QCInsts
    isPendingScInst :: QCInst -> Maybe QCInst
    isPendingScInst qci :: QCInst
    qci@(QCI { qci_pend_sc :: QCInst -> Bool
    qci_pend_sc = Bool
    True })
                      = forall a. a -> Maybe a
    Just (QCInst
    qci { qci_pend_sc :: Bool
    qci_pend_sc = Bool
    False })
    isPendingScInst QCInst
    _ = forall a. Maybe a
    Nothing

    setPendingScDict :: Ct -> Ct
    -- Set the cc_pend_sc flag to True
    setPendingScDict :: Ct -> Ct
    setPendingScDict ct :: Ct
    ct@(CDictCan { cc_pend_sc :: Ct -> Bool
    cc_pend_sc = Bool
    False })
                        = Ct
    ct { cc_pend_sc :: Bool
    cc_pend_sc = Bool
    True }
    setPendingScDict Ct
    ct = Ct
    ct

    superClassesMightHelp :: WantedConstraints -> Bool
    -- ^ True if taking superclasses of givens, or of wanteds (to perhaps
    -- expose more equalities or functional dependencies) might help to
    -- solve this constraint.  See Note [When superclasses help]
    superClassesMightHelp :: WantedConstraints -> Bool
    superClassesMightHelp (WC { wc_simple :: WantedConstraints -> Cts
    wc_simple = Cts
    simples, wc_impl :: WantedConstraints -> Bag Implication
    wc_impl = Bag Implication
    implics })
      = forall a. (a -> Bool) -> Bag a -> Bool
    anyBag Ct -> Bool
    might_help_ct Cts
    simples Bool -> Bool -> Bool
    || forall a. (a -> Bool) -> Bag a -> Bool
    anyBag Implication -> Bool
    might_help_implic Bag Implication
    implics
      where
        might_help_implic :: Implication -> Bool
    might_help_implic Implication
    ic
           | ImplicStatus
    IC_Unsolved <- Implication -> ImplicStatus
    ic_status Implication
    ic = WantedConstraints -> Bool
    superClassesMightHelp (Implication -> WantedConstraints
    ic_wanted Implication
    ic)
           | Bool
    otherwise                   = Bool
    False

        might_help_ct :: Ct -> Bool
    might_help_ct Ct
    ct = Ct -> Bool
    isWantedCt Ct
    ct Bool -> Bool -> Bool
    && Bool -> Bool
    not (Ct -> Bool
    is_ip Ct
    ct)

        is_ip :: Ct -> Bool
    is_ip (CDictCan { cc_class :: Ct -> Class
    cc_class = Class
    cls }) = Class -> Bool
    isIPClass Class
    cls
        is_ip Ct
    _                             = Bool
    False

    getPendingWantedScs :: Cts -> ([Ct], Cts)
    getPendingWantedScs :: Cts -> ([Ct], Cts)
    getPendingWantedScs Cts
    simples
      = forall acc x y.
    (acc -> x -> (acc, y)) -> acc -> Bag x -> (acc, Bag y)
    mapAccumBagL [Ct] -> Ct -> ([Ct], Ct)
    get [] Cts
    simples
      where
        get :: [Ct] -> Ct -> ([Ct], Ct)
    get [Ct]
    acc Ct
    ct | Just Ct
    ct' <- Ct -> Maybe Ct
    isPendingScDict Ct
    ct
                   = (Ct
    ct'forall a. a -> [a] -> [a]
    :[Ct]
    acc, Ct
    ct')
                   | Bool
    otherwise
                   = ([Ct]
    acc,     Ct
    ct)

    {- Note [When superclasses help]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    First read Note [The superclass story] in GHC.Tc.Solver.Canonical.

    We expand superclasses and iterate only if there is at unsolved wanted
    for which expansion of superclasses (e.g. from given constraints)
    might actually help. The function superClassesMightHelp tells if
    doing this superclass expansion might help solve this constraint.
    Note that

      * We look inside implications; maybe it'll help to expand the Givens
        at level 2 to help solve an unsolved Wanted buried inside an
        implication.  E.g.
            forall a. Ord a => forall b. [W] Eq a

      * Superclasses help only for Wanted constraints.  Derived constraints
        are not really "unsolved" and we certainly don't want them to
        trigger superclass expansion. This was a good part of the loop
        in  #11523

      * Even for Wanted constraints, we say "no" for implicit parameters.
        we have [W] ?x::ty, expanding superclasses won't help:
          - Superclasses can't be implicit parameters
          - If we have a [G] ?x:ty2, then we'll have another unsolved
            [D] ty ~ ty2 (from the functional dependency)
            which will trigger superclass expansion.

        It's a bit of a special case, but it's easy to do.  The runtime cost
        is low because the unsolved set is usually empty anyway (errors
        aside), and the first non-implicit-parameter will terminate the search.

        The special case is worth it (#11480, comment:2) because it
        applies to CallStack constraints, which aren't type errors. If we have
           f :: (C a) => blah
           f x = ...undefined...
        we'll get a CallStack constraint.  If that's the only unsolved
        constraint it'll eventually be solved by defaulting.  So we don't
        want to emit warnings about hitting the simplifier's iteration
        limit.  A CallStack constraint really isn't an unsolved
        constraint; it can always be solved by defaulting.
    -}

    singleCt :: Ct -> Cts
    singleCt :: Ct -> Cts
    singleCt = forall a. a -> Bag a
    unitBag

    andCts :: Cts -> Cts -> Cts
    andCts :: Cts -> Cts -> Cts
    andCts = forall a. Bag a -> Bag a -> Bag a
    unionBags

    listToCts :: [Ct] -> Cts
    listToCts :: [Ct] -> Cts
    listToCts = forall a. [a] -> Bag a
    listToBag

    ctsElts :: Cts -> [Ct]
    ctsElts :: Cts -> [Ct]
    ctsElts = forall a. Bag a -> [a]
    bagToList

    consCts :: Ct -> Cts -> Cts
    consCts :: Ct -> Cts -> Cts
    consCts = forall a. a -> Bag a -> Bag a
    consBag

    snocCts :: Cts -> Ct -> Cts
    snocCts :: Cts -> Ct -> Cts
    snocCts = forall a. Bag a -> a -> Bag a
    snocBag

    extendCtsList :: Cts -> [Ct] -> Cts
    extendCtsList :: Cts -> [Ct] -> Cts
    extendCtsList Cts
    cts [Ct]
    xs | forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [Ct]
    xs   = Cts
    cts
                         | Bool
    otherwise = Cts
    cts forall a. Bag a -> Bag a -> Bag a
    `unionBags` forall a. [a] -> Bag a
    listToBag [Ct]
    xs

    andManyCts :: [Cts] -> Cts
    andManyCts :: [Cts] -> Cts
    andManyCts = forall a. [Bag a] -> Bag a
    unionManyBags

    emptyCts :: Cts
    emptyCts :: Cts
    emptyCts = forall a. Bag a
    emptyBag

    isEmptyCts :: Cts -> Bool
    isEmptyCts :: Cts -> Bool
    isEmptyCts = forall a. Bag a -> Bool
    isEmptyBag

    pprCts :: Cts -> SDoc
    pprCts :: Cts -> SDoc
    pprCts Cts
    cts = [SDoc] -> SDoc
    vcat (forall a b. (a -> b) -> [a] -> [b]
    map forall a. Outputable a => a -> SDoc
    ppr (forall a. Bag a -> [a]
    bagToList Cts
    cts))

    {-
    ************************************************************************
    *                                                                      *
                    Wanted constraints
         These are forced to be in GHC.Tc.Types because
               TcLclEnv mentions WantedConstraints
               WantedConstraint mentions CtLoc
               CtLoc mentions ErrCtxt
               ErrCtxt mentions TcM
    *                                                                      *
    v%************************************************************************
    -}

    data WantedConstraints
      = WC { WantedConstraints -> Cts
    wc_simple :: Cts              -- Unsolved constraints, all wanted
           , WantedConstraints -> Bag Implication
    wc_impl   :: Bag Implication
           , WantedConstraints -> Bag Hole
    wc_holes  :: Bag Hole
        }

    emptyWC :: WantedConstraints
    emptyWC :: WantedConstraints
    emptyWC = WC { wc_simple :: Cts
    wc_simple = forall a. Bag a
    emptyBag
                 , wc_impl :: Bag Implication
    wc_impl   = forall a. Bag a
    emptyBag
                 , wc_holes :: Bag Hole
    wc_holes  = forall a. Bag a
    emptyBag }

    mkSimpleWC :: [CtEvidence] -> WantedConstraints
    mkSimpleWC :: [CtEvidence] -> WantedConstraints
    mkSimpleWC [CtEvidence]
    cts
      = WantedConstraints
    emptyWC { wc_simple :: Cts
    wc_simple = forall a. [a] -> Bag a
    listToBag (forall a b. (a -> b) -> [a] -> [b]
    map CtEvidence -> Ct
    mkNonCanonical [CtEvidence]
    cts) }

    mkImplicWC :: Bag Implication -> WantedConstraints
    mkImplicWC :: Bag Implication -> WantedConstraints
    mkImplicWC Bag Implication
    implic
      = WantedConstraints
    emptyWC { wc_impl :: Bag Implication
    wc_impl = Bag Implication
    implic }

    isEmptyWC :: WantedConstraints -> Bool
    isEmptyWC :: WantedConstraints -> Bool
    isEmptyWC (WC { wc_simple :: WantedConstraints -> Cts
    wc_simple = Cts
    f, wc_impl :: WantedConstraints -> Bag Implication
    wc_impl = Bag Implication
    i, wc_holes :: WantedConstraints -> Bag Hole
    wc_holes = Bag Hole
    holes })
      = forall a. Bag a -> Bool
    isEmptyBag Cts
    f Bool -> Bool -> Bool
    && forall a. Bag a -> Bool
    isEmptyBag Bag Implication
    i Bool -> Bool -> Bool
    && forall a. Bag a -> Bool
    isEmptyBag Bag Hole
    holes

    -- | Checks whether a the given wanted constraints are solved, i.e.
    -- that there are no simple constraints left and all the implications
    -- are solved.
    isSolvedWC :: WantedConstraints -> Bool
    isSolvedWC :: WantedConstraints -> Bool
    isSolvedWC WC {wc_simple :: WantedConstraints -> Cts
    wc_simple = Cts
    wc_simple, wc_impl :: WantedConstraints -> Bag Implication
    wc_impl = Bag Implication
    wc_impl, wc_holes :: WantedConstraints -> Bag Hole
    wc_holes = Bag Hole
    holes} =
      forall a. Bag a -> Bool
    isEmptyBag Cts
    wc_simple Bool -> Bool -> Bool
    && forall a. (a -> Bool) -> Bag a -> Bool
    allBag (ImplicStatus -> Bool
    isSolvedStatus forall b c a. (b -> c) -> (a -> b) -> a -> c
    . Implication -> ImplicStatus
    ic_status) Bag Implication
    wc_impl Bool -> Bool -> Bool
    && forall a. Bag a -> Bool
    isEmptyBag Bag Hole
    holes

    andWC :: WantedConstraints -> WantedConstraints -> WantedConstraints
    andWC :: WantedConstraints -> WantedConstraints -> WantedConstraints
    andWC (WC { wc_simple :: WantedConstraints -> Cts
    wc_simple = Cts
    f1, wc_impl :: WantedConstraints -> Bag Implication
    wc_impl = Bag Implication
    i1, wc_holes :: WantedConstraints -> Bag Hole
    wc_holes = Bag Hole
    h1 })
          (WC { wc_simple :: WantedConstraints -> Cts
    wc_simple = Cts
    f2, wc_impl :: WantedConstraints -> Bag Implication
    wc_impl = Bag Implication
    i2, wc_holes :: WantedConstraints -> Bag Hole
    wc_holes = Bag Hole
    h2 })
      = WC { wc_simple :: Cts
    wc_simple = Cts
    f1 forall a. Bag a -> Bag a -> Bag a
    `unionBags` Cts
    f2
           , wc_impl :: Bag Implication
    wc_impl   = Bag Implication
    i1 forall a. Bag a -> Bag a -> Bag a
    `unionBags` Bag Implication
    i2
           , wc_holes :: Bag Hole
    wc_holes  = Bag Hole
    h1 forall a. Bag a -> Bag a -> Bag a
    `unionBags` Bag Hole
    h2 }

    unionsWC :: [WantedConstraints] -> WantedConstraints
    unionsWC :: [WantedConstraints] -> WantedConstraints
    unionsWC = forall (t :: * -> *) a b.
    Foldable t =>
    (a -> b -> b) -> b -> t a -> b
    foldr WantedConstraints -> WantedConstraints -> WantedConstraints
    andWC WantedConstraints
    emptyWC

    addSimples :: WantedConstraints -> Bag Ct -> WantedConstraints
    addSimples :: WantedConstraints -> Cts -> WantedConstraints
    addSimples WantedConstraints
    wc Cts
    cts
      = WantedConstraints
    wc { wc_simple :: Cts
    wc_simple = WantedConstraints -> Cts
    wc_simple WantedConstraints
    wc forall a. Bag a -> Bag a -> Bag a
    `unionBags` Cts
    cts }
        -- Consider: Put the new constraints at the front, so they get solved first

    addImplics :: WantedConstraints -> Bag Implication -> WantedConstraints
    addImplics :: WantedConstraints -> Bag Implication -> WantedConstraints
    addImplics WantedConstraints
    wc Bag Implication
    implic = WantedConstraints
    wc { wc_impl :: Bag Implication
    wc_impl = WantedConstraints -> Bag Implication
    wc_impl WantedConstraints
    wc forall a. Bag a -> Bag a -> Bag a
    `unionBags` Bag Implication
    implic }

    addInsols :: WantedConstraints -> Bag Ct -> WantedConstraints
    addInsols :: WantedConstraints -> Cts -> WantedConstraints
    addInsols WantedConstraints
    wc Cts
    cts
      = WantedConstraints
    wc { wc_simple :: Cts
    wc_simple = WantedConstraints -> Cts
    wc_simple WantedConstraints
    wc forall a. Bag a -> Bag a -> Bag a
    `unionBags` Cts
    cts }

    addHoles :: WantedConstraints -> Bag Hole -> WantedConstraints
    addHoles :: WantedConstraints -> Bag Hole -> WantedConstraints
    addHoles WantedConstraints
    wc Bag Hole
    holes
      = WantedConstraints
    wc { wc_holes :: Bag Hole
    wc_holes = Bag Hole
    holes forall a. Bag a -> Bag a -> Bag a
    `unionBags` WantedConstraints -> Bag Hole
    wc_holes WantedConstraints
    wc }

    dropMisleading :: WantedConstraints -> WantedConstraints
    -- Drop misleading constraints; really just class constraints
    -- See Note [Constraints and errors] in GHC.Tc.Utils.Monad
    --   for why this function is so strange, treating the 'simples'
    --   and the implications differently.  Sigh.
    dropMisleading :: WantedConstraints -> WantedConstraints
    dropMisleading (WC { wc_simple :: WantedConstraints -> Cts
    wc_simple = Cts
    simples, wc_impl :: WantedConstraints -> Bag Implication
    wc_impl = Bag Implication
    implics, wc_holes :: WantedConstraints -> Bag Hole
    wc_holes = Bag Hole
    holes })
      = WC { wc_simple :: Cts
    wc_simple = forall a. (a -> Bool) -> Bag a -> Bag a
    filterBag Ct -> Bool
    insolubleCt Cts
    simples
           , wc_impl :: Bag Implication
    wc_impl   = forall a b. (a -> b) -> Bag a -> Bag b
    mapBag Implication -> Implication
    drop_implic Bag Implication
    implics
           , wc_holes :: Bag Hole
    wc_holes  = forall a. (a -> Bool) -> Bag a -> Bag a
    filterBag Hole -> Bool
    isOutOfScopeHole Bag Hole
    holes }
      where
        drop_implic :: Implication -> Implication
    drop_implic Implication
    implic
          = Implication
    implic { ic_wanted :: WantedConstraints
    ic_wanted = WantedConstraints -> WantedConstraints
    drop_wanted (Implication -> WantedConstraints
    ic_wanted Implication
    implic) }
        drop_wanted :: WantedConstraints -> WantedConstraints
    drop_wanted (WC { wc_simple :: WantedConstraints -> Cts
    wc_simple = Cts
    simples, wc_impl :: WantedConstraints -> Bag Implication
    wc_impl = Bag Implication
    implics, wc_holes :: WantedConstraints -> Bag Hole
    wc_holes = Bag Hole
    holes })
          = WC { wc_simple :: Cts
    wc_simple = forall a. (a -> Bool) -> Bag a -> Bag a
    filterBag Ct -> Bool
    keep_ct Cts
    simples
               , wc_impl :: Bag Implication
    wc_impl   = forall a b. (a -> b) -> Bag a -> Bag b
    mapBag Implication -> Implication
    drop_implic Bag Implication
    implics
               , wc_holes :: Bag Hole
    wc_holes  = forall a. (a -> Bool) -> Bag a -> Bag a
    filterBag Hole -> Bool
    isOutOfScopeHole Bag Hole
    holes }

        keep_ct :: Ct -> Bool
    keep_ct Ct
    ct = case Xi -> Pred
    classifyPredType (Ct -> Xi
    ctPred Ct
    ct) of
                        ClassPred {} -> Bool
    False
                        Pred
    _ -> Bool
    True

    isSolvedStatus :: ImplicStatus -> Bool
    isSolvedStatus :: ImplicStatus -> Bool
    isSolvedStatus (IC_Solved {}) = Bool
    True
    isSolvedStatus ImplicStatus
    _              = Bool
    False

    isInsolubleStatus :: ImplicStatus -> Bool
    isInsolubleStatus :: ImplicStatus -> Bool
    isInsolubleStatus ImplicStatus
    IC_Insoluble    = Bool
    True
    isInsolubleStatus ImplicStatus
    IC_BadTelescope = Bool
    True
    isInsolubleStatus ImplicStatus
    _               = Bool
    False

    insolubleImplic :: Implication -> Bool
    insolubleImplic :: Implication -> Bool
    insolubleImplic Implication
    ic = ImplicStatus -> Bool
    isInsolubleStatus (Implication -> ImplicStatus
    ic_status Implication
    ic)

    insolubleWC :: WantedConstraints -> Bool
    insolubleWC :: WantedConstraints -> Bool
    insolubleWC (WC { wc_impl :: WantedConstraints -> Bag Implication
    wc_impl = Bag Implication
    implics, wc_simple :: WantedConstraints -> Cts
    wc_simple = Cts
    simples, wc_holes :: WantedConstraints -> Bag Hole
    wc_holes = Bag Hole
    holes })
      =  forall a. (a -> Bool) -> Bag a -> Bool
    anyBag Ct -> Bool
    insolubleCt Cts
    simples
      Bool -> Bool -> Bool
    || forall a. (a -> Bool) -> Bag a -> Bool
    anyBag Implication -> Bool
    insolubleImplic Bag Implication
    implics
      Bool -> Bool -> Bool
    || forall a. (a -> Bool) -> Bag a -> Bool
    anyBag Hole -> Bool
    isOutOfScopeHole Bag Hole
    holes  -- See Note [Insoluble holes]

    insolubleCt :: Ct -> Bool
    -- Definitely insoluble, in particular /excluding/ type-hole constraints
    -- Namely: a) an equality constraint
    --         b) that is insoluble
    --         c) and does not arise from a Given
    insolubleCt :: Ct -> Bool
    insolubleCt Ct
    ct
      | Bool -> Bool
    not (Ct -> Bool
    insolubleEqCt Ct
    ct) = Bool
    False
      | Ct -> Bool
    arisesFromGivens Ct
    ct    = Bool
    False              -- See Note [Given insolubles]
      | Bool
    otherwise              = Bool
    True

    insolubleEqCt :: Ct -> Bool
    -- Returns True of /equality/ constraints
    -- that are /definitely/ insoluble
    -- It won't detect some definite errors like
    --       F a ~ T (F a)
    -- where F is a type family, which actually has an occurs check
    --
    -- The function is tuned for application /after/ constraint solving
    --       i.e. assuming canonicalisation has been done
    -- E.g.  It'll reply True  for     a ~ [a]
    --               but False for   [a] ~ a
    -- and
    --                   True for  Int ~ F a Int
    --               but False for  Maybe Int ~ F a Int Int
    --               (where F is an arity-1 type function)
    insolubleEqCt :: Ct -> Bool
    insolubleEqCt (CIrredCan { cc_reason :: Ct -> CtIrredReason
    cc_reason = CtIrredReason
    reason }) = CtIrredReason -> Bool
    isInsolubleReason CtIrredReason
    reason
    insolubleEqCt Ct
    _                                  = Bool
    False

    -- | Does this hole represent an "out of scope" error?
    -- See Note [Insoluble holes]
    isOutOfScopeHole :: Hole -> Bool
    isOutOfScopeHole :: Hole -> Bool
    isOutOfScopeHole (Hole { hole_occ :: Hole -> OccName
    hole_occ = OccName
    occ }) = Bool -> Bool
    not (OccName -> Bool
    startsWithUnderscore OccName
    occ)

    instance Outputable WantedConstraints where
      ppr :: WantedConstraints -> SDoc
    ppr (WC {wc_simple :: WantedConstraints -> Cts
    wc_simple = Cts
    s, wc_impl :: WantedConstraints -> Bag Implication
    wc_impl = Bag Implication
    i, wc_holes :: WantedConstraints -> Bag Hole
    wc_holes = Bag Hole
    h})
       = String -> SDoc
    text String
    "WC" SDoc -> SDoc -> SDoc
    <+> SDoc -> SDoc
    braces ([SDoc] -> SDoc
    vcat
            [ forall a. Outputable a => SDoc -> Bag a -> SDoc
    ppr_bag (String -> SDoc
    text String
    "wc_simple") Cts
    s
            , forall a. Outputable a => SDoc -> Bag a -> SDoc
    ppr_bag (String -> SDoc
    text String
    "wc_impl") Bag Implication
    i
            , forall a. Outputable a => SDoc -> Bag a -> SDoc
    ppr_bag (String -> SDoc
    text String
    "wc_holes") Bag Hole
    h ])

    ppr_bag :: Outputable a => SDoc -> Bag a -> SDoc
    ppr_bag :: forall a. Outputable a => SDoc -> Bag a -> SDoc
    ppr_bag SDoc
    doc Bag a
    bag
     | forall a. Bag a -> Bool
    isEmptyBag Bag a
    bag = SDoc
    empty
     | Bool
    otherwise      = SDoc -> Int -> SDoc -> SDoc
    hang (SDoc
    doc SDoc -> SDoc -> SDoc
    <+> SDoc
    equals)
                           Int
    2 (forall (t :: * -> *) a b.
    Foldable t =>
    (a -> b -> b) -> b -> t a -> b
    foldr (SDoc -> SDoc -> SDoc
    ($$) forall b c a. (b -> c) -> (a -> b) -> a -> c
    . forall a. Outputable a => a -> SDoc
    ppr) SDoc
    empty Bag a
    bag)

    {- Note [Given insolubles]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider (#14325, comment:)
        class (a~b) => C a b

        foo :: C a c => a -> c
        foo x = x

        hm3 :: C (f b) b => b -> f b
        hm3 x = foo x

    In the RHS of hm3, from the [G] C (f b) b we get the insoluble
    [G] f b ~# b.  Then we also get an unsolved [W] C b (f b).
    Residual implication looks like
        forall b. C (f b) b => [G] f b ~# b
                               [W] C f (f b)

    We do /not/ want to set the implication status to IC_Insoluble,
    because that'll suppress reports of [W] C b (f b).  But we
    may not report the insoluble [G] f b ~# b either (see Note [Given errors]
    in GHC.Tc.Errors), so we may fail to report anything at all!  Yikes.

    The same applies to Derived constraints that /arise from/ Givens.
    E.g.   f :: (C Int [a]) => blah
    where a fundep means we get
           [D] Int ~ [a]
    By the same reasoning we must not suppress other errors (#15767)

    Bottom line: insolubleWC (called in GHC.Tc.Solver.setImplicationStatus)
                 should ignore givens even if they are insoluble.

    Note [Insoluble holes]
    ~~~~~~~~~~~~~~~~~~~~~~
    Hole constraints that ARE NOT treated as truly insoluble:
      a) type holes, arising from PartialTypeSignatures,
      b) "true" expression holes arising from TypedHoles

    An "expression hole" or "type hole" isn't really an error
    at all; it's a report saying "_ :: Int" here.  But an out-of-scope
    variable masquerading as expression holes IS treated as truly
    insoluble, so that it trumps other errors during error reporting.
    Yuk!

    ************************************************************************
    *                                                                      *
                    Implication constraints
    *                                                                      *
    ************************************************************************
    -}

    data Implication
      = Implic {   -- Invariants for a tree of implications:
                   -- see TcType Note [TcLevel invariants]

          Implication -> TcLevel
    ic_tclvl :: TcLevel,       -- TcLevel of unification variables
                                     -- allocated /inside/ this implication

          Implication -> [EvVar]
    ic_skols :: [TcTyVar],     -- Introduced skolems
          Implication -> SkolemInfo
    ic_info  :: SkolemInfo,    -- See Note [Skolems in an implication]
                                     -- See Note [Shadowing in a constraint]

          Implication -> [EvVar]
    ic_given  :: [EvVar],      -- Given evidence variables
                                     --   (order does not matter)
                                     -- See Invariant (GivenInv) in GHC.Tc.Utils.TcType

          Implication -> HasGivenEqs
    ic_given_eqs :: HasGivenEqs,  -- Are there Given equalities here?

          Implication -> Bool
    ic_warn_inaccessible :: Bool,
                                     -- True  <=> -Winaccessible-code is enabled
                                     -- at construction. See
                                     -- Note [Avoid -Winaccessible-code when deriving]
                                     -- in GHC.Tc.TyCl.Instance

          Implication -> TcLclEnv
    ic_env   :: TcLclEnv,
                                     -- Records the TcLClEnv at the time of creation.
                                     --
                                     -- The TcLclEnv gives the source location
                                     -- and error context for the implication, and
                                     -- hence for all the given evidence variables.

          Implication -> WantedConstraints
    ic_wanted :: WantedConstraints,  -- The wanteds
                                           -- See Invariang (WantedInf) in GHC.Tc.Utils.TcType

          Implication -> EvBindsVar
    ic_binds  :: EvBindsVar,    -- Points to the place to fill in the
                                      -- abstraction and bindings.

          -- The ic_need fields keep track of which Given evidence
          -- is used by this implication or its children
          -- NB: including stuff used by nested implications that have since
          --     been discarded
          -- See Note [Needed evidence variables]
          Implication -> TcTyCoVarSet
    ic_need_inner :: VarSet,    -- Includes all used Given evidence
          Implication -> TcTyCoVarSet
    ic_need_outer :: VarSet,    -- Includes only the free Given evidence
                                      --  i.e. ic_need_inner after deleting
                                      --       (a) givens (b) binders of ic_binds

          Implication -> ImplicStatus
    ic_status   :: ImplicStatus
        }

    implicationPrototype :: Implication
    implicationPrototype :: Implication
    implicationPrototype
       = Implic { -- These fields must be initialised
                  ic_tclvl :: TcLevel
    ic_tclvl      = forall a. String -> a
    panic String
    "newImplic:tclvl"
                , ic_binds :: EvBindsVar
    ic_binds      = forall a. String -> a
    panic String
    "newImplic:binds"
                , ic_info :: SkolemInfo
    ic_info       = forall a. String -> a
    panic String
    "newImplic:info"
                , ic_env :: TcLclEnv
    ic_env        = forall a. String -> a
    panic String
    "newImplic:env"
                , ic_warn_inaccessible :: Bool
    ic_warn_inaccessible = forall a. String -> a
    panic String
    "newImplic:warn_inaccessible"

                  -- The rest have sensible default values
                , ic_skols :: [EvVar]
    ic_skols      = []
                , ic_given :: [EvVar]
    ic_given      = []
                , ic_wanted :: WantedConstraints
    ic_wanted     = WantedConstraints
    emptyWC
                , ic_given_eqs :: HasGivenEqs
    ic_given_eqs  = HasGivenEqs
    MaybeGivenEqs
                , ic_status :: ImplicStatus
    ic_status     = ImplicStatus
    IC_Unsolved
                , ic_need_inner :: TcTyCoVarSet
    ic_need_inner = TcTyCoVarSet
    emptyVarSet
                , ic_need_outer :: TcTyCoVarSet
    ic_need_outer = TcTyCoVarSet
    emptyVarSet }

    data ImplicStatus
      = IC_Solved     -- All wanteds in the tree are solved, all the way down
           { ImplicStatus -> [EvVar]
    ics_dead :: [EvVar] }  -- Subset of ic_given that are not needed
             -- See Note [Tracking redundant constraints] in GHC.Tc.Solver

      | IC_Insoluble  -- At least one insoluble constraint in the tree

      | IC_BadTelescope  -- Solved, but the skolems in the telescope are out of
                         -- dependency order. See Note [Checking telescopes]

      | IC_Unsolved   -- Neither of the above; might go either way

    data HasGivenEqs -- See Note [HasGivenEqs]
      = NoGivenEqs      -- Definitely no given equalities,
                        --   except by Note [Let-bound skolems] in GHC.Tc.Solver.Monad
      | LocalGivenEqs   -- Might have Given equalities, but only ones that affect only
                        --   local skolems e.g. forall a b. (a ~ F b) => ...
      | MaybeGivenEqs   -- Might have any kind of Given equalities; no floating out
                        --   is possible.
      deriving HasGivenEqs -> HasGivenEqs -> Bool
    forall a. (a -> a -> Bool) -> (a -> a -> Bool) -> Eq a
    /= :: HasGivenEqs -> HasGivenEqs -> Bool
    $c/= :: HasGivenEqs -> HasGivenEqs -> Bool
    == :: HasGivenEqs -> HasGivenEqs -> Bool
    $c== :: HasGivenEqs -> HasGivenEqs -> Bool
    Eq

    {- Note [HasGivenEqs]
    ~~~~~~~~~~~~~~~~~~~~~
    The GivenEqs data type describes the Given constraints of an implication constraint:

    * NoGivenEqs: definitely no Given equalities, except perhaps let-bound skolems
      which don't count: see Note [Let-bound skolems] in GHC.Tc.Solver.Monad
      Examples: forall a. Eq a => ...
                forall a. (Show a, Num a) => ...
                forall a. a ~ Either Int Bool => ...  -- Let-bound skolem

    * LocalGivenEqs: definitely no Given equalities that would affect principal
      types.  But may have equalities that affect only skolems of this implication
      (and hence do not affect princial types)
      Examples: forall a. F a ~ Int => ...
                forall a b. F a ~ G b => ...

    * MaybeGivenEqs: may have Given equalities that would affect principal
      types
      Examples: forall. (a ~ b) => ...
                forall a. F a ~ b => ...
                forall a. c a => ...       -- The 'c' might be instantiated to (b ~)
                forall a. C a b => ....
                   where class x~y => C a b
                   so there is an equality in the superclass of a Given

    The HasGivenEqs classifications affect two things:

    * Suppressing redundant givens during error reporting; see GHC.Tc.Errors
      Note [Suppress redundant givens during error reporting]

    * Floating in approximateWC.

    Specifically, here's how it goes:

                     Stops floating    |   Suppresses Givens in errors
                     in approximateWC  |
                     -----------------------------------------------
     NoGivenEqs         NO             |         YES
     LocalGivenEqs      NO             |         NO
     MaybeGivenEqs      YES            |         NO
    -}

    instance Outputable Implication where
      ppr :: Implication -> SDoc
    ppr (Implic { ic_tclvl :: Implication -> TcLevel
    ic_tclvl = TcLevel
    tclvl, ic_skols :: Implication -> [EvVar]
    ic_skols = [EvVar]
    skols
                  , ic_given :: Implication -> [EvVar]
    ic_given = [EvVar]
    given, ic_given_eqs :: Implication -> HasGivenEqs
    ic_given_eqs = HasGivenEqs
    given_eqs
                  , ic_wanted :: Implication -> WantedConstraints
    ic_wanted = WantedConstraints
    wanted, ic_status :: Implication -> ImplicStatus
    ic_status = ImplicStatus
    status
                  , ic_binds :: Implication -> EvBindsVar
    ic_binds = EvBindsVar
    binds
                  , ic_need_inner :: Implication -> TcTyCoVarSet
    ic_need_inner = TcTyCoVarSet
    need_in, ic_need_outer :: Implication -> TcTyCoVarSet
    ic_need_outer = TcTyCoVarSet
    need_out
                  , ic_info :: Implication -> SkolemInfo
    ic_info = SkolemInfo
    info })
       = SDoc -> Int -> SDoc -> SDoc
    hang (String -> SDoc
    text String
    "Implic" SDoc -> SDoc -> SDoc
    <+> SDoc
    lbrace)
            Int
    2 ([SDoc] -> SDoc
    sep [ String -> SDoc
    text String
    "TcLevel =" SDoc -> SDoc -> SDoc
    <+> forall a. Outputable a => a -> SDoc
    ppr TcLevel
    tclvl
                   , String -> SDoc
    text String
    "Skolems =" SDoc -> SDoc -> SDoc
    <+> [EvVar] -> SDoc
    pprTyVars [EvVar]
    skols
                   , String -> SDoc
    text String
    "Given-eqs =" SDoc -> SDoc -> SDoc
    <+> forall a. Outputable a => a -> SDoc
    ppr HasGivenEqs
    given_eqs
                   , String -> SDoc
    text String
    "Status =" SDoc -> SDoc -> SDoc
    <+> forall a. Outputable a => a -> SDoc
    ppr ImplicStatus
    status
                   , SDoc -> Int -> SDoc -> SDoc
    hang (String -> SDoc
    text String
    "Given =")  Int
    2 ([EvVar] -> SDoc
    pprEvVars [EvVar]
    given)
                   , SDoc -> Int -> SDoc -> SDoc
    hang (String -> SDoc
    text String
    "Wanted =") Int
    2 (forall a. Outputable a => a -> SDoc
    ppr WantedConstraints
    wanted)
                   , String -> SDoc
    text String
    "Binds =" SDoc -> SDoc -> SDoc
    <+> forall a. Outputable a => a -> SDoc
    ppr EvBindsVar
    binds
                   , SDoc -> SDoc
    whenPprDebug (String -> SDoc
    text String
    "Needed inner =" SDoc -> SDoc -> SDoc
    <+> forall a. Outputable a => a -> SDoc
    ppr TcTyCoVarSet
    need_in)
                   , SDoc -> SDoc
    whenPprDebug (String -> SDoc
    text String
    "Needed outer =" SDoc -> SDoc -> SDoc
    <+> forall a. Outputable a => a -> SDoc
    ppr TcTyCoVarSet
    need_out)
                   , SkolemInfo -> SDoc
    pprSkolInfo SkolemInfo
    info ] SDoc -> SDoc -> SDoc
    <+> SDoc
    rbrace)

    instance Outputable ImplicStatus where
      ppr :: ImplicStatus -> SDoc
    ppr ImplicStatus
    IC_Insoluble    = String -> SDoc
    text String
    "Insoluble"
      ppr ImplicStatus
    IC_BadTelescope = String -> SDoc
    text String
    "Bad telescope"
      ppr ImplicStatus
    IC_Unsolved     = String -> SDoc
    text String
    "Unsolved"
      ppr (IC_Solved { ics_dead :: ImplicStatus -> [EvVar]
    ics_dead = [EvVar]
    dead })
        = String -> SDoc
    text String
    "Solved" SDoc -> SDoc -> SDoc
    <+> (SDoc -> SDoc
    braces (String -> SDoc
    text String
    "Dead givens =" SDoc -> SDoc -> SDoc
    <+> forall a. Outputable a => a -> SDoc
    ppr [EvVar]
    dead))

    checkTelescopeSkol :: SkolemInfo -> Bool
    -- See Note [Checking telescopes]
    checkTelescopeSkol :: SkolemInfo -> Bool
    checkTelescopeSkol (ForAllSkol {}) = Bool
    True
    checkTelescopeSkol SkolemInfo
    _               = Bool
    False

    instance Outputable HasGivenEqs where
      ppr :: HasGivenEqs -> SDoc
    ppr HasGivenEqs
    NoGivenEqs    = String -> SDoc
    text String
    "NoGivenEqs"
      ppr HasGivenEqs
    LocalGivenEqs = String -> SDoc
    text String
    "LocalGivenEqs"
      ppr HasGivenEqs
    MaybeGivenEqs = String -> SDoc
    text String
    "MaybeGivenEqs"

    -- Used in GHC.Tc.Solver.Monad.getHasGivenEqs
    instance Semigroup HasGivenEqs where
      HasGivenEqs
    NoGivenEqs <> :: HasGivenEqs -> HasGivenEqs -> HasGivenEqs
    <> HasGivenEqs
    other = HasGivenEqs
    other
      HasGivenEqs
    other <> HasGivenEqs
    NoGivenEqs = HasGivenEqs
    other

      HasGivenEqs
    MaybeGivenEqs <> HasGivenEqs
    _other = HasGivenEqs
    MaybeGivenEqs
      HasGivenEqs
    _other <> HasGivenEqs
    MaybeGivenEqs = HasGivenEqs
    MaybeGivenEqs

      HasGivenEqs
    LocalGivenEqs <> HasGivenEqs
    LocalGivenEqs = HasGivenEqs
    LocalGivenEqs

    -- Used in GHC.Tc.Solver.Monad.getHasGivenEqs
    instance Monoid HasGivenEqs where
      mempty :: HasGivenEqs
    mempty = HasGivenEqs
    NoGivenEqs

    {- Note [Checking telescopes]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When kind-checking a /user-written/ type, we might have a "bad telescope"
    like this one:
      data SameKind :: forall k. k -> k -> Type
      type Foo :: forall a k (b :: k). SameKind a b -> Type

    The kind of 'a' mentions 'k' which is bound after 'a'.  Oops.

    One approach to doing this would be to bring each of a, k, and b into
    scope, one at a time, creating a separate implication constraint for
    each one, and bumping the TcLevel. This would work, because the kind
    of, say, a would be untouchable when k is in scope (and the constraint
    couldn't float out because k blocks it). However, it leads to terrible
    error messages, complaining about skolem escape. While it is indeed a
    problem of skolem escape, we can do better.

    Instead, our approach is to bring the block of variables into scope
    all at once, creating one implication constraint for the lot:

    * We make a single implication constraint when kind-checking
      the 'forall' in Foo's kind, something like
          forall a k (b::k). { wanted constraints }

    * Having solved {wanted}, before discarding the now-solved implication,
      the constraint solver checks the dependency order of the skolem
      variables (ic_skols).  This is done in setImplicationStatus.

    * This check is only necessary if the implication was born from a
      'forall' in a user-written signature (the HsForAllTy case in
      GHC.Tc.Gen.HsType.  If, say, it comes from checking a pattern match
      that binds existentials, where the type of the data constructor is
      known to be valid (it in tcConPat), no need for the check.

      So the check is done /if and only if/ ic_info is ForAllSkol.

    * If ic_info is (ForAllSkol dt dvs), the dvs::SDoc displays the
      original, user-written type variables.

    * Be careful /NOT/ to discard an implication with a ForAllSkol
      ic_info, even if ic_wanted is empty.  We must give the
      constraint solver a chance to make that bad-telescope test!  Hence
      the extra guard in emitResidualTvConstraint; see #16247

    * Don't mix up inferred and explicit variables in the same implication
      constraint.  E.g.
          foo :: forall a kx (b :: kx). SameKind a b
      We want an implication
          Implic { ic_skol = [(a::kx), kx, (b::kx)], ... }
      but GHC will attempt to quantify over kx, since it is free in (a::kx),
      and it's hopelessly confusing to report an error about quantified
      variables   kx (a::kx) kx (b::kx).
      Instead, the outer quantification over kx should be in a separate
      implication. TL;DR: an explicit forall should generate an implication
      quantified only over those explicitly quantified variables.

    Note [Needed evidence variables]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Th ic_need_evs field holds the free vars of ic_binds, and all the
    ic_binds in nested implications.

      * Main purpose: if one of the ic_givens is not mentioned in here, it
        is redundant.

      * solveImplication may drop an implication altogether if it has no
        remaining 'wanteds'. But we still track the free vars of its
        evidence binds, even though it has now disappeared.

    Note [Shadowing in a constraint]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We assume NO SHADOWING in a constraint.  Specifically
     * The unification variables are all implicitly quantified at top
       level, and are all unique
     * The skolem variables bound in ic_skols are all freah when the
       implication is created.
    So we can safely substitute. For example, if we have
       forall a.  a~Int => ...(forall b. ...a...)...
    we can push the (a~Int) constraint inwards in the "givens" without
    worrying that 'b' might clash.

    Note [Skolems in an implication]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The skolems in an implication are used:

    * When considering floating a constraint outside the implication in
      GHC.Tc.Solver.floatEqualities or GHC.Tc.Solver.approximateImplications
      For this, we can treat ic_skols as a set.

    * When checking that a /user-specified/ forall (ic_info = ForAllSkol tvs)
      has its variables in the correct order; see Note [Checking telescopes].
      Only for these implications does ic_skols need to be a list.

    Nota bene: Although ic_skols is a list, it is not necessarily
    in dependency order:
    - In the ic_info=ForAllSkol case, the user might have written them
      in the wrong order
    - In the case of a type signature like
          f :: [a] -> [b]
      the renamer gathers the implicit "outer" forall'd variables {a,b}, but
      does not know what order to put them in.  The type checker can sort them
      into dependency order, but only after solving all the kind constraints;
      and to do that it's convenient to create the Implication!

    So we accept that ic_skols may be out of order.  Think of it as a set or
    (in the case of ic_info=ForAllSkol, a list in user-specified, and possibly
    wrong, order.

    Note [Insoluble constraints]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Some of the errors that we get during canonicalization are best
    reported when all constraints have been simplified as much as
    possible. For instance, assume that during simplification the
    following constraints arise:

     [Wanted]   F alpha ~  uf1
     [Wanted]   beta ~ uf1 beta

    When canonicalizing the wanted (beta ~ uf1 beta), if we eagerly fail
    we will simply see a message:
        'Can't construct the infinite type  beta ~ uf1 beta'
    and the user has no idea what the uf1 variable is.

    Instead our plan is that we will NOT fail immediately, but:
        (1) Record the "frozen" error in the ic_insols field
        (2) Isolate the offending constraint from the rest of the inerts
        (3) Keep on simplifying/canonicalizing

    At the end, we will hopefully have substituted uf1 := F alpha, and we
    will be able to report a more informative error:
        'Can't construct the infinite type beta ~ F alpha beta'

    Insoluble constraints *do* include Derived constraints. For example,
    a functional dependency might give rise to [D] Int ~ Bool, and we must
    report that.  If insolubles did not contain Deriveds, reportErrors would
    never see it.

    ************************************************************************
    *                                                                      *
                Pretty printing
    *                                                                      *
    ************************************************************************
    -}

    pprEvVars :: [EvVar] -> SDoc    -- Print with their types
    pprEvVars :: [EvVar] -> SDoc
    pprEvVars [EvVar]
    ev_vars = [SDoc] -> SDoc
    vcat (forall a b. (a -> b) -> [a] -> [b]
    map EvVar -> SDoc
    pprEvVarWithType [EvVar]
    ev_vars)

    pprEvVarTheta :: [EvVar] -> SDoc
    pprEvVarTheta :: [EvVar] -> SDoc
    pprEvVarTheta [EvVar]
    ev_vars = [Xi] -> SDoc
    pprTheta (forall a b. (a -> b) -> [a] -> [b]
    map EvVar -> Xi
    evVarPred [EvVar]
    ev_vars)

    pprEvVarWithType :: EvVar -> SDoc
    pprEvVarWithType :: EvVar -> SDoc
    pprEvVarWithType EvVar
    v = forall a. Outputable a => a -> SDoc
    ppr EvVar
    v SDoc -> SDoc -> SDoc
    <+> SDoc
    dcolon SDoc -> SDoc -> SDoc
    <+> Xi -> SDoc
    pprType (EvVar -> Xi
    evVarPred EvVar
    v)

    wrapType :: Type -> [TyVar] -> [PredType] -> Type
    wrapType :: Xi -> [EvVar] -> [Xi] -> Xi
    wrapType Xi
    ty [EvVar]
    skols [Xi]
    givens = [EvVar] -> Xi -> Xi
    mkSpecForAllTys [EvVar]
    skols forall a b. (a -> b) -> a -> b
    $ [Xi] -> Xi -> Xi
    mkPhiTy [Xi]
    givens Xi
    ty

    {-
    ************************************************************************
    *                                                                      *
                CtEvidence
    *                                                                      *
    ************************************************************************

    Note [Evidence field of CtEvidence]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    During constraint solving we never look at the type of ctev_evar/ctev_dest;
    instead we look at the ctev_pred field.  The evtm/evar field
    may be un-zonked.

    Note [Bind new Givens immediately]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    For Givens we make new EvVars and bind them immediately. Two main reasons:
      * Gain sharing.  E.g. suppose we start with g :: C a b, where
           class D a => C a b
           class (E a, F a) => D a
        If we generate all g's superclasses as separate EvTerms we might
        get    selD1 (selC1 g) :: E a
               selD2 (selC1 g) :: F a
               selC1 g :: D a
        which we could do more economically as:
               g1 :: D a = selC1 g
               g2 :: E a = selD1 g1
               g3 :: F a = selD2 g1

      * For *coercion* evidence we *must* bind each given:
          class (a~b) => C a b where ....
          f :: C a b => ....
        Then in f's Givens we have g:(C a b) and the superclass sc(g,0):a~b.
        But that superclass selector can't (yet) appear in a coercion
        (see evTermCoercion), so the easy thing is to bind it to an Id.

    So a Given has EvVar inside it rather than (as previously) an EvTerm.

    -}

    -- | A place for type-checking evidence to go after it is generated.
    -- Wanted equalities are always HoleDest; other wanteds are always
    -- EvVarDest.
    data TcEvDest
      = EvVarDest EvVar         -- ^ bind this var to the evidence
                  -- EvVarDest is always used for non-type-equalities
                  -- e.g. class constraints

      | HoleDest  CoercionHole  -- ^ fill in this hole with the evidence
                  -- HoleDest is always used for type-equalities
                  -- See Note [Coercion holes] in GHC.Core.TyCo.Rep

    data CtEvidence
      = CtGiven    -- Truly given, not depending on subgoals
          { CtEvidence -> Xi
    ctev_pred :: TcPredType      -- See Note [Ct/evidence invariant]
          , CtEvidence -> EvVar
    ctev_evar :: EvVar           -- See Note [Evidence field of CtEvidence]
          , CtEvidence -> CtLoc
    ctev_loc  :: CtLoc }

      | CtWanted   -- Wanted goal
          { ctev_pred :: TcPredType     -- See Note [Ct/evidence invariant]
          , CtEvidence -> TcEvDest
    ctev_dest :: TcEvDest
          , CtEvidence -> ShadowInfo
    ctev_nosh :: ShadowInfo     -- See Note [Constraint flavours]
          , ctev_loc  :: CtLoc }

      | CtDerived  -- A goal that we don't really have to solve and can't
                   -- immediately rewrite anything other than a derived
                   -- (there's no evidence!) but if we do manage to solve
                   -- it may help in solving other goals.
          { ctev_pred :: TcPredType
          , ctev_loc  :: CtLoc }

    ctEvPred :: CtEvidence -> TcPredType
    -- The predicate of a flavor
    ctEvPred :: CtEvidence -> Xi
    ctEvPred = CtEvidence -> Xi
    ctev_pred

    ctEvLoc :: CtEvidence -> CtLoc
    ctEvLoc :: CtEvidence -> CtLoc
    ctEvLoc = CtEvidence -> CtLoc
    ctev_loc

    ctEvOrigin :: CtEvidence -> CtOrigin
    ctEvOrigin :: CtEvidence -> CtOrigin
    ctEvOrigin = CtLoc -> CtOrigin
    ctLocOrigin forall b c a. (b -> c) -> (a -> b) -> a -> c
    . CtEvidence -> CtLoc
    ctEvLoc

    -- | Get the equality relation relevant for a 'CtEvidence'
    ctEvEqRel :: CtEvidence -> EqRel
    ctEvEqRel :: CtEvidence -> EqRel
    ctEvEqRel = Xi -> EqRel
    predTypeEqRel forall b c a. (b -> c) -> (a -> b) -> a -> c
    . CtEvidence -> Xi
    ctEvPred

    -- | Get the role relevant for a 'CtEvidence'
    ctEvRole :: CtEvidence -> Role
    ctEvRole :: CtEvidence -> Role
    ctEvRole = EqRel -> Role
    eqRelRole forall b c a. (b -> c) -> (a -> b) -> a -> c
    . CtEvidence -> EqRel
    ctEvEqRel

    ctEvTerm :: CtEvidence -> EvTerm
    ctEvTerm :: CtEvidence -> EvTerm
    ctEvTerm CtEvidence
    ev = EvExpr -> EvTerm
    EvExpr (CtEvidence -> EvExpr
    ctEvExpr CtEvidence
    ev)

    ctEvExpr :: CtEvidence -> EvExpr
    ctEvExpr :: CtEvidence -> EvExpr
    ctEvExpr ev :: CtEvidence
    ev@(CtWanted { ctev_dest :: CtEvidence -> TcEvDest
    ctev_dest = HoleDest CoercionHole
    _ })
                = forall b. Coercion -> Expr b
    Coercion forall a b. (a -> b) -> a -> b
    $ HasDebugCallStack => CtEvidence -> Coercion
    ctEvCoercion CtEvidence
    ev
    ctEvExpr CtEvidence
    ev = EvVar -> EvExpr
    evId (CtEvidence -> EvVar
    ctEvEvId CtEvidence
    ev)

    ctEvCoercion :: HasDebugCallStack => CtEvidence -> TcCoercion
    ctEvCoercion :: HasDebugCallStack => CtEvidence -> Coercion
    ctEvCoercion (CtGiven { ctev_evar :: CtEvidence -> EvVar
    ctev_evar = EvVar
    ev_id })
      = EvVar -> Coercion
    mkTcCoVarCo EvVar
    ev_id
    ctEvCoercion (CtWanted { ctev_dest :: CtEvidence -> TcEvDest
    ctev_dest = TcEvDest
    dest })
      | HoleDest CoercionHole
    hole <- TcEvDest
    dest
      = -- ctEvCoercion is only called on type equalities
        -- and they always have HoleDests
        CoercionHole -> Coercion
    mkHoleCo CoercionHole
    hole
    ctEvCoercion CtEvidence
    ev
      = forall a. HasCallStack => String -> SDoc -> a
    pprPanic String
    "ctEvCoercion" (forall a. Outputable a => a -> SDoc
    ppr CtEvidence
    ev)

    ctEvEvId :: CtEvidence -> EvVar
    ctEvEvId :: CtEvidence -> EvVar
    ctEvEvId (CtWanted { ctev_dest :: CtEvidence -> TcEvDest
    ctev_dest = EvVarDest EvVar
    ev }) = EvVar
    ev
    ctEvEvId (CtWanted { ctev_dest :: CtEvidence -> TcEvDest
    ctev_dest = HoleDest CoercionHole
    h })   = CoercionHole -> EvVar
    coHoleCoVar CoercionHole
    h
    ctEvEvId (CtGiven  { ctev_evar :: CtEvidence -> EvVar
    ctev_evar = EvVar
    ev })           = EvVar
    ev
    ctEvEvId ctev :: CtEvidence
    ctev@(CtDerived {}) = forall a. HasCallStack => String -> SDoc -> a
    pprPanic String
    "ctEvId:" (forall a. Outputable a => a -> SDoc
    ppr CtEvidence
    ctev)

    instance Outputable TcEvDest where
      ppr :: TcEvDest -> SDoc
    ppr (HoleDest CoercionHole
    h)   = String -> SDoc
    text String
    "hole" SDoc -> SDoc -> SDoc
    <> forall a. Outputable a => a -> SDoc
    ppr CoercionHole
    h
      ppr (EvVarDest EvVar
    ev) = forall a. Outputable a => a -> SDoc
    ppr EvVar
    ev

    instance Outputable CtEvidence where
      ppr :: CtEvidence -> SDoc
    ppr CtEvidence
    ev = forall a. Outputable a => a -> SDoc
    ppr (CtEvidence -> CtFlavour
    ctEvFlavour CtEvidence
    ev)
               SDoc -> SDoc -> SDoc
    <+> SDoc
    pp_ev SDoc -> SDoc -> SDoc
    <+> SDoc -> SDoc
    braces (forall a. Outputable a => a -> SDoc
    ppr (CtLoc -> SubGoalDepth
    ctl_depth (CtEvidence -> CtLoc
    ctEvLoc CtEvidence
    ev)))
                             -- Show the sub-goal depth too
                   SDoc -> SDoc -> SDoc
    <> SDoc
    dcolon SDoc -> SDoc -> SDoc
    <+> forall a. Outputable a => a -> SDoc
    ppr (CtEvidence -> Xi
    ctEvPred CtEvidence
    ev)
        where
          pp_ev :: SDoc
    pp_ev = case CtEvidence
    ev of
                 CtGiven { ctev_evar :: CtEvidence -> EvVar
    ctev_evar = EvVar
    v } -> forall a. Outputable a => a -> SDoc
    ppr EvVar
    v
                 CtWanted {ctev_dest :: CtEvidence -> TcEvDest
    ctev_dest = TcEvDest
    d } -> forall a. Outputable a => a -> SDoc
    ppr TcEvDest
    d
                 CtDerived {}              -> String -> SDoc
    text String
    "_"

    isWanted :: CtEvidence -> Bool
    isWanted :: CtEvidence -> Bool
    isWanted (CtWanted {}) = Bool
    True
    isWanted CtEvidence
    _ = Bool
    False

    isGiven :: CtEvidence -> Bool
    isGiven :: CtEvidence -> Bool
    isGiven (CtGiven {})  = Bool
    True
    isGiven CtEvidence
    _ = Bool
    False

    isDerived :: CtEvidence -> Bool
    isDerived :: CtEvidence -> Bool
    isDerived (CtDerived {}) = Bool
    True
    isDerived CtEvidence
    _              = Bool
    False

    {-
    %************************************************************************
    %*                                                                      *
                CtFlavour
    %*                                                                      *
    %************************************************************************

    Note [Constraint flavours]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    Constraints come in four flavours:

    * [G] Given: we have evidence

    * [W] Wanted WOnly: we want evidence

    * [D] Derived: any solution must satisfy this constraint, but
          we don't need evidence for it.  Examples include:
            - superclasses of [W] class constraints
            - equalities arising from functional dependencies
              or injectivity

    * [WD] Wanted WDeriv: a single constraint that represents
                          both [W] and [D]
      We keep them paired as one both for efficiency

    The ctev_nosh field of a Wanted distinguishes between [W] and [WD]

    Wanted constraints are born as [WD], but are split into [W] and its
    "shadow" [D] in GHC.Tc.Solver.Monad.maybeEmitShadow.

    See Note [The improvement story and derived shadows] in GHC.Tc.Solver.Monad
    -}

    data CtFlavour  -- See Note [Constraint flavours]
      = Given
      | Wanted ShadowInfo
      | Derived
      deriving CtFlavour -> CtFlavour -> Bool
    forall a. (a -> a -> Bool) -> (a -> a -> Bool) -> Eq a
    /= :: CtFlavour -> CtFlavour -> Bool
    $c/= :: CtFlavour -> CtFlavour -> Bool
    == :: CtFlavour -> CtFlavour -> Bool
    $c== :: CtFlavour -> CtFlavour -> Bool
    Eq

    data ShadowInfo
      = WDeriv   -- [WD] This Wanted constraint has no Derived shadow,
                 -- so it behaves like a pair of a Wanted and a Derived
      | WOnly    -- [W] It has a separate derived shadow
                 -- See Note [The improvement story and derived shadows] in GHC.Tc.Solver.Monad
      deriving( ShadowInfo -> ShadowInfo -> Bool
    forall a. (a -> a -> Bool) -> (a -> a -> Bool) -> Eq a
    /= :: ShadowInfo -> ShadowInfo -> Bool
    $c/= :: ShadowInfo -> ShadowInfo -> Bool
    == :: ShadowInfo -> ShadowInfo -> Bool
    $c== :: ShadowInfo -> ShadowInfo -> Bool
    Eq )

    instance Outputable CtFlavour where
      ppr :: CtFlavour -> SDoc
    ppr CtFlavour
    Given           = String -> SDoc
    text String
    "[G]"
      ppr (Wanted ShadowInfo
    WDeriv) = String -> SDoc
    text String
    "[WD]"
      ppr (Wanted ShadowInfo
    WOnly)  = String -> SDoc
    text String
    "[W]"
      ppr CtFlavour
    Derived         = String -> SDoc
    text String
    "[D]"

    -- | Does this 'CtFlavour' subsumed 'Derived'? True of @[WD]@ and @[D]@.
    ctFlavourContainsDerived :: CtFlavour -> Bool
    ctFlavourContainsDerived :: CtFlavour -> Bool
    ctFlavourContainsDerived (Wanted ShadowInfo
    WDeriv) = Bool
    True
    ctFlavourContainsDerived CtFlavour
    Derived         = Bool
    True
    ctFlavourContainsDerived CtFlavour
    _               = Bool
    False

    ctEvFlavour :: CtEvidence -> CtFlavour
    ctEvFlavour :: CtEvidence -> CtFlavour
    ctEvFlavour (CtWanted { ctev_nosh :: CtEvidence -> ShadowInfo
    ctev_nosh = ShadowInfo
    nosh }) = ShadowInfo -> CtFlavour
    Wanted ShadowInfo
    nosh
    ctEvFlavour (CtGiven {})                    = CtFlavour
    Given
    ctEvFlavour (CtDerived {})                  = CtFlavour
    Derived

    -- | Whether or not one 'Ct' can rewrite another is determined by its
    -- flavour and its equality relation. See also
    -- Note [Flavours with roles] in "GHC.Tc.Solver.Monad"
    type CtFlavourRole = (CtFlavour, EqRel)

    -- | Extract the flavour, role, and boxity from a 'CtEvidence'
    ctEvFlavourRole :: CtEvidence -> CtFlavourRole
    ctEvFlavourRole :: CtEvidence -> CtFlavourRole
    ctEvFlavourRole CtEvidence
    ev = (CtEvidence -> CtFlavour
    ctEvFlavour CtEvidence
    ev, CtEvidence -> EqRel
    ctEvEqRel CtEvidence
    ev)

    -- | Extract the flavour and role from a 'Ct'
    ctFlavourRole :: Ct -> CtFlavourRole
    -- Uses short-cuts to role for special cases
    ctFlavourRole :: Ct -> CtFlavourRole
    ctFlavourRole (CDictCan { cc_ev :: Ct -> CtEvidence
    cc_ev = CtEvidence
    ev })
      = (CtEvidence -> CtFlavour
    ctEvFlavour CtEvidence
    ev, EqRel
    NomEq)
    ctFlavourRole (CEqCan { cc_ev :: Ct -> CtEvidence
    cc_ev = CtEvidence
    ev, cc_eq_rel :: Ct -> EqRel
    cc_eq_rel = EqRel
    eq_rel })
      = (CtEvidence -> CtFlavour
    ctEvFlavour CtEvidence
    ev, EqRel
    eq_rel)
    ctFlavourRole Ct
    ct
      = CtEvidence -> CtFlavourRole
    ctEvFlavourRole (Ct -> CtEvidence
    ctEvidence Ct
    ct)

    {- Note [eqCanRewrite]
    ~~~~~~~~~~~~~~~~~~~~~~
    (eqCanRewrite ct1 ct2) holds if the constraint ct1 (a CEqCan of form
    lhs ~ ty) can be used to rewrite ct2.  It must satisfy the properties of
    a can-rewrite relation, see Definition [Can-rewrite relation] in
    GHC.Tc.Solver.Monad.

    With the solver handling Coercible constraints like equality constraints,
    the rewrite conditions must take role into account, never allowing
    a representational equality to rewrite a nominal one.

    Note [Wanteds do not rewrite Wanteds]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We don't allow Wanteds to rewrite Wanteds, because that can give rise
    to very confusing type error messages.  A good example is #8450.
    Here's another
       f :: a -> Bool
       f x = ( [x,'c'], [x,True] ) `seq` True
    Here we get
      [W] a ~ Char
      [W] a ~ Bool
    but we do not want to complain about Bool ~ Char!

    Note [Deriveds do rewrite Deriveds]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    However we DO allow Deriveds to rewrite Deriveds, because that's how
    improvement works; see Note [The improvement story] in GHC.Tc.Solver.Interact.

    However, for now at least I'm only letting (Derived,NomEq) rewrite
    (Derived,NomEq) and not doing anything for ReprEq.  If we have
        eqCanRewriteFR (Derived, NomEq) (Derived, _)  = True
    then we lose property R2 of Definition [Can-rewrite relation]
    in GHC.Tc.Solver.Monad
      R2.  If f1 >= f, and f2 >= f,
           then either f1 >= f2 or f2 >= f1
    Consider f1 = (Given, ReprEq)
             f2 = (Derived, NomEq)
              f = (Derived, ReprEq)

    I thought maybe we could never get Derived ReprEq constraints, but
    we can; straight from the Wanteds during improvement. And from a Derived
    ReprEq we could conceivably get a Derived NomEq improvement (by decomposing
    a type constructor with Nomninal role), and hence unify.

    This restriction that (Derived, NomEq) cannot rewrite (Derived, ReprEq) bites,
    in an obscure scenario:

      data T a
      type role T nominal

      type family F a

      g :: forall b a. (F a ~ T a, Coercible (F a) (T b)) => ()
      g = ()

      f :: forall a. (F a ~ T a) => ()
      f = g @a

    The problem is in the body of f. We have

      [G]  F a ~N T a
      [WD] F alpha ~N T alpha
      [WD] F alpha ~R T a

    The Wanteds aren't of use, so let's just look at Deriveds:

      [G] F a ~N T a
      [D] F alpha ~N T alpha
      [D] F alpha ~R T a

    As written, this makes no progress, and GHC errors. But, if we
    allowed D/N to rewrite D/R, the first D could rewrite the second:

      [G] F a ~N T a
      [D] F alpha ~N T alpha
      [D] T alpha ~R T a

    Now we decompose the second D to get

      [D] alpha ~N a

    noting the role annotation on T. This causes (alpha := a), and then
    everything else unlocks.

    What to do? We could "decompose" nominal equalities into nominal-only
    ("NO") equalities and representational ones, where a NO equality rewrites
    only nominals. That is, when considering whether [D] F alpha ~N T alpha
    should rewrite [D] F alpha ~R T a, we could require splitting the first D
    into [D] F alpha ~NO T alpha, [D] F alpha ~R T alpha. Then, we use the R
    half of the split to rewrite the second D, and off we go. This splitting
    would allow the split-off R equality to be rewritten by other equalities,
    thus avoiding the problem in Note [Why R2?] in GHC.Tc.Solver.Monad.

    This infelicity is #19665 and tested in typecheck/should_compile/T19665
    (marked as expect_broken).

    -}

    eqCanRewrite :: EqRel -> EqRel -> Bool
    eqCanRewrite :: EqRel -> EqRel -> Bool
    eqCanRewrite EqRel
    NomEq  EqRel
    _      = Bool
    True
    eqCanRewrite EqRel
    ReprEq EqRel
    ReprEq = Bool
    True
    eqCanRewrite EqRel
    ReprEq EqRel
    NomEq  = Bool
    False

    eqCanRewriteFR :: CtFlavourRole -> CtFlavourRole -> Bool
    -- Can fr1 actually rewrite fr2?
    -- Very important function!
    -- See Note [eqCanRewrite]
    -- See Note [Wanteds do not rewrite Wanteds]
    -- See Note [Deriveds do rewrite Deriveds]
    eqCanRewriteFR :: CtFlavourRole -> CtFlavourRole -> Bool
    eqCanRewriteFR (CtFlavour
    Given,         EqRel
    r1)    (CtFlavour
    _,       EqRel
    r2)    = EqRel -> EqRel -> Bool
    eqCanRewrite EqRel
    r1 EqRel
    r2
    eqCanRewriteFR (Wanted ShadowInfo
    WDeriv, EqRel
    NomEq) (CtFlavour
    Derived, EqRel
    NomEq) = Bool
    True
    eqCanRewriteFR (CtFlavour
    Derived,       EqRel
    NomEq) (CtFlavour
    Derived, EqRel
    NomEq) = Bool
    True
    eqCanRewriteFR CtFlavourRole
    _                      CtFlavourRole
    _                = Bool
    False

    eqMayRewriteFR :: CtFlavourRole -> CtFlavourRole -> Bool
    -- Is it /possible/ that fr1 can rewrite fr2?
    -- This is used when deciding which inerts to kick out,
    -- at which time a [WD] inert may be split into [W] and [D]
    eqMayRewriteFR :: CtFlavourRole -> CtFlavourRole -> Bool
    eqMayRewriteFR (Wanted ShadowInfo
    WDeriv, EqRel
    NomEq) (Wanted ShadowInfo
    WDeriv, EqRel
    NomEq) = Bool
    True
    eqMayRewriteFR (CtFlavour
    Derived,       EqRel
    NomEq) (Wanted ShadowInfo
    WDeriv, EqRel
    NomEq) = Bool
    True
    eqMayRewriteFR CtFlavourRole
    fr1 CtFlavourRole
    fr2 = CtFlavourRole -> CtFlavourRole -> Bool
    eqCanRewriteFR CtFlavourRole
    fr1 CtFlavourRole
    fr2

    {- Note [eqCanDischarge]
    ~~~~~~~~~~~~~~~~~~~~~~~~
    Suppose we have two identical CEqCan equality constraints
    (i.e. both LHS and RHS are the same)
          (x1:lhs~t) `eqCanDischarge` (xs:lhs~t)
    Can we just drop x2 in favour of x1?

    Answer: yes if eqCanDischarge is true.

    Note that we do /not/ allow Wanted to discharge Derived.
    We must keep both.  Why?  Because the Derived may rewrite
    other Deriveds in the model whereas the Wanted cannot.

    However a Wanted can certainly discharge an identical Wanted.  So
    eqCanDischarge does /not/ define a can-rewrite relation in the
    sense of Definition [Can-rewrite relation] in GHC.Tc.Solver.Monad.

    We /do/ say that a [W] can discharge a [WD].  In evidence terms it
    certainly can, and the /caller/ arranges that the otherwise-lost [D]
    is spat out as a new Derived.  -}

    eqCanDischargeFR :: CtFlavourRole -> CtFlavourRole -> Bool
    -- See Note [eqCanDischarge]
    eqCanDischargeFR :: CtFlavourRole -> CtFlavourRole -> Bool
    eqCanDischargeFR (CtFlavour
    f1,EqRel
    r1) (CtFlavour
    f2, EqRel
    r2) =  EqRel -> EqRel -> Bool
    eqCanRewrite EqRel
    r1 EqRel
    r2
                                      Bool -> Bool -> Bool
    && CtFlavour -> CtFlavour -> Bool
    eqCanDischargeF CtFlavour
    f1 CtFlavour
    f2

    eqCanDischargeF :: CtFlavour -> CtFlavour -> Bool
    eqCanDischargeF :: CtFlavour -> CtFlavour -> Bool
    eqCanDischargeF CtFlavour
    Given   CtFlavour
    _                  = Bool
    True
    eqCanDischargeF (Wanted ShadowInfo
    _)      (Wanted ShadowInfo
    _) = Bool
    True
    eqCanDischargeF (Wanted ShadowInfo
    WDeriv) CtFlavour
    Derived    = Bool
    True
    eqCanDischargeF CtFlavour
    Derived         CtFlavour
    Derived    = Bool
    True
    eqCanDischargeF CtFlavour
    _               CtFlavour
    _          = Bool
    False

    {-
    ************************************************************************
    *                                                                      *
                SubGoalDepth
    *                                                                      *
    ************************************************************************

    Note [SubGoalDepth]
    ~~~~~~~~~~~~~~~~~~~
    The 'SubGoalDepth' takes care of stopping the constraint solver from looping.

    The counter starts at zero and increases. It includes dictionary constraints,
    equality simplification, and type family reduction. (Why combine these? Because
    it's actually quite easy to mistake one for another, in sufficiently involved
    scenarios, like ConstraintKinds.)

    The flag -freduction-depth=n fixes the maximium level.

    * The counter includes the depth of type class instance declarations.  Example:
         [W] d{7} : Eq [Int]
      That is d's dictionary-constraint depth is 7.  If we use the instance
         $dfEqList :: Eq a => Eq [a]
      to simplify it, we get
         d{7} = $dfEqList d'{8}
      where d'{8} : Eq Int, and d' has depth 8.

      For civilised (decidable) instance declarations, each increase of
      depth removes a type constructor from the type, so the depth never
      gets big; i.e. is bounded by the structural depth of the type.

    * The counter also increments when resolving
    equalities involving type functions. Example:
      Assume we have a wanted at depth 7:
        [W] d{7} : F () ~ a
      If there is a type function equation "F () = Int", this would be rewritten to
        [W] d{8} : Int ~ a
      and remembered as having depth 8.

      Again, without UndecidableInstances, this counter is bounded, but without it
      can resolve things ad infinitum. Hence there is a maximum level.

    * Lastly, every time an equality is rewritten, the counter increases. Again,
      rewriting an equality constraint normally makes progress, but it's possible
      the "progress" is just the reduction of an infinitely-reducing type family.
      Hence we need to track the rewrites.

    When compiling a program requires a greater depth, then GHC recommends turning
    off this check entirely by setting -freduction-depth=0. This is because the
    exact number that works is highly variable, and is likely to change even between
    minor releases. Because this check is solely to prevent infinite compilation
    times, it seems safe to disable it when a user has ascertained that their program
    doesn't loop at the type level.

    -}

    -- | See Note [SubGoalDepth]
    newtype SubGoalDepth = SubGoalDepth Int
      deriving (SubGoalDepth -> SubGoalDepth -> Bool
    forall a. (a -> a -> Bool) -> (a -> a -> Bool) -> Eq a
    /= :: SubGoalDepth -> SubGoalDepth -> Bool
    $c/= :: SubGoalDepth -> SubGoalDepth -> Bool
    == :: SubGoalDepth -> SubGoalDepth -> Bool
    $c== :: SubGoalDepth -> SubGoalDepth -> Bool
    Eq, Eq SubGoalDepth
    SubGoalDepth -> SubGoalDepth -> Bool
    SubGoalDepth -> SubGoalDepth -> Ordering
    SubGoalDepth -> SubGoalDepth -> SubGoalDepth
    forall a.
    Eq a
    -> (a -> a -> Ordering)
    -> (a -> a -> Bool)
    -> (a -> a -> Bool)
    -> (a -> a -> Bool)
    -> (a -> a -> Bool)
    -> (a -> a -> a)
    -> (a -> a -> a)
    -> Ord a
    min :: SubGoalDepth -> SubGoalDepth -> SubGoalDepth
    $cmin :: SubGoalDepth -> SubGoalDepth -> SubGoalDepth
    max :: SubGoalDepth -> SubGoalDepth -> SubGoalDepth
    $cmax :: SubGoalDepth -> SubGoalDepth -> SubGoalDepth
    >= :: SubGoalDepth -> SubGoalDepth -> Bool
    $c>= :: SubGoalDepth -> SubGoalDepth -> Bool
    > :: SubGoalDepth -> SubGoalDepth -> Bool
    $c> :: SubGoalDepth -> SubGoalDepth -> Bool
    <= :: SubGoalDepth -> SubGoalDepth -> Bool
    $c<= :: SubGoalDepth -> SubGoalDepth -> Bool
    < :: SubGoalDepth -> SubGoalDepth -> Bool
    $c< :: SubGoalDepth -> SubGoalDepth -> Bool
    compare :: SubGoalDepth -> SubGoalDepth -> Ordering
    $ccompare :: SubGoalDepth -> SubGoalDepth -> Ordering
    Ord, SubGoalDepth -> SDoc
    forall a. (a -> SDoc) -> Outputable a
    ppr :: SubGoalDepth -> SDoc
    $cppr :: SubGoalDepth -> SDoc
    Outputable)

    initialSubGoalDepth :: SubGoalDepth
    initialSubGoalDepth :: SubGoalDepth
    initialSubGoalDepth = Int -> SubGoalDepth
    SubGoalDepth Int
    0

    bumpSubGoalDepth :: SubGoalDepth -> SubGoalDepth
    bumpSubGoalDepth :: SubGoalDepth -> SubGoalDepth
    bumpSubGoalDepth (SubGoalDepth Int
    n) = Int -> SubGoalDepth
    SubGoalDepth (Int
    n forall a. Num a => a -> a -> a
    + Int
    1)

    maxSubGoalDepth :: SubGoalDepth -> SubGoalDepth -> SubGoalDepth
    maxSubGoalDepth :: SubGoalDepth -> SubGoalDepth -> SubGoalDepth
    maxSubGoalDepth (SubGoalDepth Int
    n) (SubGoalDepth Int
    m) = Int -> SubGoalDepth
    SubGoalDepth (Int
    n forall a. Ord a => a -> a -> a
    `max` Int
    m)

    subGoalDepthExceeded :: DynFlags -> SubGoalDepth -> Bool
    subGoalDepthExceeded :: DynFlags -> SubGoalDepth -> Bool
    subGoalDepthExceeded DynFlags
    dflags (SubGoalDepth Int
    d)
      = Int -> IntWithInf
    mkIntWithInf Int
    d forall a. Ord a => a -> a -> Bool
    > DynFlags -> IntWithInf
    reductionDepth DynFlags
    dflags

    {-
    ************************************************************************
    *                                                                      *
                CtLoc
    *                                                                      *
    ************************************************************************

    The 'CtLoc' gives information about where a constraint came from.
    This is important for decent error message reporting because
    dictionaries don't appear in the original source code.
    type will evolve...

    -}

    data CtLoc = CtLoc { CtLoc -> CtOrigin
    ctl_origin :: CtOrigin
                       , CtLoc -> TcLclEnv
    ctl_env    :: TcLclEnv
                       , CtLoc -> Maybe TypeOrKind
    ctl_t_or_k :: Maybe TypeOrKind  -- OK if we're not sure
                       , CtLoc -> SubGoalDepth
    ctl_depth  :: !SubGoalDepth }

      -- The TcLclEnv includes particularly
      --    source location:  tcl_loc   :: RealSrcSpan
      --    context:          tcl_ctxt  :: [ErrCtxt]
      --    binder stack:     tcl_bndrs :: TcBinderStack
      --    level:            tcl_tclvl :: TcLevel

    mkKindLoc :: TcType -> TcType   -- original *types* being compared
              -> CtLoc -> CtLoc
    mkKindLoc :: Xi -> Xi -> CtLoc -> CtLoc
    mkKindLoc Xi
    s1 Xi
    s2 CtLoc
    loc = CtLoc -> CtOrigin -> CtLoc
    setCtLocOrigin (CtLoc -> CtLoc
    toKindLoc CtLoc
    loc)
                            (Xi -> Xi -> CtOrigin -> Maybe TypeOrKind -> CtOrigin
    KindEqOrigin Xi
    s1 Xi
    s2 (CtLoc -> CtOrigin
    ctLocOrigin CtLoc
    loc)
                                          (CtLoc -> Maybe TypeOrKind
    ctLocTypeOrKind_maybe CtLoc
    loc))

    -- | Take a CtLoc and moves it to the kind level
    toKindLoc :: CtLoc -> CtLoc
    toKindLoc :: CtLoc -> CtLoc
    toKindLoc CtLoc
    loc = CtLoc
    loc { ctl_t_or_k :: Maybe TypeOrKind
    ctl_t_or_k = forall a. a -> Maybe a
    Just TypeOrKind
    KindLevel }

    mkGivenLoc :: TcLevel -> SkolemInfo -> TcLclEnv -> CtLoc
    mkGivenLoc :: TcLevel -> SkolemInfo -> TcLclEnv -> CtLoc
    mkGivenLoc TcLevel
    tclvl SkolemInfo
    skol_info TcLclEnv
    env
      = CtLoc { ctl_origin :: CtOrigin
    ctl_origin = SkolemInfo -> CtOrigin
    GivenOrigin SkolemInfo
    skol_info
              , ctl_env :: TcLclEnv
    ctl_env    = TcLclEnv -> TcLevel -> TcLclEnv
    setLclEnvTcLevel TcLclEnv
    env TcLevel
    tclvl
              , ctl_t_or_k :: Maybe TypeOrKind
    ctl_t_or_k = forall a. Maybe a
    Nothing    -- this only matters for error msgs
              , ctl_depth :: SubGoalDepth
    ctl_depth  = SubGoalDepth
    initialSubGoalDepth }

    ctLocEnv :: CtLoc -> TcLclEnv
    ctLocEnv :: CtLoc -> TcLclEnv
    ctLocEnv = CtLoc -> TcLclEnv
    ctl_env

    ctLocLevel :: CtLoc -> TcLevel
    ctLocLevel :: CtLoc -> TcLevel
    ctLocLevel CtLoc
    loc = TcLclEnv -> TcLevel
    getLclEnvTcLevel (CtLoc -> TcLclEnv
    ctLocEnv CtLoc
    loc)

    ctLocDepth :: CtLoc -> SubGoalDepth
    ctLocDepth :: CtLoc -> SubGoalDepth
    ctLocDepth = CtLoc -> SubGoalDepth
    ctl_depth

    ctLocOrigin :: CtLoc -> CtOrigin
    ctLocOrigin :: CtLoc -> CtOrigin
    ctLocOrigin = CtLoc -> CtOrigin
    ctl_origin

    ctLocSpan :: CtLoc -> RealSrcSpan
    ctLocSpan :: CtLoc -> RealSrcSpan
    ctLocSpan (CtLoc { ctl_env :: CtLoc -> TcLclEnv
    ctl_env = TcLclEnv
    lcl}) = TcLclEnv -> RealSrcSpan
    getLclEnvLoc TcLclEnv
    lcl

    ctLocTypeOrKind_maybe :: CtLoc -> Maybe TypeOrKind
    ctLocTypeOrKind_maybe :: CtLoc -> Maybe TypeOrKind
    ctLocTypeOrKind_maybe = CtLoc -> Maybe TypeOrKind
    ctl_t_or_k

    setCtLocSpan :: CtLoc -> RealSrcSpan -> CtLoc
    setCtLocSpan :: CtLoc -> RealSrcSpan -> CtLoc
    setCtLocSpan ctl :: CtLoc
    ctl@(CtLoc { ctl_env :: CtLoc -> TcLclEnv
    ctl_env = TcLclEnv
    lcl }) RealSrcSpan
    loc = CtLoc -> TcLclEnv -> CtLoc
    setCtLocEnv CtLoc
    ctl (TcLclEnv -> RealSrcSpan -> TcLclEnv
    setLclEnvLoc TcLclEnv
    lcl RealSrcSpan
    loc)

    bumpCtLocDepth :: CtLoc -> CtLoc
    bumpCtLocDepth :: CtLoc -> CtLoc
    bumpCtLocDepth loc :: CtLoc
    loc@(CtLoc { ctl_depth :: CtLoc -> SubGoalDepth
    ctl_depth = SubGoalDepth
    d }) = CtLoc
    loc { ctl_depth :: SubGoalDepth
    ctl_depth = SubGoalDepth -> SubGoalDepth
    bumpSubGoalDepth SubGoalDepth
    d }

    setCtLocOrigin :: CtLoc -> CtOrigin -> CtLoc
    setCtLocOrigin :: CtLoc -> CtOrigin -> CtLoc
    setCtLocOrigin CtLoc
    ctl CtOrigin
    orig = CtLoc
    ctl { ctl_origin :: CtOrigin
    ctl_origin = CtOrigin
    orig }

    updateCtLocOrigin :: CtLoc -> (CtOrigin -> CtOrigin) -> CtLoc
    updateCtLocOrigin :: CtLoc -> (CtOrigin -> CtOrigin) -> CtLoc
    updateCtLocOrigin ctl :: CtLoc
    ctl@(CtLoc { ctl_origin :: CtLoc -> CtOrigin
    ctl_origin = CtOrigin
    orig }) CtOrigin -> CtOrigin
    upd
      = CtLoc
    ctl { ctl_origin :: CtOrigin
    ctl_origin = CtOrigin -> CtOrigin
    upd CtOrigin
    orig }

    setCtLocEnv :: CtLoc -> TcLclEnv -> CtLoc
    setCtLocEnv :: CtLoc -> TcLclEnv -> CtLoc
    setCtLocEnv CtLoc
    ctl TcLclEnv
    env = CtLoc
    ctl { ctl_env :: TcLclEnv
    ctl_env = TcLclEnv
    env }

    pprCtLoc :: CtLoc -> SDoc
    -- "arising from ... at ..."
    -- Not an instance of Outputable because of the "arising from" prefix
    pprCtLoc :: CtLoc -> SDoc
    pprCtLoc (CtLoc { ctl_origin :: CtLoc -> CtOrigin
    ctl_origin = CtOrigin
    o, ctl_env :: CtLoc -> TcLclEnv
    ctl_env = TcLclEnv
    lcl})
      = [SDoc] -> SDoc
    sep [ CtOrigin -> SDoc
    pprCtOrigin CtOrigin
    o
            , String -> SDoc
    text String
    "at" SDoc -> SDoc -> SDoc
    <+> forall a. Outputable a => a -> SDoc
    ppr (TcLclEnv -> RealSrcSpan
    getLclEnvLoc TcLclEnv
    lcl)]