    {-# LANGUAGE CPP #-}
    {-# LANGUAGE DeriveFunctor #-}
    {-# LANGUAGE MultiWayIf #-}

    module GHC.Tc.Solver.Canonical(
         canonicalize,
         unifyDerived,
         makeSuperClasses,
         StopOrContinue(..), stopWith, continueWith, andWhenContinue,
         solveCallStack    -- For GHC.Tc.Solver
      ) where

    #include "HsVersions.h"

    import GHC.Prelude

    import GHC.Tc.Types.Constraint
    import GHC.Core.Predicate
    import GHC.Tc.Types.Origin
    import GHC.Tc.Utils.Unify
    import GHC.Tc.Utils.TcType
    import GHC.Core.Type
    import GHC.Tc.Solver.Rewrite
    import GHC.Tc.Solver.Monad
    import GHC.Tc.Types.Evidence
    import GHC.Tc.Types.EvTerm
    import GHC.Core.Class
    import GHC.Core.TyCon
    import GHC.Core.Multiplicity
    import GHC.Core.TyCo.Rep   -- cleverly decomposes types, good for completeness checking
    import GHC.Core.Coercion
    import GHC.Core.Coercion.Axiom
    import GHC.Core
    import GHC.Types.Id( mkTemplateLocals )
    import GHC.Core.FamInstEnv ( FamInstEnvs )
    import GHC.Tc.Instance.Family ( tcTopNormaliseNewTypeTF_maybe )
    import GHC.Types.Var
    import GHC.Types.Var.Env( mkInScopeSet )
    import GHC.Types.Var.Set( delVarSetList, anyVarSet )
    import GHC.Utils.Outputable
    import GHC.Utils.Panic
    import GHC.Builtin.Types ( anyTypeOfKind )
    import GHC.Types.Name.Set
    import GHC.Types.Name.Reader
    import GHC.Hs.Type( HsIPName(..) )

    import GHC.Data.Pair
    import GHC.Utils.Misc
    import GHC.Data.Bag
    import GHC.Utils.Monad
    import Control.Monad
    import Data.Maybe ( isJust, isNothing )
    import Data.List  ( zip4 )
    import GHC.Types.Basic

    import Data.Bifunctor ( bimap )
    import Data.Foldable ( traverse_ )

    {-
    ************************************************************************
    *                                                                      *
    *                      The Canonicaliser                               *
    *                                                                      *
    ************************************************************************

    Note [Canonicalization]
    ~~~~~~~~~~~~~~~~~~~~~~~

    Canonicalization converts a simple constraint to a canonical form. It is
    unary (i.e. treats individual constraints one at a time).

    Constraints originating from user-written code come into being as
    CNonCanonicals. We know nothing about these constraints. So, first:

         Classify CNonCanoncal constraints, depending on whether they
         are equalities, class predicates, or other.

    Then proceed depending on the shape of the constraint. Generally speaking,
    each constraint gets rewritten and then decomposed into one of several forms
    (see type Ct in GHC.Tc.Types).

    When an already-canonicalized constraint gets kicked out of the inert set,
    it must be recanonicalized. But we know a bit about its shape from the
    last time through, so we can skip the classification step.

    -}

    -- Top-level canonicalization
    -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    canonicalize :: Ct -> TcS (StopOrContinue Ct)
    canonicalize :: Ct -> TcS (StopOrContinue Ct)
    canonicalize (CNonCanonical { cc_ev :: Ct -> CtEvidence
    cc_ev = CtEvidence
    ev })
      = {-# SCC "canNC" #-}
        CtEvidence -> TcS (StopOrContinue Ct)
    canNC CtEvidence
    ev

    canonicalize (CQuantCan (QCI { qci_ev :: QCInst -> CtEvidence
    qci_ev = CtEvidence
    ev, qci_pend_sc :: QCInst -> Bool
    qci_pend_sc = Bool
    pend_sc }))
      = CtEvidence -> Bool -> TcS (StopOrContinue Ct)
    canForAll CtEvidence
    ev Bool
    pend_sc

    canonicalize (CIrredCan { cc_ev :: Ct -> CtEvidence
    cc_ev = CtEvidence
    ev })
      = CtEvidence -> TcS (StopOrContinue Ct)
    canNC CtEvidence
    ev
        -- Instead of rewriting the evidence before classifying, it's possible we
        -- can make progress without the rewrite. Try this first.
        -- For insolubles (all of which are equalities), do /not/ rewrite the arguments
        -- In #14350 doing so led entire-unnecessary and ridiculously large
        -- type function expansion.  Instead, canEqNC just applies
        -- the substitution to the predicate, and may do decomposition;
        --    e.g. a ~ [a], where [G] a ~ [Int], can decompose

    canonicalize (CDictCan { cc_ev :: Ct -> CtEvidence
    cc_ev = CtEvidence
    ev, cc_class :: Ct -> Class
    cc_class  = Class
    cls
                           , cc_tyargs :: Ct -> [TcType]
    cc_tyargs = [TcType]
    xis, cc_pend_sc :: Ct -> Bool
    cc_pend_sc = Bool
    pend_sc })
      = {-# SCC "canClass" #-}
        CtEvidence -> Class -> [TcType] -> Bool -> TcS (StopOrContinue Ct)
    canClass CtEvidence
    ev Class
    cls [TcType]
    xis Bool
    pend_sc

    canonicalize (CEqCan { cc_ev :: Ct -> CtEvidence
    cc_ev     = CtEvidence
    ev
                         , cc_lhs :: Ct -> CanEqLHS
    cc_lhs    = CanEqLHS
    lhs
                         , cc_rhs :: Ct -> TcType
    cc_rhs    = TcType
    rhs
                         , cc_eq_rel :: Ct -> EqRel
    cc_eq_rel = EqRel
    eq_rel })
      = {-# SCC "canEqLeafTyVarEq" #-}
        CtEvidence -> EqRel -> TcType -> TcType -> TcS (StopOrContinue Ct)
    canEqNC CtEvidence
    ev EqRel
    eq_rel (CanEqLHS -> TcType
    canEqLHSType CanEqLHS
    lhs) TcType
    rhs

    canNC :: CtEvidence -> TcS (StopOrContinue Ct)
    canNC :: CtEvidence -> TcS (StopOrContinue Ct)
    canNC CtEvidence
    ev =
      case TcType -> Pred
    classifyPredType TcType
    pred of
          ClassPred Class
    cls [TcType]
    tys     -> do String -> SDoc -> TcS ()
    traceTcS String
    "canEvNC:cls" (forall a. Outputable a => a -> SDoc
    ppr Class
    cls SDoc -> SDoc -> SDoc
    <+> forall a. Outputable a => a -> SDoc
    ppr [TcType]
    tys)
                                      CtEvidence -> Class -> [TcType] -> TcS (StopOrContinue Ct)
    canClassNC CtEvidence
    ev Class
    cls [TcType]
    tys
          EqPred EqRel
    eq_rel TcType
    ty1 TcType
    ty2 -> do String -> SDoc -> TcS ()
    traceTcS String
    "canEvNC:eq" (forall a. Outputable a => a -> SDoc
    ppr TcType
    ty1 SDoc -> SDoc -> SDoc
    $$ forall a. Outputable a => a -> SDoc
    ppr TcType
    ty2)
                                      CtEvidence -> EqRel -> TcType -> TcType -> TcS (StopOrContinue Ct)
    canEqNC    CtEvidence
    ev EqRel
    eq_rel TcType
    ty1 TcType
    ty2
          IrredPred {}          -> do String -> SDoc -> TcS ()
    traceTcS String
    "canEvNC:irred" (forall a. Outputable a => a -> SDoc
    ppr TcType
    pred)
                                      CtEvidence -> TcS (StopOrContinue Ct)
    canIrred CtEvidence
    ev
          ForAllPred [TcTyVar]
    tvs [TcType]
    th TcType
    p   -> do String -> SDoc -> TcS ()
    traceTcS String
    "canEvNC:forall" (forall a. Outputable a => a -> SDoc
    ppr TcType
    pred)
                                      CtEvidence
    -> [TcTyVar] -> [TcType] -> TcType -> TcS (StopOrContinue Ct)
    canForAllNC CtEvidence
    ev [TcTyVar]
    tvs [TcType]
    th TcType
    p
      where
        pred :: TcType
    pred = CtEvidence -> TcType
    ctEvPred CtEvidence
    ev

    {-
    ************************************************************************
    *                                                                      *
    *                      Class Canonicalization
    *                                                                      *
    ************************************************************************
    -}

    canClassNC :: CtEvidence -> Class -> [Type] -> TcS (StopOrContinue Ct)
    -- "NC" means "non-canonical"; that is, we have got here
    -- from a NonCanonical constraint, not from a CDictCan
    -- Precondition: EvVar is class evidence
    canClassNC :: CtEvidence -> Class -> [TcType] -> TcS (StopOrContinue Ct)
    canClassNC CtEvidence
    ev Class
    cls [TcType]
    tys
      | CtEvidence -> Bool
    isGiven CtEvidence
    ev  -- See Note [Eagerly expand given superclasses]
      = do { [Ct]
    sc_cts <- CtEvidence
    -> [TcTyVar] -> [TcType] -> Class -> [TcType] -> TcS [Ct]
    mkStrictSuperClasses CtEvidence
    ev [] [] Class
    cls [TcType]
    tys
           ; [Ct] -> TcS ()
    emitWork [Ct]
    sc_cts
           ; CtEvidence -> Class -> [TcType] -> Bool -> TcS (StopOrContinue Ct)
    canClass CtEvidence
    ev Class
    cls [TcType]
    tys Bool
    False }

      | CtEvidence -> Bool
    isWanted CtEvidence
    ev
      , Just FastString
    ip_name <- Class -> [TcType] -> Maybe FastString
    isCallStackPred Class
    cls [TcType]
    tys
      , OccurrenceOf Name
    func <- CtLoc -> CtOrigin
    ctLocOrigin CtLoc
    loc
      -- If we're given a CallStack constraint that arose from a function
      -- call, we need to push the current call-site onto the stack instead
      -- of solving it directly from a given.
      -- See Note [Overview of implicit CallStacks] in GHC.Tc.Types.Evidence
      -- and Note [Solving CallStack constraints] in GHC.Tc.Solver.Monad
      = do { -- First we emit a new constraint that will capture the
             -- given CallStack.
           ; let new_loc :: CtLoc
    new_loc = CtLoc -> CtOrigin -> CtLoc
    setCtLocOrigin CtLoc
    loc (HsIPName -> CtOrigin
    IPOccOrigin (FastString -> HsIPName
    HsIPName FastString
    ip_name))
                                -- We change the origin to IPOccOrigin so
                                -- this rule does not fire again.
                                -- See Note [Overview of implicit CallStacks]

           ; CtEvidence
    new_ev <- CtLoc -> TcType -> TcS CtEvidence
    newWantedEvVarNC CtLoc
    new_loc TcType
    pred

             -- Then we solve the wanted by pushing the call-site
             -- onto the newly emitted CallStack
           ; let ev_cs :: EvCallStack
    ev_cs = Name -> RealSrcSpan -> EvExpr -> EvCallStack
    EvCsPushCall Name
    func (CtLoc -> RealSrcSpan
    ctLocSpan CtLoc
    loc) (CtEvidence -> EvExpr
    ctEvExpr CtEvidence
    new_ev)
           ; CtEvidence -> EvCallStack -> TcS ()
    solveCallStack CtEvidence
    ev EvCallStack
    ev_cs

           ; CtEvidence -> Class -> [TcType] -> Bool -> TcS (StopOrContinue Ct)
    canClass CtEvidence
    new_ev Class
    cls [TcType]
    tys Bool
    False }

      | Bool
    otherwise
      = CtEvidence -> Class -> [TcType] -> Bool -> TcS (StopOrContinue Ct)
    canClass CtEvidence
    ev Class
    cls [TcType]
    tys (Class -> Bool
    has_scs Class
    cls)

      where
        has_scs :: Class -> Bool
    has_scs Class
    cls = Bool -> Bool
    not (forall (t :: * -> *) a. Foldable t => t a -> Bool
    null (Class -> [TcType]
    classSCTheta Class
    cls))
        loc :: CtLoc
    loc  = CtEvidence -> CtLoc
    ctEvLoc CtEvidence
    ev
        pred :: TcType
    pred = CtEvidence -> TcType
    ctEvPred CtEvidence
    ev

    solveCallStack :: CtEvidence -> EvCallStack -> TcS ()
    -- Also called from GHC.Tc.Solver when defaulting call stacks
    solveCallStack :: CtEvidence -> EvCallStack -> TcS ()
    solveCallStack CtEvidence
    ev EvCallStack
    ev_cs = do
      -- We're given ev_cs :: CallStack, but the evidence term should be a
      -- dictionary, so we have to coerce ev_cs to a dictionary for
      -- `IP ip CallStack`. See Note [Overview of implicit CallStacks]
      EvExpr
    cs_tm <- forall (m :: * -> *).
    (MonadThings m, HasModule m, HasDynFlags m) =>
    EvCallStack -> m EvExpr
    evCallStack EvCallStack
    ev_cs
      let ev_tm :: EvTerm
    ev_tm = EvExpr -> TcCoercion -> EvTerm
    mkEvCast EvExpr
    cs_tm (TcType -> TcCoercion
    wrapIP (CtEvidence -> TcType
    ctEvPred CtEvidence
    ev))
      CtEvidence -> EvTerm -> TcS ()
    setEvBindIfWanted CtEvidence
    ev EvTerm
    ev_tm

    canClass :: CtEvidence
             -> Class -> [Type]
             -> Bool            -- True <=> un-explored superclasses
             -> TcS (StopOrContinue Ct)
    -- Precondition: EvVar is class evidence

    canClass :: CtEvidence -> Class -> [TcType] -> Bool -> TcS (StopOrContinue Ct)
    canClass CtEvidence
    ev Class
    cls [TcType]
    tys Bool
    pend_sc
      =   -- all classes do *nominal* matching
        ASSERT2( ctEvRole ev == Nominal, ppr ev $$ ppr cls $$ ppr tys )
        do { ([TcType]
    xis, [TcCoercion]
    cos) <- CtEvidence -> TyCon -> [TcType] -> TcS ([TcType], [TcCoercion])
    rewriteArgsNom CtEvidence
    ev TyCon
    cls_tc [TcType]
    tys
           ; let co :: TcCoercion
    co = Role -> TyCon -> [TcCoercion] -> TcCoercion
    mkTcTyConAppCo Role
    Nominal TyCon
    cls_tc [TcCoercion]
    cos
                 xi :: TcType
    xi = Class -> [TcType] -> TcType
    mkClassPred Class
    cls [TcType]
    xis
                 mk_ct :: CtEvidence -> Ct
    mk_ct CtEvidence
    new_ev = CDictCan { cc_ev :: CtEvidence
    cc_ev = CtEvidence
    new_ev
                                         , cc_tyargs :: [TcType]
    cc_tyargs = [TcType]
    xis
                                         , cc_class :: Class
    cc_class = Class
    cls
                                         , cc_pend_sc :: Bool
    cc_pend_sc = Bool
    pend_sc }
           ; StopOrContinue CtEvidence
    mb <- CtEvidence
    -> TcType -> TcCoercion -> TcS (StopOrContinue CtEvidence)
    rewriteEvidence CtEvidence
    ev TcType
    xi TcCoercion
    co
           ; String -> SDoc -> TcS ()
    traceTcS String
    "canClass" ([SDoc] -> SDoc
    vcat [ forall a. Outputable a => a -> SDoc
    ppr CtEvidence
    ev
                                       , forall a. Outputable a => a -> SDoc
    ppr TcType
    xi, forall a. Outputable a => a -> SDoc
    ppr StopOrContinue CtEvidence
    mb ])
           ; forall (m :: * -> *) a. Monad m => a -> m a
    return (forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    fmap CtEvidence -> Ct
    mk_ct StopOrContinue CtEvidence
    mb) }
      where
        cls_tc :: TyCon
    cls_tc = Class -> TyCon
    classTyCon Class
    cls

    {- Note [The superclass story]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We need to add superclass constraints for two reasons:

    * For givens [G], they give us a route to proof.  E.g.
        f :: Ord a => a -> Bool
        f x = x == x
      We get a Wanted (Eq a), which can only be solved from the superclass
      of the Given (Ord a).

    * For wanteds [W], and deriveds [WD], [D], they may give useful
      functional dependencies.  E.g.
         class C a b | a -> b where ...
         class C a b => D a b where ...
      Now a [W] constraint (D Int beta) has (C Int beta) as a superclass
      and that might tell us about beta, via C's fundeps.  We can get this
      by generating a [D] (C Int beta) constraint.  It's derived because
      we don't actually have to cough up any evidence for it; it's only there
      to generate fundep equalities.

    See Note [Why adding superclasses can help].

    For these reasons we want to generate superclass constraints for both
    Givens and Wanteds. But:

    * (Minor) they are often not needed, so generating them aggressively
      is a waste of time.

    * (Major) if we want recursive superclasses, there would be an infinite
      number of them.  Here is a real-life example (#10318);

         class (Frac (Frac a) ~ Frac a,
                Fractional (Frac a),
                IntegralDomain (Frac a))
             => IntegralDomain a where
          type Frac a :: *

      Notice that IntegralDomain has an associated type Frac, and one
      of IntegralDomain's superclasses is another IntegralDomain constraint.

    So here's the plan:

    1. Eagerly generate superclasses for given (but not wanted)
       constraints; see Note [Eagerly expand given superclasses].
       This is done using mkStrictSuperClasses in canClassNC, when
       we take a non-canonical Given constraint and cannonicalise it.

       However stop if you encounter the same class twice.  That is,
       mkStrictSuperClasses expands eagerly, but has a conservative
       termination condition: see Note [Expanding superclasses] in GHC.Tc.Utils.TcType.

    2. Solve the wanteds as usual, but do no further expansion of
       superclasses for canonical CDictCans in solveSimpleGivens or
       solveSimpleWanteds; Note [Danger of adding superclasses during solving]

       However, /do/ continue to eagerly expand superclasses for new /given/
       /non-canonical/ constraints (canClassNC does this).  As #12175
       showed, a type-family application can expand to a class constraint,
       and we want to see its superclasses for just the same reason as
       Note [Eagerly expand given superclasses].

    3. If we have any remaining unsolved wanteds
            (see Note [When superclasses help] in GHC.Tc.Types.Constraint)
       try harder: take both the Givens and Wanteds, and expand
       superclasses again.  See the calls to expandSuperClasses in
       GHC.Tc.Solver.simpl_loop and solveWanteds.

       This may succeed in generating (a finite number of) extra Givens,
       and extra Deriveds. Both may help the proof.

    3a An important wrinkle: only expand Givens from the current level.
       Two reasons:
          - We only want to expand it once, and that is best done at
            the level it is bound, rather than repeatedly at the leaves
            of the implication tree
          - We may be inside a type where we can't create term-level
            evidence anyway, so we can't superclass-expand, say,
            (a ~ b) to get (a ~# b).  This happened in #15290.

    4. Go round to (2) again.  This loop (2,3,4) is implemented
       in GHC.Tc.Solver.simpl_loop.

    The cc_pend_sc flag in a CDictCan records whether the superclasses of
    this constraint have been expanded.  Specifically, in Step 3 we only
    expand superclasses for constraints with cc_pend_sc set to true (i.e.
    isPendingScDict holds).

    Why do we do this?  Two reasons:

    * To avoid repeated work, by repeatedly expanding the superclasses of
      same constraint,

    * To terminate the above loop, at least in the -XNoRecursiveSuperClasses
      case.  If there are recursive superclasses we could, in principle,
      expand forever, always encountering new constraints.

    When we take a CNonCanonical or CIrredCan, but end up classifying it
    as a CDictCan, we set the cc_pend_sc flag to False.

    Note [Superclass loops]
    ~~~~~~~~~~~~~~~~~~~~~~~
    Suppose we have
      class C a => D a
      class D a => C a

    Then, when we expand superclasses, we'll get back to the self-same
    predicate, so we have reached a fixpoint in expansion and there is no
    point in fruitlessly expanding further.  This case just falls out from
    our strategy.  Consider
      f :: C a => a -> Bool
      f x = x==x
    Then canClassNC gets the [G] d1: C a constraint, and eager emits superclasses
    G] d2: D a, [G] d3: C a (psc).  (The "psc" means it has its sc_pend flag set.)
    When processing d3 we find a match with d1 in the inert set, and we always
    keep the inert item (d1) if possible: see Note [Replacement vs keeping] in
    GHC.Tc.Solver.Interact.  So d3 dies a quick, happy death.

    Note [Eagerly expand given superclasses]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In step (1) of Note [The superclass story], why do we eagerly expand
    Given superclasses by one layer?  (By "one layer" we mean expand transitively
    until you meet the same class again -- the conservative criterion embodied
    in expandSuperClasses.  So a "layer" might be a whole stack of superclasses.)
    We do this eagerly for Givens mainly because of some very obscure
    cases like this:

       instance Bad a => Eq (T a)

       f :: (Ord (T a)) => blah
       f x = ....needs Eq (T a), Ord (T a)....

    Here if we can't satisfy (Eq (T a)) from the givens we'll use the
    instance declaration; but then we are stuck with (Bad a).  Sigh.
    This is really a case of non-confluent proofs, but to stop our users
    complaining we expand one layer in advance.

    Note [Instance and Given overlap] in GHC.Tc.Solver.Interact.

    We also want to do this if we have

       f :: F (T a) => blah

    where
       type instance F (T a) = Ord (T a)

    So we may need to do a little work on the givens to expose the
    class that has the superclasses.  That's why the superclass
    expansion for Givens happens in canClassNC.

    This same scenario happens with quantified constraints, whose superclasses
    are also eagerly expanded. Test case: typecheck/should_compile/T16502b
    These are handled in canForAllNC, analogously to canClassNC.

    Note [Why adding superclasses can help]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Examples of how adding superclasses can help:

        --- Example 1
            class C a b | a -> b
        Suppose we want to solve
             [G] C a b
             [W] C a beta
        Then adding [D] beta~b will let us solve it.

        -- Example 2 (similar but using a type-equality superclass)
            class (F a ~ b) => C a b
        And try to sllve:
             [G] C a b
             [W] C a beta
        Follow the superclass rules to add
             [G] F a ~ b
             [D] F a ~ beta
        Now we get [D] beta ~ b, and can solve that.

        -- Example (tcfail138)
          class L a b | a -> b
          class (G a, L a b) => C a b

          instance C a b' => G (Maybe a)
          instance C a b  => C (Maybe a) a
          instance L (Maybe a) a

        When solving the superclasses of the (C (Maybe a) a) instance, we get
          [G] C a b, and hance by superclasses, [G] G a, [G] L a b
          [W] G (Maybe a)
        Use the instance decl to get
          [W] C a beta
        Generate its derived superclass
          [D] L a beta.  Now using fundeps, combine with [G] L a b to get
          [D] beta ~ b
        which is what we want.

    Note [Danger of adding superclasses during solving]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Here's a serious, but now out-dated example, from #4497:

       class Num (RealOf t) => Normed t
       type family RealOf x

    Assume the generated wanted constraint is:
       [W] RealOf e ~ e
       [W] Normed e

    If we were to be adding the superclasses during simplification we'd get:
       [W] RealOf e ~ e
       [W] Normed e
       [D] RealOf e ~ fuv
       [D] Num fuv
    ==>
       e := fuv, Num fuv, Normed fuv, RealOf fuv ~ fuv

    While looks exactly like our original constraint. If we add the
    superclass of (Normed fuv) again we'd loop.  By adding superclasses
    definitely only once, during canonicalisation, this situation can't
    happen.

    Mind you, now that Wanteds cannot rewrite Derived, I think this particular
    situation can't happen.

    Note [Nested quantified constraint superclasses]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider (typecheck/should_compile/T17202)

      class C1 a
      class (forall c. C1 c) => C2 a
      class (forall b. (b ~ F a) => C2 a) => C3 a

    Elsewhere in the code, we get a [G] g1 :: C3 a. We expand its superclass
    to get [G] g2 :: (forall b. (b ~ F a) => C2 a). This constraint has a
    superclass, as well. But we now must be careful: we cannot just add
    (forall c. C1 c) as a Given, because we need to remember g2's context.
    That new constraint is Given only when forall b. (b ~ F a) is true.

    It's tempting to make the new Given be (forall b. (b ~ F a) => forall c. C1 c),
    but that's problematic, because it's nested, and ForAllPred is not capable
    of representing a nested quantified constraint. (We could change ForAllPred
    to allow this, but the solution in this Note is much more local and simpler.)

    So, we swizzle it around to get (forall b c. (b ~ F a) => C1 c).

    More generally, if we are expanding the superclasses of
      g0 :: forall tvs. theta => cls tys
    and find a superclass constraint
      forall sc_tvs. sc_theta => sc_inner_pred
    we must have a selector
      sel_id :: forall cls_tvs. cls cls_tvs -> forall sc_tvs. sc_theta => sc_inner_pred
    and thus build
      g_sc :: forall tvs sc_tvs. theta => sc_theta => sc_inner_pred
      g_sc = /\ tvs. /\ sc_tvs. \ theta_ids. \ sc_theta_ids.
             sel_id tys (g0 tvs theta_ids) sc_tvs sc_theta_ids

    Actually, we cheat a bit by eta-reducing: note that sc_theta_ids are both the
    last bound variables and the last arguments. This avoids the need to produce
    the sc_theta_ids at all. So our final construction is

      g_sc = /\ tvs. /\ sc_tvs. \ theta_ids.
             sel_id tys (g0 tvs theta_ids) sc_tvs

      -}

    makeSuperClasses :: [Ct] -> TcS [Ct]
    -- Returns strict superclasses, transitively, see Note [The superclasses story]
    -- See Note [The superclass story]
    -- The loop-breaking here follows Note [Expanding superclasses] in GHC.Tc.Utils.TcType
    -- Specifically, for an incoming (C t) constraint, we return all of (C t)'s
    --    superclasses, up to /and including/ the first repetition of C
    --
    -- Example:  class D a => C a
    --           class C [a] => D a
    -- makeSuperClasses (C x) will return (D x, C [x])
    --
    -- NB: the incoming constraints have had their cc_pend_sc flag already
    --     flipped to False, by isPendingScDict, so we are /obliged/ to at
    --     least produce the immediate superclasses
    makeSuperClasses :: [Ct] -> TcS [Ct]
    makeSuperClasses [Ct]
    cts = forall (m :: * -> *) a b. Monad m => (a -> m [b]) -> [a] -> m [b]
    concatMapM Ct -> TcS [Ct]
    go [Ct]
    cts
      where
        go :: Ct -> TcS [Ct]
    go (CDictCan { cc_ev :: Ct -> CtEvidence
    cc_ev = CtEvidence
    ev, cc_class :: Ct -> Class
    cc_class = Class
    cls, cc_tyargs :: Ct -> [TcType]
    cc_tyargs = [TcType]
    tys })
          = CtEvidence
    -> [TcTyVar] -> [TcType] -> Class -> [TcType] -> TcS [Ct]
    mkStrictSuperClasses CtEvidence
    ev [] [] Class
    cls [TcType]
    tys
        go (CQuantCan (QCI { qci_pred :: QCInst -> TcType
    qci_pred = TcType
    pred, qci_ev :: QCInst -> CtEvidence
    qci_ev = CtEvidence
    ev }))
          = ASSERT2( isClassPred pred, ppr pred )  -- The cts should all have
                                                   -- class pred heads
            CtEvidence
    -> [TcTyVar] -> [TcType] -> Class -> [TcType] -> TcS [Ct]
    mkStrictSuperClasses CtEvidence
    ev [TcTyVar]
    tvs [TcType]
    theta Class
    cls [TcType]
    tys
          where
            ([TcTyVar]
    tvs, [TcType]
    theta, Class
    cls, [TcType]
    tys) = TcType -> ([TcTyVar], [TcType], Class, [TcType])
    tcSplitDFunTy (CtEvidence -> TcType
    ctEvPred CtEvidence
    ev)
        go Ct
    ct = forall a. HasCallStack => String -> SDoc -> a
    pprPanic String
    "makeSuperClasses" (forall a. Outputable a => a -> SDoc
    ppr Ct
    ct)

    mkStrictSuperClasses
        :: CtEvidence
        -> [TyVar] -> ThetaType  -- These two args are non-empty only when taking
                                 -- superclasses of a /quantified/ constraint
        -> Class -> [Type] -> TcS [Ct]
    -- Return constraints for the strict superclasses of
    --   ev :: forall as. theta => cls tys
    mkStrictSuperClasses :: CtEvidence
    -> [TcTyVar] -> [TcType] -> Class -> [TcType] -> TcS [Ct]
    mkStrictSuperClasses CtEvidence
    ev [TcTyVar]
    tvs [TcType]
    theta Class
    cls [TcType]
    tys
      = NameSet
    -> CtEvidence
    -> [TcTyVar]
    -> [TcType]
    -> Class
    -> [TcType]
    -> TcS [Ct]
    mk_strict_superclasses (Name -> NameSet
    unitNameSet (Class -> Name
    className Class
    cls))
                               CtEvidence
    ev [TcTyVar]
    tvs [TcType]
    theta Class
    cls [TcType]
    tys

    mk_strict_superclasses :: NameSet -> CtEvidence
                           -> [TyVar] -> ThetaType
                           -> Class -> [Type] -> TcS [Ct]
    -- Always return the immediate superclasses of (cls tys);
    -- and expand their superclasses, provided none of them are in rec_clss
    -- nor are repeated
    mk_strict_superclasses :: NameSet
    -> CtEvidence
    -> [TcTyVar]
    -> [TcType]
    -> Class
    -> [TcType]
    -> TcS [Ct]
    mk_strict_superclasses NameSet
    rec_clss (CtGiven { ctev_evar :: CtEvidence -> TcTyVar
    ctev_evar = TcTyVar
    evar, ctev_loc :: CtEvidence -> CtLoc
    ctev_loc = CtLoc
    loc })
                           [TcTyVar]
    tvs [TcType]
    theta Class
    cls [TcType]
    tys
      = forall (m :: * -> *) a b. Monad m => (a -> m [b]) -> [a] -> m [b]
    concatMapM (CtLoc -> TcTyVar -> TcS [Ct]
    do_one_given (CtLoc -> CtLoc
    mk_given_loc CtLoc
    loc)) forall a b. (a -> b) -> a -> b
    $
        Class -> [TcTyVar]
    classSCSelIds Class
    cls
      where
        dict_ids :: [TcTyVar]
    dict_ids  = [TcType] -> [TcTyVar]
    mkTemplateLocals [TcType]
    theta
        size :: TypeSize
    size      = [TcType] -> TypeSize
    sizeTypes [TcType]
    tys

        do_one_given :: CtLoc -> TcTyVar -> TcS [Ct]
    do_one_given CtLoc
    given_loc TcTyVar
    sel_id
          | HasDebugCallStack => TcType -> Bool
    isUnliftedType TcType
    sc_pred
          , Bool -> Bool
    not (forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [TcTyVar]
    tvs Bool -> Bool -> Bool
    && forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [TcType]
    theta)
          = -- See Note [Equality superclasses in quantified constraints]
            forall (m :: * -> *) a. Monad m => a -> m a
    return []
          | Bool
    otherwise
          = do { CtEvidence
    given_ev <- CtLoc -> (TcType, EvTerm) -> TcS CtEvidence
    newGivenEvVar CtLoc
    given_loc forall a b. (a -> b) -> a -> b
    $
                             TcTyVar -> TcType -> (TcType, EvTerm)
    mk_given_desc TcTyVar
    sel_id TcType
    sc_pred
               ; NameSet
    -> CtEvidence -> [TcTyVar] -> [TcType] -> TcType -> TcS [Ct]
    mk_superclasses NameSet
    rec_clss CtEvidence
    given_ev [TcTyVar]
    tvs [TcType]
    theta TcType
    sc_pred }
          where
            sc_pred :: TcType
    sc_pred = TcTyVar -> [TcType] -> TcType
    classMethodInstTy TcTyVar
    sel_id [TcType]
    tys

          -- See Note [Nested quantified constraint superclasses]
        mk_given_desc :: Id -> PredType -> (PredType, EvTerm)
        mk_given_desc :: TcTyVar -> TcType -> (TcType, EvTerm)
    mk_given_desc TcTyVar
    sel_id TcType
    sc_pred
          = (TcType
    swizzled_pred, EvTerm
    swizzled_evterm)
          where
            ([TcTyVar]
    sc_tvs, TcType
    sc_rho)          = TcType -> ([TcTyVar], TcType)
    splitForAllTyCoVars TcType
    sc_pred
            ([Scaled TcType]
    sc_theta, TcType
    sc_inner_pred) = TcType -> ([Scaled TcType], TcType)
    splitFunTys TcType
    sc_rho

            all_tvs :: [TcTyVar]
    all_tvs       = [TcTyVar]
    tvs forall a. [a] -> [a] -> [a]
    `chkAppend` [TcTyVar]
    sc_tvs
            all_theta :: [TcType]
    all_theta     = [TcType]
    theta forall a. [a] -> [a] -> [a]
    `chkAppend` (forall a b. (a -> b) -> [a] -> [b]
    map forall a. Scaled a -> a
    scaledThing [Scaled TcType]
    sc_theta)
            swizzled_pred :: TcType
    swizzled_pred = [TcTyVar] -> [TcType] -> TcType -> TcType
    mkInfSigmaTy [TcTyVar]
    all_tvs [TcType]
    all_theta TcType
    sc_inner_pred

            -- evar :: forall tvs. theta => cls tys
            -- sel_id :: forall cls_tvs. cls cls_tvs
            --                        -> forall sc_tvs. sc_theta => sc_inner_pred
            -- swizzled_evterm :: forall tvs sc_tvs. theta => sc_theta => sc_inner_pred
            swizzled_evterm :: EvTerm
    swizzled_evterm = EvExpr -> EvTerm
    EvExpr forall a b. (a -> b) -> a -> b
    $
              forall b. [b] -> Expr b -> Expr b
    mkLams [TcTyVar]
    all_tvs forall a b. (a -> b) -> a -> b
    $
              forall b. [b] -> Expr b -> Expr b
    mkLams [TcTyVar]
    dict_ids forall a b. (a -> b) -> a -> b
    $
              forall b. TcTyVar -> Expr b
    Var TcTyVar
    sel_id
                forall b. Expr b -> [TcType] -> Expr b
    `mkTyApps` [TcType]
    tys
                forall b. Expr b -> Expr b -> Expr b
    `App` (TcTyVar -> EvExpr
    evId TcTyVar
    evar forall b. Expr b -> [TcTyVar] -> Expr b
    `mkVarApps` ([TcTyVar]
    tvs forall a. [a] -> [a] -> [a]
    ++ [TcTyVar]
    dict_ids))
                forall b. Expr b -> [TcTyVar] -> Expr b
    `mkVarApps` [TcTyVar]
    sc_tvs

        mk_given_loc :: CtLoc -> CtLoc
    mk_given_loc CtLoc
    loc
           | Class -> Bool
    isCTupleClass Class
    cls
           = CtLoc
    loc   -- For tuple predicates, just take them apart, without
                   -- adding their (large) size into the chain.  When we
                   -- get down to a base predicate, we'll include its size.
                   -- #10335

             -- See Note [Solving superclass constraints] in GHC.Tc.TyCl.Instance
             -- for explantation of InstSCOrigin and Note [Replacement vs keeping] in
             -- GHC.Tc.Solver.Interact for why we need OtherSCOrigin and depths
           | Bool
    otherwise
           = CtLoc
    loc { ctl_origin :: CtOrigin
    ctl_origin = CtOrigin
    new_orig }
           where
             new_orig :: CtOrigin
    new_orig = case CtLoc -> CtOrigin
    ctLocOrigin CtLoc
    loc of
                -- these cases are when we have something that's already a superclass constraint
               InstSCOrigin  Int
    sc_depth TypeSize
    n  -> Int -> TypeSize -> CtOrigin
    InstSCOrigin  (Int
    sc_depth forall a. Num a => a -> a -> a
    + Int
    1) (TypeSize
    n forall a. Ord a => a -> a -> a
    `max` TypeSize
    size)
               OtherSCOrigin Int
    sc_depth SkolemInfo
    si -> Int -> SkolemInfo -> CtOrigin
    OtherSCOrigin (Int
    sc_depth forall a. Num a => a -> a -> a
    + Int
    1) SkolemInfo
    si

                -- these cases do not already have a superclass constraint: depth starts at 1
               GivenOrigin SkolemInfo
    InstSkol      -> Int -> TypeSize -> CtOrigin
    InstSCOrigin  Int
    1 TypeSize
    size
               GivenOrigin SkolemInfo
    other_skol    -> Int -> SkolemInfo -> CtOrigin
    OtherSCOrigin Int
    1 SkolemInfo
    other_skol

               CtOrigin
    other_orig                -> forall a. HasCallStack => String -> SDoc -> a
    pprPanic String
    "Given constraint without given origin" forall a b. (a -> b) -> a -> b
    $
                                            forall a. Outputable a => a -> SDoc
    ppr TcTyVar
    evar SDoc -> SDoc -> SDoc
    $$ forall a. Outputable a => a -> SDoc
    ppr CtOrigin
    other_orig

    mk_strict_superclasses NameSet
    rec_clss CtEvidence
    ev [TcTyVar]
    tvs [TcType]
    theta Class
    cls [TcType]
    tys
      | forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
    all TcType -> Bool
    noFreeVarsOfType [TcType]
    tys
      = forall (m :: * -> *) a. Monad m => a -> m a
    return [] -- Wanteds with no variables yield no deriveds.
                  -- See Note [Improvement from Ground Wanteds]

      | Bool
    otherwise -- Wanted/Derived case, just add Derived superclasses
                  -- that can lead to improvement.
      = ASSERT2( null tvs && null theta, ppr tvs $$ ppr theta )
        forall (m :: * -> *) a b. Monad m => (a -> m [b]) -> [a] -> m [b]
    concatMapM TcType -> TcS [Ct]
    do_one_derived (Class -> [TcType] -> [TcType]
    immSuperClasses Class
    cls [TcType]
    tys)
      where
        loc :: CtLoc
    loc = CtEvidence -> CtLoc
    ctEvLoc CtEvidence
    ev

        do_one_derived :: TcType -> TcS [Ct]
    do_one_derived TcType
    sc_pred
          = do { CtEvidence
    sc_ev <- CtLoc -> TcType -> TcS CtEvidence
    newDerivedNC CtLoc
    loc TcType
    sc_pred
               ; NameSet
    -> CtEvidence -> [TcTyVar] -> [TcType] -> TcType -> TcS [Ct]
    mk_superclasses NameSet
    rec_clss CtEvidence
    sc_ev [] [] TcType
    sc_pred }

    {- Note [Improvement from Ground Wanteds]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Suppose class C b a => D a b
    and consider
      [W] D Int Bool
    Is there any point in emitting [D] C Bool Int?  No!  The only point of
    emitting superclass constraints for W/D constraints is to get
    improvement, extra unifications that result from functional
    dependencies.  See Note [Why adding superclasses can help] above.

    But no variables means no improvement; case closed.
    -}

    mk_superclasses :: NameSet -> CtEvidence
                    -> [TyVar] -> ThetaType -> PredType -> TcS [Ct]
    -- Return this constraint, plus its superclasses, if any
    mk_superclasses :: NameSet
    -> CtEvidence -> [TcTyVar] -> [TcType] -> TcType -> TcS [Ct]
    mk_superclasses NameSet
    rec_clss CtEvidence
    ev [TcTyVar]
    tvs [TcType]
    theta TcType
    pred
      | ClassPred Class
    cls [TcType]
    tys <- TcType -> Pred
    classifyPredType TcType
    pred
      = NameSet
    -> CtEvidence
    -> [TcTyVar]
    -> [TcType]
    -> Class
    -> [TcType]
    -> TcS [Ct]
    mk_superclasses_of NameSet
    rec_clss CtEvidence
    ev [TcTyVar]
    tvs [TcType]
    theta Class
    cls [TcType]
    tys

      | Bool
    otherwise   -- Superclass is not a class predicate
      = forall (m :: * -> *) a. Monad m => a -> m a
    return [CtEvidence -> Ct
    mkNonCanonical CtEvidence
    ev]

    mk_superclasses_of :: NameSet -> CtEvidence
                       -> [TyVar] -> ThetaType -> Class -> [Type]
                       -> TcS [Ct]
    -- Always return this class constraint,
    -- and expand its superclasses
    mk_superclasses_of :: NameSet
    -> CtEvidence
    -> [TcTyVar]
    -> [TcType]
    -> Class
    -> [TcType]
    -> TcS [Ct]
    mk_superclasses_of NameSet
    rec_clss CtEvidence
    ev [TcTyVar]
    tvs [TcType]
    theta Class
    cls [TcType]
    tys
      | Bool
    loop_found = do { String -> SDoc -> TcS ()
    traceTcS String
    "mk_superclasses_of: loop" (forall a. Outputable a => a -> SDoc
    ppr Class
    cls SDoc -> SDoc -> SDoc
    <+> forall a. Outputable a => a -> SDoc
    ppr [TcType]
    tys)
                        ; forall (m :: * -> *) a. Monad m => a -> m a
    return [Ct
    this_ct] }  -- cc_pend_sc of this_ct = True
      | Bool
    otherwise  = do { String -> SDoc -> TcS ()
    traceTcS String
    "mk_superclasses_of" ([SDoc] -> SDoc
    vcat [ forall a. Outputable a => a -> SDoc
    ppr Class
    cls SDoc -> SDoc -> SDoc
    <+> forall a. Outputable a => a -> SDoc
    ppr [TcType]
    tys
                                                              , forall a. Outputable a => a -> SDoc
    ppr (Class -> Bool
    isCTupleClass Class
    cls)
                                                              , forall a. Outputable a => a -> SDoc
    ppr NameSet
    rec_clss
                                                              ])
                        ; [Ct]
    sc_cts <- NameSet
    -> CtEvidence
    -> [TcTyVar]
    -> [TcType]
    -> Class
    -> [TcType]
    -> TcS [Ct]
    mk_strict_superclasses NameSet
    rec_clss' CtEvidence
    ev [TcTyVar]
    tvs [TcType]
    theta Class
    cls [TcType]
    tys
                        ; forall (m :: * -> *) a. Monad m => a -> m a
    return (Ct
    this_ct forall a. a -> [a] -> [a]
    : [Ct]
    sc_cts) }
                                       -- cc_pend_sc of this_ct = False
      where
        cls_nm :: Name
    cls_nm     = Class -> Name
    className Class
    cls
        loop_found :: Bool
    loop_found = Bool -> Bool
    not (Class -> Bool
    isCTupleClass Class
    cls) Bool -> Bool -> Bool
    && Name
    cls_nm Name -> NameSet -> Bool
    `elemNameSet` NameSet
    rec_clss
                     -- Tuples never contribute to recursion, and can be nested
        rec_clss' :: NameSet
    rec_clss'  = NameSet
    rec_clss NameSet -> Name -> NameSet
    `extendNameSet` Name
    cls_nm

        this_ct :: Ct
    this_ct | forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [TcTyVar]
    tvs, forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [TcType]
    theta
                = CDictCan { cc_ev :: CtEvidence
    cc_ev = CtEvidence
    ev, cc_class :: Class
    cc_class = Class
    cls, cc_tyargs :: [TcType]
    cc_tyargs = [TcType]
    tys
                           , cc_pend_sc :: Bool
    cc_pend_sc = Bool
    loop_found }
                     -- NB: If there is a loop, we cut off, so we have not
                     --     added the superclasses, hence cc_pend_sc = True
                | Bool
    otherwise
                = QCInst -> Ct
    CQuantCan (QCI { qci_tvs :: [TcTyVar]
    qci_tvs = [TcTyVar]
    tvs, qci_pred :: TcType
    qci_pred = Class -> [TcType] -> TcType
    mkClassPred Class
    cls [TcType]
    tys
                                 , qci_ev :: CtEvidence
    qci_ev = CtEvidence
    ev
                                 , qci_pend_sc :: Bool
    qci_pend_sc = Bool
    loop_found })

    {- Note [Equality superclasses in quantified constraints]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider (#15359, #15593, #15625)
      f :: (forall a. theta => a ~ b) => stuff

    It's a bit odd to have a local, quantified constraint for `(a~b)`,
    but some people want such a thing (see the tickets). And for
    Coercible it is definitely useful
      f :: forall m. (forall p q. Coercible p q => Coercible (m p) (m q)))
                     => stuff

    Moreover it's not hard to arrange; we just need to look up /equality/
    constraints in the quantified-constraint environment, which we do in
    GHC.Tc.Solver.Interact.doTopReactOther.

    There is a wrinkle though, in the case where 'theta' is empty, so
    we have
      f :: (forall a. a~b) => stuff

    Now, potentially, the superclass machinery kicks in, in
    makeSuperClasses, giving us a a second quantified constraint
           (forall a. a ~# b)
    BUT this is an unboxed value!  And nothing has prepared us for
    dictionary "functions" that are unboxed.  Actually it does just
    about work, but the simplifier ends up with stuff like
       case (/\a. eq_sel d) of df -> ...(df @Int)...
    and fails to simplify that any further.  And it doesn't satisfy
    isPredTy any more.

    So for now we simply decline to take superclasses in the quantified
    case.  Instead we have a special case in GHC.Tc.Solver.Interact.doTopReactOther,
    which looks for primitive equalities specially in the quantified
    constraints.

    See also Note [Evidence for quantified constraints] in GHC.Core.Predicate.

    ************************************************************************
    *                                                                      *
    *                      Irreducibles canonicalization
    *                                                                      *
    ************************************************************************
    -}

    canIrred :: CtEvidence -> TcS (StopOrContinue Ct)
    -- Precondition: ty not a tuple and no other evidence form
    canIrred :: CtEvidence -> TcS (StopOrContinue Ct)
    canIrred CtEvidence
    ev
      = do { let pred :: TcType
    pred = CtEvidence -> TcType
    ctEvPred CtEvidence
    ev
           ; String -> SDoc -> TcS ()
    traceTcS String
    "can_pred" (String -> SDoc
    text String
    "IrredPred = " SDoc -> SDoc -> SDoc
    <+> forall a. Outputable a => a -> SDoc
    ppr TcType
    pred)
           ; (TcType
    xi,TcCoercion
    co) <- CtEvidence -> TcType -> TcS (TcType, TcCoercion)
    rewrite CtEvidence
    ev TcType
    pred -- co :: xi ~ pred
           ; CtEvidence
    -> TcType -> TcCoercion -> TcS (StopOrContinue CtEvidence)
    rewriteEvidence CtEvidence
    ev TcType
    xi TcCoercion
    co forall a b.
    TcS (StopOrContinue a)
    -> (a -> TcS (StopOrContinue b)) -> TcS (StopOrContinue b)
    `andWhenContinue` \ CtEvidence
    new_ev ->

        do { -- Re-classify, in case rewriting has improved its shape
             -- Code is like the canNC, except
             -- that the IrredPred branch stops work
           ; case TcType -> Pred
    classifyPredType (CtEvidence -> TcType
    ctEvPred CtEvidence
    new_ev) of
               ClassPred Class
    cls [TcType]
    tys     -> CtEvidence -> Class -> [TcType] -> TcS (StopOrContinue Ct)
    canClassNC CtEvidence
    new_ev Class
    cls [TcType]
    tys
               EqPred EqRel
    eq_rel TcType
    ty1 TcType
    ty2 -> CtEvidence -> EqRel -> TcType -> TcType -> TcS (StopOrContinue Ct)
    canEqNC CtEvidence
    new_ev EqRel
    eq_rel TcType
    ty1 TcType
    ty2
               ForAllPred [TcTyVar]
    tvs [TcType]
    th TcType
    p   -> -- this is highly suspect; Quick Look
                                        -- should never leave a meta-var filled
                                        -- in with a polytype. This is #18987.
                                        do String -> SDoc -> TcS ()
    traceTcS String
    "canEvNC:forall" (forall a. Outputable a => a -> SDoc
    ppr TcType
    pred)
                                           CtEvidence
    -> [TcTyVar] -> [TcType] -> TcType -> TcS (StopOrContinue Ct)
    canForAllNC CtEvidence
    ev [TcTyVar]
    tvs [TcType]
    th TcType
    p
               IrredPred {}          -> forall a. a -> TcS (StopOrContinue a)
    continueWith forall a b. (a -> b) -> a -> b
    $
                                        CtIrredReason -> CtEvidence -> Ct
    mkIrredCt CtIrredReason
    IrredShapeReason CtEvidence
    new_ev } }

    {- *********************************************************************
    *                                                                      *
    *                      Quantified predicates
    *                                                                      *
    ********************************************************************* -}

    {- Note [Quantified constraints]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The -XQuantifiedConstraints extension allows type-class contexts like this:

      data Rose f x = Rose x (f (Rose f x))

      instance (Eq a, forall b. Eq b => Eq (f b))
            => Eq (Rose f a)  where
        (Rose x1 rs1) == (Rose x2 rs2) = x1==x2 && rs1 == rs2

    Note the (forall b. Eq b => Eq (f b)) in the instance contexts.
    This quantified constraint is needed to solve the
     [W] (Eq (f (Rose f x)))
    constraint which arises form the (==) definition.

    The wiki page is
      https://gitlab.haskell.org/ghc/ghc/wikis/quantified-constraints
    which in turn contains a link to the GHC Proposal where the change
    is specified, and a Haskell Symposium paper about it.

    We implement two main extensions to the design in the paper:

     1. We allow a variable in the instance head, e.g.
          f :: forall m a. (forall b. m b) => D (m a)
        Notice the 'm' in the head of the quantified constraint, not
        a class.

     2. We support superclasses to quantified constraints.
        For example (contrived):
          f :: (Ord b, forall b. Ord b => Ord (m b)) => m a -> m a -> Bool
          f x y = x==y
        Here we need (Eq (m a)); but the quantified constraint deals only
        with Ord.  But we can make it work by using its superclass.

    Here are the moving parts
      * Language extension {-# LANGUAGE QuantifiedConstraints #-}
        and add it to ghc-boot-th:GHC.LanguageExtensions.Type.Extension

      * A new form of evidence, EvDFun, that is used to discharge
        such wanted constraints

      * checkValidType gets some changes to accept forall-constraints
        only in the right places.

      * Predicate.Pred gets a new constructor ForAllPred, and
        and classifyPredType analyses a PredType to decompose
        the new forall-constraints

      * GHC.Tc.Solver.Monad.InertCans gets an extra field, inert_insts,
        which holds all the Given forall-constraints.  In effect,
        such Given constraints are like local instance decls.

      * When trying to solve a class constraint, via
        GHC.Tc.Solver.Interact.matchInstEnv, use the InstEnv from inert_insts
        so that we include the local Given forall-constraints
        in the lookup.  (See GHC.Tc.Solver.Monad.getInstEnvs.)

      * GHC.Tc.Solver.Canonical.canForAll deals with solving a
        forall-constraint.  See
           Note [Solving a Wanted forall-constraint]

      * We augment the kick-out code to kick out an inert
        forall constraint if it can be rewritten by a new
        type equality; see GHC.Tc.Solver.Monad.kick_out_rewritable

    Note that a quantified constraint is never /inferred/
    (by GHC.Tc.Solver.simplifyInfer).  A function can only have a
    quantified constraint in its type if it is given an explicit
    type signature.

    -}

    canForAllNC :: CtEvidence -> [TyVar] -> TcThetaType -> TcPredType
                -> TcS (StopOrContinue Ct)
    canForAllNC :: CtEvidence
    -> [TcTyVar] -> [TcType] -> TcType -> TcS (StopOrContinue Ct)
    canForAllNC CtEvidence
    ev [TcTyVar]
    tvs [TcType]
    theta TcType
    pred
      | CtEvidence -> Bool
    isGiven CtEvidence
    ev  -- See Note [Eagerly expand given superclasses]
      , Just (Class
    cls, [TcType]
    tys) <- Maybe (Class, [TcType])
    cls_pred_tys_maybe
      = do { [Ct]
    sc_cts <- CtEvidence
    -> [TcTyVar] -> [TcType] -> Class -> [TcType] -> TcS [Ct]
    mkStrictSuperClasses CtEvidence
    ev [TcTyVar]
    tvs [TcType]
    theta Class
    cls [TcType]
    tys
           ; [Ct] -> TcS ()
    emitWork [Ct]
    sc_cts
           ; CtEvidence -> Bool -> TcS (StopOrContinue Ct)
    canForAll CtEvidence
    ev Bool
    False }

      | Bool
    otherwise
      = CtEvidence -> Bool -> TcS (StopOrContinue Ct)
    canForAll CtEvidence
    ev (forall a. Maybe a -> Bool
    isJust Maybe (Class, [TcType])
    cls_pred_tys_maybe)

      where
        cls_pred_tys_maybe :: Maybe (Class, [TcType])
    cls_pred_tys_maybe = TcType -> Maybe (Class, [TcType])
    getClassPredTys_maybe TcType
    pred

    canForAll :: CtEvidence -> Bool -> TcS (StopOrContinue Ct)
    -- We have a constraint (forall as. blah => C tys)
    canForAll :: CtEvidence -> Bool -> TcS (StopOrContinue Ct)
    canForAll CtEvidence
    ev Bool
    pend_sc
      = do { -- First rewrite it to apply the current substitution
             let pred :: TcType
    pred = CtEvidence -> TcType
    ctEvPred CtEvidence
    ev
           ; (TcType
    xi,TcCoercion
    co) <- CtEvidence -> TcType -> TcS (TcType, TcCoercion)
    rewrite CtEvidence
    ev TcType
    pred -- co :: xi ~ pred
           ; CtEvidence
    -> TcType -> TcCoercion -> TcS (StopOrContinue CtEvidence)
    rewriteEvidence CtEvidence
    ev TcType
    xi TcCoercion
    co forall a b.
    TcS (StopOrContinue a)
    -> (a -> TcS (StopOrContinue b)) -> TcS (StopOrContinue b)
    `andWhenContinue` \ CtEvidence
    new_ev ->

        do { -- Now decompose into its pieces and solve it
             -- (It takes a lot less code to rewrite before decomposing.)
           ; case TcType -> Pred
    classifyPredType (CtEvidence -> TcType
    ctEvPred CtEvidence
    new_ev) of
               ForAllPred [TcTyVar]
    tvs [TcType]
    theta TcType
    pred
                  -> CtEvidence
    -> [TcTyVar]
    -> [TcType]
    -> TcType
    -> Bool
    -> TcS (StopOrContinue Ct)
    solveForAll CtEvidence
    new_ev [TcTyVar]
    tvs [TcType]
    theta TcType
    pred Bool
    pend_sc
               Pred
    _  -> forall a. HasCallStack => String -> SDoc -> a
    pprPanic String
    "canForAll" (forall a. Outputable a => a -> SDoc
    ppr CtEvidence
    new_ev)
        } }

    solveForAll :: CtEvidence -> [TyVar] -> TcThetaType -> PredType -> Bool
                -> TcS (StopOrContinue Ct)
    solveForAll :: CtEvidence
    -> [TcTyVar]
    -> [TcType]
    -> TcType
    -> Bool
    -> TcS (StopOrContinue Ct)
    solveForAll CtEvidence
    ev [TcTyVar]
    tvs [TcType]
    theta TcType
    pred Bool
    pend_sc
      | CtWanted { ctev_dest :: CtEvidence -> TcEvDest
    ctev_dest = TcEvDest
    dest } <- CtEvidence
    ev
      = -- See Note [Solving a Wanted forall-constraint]
        do { let skol_info :: SkolemInfo
    skol_info = SkolemInfo
    QuantCtxtSkol
                 empty_subst :: TCvSubst
    empty_subst = InScopeSet -> TCvSubst
    mkEmptyTCvSubst forall a b. (a -> b) -> a -> b
    $ VarSet -> InScopeSet
    mkInScopeSet forall a b. (a -> b) -> a -> b
    $
                               [TcType] -> VarSet
    tyCoVarsOfTypes (TcType
    predforall a. a -> [a] -> [a]
    :[TcType]
    theta) VarSet -> [TcTyVar] -> VarSet
    `delVarSetList` [TcTyVar]
    tvs
           ; (TCvSubst
    subst, [TcTyVar]
    skol_tvs) <- TCvSubst -> [TcTyVar] -> TcS (TCvSubst, [TcTyVar])
    tcInstSkolTyVarsX TCvSubst
    empty_subst [TcTyVar]
    tvs
           ; [TcTyVar]
    given_ev_vars <- forall (t :: * -> *) (m :: * -> *) a b.
    (Traversable t, Monad m) =>
    (a -> m b) -> t a -> m (t b)
    mapM TcType -> TcS TcTyVar
    newEvVar (HasCallStack => TCvSubst -> [TcType] -> [TcType]
    substTheta TCvSubst
    subst [TcType]
    theta)

           ; (TcLevel
    lvl, (TcTyVar
    w_id, Bag Ct
    wanteds))
                 <- forall a. SDoc -> TcS a -> TcS (TcLevel, a)
    pushLevelNoWorkList (forall a. Outputable a => a -> SDoc
    ppr SkolemInfo
    skol_info) forall a b. (a -> b) -> a -> b
    $
                    do { CtEvidence
    wanted_ev <- CtLoc -> TcType -> TcS CtEvidence
    newWantedEvVarNC CtLoc
    loc forall a b. (a -> b) -> a -> b
    $
                                      HasCallStack => TCvSubst -> TcType -> TcType
    substTy TCvSubst
    subst TcType
    pred
                       ; forall (m :: * -> *) a. Monad m => a -> m a
    return ( CtEvidence -> TcTyVar
    ctEvEvId CtEvidence
    wanted_ev
                                , forall a. a -> Bag a
    unitBag (CtEvidence -> Ct
    mkNonCanonical CtEvidence
    wanted_ev)) }

          ; TcEvBinds
    ev_binds <- TcLevel
    -> SkolemInfo -> [TcTyVar] -> [TcTyVar] -> Bag Ct -> TcS TcEvBinds
    emitImplicationTcS TcLevel
    lvl SkolemInfo
    skol_info [TcTyVar]
    skol_tvs
                                           [TcTyVar]
    given_ev_vars Bag Ct
    wanteds

          ; TcEvDest -> EvTerm -> TcS ()
    setWantedEvTerm TcEvDest
    dest forall a b. (a -> b) -> a -> b
    $
            EvFun { et_tvs :: [TcTyVar]
    et_tvs = [TcTyVar]
    skol_tvs, et_given :: [TcTyVar]
    et_given = [TcTyVar]
    given_ev_vars
                  , et_binds :: TcEvBinds
    et_binds = TcEvBinds
    ev_binds, et_body :: TcTyVar
    et_body = TcTyVar
    w_id }

          ; forall a. CtEvidence -> String -> TcS (StopOrContinue a)
    stopWith CtEvidence
    ev String
    "Wanted forall-constraint" }

      | CtEvidence -> Bool
    isGiven CtEvidence
    ev   -- See Note [Solving a Given forall-constraint]
      = do { QCInst -> TcS ()
    addInertForAll QCInst
    qci
           ; forall a. CtEvidence -> String -> TcS (StopOrContinue a)
    stopWith CtEvidence
    ev String
    "Given forall-constraint" }

      | Bool
    otherwise
      = do { String -> SDoc -> TcS ()
    traceTcS String
    "discarding derived forall-constraint" (forall a. Outputable a => a -> SDoc
    ppr CtEvidence
    ev)
           ; forall a. CtEvidence -> String -> TcS (StopOrContinue a)
    stopWith CtEvidence
    ev String
    "Derived forall-constraint" }
      where
        loc :: CtLoc
    loc = CtEvidence -> CtLoc
    ctEvLoc CtEvidence
    ev
        qci :: QCInst
    qci = QCI { qci_ev :: CtEvidence
    qci_ev = CtEvidence
    ev, qci_tvs :: [TcTyVar]
    qci_tvs = [TcTyVar]
    tvs
                  , qci_pred :: TcType
    qci_pred = TcType
    pred, qci_pend_sc :: Bool
    qci_pend_sc = Bool
    pend_sc }

    {- Note [Solving a Wanted forall-constraint]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Solving a wanted forall (quantified) constraint
      [W] df :: forall ab. (Eq a, Ord b) => C x a b
    is delightfully easy.   Just build an implication constraint
        forall ab. (g1::Eq a, g2::Ord b) => [W] d :: C x a
    and discharge df thus:
        df = /\ab. \g1 g2. let <binds> in d
    where <binds> is filled in by solving the implication constraint.
    All the machinery is to hand; there is little to do.

    Note [Solving a Given forall-constraint]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    For a Given constraint
      [G] df :: forall ab. (Eq a, Ord b) => C x a b
    we just add it to TcS's local InstEnv of known instances,
    via addInertForall.  Then, if we look up (C x Int Bool), say,
    we'll find a match in the InstEnv.

    ************************************************************************
    *                                                                      *
    *        Equalities
    *                                                                      *
    ************************************************************************

    Note [Canonicalising equalities]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In order to canonicalise an equality, we look at the structure of the
    two types at hand, looking for similarities. A difficulty is that the
    types may look dissimilar before rewriting but similar after rewriting.
    However, we don't just want to jump in and rewrite right away, because
    this might be wasted effort. So, after looking for similarities and failing,
    we rewrite and then try again. Of course, we don't want to loop, so we
    track whether or not we've already rewritten.

    It is conceivable to do a better job at tracking whether or not a type
    is rewritten, but this is left as future work. (Mar '15)

    Note [Decomposing FunTy]
    ~~~~~~~~~~~~~~~~~~~~~~~~
    can_eq_nc' may attempt to decompose a FunTy that is un-zonked.  This
    means that we may very well have a FunTy containing a type of some
    unknown kind. For instance, we may have,

        FunTy (a :: k) Int

    Where k is a unification variable. So the calls to getRuntimeRep_maybe may
    fail (returning Nothing).  In that case we'll fall through, zonk, and try again.
    Zonking should fill the variable k, meaning that decomposition will succeed the
    second time around.

    Also note that we require the AnonArgFlag to match.  This will stop
    us decomposing
       (Int -> Bool)  ~  (Show a => blah)
    It's as if we treat (->) and (=>) as different type constructors.
    -}

    canEqNC :: CtEvidence -> EqRel -> Type -> Type -> TcS (StopOrContinue Ct)
    canEqNC :: CtEvidence -> EqRel -> TcType -> TcType -> TcS (StopOrContinue Ct)
    canEqNC CtEvidence
    ev EqRel
    eq_rel TcType
    ty1 TcType
    ty2
      = do { Either (Pair TcType) TcType
    result <- TcType -> TcType -> TcS (Either (Pair TcType) TcType)
    zonk_eq_types TcType
    ty1 TcType
    ty2
           ; case Either (Pair TcType) TcType
    result of
               Left (Pair TcType
    ty1' TcType
    ty2') -> Bool
    -> CtEvidence
    -> EqRel
    -> TcType
    -> TcType
    -> TcType
    -> TcType
    -> TcS (StopOrContinue Ct)
    can_eq_nc Bool
    False CtEvidence
    ev EqRel
    eq_rel TcType
    ty1' TcType
    ty1 TcType
    ty2' TcType
    ty2
               Right TcType
    ty              -> CtEvidence -> EqRel -> TcType -> TcS (StopOrContinue Ct)
    canEqReflexive CtEvidence
    ev EqRel
    eq_rel TcType
    ty }

    can_eq_nc
       :: Bool            -- True => both types are rewritten
       -> CtEvidence
       -> EqRel
       -> Type -> Type    -- LHS, after and before type-synonym expansion, resp
       -> Type -> Type    -- RHS, after and before type-synonym expansion, resp
       -> TcS (StopOrContinue Ct)
    can_eq_nc :: Bool
    -> CtEvidence
    -> EqRel
    -> TcType
    -> TcType
    -> TcType
    -> TcType
    -> TcS (StopOrContinue Ct)
    can_eq_nc Bool
    rewritten CtEvidence
    ev EqRel
    eq_rel TcType
    ty1 TcType
    ps_ty1 TcType
    ty2 TcType
    ps_ty2
      = do { String -> SDoc -> TcS ()
    traceTcS String
    "can_eq_nc" forall a b. (a -> b) -> a -> b
    $
             [SDoc] -> SDoc
    vcat [ forall a. Outputable a => a -> SDoc
    ppr Bool
    rewritten, forall a. Outputable a => a -> SDoc
    ppr CtEvidence
    ev, forall a. Outputable a => a -> SDoc
    ppr EqRel
    eq_rel, forall a. Outputable a => a -> SDoc
    ppr TcType
    ty1, forall a. Outputable a => a -> SDoc
    ppr TcType
    ps_ty1, forall a. Outputable a => a -> SDoc
    ppr TcType
    ty2, forall a. Outputable a => a -> SDoc
    ppr TcType
    ps_ty2 ]
           ; GlobalRdrEnv
    rdr_env <- TcS GlobalRdrEnv
    getGlobalRdrEnvTcS
           ; (FamInstEnv, FamInstEnv)
    fam_insts <- TcS (FamInstEnv, FamInstEnv)
    getFamInstEnvs
           ; Bool
    -> GlobalRdrEnv
    -> (FamInstEnv, FamInstEnv)
    -> CtEvidence
    -> EqRel
    -> TcType
    -> TcType
    -> TcType
    -> TcType
    -> TcS (StopOrContinue Ct)
    can_eq_nc' Bool
    rewritten GlobalRdrEnv
    rdr_env (FamInstEnv, FamInstEnv)
    fam_insts CtEvidence
    ev EqRel
    eq_rel TcType
    ty1 TcType
    ps_ty1 TcType
    ty2 TcType
    ps_ty2 }

    can_eq_nc'
       :: Bool           -- True => both input types are rewritten
       -> GlobalRdrEnv   -- needed to see which newtypes are in scope
       -> FamInstEnvs    -- needed to unwrap data instances
       -> CtEvidence
       -> EqRel
       -> Type -> Type    -- LHS, after and before type-synonym expansion, resp
       -> Type -> Type    -- RHS, after and before type-synonym expansion, resp
       -> TcS (StopOrContinue Ct)

    -- See Note [Comparing nullary type synonyms] in GHC.Core.Type.
    can_eq_nc' :: Bool
    -> GlobalRdrEnv
    -> (FamInstEnv, FamInstEnv)
    -> CtEvidence
    -> EqRel
    -> TcType
    -> TcType
    -> TcType
    -> TcType
    -> TcS (StopOrContinue Ct)
    can_eq_nc' Bool
    _flat GlobalRdrEnv
    _rdr_env (FamInstEnv, FamInstEnv)
    _envs CtEvidence
    ev EqRel
    eq_rel ty1 :: TcType
    ty1@(TyConApp TyCon
    tc1 []) TcType
    _ps_ty1 (TyConApp TyCon
    tc2 []) TcType
    _ps_ty2
      | TyCon
    tc1 forall a. Eq a => a -> a -> Bool
    == TyCon
    tc2
      = CtEvidence -> EqRel -> TcType -> TcS (StopOrContinue Ct)
    canEqReflexive CtEvidence
    ev EqRel
    eq_rel TcType
    ty1

    -- Expand synonyms first; see Note [Type synonyms and canonicalization]
    can_eq_nc' Bool
    rewritten GlobalRdrEnv
    rdr_env (FamInstEnv, FamInstEnv)
    envs CtEvidence
    ev EqRel
    eq_rel TcType
    ty1 TcType
    ps_ty1 TcType
    ty2 TcType
    ps_ty2
      | Just TcType
    ty1' <- TcType -> Maybe TcType
    tcView TcType
    ty1 = Bool
    -> GlobalRdrEnv
    -> (FamInstEnv, FamInstEnv)
    -> CtEvidence
    -> EqRel
    -> TcType
    -> TcType
    -> TcType
    -> TcType
    -> TcS (StopOrContinue Ct)
    can_eq_nc' Bool
    rewritten GlobalRdrEnv
    rdr_env (FamInstEnv, FamInstEnv)
    envs CtEvidence
    ev EqRel
    eq_rel TcType
    ty1' TcType
    ps_ty1 TcType
    ty2  TcType
    ps_ty2
      | Just TcType
    ty2' <- TcType -> Maybe TcType
    tcView TcType
    ty2 = Bool
    -> GlobalRdrEnv
    -> (FamInstEnv, FamInstEnv)
    -> CtEvidence
    -> EqRel
    -> TcType
    -> TcType
    -> TcType
    -> TcType
    -> TcS (StopOrContinue Ct)
    can_eq_nc' Bool
    rewritten GlobalRdrEnv
    rdr_env (FamInstEnv, FamInstEnv)
    envs CtEvidence
    ev EqRel
    eq_rel TcType
    ty1  TcType
    ps_ty1 TcType
    ty2' TcType
    ps_ty2

    -- need to check for reflexivity in the ReprEq case.
    -- See Note [Eager reflexivity check]
    -- Check only when rewritten because the zonk_eq_types check in canEqNC takes
    -- care of the non-rewritten case.
    can_eq_nc' Bool
    True GlobalRdrEnv
    _rdr_env (FamInstEnv, FamInstEnv)
    _envs CtEvidence
    ev EqRel
    ReprEq TcType
    ty1 TcType
    _ TcType
    ty2 TcType
    _
      | TcType
    ty1 HasDebugCallStack => TcType -> TcType -> Bool
    `tcEqType` TcType
    ty2
      = CtEvidence -> EqRel -> TcType -> TcS (StopOrContinue Ct)
    canEqReflexive CtEvidence
    ev EqRel
    ReprEq TcType
    ty1

    -- When working with ReprEq, unwrap newtypes.
    -- See Note [Unwrap newtypes first]
    -- This must be above the TyVarTy case, in order to guarantee (TyEq:N)
    can_eq_nc' Bool
    _rewritten GlobalRdrEnv
    rdr_env (FamInstEnv, FamInstEnv)
    envs CtEvidence
    ev EqRel
    eq_rel TcType
    ty1 TcType
    ps_ty1 TcType
    ty2 TcType
    ps_ty2
      | EqRel
    ReprEq <- EqRel
    eq_rel
      , Just ((Bag GlobalRdrElt, TcCoercion), TcType)
    stuff1 <- (FamInstEnv, FamInstEnv)
    -> GlobalRdrEnv
    -> TcType
    -> Maybe ((Bag GlobalRdrElt, TcCoercion), TcType)
    tcTopNormaliseNewTypeTF_maybe (FamInstEnv, FamInstEnv)
    envs GlobalRdrEnv
    rdr_env TcType
    ty1
      = CtEvidence
    -> SwapFlag
    -> TcType
    -> ((Bag GlobalRdrElt, TcCoercion), TcType)
    -> TcType
    -> TcType
    -> TcS (StopOrContinue Ct)
    can_eq_newtype_nc CtEvidence
    ev SwapFlag
    NotSwapped TcType
    ty1 ((Bag GlobalRdrElt, TcCoercion), TcType)
    stuff1 TcType
    ty2 TcType
    ps_ty2

      | EqRel
    ReprEq <- EqRel
    eq_rel
      , Just ((Bag GlobalRdrElt, TcCoercion), TcType)
    stuff2 <- (FamInstEnv, FamInstEnv)
    -> GlobalRdrEnv
    -> TcType
    -> Maybe ((Bag GlobalRdrElt, TcCoercion), TcType)
    tcTopNormaliseNewTypeTF_maybe (FamInstEnv, FamInstEnv)
    envs GlobalRdrEnv
    rdr_env TcType
    ty2
      = CtEvidence
    -> SwapFlag
    -> TcType
    -> ((Bag GlobalRdrElt, TcCoercion), TcType)
    -> TcType
    -> TcType
    -> TcS (StopOrContinue Ct)
    can_eq_newtype_nc CtEvidence
    ev SwapFlag
    IsSwapped  TcType
    ty2 ((Bag GlobalRdrElt, TcCoercion), TcType)
    stuff2 TcType
    ty1 TcType
    ps_ty1

    -- Then, get rid of casts
    can_eq_nc' Bool
    rewritten GlobalRdrEnv
    _rdr_env (FamInstEnv, FamInstEnv)
    _envs CtEvidence
    ev EqRel
    eq_rel (CastTy TcType
    ty1 TcCoercion
    co1) TcType
    _ TcType
    ty2 TcType
    ps_ty2
      | forall a. Maybe a -> Bool
    isNothing (TcType -> Maybe CanEqLHS
    canEqLHS_maybe TcType
    ty2)  -- See (3) in Note [Equalities with incompatible kinds]
      = Bool
    -> CtEvidence
    -> EqRel
    -> SwapFlag
    -> TcType
    -> TcCoercion
    -> TcType
    -> TcType
    -> TcS (StopOrContinue Ct)
    canEqCast Bool
    rewritten CtEvidence
    ev EqRel
    eq_rel SwapFlag
    NotSwapped TcType
    ty1 TcCoercion
    co1 TcType
    ty2 TcType
    ps_ty2
    can_eq_nc' Bool
    rewritten GlobalRdrEnv
    _rdr_env (FamInstEnv, FamInstEnv)
    _envs CtEvidence
    ev EqRel
    eq_rel TcType
    ty1 TcType
    ps_ty1 (CastTy TcType
    ty2 TcCoercion
    co2) TcType
    _
      | forall a. Maybe a -> Bool
    isNothing (TcType -> Maybe CanEqLHS
    canEqLHS_maybe TcType
    ty1)  -- See (3) in Note [Equalities with incompatible kinds]
      = Bool
    -> CtEvidence
    -> EqRel
    -> SwapFlag
    -> TcType
    -> TcCoercion
    -> TcType
    -> TcType
    -> TcS (StopOrContinue Ct)
    canEqCast Bool
    rewritten CtEvidence
    ev EqRel
    eq_rel SwapFlag
    IsSwapped TcType
    ty2 TcCoercion
    co2 TcType
    ty1 TcType
    ps_ty1

    ----------------------
    -- Otherwise try to decompose
    ----------------------

    -- Literals
    can_eq_nc' Bool
    _rewritten GlobalRdrEnv
    _rdr_env (FamInstEnv, FamInstEnv)
    _envs CtEvidence
    ev EqRel
    eq_rel ty1 :: TcType
    ty1@(LitTy TyLit
    l1) TcType
    _ (LitTy TyLit
    l2) TcType
    _
     | TyLit
    l1 forall a. Eq a => a -> a -> Bool
    == TyLit
    l2
      = do { CtEvidence -> EvTerm -> TcS ()
    setEvBindIfWanted CtEvidence
    ev (TcCoercion -> EvTerm
    evCoercion forall a b. (a -> b) -> a -> b
    $ Role -> TcType -> TcCoercion
    mkReflCo (EqRel -> Role
    eqRelRole EqRel
    eq_rel) TcType
    ty1)
           ; forall a. CtEvidence -> String -> TcS (StopOrContinue a)
    stopWith CtEvidence
    ev String
    "Equal LitTy" }

    -- Decompose FunTy: (s -> t) and (c => t)
    -- NB: don't decompose (Int -> blah) ~ (Show a => blah)
    can_eq_nc' Bool
    _rewritten GlobalRdrEnv
    _rdr_env (FamInstEnv, FamInstEnv)
    _envs CtEvidence
    ev EqRel
    eq_rel
               (FunTy { ft_mult :: TcType -> TcType
    ft_mult = TcType
    am1, ft_af :: TcType -> AnonArgFlag
    ft_af = AnonArgFlag
    af1, ft_arg :: TcType -> TcType
    ft_arg = TcType
    ty1a, ft_res :: TcType -> TcType
    ft_res = TcType
    ty1b }) TcType
    _ps_ty1
               (FunTy { ft_mult :: TcType -> TcType
    ft_mult = TcType
    am2, ft_af :: TcType -> AnonArgFlag
    ft_af = AnonArgFlag
    af2, ft_arg :: TcType -> TcType
    ft_arg = TcType
    ty2a, ft_res :: TcType -> TcType
    ft_res = TcType
    ty2b }) TcType
    _ps_ty2
      | AnonArgFlag
    af1 forall a. Eq a => a -> a -> Bool
    == AnonArgFlag
    af2   -- Don't decompose (Int -> blah) ~ (Show a => blah)
      , Just TcType
    ty1a_rep <- HasDebugCallStack => TcType -> Maybe TcType
    getRuntimeRep_maybe TcType
    ty1a  -- getRutimeRep_maybe:
      , Just TcType
    ty1b_rep <- HasDebugCallStack => TcType -> Maybe TcType
    getRuntimeRep_maybe TcType
    ty1b  -- see Note [Decomposing FunTy]
      , Just TcType
    ty2a_rep <- HasDebugCallStack => TcType -> Maybe TcType
    getRuntimeRep_maybe TcType
    ty2a
      , Just TcType
    ty2b_rep <- HasDebugCallStack => TcType -> Maybe TcType
    getRuntimeRep_maybe TcType
    ty2b
      = CtEvidence
    -> EqRel
    -> TyCon
    -> [TcType]
    -> [TcType]
    -> TcS (StopOrContinue Ct)
    canDecomposableTyConAppOK CtEvidence
    ev EqRel
    eq_rel TyCon
    funTyCon
                                  [TcType
    am1, TcType
    ty1a_rep, TcType
    ty1b_rep, TcType
    ty1a, TcType
    ty1b]
                                  [TcType
    am2, TcType
    ty2a_rep, TcType
    ty2b_rep, TcType
    ty2a, TcType
    ty2b]

    -- Decompose type constructor applications
    -- NB: we have expanded type synonyms already
    can_eq_nc' Bool
    _rewritten GlobalRdrEnv
    _rdr_env (FamInstEnv, FamInstEnv)
    _envs CtEvidence
    ev EqRel
    eq_rel TcType
    ty1 TcType
    _ TcType
    ty2 TcType
    _
      | Just (TyCon
    tc1, [TcType]
    tys1) <- HasCallStack => TcType -> Maybe (TyCon, [TcType])
    tcSplitTyConApp_maybe TcType
    ty1
      , Just (TyCon
    tc2, [TcType]
    tys2) <- HasCallStack => TcType -> Maybe (TyCon, [TcType])
    tcSplitTyConApp_maybe TcType
    ty2
       -- we want to catch e.g. Maybe Int ~ (Int -> Int) here for better
       -- error messages rather than decomposing into AppTys;
       -- hence no direct match on TyConApp
      , Bool -> Bool
    not (TyCon -> Bool
    isTypeFamilyTyCon TyCon
    tc1)
      , Bool -> Bool
    not (TyCon -> Bool
    isTypeFamilyTyCon TyCon
    tc2)
      = CtEvidence
    -> EqRel
    -> TyCon
    -> [TcType]
    -> TyCon
    -> [TcType]
    -> TcS (StopOrContinue Ct)
    canTyConApp CtEvidence
    ev EqRel
    eq_rel TyCon
    tc1 [TcType]
    tys1 TyCon
    tc2 [TcType]
    tys2

    can_eq_nc' Bool
    _rewritten GlobalRdrEnv
    _rdr_env (FamInstEnv, FamInstEnv)
    _envs CtEvidence
    ev EqRel
    eq_rel
               s1 :: TcType
    s1@(ForAllTy (Bndr TcTyVar
    _ ArgFlag
    vis1) TcType
    _) TcType
    _
               s2 :: TcType
    s2@(ForAllTy (Bndr TcTyVar
    _ ArgFlag
    vis2) TcType
    _) TcType
    _
      | ArgFlag
    vis1 ArgFlag -> ArgFlag -> Bool
    `sameVis` ArgFlag
    vis2 -- Note [ForAllTy and typechecker equality]
      = CtEvidence -> EqRel -> TcType -> TcType -> TcS (StopOrContinue Ct)
    can_eq_nc_forall CtEvidence
    ev EqRel
    eq_rel TcType
    s1 TcType
    s2

    -- See Note [Canonicalising type applications] about why we require rewritten types
    -- Use tcSplitAppTy, not matching on AppTy, to catch oversaturated type families
    -- NB: Only decompose AppTy for nominal equality. See Note [Decomposing equality]
    can_eq_nc' Bool
    True GlobalRdrEnv
    _rdr_env (FamInstEnv, FamInstEnv)
    _envs CtEvidence
    ev EqRel
    NomEq TcType
    ty1 TcType
    _ TcType
    ty2 TcType
    _
      | Just (TcType
    t1, TcType
    s1) <- TcType -> Maybe (TcType, TcType)
    tcSplitAppTy_maybe TcType
    ty1
      , Just (TcType
    t2, TcType
    s2) <- TcType -> Maybe (TcType, TcType)
    tcSplitAppTy_maybe TcType
    ty2
      = CtEvidence
    -> TcType -> TcType -> TcType -> TcType -> TcS (StopOrContinue Ct)
    can_eq_app CtEvidence
    ev TcType
    t1 TcType
    s1 TcType
    t2 TcType
    s2

    -------------------
    -- Can't decompose.
    -------------------

    -- No similarity in type structure detected. Rewrite and try again.
    can_eq_nc' Bool
    False GlobalRdrEnv
    rdr_env (FamInstEnv, FamInstEnv)
    envs CtEvidence
    ev EqRel
    eq_rel TcType
    _ TcType
    ps_ty1 TcType
    _ TcType
    ps_ty2
      = do { (TcType
    xi1, TcCoercion
    co1) <- CtEvidence -> TcType -> TcS (TcType, TcCoercion)
    rewrite CtEvidence
    ev TcType
    ps_ty1
           ; (TcType
    xi2, TcCoercion
    co2) <- CtEvidence -> TcType -> TcS (TcType, TcCoercion)
    rewrite CtEvidence
    ev TcType
    ps_ty2
           ; CtEvidence
    new_ev <- CtEvidence
    -> SwapFlag
    -> TcType
    -> TcType
    -> TcCoercion
    -> TcCoercion
    -> TcS CtEvidence
    rewriteEqEvidence CtEvidence
    ev SwapFlag
    NotSwapped TcType
    xi1 TcType
    xi2 TcCoercion
    co1 TcCoercion
    co2
           ; Bool
    -> GlobalRdrEnv
    -> (FamInstEnv, FamInstEnv)
    -> CtEvidence
    -> EqRel
    -> TcType
    -> TcType
    -> TcType
    -> TcType
    -> TcS (StopOrContinue Ct)
    can_eq_nc' Bool
    True GlobalRdrEnv
    rdr_env (FamInstEnv, FamInstEnv)
    envs CtEvidence
    new_ev EqRel
    eq_rel TcType
    xi1 TcType
    xi1 TcType
    xi2 TcType
    xi2 }

    ----------------------------
    -- Look for a canonical LHS. See Note [Canonical LHS].
    -- Only rewritten types end up below here.
    ----------------------------

    -- NB: pattern match on True: we want only rewritten types sent to canEqLHS
    -- This means we've rewritten any variables and reduced any type family redexes
    -- See also Note [No top-level newtypes on RHS of representational equalities]
    can_eq_nc' Bool
    True GlobalRdrEnv
    _rdr_env (FamInstEnv, FamInstEnv)
    _envs CtEvidence
    ev EqRel
    eq_rel TcType
    ty1 TcType
    ps_ty1 TcType
    ty2 TcType
    ps_ty2
      | Just CanEqLHS
    can_eq_lhs1 <- TcType -> Maybe CanEqLHS
    canEqLHS_maybe TcType
    ty1
      = CtEvidence
    -> EqRel
    -> SwapFlag
    -> CanEqLHS
    -> TcType
    -> TcType
    -> TcType
    -> TcS (StopOrContinue Ct)
    canEqCanLHS CtEvidence
    ev EqRel
    eq_rel SwapFlag
    NotSwapped CanEqLHS
    can_eq_lhs1 TcType
    ps_ty1 TcType
    ty2 TcType
    ps_ty2

      | Just CanEqLHS
    can_eq_lhs2 <- TcType -> Maybe CanEqLHS
    canEqLHS_maybe TcType
    ty2
      = CtEvidence
    -> EqRel
    -> SwapFlag
    -> CanEqLHS
    -> TcType
    -> TcType
    -> TcType
    -> TcS (StopOrContinue Ct)
    canEqCanLHS CtEvidence
    ev EqRel
    eq_rel SwapFlag
    IsSwapped CanEqLHS
    can_eq_lhs2 TcType
    ps_ty2 TcType
    ty1 TcType
    ps_ty1

         -- If the type is TyConApp tc1 args1, then args1 really can't be less
         -- than tyConArity tc1. It could be *more* than tyConArity, but then we
         -- should have handled the case as an AppTy. That case only fires if
         -- _both_ sides of the equality are AppTy-like... but if one side is
         -- AppTy-like and the other isn't (and it also isn't a variable or
         -- saturated type family application, both of which are handled by
         -- can_eq_nc'), we're in a failure mode and can just fall through.

    ----------------------------
    -- Fall-through. Give up.
    ----------------------------

    -- We've rewritten and the types don't match. Give up.
    can_eq_nc' Bool
    True GlobalRdrEnv
    _rdr_env (FamInstEnv, FamInstEnv)
    _envs CtEvidence
    ev EqRel
    eq_rel TcType
    _ TcType
    ps_ty1 TcType
    _ TcType
    ps_ty2
      = do { String -> SDoc -> TcS ()
    traceTcS String
    "can_eq_nc' catch-all case" (forall a. Outputable a => a -> SDoc
    ppr TcType
    ps_ty1 SDoc -> SDoc -> SDoc
    $$ forall a. Outputable a => a -> SDoc
    ppr TcType
    ps_ty2)
           ; case EqRel
    eq_rel of -- See Note [Unsolved equalities]
                EqRel
    ReprEq -> forall a. a -> TcS (StopOrContinue a)
    continueWith (CtIrredReason -> CtEvidence -> Ct
    mkIrredCt CtIrredReason
    ReprEqReason CtEvidence
    ev)
                EqRel
    NomEq  -> forall a. a -> TcS (StopOrContinue a)
    continueWith (CtIrredReason -> CtEvidence -> Ct
    mkIrredCt CtIrredReason
    ShapeMismatchReason CtEvidence
    ev) }
              -- No need to call canEqFailure/canEqHardFailure because they
              -- rewrite, and the types involved here are already rewritten

    {- Note [Unsolved equalities]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If we have an unsolved equality like
      (a b ~R# Int)
    that is not necessarily insoluble!  Maybe 'a' will turn out to be a newtype.
    So we want to make it a potentially-soluble Irred not an insoluble one.
    Missing this point is what caused #15431

    Note [ForAllTy and typechecker equality]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Should GHC type-check the following program (adapted from #15740)?

      {-# LANGUAGE PolyKinds, ... #-}
      data D a
      type family F :: forall k. k -> Type
      type instance F = D

    Due to the way F is declared, any instance of F must have a right-hand side
    whose kind is equal to `forall k. k -> Type`. The kind of D is
    `forall {k}. k -> Type`, which is very close, but technically uses distinct
    Core:

      -----------------------------------------------------------
      | Source Haskell    | Core                                |
      -----------------------------------------------------------
      | forall  k.  <...> | ForAllTy (Bndr k Specified) (<...>) |
      | forall {k}. <...> | ForAllTy (Bndr k Inferred)  (<...>) |
      -----------------------------------------------------------

    We could deem these kinds to be unequal, but that would imply rejecting
    programs like the one above. Whether a kind variable binder ends up being
    specified or inferred can be somewhat subtle, however, especially for kinds
    that aren't explicitly written out in the source code (like in D above).
    For now, we decide to not make the specified/inferred status of an invisible
    type variable binder affect GHC's notion of typechecker equality
    (see Note [Typechecker equality vs definitional equality] in
    GHC.Tc.Utils.TcType). That is, we have the following:

      --------------------------------------------------
      | Type 1            | Type 2            | Equal? |
      --------------------|-----------------------------
      | forall k. <...>   | forall k. <...>   | Yes    |
      |                   | forall {k}. <...> | Yes    |
      |                   | forall k -> <...> | No     |
      --------------------------------------------------
      | forall {k}. <...> | forall k. <...>   | Yes    |
      |                   | forall {k}. <...> | Yes    |
      |                   | forall k -> <...> | No     |
      --------------------------------------------------
      | forall k -> <...> | forall k. <...>   | No     |
      |                   | forall {k}. <...> | No     |
      |                   | forall k -> <...> | Yes    |
      --------------------------------------------------

    We implement this nuance by using the GHC.Types.Var.sameVis function in
    GHC.Tc.Solver.Canonical.canEqNC and GHC.Tc.Utils.TcType.tcEqType, which
    respect typechecker equality. sameVis puts both forms of invisible type
    variable binders into the same equivalence class.

    Note that we do /not/ use sameVis in GHC.Core.Type.eqType, which implements
    /definitional/ equality, a slighty more coarse-grained notion of equality
    (see Note [Non-trivial definitional equality] in GHC.Core.TyCo.Rep) that does
    not consider the ArgFlag of ForAllTys at all. That is, eqType would equate all
    of forall k. <...>, forall {k}. <...>, and forall k -> <...>.
    -}

    ---------------------------------
    can_eq_nc_forall :: CtEvidence -> EqRel
                     -> Type -> Type    -- LHS and RHS
                     -> TcS (StopOrContinue Ct)
    -- (forall as. phi1) ~ (forall bs. phi2)
    -- Check for length match of as, bs
    -- Then build an implication constraint: forall as. phi1 ~ phi2[as/bs]
    -- But remember also to unify the kinds of as and bs
    --  (this is the 'go' loop), and actually substitute phi2[as |> cos / bs]
    -- Remember also that we might have forall z (a:z). blah
    --  so we must proceed one binder at a time (#13879)

    can_eq_nc_forall :: CtEvidence -> EqRel -> TcType -> TcType -> TcS (StopOrContinue Ct)
    can_eq_nc_forall CtEvidence
    ev EqRel
    eq_rel TcType
    s1 TcType
    s2
     | CtWanted { ctev_loc :: CtEvidence -> CtLoc
    ctev_loc = CtLoc
    loc, ctev_dest :: CtEvidence -> TcEvDest
    ctev_dest = TcEvDest
    orig_dest } <- CtEvidence
    ev
     = do { let free_tvs :: VarSet
    free_tvs       = [TcType] -> VarSet
    tyCoVarsOfTypes [TcType
    s1,TcType
    s2]
                ([TyVarBinder]
    bndrs1, TcType
    phi1) = TcType -> ([TyVarBinder], TcType)
    tcSplitForAllTyVarBinders TcType
    s1
                ([TyVarBinder]
    bndrs2, TcType
    phi2) = TcType -> ([TyVarBinder], TcType)
    tcSplitForAllTyVarBinders TcType
    s2
          ; if Bool -> Bool
    not (forall a b. [a] -> [b] -> Bool
    equalLength [TyVarBinder]
    bndrs1 [TyVarBinder]
    bndrs2)
            then do { String -> SDoc -> TcS ()
    traceTcS String
    "Forall failure" forall a b. (a -> b) -> a -> b
    $
                         [SDoc] -> SDoc
    vcat [ forall a. Outputable a => a -> SDoc
    ppr TcType
    s1, forall a. Outputable a => a -> SDoc
    ppr TcType
    s2, forall a. Outputable a => a -> SDoc
    ppr [TyVarBinder]
    bndrs1, forall a. Outputable a => a -> SDoc
    ppr [TyVarBinder]
    bndrs2
                              , forall a. Outputable a => a -> SDoc
    ppr (forall a b. (a -> b) -> [a] -> [b]
    map forall tv argf. VarBndr tv argf -> argf
    binderArgFlag [TyVarBinder]
    bndrs1)
                              , forall a. Outputable a => a -> SDoc
    ppr (forall a b. (a -> b) -> [a] -> [b]
    map forall tv argf. VarBndr tv argf -> argf
    binderArgFlag [TyVarBinder]
    bndrs2) ]
                    ; CtEvidence -> TcType -> TcType -> TcS (StopOrContinue Ct)
    canEqHardFailure CtEvidence
    ev TcType
    s1 TcType
    s2 }
            else
       do { String -> SDoc -> TcS ()
    traceTcS String
    "Creating implication for polytype equality" forall a b. (a -> b) -> a -> b
    $ forall a. Outputable a => a -> SDoc
    ppr CtEvidence
    ev
          ; let empty_subst1 :: TCvSubst
    empty_subst1 = InScopeSet -> TCvSubst
    mkEmptyTCvSubst forall a b. (a -> b) -> a -> b
    $ VarSet -> InScopeSet
    mkInScopeSet VarSet
    free_tvs
          ; (TCvSubst
    subst1, [TcTyVar]
    skol_tvs) <- TCvSubst -> [TcTyVar] -> TcS (TCvSubst, [TcTyVar])
    tcInstSkolTyVarsX TCvSubst
    empty_subst1 forall a b. (a -> b) -> a -> b
    $
                                  forall tv argf. [VarBndr tv argf] -> [tv]
    binderVars [TyVarBinder]
    bndrs1

          ; let skol_info :: SkolemInfo
    skol_info = TcType -> SkolemInfo
    UnifyForAllSkol TcType
    phi1
                phi1' :: TcType
    phi1' = HasCallStack => TCvSubst -> TcType -> TcType
    substTy TCvSubst
    subst1 TcType
    phi1

                -- Unify the kinds, extend the substitution
                go :: [TcTyVar] -> TCvSubst -> [TyVarBinder]
                   -> TcS (TcCoercion, Cts)
                go :: [TcTyVar] -> TCvSubst -> [TyVarBinder] -> TcS (TcCoercion, Bag Ct)
    go (TcTyVar
    skol_tv:[TcTyVar]
    skol_tvs) TCvSubst
    subst (TyVarBinder
    bndr2:[TyVarBinder]
    bndrs2)
                  = do { let tv2 :: TcTyVar
    tv2 = forall tv argf. VarBndr tv argf -> tv
    binderVar TyVarBinder
    bndr2
                       ; (TcCoercion
    kind_co, Bag Ct
    wanteds1) <- CtLoc -> Role -> TcType -> TcType -> TcS (TcCoercion, Bag Ct)
    unify CtLoc
    loc Role
    Nominal (TcTyVar -> TcType
    tyVarKind TcTyVar
    skol_tv)
                                                      (HasCallStack => TCvSubst -> TcType -> TcType
    substTy TCvSubst
    subst (TcTyVar -> TcType
    tyVarKind TcTyVar
    tv2))
                       ; let subst' :: TCvSubst
    subst' = TCvSubst -> TcTyVar -> TcType -> TCvSubst
    extendTvSubstAndInScope TCvSubst
    subst TcTyVar
    tv2
                                           (TcType -> TcCoercion -> TcType
    mkCastTy (TcTyVar -> TcType
    mkTyVarTy TcTyVar
    skol_tv) TcCoercion
    kind_co)
                             -- skol_tv is already in the in-scope set, but the
                             -- free vars of kind_co are not; hence "...AndInScope"
                       ; (TcCoercion
    co, Bag Ct
    wanteds2) <- [TcTyVar] -> TCvSubst -> [TyVarBinder] -> TcS (TcCoercion, Bag Ct)
    go [TcTyVar]
    skol_tvs TCvSubst
    subst' [TyVarBinder]
    bndrs2
                       ; forall (m :: * -> *) a. Monad m => a -> m a
    return ( TcTyVar -> TcCoercion -> TcCoercion -> TcCoercion
    mkTcForAllCo TcTyVar
    skol_tv TcCoercion
    kind_co TcCoercion
    co
                                , Bag Ct
    wanteds1 forall a. Bag a -> Bag a -> Bag a
    `unionBags` Bag Ct
    wanteds2 ) }

                -- Done: unify phi1 ~ phi2
                go [] TCvSubst
    subst [TyVarBinder]
    bndrs2
                  = ASSERT( null bndrs2 )
                    CtLoc -> Role -> TcType -> TcType -> TcS (TcCoercion, Bag Ct)
    unify CtLoc
    loc (EqRel -> Role
    eqRelRole EqRel
    eq_rel) TcType
    phi1' (TCvSubst -> TcType -> TcType
    substTyUnchecked TCvSubst
    subst TcType
    phi2)

                go [TcTyVar]
    _ TCvSubst
    _ [TyVarBinder]
    _ = forall a. String -> a
    panic String
    "cna_eq_nc_forall"  -- case (s:ss) []

                empty_subst2 :: TCvSubst
    empty_subst2 = InScopeSet -> TCvSubst
    mkEmptyTCvSubst (TCvSubst -> InScopeSet
    getTCvInScope TCvSubst
    subst1)

          ; (TcLevel
    lvl, (TcCoercion
    all_co, Bag Ct
    wanteds)) <- forall a. SDoc -> TcS a -> TcS (TcLevel, a)
    pushLevelNoWorkList (forall a. Outputable a => a -> SDoc
    ppr SkolemInfo
    skol_info) forall a b. (a -> b) -> a -> b
    $
                                        [TcTyVar] -> TCvSubst -> [TyVarBinder] -> TcS (TcCoercion, Bag Ct)
    go [TcTyVar]
    skol_tvs TCvSubst
    empty_subst2 [TyVarBinder]
    bndrs2
          ; TcLevel -> SkolemInfo -> [TcTyVar] -> Bag Ct -> TcS ()
    emitTvImplicationTcS TcLevel
    lvl SkolemInfo
    skol_info [TcTyVar]
    skol_tvs Bag Ct
    wanteds

          ; TcEvDest -> TcCoercion -> TcS ()
    setWantedEq TcEvDest
    orig_dest TcCoercion
    all_co
          ; forall a. CtEvidence -> String -> TcS (StopOrContinue a)
    stopWith CtEvidence
    ev String
    "Deferred polytype equality" } }

     | Bool
    otherwise
     = do { String -> SDoc -> TcS ()
    traceTcS String
    "Omitting decomposition of given polytype equality" forall a b. (a -> b) -> a -> b
    $
            TcType -> TcType -> SDoc
    pprEq TcType
    s1 TcType
    s2    -- See Note [Do not decompose given polytype equalities]
          ; forall a. CtEvidence -> String -> TcS (StopOrContinue a)
    stopWith CtEvidence
    ev String
    "Discard given polytype equality" }

     where
        unify :: CtLoc -> Role -> TcType -> TcType -> TcS (TcCoercion, Cts)
        -- This version returns the wanted constraint rather
        -- than putting it in the work list
        unify :: CtLoc -> Role -> TcType -> TcType -> TcS (TcCoercion, Bag Ct)
    unify CtLoc
    loc Role
    role TcType
    ty1 TcType
    ty2
          | TcType
    ty1 HasDebugCallStack => TcType -> TcType -> Bool
    `tcEqType` TcType
    ty2
          = forall (m :: * -> *) a. Monad m => a -> m a
    return (Role -> TcType -> TcCoercion
    mkTcReflCo Role
    role TcType
    ty1, forall a. Bag a
    emptyBag)
          | Bool
    otherwise
          = do { (CtEvidence
    wanted, TcCoercion
    co) <- CtLoc -> Role -> TcType -> TcType -> TcS (CtEvidence, TcCoercion)
    newWantedEq CtLoc
    loc Role
    role TcType
    ty1 TcType
    ty2
               ; forall (m :: * -> *) a. Monad m => a -> m a
    return (TcCoercion
    co, forall a. a -> Bag a
    unitBag (CtEvidence -> Ct
    mkNonCanonical CtEvidence
    wanted)) }

    ---------------------------------
    -- | Compare types for equality, while zonking as necessary. Gives up
    -- as soon as it finds that two types are not equal.
    -- This is quite handy when some unification has made two
    -- types in an inert Wanted to be equal. We can discover the equality without
    -- rewriting, which is sometimes very expensive (in the case of type functions).
    -- In particular, this function makes a ~20% improvement in test case
    -- perf/compiler/T5030.
    --
    -- Returns either the (partially zonked) types in the case of
    -- inequality, or the one type in the case of equality. canEqReflexive is
    -- a good next step in the 'Right' case. Returning 'Left' is always safe.
    --
    -- NB: This does *not* look through type synonyms. In fact, it treats type
    -- synonyms as rigid constructors. In the future, it might be convenient
    -- to look at only those arguments of type synonyms that actually appear
    -- in the synonym RHS. But we're not there yet.
    zonk_eq_types :: TcType -> TcType -> TcS (Either (Pair TcType) TcType)
    zonk_eq_types :: TcType -> TcType -> TcS (Either (Pair TcType) TcType)
    zonk_eq_types = TcType -> TcType -> TcS (Either (Pair TcType) TcType)
    go
      where
        go :: TcType -> TcType -> TcS (Either (Pair TcType) TcType)
    go (TyVarTy TcTyVar
    tv1) (TyVarTy TcTyVar
    tv2) = TcTyVar -> TcTyVar -> TcS (Either (Pair TcType) TcType)
    tyvar_tyvar TcTyVar
    tv1 TcTyVar
    tv2
        go (TyVarTy TcTyVar
    tv1) TcType
    ty2           = SwapFlag -> TcTyVar -> TcType -> TcS (Either (Pair TcType) TcType)
    tyvar SwapFlag
    NotSwapped TcTyVar
    tv1 TcType
    ty2
        go TcType
    ty1 (TyVarTy TcTyVar
    tv2)           = SwapFlag -> TcTyVar -> TcType -> TcS (Either (Pair TcType) TcType)
    tyvar SwapFlag
    IsSwapped  TcTyVar
    tv2 TcType
    ty1

        -- We handle FunTys explicitly here despite the fact that they could also be
        -- treated as an application. Why? Well, for one it's cheaper to just look
        -- at two types (the argument and result types) than four (the argument,
        -- result, and their RuntimeReps). Also, we haven't completely zonked yet,
        -- so we may run into an unzonked type variable while trying to compute the
        -- RuntimeReps of the argument and result types. This can be observed in
        -- testcase tc269.
        go TcType
    ty1 TcType
    ty2
          | Just (Scaled TcType
    w1 TcType
    arg1, TcType
    res1) <- Maybe (Scaled TcType, TcType)
    split1
          , Just (Scaled TcType
    w2 TcType
    arg2, TcType
    res2) <- Maybe (Scaled TcType, TcType)
    split2
          , TcType -> TcType -> Bool
    eqType TcType
    w1 TcType
    w2
          = do { Either (Pair TcType) TcType
    res_a <- TcType -> TcType -> TcS (Either (Pair TcType) TcType)
    go TcType
    arg1 TcType
    arg2
               ; Either (Pair TcType) TcType
    res_b <- TcType -> TcType -> TcS (Either (Pair TcType) TcType)
    go TcType
    res1 TcType
    res2
               ; forall (m :: * -> *) a. Monad m => a -> m a
    return forall a b. (a -> b) -> a -> b
    $ forall a b c.
    (a -> b -> c)
    -> Either (Pair b) b -> Either (Pair a) a -> Either (Pair c) c
    combine_rev (TcType -> TcType -> TcType -> TcType
    mkVisFunTy TcType
    w1) Either (Pair TcType) TcType
    res_b Either (Pair TcType) TcType
    res_a
               }
          | forall a. Maybe a -> Bool
    isJust Maybe (Scaled TcType, TcType)
    split1 Bool -> Bool -> Bool
    || forall a. Maybe a -> Bool
    isJust Maybe (Scaled TcType, TcType)
    split2
          = forall {m :: * -> *} {a} {b}.
    Monad m =>
    a -> a -> m (Either (Pair a) b)
    bale_out TcType
    ty1 TcType
    ty2
          where
            split1 :: Maybe (Scaled TcType, TcType)
    split1 = TcType -> Maybe (Scaled TcType, TcType)
    tcSplitFunTy_maybe TcType
    ty1
            split2 :: Maybe (Scaled TcType, TcType)
    split2 = TcType -> Maybe (Scaled TcType, TcType)
    tcSplitFunTy_maybe TcType
    ty2

        go TcType
    ty1 TcType
    ty2
          | Just (TyCon
    tc1, [TcType]
    tys1) <- HasDebugCallStack => TcType -> Maybe (TyCon, [TcType])
    tcRepSplitTyConApp_maybe TcType
    ty1
          , Just (TyCon
    tc2, [TcType]
    tys2) <- HasDebugCallStack => TcType -> Maybe (TyCon, [TcType])
    tcRepSplitTyConApp_maybe TcType
    ty2
          = if TyCon
    tc1 forall a. Eq a => a -> a -> Bool
    == TyCon
    tc2 Bool -> Bool -> Bool
    && [TcType]
    tys1 forall a b. [a] -> [b] -> Bool
    `equalLength` [TcType]
    tys2
              -- Crucial to check for equal-length args, because
              -- we cannot assume that the two args to 'go' have
              -- the same kind.  E.g go (Proxy *      (Maybe Int))
              --                        (Proxy (*->*) Maybe)
              -- We'll call (go (Maybe Int) Maybe)
              -- See #13083
            then TyCon -> [TcType] -> [TcType] -> TcS (Either (Pair TcType) TcType)
    tycon TyCon
    tc1 [TcType]
    tys1 [TcType]
    tys2
            else forall {m :: * -> *} {a} {b}.
    Monad m =>
    a -> a -> m (Either (Pair a) b)
    bale_out TcType
    ty1 TcType
    ty2

        go TcType
    ty1 TcType
    ty2
          | Just (TcType
    ty1a, TcType
    ty1b) <- TcType -> Maybe (TcType, TcType)
    tcRepSplitAppTy_maybe TcType
    ty1
          , Just (TcType
    ty2a, TcType
    ty2b) <- TcType -> Maybe (TcType, TcType)
    tcRepSplitAppTy_maybe TcType
    ty2
          = do { Either (Pair TcType) TcType
    res_a <- TcType -> TcType -> TcS (Either (Pair TcType) TcType)
    go TcType
    ty1a TcType
    ty2a
               ; Either (Pair TcType) TcType
    res_b <- TcType -> TcType -> TcS (Either (Pair TcType) TcType)
    go TcType
    ty1b TcType
    ty2b
               ; forall (m :: * -> *) a. Monad m => a -> m a
    return forall a b. (a -> b) -> a -> b
    $ forall a b c.
    (a -> b -> c)
    -> Either (Pair b) b -> Either (Pair a) a -> Either (Pair c) c
    combine_rev TcType -> TcType -> TcType
    mkAppTy Either (Pair TcType) TcType
    res_b Either (Pair TcType) TcType
    res_a }

        go ty1 :: TcType
    ty1@(LitTy TyLit
    lit1) (LitTy TyLit
    lit2)
          | TyLit
    lit1 forall a. Eq a => a -> a -> Bool
    == TyLit
    lit2
          = forall (m :: * -> *) a. Monad m => a -> m a
    return (forall a b. b -> Either a b
    Right TcType
    ty1)

        go TcType
    ty1 TcType
    ty2 = forall {m :: * -> *} {a} {b}.
    Monad m =>
    a -> a -> m (Either (Pair a) b)
    bale_out TcType
    ty1 TcType
    ty2
          -- We don't handle more complex forms here

        bale_out :: a -> a -> m (Either (Pair a) b)
    bale_out a
    ty1 a
    ty2 = forall (m :: * -> *) a. Monad m => a -> m a
    return forall a b. (a -> b) -> a -> b
    $ forall a b. a -> Either a b
    Left (forall a. a -> a -> Pair a
    Pair a
    ty1 a
    ty2)

        tyvar :: SwapFlag -> TcTyVar -> TcType
              -> TcS (Either (Pair TcType) TcType)
          -- Try to do as little as possible, as anything we do here is redundant
          -- with rewriting. In particular, no need to zonk kinds. That's why
          -- we don't use the already-defined zonking functions
        tyvar :: SwapFlag -> TcTyVar -> TcType -> TcS (Either (Pair TcType) TcType)
    tyvar SwapFlag
    swapped TcTyVar
    tv TcType
    ty
          = case TcTyVar -> TcTyVarDetails
    tcTyVarDetails TcTyVar
    tv of
              MetaTv { mtv_ref :: TcTyVarDetails -> IORef MetaDetails
    mtv_ref = IORef MetaDetails
    ref }
                -> do { MetaDetails
    cts <- forall a. TcRef a -> TcS a
    readTcRef IORef MetaDetails
    ref
                      ; case MetaDetails
    cts of
                          MetaDetails
    Flexi        -> TcS (Either (Pair TcType) TcType)
    give_up
                          Indirect TcType
    ty' -> do { forall {a} {a}. (Outputable a, Outputable a) => a -> a -> TcS ()
    trace_indirect TcTyVar
    tv TcType
    ty'
                                             ; forall a b. SwapFlag -> (a -> a -> b) -> a -> a -> b
    unSwap SwapFlag
    swapped TcType -> TcType -> TcS (Either (Pair TcType) TcType)
    go TcType
    ty' TcType
    ty } }
              TcTyVarDetails
    _ -> TcS (Either (Pair TcType) TcType)
    give_up
          where
            give_up :: TcS (Either (Pair TcType) TcType)
    give_up = forall (m :: * -> *) a. Monad m => a -> m a
    return forall a b. (a -> b) -> a -> b
    $ forall a b. a -> Either a b
    Left forall a b. (a -> b) -> a -> b
    $ forall a b. SwapFlag -> (a -> a -> b) -> a -> a -> b
    unSwap SwapFlag
    swapped forall a. a -> a -> Pair a
    Pair (TcTyVar -> TcType
    mkTyVarTy TcTyVar
    tv) TcType
    ty

        tyvar_tyvar :: TcTyVar -> TcTyVar -> TcS (Either (Pair TcType) TcType)
    tyvar_tyvar TcTyVar
    tv1 TcTyVar
    tv2
          | TcTyVar
    tv1 forall a. Eq a => a -> a -> Bool
    == TcTyVar
    tv2 = forall (m :: * -> *) a. Monad m => a -> m a
    return (forall a b. b -> Either a b
    Right (TcTyVar -> TcType
    mkTyVarTy TcTyVar
    tv1))
          | Bool
    otherwise  = do { (TcType
    ty1', Bool
    progress1) <- TcTyVar -> TcS (TcType, Bool)
    quick_zonk TcTyVar
    tv1
                            ; (TcType
    ty2', Bool
    progress2) <- TcTyVar -> TcS (TcType, Bool)
    quick_zonk TcTyVar
    tv2
                            ; if Bool
    progress1 Bool -> Bool -> Bool
    || Bool
    progress2
                              then TcType -> TcType -> TcS (Either (Pair TcType) TcType)
    go TcType
    ty1' TcType
    ty2'
                              else forall (m :: * -> *) a. Monad m => a -> m a
    return forall a b. (a -> b) -> a -> b
    $ forall a b. a -> Either a b
    Left (forall a. a -> a -> Pair a
    Pair (TcTyVar -> TcType
    TyVarTy TcTyVar
    tv1) (TcTyVar -> TcType
    TyVarTy TcTyVar
    tv2)) }

        trace_indirect :: a -> a -> TcS ()
    trace_indirect a
    tv a
    ty
           = String -> SDoc -> TcS ()
    traceTcS String
    "Following filled tyvar (zonk_eq_types)"
                      (forall a. Outputable a => a -> SDoc
    ppr a
    tv SDoc -> SDoc -> SDoc
    <+> SDoc
    equals SDoc -> SDoc -> SDoc
    <+> forall a. Outputable a => a -> SDoc
    ppr a
    ty)

        quick_zonk :: TcTyVar -> TcS (TcType, Bool)
    quick_zonk TcTyVar
    tv = case TcTyVar -> TcTyVarDetails
    tcTyVarDetails TcTyVar
    tv of
          MetaTv { mtv_ref :: TcTyVarDetails -> IORef MetaDetails
    mtv_ref = IORef MetaDetails
    ref }
            -> do { MetaDetails
    cts <- forall a. TcRef a -> TcS a
    readTcRef IORef MetaDetails
    ref
                  ; case MetaDetails
    cts of
                      MetaDetails
    Flexi        -> forall (m :: * -> *) a. Monad m => a -> m a
    return (TcTyVar -> TcType
    TyVarTy TcTyVar
    tv, Bool
    False)
                      Indirect TcType
    ty' -> do { forall {a} {a}. (Outputable a, Outputable a) => a -> a -> TcS ()
    trace_indirect TcTyVar
    tv TcType
    ty'
                                         ; forall (m :: * -> *) a. Monad m => a -> m a
    return (TcType
    ty', Bool
    True) } }
          TcTyVarDetails
    _ -> forall (m :: * -> *) a. Monad m => a -> m a
    return (TcTyVar -> TcType
    TyVarTy TcTyVar
    tv, Bool
    False)

          -- This happens for type families, too. But recall that failure
          -- here just means to try harder, so it's OK if the type function
          -- isn't injective.
        tycon :: TyCon -> [TcType] -> [TcType]
              -> TcS (Either (Pair TcType) TcType)
        tycon :: TyCon -> [TcType] -> [TcType] -> TcS (Either (Pair TcType) TcType)
    tycon TyCon
    tc [TcType]
    tys1 [TcType]
    tys2
          = do { [Either (Pair TcType) TcType]
    results <- forall (m :: * -> *) a b c.
    Applicative m =>
    (a -> b -> m c) -> [a] -> [b] -> m [c]
    zipWithM TcType -> TcType -> TcS (Either (Pair TcType) TcType)
    go [TcType]
    tys1 [TcType]
    tys2
               ; forall (m :: * -> *) a. Monad m => a -> m a
    return forall a b. (a -> b) -> a -> b
    $ case [Either (Pair TcType) TcType] -> Either (Pair [TcType]) [TcType]
    combine_results [Either (Pair TcType) TcType]
    results of
                   Left Pair [TcType]
    tys  -> forall a b. a -> Either a b
    Left (TyCon -> [TcType] -> TcType
    mkTyConApp TyCon
    tc forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> Pair [TcType]
    tys)
                   Right [TcType]
    tys -> forall a b. b -> Either a b
    Right (TyCon -> [TcType] -> TcType
    mkTyConApp TyCon
    tc [TcType]
    tys) }

        combine_results :: [Either (Pair TcType) TcType]
                        -> Either (Pair [TcType]) [TcType]
        combine_results :: [Either (Pair TcType) TcType] -> Either (Pair [TcType]) [TcType]
    combine_results = forall (p :: * -> * -> *) a b c d.
    Bifunctor p =>
    (a -> b) -> (c -> d) -> p a c -> p b d
    bimap (forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    fmap forall a. [a] -> [a]
    reverse) forall a. [a] -> [a]
    reverse forall b c a. (b -> c) -> (a -> b) -> a -> c
    .
                          forall (t :: * -> *) b a.
    Foldable t =>
    (b -> a -> b) -> b -> t a -> b
    foldl' (forall a b c.
    (a -> b -> c)
    -> Either (Pair b) b -> Either (Pair a) a -> Either (Pair c) c
    combine_rev (:)) (forall a b. b -> Either a b
    Right [])

          -- combine (in reverse) a new result onto an already-combined result
        combine_rev :: (a -> b -> c)
                    -> Either (Pair b) b
                    -> Either (Pair a) a
                    -> Either (Pair c) c
        combine_rev :: forall a b c.
    (a -> b -> c)
    -> Either (Pair b) b -> Either (Pair a) a -> Either (Pair c) c
    combine_rev a -> b -> c
    f (Left Pair b
    list) (Left Pair a
    elt) = forall a b. a -> Either a b
    Left (a -> b -> c
    f forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> Pair a
    elt     forall (f :: * -> *) a b. Applicative f => f (a -> b) -> f a -> f b
    <*> Pair b
    list)
        combine_rev a -> b -> c
    f (Left Pair b
    list) (Right a
    ty) = forall a b. a -> Either a b
    Left (a -> b -> c
    f forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> forall (f :: * -> *) a. Applicative f => a -> f a
    pure a
    ty forall (f :: * -> *) a b. Applicative f => f (a -> b) -> f a -> f b
    <*> Pair b
    list)
        combine_rev a -> b -> c
    f (Right b
    tys) (Left Pair a
    elt) = forall a b. a -> Either a b
    Left (a -> b -> c
    f forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> Pair a
    elt     forall (f :: * -> *) a b. Applicative f => f (a -> b) -> f a -> f b
    <*> forall (f :: * -> *) a. Applicative f => a -> f a
    pure b
    tys)
        combine_rev a -> b -> c
    f (Right b
    tys) (Right a
    ty) = forall a b. b -> Either a b
    Right (a -> b -> c
    f a
    ty b
    tys)

    {- See Note [Unwrap newtypes first]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
      newtype N m a = MkN (m a)
    Then N will get a conservative, Nominal role for its second parameter 'a',
    because it appears as an argument to the unknown 'm'. Now consider
      [W] N Maybe a  ~R#  N Maybe b

    If we decompose, we'll get
      [W] a ~N# b

    But if instead we unwrap we'll get
      [W] Maybe a ~R# Maybe b
    which in turn gives us
      [W] a ~R# b
    which is easier to satisfy.

    Bottom line: unwrap newtypes before decomposing them!
    c.f. #9123 comment:52,53 for a compelling example.

    Note [Newtypes can blow the stack]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Suppose we have

      newtype X = MkX (Int -> X)
      newtype Y = MkY (Int -> Y)

    and now wish to prove

      [W] X ~R Y

    This Wanted will loop, expanding out the newtypes ever deeper looking
    for a solid match or a solid discrepancy. Indeed, there is something
    appropriate to this looping, because X and Y *do* have the same representation,
    in the limit -- they're both (Fix ((->) Int)). However, no finitely-sized
    coercion will ever witness it. This loop won't actually cause GHC to hang,
    though, because we check our depth when unwrapping newtypes.

    Note [Eager reflexivity check]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Suppose we have

      newtype X = MkX (Int -> X)

    and

      [W] X ~R X

    Naively, we would start unwrapping X and end up in a loop. Instead,
    we do this eager reflexivity check. This is necessary only for representational
    equality because the rewriter technology deals with the similar case
    (recursive type families) for nominal equality.

    Note that this check does not catch all cases, but it will catch the cases
    we're most worried about, types like X above that are actually inhabited.

    Here's another place where this reflexivity check is key:
    Consider trying to prove (f a) ~R (f a). The AppTys in there can't
    be decomposed, because representational equality isn't congruent with respect
    to AppTy. So, when canonicalising the equality above, we get stuck and
    would normally produce a CIrredCan. However, we really do want to
    be able to solve (f a) ~R (f a). So, in the representational case only,
    we do a reflexivity check.

    (This would be sound in the nominal case, but unnecessary, and I [Richard
    E.] am worried that it would slow down the common case.)
    -}

    ------------------------
    -- | We're able to unwrap a newtype. Update the bits accordingly.
    can_eq_newtype_nc :: CtEvidence           -- ^ :: ty1 ~ ty2
                      -> SwapFlag
                      -> TcType                                    -- ^ ty1
                      -> ((Bag GlobalRdrElt, TcCoercion), TcType)  -- ^ :: ty1 ~ ty1'
                      -> TcType               -- ^ ty2
                      -> TcType               -- ^ ty2, with type synonyms
                      -> TcS (StopOrContinue Ct)
    can_eq_newtype_nc :: CtEvidence
    -> SwapFlag
    -> TcType
    -> ((Bag GlobalRdrElt, TcCoercion), TcType)
    -> TcType
    -> TcType
    -> TcS (StopOrContinue Ct)
    can_eq_newtype_nc CtEvidence
    ev SwapFlag
    swapped TcType
    ty1 ((Bag GlobalRdrElt
    gres, TcCoercion
    co), TcType
    ty1') TcType
    ty2 TcType
    ps_ty2
      = do { String -> SDoc -> TcS ()
    traceTcS String
    "can_eq_newtype_nc" forall a b. (a -> b) -> a -> b
    $
             [SDoc] -> SDoc
    vcat [ forall a. Outputable a => a -> SDoc
    ppr CtEvidence
    ev, forall a. Outputable a => a -> SDoc
    ppr SwapFlag
    swapped, forall a. Outputable a => a -> SDoc
    ppr TcCoercion
    co, forall a. Outputable a => a -> SDoc
    ppr Bag GlobalRdrElt
    gres, forall a. Outputable a => a -> SDoc
    ppr TcType
    ty1', forall a. Outputable a => a -> SDoc
    ppr TcType
    ty2 ]

             -- check for blowing our stack:
             -- See Note [Newtypes can blow the stack]
           ; CtLoc -> TcType -> TcS ()
    checkReductionDepth (CtEvidence -> CtLoc
    ctEvLoc CtEvidence
    ev) TcType
    ty1

             -- Next, we record uses of newtype constructors, since coercing
             -- through newtypes is tantamount to using their constructors.
           ; [GlobalRdrElt] -> TcS ()
    addUsedGREs [GlobalRdrElt]
    gre_list
             -- If a newtype constructor was imported, don't warn about not
             -- importing it...
           ; forall (t :: * -> *) (f :: * -> *) a b.
    (Foldable t, Applicative f) =>
    (a -> f b) -> t a -> f ()
    traverse_ Name -> TcS ()
    keepAlive forall a b. (a -> b) -> a -> b
    $ forall a b. (a -> b) -> [a] -> [b]
    map GlobalRdrElt -> Name
    greMangledName [GlobalRdrElt]
    gre_list
             -- ...and similarly, if a newtype constructor was defined in the same
             -- module, don't warn about it being unused.
             -- See Note [Tracking unused binding and imports] in GHC.Tc.Utils.

           ; CtEvidence
    new_ev <- CtEvidence
    -> SwapFlag
    -> TcType
    -> TcType
    -> TcCoercion
    -> TcCoercion
    -> TcS CtEvidence
    rewriteEqEvidence CtEvidence
    ev SwapFlag
    swapped TcType
    ty1' TcType
    ps_ty2
                                         (TcCoercion -> TcCoercion
    mkTcSymCo TcCoercion
    co) (Role -> TcType -> TcCoercion
    mkTcReflCo Role
    Representational TcType
    ps_ty2)
           ; Bool
    -> CtEvidence
    -> EqRel
    -> TcType
    -> TcType
    -> TcType
    -> TcType
    -> TcS (StopOrContinue Ct)
    can_eq_nc Bool
    False CtEvidence
    new_ev EqRel
    ReprEq TcType
    ty1' TcType
    ty1' TcType
    ty2 TcType
    ps_ty2 }
      where
        gre_list :: [GlobalRdrElt]
    gre_list = forall a. Bag a -> [a]
    bagToList Bag GlobalRdrElt
    gres

    ---------
    -- ^ Decompose a type application.
    -- All input types must be rewritten. See Note [Canonicalising type applications]
    -- Nominal equality only!
    can_eq_app :: CtEvidence       -- :: s1 t1 ~N s2 t2
               -> Xi -> Xi         -- s1 t1
               -> Xi -> Xi         -- s2 t2
               -> TcS (StopOrContinue Ct)

    -- AppTys only decompose for nominal equality, so this case just leads
    -- to an irreducible constraint; see typecheck/should_compile/T10494
    -- See Note [Decomposing AppTy at representational role]
    can_eq_app :: CtEvidence
    -> TcType -> TcType -> TcType -> TcType -> TcS (StopOrContinue Ct)
    can_eq_app CtEvidence
    ev TcType
    s1 TcType
    t1 TcType
    s2 TcType
    t2
      | CtDerived {} <- CtEvidence
    ev
      = do { CtLoc -> [Role] -> [TcType] -> [TcType] -> TcS ()
    unifyDeriveds CtLoc
    loc [Role
    Nominal, Role
    Nominal] [TcType
    s1, TcType
    t1] [TcType
    s2, TcType
    t2]
           ; forall a. CtEvidence -> String -> TcS (StopOrContinue a)
    stopWith CtEvidence
    ev String
    "Decomposed [D] AppTy" }

      | CtWanted { ctev_dest :: CtEvidence -> TcEvDest
    ctev_dest = TcEvDest
    dest } <- CtEvidence
    ev
      = do { TcCoercion
    co_s <- CtLoc -> Role -> TcType -> TcType -> TcS TcCoercion
    unifyWanted CtLoc
    loc Role
    Nominal TcType
    s1 TcType
    s2
           ; let arg_loc :: CtLoc
    arg_loc
                   | TcType -> Bool
    isNextArgVisible TcType
    s1 = CtLoc
    loc
                   | Bool
    otherwise           = CtLoc -> (CtOrigin -> CtOrigin) -> CtLoc
    updateCtLocOrigin CtLoc
    loc CtOrigin -> CtOrigin
    toInvisibleOrigin
           ; TcCoercion
    co_t <- CtLoc -> Role -> TcType -> TcType -> TcS TcCoercion
    unifyWanted CtLoc
    arg_loc Role
    Nominal TcType
    t1 TcType
    t2
           ; let co :: TcCoercion
    co = TcCoercion -> TcCoercion -> TcCoercion
    mkAppCo TcCoercion
    co_s TcCoercion
    co_t
           ; TcEvDest -> TcCoercion -> TcS ()
    setWantedEq TcEvDest
    dest TcCoercion
    co
           ; forall a. CtEvidence -> String -> TcS (StopOrContinue a)
    stopWith CtEvidence
    ev String
    "Decomposed [W] AppTy" }

        -- If there is a ForAll/(->) mismatch, the use of the Left coercion
        -- below is ill-typed, potentially leading to a panic in splitTyConApp
        -- Test case: typecheck/should_run/Typeable1
        -- We could also include this mismatch check above (for W and D), but it's slow
        -- and we'll get a better error message not doing it
      | TcType
    s1k TcType -> TcType -> Bool
    `mismatches` TcType
    s2k
      = CtEvidence -> TcType -> TcType -> TcS (StopOrContinue Ct)
    canEqHardFailure CtEvidence
    ev (TcType
    s1 TcType -> TcType -> TcType
    `mkAppTy` TcType
    t1) (TcType
    s2 TcType -> TcType -> TcType
    `mkAppTy` TcType
    t2)

      | CtGiven { ctev_evar :: CtEvidence -> TcTyVar
    ctev_evar = TcTyVar
    evar } <- CtEvidence
    ev
      = do { let co :: TcCoercion
    co   = TcTyVar -> TcCoercion
    mkTcCoVarCo TcTyVar
    evar
                 co_s :: TcCoercion
    co_s = LeftOrRight -> TcCoercion -> TcCoercion
    mkTcLRCo LeftOrRight
    CLeft  TcCoercion
    co
                 co_t :: TcCoercion
    co_t = LeftOrRight -> TcCoercion -> TcCoercion
    mkTcLRCo LeftOrRight
    CRight TcCoercion
    co
           ; CtEvidence
    evar_s <- CtLoc -> (TcType, EvTerm) -> TcS CtEvidence
    newGivenEvVar CtLoc
    loc ( CtEvidence -> TcType -> TcType -> TcType
    mkTcEqPredLikeEv CtEvidence
    ev TcType
    s1 TcType
    s2
                                         , TcCoercion -> EvTerm
    evCoercion TcCoercion
    co_s )
           ; CtEvidence
    evar_t <- CtLoc -> (TcType, EvTerm) -> TcS CtEvidence
    newGivenEvVar CtLoc
    loc ( CtEvidence -> TcType -> TcType -> TcType
    mkTcEqPredLikeEv CtEvidence
    ev TcType
    t1 TcType
    t2
                                         , TcCoercion -> EvTerm
    evCoercion TcCoercion
    co_t )
           ; [CtEvidence] -> TcS ()
    emitWorkNC [CtEvidence
    evar_t]
           ; CtEvidence -> EqRel -> TcType -> TcType -> TcS (StopOrContinue Ct)
    canEqNC CtEvidence
    evar_s EqRel
    NomEq TcType
    s1 TcType
    s2 }

      where
        loc :: CtLoc
    loc = CtEvidence -> CtLoc
    ctEvLoc CtEvidence
    ev

        s1k :: TcType
    s1k = HasDebugCallStack => TcType -> TcType
    tcTypeKind TcType
    s1
        s2k :: TcType
    s2k = HasDebugCallStack => TcType -> TcType
    tcTypeKind TcType
    s2

        TcType
    k1 mismatches :: TcType -> TcType -> Bool
    `mismatches` TcType
    k2
          =  TcType -> Bool
    isForAllTy TcType
    k1 Bool -> Bool -> Bool
    && Bool -> Bool
    not (TcType -> Bool
    isForAllTy TcType
    k2)
          Bool -> Bool -> Bool
    || Bool -> Bool
    not (TcType -> Bool
    isForAllTy TcType
    k1) Bool -> Bool -> Bool
    && TcType -> Bool
    isForAllTy TcType
    k2

    -----------------------
    -- | Break apart an equality over a casted type
    -- looking like   (ty1 |> co1) ~ ty2   (modulo a swap-flag)
    canEqCast :: Bool         -- are both types rewritten?
              -> CtEvidence
              -> EqRel
              -> SwapFlag
              -> TcType -> Coercion   -- LHS (res. RHS), ty1 |> co1
              -> TcType -> TcType     -- RHS (res. LHS), ty2 both normal and pretty
              -> TcS (StopOrContinue Ct)
    canEqCast :: Bool
    -> CtEvidence
    -> EqRel
    -> SwapFlag
    -> TcType
    -> TcCoercion
    -> TcType
    -> TcType
    -> TcS (StopOrContinue Ct)
    canEqCast Bool
    rewritten CtEvidence
    ev EqRel
    eq_rel SwapFlag
    swapped TcType
    ty1 TcCoercion
    co1 TcType
    ty2 TcType
    ps_ty2
      = do { String -> SDoc -> TcS ()
    traceTcS String
    "Decomposing cast" ([SDoc] -> SDoc
    vcat [ forall a. Outputable a => a -> SDoc
    ppr CtEvidence
    ev
                                               , forall a. Outputable a => a -> SDoc
    ppr TcType
    ty1 SDoc -> SDoc -> SDoc
    <+> String -> SDoc
    text String
    "|>" SDoc -> SDoc -> SDoc
    <+> forall a. Outputable a => a -> SDoc
    ppr TcCoercion
    co1
                                               , forall a. Outputable a => a -> SDoc
    ppr TcType
    ps_ty2 ])
           ; CtEvidence
    new_ev <- CtEvidence
    -> SwapFlag
    -> TcType
    -> TcType
    -> TcCoercion
    -> TcCoercion
    -> TcS CtEvidence
    rewriteEqEvidence CtEvidence
    ev SwapFlag
    swapped TcType
    ty1 TcType
    ps_ty2
                                         (Role -> TcType -> TcCoercion -> TcCoercion
    mkTcGReflRightCo Role
    role TcType
    ty1 TcCoercion
    co1)
                                         (Role -> TcType -> TcCoercion
    mkTcReflCo Role
    role TcType
    ps_ty2)
           ; Bool
    -> CtEvidence
    -> EqRel
    -> TcType
    -> TcType
    -> TcType
    -> TcType
    -> TcS (StopOrContinue Ct)
    can_eq_nc Bool
    rewritten CtEvidence
    new_ev EqRel
    eq_rel TcType
    ty1 TcType
    ty1 TcType
    ty2 TcType
    ps_ty2 }
      where
        role :: Role
    role = EqRel -> Role
    eqRelRole EqRel
    eq_rel

    ------------------------
    canTyConApp :: CtEvidence -> EqRel
                -> TyCon -> [TcType]
                -> TyCon -> [TcType]
                -> TcS (StopOrContinue Ct)
    -- See Note [Decomposing TyConApps]
    -- Neither tc1 nor tc2 is a saturated funTyCon
    canTyConApp :: CtEvidence
    -> EqRel
    -> TyCon
    -> [TcType]
    -> TyCon
    -> [TcType]
    -> TcS (StopOrContinue Ct)
    canTyConApp CtEvidence
    ev EqRel
    eq_rel TyCon
    tc1 [TcType]
    tys1 TyCon
    tc2 [TcType]
    tys2
      | TyCon
    tc1 forall a. Eq a => a -> a -> Bool
    == TyCon
    tc2
      , [TcType]
    tys1 forall a b. [a] -> [b] -> Bool
    `equalLength` [TcType]
    tys2
      = do { InertSet
    inerts <- TcS InertSet
    getTcSInerts
           ; if InertSet -> Bool
    can_decompose InertSet
    inerts
             then CtEvidence
    -> EqRel
    -> TyCon
    -> [TcType]
    -> [TcType]
    -> TcS (StopOrContinue Ct)
    canDecomposableTyConAppOK CtEvidence
    ev EqRel
    eq_rel TyCon
    tc1 [TcType]
    tys1 [TcType]
    tys2
             else CtEvidence -> EqRel -> TcType -> TcType -> TcS (StopOrContinue Ct)
    canEqFailure CtEvidence
    ev EqRel
    eq_rel TcType
    ty1 TcType
    ty2 }

      -- See Note [Skolem abstract data] in GHC.Core.Tycon
      | TyCon -> Bool
    tyConSkolem TyCon
    tc1 Bool -> Bool -> Bool
    || TyCon -> Bool
    tyConSkolem TyCon
    tc2
      = do { String -> SDoc -> TcS ()
    traceTcS String
    "canTyConApp: skolem abstract" (forall a. Outputable a => a -> SDoc
    ppr TyCon
    tc1 SDoc -> SDoc -> SDoc
    $$ forall a. Outputable a => a -> SDoc
    ppr TyCon
    tc2)
           ; forall a. a -> TcS (StopOrContinue a)
    continueWith (CtIrredReason -> CtEvidence -> Ct
    mkIrredCt CtIrredReason
    AbstractTyConReason CtEvidence
    ev) }

      -- Fail straight away for better error messages
      -- See Note [Use canEqFailure in canDecomposableTyConApp]
      | EqRel
    eq_rel forall a. Eq a => a -> a -> Bool
    == EqRel
    ReprEq Bool -> Bool -> Bool
    && Bool -> Bool
    not (TyCon -> Role -> Bool
    isGenerativeTyCon TyCon
    tc1 Role
    Representational Bool -> Bool -> Bool
    &&
                                 TyCon -> Role -> Bool
    isGenerativeTyCon TyCon
    tc2 Role
    Representational)
      = CtEvidence -> EqRel -> TcType -> TcType -> TcS (StopOrContinue Ct)
    canEqFailure CtEvidence
    ev EqRel
    eq_rel TcType
    ty1 TcType
    ty2
      | Bool
    otherwise
      = CtEvidence -> TcType -> TcType -> TcS (StopOrContinue Ct)
    canEqHardFailure CtEvidence
    ev TcType
    ty1 TcType
    ty2
      where
        -- Reconstruct the types for error messages. This would do
        -- the wrong thing (from a pretty printing point of view)
        -- for functions, because we've lost the AnonArgFlag; but
        -- in fact we never call canTyConApp on a saturated FunTyCon
        ty1 :: TcType
    ty1 = TyCon -> [TcType] -> TcType
    mkTyConApp TyCon
    tc1 [TcType]
    tys1
        ty2 :: TcType
    ty2 = TyCon -> [TcType] -> TcType
    mkTyConApp TyCon
    tc2 [TcType]
    tys2

        loc :: CtLoc
    loc  = CtEvidence -> CtLoc
    ctEvLoc CtEvidence
    ev
        pred :: TcType
    pred = CtEvidence -> TcType
    ctEvPred CtEvidence
    ev

         -- See Note [Decomposing equality]
        can_decompose :: InertSet -> Bool
    can_decompose InertSet
    inerts
          =  TyCon -> Role -> Bool
    isInjectiveTyCon TyCon
    tc1 (EqRel -> Role
    eqRelRole EqRel
    eq_rel)
          Bool -> Bool -> Bool
    || (CtEvidence -> CtFlavour
    ctEvFlavour CtEvidence
    ev forall a. Eq a => a -> a -> Bool
    /= CtFlavour
    Given Bool -> Bool -> Bool
    && forall a. Bag a -> Bool
    isEmptyBag (CtLoc -> TcType -> InertSet -> Bag Ct
    matchableGivens CtLoc
    loc TcType
    pred InertSet
    inerts))

    {-
    Note [Use canEqFailure in canDecomposableTyConApp]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We must use canEqFailure, not canEqHardFailure here, because there is
    the possibility of success if working with a representational equality.
    Here is one case:

      type family TF a where TF Char = Bool
      data family DF a
      newtype instance DF Bool = MkDF Int

    Suppose we are canonicalising (Int ~R DF (TF a)), where we don't yet
    know `a`. This is *not* a hard failure, because we might soon learn
    that `a` is, in fact, Char, and then the equality succeeds.

    Here is another case:

      [G] Age ~R Int

    where Age's constructor is not in scope. We don't want to report
    an "inaccessible code" error in the context of this Given!

    For example, see typecheck/should_compile/T10493, repeated here:

      import Data.Ord (Down)  -- no constructor

      foo :: Coercible (Down Int) Int => Down Int -> Int
      foo = coerce

    That should compile, but only because we use canEqFailure and not
    canEqHardFailure.

    Note [Decomposing equality]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If we have a constraint (of any flavour and role) that looks like
    T tys1 ~ T tys2, what can we conclude about tys1 and tys2? The answer,
    of course, is "it depends". This Note spells it all out.

    In this Note, "decomposition" refers to taking the constraint
      [fl] (T tys1 ~X T tys2)
    (for some flavour fl and some role X) and replacing it with
      [fls'] (tys1 ~Xs' tys2)
    where that notation indicates a list of new constraints, where the
    new constraints may have different flavours and different roles.

    The key property to consider is injectivity. When decomposing a Given, the
    decomposition is sound if and only if T is injective in all of its type
    arguments. When decomposing a Wanted, the decomposition is sound (assuming the
    correct roles in the produced equality constraints), but it may be a guess --
    that is, an unforced decision by the constraint solver. Decomposing Wanteds
    over injective TyCons does not entail guessing. But sometimes we want to
    decompose a Wanted even when the TyCon involved is not injective! (See below.)

    So, in broad strokes, we want this rule:

    (*) Decompose a constraint (T tys1 ~X T tys2) if and only if T is injective
    at role X.

    Pursuing the details requires exploring three axes:
    * Flavour: Given vs. Derived vs. Wanted
    * Role: Nominal vs. Representational
    * TyCon species: datatype vs. newtype vs. data family vs. type family vs. type variable

    (A type variable isn't a TyCon, of course, but it's convenient to put the AppTy case
    in the same table.)

    Right away, we can say that Derived behaves just as Wanted for the purposes
    of decomposition. The difference between Derived and Wanted is the handling of
    evidence. Since decomposition in these cases isn't a matter of soundness but of
    guessing, we want the same behaviour regardless of evidence.

    Here is a table (discussion following) detailing where decomposition of
       (T s1 ... sn) ~r (T t1 .. tn)
    is allowed.  The first four lines (Data types ... type family) refer
    to TyConApps with various TyCons T; the last line is for AppTy, covering
    both where there is a type variable at the head and the case for an over-
    saturated type family.

    NOMINAL               GIVEN        WANTED                         WHERE

    Datatype               YES          YES                           canTyConApp
    Newtype                YES          YES                           canTyConApp
    Data family            YES          YES                           canTyConApp
    Type family            NO{1}        YES, in injective args{1}     canEqCanLHS2
    AppTy                  YES          YES                           can_eq_app

    REPRESENTATIONAL      GIVEN        WANTED

    Datatype               YES          YES                           canTyConApp
    Newtype                NO{2}       MAYBE{2}                canTyConApp(can_decompose)
    Data family            NO{3}       MAYBE{3}                canTyConApp(can_decompose)
    Type family            NO           NO                            canEqCanLHS2
    AppTy                  NO{4}        NO{4}                         can_eq_nc'

    {1}: Type families can be injective in some, but not all, of their arguments,
    so we want to do partial decomposition. This is quite different than the way
    other decomposition is done, where the decomposed equalities replace the original
    one. We thus proceed much like we do with superclasses, emitting new Deriveds
    when "decomposing" a partially-injective type family Wanted. Injective type
    families have no corresponding evidence of their injectivity, so we cannot
    decompose an injective-type-family Given.

    {2}: See Note [Decomposing newtypes at representational role]

    {3}: Because of the possibility of newtype instances, we must treat
    data families like newtypes. See also
    Note [Decomposing newtypes at representational role]. See #10534 and
    test case typecheck/should_fail/T10534.

    {4}: See Note [Decomposing AppTy at representational role]

    In the implementation of can_eq_nc and friends, we don't directly pattern
    match using lines like in the tables above, as those tables don't cover
    all cases (what about PrimTyCon? tuples?). Instead we just ask about injectivity,
    boiling the tables above down to rule (*). The exceptions to rule (*) are for
    injective type families, which are handled separately from other decompositions,
    and the MAYBE entries above.

    Note [Decomposing newtypes at representational role]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    This note discusses the 'newtype' line in the REPRESENTATIONAL table
    in Note [Decomposing equality]. (At nominal role, newtypes are fully
    decomposable.)

    Here is a representative example of why representational equality over
    newtypes is tricky:

      newtype Nt a = Mk Bool         -- NB: a is not used in the RHS,
      type role Nt representational  -- but the user gives it an R role anyway

    If we have [W] Nt alpha ~R Nt beta, we *don't* want to decompose to
    [W] alpha ~R beta, because it's possible that alpha and beta aren't
    representationally equal. Here's another example.

      newtype Nt a = MkNt (Id a)
      type family Id a where Id a = a

      [W] Nt Int ~R Nt Age

    Because of its use of a type family, Nt's parameter will get inferred to have
    a nominal role. Thus, decomposing the wanted will yield [W] Int ~N Age, which
    is unsatisfiable. Unwrapping, though, leads to a solution.

    Conclusion:
     * Unwrap newtypes before attempting to decompose them.
       This is done in can_eq_nc'.

    It all comes from the fact that newtypes aren't necessarily injective
    w.r.t. representational equality.

    Furthermore, as explained in Note [NthCo and newtypes] in GHC.Core.TyCo.Rep, we can't use
    NthCo on representational coercions over newtypes. NthCo comes into play
    only when decomposing givens.

    Conclusion:
     * Do not decompose [G] N s ~R N t

    Is it sensible to decompose *Wanted* constraints over newtypes?  Yes!
    It's the only way we could ever prove (IO Int ~R IO Age), recalling
    that IO is a newtype.

    However we must be careful.  Consider

      type role Nt representational

      [G] Nt a ~R Nt b       (1)
      [W] NT alpha ~R Nt b   (2)
      [W] alpha ~ a          (3)

    If we focus on (3) first, we'll substitute in (2), and now it's
    identical to the given (1), so we succeed.  But if we focus on (2)
    first, and decompose it, we'll get (alpha ~R b), which is not soluble.
    This is exactly like the question of overlapping Givens for class
    constraints: see Note [Instance and Given overlap] in GHC.Tc.Solver.Interact.

    Conclusion:
      * Decompose [W] N s ~R N t  iff there no given constraint that could
        later solve it.

    Note [Decomposing AppTy at representational role]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We never decompose AppTy at a representational role. For Givens, doing
    so is simply unsound: the LRCo coercion former requires a nominal-roled
    arguments. (See (1) for an example of why.) For Wanteds, decomposing
    would be sound, but it would be a guess, and a non-confluent one at that.

    Here is an example:

        [G] g1 :: a ~R b
        [W] w1 :: Maybe b ~R alpha a
        [W] w2 :: alpha ~ Maybe

    Suppose we see w1 before w2. If we were to decompose, we would decompose
    this to become

        [W] w3 :: Maybe ~R alpha
        [W] w4 :: b ~ a

    Note that w4 is *nominal*. A nominal role here is necessary because AppCo
    requires a nominal role on its second argument. (See (2) for an example of
    why.) If we decomposed w1 to w3,w4, we would then get stuck, because w4
    is insoluble. On the other hand, if we see w2 first, setting alpha := Maybe,
    all is well, as we can decompose Maybe b ~R Maybe a into b ~R a.

    Another example:

        newtype Phant x = MkPhant Int

        [W] w1 :: Phant Int ~R alpha Bool
        [W] w2 :: alpha ~ Phant

    If we see w1 first, decomposing would be disastrous, as we would then try
    to solve Int ~ Bool. Instead, spotting w2 allows us to simplify w1 to become

        [W] w1' :: Phant Int ~R Phant Bool

    which can then (assuming MkPhant is in scope) be simplified to Int ~R Int,
    and all will be well. See also Note [Unwrap newtypes first].

    Bottom line: never decompose AppTy with representational roles.

    (1) Decomposing a Given AppTy over a representational role is simply
    unsound. For example, if we have co1 :: Phant Int ~R a Bool (for
    the newtype Phant, above), then we surely don't want any relationship
    between Int and Bool, lest we also have co2 :: Phant ~ a around.

    (2) The role on the AppCo coercion is a conservative choice, because we don't
    know the role signature of the function. For example, let's assume we could
    have a representational role on the second argument of AppCo. Then, consider

        data G a where    -- G will have a nominal role, as G is a GADT
          MkG :: G Int
        newtype Age = MkAge Int

        co1 :: G ~R a        -- by assumption
        co2 :: Age ~R Int    -- by newtype axiom
        co3 = AppCo co1 co2 :: G Age ~R a Int    -- by our broken AppCo

    and now co3 can be used to cast MkG to have type G Age, in violation of
    the way GADTs are supposed to work (which is to use nominal equality).

    -}

    canDecomposableTyConAppOK :: CtEvidence -> EqRel
                              -> TyCon -> [TcType] -> [TcType]
                              -> TcS (StopOrContinue Ct)
    -- Precondition: tys1 and tys2 are the same length, hence "OK"
    canDecomposableTyConAppOK :: CtEvidence
    -> EqRel
    -> TyCon
    -> [TcType]
    -> [TcType]
    -> TcS (StopOrContinue Ct)
    canDecomposableTyConAppOK CtEvidence
    ev EqRel
    eq_rel TyCon
    tc [TcType]
    tys1 [TcType]
    tys2
      = ASSERT( tys1 `equalLength` tys2 )
        do { String -> SDoc -> TcS ()
    traceTcS String
    "canDecomposableTyConAppOK"
                      (forall a. Outputable a => a -> SDoc
    ppr CtEvidence
    ev SDoc -> SDoc -> SDoc
    $$ forall a. Outputable a => a -> SDoc
    ppr EqRel
    eq_rel SDoc -> SDoc -> SDoc
    $$ forall a. Outputable a => a -> SDoc
    ppr TyCon
    tc SDoc -> SDoc -> SDoc
    $$ forall a. Outputable a => a -> SDoc
    ppr [TcType]
    tys1 SDoc -> SDoc -> SDoc
    $$ forall a. Outputable a => a -> SDoc
    ppr [TcType]
    tys2)
           ; case CtEvidence
    ev of
               CtDerived {}
                 -> CtLoc -> [Role] -> [TcType] -> [TcType] -> TcS ()
    unifyDeriveds CtLoc
    loc [Role]
    tc_roles [TcType]
    tys1 [TcType]
    tys2

               CtWanted { ctev_dest :: CtEvidence -> TcEvDest
    ctev_dest = TcEvDest
    dest }
                      -- new_locs and tc_roles are both infinite, so
                      -- we are guaranteed that cos has the same length
                      -- as tys1 and tys2
                 -> do { [TcCoercion]
    cos <- forall (m :: * -> *) a b c d e.
    Monad m =>
    (a -> b -> c -> d -> m e) -> [a] -> [b] -> [c] -> [d] -> m [e]
    zipWith4M CtLoc -> Role -> TcType -> TcType -> TcS TcCoercion
    unifyWanted [CtLoc]
    new_locs [Role]
    tc_roles [TcType]
    tys1 [TcType]
    tys2
                       ; TcEvDest -> TcCoercion -> TcS ()
    setWantedEq TcEvDest
    dest (HasDebugCallStack => Role -> TyCon -> [TcCoercion] -> TcCoercion
    mkTyConAppCo Role
    role TyCon
    tc [TcCoercion]
    cos) }

               CtGiven { ctev_evar :: CtEvidence -> TcTyVar
    ctev_evar = TcTyVar
    evar }
                 -> do { let ev_co :: TcCoercion
    ev_co = TcTyVar -> TcCoercion
    mkCoVarCo TcTyVar
    evar
                       ; [CtEvidence]
    given_evs <- CtLoc -> [(TcType, EvTerm)] -> TcS [CtEvidence]
    newGivenEvVars CtLoc
    loc forall a b. (a -> b) -> a -> b
    $
                                      [ ( Role -> TcType -> TcType -> TcType
    mkPrimEqPredRole Role
    r TcType
    ty1 TcType
    ty2
                                        , TcCoercion -> EvTerm
    evCoercion forall a b. (a -> b) -> a -> b
    $ HasDebugCallStack => Role -> Int -> TcCoercion -> TcCoercion
    mkNthCo Role
    r Int
    i TcCoercion
    ev_co )
                                      | (Role
    r, TcType
    ty1, TcType
    ty2, Int
    i) <- forall a b c d. [a] -> [b] -> [c] -> [d] -> [(a, b, c, d)]
    zip4 [Role]
    tc_roles [TcType]
    tys1 [TcType]
    tys2 [Int
    0..]
                                      , Role
    r forall a. Eq a => a -> a -> Bool
    /= Role
    Phantom
                                      , Bool -> Bool
    not (TcType -> Bool
    isCoercionTy TcType
    ty1) Bool -> Bool -> Bool
    && Bool -> Bool
    not (TcType -> Bool
    isCoercionTy TcType
    ty2) ]
                       ; [CtEvidence] -> TcS ()
    emitWorkNC [CtEvidence]
    given_evs }

        ; forall a. CtEvidence -> String -> TcS (StopOrContinue a)
    stopWith CtEvidence
    ev String
    "Decomposed TyConApp" }

      where
        loc :: CtLoc
    loc        = CtEvidence -> CtLoc
    ctEvLoc CtEvidence
    ev
        role :: Role
    role       = EqRel -> Role
    eqRelRole EqRel
    eq_rel

          -- infinite, as tyConRolesX returns an infinite tail of Nominal
        tc_roles :: [Role]
    tc_roles   = Role -> TyCon -> [Role]
    tyConRolesX Role
    role TyCon
    tc

          -- Add nuances to the location during decomposition:
          --  * if the argument is a kind argument, remember this, so that error
          --    messages say "kind", not "type". This is determined based on whether
          --    the corresponding tyConBinder is named (that is, dependent)
          --  * if the argument is invisible, note this as well, again by
          --    looking at the corresponding binder
          -- For oversaturated tycons, we need the (repeat loc) tail, which doesn't
          -- do either of these changes. (Forgetting to do so led to #16188)
          --
          -- NB: infinite in length
        new_locs :: [CtLoc]
    new_locs = [ CtLoc
    new_loc
                   | TyConBinder
    bndr <- TyCon -> [TyConBinder]
    tyConBinders TyCon
    tc
                   , let new_loc0 :: CtLoc
    new_loc0 | TyConBinder -> Bool
    isNamedTyConBinder TyConBinder
    bndr = CtLoc -> CtLoc
    toKindLoc CtLoc
    loc
                                  | Bool
    otherwise               = CtLoc
    loc
                         new_loc :: CtLoc
    new_loc  | forall tv. VarBndr tv TyConBndrVis -> Bool
    isInvisibleTyConBinder TyConBinder
    bndr
                                  = CtLoc -> (CtOrigin -> CtOrigin) -> CtLoc
    updateCtLocOrigin CtLoc
    new_loc0 CtOrigin -> CtOrigin
    toInvisibleOrigin
                                  | Bool
    otherwise
                                  = CtLoc
    new_loc0 ]
                   forall a. [a] -> [a] -> [a]
    ++ forall a. a -> [a]
    repeat CtLoc
    loc

    -- | Call when canonicalizing an equality fails, but if the equality is
    -- representational, there is some hope for the future.
    -- Examples in Note [Use canEqFailure in canDecomposableTyConApp]
    canEqFailure :: CtEvidence -> EqRel
                 -> TcType -> TcType -> TcS (StopOrContinue Ct)
    canEqFailure :: CtEvidence -> EqRel -> TcType -> TcType -> TcS (StopOrContinue Ct)
    canEqFailure CtEvidence
    ev EqRel
    NomEq TcType
    ty1 TcType
    ty2
      = CtEvidence -> TcType -> TcType -> TcS (StopOrContinue Ct)
    canEqHardFailure CtEvidence
    ev TcType
    ty1 TcType
    ty2
    canEqFailure CtEvidence
    ev EqRel
    ReprEq TcType
    ty1 TcType
    ty2
      = do { (TcType
    xi1, TcCoercion
    co1) <- CtEvidence -> TcType -> TcS (TcType, TcCoercion)
    rewrite CtEvidence
    ev TcType
    ty1
           ; (TcType
    xi2, TcCoercion
    co2) <- CtEvidence -> TcType -> TcS (TcType, TcCoercion)
    rewrite CtEvidence
    ev TcType
    ty2
                -- We must rewrite the types before putting them in the
                -- inert set, so that we are sure to kick them out when
                -- new equalities become available
           ; String -> SDoc -> TcS ()
    traceTcS String
    "canEqFailure with ReprEq" forall a b. (a -> b) -> a -> b
    $
             [SDoc] -> SDoc
    vcat [ forall a. Outputable a => a -> SDoc
    ppr CtEvidence
    ev, forall a. Outputable a => a -> SDoc
    ppr TcType
    ty1, forall a. Outputable a => a -> SDoc
    ppr TcType
    ty2, forall a. Outputable a => a -> SDoc
    ppr TcType
    xi1, forall a. Outputable a => a -> SDoc
    ppr TcType
    xi2 ]
           ; CtEvidence
    new_ev <- CtEvidence
    -> SwapFlag
    -> TcType
    -> TcType
    -> TcCoercion
    -> TcCoercion
    -> TcS CtEvidence
    rewriteEqEvidence CtEvidence
    ev SwapFlag
    NotSwapped TcType
    xi1 TcType
    xi2 TcCoercion
    co1 TcCoercion
    co2
           ; forall a. a -> TcS (StopOrContinue a)
    continueWith (CtIrredReason -> CtEvidence -> Ct
    mkIrredCt CtIrredReason
    ReprEqReason CtEvidence
    new_ev) }

    -- | Call when canonicalizing an equality fails with utterly no hope.
    canEqHardFailure :: CtEvidence
                     -> TcType -> TcType -> TcS (StopOrContinue Ct)
    -- See Note [Make sure that insolubles are fully rewritten]
    canEqHardFailure :: CtEvidence -> TcType -> TcType -> TcS (StopOrContinue Ct)
    canEqHardFailure CtEvidence
    ev TcType
    ty1 TcType
    ty2
      = do { String -> SDoc -> TcS ()
    traceTcS String
    "canEqHardFailure" (forall a. Outputable a => a -> SDoc
    ppr TcType
    ty1 SDoc -> SDoc -> SDoc
    $$ forall a. Outputable a => a -> SDoc
    ppr TcType
    ty2)
           ; (TcType
    s1, TcCoercion
    co1) <- CtEvidence -> TcType -> TcS (TcType, TcCoercion)
    rewrite CtEvidence
    ev TcType
    ty1
           ; (TcType
    s2, TcCoercion
    co2) <- CtEvidence -> TcType -> TcS (TcType, TcCoercion)
    rewrite CtEvidence
    ev TcType
    ty2
           ; CtEvidence
    new_ev <- CtEvidence
    -> SwapFlag
    -> TcType
    -> TcType
    -> TcCoercion
    -> TcCoercion
    -> TcS CtEvidence
    rewriteEqEvidence CtEvidence
    ev SwapFlag
    NotSwapped TcType
    s1 TcType
    s2 TcCoercion
    co1 TcCoercion
    co2
           ; forall a. a -> TcS (StopOrContinue a)
    continueWith (CtIrredReason -> CtEvidence -> Ct
    mkIrredCt CtIrredReason
    ShapeMismatchReason CtEvidence
    new_ev) }

    {-
    Note [Decomposing TyConApps]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If we see (T s1 t1 ~ T s2 t2), then we can just decompose to
      (s1 ~ s2, t1 ~ t2)
    and push those back into the work list.  But if
      s1 = K k1    s2 = K k2
    then we will just decomopose s1~s2, and it might be better to
    do so on the spot.  An important special case is where s1=s2,
    and we get just Refl.

    So canDecomposableTyCon is a fast-path decomposition that uses
    unifyWanted etc to short-cut that work.

    Note [Canonicalising type applications]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Given (s1 t1) ~ ty2, how should we proceed?
    The simple thing is to see if ty2 is of form (s2 t2), and
    decompose.

    However, over-eager decomposition gives bad error messages
    for things like
       a b ~ Maybe c
       e f ~ p -> q
    Suppose (in the first example) we already know a~Array.  Then if we
    decompose the application eagerly, yielding
       a ~ Maybe
       b ~ c
    we get an error        "Can't match Array ~ Maybe",
    but we'd prefer to get "Can't match Array b ~ Maybe c".

    So instead can_eq_wanted_app rewrites the LHS and RHS, in the hope of
    replacing (a b) by (Array b), before using try_decompose_app to
    decompose it.

    Note [Make sure that insolubles are fully rewritten]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When an equality fails, we still want to rewrite the equality
    all the way down, so that it accurately reflects
     (a) the mutable reference substitution in force at start of solving
     (b) any ty-binds in force at this point in solving
    See Note [Rewrite insolubles] in GHC.Tc.Solver.Monad.
    And if we don't do this there is a bad danger that
    GHC.Tc.Solver.applyTyVarDefaulting will find a variable
    that has in fact been substituted.

    Note [Do not decompose Given polytype equalities]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider [G] (forall a. t1 ~ forall a. t2).  Can we decompose this?
    No -- what would the evidence look like?  So instead we simply discard
    this given evidence.

    Note [Combining insoluble constraints]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    As this point we have an insoluble constraint, like Int~Bool.

     * If it is Wanted, delete it from the cache, so that subsequent
       Int~Bool constraints give rise to separate error messages

     * But if it is Derived, DO NOT delete from cache.  A class constraint
       may get kicked out of the inert set, and then have its functional
       dependency Derived constraints generated a second time. In that
       case we don't want to get two (or more) error messages by
       generating two (or more) insoluble fundep constraints from the same
       class constraint.

    Note [No top-level newtypes on RHS of representational equalities]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Suppose we're in this situation:

     work item:  [W] c1 : a ~R b
         inert:  [G] c2 : b ~R Id a

    where
      newtype Id a = Id a

    We want to make sure canEqCanLHS sees [W] a ~R a, after b is rewritten
    and the Id newtype is unwrapped. This is assured by requiring only rewritten
    types in canEqCanLHS *and* having the newtype-unwrapping check above
    the tyvar check in can_eq_nc.

    Note [Put touchable variables on the left]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Ticket #10009, a very nasty example:

        f :: (UnF (F b) ~ b) => F b -> ()

        g :: forall a. (UnF (F a) ~ a) => a -> ()
        g _ = f (undefined :: F a)

    For g we get [G]  g1 : UnF (F a) ~ a
                 [WD] w1 : UnF (F beta) ~ beta
                 [WD] w2 : F a ~ F beta

    g1 is canonical (CEqCan). It is oriented as above because a is not touchable.
    See canEqTyVarFunEq.

    w1 is similarly canonical, though the occurs-check in canEqTyVarFunEq is key
    here.

    w2 is canonical. But which way should it be oriented? As written, we'll be
    stuck. When w2 is added to the inert set, nothing gets kicked out: g1 is
    a Given (and Wanteds don't rewrite Givens), and w2 doesn't mention the LHS
    of w2. We'll thus lose.

    But if w2 is swapped around, to

        [D] w3 : F beta ~ F a

    then (after emitting shadow Deriveds, etc. See GHC.Tc.Solver.Monad
    Note [The improvement story and derived shadows]) we'll kick w1 out of the inert
    set (it mentions the LHS of w3). We then rewrite w1 to

        [D] w4 : UnF (F a) ~ beta

    and then, using g1, to

        [D] w5 : a ~ beta

    at which point we can unify and go on to glory. (This rewriting actually
    happens all at once, in the call to rewrite during canonicalisation.)

    But what about the new LHS makes it better? It mentions a variable (beta)
    that can appear in a Wanted -- a touchable metavariable never appears
    in a Given. On the other hand, the original LHS mentioned only variables
    that appear in Givens. We thus choose to put variables that can appear
    in Wanteds on the left.

    Ticket #12526 is another good example of this in action.

    -}

    ---------------------
    canEqCanLHS :: CtEvidence          -- ev :: lhs ~ rhs
                -> EqRel -> SwapFlag
                -> CanEqLHS              -- lhs (or, if swapped, rhs)
                -> TcType                -- lhs: pretty lhs, already rewritten
                -> TcType -> TcType      -- rhs: already rewritten
                -> TcS (StopOrContinue Ct)
    canEqCanLHS :: CtEvidence
    -> EqRel
    -> SwapFlag
    -> CanEqLHS
    -> TcType
    -> TcType
    -> TcType
    -> TcS (StopOrContinue Ct)
    canEqCanLHS CtEvidence
    ev EqRel
    eq_rel SwapFlag
    swapped CanEqLHS
    lhs1 TcType
    ps_xi1 TcType
    xi2 TcType
    ps_xi2
      | TcType
    k1 HasDebugCallStack => TcType -> TcType -> Bool
    `tcEqType` TcType
    k2
      = CtEvidence
    -> EqRel
    -> SwapFlag
    -> CanEqLHS
    -> TcType
    -> TcType
    -> TcType
    -> TcS (StopOrContinue Ct)
    canEqCanLHSHomo CtEvidence
    ev EqRel
    eq_rel SwapFlag
    swapped CanEqLHS
    lhs1 TcType
    ps_xi1 TcType
    xi2 TcType
    ps_xi2

      | Bool
    otherwise
      = CtEvidence
    -> EqRel
    -> SwapFlag
    -> CanEqLHS
    -> TcType
    -> TcType
    -> TcType
    -> TcType
    -> TcType
    -> TcS (StopOrContinue Ct)
    canEqCanLHSHetero CtEvidence
    ev EqRel
    eq_rel SwapFlag
    swapped CanEqLHS
    lhs1 TcType
    ps_xi1 TcType
    k1 TcType
    xi2 TcType
    ps_xi2 TcType
    k2

      where
        k1 :: TcType
    k1 = CanEqLHS -> TcType
    canEqLHSKind CanEqLHS
    lhs1
        k2 :: TcType
    k2 = HasDebugCallStack => TcType -> TcType
    tcTypeKind TcType
    xi2

    canEqCanLHSHetero :: CtEvidence         -- :: (xi1 :: ki1) ~ (xi2 :: ki2)
                      -> EqRel -> SwapFlag
                      -> CanEqLHS -> TcType -- xi1, pretty xi1
                      -> TcKind             -- ki1
                      -> TcType -> TcType   -- xi2, pretty xi2 :: ki2
                      -> TcKind             -- ki2
                      -> TcS (StopOrContinue Ct)
    canEqCanLHSHetero :: CtEvidence
    -> EqRel
    -> SwapFlag
    -> CanEqLHS
    -> TcType
    -> TcType
    -> TcType
    -> TcType
    -> TcType
    -> TcS (StopOrContinue Ct)
    canEqCanLHSHetero CtEvidence
    ev EqRel
    eq_rel SwapFlag
    swapped CanEqLHS
    lhs1 TcType
    ps_xi1 TcType
    ki1 TcType
    xi2 TcType
    ps_xi2 TcType
    ki2
      -- See Note [Equalities with incompatible kinds]
      = do { TcCoercion
    kind_co <- TcS TcCoercion
    emit_kind_co   -- :: ki2 ~N ki1

           ; let  -- kind_co :: (ki2 :: *) ~N (ki1 :: *)   (whether swapped or not)
                  -- co1     :: kind(tv1) ~N ki1
                 rhs' :: TcType
    rhs'    = TcType
    xi2    TcType -> TcCoercion -> TcType
    `mkCastTy` TcCoercion
    kind_co   -- :: ki1
                 ps_rhs' :: TcType
    ps_rhs' = TcType
    ps_xi2 TcType -> TcCoercion -> TcType
    `mkCastTy` TcCoercion
    kind_co   -- :: ki1
                 rhs_co :: TcCoercion
    rhs_co  = Role -> TcType -> TcCoercion -> TcCoercion
    mkTcGReflLeftCo Role
    role TcType
    xi2 TcCoercion
    kind_co
                   -- rhs_co :: (xi2 |> kind_co) ~ xi2

                 lhs_co :: TcCoercion
    lhs_co = Role -> TcType -> TcCoercion
    mkTcReflCo Role
    role TcType
    xi1

           ; String -> SDoc -> TcS ()
    traceTcS String
    "Hetero equality gives rise to kind equality"
               (forall a. Outputable a => a -> SDoc
    ppr TcCoercion
    kind_co SDoc -> SDoc -> SDoc
    <+> SDoc
    dcolon SDoc -> SDoc -> SDoc
    <+> [SDoc] -> SDoc
    sep [ forall a. Outputable a => a -> SDoc
    ppr TcType
    ki2, String -> SDoc
    text String
    "~#", forall a. Outputable a => a -> SDoc
    ppr TcType
    ki1 ])
           ; CtEvidence
    type_ev <- CtEvidence
    -> SwapFlag
    -> TcType
    -> TcType
    -> TcCoercion
    -> TcCoercion
    -> TcS CtEvidence
    rewriteEqEvidence CtEvidence
    ev SwapFlag
    swapped TcType
    xi1 TcType
    rhs' TcCoercion
    lhs_co TcCoercion
    rhs_co

              -- rewriteEqEvidence carries out the swap, so we're NotSwapped any more
           ; CtEvidence
    -> EqRel
    -> SwapFlag
    -> CanEqLHS
    -> TcType
    -> TcType
    -> TcType
    -> TcS (StopOrContinue Ct)
    canEqCanLHSHomo CtEvidence
    type_ev EqRel
    eq_rel SwapFlag
    NotSwapped CanEqLHS
    lhs1 TcType
    ps_xi1 TcType
    rhs' TcType
    ps_rhs' }
      where
        emit_kind_co :: TcS CoercionN
        emit_kind_co :: TcS TcCoercion
    emit_kind_co
          | CtGiven { ctev_evar :: CtEvidence -> TcTyVar
    ctev_evar = TcTyVar
    evar } <- CtEvidence
    ev
          = do { let kind_co :: TcCoercion
    kind_co = TcCoercion -> TcCoercion
    maybe_sym forall a b. (a -> b) -> a -> b
    $ TcCoercion -> TcCoercion
    mkTcKindCo (TcTyVar -> TcCoercion
    mkTcCoVarCo TcTyVar
    evar)  -- :: k2 ~ k1
               ; CtEvidence
    kind_ev <- CtLoc -> (TcType, EvTerm) -> TcS CtEvidence
    newGivenEvVar CtLoc
    kind_loc (TcType
    kind_pty, TcCoercion -> EvTerm
    evCoercion TcCoercion
    kind_co)
               ; [CtEvidence] -> TcS ()
    emitWorkNC [CtEvidence
    kind_ev]
               ; forall (m :: * -> *) a. Monad m => a -> m a
    return (HasDebugCallStack => CtEvidence -> TcCoercion
    ctEvCoercion CtEvidence
    kind_ev) }

          | Bool
    otherwise
          = CtLoc -> Role -> TcType -> TcType -> TcS TcCoercion
    unifyWanted CtLoc
    kind_loc Role
    Nominal TcType
    ki2 TcType
    ki1

        xi1 :: TcType
    xi1      = CanEqLHS -> TcType
    canEqLHSType CanEqLHS
    lhs1
        loc :: CtLoc
    loc      = CtEvidence -> CtLoc
    ctev_loc CtEvidence
    ev
        role :: Role
    role     = EqRel -> Role
    eqRelRole EqRel
    eq_rel
        kind_loc :: CtLoc
    kind_loc = TcType -> TcType -> CtLoc -> CtLoc
    mkKindLoc TcType
    xi1 TcType
    xi2 CtLoc
    loc
        kind_pty :: TcType
    kind_pty = TcType -> TcType -> TcType -> TcType -> TcType
    mkHeteroPrimEqPred TcType
    liftedTypeKind TcType
    liftedTypeKind TcType
    ki2 TcType
    ki1

        maybe_sym :: TcCoercion -> TcCoercion
    maybe_sym = case SwapFlag
    swapped of
              SwapFlag
    IsSwapped  -> forall a. a -> a
    id         -- if the input is swapped, then we already
                                       -- will have k2 ~ k1
              SwapFlag
    NotSwapped -> TcCoercion -> TcCoercion
    mkTcSymCo

    -- guaranteed that tcTypeKind lhs == tcTypeKind rhs
    canEqCanLHSHomo :: CtEvidence
                    -> EqRel -> SwapFlag
                    -> CanEqLHS           -- lhs (or, if swapped, rhs)
                    -> TcType             -- pretty lhs
                    -> TcType -> TcType   -- rhs, pretty rhs
                    -> TcS (StopOrContinue Ct)
    canEqCanLHSHomo :: CtEvidence
    -> EqRel
    -> SwapFlag
    -> CanEqLHS
    -> TcType
    -> TcType
    -> TcType
    -> TcS (StopOrContinue Ct)
    canEqCanLHSHomo CtEvidence
    ev EqRel
    eq_rel SwapFlag
    swapped CanEqLHS
    lhs1 TcType
    ps_xi1 TcType
    xi2 TcType
    ps_xi2
      | (TcType
    xi2', MCoercion
    mco) <- TcType -> (TcType, MCoercion)
    split_cast_ty TcType
    xi2
      , Just CanEqLHS
    lhs2 <- TcType -> Maybe CanEqLHS
    canEqLHS_maybe TcType
    xi2'
      = CtEvidence
    -> EqRel
    -> SwapFlag
    -> CanEqLHS
    -> TcType
    -> CanEqLHS
    -> TcType
    -> MCoercion
    -> TcS (StopOrContinue Ct)
    canEqCanLHS2 CtEvidence
    ev EqRel
    eq_rel SwapFlag
    swapped CanEqLHS
    lhs1 TcType
    ps_xi1 CanEqLHS
    lhs2 (TcType
    ps_xi2 TcType -> MCoercion -> TcType
    `mkCastTyMCo` MCoercion -> MCoercion
    mkTcSymMCo MCoercion
    mco) MCoercion
    mco

      | Bool
    otherwise
      = CtEvidence
    -> EqRel
    -> SwapFlag
    -> CanEqLHS
    -> TcType
    -> TcS (StopOrContinue Ct)
    canEqCanLHSFinish CtEvidence
    ev EqRel
    eq_rel SwapFlag
    swapped CanEqLHS
    lhs1 TcType
    ps_xi2

      where
        split_cast_ty :: TcType -> (TcType, MCoercion)
    split_cast_ty (CastTy TcType
    ty TcCoercion
    co) = (TcType
    ty, TcCoercion -> MCoercion
    MCo TcCoercion
    co)
        split_cast_ty TcType
    other          = (TcType
    other, MCoercion
    MRefl)

    -- This function deals with the case that both LHS and RHS are potential
    -- CanEqLHSs.
    canEqCanLHS2 :: CtEvidence              -- lhs ~ (rhs |> mco)
                                            -- or, if swapped: (rhs |> mco) ~ lhs
                 -> EqRel -> SwapFlag
                 -> CanEqLHS                -- lhs (or, if swapped, rhs)
                 -> TcType                  -- pretty lhs
                 -> CanEqLHS                -- rhs
                 -> TcType                  -- pretty rhs
                 -> MCoercion               -- :: kind(rhs) ~N kind(lhs)
                 -> TcS (StopOrContinue Ct)
    canEqCanLHS2 :: CtEvidence
    -> EqRel
    -> SwapFlag
    -> CanEqLHS
    -> TcType
    -> CanEqLHS
    -> TcType
    -> MCoercion
    -> TcS (StopOrContinue Ct)
    canEqCanLHS2 CtEvidence
    ev EqRel
    eq_rel SwapFlag
    swapped CanEqLHS
    lhs1 TcType
    ps_xi1 CanEqLHS
    lhs2 TcType
    ps_xi2 MCoercion
    mco
      | CanEqLHS
    lhs1 CanEqLHS -> CanEqLHS -> Bool
    `eqCanEqLHS` CanEqLHS
    lhs2
        -- It must be the case that mco is reflexive
      = CtEvidence -> EqRel -> TcType -> TcS (StopOrContinue Ct)
    canEqReflexive CtEvidence
    ev EqRel
    eq_rel (CanEqLHS -> TcType
    canEqLHSType CanEqLHS
    lhs1)

      | TyVarLHS TcTyVar
    tv1 <- CanEqLHS
    lhs1
      , TyVarLHS TcTyVar
    tv2 <- CanEqLHS
    lhs2
      , Bool -> TcTyVar -> TcTyVar -> Bool
    swapOverTyVars (CtEvidence -> Bool
    isGiven CtEvidence
    ev) TcTyVar
    tv1 TcTyVar
    tv2
      = do { String -> SDoc -> TcS ()
    traceTcS String
    "canEqLHS2 swapOver" (forall a. Outputable a => a -> SDoc
    ppr TcTyVar
    tv1 SDoc -> SDoc -> SDoc
    $$ forall a. Outputable a => a -> SDoc
    ppr TcTyVar
    tv2 SDoc -> SDoc -> SDoc
    $$ forall a. Outputable a => a -> SDoc
    ppr SwapFlag
    swapped)
           ; CtEvidence
    new_ev <- TcS CtEvidence
    do_swap
           ; CtEvidence
    -> EqRel
    -> SwapFlag
    -> CanEqLHS
    -> TcType
    -> TcS (StopOrContinue Ct)
    canEqCanLHSFinish CtEvidence
    new_ev EqRel
    eq_rel SwapFlag
    IsSwapped (TcTyVar -> CanEqLHS
    TyVarLHS TcTyVar
    tv2)
                                                       (TcType
    ps_xi1 TcType -> MCoercion -> TcType
    `mkCastTyMCo` MCoercion
    sym_mco) }

      | TyVarLHS TcTyVar
    tv1 <- CanEqLHS
    lhs1
      , TyFamLHS TyCon
    fun_tc2 [TcType]
    fun_args2 <- CanEqLHS
    lhs2
      = CtEvidence
    -> EqRel
    -> SwapFlag
    -> TcTyVar
    -> TcType
    -> TyCon
    -> [TcType]
    -> TcType
    -> MCoercion
    -> TcS (StopOrContinue Ct)
    canEqTyVarFunEq CtEvidence
    ev EqRel
    eq_rel SwapFlag
    swapped TcTyVar
    tv1 TcType
    ps_xi1 TyCon
    fun_tc2 [TcType]
    fun_args2 TcType
    ps_xi2 MCoercion
    mco

      | TyFamLHS TyCon
    fun_tc1 [TcType]
    fun_args1 <- CanEqLHS
    lhs1
      , TyVarLHS TcTyVar
    tv2 <- CanEqLHS
    lhs2
      = do { CtEvidence
    new_ev <- TcS CtEvidence
    do_swap
           ; CtEvidence
    -> EqRel
    -> SwapFlag
    -> TcTyVar
    -> TcType
    -> TyCon
    -> [TcType]
    -> TcType
    -> MCoercion
    -> TcS (StopOrContinue Ct)
    canEqTyVarFunEq CtEvidence
    new_ev EqRel
    eq_rel SwapFlag
    IsSwapped TcTyVar
    tv2 TcType
    ps_xi2
                                                     TyCon
    fun_tc1 [TcType]
    fun_args1 TcType
    ps_xi1 MCoercion
    sym_mco }

      | TyFamLHS TyCon
    fun_tc1 [TcType]
    fun_args1 <- CanEqLHS
    lhs1
      , TyFamLHS TyCon
    fun_tc2 [TcType]
    fun_args2 <- CanEqLHS
    lhs2
      = do { String -> SDoc -> TcS ()
    traceTcS String
    "canEqCanLHS2 two type families" (forall a. Outputable a => a -> SDoc
    ppr CanEqLHS
    lhs1 SDoc -> SDoc -> SDoc
    $$ forall a. Outputable a => a -> SDoc
    ppr CanEqLHS
    lhs2)

             -- emit derived equalities for injective type families
           ; let inj_eqns :: [TypeEqn]  -- TypeEqn = Pair Type
                 inj_eqns :: [Pair TcType]
    inj_eqns
                   | EqRel
    ReprEq <- EqRel
    eq_rel   = []   -- injectivity applies only for nom. eqs.
                   | TyCon
    fun_tc1 forall a. Eq a => a -> a -> Bool
    /= TyCon
    fun_tc2 = []   -- if the families don't match, stop.

                   | Injective [Bool]
    inj <- TyCon -> Injectivity
    tyConInjectivityInfo TyCon
    fun_tc1
                   = [ forall a. a -> a -> Pair a
    Pair TcType
    arg1 TcType
    arg2
                     | (TcType
    arg1, TcType
    arg2, Bool
    True) <- forall a b c. [a] -> [b] -> [c] -> [(a, b, c)]
    zip3 [TcType]
    fun_args1 [TcType]
    fun_args2 [Bool]
    inj ]

                     -- built-in synonym families don't have an entry point
                     -- for this use case. So, we just use sfInteractInert
                     -- and pass two equal RHSs. We *could* add another entry
                     -- point, but then there would be a burden to make
                     -- sure the new entry point and existing ones were
                     -- internally consistent. This is slightly distasteful,
                     -- but it works well in practice and localises the
                     -- problem.
                   | Just BuiltInSynFamily
    ops <- TyCon -> Maybe BuiltInSynFamily
    isBuiltInSynFamTyCon_maybe TyCon
    fun_tc1
                   = let ki1 :: TcType
    ki1 = CanEqLHS -> TcType
    canEqLHSKind CanEqLHS
    lhs1
                         ki2 :: TcType
    ki2 | MCoercion
    MRefl <- MCoercion
    mco
                             = TcType
    ki1   -- just a small optimisation
                             | Bool
    otherwise
                             = CanEqLHS -> TcType
    canEqLHSKind CanEqLHS
    lhs2

                         fake_rhs1 :: TcType
    fake_rhs1 = TcType -> TcType
    anyTypeOfKind TcType
    ki1
                         fake_rhs2 :: TcType
    fake_rhs2 = TcType -> TcType
    anyTypeOfKind TcType
    ki2
                     in
                     BuiltInSynFamily
    -> [TcType] -> TcType -> [TcType] -> TcType -> [Pair TcType]
    sfInteractInert BuiltInSynFamily
    ops [TcType]
    fun_args1 TcType
    fake_rhs1 [TcType]
    fun_args2 TcType
    fake_rhs2

                   | Bool
    otherwise  -- ordinary, non-injective type family
                   = []

           ; forall (f :: * -> *). Applicative f => Bool -> f () -> f ()
    unless (CtEvidence -> Bool
    isGiven CtEvidence
    ev) forall a b. (a -> b) -> a -> b
    $
             forall (t :: * -> *) (m :: * -> *) a b.
    (Foldable t, Monad m) =>
    (a -> m b) -> t a -> m ()
    mapM_ (CtLoc -> Role -> Pair TcType -> TcS ()
    unifyDerived (CtEvidence -> CtLoc
    ctEvLoc CtEvidence
    ev) Role
    Nominal) [Pair TcType]
    inj_eqns

           ; TcLevel
    tclvl <- TcS TcLevel
    getTcLevel
           ; DynFlags
    dflags <- forall (m :: * -> *). HasDynFlags m => m DynFlags
    getDynFlags
           ; let tvs1 :: VarSet
    tvs1 = [TcType] -> VarSet
    tyCoVarsOfTypes [TcType]
    fun_args1
                 tvs2 :: VarSet
    tvs2 = [TcType] -> VarSet
    tyCoVarsOfTypes [TcType]
    fun_args2

                 swap_for_rewriting :: Bool
    swap_for_rewriting = (TcTyVar -> Bool) -> VarSet -> Bool
    anyVarSet (TcLevel -> TcTyVar -> Bool
    isTouchableMetaTyVar TcLevel
    tclvl) VarSet
    tvs2 Bool -> Bool -> Bool
    &&
                              -- swap 'em: Note [Put touchable variables on the left]
                                      Bool -> Bool
    not ((TcTyVar -> Bool) -> VarSet -> Bool
    anyVarSet (TcLevel -> TcTyVar -> Bool
    isTouchableMetaTyVar TcLevel
    tclvl) VarSet
    tvs1)
                              -- this check is just to avoid unfruitful swapping

                   -- If we have F a ~ F (F a), we want to swap.
                 swap_for_occurs :: Bool
    swap_for_occurs
                   | CheckTyEqResult -> Bool
    cterHasNoProblem   forall a b. (a -> b) -> a -> b
    $ DynFlags -> TyCon -> [TcType] -> TcType -> CheckTyEqResult
    checkTyFamEq DynFlags
    dflags TyCon
    fun_tc2 [TcType]
    fun_args2
                                                       (TyCon -> [TcType] -> TcType
    mkTyConApp TyCon
    fun_tc1 [TcType]
    fun_args1)
                   , CheckTyEqResult -> Bool
    cterHasOccursCheck forall a b. (a -> b) -> a -> b
    $ DynFlags -> TyCon -> [TcType] -> TcType -> CheckTyEqResult
    checkTyFamEq DynFlags
    dflags TyCon
    fun_tc1 [TcType]
    fun_args1
                                                       (TyCon -> [TcType] -> TcType
    mkTyConApp TyCon
    fun_tc2 [TcType]
    fun_args2)
                   = Bool
    True

                   | Bool
    otherwise
                   = Bool
    False

           ; if Bool
    swap_for_rewriting Bool -> Bool -> Bool
    || Bool
    swap_for_occurs
             then do { CtEvidence
    new_ev <- TcS CtEvidence
    do_swap
                     ; CtEvidence
    -> EqRel
    -> SwapFlag
    -> CanEqLHS
    -> TcType
    -> TcS (StopOrContinue Ct)
    canEqCanLHSFinish CtEvidence
    new_ev EqRel
    eq_rel SwapFlag
    IsSwapped CanEqLHS
    lhs2 (TcType
    ps_xi1 TcType -> MCoercion -> TcType
    `mkCastTyMCo` MCoercion
    sym_mco) }
             else TcS (StopOrContinue Ct)
    finish_without_swapping }

      -- that's all the special cases. Now we just figure out which non-special case
      -- to continue to.
      | Bool
    otherwise
      = TcS (StopOrContinue Ct)
    finish_without_swapping

      where
        sym_mco :: MCoercion
    sym_mco = MCoercion -> MCoercion
    mkTcSymMCo MCoercion
    mco

        do_swap :: TcS CtEvidence
    do_swap = CtEvidence
    -> EqRel
    -> SwapFlag
    -> TcType
    -> TcType
    -> MCoercion
    -> TcS CtEvidence
    rewriteCastedEquality CtEvidence
    ev EqRel
    eq_rel SwapFlag
    swapped (CanEqLHS -> TcType
    canEqLHSType CanEqLHS
    lhs1) (CanEqLHS -> TcType
    canEqLHSType CanEqLHS
    lhs2) MCoercion
    mco
        finish_without_swapping :: TcS (StopOrContinue Ct)
    finish_without_swapping = CtEvidence
    -> EqRel
    -> SwapFlag
    -> CanEqLHS
    -> TcType
    -> TcS (StopOrContinue Ct)
    canEqCanLHSFinish CtEvidence
    ev EqRel
    eq_rel SwapFlag
    swapped CanEqLHS
    lhs1 (TcType
    ps_xi2 TcType -> MCoercion -> TcType
    `mkCastTyMCo` MCoercion
    mco)

    -- This function handles the case where one side is a tyvar and the other is
    -- a type family application. Which to put on the left?
    --   If the tyvar is a touchable meta-tyvar, put it on the left, as this may
    --   be our only shot to unify.
    --   Otherwise, put the function on the left, because it's generally better to
    --   rewrite away function calls. This makes types smaller. And it seems necessary:
    --     [W] F alpha ~ alpha
    --     [W] F alpha ~ beta
    --     [W] G alpha beta ~ Int   ( where we have type instance G a a = a )
    --   If we end up with a stuck alpha ~ F alpha, we won't be able to solve this.
    --   Test case: indexed-types/should_compile/CEqCanOccursCheck
    canEqTyVarFunEq :: CtEvidence               -- :: lhs ~ (rhs |> mco)
                                                -- or (rhs |> mco) ~ lhs if swapped
                    -> EqRel -> SwapFlag
                    -> TyVar -> TcType          -- lhs (or if swapped rhs), pretty lhs
                    -> TyCon -> [Xi] -> TcType  -- rhs (or if swapped lhs) fun and args, pretty rhs
                    -> MCoercion                -- :: kind(rhs) ~N kind(lhs)
                    -> TcS (StopOrContinue Ct)
    canEqTyVarFunEq :: CtEvidence
    -> EqRel
    -> SwapFlag
    -> TcTyVar
    -> TcType
    -> TyCon
    -> [TcType]
    -> TcType
    -> MCoercion
    -> TcS (StopOrContinue Ct)
    canEqTyVarFunEq CtEvidence
    ev EqRel
    eq_rel SwapFlag
    swapped TcTyVar
    tv1 TcType
    ps_xi1 TyCon
    fun_tc2 [TcType]
    fun_args2 TcType
    ps_xi2 MCoercion
    mco
      = do { TouchabilityTestResult
    is_touchable <- CtFlavour -> TcTyVar -> TcType -> TcS TouchabilityTestResult
    touchabilityTest (CtEvidence -> CtFlavour
    ctEvFlavour CtEvidence
    ev) TcTyVar
    tv1 TcType
    rhs
           ; DynFlags
    dflags       <- forall (m :: * -> *). HasDynFlags m => m DynFlags
    getDynFlags
           ; if | case TouchabilityTestResult
    is_touchable of { TouchabilityTestResult
    Untouchable -> Bool
    False; TouchabilityTestResult
    _ -> Bool
    True }
                , CheckTyEqResult -> Bool
    cterHasNoProblem forall a b. (a -> b) -> a -> b
    $
                    DynFlags -> TcTyVar -> TcType -> CheckTyEqResult
    checkTyVarEq DynFlags
    dflags TcTyVar
    tv1 TcType
    rhs CheckTyEqResult -> CheckTyEqProblem -> CheckTyEqResult
    `cterRemoveProblem` CheckTyEqProblem
    cteTypeFamily
                -> CtEvidence
    -> EqRel
    -> SwapFlag
    -> CanEqLHS
    -> TcType
    -> TcS (StopOrContinue Ct)
    canEqCanLHSFinish CtEvidence
    ev EqRel
    eq_rel SwapFlag
    swapped (TcTyVar -> CanEqLHS
    TyVarLHS TcTyVar
    tv1) TcType
    rhs

                | Bool
    otherwise
                  -> do { CtEvidence
    new_ev <- CtEvidence
    -> EqRel
    -> SwapFlag
    -> TcType
    -> TcType
    -> MCoercion
    -> TcS CtEvidence
    rewriteCastedEquality CtEvidence
    ev EqRel
    eq_rel SwapFlag
    swapped
                                      (TcTyVar -> TcType
    mkTyVarTy TcTyVar
    tv1) (TyCon -> [TcType] -> TcType
    mkTyConApp TyCon
    fun_tc2 [TcType]
    fun_args2)
                                      MCoercion
    mco
                        ; CtEvidence
    -> EqRel
    -> SwapFlag
    -> CanEqLHS
    -> TcType
    -> TcS (StopOrContinue Ct)
    canEqCanLHSFinish CtEvidence
    new_ev EqRel
    eq_rel SwapFlag
    IsSwapped
                                      (TyCon -> [TcType] -> CanEqLHS
    TyFamLHS TyCon
    fun_tc2 [TcType]
    fun_args2)
                                      (TcType
    ps_xi1 TcType -> MCoercion -> TcType
    `mkCastTyMCo` MCoercion
    sym_mco) } }
      where
        sym_mco :: MCoercion
    sym_mco = MCoercion -> MCoercion
    mkTcSymMCo MCoercion
    mco
        rhs :: TcType
    rhs = TcType
    ps_xi2 TcType -> MCoercion -> TcType
    `mkCastTyMCo` MCoercion
    mco

    -- The RHS here is either not CanEqLHS, or it's one that we
    -- want to rewrite the LHS to (as per e.g. swapOverTyVars)
    canEqCanLHSFinish :: CtEvidence
                      -> EqRel -> SwapFlag
                      -> CanEqLHS              -- lhs (or, if swapped, rhs)
                      -> TcType          -- rhs, pretty rhs
                      -> TcS (StopOrContinue Ct)
    canEqCanLHSFinish :: CtEvidence
    -> EqRel
    -> SwapFlag
    -> CanEqLHS
    -> TcType
    -> TcS (StopOrContinue Ct)
    canEqCanLHSFinish CtEvidence
    ev EqRel
    eq_rel SwapFlag
    swapped CanEqLHS
    lhs TcType
    rhs
    -- RHS is fully rewritten, but with type synonyms
    -- preserved as much as possible
    -- guaranteed that tyVarKind lhs == typeKind rhs, for (TyEq:K)
    -- (TyEq:N) is checked in can_eq_nc', and (TyEq:TV) is handled in canEqCanLHS2

      = do { DynFlags
    dflags <- forall (m :: * -> *). HasDynFlags m => m DynFlags
    getDynFlags
           ; CtEvidence
    new_ev <- CtEvidence
    -> SwapFlag
    -> TcType
    -> TcType
    -> TcCoercion
    -> TcCoercion
    -> TcS CtEvidence
    rewriteEqEvidence CtEvidence
    ev SwapFlag
    swapped TcType
    lhs_ty TcType
    rhs TcCoercion
    rewrite_co1 TcCoercion
    rewrite_co2

         -- by now, (TyEq:K) is already satisfied
           ; MASSERT(canEqLHSKind lhs `eqType` tcTypeKind rhs)

         -- by now, (TyEq:N) is already satisfied (if applicable)
           ; MASSERT(not bad_newtype)

         -- guarantees (TyEq:OC), (TyEq:F)
         -- Must do the occurs check even on tyvar/tyvar
         -- equalities, in case have  x ~ (y :: ..x...); this is #12593.
         -- This next line checks also for coercion holes (TyEq:H); see
         -- Note [Equalities with incompatible kinds]
           ; let result0 :: CheckTyEqResult
    result0 = DynFlags -> CanEqLHS -> TcType -> CheckTyEqResult
    checkTypeEq DynFlags
    dflags CanEqLHS
    lhs TcType
    rhs CheckTyEqResult -> CheckTyEqProblem -> CheckTyEqResult
    `cterRemoveProblem` CheckTyEqProblem
    cteTypeFamily
         -- type families are OK here
         -- NB: no occCheckExpand here; see Note [Rewriting synonyms] in GHC.Tc.Solver.Rewrite

                  -- a ~R# b a is soluble if b later turns out to be Identity
                 result :: CheckTyEqResult
    result = case EqRel
    eq_rel of
                            EqRel
    NomEq  -> CheckTyEqResult
    result0
                            EqRel
    ReprEq -> CheckTyEqResult -> CheckTyEqResult
    cterSetOccursCheckSoluble CheckTyEqResult
    result0

                 reason :: CtIrredReason
    reason | CheckTyEqResult
    result CheckTyEqResult -> CheckTyEqProblem -> Bool
    `cterHasOnlyProblem` CheckTyEqProblem
    cteHoleBlocker
                        = HoleSet -> CtIrredReason
    HoleBlockerReason (TcType -> HoleSet
    coercionHolesOfType TcType
    rhs)
                        | Bool
    otherwise
                        = CheckTyEqResult -> CtIrredReason
    NonCanonicalReason CheckTyEqResult
    result

           ; if CheckTyEqResult -> Bool
    cterHasNoProblem CheckTyEqResult
    result
             then do { String -> SDoc -> TcS ()
    traceTcS String
    "CEqCan" (forall a. Outputable a => a -> SDoc
    ppr CanEqLHS
    lhs SDoc -> SDoc -> SDoc
    $$ forall a. Outputable a => a -> SDoc
    ppr TcType
    rhs)
                     ; forall a. a -> TcS (StopOrContinue a)
    continueWith (CEqCan { cc_ev :: CtEvidence
    cc_ev = CtEvidence
    new_ev, cc_lhs :: CanEqLHS
    cc_lhs = CanEqLHS
    lhs
                                            , cc_rhs :: TcType
    cc_rhs = TcType
    rhs, cc_eq_rel :: EqRel
    cc_eq_rel = EqRel
    eq_rel }) }

             else do { Maybe (TcCoercion, TcType)
    m_stuff <- CtEvidence
    -> CheckTyEqResult
    -> CanEqLHS
    -> TcType
    -> TcS (Maybe (TcCoercion, TcType))
    breakTyEqCycle_maybe CtEvidence
    ev CheckTyEqResult
    result CanEqLHS
    lhs TcType
    rhs
                               -- See Note [Type equality cycles];
                               -- returning Nothing is the vastly common case
                     ; case Maybe (TcCoercion, TcType)
    m_stuff of
                         { Maybe (TcCoercion, TcType)
    Nothing ->
                             do { String -> SDoc -> TcS ()
    traceTcS String
    "canEqCanLHSFinish can't make a canonical"
                                           (forall a. Outputable a => a -> SDoc
    ppr CanEqLHS
    lhs SDoc -> SDoc -> SDoc
    $$ forall a. Outputable a => a -> SDoc
    ppr TcType
    rhs)
                                ; forall a. a -> TcS (StopOrContinue a)
    continueWith (CtIrredReason -> CtEvidence -> Ct
    mkIrredCt CtIrredReason
    reason CtEvidence
    new_ev) }
                         ; Just (TcCoercion
    co, TcType
    new_rhs) ->
                  do { String -> SDoc -> TcS ()
    traceTcS String
    "canEqCanLHSFinish breaking a cycle" forall a b. (a -> b) -> a -> b
    $
                                forall a. Outputable a => a -> SDoc
    ppr CanEqLHS
    lhs SDoc -> SDoc -> SDoc
    $$ forall a. Outputable a => a -> SDoc
    ppr TcType
    rhs
                     ; String -> SDoc -> TcS ()
    traceTcS String
    "new RHS:" (forall a. Outputable a => a -> SDoc
    ppr TcType
    new_rhs)

                       -- This check is Detail (1) in the Note
                     ; if CheckTyEqResult -> Bool
    cterHasOccursCheck (DynFlags -> CanEqLHS -> TcType -> CheckTyEqResult
    checkTypeEq DynFlags
    dflags CanEqLHS
    lhs TcType
    new_rhs)

                       then do { String -> SDoc -> TcS ()
    traceTcS String
    "Note [Type equality cycles] Detail (1)"
                                          (forall a. Outputable a => a -> SDoc
    ppr TcType
    new_rhs)
                               ; forall a. a -> TcS (StopOrContinue a)
    continueWith (CtIrredReason -> CtEvidence -> Ct
    mkIrredCt CtIrredReason
    reason CtEvidence
    new_ev) }

                       else do { -- See Detail (6) of Note [Type equality cycles]
                                 CtEvidence
    new_new_ev <- CtEvidence
    -> SwapFlag
    -> TcType
    -> TcType
    -> TcCoercion
    -> TcCoercion
    -> TcS CtEvidence
    rewriteEqEvidence CtEvidence
    new_ev SwapFlag
    NotSwapped
                                                 TcType
    lhs_ty TcType
    new_rhs
                                                 (TcType -> TcCoercion
    mkTcNomReflCo TcType
    lhs_ty) TcCoercion
    co

                               ; forall a. a -> TcS (StopOrContinue a)
    continueWith (CEqCan { cc_ev :: CtEvidence
    cc_ev = CtEvidence
    new_new_ev
                                                      , cc_lhs :: CanEqLHS
    cc_lhs = CanEqLHS
    lhs
                                                      , cc_rhs :: TcType
    cc_rhs = TcType
    new_rhs
                                                      , cc_eq_rel :: EqRel
    cc_eq_rel = EqRel
    eq_rel }) }}}}}
      where
        role :: Role
    role = EqRel -> Role
    eqRelRole EqRel
    eq_rel

        lhs_ty :: TcType
    lhs_ty = CanEqLHS -> TcType
    canEqLHSType CanEqLHS
    lhs

        rewrite_co1 :: TcCoercion
    rewrite_co1  = Role -> TcType -> TcCoercion
    mkTcReflCo Role
    role TcType
    lhs_ty
        rewrite_co2 :: TcCoercion
    rewrite_co2  = Role -> TcType -> TcCoercion
    mkTcReflCo Role
    role TcType
    rhs

        -- This is about (TyEq:N)
        bad_newtype :: Bool
    bad_newtype | EqRel
    ReprEq <- EqRel
    eq_rel
                    , Just TyCon
    tc <- TcType -> Maybe TyCon
    tyConAppTyCon_maybe TcType
    rhs
                    = TyCon -> Bool
    isNewTyCon TyCon
    tc
                    | Bool
    otherwise
                    = Bool
    False

    -- | Solve a reflexive equality constraint
    canEqReflexive :: CtEvidence    -- ty ~ ty
                   -> EqRel
                   -> TcType        -- ty
                   -> TcS (StopOrContinue Ct)   -- always Stop
    canEqReflexive :: CtEvidence -> EqRel -> TcType -> TcS (StopOrContinue Ct)
    canEqReflexive CtEvidence
    ev EqRel
    eq_rel TcType
    ty
      = do { CtEvidence -> EvTerm -> TcS ()
    setEvBindIfWanted CtEvidence
    ev (TcCoercion -> EvTerm
    evCoercion forall a b. (a -> b) -> a -> b
    $
                                   Role -> TcType -> TcCoercion
    mkTcReflCo (EqRel -> Role
    eqRelRole EqRel
    eq_rel) TcType
    ty)
           ; forall a. CtEvidence -> String -> TcS (StopOrContinue a)
    stopWith CtEvidence
    ev String
    "Solved by reflexivity" }

    rewriteCastedEquality :: CtEvidence     -- :: lhs ~ (rhs |> mco), or (rhs |> mco) ~ lhs
                          -> EqRel -> SwapFlag
                          -> TcType         -- lhs
                          -> TcType         -- rhs
                          -> MCoercion      -- mco
                          -> TcS CtEvidence -- :: (lhs |> sym mco) ~ rhs
                                            -- result is independent of SwapFlag
    rewriteCastedEquality :: CtEvidence
    -> EqRel
    -> SwapFlag
    -> TcType
    -> TcType
    -> MCoercion
    -> TcS CtEvidence
    rewriteCastedEquality CtEvidence
    ev EqRel
    eq_rel SwapFlag
    swapped TcType
    lhs TcType
    rhs MCoercion
    mco
      = CtEvidence
    -> SwapFlag
    -> TcType
    -> TcType
    -> TcCoercion
    -> TcCoercion
    -> TcS CtEvidence
    rewriteEqEvidence CtEvidence
    ev SwapFlag
    swapped TcType
    new_lhs TcType
    new_rhs TcCoercion
    lhs_co TcCoercion
    rhs_co
      where
        new_lhs :: TcType
    new_lhs = TcType
    lhs TcType -> MCoercion -> TcType
    `mkCastTyMCo` MCoercion
    sym_mco
        lhs_co :: TcCoercion
    lhs_co  = Role -> TcType -> MCoercion -> TcCoercion
    mkTcGReflLeftMCo Role
    role TcType
    lhs MCoercion
    sym_mco

        new_rhs :: TcType
    new_rhs = TcType
    rhs
        rhs_co :: TcCoercion
    rhs_co  = Role -> TcType -> MCoercion -> TcCoercion
    mkTcGReflRightMCo Role
    role TcType
    rhs MCoercion
    mco

        sym_mco :: MCoercion
    sym_mco = MCoercion -> MCoercion
    mkTcSymMCo MCoercion
    mco
        role :: Role
    role    = EqRel -> Role
    eqRelRole EqRel
    eq_rel

    {- Note [Equalities with incompatible kinds]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    What do we do when we have an equality

      (tv :: k1) ~ (rhs :: k2)

    where k1 and k2 differ? Easy: we create a coercion that relates k1 and
    k2 and use this to cast. To wit, from

      [X] (tv :: k1) ~ (rhs :: k2)

    (where [X] is [G], [W], or [D]), we go to

      [noDerived X] co :: k2 ~ k1
      [X]           (tv :: k1) ~ ((rhs |> co) :: k1)

    where

      noDerived G = G
      noDerived _ = W

    For reasons described in Wrinkle (2) below, we want the [X] constraint to be "blocked";
    that is, it should be put aside, and not used to rewrite any other constraint,
    until the kind-equality on which it depends (namely 'co' above) is solved.
    To achieve this
    * The [X] constraint is a CIrredCan
    * With a cc_reason of HoleBlockerReason bchs
    * Where 'bchs' is the set of "blocking coercion holes".  The blocking coercion
      holes are the free coercion holes of [X]'s type
    * When all the blocking coercion holes in the CIrredCan are filled (solved),
      we convert [X] to a CNonCanonical and put it in the work list.
    All this is described in more detail in Wrinkle (2).

    Wrinkles:

     (1) The noDerived step is because Derived equalities have no evidence.
         And yet we absolutely need evidence to be able to proceed here.
         Given evidence will use the KindCo coercion; Wanted evidence will
         be a coercion hole. Even a Derived hetero equality begets a Wanted
         kind equality.

     (2) Though it would be sound to do so, we must not mark the rewritten Wanted
           [W] (tv :: k1) ~ ((rhs |> co) :: k1)
         as canonical in the inert set. In particular, we must not unify tv.
         If we did, the Wanted becomes a Given (effectively), and then can
         rewrite other Wanteds. But that's bad: See Note [Wanteds do not rewrite Wanteds]
         in GHC.Tc.Types.Constraint. The problem is about poor error messages. See #11198 for
         tales of destruction.

         So, we have an invariant on CEqCan (TyEq:H) that the RHS does not have
         any coercion holes. This is checked in checkTypeEq. Any equalities that
         have such an RHS are turned into CIrredCans with a HoleBlockerReason. We also
         must be sure to kick out any such CIrredCan constraints that mention coercion holes
         when those holes get filled in, so that the unification step can now proceed.

         The kicking out is done in kickOutAfterFillingCoercionHole, and the inerts
         are stored in the inert_blocked field of InertCans.

         However, we must be careful: we kick out only when no coercion holes are
         left. The holes in the type are stored in the HoleBlockerReason CtIrredReason.
         The extra check that there are no more remaining holes avoids
         needless work when rewriting evidence (which fills coercion holes) and
         aids efficiency.

         Moreover, kicking out when there are remaining unfilled holes can
         cause a loop in the solver in this case:
              [W] w1 :: (ty1 :: F a) ~ (ty2 :: s)
         After canonicalisation, we discover that this equality is heterogeneous.
         So we emit
              [W] co_abc :: F a ~ s
         and preserve the original as
              [W] w2 :: (ty1 |> co_abc) ~ ty2    (blocked on co_abc)
         Then, co_abc comes becomes the work item. It gets swapped in
         canEqCanLHS2 and then back again in canEqTyVarFunEq. We thus get
         co_abc := sym co_abd, and then co_abd := sym co_abe, with
              [W] co_abe :: F a ~ s
         This process has filled in co_abc. Suppose w2 were kicked out.
         When it gets processed,
         would get this whole chain going again. The solution is to
         kick out a blocked constraint only when the result of filling
         in the blocking coercion involves no further blocking coercions.
         Alternatively, we could be careful not to do unnecessary swaps during
         canonicalisation, but that seems hard to do, in general.

     (3) Suppose we have [W] (a :: k1) ~ (rhs :: k2). We duly follow the
         algorithm detailed here, producing [W] co :: k2 ~ k1, and adding
         [W] (a :: k1) ~ ((rhs |> co) :: k1) to the irreducibles. Some time
         later, we solve co, and fill in co's coercion hole. This kicks out
         the irreducible as described in (2).
         But now, during canonicalization, we see the cast
         and remove it, in canEqCast. By the time we get into canEqCanLHS, the equality
         is heterogeneous again, and the process repeats.

         To avoid this, we don't strip casts off a type if the other type
         in the equality is a CanEqLHS (the scenario above can happen with a
         type family, too. testcase: typecheck/should_compile/T13822).
         And this is an improvement regardless:
         because tyvars can, generally, unify with casted types, there's no
         reason to go through the work of stripping off the cast when the
         cast appears opposite a tyvar. This is implemented in the cast case
         of can_eq_nc'.

     (4) Reporting an error for a constraint that is blocked with HoleBlockerReason
         is hard: what would we say to users? And we don't
         really need to report, because if a constraint is blocked, then
         there is unsolved wanted blocking it; that unsolved wanted will
         be reported. We thus push such errors to the bottom of the queue
         in the error-reporting code; they should never be printed.

         (4a) It would seem possible to do this filtering just based on the
              presence of a blocking coercion hole. However, this is no good,
              as it suppresses e.g. no-instance-found errors. We thus record
              a CtIrredReason in CIrredCan and filter based on this status.
              This happened in T14584. An alternative approach is to expressly
              look for *equalities* with blocking coercion holes, but actually
              recording the blockage in a status field seems nicer.

         (4b) The error message might be printed with -fdefer-type-errors,
              so it still must exist. This is the only reason why there is
              a message at all. Otherwise, we could simply do nothing.

    Historical note:

    We used to do this via emitting a Derived kind equality and then parking
    the heterogeneous equality as irreducible. But this new approach is much
    more direct. And it doesn't produce duplicate Deriveds (as the old one did).

    Note [Type synonyms and canonicalization]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We treat type synonym applications as xi types, that is, they do not
    count as type function applications.  However, we do need to be a bit
    careful with type synonyms: like type functions they may not be
    generative or injective.  However, unlike type functions, they are
    parametric, so there is no problem in expanding them whenever we see
    them, since we do not need to know anything about their arguments in
    order to expand them; this is what justifies not having to treat them
    as specially as type function applications.  The thing that causes
    some subtleties is that we prefer to leave type synonym applications
    *unexpanded* whenever possible, in order to generate better error
    messages.

    If we encounter an equality constraint with type synonym applications
    on both sides, or a type synonym application on one side and some sort
    of type application on the other, we simply must expand out the type
    synonyms in order to continue decomposing the equality constraint into
    primitive equality constraints.  For example, suppose we have

      type F a = [Int]

    and we encounter the equality

      F a ~ [b]

    In order to continue we must expand F a into [Int], giving us the
    equality

      [Int] ~ [b]

    which we can then decompose into the more primitive equality
    constraint

      Int ~ b.

    However, if we encounter an equality constraint with a type synonym
    application on one side and a variable on the other side, we should
    NOT (necessarily) expand the type synonym, since for the purpose of
    good error messages we want to leave type synonyms unexpanded as much
    as possible.  Hence the ps_xi1, ps_xi2 argument passed to canEqCanLHS.

    Note [Type equality cycles]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider this situation (from indexed-types/should_compile/GivenLoop):

      instance C (Maybe b)
      *[G] a ~ Maybe (F a)
      [W] C a

    or (typecheck/should_compile/T19682b):

      instance C (a -> b)
      *[WD] alpha ~ (Arg alpha -> Res alpha)
      [W] C alpha

    or (typecheck/should_compile/T21515):

      type family Code a
      *[G] Code a ~ '[ '[ Head (Head (Code a)) ] ]
      [W] Code a ~ '[ '[ alpha ] ]

    In order to solve the final Wanted, we must use the starred constraint
    for rewriting. But note that all starred constraints have occurs-check failures,
    and so we can't straightforwardly add these to the inert set and
    use them for rewriting. (NB: A rigid type constructor is at the
    top of all RHSs, preventing reorienting in canEqTyVarFunEq in the tyvar
    cases.)

    The key idea is to replace the outermost type family applications in the RHS of the
    starred constraints with a fresh variable, which we'll call a cycle-breaker
    variable, or cbv. Then, relate the cbv back with the original type family application
    via new equality constraints. Our situations thus become:

      instance C (Maybe b)
      [G] a ~ Maybe cbv
      [G] F a ~ cbv
      [W] C a

    or

      instance C (a -> b)
      [WD] alpha ~ (cbv1 -> cbv2)
      [WD] Arg alpha ~ cbv1
      [WD] Res alpha ~ cbv2
      [W] C alpha

    or

      [G] Code a ~ '[ '[ cbv ] ]
      [G] Head (Head (Code a)) ~ cbv
      [W] Code a ~ '[ '[ alpha ] ]

    This transformation (creating the new types and emitting new equality
    constraints) is done in breakTyEqCycle_maybe.

    The details depend on whether we're working with a Given or a Derived.
    (Note that the Wanteds are really WDs, above. This is because Wanteds
    are not used for rewriting.)

    Given
    -----

    We emit a new Given, [G] F a ~ cbv, equating the type family application to
    our new cbv. Note its orientation: The type family ends up on the left; see
    commentary on canEqTyVarFunEq, which decides how to orient such cases. No
    special treatment for CycleBreakerTvs is necessary. This scenario is now
    easily soluble, by using the first Given to rewrite the Wanted, which can now
    be solved.

    (The first Given actually also rewrites the second one, giving
    [G] F (Maybe cbv) ~ cbv, but this causes no trouble.)

    Of course, we don't want our fresh variables leaking into e.g. error messages.
    So we fill in the metavariables with their original type family applications
    after we're done running the solver (in nestImplicTcS and runTcSWithEvBinds).
    This is done by restoreTyVarCycles, which uses the inert_cycle_breakers field in
    InertSet, which contains the pairings invented in breakTyEqCycle_maybe.

    That is:

    We transform
      [G] g : lhs ~ ...(F lhs)...
    to
      [G] (Refl lhs) : F lhs ~ cbv      -- CEqCan
      [G] g          : lhs ~ ...cbv...  -- CEqCan

    Note that
    * `cbv` is a fresh cycle breaker variable.
    * `cbv` is a is a meta-tyvar, but it is completely untouchable.
    * We track the cycle-breaker variables in inert_cycle_breakers in InertSet
    * We eventually fill in the cycle-breakers, with `cbv := F lhs`.
      No one else fills in cycle-breakers!
    * The evidence for the new `F lhs ~ cbv` constraint is Refl, because we know
      this fill-in is ultimately going to happen.
    * In inert_cycle_breakers, we remember the (cbv, F lhs) pair; that is, we
      remember the /original/ type.  The [G] F lhs ~ cbv constraint may be rewritten
      by other givens (eg if we have another [G] lhs ~ (b,c)), but at the end we
      still fill in with cbv := F lhs
    * This fill-in is done when solving is complete, by restoreTyVarCycles
      in nestImplicTcS and runTcSWithEvBinds.

    Wanted/Derived
    --------------
    The fresh cycle-breaker variables here must actually be normal, touchable
    metavariables. That is, they are TauTvs. Nothing at all unusual. Repeating
    the example from above, we have

      *[WD] alpha ~ (Arg alpha -> Res alpha)

    and we turn this into

      *[WD] alpha ~ (cbv1 -> cbv2)
      [WD] Arg alpha ~ cbv1
      [WD] Res alpha ~ cbv2

    where cbv1 and cbv2 are fresh TauTvs. Why TauTvs? See [Why TauTvs] below.

    Critically, we emit the two new constraints (the last two above)
    directly instead of calling unifyWanted. (Otherwise, we'd end up unifying cbv1
    and cbv2 immediately, achieving nothing.)
    Next, we unify alpha := cbv1 -> cbv2, having eliminated the occurs check. This
    unification -- which must be the next step after breaking the cycles --
    happens in the course of normal behavior of top-level
    interactions, later in the solver pipeline. We know this unification will
    indeed happen because breakTyEqCycle_maybe, which decides whether to apply
    this logic, checks to ensure unification will succeed in its final_check.
    (In particular, the LHS must be a touchable tyvar, never a type family. We don't
    yet have an example of where this logic is needed with a type family, and it's
    unclear how to handle this case, so we're skipping for now.) Now, we're
    here (including further context from our original example, from the top of the
    Note):

      instance C (a -> b)
      [WD] Arg (cbv1 -> cbv2) ~ cbv1
      [WD] Res (cbv1 -> cbv2) ~ cbv2
      [W] C (cbv1 -> cbv2)

    The first two WD constraints reduce to reflexivity and are discarded,
    and the last is easily soluble.

    [Why TauTvs]:
    Let's look at another example (typecheck/should_compile/T19682) where we need
    to unify the cbvs:

      class    (AllEqF xs ys, SameShapeAs xs ys) => AllEq xs ys
      instance (AllEqF xs ys, SameShapeAs xs ys) => AllEq xs ys

      type family SameShapeAs xs ys :: Constraint where
        SameShapeAs '[] ys      = (ys ~ '[])
        SameShapeAs (x : xs) ys = (ys ~ (Head ys : Tail ys))

      type family AllEqF xs ys :: Constraint where
        AllEqF '[]      '[]      = ()
        AllEqF (x : xs) (y : ys) = (x ~ y, AllEq xs ys)

      [WD] alpha ~ (Head alpha : Tail alpha)
      [WD] AllEqF '[Bool] alpha

    Without the logic detailed in this Note, we're stuck here, as AllEqF cannot
    reduce and alpha cannot unify. Let's instead apply our cycle-breaker approach,
    just as described above. We thus invent cbv1 and cbv2 and unify
    alpha := cbv1 -> cbv2, yielding (after zonking)

      [WD] Head (cbv1 : cbv2) ~ cbv1
      [WD] Tail (cbv1 : cbv2) ~ cbv2
      [WD] AllEqF '[Bool] (cbv1 : cbv2)

    The first two WD constraints simplify to reflexivity and are discarded.
    But the last reduces:

      [WD] Bool ~ cbv1
      [WD] AllEq '[] cbv2

    The first of these is solved by unification: cbv1 := Bool. The second
    is solved by the instance for AllEq to become

      [WD] AllEqF '[] cbv2
      [WD] SameShapeAs '[] cbv2

    While the first of these is stuck, the second makes progress, to lead to

      [WD] AllEqF '[] cbv2
      [WD] cbv2 ~ '[]

    This second constraint is solved by unification: cbv2 := '[]. We now
    have

      [WD] AllEqF '[] '[]

    which reduces to

      [WD] ()

    which is trivially satisfiable. Hooray!

    Note that we need to unify the cbvs here; if we did not, there would be
    no way to solve those constraints. That's why the cycle-breakers are
    ordinary TauTvs.

    In all cases
    ------------

    We detect this scenario by the following characteristics:
     - a constraint with a soluble occurs-check failure
       (as indicated by the cteSolubleOccurs bit set in a CheckTyEqResult
       from checkTypeEq)
     - and a nominal equality
     - and either
        - a Given flavour (but see also Detail (7) below)
        - a Wanted/Derived or just plain Derived flavour, with a touchable metavariable
          on the left

    We don't use this trick for representational equalities, as there is no
    concrete use case where it is helpful (unlike for nominal equalities).
    Furthermore, because function applications can be CanEqLHSs, but newtype
    applications cannot, the disparities between the cases are enough that it
    would be effortful to expand the idea to representational equalities. A quick
    attempt, with

          data family N a b

          f :: (Coercible a (N a b), Coercible (N a b) b) => a -> b
          f = coerce

    failed with "Could not match 'b' with 'b'." Further work is held off
    until when we have a concrete incentive to explore this dark corner.

    Details:

     (1) We don't look under foralls, at all, when substituting away type family
         applications, because doing so can never be fruitful. Recall that we
         are in a case like [G] lhs ~ forall b. ... lhs ....   Until we have a type
         family that can pull the body out from a forall (e.g. type instance F (forall b. ty) = ty),
         this will always be
         insoluble. Note also that the forall cannot be in an argument to a
         type family, or that outer type family application would already have
         been substituted away.

         However, we still must check to make sure that breakTyEqCycle_maybe actually
         succeeds in getting rid of all occurrences of the offending lhs. If
         one is hidden under a forall, this won't be true. A similar problem can
         happen if the variable appears only in a kind
         (e.g. k ~ ... (a :: k) ...). So we perform an additional check after
         performing the substitution. It is tiresome to re-run all of checkTypeEq
         here, but reimplementing just the occurs-check is even more tiresome.

         Skipping this check causes typecheck/should_fail/GivenForallLoop and
         polykinds/T18451 to loop.

     (2) Our goal here is to avoid loops in rewriting. We can thus skip looking
         in coercions, as we don't rewrite in coercions in the algorithm in
         GHC.Solver.Rewrite. (This is another reason
         we need to re-check that we've gotten rid of all occurrences of the
         offending variable.)

     (3) As we're substituting as described in this Note, we can build ill-kinded
         types. For example, if we have Proxy (F a) b, where (b :: F a), then
         replacing this with Proxy cbv b is ill-kinded. However, we will later
         set cbv := F a, and so the zonked type will be well-kinded again.
         The temporary ill-kinded type hurts no one, and avoiding this would
         be quite painfully difficult.

         Specifically, this detail does not contravene the Purely Kinded Type Invariant
         (Note [The Purely Kinded Type Invariant (PKTI)] in GHC.Tc.Gen.HsType).
         The PKTI says that we can call typeKind on any type, without failure.
         It would be violated if we, say, replaced a kind (a -> b) with a kind c,
         because an arrow kind might be consulted in piResultTys. Here, we are
         replacing one opaque type like (F a b c) with another, cbv (opaque in
         that we never assume anything about its structure, like that it has a
         result type or a RuntimeRep argument).

     (4) The evidence for the produced Givens is all just reflexive, because
         we will eventually set the cycle-breaker variable to be the type family,
         and then, after the zonk, all will be well.

     (5) The approach here is inefficient because it replaces every (outermost)
         type family application with a type variable, regardless of whether that
         particular appplication is implicated in the occurs check.  An alternative
         would be to replce only type-family applications that meantion the offending LHS.
         For instance, we could choose to
         affect only type family applications that mention the offending LHS:
         e.g. in a ~ (F b, G a), we need to replace only G a, not F b. Furthermore,
         we could try to detect cases like a ~ (F a, F a) and use the same
         tyvar to replace F a. (Cf.
         Note [Flattening type-family applications when matching instances]
         in GHC.Core.Unify, which
         goes to this extra effort.) There may be other opportunities for
         improvement. However, this is really a very small corner case.
         The investment to craft a clever,
         performant solution seems unworthwhile.

     (6) We often get the predicate associated with a constraint from its
         evidence with ctPred. We thus must not only make sure the generated
         CEqCan's fields have the updated RHS type (that is, the one produced
         by replacing type family applications with fresh variables),
         but we must also update the evidence itself. This is done by the call to rewriteEqEvidence
         in canEqCanLHSFinish.

     (7) We don't wish to apply this magic on the equalities created
         by this very same process.
         Consider this, from typecheck/should_compile/ContextStack2:

           type instance TF (a, b) = (TF a, TF b)
           t :: (a ~ TF (a, Int)) => ...

           [G] a ~ TF (a, Int)

         The RHS reduces, so we get

           [G] a ~ (TF a, TF Int)

         We then break cycles, to get

           [G] g1 :: a ~ (cbv1, cbv2)
           [G] g2 :: TF a ~ cbv1
           [G] g3 :: TF Int ~ cbv2

         g1 gets added to the inert set, as written. But then g2 becomes
         the work item. g1 rewrites g2 to become

           [G] TF (cbv1, cbv2) ~ cbv1

         which then uses the type instance to become

           [G] (TF cbv1, TF cbv2) ~ cbv1

         which looks remarkably like the Given we started with. If left
         unchecked, this will end up breaking cycles again, looping ad
         infinitum (and resulting in a context-stack reduction error,
         not an outright loop). The solution is easy: don't break cycles
         on an equality generated by breaking cycles. Instead, we mark this
         final Given as a CIrredCan with a NonCanonicalReason with the soluble
         occurs-check bit set (only).

         We track these equalities by giving them a special CtOrigin,
         CycleBreakerOrigin. This works for both Givens and Wanteds, as
         we need the logic in the W case for e.g. typecheck/should_fail/T17139.

     (8) We really want to do this all only when there is a soluble occurs-check
         failure, not when other problems arise (such as an impredicative
         equality like alpha ~ forall a. a -> a). That is why breakTyEqCycle_maybe
         uses cterHasOnlyProblem when looking at the result of checkTypeEq, which
         checks for many of the invariants on a CEqCan.
    -}

    {-
    ************************************************************************
    *                                                                      *
                      Evidence transformation
    *                                                                      *
    ************************************************************************
    -}

    data StopOrContinue a
      = ContinueWith a    -- The constraint was not solved, although it may have
                          --   been rewritten

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

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

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

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

    andWhenContinue :: TcS (StopOrContinue a)
                    -> (a -> TcS (StopOrContinue b))
                    -> TcS (StopOrContinue b)
    andWhenContinue :: forall a b.
    TcS (StopOrContinue a)
    -> (a -> TcS (StopOrContinue b)) -> TcS (StopOrContinue b)
    andWhenContinue TcS (StopOrContinue a)
    tcs1 a -> TcS (StopOrContinue b)
    tcs2
      = do { StopOrContinue a
    r <- TcS (StopOrContinue a)
    tcs1
           ; case StopOrContinue a
    r of
               Stop CtEvidence
    ev SDoc
    s       -> forall (m :: * -> *) a. Monad m => a -> m a
    return (forall a. CtEvidence -> SDoc -> StopOrContinue a
    Stop CtEvidence
    ev SDoc
    s)
               ContinueWith a
    ct -> a -> TcS (StopOrContinue b)
    tcs2 a
    ct }
    infixr 0 `andWhenContinue`    -- allow chaining with ($)

    rewriteEvidence :: CtEvidence   -- old evidence
                    -> TcPredType   -- new predicate
                    -> TcCoercion   -- Of type :: new predicate ~ <type of old evidence>
                    -> TcS (StopOrContinue CtEvidence)
    -- Returns Just new_ev iff either (i)  'co' is reflexivity
    --                             or (ii) 'co' is not reflexivity, and 'new_pred' not cached
    -- In either case, there is nothing new to do with new_ev
    {-
         rewriteEvidence old_ev new_pred co
    Main purpose: create new evidence for new_pred;
                  unless new_pred is cached already
    * Returns a new_ev : new_pred, with same wanted/given/derived flag as old_ev
    * If old_ev was wanted, create a binding for old_ev, in terms of new_ev
    * If old_ev was given, AND not cached, create a binding for new_ev, in terms of old_ev
    * Returns Nothing if new_ev is already cached

            Old evidence    New predicate is               Return new evidence
            flavour                                        of same flavor
            -------------------------------------------------------------------
            Wanted          Already solved or in inert     Nothing
            or Derived      Not                            Just new_evidence

            Given           Already in inert               Nothing
                            Not                            Just new_evidence

    Note [Rewriting with Refl]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    If the coercion is just reflexivity then you may re-use the same
    variable.  But be careful!  Although the coercion is Refl, new_pred
    may reflect the result of unification alpha := ty, so new_pred might
    not _look_ the same as old_pred, and it's vital to proceed from now on
    using new_pred.

    The rewriter preserves type synonyms, so they should appear in new_pred
    as well as in old_pred; that is important for good error messages.
     -}

    rewriteEvidence :: CtEvidence
    -> TcType -> TcCoercion -> TcS (StopOrContinue CtEvidence)
    rewriteEvidence old_ev :: CtEvidence
    old_ev@(CtDerived {}) TcType
    new_pred TcCoercion
    _co
      = -- If derived, don't even look at the coercion.
        -- This is very important, DO NOT re-order the equations for
        -- rewriteEvidence to put the isTcReflCo test first!
        -- Why?  Because for *Derived* constraints, c, the coercion, which
        -- was produced by rewriting, may contain suspended calls to
        -- (ctEvExpr c), which fails for Derived constraints.
        -- (Getting this wrong caused #7384.)
        forall a. a -> TcS (StopOrContinue a)
    continueWith (CtEvidence
    old_ev { ctev_pred :: TcType
    ctev_pred = TcType
    new_pred })

    rewriteEvidence CtEvidence
    old_ev TcType
    new_pred TcCoercion
    co
      | TcCoercion -> Bool
    isTcReflCo TcCoercion
    co -- See Note [Rewriting with Refl]
      = forall a. a -> TcS (StopOrContinue a)
    continueWith (CtEvidence
    old_ev { ctev_pred :: TcType
    ctev_pred = TcType
    new_pred })

    rewriteEvidence ev :: CtEvidence
    ev@(CtGiven { ctev_evar :: CtEvidence -> TcTyVar
    ctev_evar = TcTyVar
    old_evar, ctev_loc :: CtEvidence -> CtLoc
    ctev_loc = CtLoc
    loc }) TcType
    new_pred TcCoercion
    co
      = do { CtEvidence
    new_ev <- CtLoc -> (TcType, EvTerm) -> TcS CtEvidence
    newGivenEvVar CtLoc
    loc (TcType
    new_pred, EvTerm
    new_tm)
           ; forall a. a -> TcS (StopOrContinue a)
    continueWith CtEvidence
    new_ev }
      where
        -- mkEvCast optimises ReflCo
        new_tm :: EvTerm
    new_tm = EvExpr -> TcCoercion -> EvTerm
    mkEvCast (TcTyVar -> EvExpr
    evId TcTyVar
    old_evar) (Role -> Role -> TcCoercion -> TcCoercion
    tcDowngradeRole Role
    Representational
                                                           (CtEvidence -> Role
    ctEvRole CtEvidence
    ev)
                                                           (TcCoercion -> TcCoercion
    mkTcSymCo TcCoercion
    co))

    rewriteEvidence ev :: CtEvidence
    ev@(CtWanted { ctev_dest :: CtEvidence -> TcEvDest
    ctev_dest = TcEvDest
    dest
                                 , ctev_nosh :: CtEvidence -> ShadowInfo
    ctev_nosh = ShadowInfo
    si
                                 , ctev_loc :: CtEvidence -> CtLoc
    ctev_loc = CtLoc
    loc }) TcType
    new_pred TcCoercion
    co
      = do { MaybeNew
    mb_new_ev <- ShadowInfo -> CtLoc -> TcType -> TcS MaybeNew
    newWanted_SI ShadowInfo
    si CtLoc
    loc TcType
    new_pred
                   -- The "_SI" variant ensures that we make a new Wanted
                   -- with the same shadow-info as the existing one
                   -- with the same shadow-info as the existing one (#16735)
           ; MASSERT( tcCoercionRole co == ctEvRole ev )
           ; TcEvDest -> EvTerm -> TcS ()
    setWantedEvTerm TcEvDest
    dest
                (EvExpr -> TcCoercion -> EvTerm
    mkEvCast (MaybeNew -> EvExpr
    getEvExpr MaybeNew
    mb_new_ev)
                          (Role -> Role -> TcCoercion -> TcCoercion
    tcDowngradeRole Role
    Representational (CtEvidence -> Role
    ctEvRole CtEvidence
    ev) TcCoercion
    co))
           ; case MaybeNew
    mb_new_ev of
                Fresh  CtEvidence
    new_ev -> forall a. a -> TcS (StopOrContinue a)
    continueWith CtEvidence
    new_ev
                Cached EvExpr
    _      -> forall a. CtEvidence -> String -> TcS (StopOrContinue a)
    stopWith CtEvidence
    ev String
    "Cached wanted" }

    rewriteEqEvidence :: CtEvidence         -- Old evidence :: olhs ~ orhs (not swapped)
                                            --              or orhs ~ olhs (swapped)
                      -> SwapFlag
                      -> TcType -> TcType   -- New predicate  nlhs ~ nrhs
                      -> TcCoercion         -- lhs_co, of type :: nlhs ~ olhs
                      -> TcCoercion         -- rhs_co, of type :: nrhs ~ orhs
                      -> TcS CtEvidence     -- Of type nlhs ~ nrhs
    -- For (rewriteEqEvidence (Given g olhs orhs) False nlhs nrhs lhs_co rhs_co)
    -- we generate
    -- If not swapped
    --      g1 : nlhs ~ nrhs = lhs_co ; g ; sym rhs_co
    -- If 'swapped'
    --      g1 : nlhs ~ nrhs = lhs_co ; Sym g ; sym rhs_co
    --
    -- For (Wanted w) we do the dual thing.
    -- New  w1 : nlhs ~ nrhs
    -- If not swapped
    --      w : olhs ~ orhs = sym lhs_co ; w1 ; rhs_co
    -- If swapped
    --      w : orhs ~ olhs = sym rhs_co ; sym w1 ; lhs_co
    --
    -- It's all a form of rewwriteEvidence, specialised for equalities
    rewriteEqEvidence :: CtEvidence
    -> SwapFlag
    -> TcType
    -> TcType
    -> TcCoercion
    -> TcCoercion
    -> TcS CtEvidence
    rewriteEqEvidence CtEvidence
    old_ev SwapFlag
    swapped TcType
    nlhs TcType
    nrhs TcCoercion
    lhs_co TcCoercion
    rhs_co
      | CtDerived {} <- CtEvidence
    old_ev  -- Don't force the evidence for a Derived
      = forall (m :: * -> *) a. Monad m => a -> m a
    return (CtEvidence
    old_ev { ctev_pred :: TcType
    ctev_pred = TcType
    new_pred })

      | SwapFlag
    NotSwapped <- SwapFlag
    swapped
      , TcCoercion -> Bool
    isTcReflCo TcCoercion
    lhs_co      -- See Note [Rewriting with Refl]
      , TcCoercion -> Bool
    isTcReflCo TcCoercion
    rhs_co
      = forall (m :: * -> *) a. Monad m => a -> m a
    return (CtEvidence
    old_ev { ctev_pred :: TcType
    ctev_pred = TcType
    new_pred })

      | CtGiven { ctev_evar :: CtEvidence -> TcTyVar
    ctev_evar = TcTyVar
    old_evar } <- CtEvidence
    old_ev
      = do { let new_tm :: EvTerm
    new_tm = TcCoercion -> EvTerm
    evCoercion (TcCoercion
    lhs_co
                                      TcCoercion -> TcCoercion -> TcCoercion
    `mkTcTransCo` SwapFlag -> TcCoercion -> TcCoercion
    maybeTcSymCo SwapFlag
    swapped (TcTyVar -> TcCoercion
    mkTcCoVarCo TcTyVar
    old_evar)
                                      TcCoercion -> TcCoercion -> TcCoercion
    `mkTcTransCo` TcCoercion -> TcCoercion
    mkTcSymCo TcCoercion
    rhs_co)
           ; CtLoc -> (TcType, EvTerm) -> TcS CtEvidence
    newGivenEvVar CtLoc
    loc' (TcType
    new_pred, EvTerm
    new_tm) }

      | CtWanted { ctev_dest :: CtEvidence -> TcEvDest
    ctev_dest = TcEvDest
    dest, ctev_nosh :: CtEvidence -> ShadowInfo
    ctev_nosh = ShadowInfo
    si } <- CtEvidence
    old_ev
      = do { (CtEvidence
    new_ev, TcCoercion
    hole_co) <- ShadowInfo
    -> CtLoc
    -> Role
    -> TcType
    -> TcType
    -> TcS (CtEvidence, TcCoercion)
    newWantedEq_SI ShadowInfo
    si CtLoc
    loc'
                                                 (CtEvidence -> Role
    ctEvRole CtEvidence
    old_ev) TcType
    nlhs TcType
    nrhs
                   -- The "_SI" variant ensures that we make a new Wanted
                   -- with the same shadow-info as the existing one (#16735)
           ; let co :: TcCoercion
    co = SwapFlag -> TcCoercion -> TcCoercion
    maybeTcSymCo SwapFlag
    swapped forall a b. (a -> b) -> a -> b
    $
                      TcCoercion -> TcCoercion
    mkSymCo TcCoercion
    lhs_co
                      TcCoercion -> TcCoercion -> TcCoercion
    `mkTransCo` TcCoercion
    hole_co
                      TcCoercion -> TcCoercion -> TcCoercion
    `mkTransCo` TcCoercion
    rhs_co
           ; TcEvDest -> TcCoercion -> TcS ()
    setWantedEq TcEvDest
    dest TcCoercion
    co
           ; String -> SDoc -> TcS ()
    traceTcS String
    "rewriteEqEvidence" ([SDoc] -> SDoc
    vcat [forall a. Outputable a => a -> SDoc
    ppr CtEvidence
    old_ev, forall a. Outputable a => a -> SDoc
    ppr TcType
    nlhs, forall a. Outputable a => a -> SDoc
    ppr TcType
    nrhs, forall a. Outputable a => a -> SDoc
    ppr TcCoercion
    co])
           ; forall (m :: * -> *) a. Monad m => a -> m a
    return CtEvidence
    new_ev }

    #if __GLASGOW_HASKELL__ <= 810
      | otherwise
      = panic "rewriteEvidence"
    #endif
      where
        new_pred :: TcType
    new_pred = CtEvidence -> TcType -> TcType -> TcType
    mkTcEqPredLikeEv CtEvidence
    old_ev TcType
    nlhs TcType
    nrhs

          -- equality is like a type class. Bumping the depth is necessary because
          -- of recursive newtypes, where "reducing" a newtype can actually make
          -- it bigger. See Note [Newtypes can blow the stack].
        loc :: CtLoc
    loc      = CtEvidence -> CtLoc
    ctEvLoc CtEvidence
    old_ev
        loc' :: CtLoc
    loc'     = CtLoc -> CtLoc
    bumpCtLocDepth CtLoc
    loc

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

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

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

    unifyWanted :: CtLoc -> Role
                -> TcType -> TcType -> TcS Coercion
    -- Return coercion witnessing the equality of the two types,
    -- emitting new work equalities where necessary to achieve that
    -- Very good short-cut when the two types are equal, or nearly so
    -- See Note [unifyWanted and unifyDerived]
    -- The returned coercion's role matches the input parameter
    unifyWanted :: CtLoc -> Role -> TcType -> TcType -> TcS TcCoercion
    unifyWanted CtLoc
    loc Role
    Phantom TcType
    ty1 TcType
    ty2
      = do { TcCoercion
    kind_co <- CtLoc -> Role -> TcType -> TcType -> TcS TcCoercion
    unifyWanted CtLoc
    loc Role
    Nominal (HasDebugCallStack => TcType -> TcType
    tcTypeKind TcType
    ty1) (HasDebugCallStack => TcType -> TcType
    tcTypeKind TcType
    ty2)
           ; forall (m :: * -> *) a. Monad m => a -> m a
    return (TcCoercion -> TcType -> TcType -> TcCoercion
    mkPhantomCo TcCoercion
    kind_co TcType
    ty1 TcType
    ty2) }

    unifyWanted CtLoc
    loc Role
    role TcType
    orig_ty1 TcType
    orig_ty2
      = TcType -> TcType -> TcS TcCoercion
    go TcType
    orig_ty1 TcType
    orig_ty2
      where
        go :: TcType -> TcType -> TcS TcCoercion
    go TcType
    ty1 TcType
    ty2 | Just TcType
    ty1' <- TcType -> Maybe TcType
    tcView TcType
    ty1 = TcType -> TcType -> TcS TcCoercion
    go TcType
    ty1' TcType
    ty2
        go TcType
    ty1 TcType
    ty2 | Just TcType
    ty2' <- TcType -> Maybe TcType
    tcView TcType
    ty2 = TcType -> TcType -> TcS TcCoercion
    go TcType
    ty1 TcType
    ty2'

        go (FunTy AnonArgFlag
    _ TcType
    w1 TcType
    s1 TcType
    t1) (FunTy AnonArgFlag
    _ TcType
    w2 TcType
    s2 TcType
    t2)
          = do { TcCoercion
    co_s <- CtLoc -> Role -> TcType -> TcType -> TcS TcCoercion
    unifyWanted CtLoc
    loc Role
    role TcType
    s1 TcType
    s2
               ; TcCoercion
    co_t <- CtLoc -> Role -> TcType -> TcType -> TcS TcCoercion
    unifyWanted CtLoc
    loc Role
    role TcType
    t1 TcType
    t2
               ; TcCoercion
    co_w <- CtLoc -> Role -> TcType -> TcType -> TcS TcCoercion
    unifyWanted CtLoc
    loc Role
    Nominal TcType
    w1 TcType
    w2
               ; forall (m :: * -> *) a. Monad m => a -> m a
    return (Role -> TcCoercion -> TcCoercion -> TcCoercion -> TcCoercion
    mkFunCo Role
    role TcCoercion
    co_w TcCoercion
    co_s TcCoercion
    co_t) }
        go (TyConApp TyCon
    tc1 [TcType]
    tys1) (TyConApp TyCon
    tc2 [TcType]
    tys2)
          | TyCon
    tc1 forall a. Eq a => a -> a -> Bool
    == TyCon
    tc2, [TcType]
    tys1 forall a b. [a] -> [b] -> Bool
    `equalLength` [TcType]
    tys2
          , TyCon -> Role -> Bool
    isInjectiveTyCon TyCon
    tc1 Role
    role -- don't look under newtypes at Rep equality
          = do { [TcCoercion]
    cos <- forall (m :: * -> *) a b c d.
    Monad m =>
    (a -> b -> c -> m d) -> [a] -> [b] -> [c] -> m [d]
    zipWith3M (CtLoc -> Role -> TcType -> TcType -> TcS TcCoercion
    unifyWanted CtLoc
    loc)
                                  (Role -> TyCon -> [Role]
    tyConRolesX Role
    role TyCon
    tc1) [TcType]
    tys1 [TcType]
    tys2
               ; forall (m :: * -> *) a. Monad m => a -> m a
    return (HasDebugCallStack => Role -> TyCon -> [TcCoercion] -> TcCoercion
    mkTyConAppCo Role
    role TyCon
    tc1 [TcCoercion]
    cos) }

        go ty1 :: TcType
    ty1@(TyVarTy TcTyVar
    tv) TcType
    ty2
          = do { Maybe TcType
    mb_ty <- TcTyVar -> TcS (Maybe TcType)
    isFilledMetaTyVar_maybe TcTyVar
    tv
               ; case Maybe TcType
    mb_ty of
                    Just TcType
    ty1' -> TcType -> TcType -> TcS TcCoercion
    go TcType
    ty1' TcType
    ty2
                    Maybe TcType
    Nothing   -> TcType -> TcType -> TcS TcCoercion
    bale_out TcType
    ty1 TcType
    ty2}
        go TcType
    ty1 ty2 :: TcType
    ty2@(TyVarTy TcTyVar
    tv)
          = do { Maybe TcType
    mb_ty <- TcTyVar -> TcS (Maybe TcType)
    isFilledMetaTyVar_maybe TcTyVar
    tv
               ; case Maybe TcType
    mb_ty of
                    Just TcType
    ty2' -> TcType -> TcType -> TcS TcCoercion
    go TcType
    ty1 TcType
    ty2'
                    Maybe TcType
    Nothing   -> TcType -> TcType -> TcS TcCoercion
    bale_out TcType
    ty1 TcType
    ty2 }

        go ty1 :: TcType
    ty1@(CoercionTy {}) (CoercionTy {})
          = forall (m :: * -> *) a. Monad m => a -> m a
    return (Role -> TcType -> TcCoercion
    mkReflCo Role
    role TcType
    ty1) -- we just don't care about coercions!

        go TcType
    ty1 TcType
    ty2 = TcType -> TcType -> TcS TcCoercion
    bale_out TcType
    ty1 TcType
    ty2

        bale_out :: TcType -> TcType -> TcS TcCoercion
    bale_out TcType
    ty1 TcType
    ty2
           | TcType
    ty1 HasDebugCallStack => TcType -> TcType -> Bool
    `tcEqType` TcType
    ty2 = forall (m :: * -> *) a. Monad m => a -> m a
    return (Role -> TcType -> TcCoercion
    mkTcReflCo Role
    role TcType
    ty1)
            -- Check for equality; e.g. a ~ a, or (m a) ~ (m a)
           | Bool
    otherwise = CtLoc -> Role -> TcType -> TcType -> TcS TcCoercion
    emitNewWantedEq CtLoc
    loc Role
    role TcType
    orig_ty1 TcType
    orig_ty2

    unifyDeriveds :: CtLoc -> [Role] -> [TcType] -> [TcType] -> TcS ()
    -- See Note [unifyWanted and unifyDerived]
    unifyDeriveds :: CtLoc -> [Role] -> [TcType] -> [TcType] -> TcS ()
    unifyDeriveds CtLoc
    loc [Role]
    roles [TcType]
    tys1 [TcType]
    tys2 = forall (m :: * -> *) a b c d.
    Monad m =>
    (a -> b -> c -> m d) -> [a] -> [b] -> [c] -> m ()
    zipWith3M_ (CtLoc -> Role -> TcType -> TcType -> TcS ()
    unify_derived CtLoc
    loc) [Role]
    roles [TcType]
    tys1 [TcType]
    tys2

    unifyDerived :: CtLoc -> Role -> Pair TcType -> TcS ()
    -- See Note [unifyWanted and unifyDerived]
    unifyDerived :: CtLoc -> Role -> Pair TcType -> TcS ()
    unifyDerived CtLoc
    loc Role
    role (Pair TcType
    ty1 TcType
    ty2) = CtLoc -> Role -> TcType -> TcType -> TcS ()
    unify_derived CtLoc
    loc Role
    role TcType
    ty1 TcType
    ty2

    unify_derived :: CtLoc -> Role -> TcType -> TcType -> TcS ()
    -- Create new Derived and put it in the work list
    -- Should do nothing if the two types are equal
    -- See Note [unifyWanted and unifyDerived]
    unify_derived :: CtLoc -> Role -> TcType -> TcType -> TcS ()
    unify_derived CtLoc
    _   Role
    Phantom TcType
    _        TcType
    _        = forall (m :: * -> *) a. Monad m => a -> m a
    return ()
    unify_derived CtLoc
    loc Role
    role    TcType
    orig_ty1 TcType
    orig_ty2
      = TcType -> TcType -> TcS ()
    go TcType
    orig_ty1 TcType
    orig_ty2
      where
        go :: TcType -> TcType -> TcS ()
    go TcType
    ty1 TcType
    ty2 | Just TcType
    ty1' <- TcType -> Maybe TcType
    tcView TcType
    ty1 = TcType -> TcType -> TcS ()
    go TcType
    ty1' TcType
    ty2
        go TcType
    ty1 TcType
    ty2 | Just TcType
    ty2' <- TcType -> Maybe TcType
    tcView TcType
    ty2 = TcType -> TcType -> TcS ()
    go TcType
    ty1 TcType
    ty2'

        go (FunTy AnonArgFlag
    _ TcType
    w1 TcType
    s1 TcType
    t1) (FunTy AnonArgFlag
    _ TcType
    w2 TcType
    s2 TcType
    t2)
          = do { CtLoc -> Role -> TcType -> TcType -> TcS ()
    unify_derived CtLoc
    loc Role
    role TcType
    s1 TcType
    s2
               ; CtLoc -> Role -> TcType -> TcType -> TcS ()
    unify_derived CtLoc
    loc Role
    role TcType
    t1 TcType
    t2
               ; CtLoc -> Role -> TcType -> TcType -> TcS ()
    unify_derived CtLoc
    loc Role
    Nominal TcType
    w1 TcType
    w2 }
        go (TyConApp TyCon
    tc1 [TcType]
    tys1) (TyConApp TyCon
    tc2 [TcType]
    tys2)
          | TyCon
    tc1 forall a. Eq a => a -> a -> Bool
    == TyCon
    tc2, [TcType]
    tys1 forall a b. [a] -> [b] -> Bool
    `equalLength` [TcType]
    tys2
          , TyCon -> Role -> Bool
    isInjectiveTyCon TyCon
    tc1 Role
    role
          = CtLoc -> [Role] -> [TcType] -> [TcType] -> TcS ()
    unifyDeriveds CtLoc
    loc (Role -> TyCon -> [Role]
    tyConRolesX Role
    role TyCon
    tc1) [TcType]
    tys1 [TcType]
    tys2
        go ty1 :: TcType
    ty1@(TyVarTy TcTyVar
    tv) TcType
    ty2
          = do { Maybe TcType
    mb_ty <- TcTyVar -> TcS (Maybe TcType)
    isFilledMetaTyVar_maybe TcTyVar
    tv
               ; case Maybe TcType
    mb_ty of
                    Just TcType
    ty1' -> TcType -> TcType -> TcS ()
    go TcType
    ty1' TcType
    ty2
                    Maybe TcType
    Nothing   -> TcType -> TcType -> TcS ()
    bale_out TcType
    ty1 TcType
    ty2 }
        go TcType
    ty1 ty2 :: TcType
    ty2@(TyVarTy TcTyVar
    tv)
          = do { Maybe TcType
    mb_ty <- TcTyVar -> TcS (Maybe TcType)
    isFilledMetaTyVar_maybe TcTyVar
    tv
               ; case Maybe TcType
    mb_ty of
                    Just TcType
    ty2' -> TcType -> TcType -> TcS ()
    go TcType
    ty1 TcType
    ty2'
                    Maybe TcType
    Nothing   -> TcType -> TcType -> TcS ()
    bale_out TcType
    ty1 TcType
    ty2 }
        go TcType
    ty1 TcType
    ty2 = TcType -> TcType -> TcS ()
    bale_out TcType
    ty1 TcType
    ty2

        bale_out :: TcType -> TcType -> TcS ()
    bale_out TcType
    ty1 TcType
    ty2
           | TcType
    ty1 HasDebugCallStack => TcType -> TcType -> Bool
    `tcEqType` TcType
    ty2 = forall (m :: * -> *) a. Monad m => a -> m a
    return ()
            -- Check for equality; e.g. a ~ a, or (m a) ~ (m a)
           | Bool
    otherwise = CtLoc -> Role -> TcType -> TcType -> TcS ()
    emitNewDerivedEq CtLoc
    loc Role
    role TcType
    orig_ty1 TcType
    orig_ty2