types/Unify.hs

    -- (c) The University of Glasgow 2006

    {-# LANGUAGE ScopedTypeVariables #-}
    {-# LANGUAGE CPP #-}
    {-# LANGUAGE DeriveFunctor #-}

    module Unify (
            tcMatchTy, tcMatchTyKi,
            tcMatchTys, tcMatchTyKis,
            tcMatchTyX, tcMatchTysX, tcMatchTyKisX,
            ruleMatchTyKiX,

            -- * Rough matching
            roughMatchTcs, instanceCantMatch,
            typesCantMatch,

            -- Side-effect free unification
            tcUnifyTy, tcUnifyTyKi, tcUnifyTys, tcUnifyTyKis,
            tcUnifyTysFG, tcUnifyTyWithTFs,
            BindFlag(..),
            UnifyResult, UnifyResultM(..),

            -- Matching a type against a lifted type (coercion)
            liftCoMatch
       ) where

    #include "HsVersions.h"

    import GhcPrelude

    import Var
    import VarEnv
    import VarSet
    import Name( Name )
    import Type hiding ( getTvSubstEnv )
    import Coercion hiding ( getCvSubstEnv )
    import TyCon
    import TyCoRep hiding ( getTvSubstEnv, getCvSubstEnv )
    import FV( FV, fvVarSet, fvVarList )
    import Util
    import Pair
    import Outputable
    import UniqFM
    import UniqSet

    import Control.Monad
    import qualified Control.Monad.Fail as MonadFail
    import Control.Applicative hiding ( empty )
    import qualified Control.Applicative

    {-

    Unification is much tricker than you might think.

    1. The substitution we generate binds the *template type variables*
       which are given to us explicitly.

    2. We want to match in the presence of foralls;
            e.g     (forall a. t1) ~ (forall b. t2)

       That is what the RnEnv2 is for; it does the alpha-renaming
       that makes it as if a and b were the same variable.
       Initialising the RnEnv2, so that it can generate a fresh
       binder when necessary, entails knowing the free variables of
       both types.

    3. We must be careful not to bind a template type variable to a
       locally bound variable.  E.g.
            (forall a. x) ~ (forall b. b)
       where x is the template type variable.  Then we do not want to
       bind x to a/b!  This is a kind of occurs check.
       The necessary locals accumulate in the RnEnv2.

    Note [tcMatchTy vs tcMatchTyKi]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    This module offers two variants of matching: with kinds and without.
    The TyKi variant takes two types, of potentially different kinds,
    and matches them. Along the way, it necessarily also matches their
    kinds. The Ty variant instead assumes that the kinds are already
    eqType and so skips matching up the kinds.

    How do you choose between them?

    1. If you know that the kinds of the two types are eqType, use
       the Ty variant. It is more efficient, as it does less work.

    2. If the kinds of variables in the  template type might mention type families,
       use the Ty variant (and do other work to make sure the kinds
       work out). These pure unification functions do a straightforward
       syntactic unification and do no complex reasoning about type
       families. Note that the types of the variables in instances can indeed
       mention type families, so instance lookup must use the Ty variant.

       (Nothing goes terribly wrong -- no panics -- if there might be type
       families in kinds in the TyKi variant. You just might get match
       failure even though a reducing a type family would lead to success.)

    3. Otherwise, if you're sure that the variable kinds to not mention
       type families and you're not already sure that the kind of the template
       equals the kind of the target, then use the TyKi versio.n
    -}

    -- | @tcMatchTy t1 t2@ produces a substitution (over fvs(t1))
    -- @s@ such that @s(t1)@ equals @t2@.
    -- The returned substitution might bind coercion variables,
    -- if the variable is an argument to a GADT constructor.
    --
    -- Precondition: typeKind ty1 `eqType` typeKind ty2
    --
    -- We don't pass in a set of "template variables" to be bound
    -- by the match, because tcMatchTy (and similar functions) are
    -- always used on top-level types, so we can bind any of the
    -- free variables of the LHS.
    -- See also Note [tcMatchTy vs tcMatchTyKi]
    tcMatchTy :: Type -> Type -> Maybe TCvSubst
    tcMatchTy ty1 ty2 = tcMatchTys [ty1] [ty2]

    -- | Like 'tcMatchTy', but allows the kinds of the types to differ,
    -- and thus matches them as well.
    -- See also Note [tcMatchTy vs tcMatchTyKi]
    tcMatchTyKi :: Type -> Type -> Maybe TCvSubst
    tcMatchTyKi ty1 ty2 = tcMatchTyKis [ty1] [ty2]

    -- | This is similar to 'tcMatchTy', but extends a substitution
    -- See also Note [tcMatchTy vs tcMatchTyKi]
    tcMatchTyX :: TCvSubst            -- ^ Substitution to extend
               -> Type                -- ^ Template
               -> Type                -- ^ Target
               -> Maybe TCvSubst
    tcMatchTyX subst ty1 ty2 = tcMatchTysX subst [ty1] [ty2]

    -- | Like 'tcMatchTy' but over a list of types.
    -- See also Note [tcMatchTy vs tcMatchTyKi]
    tcMatchTys :: [Type]         -- ^ Template
               -> [Type]         -- ^ Target
               -> Maybe TCvSubst -- ^ One-shot; in principle the template
                                 -- variables could be free in the target
    tcMatchTys tys1 tys2
      = tcMatchTysX (mkEmptyTCvSubst in_scope) tys1 tys2
      where
        in_scope = mkInScopeSet (tyCoVarsOfTypes tys1 `unionVarSet` tyCoVarsOfTypes tys2)

    -- | Like 'tcMatchTyKi' but over a list of types.
    -- See also Note [tcMatchTy vs tcMatchTyKi]
    tcMatchTyKis :: [Type]         -- ^ Template
                 -> [Type]         -- ^ Target
                 -> Maybe TCvSubst -- ^ One-shot substitution
    tcMatchTyKis tys1 tys2
      = tcMatchTyKisX (mkEmptyTCvSubst in_scope) tys1 tys2
      where
        in_scope = mkInScopeSet (tyCoVarsOfTypes tys1 `unionVarSet` tyCoVarsOfTypes tys2)

    -- | Like 'tcMatchTys', but extending a substitution
    -- See also Note [tcMatchTy vs tcMatchTyKi]
    tcMatchTysX :: TCvSubst       -- ^ Substitution to extend
                -> [Type]         -- ^ Template
                -> [Type]         -- ^ Target
                -> Maybe TCvSubst -- ^ One-shot substitution
    tcMatchTysX subst tys1 tys2
      = tc_match_tys_x False subst tys1 tys2

    -- | Like 'tcMatchTyKis', but extending a substitution
    -- See also Note [tcMatchTy vs tcMatchTyKi]
    tcMatchTyKisX :: TCvSubst        -- ^ Substitution to extend
                  -> [Type]          -- ^ Template
                  -> [Type]          -- ^ Target
                  -> Maybe TCvSubst  -- ^ One-shot substitution
    tcMatchTyKisX subst tys1 tys2
      = tc_match_tys_x True subst tys1 tys2

    -- | Worker for 'tcMatchTysX' and 'tcMatchTyKisX'
    tc_match_tys_x :: Bool          -- ^ match kinds?
                   -> TCvSubst
                   -> [Type]
                   -> [Type]
                   -> Maybe TCvSubst
    tc_match_tys_x match_kis (TCvSubst in_scope tv_env cv_env) tys1 tys2
      = case tc_unify_tys (const BindMe)
                          False  -- Matching, not unifying
                          False  -- Not an injectivity check
                          match_kis
                          (mkRnEnv2 in_scope) tv_env cv_env tys1 tys2 of
          Unifiable (tv_env', cv_env')
            -> Just $ TCvSubst in_scope tv_env' cv_env'
          _ -> Nothing

    -- | This one is called from the expression matcher,
    -- which already has a MatchEnv in hand
    ruleMatchTyKiX
      :: TyCoVarSet          -- ^ template variables
      -> RnEnv2
      -> TvSubstEnv          -- ^ type substitution to extend
      -> Type                -- ^ Template
      -> Type                -- ^ Target
      -> Maybe TvSubstEnv
    ruleMatchTyKiX tmpl_tvs rn_env tenv tmpl target
    -- See Note [Kind coercions in Unify]
      = case tc_unify_tys (matchBindFun tmpl_tvs) False False
                          True -- <-- this means to match the kinds
                          rn_env tenv emptyCvSubstEnv [tmpl] [target] of
          Unifiable (tenv', _) -> Just tenv'
          _                    -> Nothing

    matchBindFun :: TyCoVarSet -> TyVar -> BindFlag
    matchBindFun tvs tv = if tv `elemVarSet` tvs then BindMe else Skolem

    {- *********************************************************************
    *                                                                      *
                    Rough matching
    *                                                                      *
    ********************************************************************* -}

    -- See Note [Rough match] field in InstEnv

    roughMatchTcs :: [Type] -> [Maybe Name]
    roughMatchTcs tys = map rough tys
      where
        rough ty
          | Just (ty', _) <- splitCastTy_maybe ty   = rough ty'
          | Just (tc,_)   <- splitTyConApp_maybe ty = Just (tyConName tc)
          | otherwise                               = Nothing

    instanceCantMatch :: [Maybe Name] -> [Maybe Name] -> Bool
    -- (instanceCantMatch tcs1 tcs2) returns True if tcs1 cannot
    -- possibly be instantiated to actual, nor vice versa;
    -- False is non-committal
    instanceCantMatch (mt : ts) (ma : as) = itemCantMatch mt ma || instanceCantMatch ts as
    instanceCantMatch _         _         =  False  -- Safe

    itemCantMatch :: Maybe Name -> Maybe Name -> Bool
    itemCantMatch (Just t) (Just a) = t /= a
    itemCantMatch _        _        = False

    {-
    ************************************************************************
    *                                                                      *
                    GADTs
    *                                                                      *
    ************************************************************************

    Note [Pruning dead case alternatives]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider        data T a where
                       T1 :: T Int
                       T2 :: T a

                    newtype X = MkX Int
                    newtype Y = MkY Char

                    type family F a
                    type instance F Bool = Int

    Now consider    case x of { T1 -> e1; T2 -> e2 }

    The question before the house is this: if I know something about the type
    of x, can I prune away the T1 alternative?

    Suppose x::T Char.  It's impossible to construct a (T Char) using T1,
            Answer = YES we can prune the T1 branch (clearly)

    Suppose x::T (F a), where 'a' is in scope.  Then 'a' might be instantiated
    to 'Bool', in which case x::T Int, so
            ANSWER = NO (clearly)

    We see here that we want precisely the apartness check implemented within
    tcUnifyTysFG. So that's what we do! Two types cannot match if they are surely
    apart. Note that since we are simply dropping dead code, a conservative test
    suffices.
    -}

    -- | Given a list of pairs of types, are any two members of a pair surely
    -- apart, even after arbitrary type function evaluation and substitution?
    typesCantMatch :: [(Type,Type)] -> Bool
    -- See Note [Pruning dead case alternatives]
    typesCantMatch prs = any (uncurry cant_match) prs
      where
        cant_match :: Type -> Type -> Bool
        cant_match t1 t2 = case tcUnifyTysFG (const BindMe) [t1] [t2] of
          SurelyApart -> True
          _           -> False

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

    Note [Fine-grained unification]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Do the types (x, x) and ([y], y) unify? The answer is seemingly "no" --
    no substitution to finite types makes these match. But, a substitution to
    *infinite* types can unify these two types: [x |-> [[[...]]], y |-> [[[...]]] ].
    Why do we care? Consider these two type family instances:

    type instance F x x   = Int
    type instance F [y] y = Bool

    If we also have

    type instance Looper = [Looper]

    then the instances potentially overlap. The solution is to use unification
    over infinite terms. This is possible (see [1] for lots of gory details), but
    a full algorithm is a little more power than we need. Instead, we make a
    conservative approximation and just omit the occurs check.

    [1]: http://research.microsoft.com/en-us/um/people/simonpj/papers/ext-f/axioms-extended.pdf

    tcUnifyTys considers an occurs-check problem as the same as general unification
    failure.

    tcUnifyTysFG ("fine-grained") returns one of three results: success, occurs-check
    failure ("MaybeApart"), or general failure ("SurelyApart").

    See also Trac #8162.

    It's worth noting that unification in the presence of infinite types is not
    complete. This means that, sometimes, a closed type family does not reduce
    when it should. See test case indexed-types/should_fail/Overlap15 for an
    example.

    Note [The substitution in MaybeApart]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The constructor MaybeApart carries data with it, typically a TvSubstEnv. Why?
    Because consider unifying these:

    (a, a, Int) ~ (b, [b], Bool)

    If we go left-to-right, we start with [a |-> b]. Then, on the middle terms, we
    apply the subst we have so far and discover that we need [b |-> [b]]. Because
    this fails the occurs check, we say that the types are MaybeApart (see above
    Note [Fine-grained unification]). But, we can't stop there! Because if we
    continue, we discover that Int is SurelyApart from Bool, and therefore the
    types are apart. This has practical consequences for the ability for closed
    type family applications to reduce. See test case
    indexed-types/should_compile/Overlap14.

    Note [Unifying with skolems]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If we discover that two types unify if and only if a skolem variable is
    substituted, we can't properly unify the types. But, that skolem variable
    may later be instantiated with a unifyable type. So, we return maybeApart
    in these cases.

    Note [Lists of different lengths are MaybeApart]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    It is unusual to call tcUnifyTys or tcUnifyTysFG with lists of different
    lengths. The place where we know this can happen is from compatibleBranches in
    FamInstEnv, when checking data family instances. Data family instances may be
    eta-reduced; see Note [Eta reduction for data family axioms] in TcInstDcls.

    We wish to say that

      D :: * -> * -> *
      axDF1 :: D Int ~ DFInst1
      axDF2 :: D Int Bool ~ DFInst2

    overlap. If we conclude that lists of different lengths are SurelyApart, then
    it will look like these do *not* overlap, causing disaster. See Trac #9371.

    In usages of tcUnifyTys outside of family instances, we always use tcUnifyTys,
    which can't tell the difference between MaybeApart and SurelyApart, so those
    usages won't notice this design choice.
    -}

    -- | Simple unification of two types; all type variables are bindable
    -- Precondition: the kinds are already equal
    tcUnifyTy :: Type -> Type       -- All tyvars are bindable
              -> Maybe TCvSubst
                           -- A regular one-shot (idempotent) substitution
    tcUnifyTy t1 t2 = tcUnifyTys (const BindMe) [t1] [t2]

    -- | Like 'tcUnifyTy', but also unifies the kinds
    tcUnifyTyKi :: Type -> Type -> Maybe TCvSubst
    tcUnifyTyKi t1 t2 = tcUnifyTyKis (const BindMe) [t1] [t2]

    -- | Unify two types, treating type family applications as possibly unifying
    -- with anything and looking through injective type family applications.
    -- Precondition: kinds are the same
    tcUnifyTyWithTFs :: Bool  -- ^ True <=> do two-way unification;
                              --   False <=> do one-way matching.
                              --   See end of sec 5.2 from the paper
                     -> Type -> Type -> Maybe TCvSubst
    -- This algorithm is an implementation of the "Algorithm U" presented in
    -- the paper "Injective type families for Haskell", Figures 2 and 3.
    -- The code is incorporated with the standard unifier for convenience, but
    -- its operation should match the specification in the paper.
    tcUnifyTyWithTFs twoWay t1 t2
      = case tc_unify_tys (const BindMe) twoWay True False
                           rn_env emptyTvSubstEnv emptyCvSubstEnv
                           [t1] [t2] of
          Unifiable  (subst, _) -> Just $ niFixTCvSubst subst
          MaybeApart (subst, _) -> Just $ niFixTCvSubst subst
          -- we want to *succeed* in questionable cases. This is a
          -- pre-unification algorithm.
          SurelyApart      -> Nothing
      where
        rn_env = mkRnEnv2 $ mkInScopeSet $ tyCoVarsOfTypes [t1, t2]

    -----------------
    tcUnifyTys :: (TyCoVar -> BindFlag)
               -> [Type] -> [Type]
               -> Maybe TCvSubst
                                    -- ^ A regular one-shot (idempotent) substitution
                                    -- that unifies the erased types. See comments
                                    -- for 'tcUnifyTysFG'

    -- The two types may have common type variables, and indeed do so in the
    -- second call to tcUnifyTys in FunDeps.checkClsFD
    tcUnifyTys bind_fn tys1 tys2
      = case tcUnifyTysFG bind_fn tys1 tys2 of
          Unifiable result -> Just result
          _                -> Nothing

    -- | Like 'tcUnifyTys' but also unifies the kinds
    tcUnifyTyKis :: (TyCoVar -> BindFlag)
                 -> [Type] -> [Type]
                 -> Maybe TCvSubst
    tcUnifyTyKis bind_fn tys1 tys2
      = case tcUnifyTyKisFG bind_fn tys1 tys2 of
          Unifiable result -> Just result
          _                -> Nothing

    -- This type does double-duty. It is used in the UM (unifier monad) and to
    -- return the final result. See Note [Fine-grained unification]
    type UnifyResult = UnifyResultM TCvSubst
    data UnifyResultM a = Unifiable a        -- the subst that unifies the types
                        | MaybeApart a       -- the subst has as much as we know
                                             -- it must be part of a most general unifier
                                             -- See Note [The substitution in MaybeApart]
                        | SurelyApart
                        deriving Functor

    instance Applicative UnifyResultM where
      pure  = Unifiable
      (<*>) = ap

    instance Monad UnifyResultM where

      SurelyApart  >>= _ = SurelyApart
      MaybeApart x >>= f = case f x of
                             Unifiable y -> MaybeApart y
                             other       -> other
      Unifiable x  >>= f = f x

    instance Alternative UnifyResultM where
      empty = SurelyApart

      a@(Unifiable {})  <|> _                 = a
      _                 <|> b@(Unifiable {})  = b
      a@(MaybeApart {}) <|> _                 = a
      _                 <|> b@(MaybeApart {}) = b
      SurelyApart       <|> SurelyApart       = SurelyApart

    instance MonadPlus UnifyResultM

    -- | @tcUnifyTysFG bind_tv tys1 tys2@ attepts to find a substitution @s@ (whose
    -- domain elements all respond 'BindMe' to @bind_tv@) such that
    -- @s(tys1)@ and that of @s(tys2)@ are equal, as witnessed by the returned
    -- Coercions. This version requires that the kinds of the types are the same,
    -- if you unify left-to-right.
    tcUnifyTysFG :: (TyVar -> BindFlag)
                 -> [Type] -> [Type]
                 -> UnifyResult
    tcUnifyTysFG bind_fn tys1 tys2
      = tc_unify_tys_fg False bind_fn tys1 tys2

    tcUnifyTyKisFG :: (TyVar -> BindFlag)
                   -> [Type] -> [Type]
                   -> UnifyResult
    tcUnifyTyKisFG bind_fn tys1 tys2
      = tc_unify_tys_fg True bind_fn tys1 tys2

    tc_unify_tys_fg :: Bool
                    -> (TyVar -> BindFlag)
                    -> [Type] -> [Type]
                    -> UnifyResult
    tc_unify_tys_fg match_kis bind_fn tys1 tys2
      = do { (env, _) <- tc_unify_tys bind_fn True False match_kis env
                                      emptyTvSubstEnv emptyCvSubstEnv
                                      tys1 tys2
           ; return $ niFixTCvSubst env }
      where
        vars = tyCoVarsOfTypes tys1 `unionVarSet` tyCoVarsOfTypes tys2
        env  = mkRnEnv2 $ mkInScopeSet vars

    -- | This function is actually the one to call the unifier -- a little
    -- too general for outside clients, though.
    tc_unify_tys :: (TyVar -> BindFlag)
                 -> AmIUnifying -- ^ True <=> unify; False <=> match
                 -> Bool        -- ^ True <=> doing an injectivity check
                 -> Bool        -- ^ True <=> treat the kinds as well
                 -> RnEnv2
                 -> TvSubstEnv  -- ^ substitution to extend
                 -> CvSubstEnv
                 -> [Type] -> [Type]
                 -> UnifyResultM (TvSubstEnv, CvSubstEnv)
    -- NB: It's tempting to ASSERT here that, if we're not matching kinds, then
    -- the kinds of the types should be the same. However, this doesn't work,
    -- as the types may be a dependent telescope, where later types have kinds
    -- that mention variables occuring earlier in the list of types. Here's an
    -- example (from typecheck/should_fail/T12709):
    --   template: [rep :: RuntimeRep,       a :: TYPE rep]
    --   target:   [LiftedRep :: RuntimeRep, Int :: TYPE LiftedRep]
    -- We can see that matching the first pair will make the kinds of the second
    -- pair equal. Yet, we still don't need a separate pass to unify the kinds
    -- of these types, so it's appropriate to use the Ty variant of unification.
    -- See also Note [tcMatchTy vs tcMatchTyKi].
    tc_unify_tys bind_fn unif inj_check match_kis rn_env tv_env cv_env tys1 tys2
      = initUM tv_env cv_env $
        do { when match_kis $
             unify_tys env kis1 kis2
           ; unify_tys env tys1 tys2
           ; (,) <$> getTvSubstEnv <*> getCvSubstEnv }
      where
        env = UMEnv { um_bind_fun = bind_fn
                    , um_skols    = emptyVarSet
                    , um_unif     = unif
                    , um_inj_tf   = inj_check
                    , um_rn_env   = rn_env }

        kis1 = map typeKind tys1
        kis2 = map typeKind tys2

    instance Outputable a => Outputable (UnifyResultM a) where
      ppr SurelyApart    = text "SurelyApart"
      ppr (Unifiable x)  = text "Unifiable" <+> ppr x
      ppr (MaybeApart x) = text "MaybeApart" <+> ppr x

    {-
    ************************************************************************
    *                                                                      *
                    Non-idempotent substitution
    *                                                                      *
    ************************************************************************

    Note [Non-idempotent substitution]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    During unification we use a TvSubstEnv/CvSubstEnv pair that is
      (a) non-idempotent
      (b) loop-free; ie repeatedly applying it yields a fixed point

    Note [Finding the substitution fixpoint]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Finding the fixpoint of a non-idempotent substitution arising from a
    unification is much trickier than it looks, because of kinds.  Consider
       T k (H k (f:k)) ~ T * (g:*)
    If we unify, we get the substitution
       [ k -> *
       , g -> H k (f:k) ]
    To make it idempotent we don't want to get just
       [ k -> *
       , g -> H * (f:k) ]
    We also want to substitute inside f's kind, to get
       [ k -> *
       , g -> H k (f:*) ]
    If we don't do this, we may apply the substitution to something,
    and get an ill-formed type, i.e. one where typeKind will fail.
    This happened, for example, in Trac #9106.

    It gets worse.  In Trac #14164 we wanted to take the fixpoint of
    this substitution
       [ xs_asV :-> F a_aY6 (z_aY7 :: a_aY6)
                            (rest_aWF :: G a_aY6 (z_aY7 :: a_aY6))
       , a_aY6  :-> a_aXQ ]

    We have to apply the substitution for a_aY6 two levels deep inside
    the invocation of F!  We don't have a function that recursively
    applies substitutions inside the kinds of variable occurrences (and
    probably rightly so).

    So, we work as follows:

     1. Start with the current substitution (which we are
        trying to fixpoint
           [ xs :-> F a (z :: a) (rest :: G a (z :: a))
           , a  :-> b ]

     2. Take all the free vars of the range of the substitution:
           {a, z, rest, b}
        NB: the free variable finder closes over
        the kinds of variable occurrences

     3. If none are in the domain of the substitution, stop.
        We have found a fixpoint.

     4. Remove the variables that are bound by the substitution, leaving
           {z, rest, b}

     5. Do a topo-sort to put them in dependency order:
           [ b :: *, z :: a, rest :: G a z ]

     6. Apply the substitution left-to-right to the kinds of these
        tyvars, extending it each time with a new binding, so we
        finish up with
           [ xs   :-> ..as before..
           , a    :-> b
           , b    :-> b    :: *
           , z    :-> z    :: b
           , rest :-> rest :: G b (z :: b) ]
        Note that rest now has the right kind

     7. Apply this extended substitution (once) to the range of
        the /original/ substitution.  (Note that we do the
        extended substitution would go on forever if you tried
        to find its fixpoint, because it maps z to z.)

     8. And go back to step 1

    In Step 6 we use the free vars from Step 2 as the initial
    in-scope set, because all of those variables appear in the
    range of the substitution, so they must all be in the in-scope
    set.  But NB that the type substitution engine does not look up
    variables in the in-scope set; it is used only to ensure no
    shadowing.
    -}

    niFixTCvSubst :: TvSubstEnv -> TCvSubst
    -- Find the idempotent fixed point of the non-idempotent substitution
    -- This is surprisingly tricky:
    --   see Note [Finding the substitution fixpoint]
    -- ToDo: use laziness instead of iteration?
    niFixTCvSubst tenv
      | not_fixpoint = niFixTCvSubst (mapVarEnv (substTy subst) tenv)
      | otherwise    = subst
      where
        range_fvs :: FV
        range_fvs = tyCoFVsOfTypes (nonDetEltsUFM tenv)
              -- It's OK to use nonDetEltsUFM here because the
              -- order of range_fvs, range_tvs is immaterial

        range_tvs :: [TyVar]
        range_tvs = fvVarList range_fvs

        not_fixpoint  = any in_domain range_tvs
        in_domain tv  = tv `elemVarEnv` tenv

        free_tvs = toposortTyVars (filterOut in_domain range_tvs)

        -- See Note [Finding the substitution fixpoint], Step 6
        init_in_scope = mkInScopeSet (fvVarSet range_fvs)
        subst = foldl add_free_tv
                      (mkTvSubst init_in_scope tenv)
                      free_tvs

        add_free_tv :: TCvSubst -> TyVar -> TCvSubst
        add_free_tv subst tv
          = extendTvSubst subst tv (mkTyVarTy tv')
         where
            tv' = updateTyVarKind (substTy subst) tv

    niSubstTvSet :: TvSubstEnv -> TyCoVarSet -> TyCoVarSet
    -- Apply the non-idempotent substitution to a set of type variables,
    -- remembering that the substitution isn't necessarily idempotent
    -- This is used in the occurs check, before extending the substitution
    niSubstTvSet tsubst tvs
      = nonDetFoldUniqSet (unionVarSet . get) emptyVarSet tvs
      -- It's OK to nonDetFoldUFM here because we immediately forget the
      -- ordering by creating a set.
      where
        get tv
          | Just ty <- lookupVarEnv tsubst tv
          = niSubstTvSet tsubst (tyCoVarsOfType ty)

          | otherwise
          = unitVarSet tv

    {-
    ************************************************************************
    *                                                                      *
                    unify_ty: the main workhorse
    *                                                                      *
    ************************************************************************

    Note [Specification of unification]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The pure unifier, unify_ty, defined in this module, tries to work out
    a substitution to make two types say True to eqType. NB: eqType is
    itself not purely syntactic; it accounts for CastTys;
    see Note [Non-trivial definitional equality] in TyCoRep

    Unlike the "impure unifiers" in the typechecker (the eager unifier in
    TcUnify, and the constraint solver itself in TcCanonical), the pure
    unifier It does /not/ work up to ~.

    The algorithm implemented here is rather delicate, and we depend on it
    to uphold certain properties. This is a summary of these required
    properties. Any reference to "flattening" refers to the flattening
    algorithm in FamInstEnv (See Note [Flattening] in FamInstEnv), not
    the flattening algorithm in the solver.

    Notation:
     θ,φ    substitutions
     ξ    type-function-free types
     τ,σ  other types
     τ♭   type τ, flattened

     ≡    eqType

    (U1) Soundness.
         If (unify τ₁ τ₂) = Unifiable θ, then θ(τ₁) ≡ θ(τ₂).
         θ is a most general unifier for τ₁ and τ₂.

    (U2) Completeness.
         If (unify ξ₁ ξ₂) = SurelyApart,
         then there exists no substitution θ such that θ(ξ₁) ≡ θ(ξ₂).

    These two properties are stated as Property 11 in the "Closed Type Families"
    paper (POPL'14). Below, this paper is called [CTF].

    (U3) Apartness under substitution.
         If (unify ξ τ♭) = SurelyApart, then (unify ξ θ(τ)♭) = SurelyApart,
         for any θ. (Property 12 from [CTF])

    (U4) Apart types do not unify.
         If (unify ξ τ♭) = SurelyApart, then there exists no θ
         such that θ(ξ) = θ(τ). (Property 13 from [CTF])

    THEOREM. Completeness w.r.t ~
        If (unify τ₁♭ τ₂♭) = SurelyApart,
        then there exists no proof that (τ₁ ~ τ₂).

    PROOF. See appendix of [CTF].

    The unification algorithm is used for type family injectivity, as described
    in the "Injective Type Families" paper (Haskell'15), called [ITF]. When run
    in this mode, it has the following properties.

    (I1) If (unify σ τ) = SurelyApart, then σ and τ are not unifiable, even
         after arbitrary type family reductions. Note that σ and τ are
         not flattened here.

    (I2) If (unify σ τ) = MaybeApart θ, and if some
         φ exists such that φ(σ) ~ φ(τ), then φ extends θ.

    Furthermore, the RULES matching algorithm requires this property,
    but only when using this algorithm for matching:

    (M1) If (match σ τ) succeeds with θ, then all matchable tyvars
         in σ are bound in θ.

         Property M1 means that we must extend the substitution with,
         say (a ↦ a) when appropriate during matching.
         See also Note [Self-substitution when matching].

    (M2) Completeness of matching.
         If θ(σ) = τ, then (match σ τ) = Unifiable φ,
         where θ is an extension of φ.

    Sadly, property M2 and I2 conflict. Consider

    type family F1 a b where
      F1 Int    Bool   = Char
      F1 Double String = Char

    Consider now two matching problems:

    P1. match (F1 a Bool) (F1 Int Bool)
    P2. match (F1 a Bool) (F1 Double String)

    In case P1, we must find (a ↦ Int) to satisfy M2.
    In case P2, we must /not/ find (a ↦ Double), in order to satisfy I2. (Note
    that the correct mapping for I2 is (a ↦ Int). There is no way to discover
    this, but we musn't map a to anything else!)

    We thus must parameterize the algorithm over whether it's being used
    for an injectivity check (refrain from looking at non-injective arguments
    to type families) or not (do indeed look at those arguments).  This is
    implemented  by the uf_inj_tf field of UmEnv.

    (It's all a question of whether or not to include equation (7) from Fig. 2
    of [ITF].)

    This extra parameter is a bit fiddly, perhaps, but seemingly less so than
    having two separate, almost-identical algorithms.

    Note [Self-substitution when matching]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    What should happen when we're *matching* (not unifying) a1 with a1? We
    should get a substitution [a1 |-> a1]. A successful match should map all
    the template variables (except ones that disappear when expanding synonyms).
    But when unifying, we don't want to do this, because we'll then fall into
    a loop.

    This arrangement affects the code in three places:
     - If we're matching a refined template variable, don't recur. Instead, just
       check for equality. That is, if we know [a |-> Maybe a] and are matching
       (a ~? Maybe Int), we want to just fail.

     - Skip the occurs check when matching. This comes up in two places, because
       matching against variables is handled separately from matching against
       full-on types.

    Note that this arrangement was provoked by a real failure, where the same
    unique ended up in the template as in the target. (It was a rule firing when
    compiling Data.List.NonEmpty.)

    Note [Matching coercion variables]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider this:

       type family F a

       data G a where
         MkG :: F a ~ Bool => G a

       type family Foo (x :: G a) :: F a
       type instance Foo MkG = False

    We would like that to be accepted. For that to work, we need to introduce
    a coercion variable on the left and then use it on the right. Accordingly,
    at use sites of Foo, we need to be able to use matching to figure out the
    value for the coercion. (See the desugared version:

       axFoo :: [a :: *, c :: F a ~ Bool]. Foo (MkG c) = False |> (sym c)

    ) We never want this action to happen during *unification* though, when
    all bets are off.

    Note [Kind coercions in Unify]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We wish to match/unify while ignoring casts. But, we can't just ignore
    them completely, or we'll end up with ill-kinded substitutions. For example,
    say we're matching `a` with `ty |> co`. If we just drop the cast, we'll
    return [a |-> ty], but `a` and `ty` might have different kinds. We can't
    just match/unify their kinds, either, because this might gratuitously
    fail. After all, `co` is the witness that the kinds are the same -- they
    may look nothing alike.

    So, we pass a kind coercion to the match/unify worker. This coercion witnesses
    the equality between the substed kind of the left-hand type and the substed
    kind of the right-hand type. Note that we do not unify kinds at the leaves
    (as we did previously). We thus have

    INVARIANT: In the call
        unify_ty ty1 ty2 kco
    it must be that subst(kco) :: subst(kind(ty1)) ~N subst(kind(ty2)), where
    `subst` is the ambient substitution in the UM monad.

    To get this coercion, we first have to match/unify
    the kinds before looking at the types. Happily, we need look only one level
    up, as all kinds are guaranteed to have kind *.

    When we're working with type applications (either TyConApp or AppTy) we
    need to worry about establishing INVARIANT, as the kinds of the function
    & arguments aren't (necessarily) included in the kind of the result.
    When unifying two TyConApps, this is easy, because the two TyCons are
    the same. Their kinds are thus the same. As long as we unify left-to-right,
    we'll be sure to unify types' kinds before the types themselves. (For example,
    think about Proxy :: forall k. k -> *. Unifying the first args matches up
    the kinds of the second args.)

    For AppTy, we must unify the kinds of the functions, but once these are
    unified, we can continue unifying arguments without worrying further about
    kinds.

    The interface to this module includes both "...Ty" functions and
    "...TyKi" functions. The former assume that INVARIANT is already
    established, either because the kinds are the same or because the
    list of types being passed in are the well-typed arguments to some
    type constructor (see two paragraphs above). The latter take a separate
    pre-pass over the kinds to establish INVARIANT. Sometimes, it's important
    not to take the second pass, as it caused #12442.

    We thought, at one point, that this was all unnecessary: why should
    casts be in types in the first place? But they are sometimes. In
    dependent/should_compile/KindEqualities2, we see, for example the
    constraint Num (Int |> (blah ; sym blah)).  We naturally want to find
    a dictionary for that constraint, which requires dealing with
    coercions in this manner.

    Note [Matching in the presence of casts]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When matching, it is crucial that no variables from the template
    end up in the range of the matching substitution (obviously!).
    When unifying, that's not a constraint; instead we take the fixpoint
    of the substitution at the end.

    So what should we do with this, when matching?
       unify_ty (tmpl |> co) tgt kco

    Previously, wrongly, we pushed 'co' in the (horrid) accumulating
    'kco' argument like this:
       unify_ty (tmpl |> co) tgt kco
         = unify_ty tmpl tgt (kco ; co)

    But that is obviously wrong because 'co' (from the template) ends
    up in 'kco', which in turn ends up in the range of the substitution.

    This all came up in Trac #13910.  Because we match tycon arguments
    left-to-right, the ambient substitution will already have a matching
    substitution for any kinds; so there is an easy fix: just apply
    the substitution-so-far to the coercion from the LHS.

    Note that

    * When matching, the first arg of unify_ty is always the template;
      we never swap round.

    * The above argument is distressingly indirect. We seek a
      better way.

    * One better way is to ensure that type patterns (the template
      in the matching process) have no casts.  See Trac #14119.

    -}

    -------------- unify_ty: the main workhorse -----------

    type AmIUnifying = Bool   -- True  <=> Unifying
                              -- False <=> Matching

    unify_ty :: UMEnv
             -> Type -> Type  -- Types to be unified and a co
             -> CoercionN     -- A coercion between their kinds
                              -- See Note [Kind coercions in Unify]
             -> UM ()
    -- See Note [Specification of unification]
    -- Respects newtypes, PredTypes

    unify_ty env ty1 ty2 kco
        -- TODO: More commentary needed here
      | Just ty1' <- tcView ty1   = unify_ty env ty1' ty2 kco
      | Just ty2' <- tcView ty2   = unify_ty env ty1 ty2' kco
      | CastTy ty1' co <- ty1     = if um_unif env
                                    then unify_ty env ty1' ty2 (co `mkTransCo` kco)
                                    else -- See Note [Matching in the presence of casts]
                                         do { subst <- getSubst env
                                            ; let co' = substCo subst co
                                            ; unify_ty env ty1' ty2 (co' `mkTransCo` kco) }
      | CastTy ty2' co <- ty2     = unify_ty env ty1 ty2' (kco `mkTransCo` mkSymCo co)

    unify_ty env (TyVarTy tv1) ty2 kco
      = uVar env tv1 ty2 kco
    unify_ty env ty1 (TyVarTy tv2) kco
      | um_unif env  -- If unifying, can swap args
      = uVar (umSwapRn env) tv2 ty1 (mkSymCo kco)

    unify_ty env ty1 ty2 _kco
      | Just (tc1, tys1) <- mb_tc_app1
      , Just (tc2, tys2) <- mb_tc_app2
      , tc1 == tc2 || (tcIsLiftedTypeKind ty1 && tcIsLiftedTypeKind ty2)
      = if isInjectiveTyCon tc1 Nominal
        then unify_tys env tys1 tys2
        else do { let inj | isTypeFamilyTyCon tc1
                          = case tyConInjectivityInfo tc1 of
                                   NotInjective -> repeat False
                                   Injective bs -> bs
                          | otherwise
                          = repeat False

                      (inj_tys1, noninj_tys1) = partitionByList inj tys1
                      (inj_tys2, noninj_tys2) = partitionByList inj tys2

                ; unify_tys env inj_tys1 inj_tys2
                ; unless (um_inj_tf env) $ -- See (end of) Note [Specification of unification]
                  don'tBeSoSure $ unify_tys env noninj_tys1 noninj_tys2 }

      | Just (tc1, _) <- mb_tc_app1
      , not (isGenerativeTyCon tc1 Nominal)
        -- E.g.   unify_ty (F ty1) b  =  MaybeApart
        --        because the (F ty1) behaves like a variable
        --        NB: if unifying, we have already dealt
        --            with the 'ty2 = variable' case
      = maybeApart

      | Just (tc2, _) <- mb_tc_app2
      , not (isGenerativeTyCon tc2 Nominal)
      , um_unif env
        -- E.g.   unify_ty [a] (F ty2) =  MaybeApart, when unifying (only)
        --        because the (F ty2) behaves like a variable
        --        NB: we have already dealt with the 'ty1 = variable' case
      = maybeApart

      where
        mb_tc_app1 = tcSplitTyConApp_maybe ty1
        mb_tc_app2 = tcSplitTyConApp_maybe ty2

            -- Applications need a bit of care!
            -- They can match FunTy and TyConApp, so use splitAppTy_maybe
            -- NB: we've already dealt with type variables,
            -- so if one type is an App the other one jolly well better be too
    unify_ty env (AppTy ty1a ty1b) ty2 _kco
      | Just (ty2a, ty2b) <- tcRepSplitAppTy_maybe ty2
      = unify_ty_app env ty1a [ty1b] ty2a [ty2b]

    unify_ty env ty1 (AppTy ty2a ty2b) _kco
      | Just (ty1a, ty1b) <- tcRepSplitAppTy_maybe ty1
      = unify_ty_app env ty1a [ty1b] ty2a [ty2b]

    unify_ty _ (LitTy x) (LitTy y) _kco | x == y = return ()

    unify_ty env (ForAllTy (TvBndr tv1 _) ty1) (ForAllTy (TvBndr tv2 _) ty2) kco
      = do { unify_ty env (tyVarKind tv1) (tyVarKind tv2) (mkNomReflCo liftedTypeKind)
           ; let env' = umRnBndr2 env tv1 tv2
           ; unify_ty env' ty1 ty2 kco }

    -- See Note [Matching coercion variables]
    unify_ty env (CoercionTy co1) (CoercionTy co2) kco
      = do { c_subst <- getCvSubstEnv
           ; case co1 of
               CoVarCo cv
                 | not (um_unif env)
                 , not (cv `elemVarEnv` c_subst)
                 , BindMe <- tvBindFlag env cv
                 -> do { checkRnEnv env (tyCoVarsOfCo co2)
                       ; let (co_l, co_r) = decomposeFunCo Nominal kco
                          -- cv :: t1 ~ t2
                          -- co2 :: s1 ~ s2
                          -- co_l :: t1 ~ s1
                          -- co_r :: t2 ~ s2
                       ; extendCvEnv cv (co_l `mkTransCo`
                                         co2 `mkTransCo`
                                         mkSymCo co_r) }
               _ -> return () }

    unify_ty _ _ _ _ = surelyApart

    unify_ty_app :: UMEnv -> Type -> [Type] -> Type -> [Type] -> UM ()
    unify_ty_app env ty1 ty1args ty2 ty2args
      | Just (ty1', ty1a) <- repSplitAppTy_maybe ty1
      , Just (ty2', ty2a) <- repSplitAppTy_maybe ty2
      = unify_ty_app env ty1' (ty1a : ty1args) ty2' (ty2a : ty2args)

      | otherwise
      = do { let ki1 = typeKind ty1
                 ki2 = typeKind ty2
               -- See Note [Kind coercions in Unify]
           ; unify_ty  env ki1 ki2 (mkNomReflCo liftedTypeKind)
           ; unify_ty  env ty1 ty2 (mkNomReflCo ki1)
           ; unify_tys env ty1args ty2args }

    unify_tys :: UMEnv -> [Type] -> [Type] -> UM ()
    unify_tys env orig_xs orig_ys
      = go orig_xs orig_ys
      where
        go []     []     = return ()
        go (x:xs) (y:ys)
          -- See Note [Kind coercions in Unify]
          = do { unify_ty env x y (mkNomReflCo $ typeKind x)
               ; go xs ys }
        go _ _ = maybeApart  -- See Note [Lists of different lengths are MaybeApart]

    ---------------------------------
    uVar :: UMEnv
         -> InTyVar         -- Variable to be unified
         -> Type            -- with this Type
         -> Coercion        -- :: kind tv ~N kind ty
         -> UM ()

    uVar env tv1 ty kco
     = do { -- Apply the ambient renaming
            let tv1' = umRnOccL env tv1

            -- Check to see whether tv1 is refined by the substitution
          ; subst <- getTvSubstEnv
          ; case (lookupVarEnv subst tv1') of
              Just ty' | um_unif env                -- Unifying, so call
                       -> unify_ty env ty' ty kco   -- back into unify
                       | otherwise
                       -> -- Matching, we don't want to just recur here.
                          -- this is because the range of the subst is the target
                          -- type, not the template type. So, just check for
                          -- normal type equality.
                          guard ((ty' `mkCastTy` kco) `eqType` ty)
              Nothing  -> uUnrefined env tv1' ty ty kco } -- No, continue

    uUnrefined :: UMEnv
               -> OutTyVar          -- variable to be unified
               -> Type              -- with this Type
               -> Type              -- (version w/ expanded synonyms)
               -> Coercion          -- :: kind tv ~N kind ty
               -> UM ()

    -- We know that tv1 isn't refined

    uUnrefined env tv1' ty2 ty2' kco
      | Just ty2'' <- coreView ty2'
      = uUnrefined env tv1' ty2 ty2'' kco    -- Unwrap synonyms
                    -- This is essential, in case we have
                    --      type Foo a = a
                    -- and then unify a ~ Foo a

      | TyVarTy tv2 <- ty2'
      = do { let tv2' = umRnOccR env tv2
           ; unless (tv1' == tv2' && um_unif env) $ do
               -- If we are unifying a ~ a, just return immediately
               -- Do not extend the substitution
               -- See Note [Self-substitution when matching]

              -- Check to see whether tv2 is refined
           { subst <- getTvSubstEnv
           ; case lookupVarEnv subst tv2 of
             {  Just ty' | um_unif env -> uUnrefined env tv1' ty' ty' kco
             ;  _ ->

        do {   -- So both are unrefined
               -- Bind one or the other, depending on which is bindable
           ; let b1  = tvBindFlag env tv1'
                 b2  = tvBindFlag env tv2'
                 ty1 = mkTyVarTy tv1'
           ; case (b1, b2) of
               (BindMe, _) -> bindTv env tv1' (ty2 `mkCastTy` mkSymCo kco)
               (_, BindMe) | um_unif env
                           -> bindTv (umSwapRn env) tv2 (ty1 `mkCastTy` kco)

               _ | tv1' == tv2' -> return ()
                 -- How could this happen? If we're only matching and if
                 -- we're comparing forall-bound variables.

               _ -> maybeApart -- See Note [Unification with skolems]
      }}}}

    uUnrefined env tv1' ty2 _ kco -- ty2 is not a type variable
      = case tvBindFlag env tv1' of
          Skolem -> maybeApart  -- See Note [Unification with skolems]
          BindMe -> bindTv env tv1' (ty2 `mkCastTy` mkSymCo kco)

    bindTv :: UMEnv -> OutTyVar -> Type -> UM ()
    -- OK, so we want to extend the substitution with tv := ty
    -- But first, we must do a couple of checks
    bindTv env tv1 ty2
      = do  { let free_tvs2 = tyCoVarsOfType ty2

            -- Make sure tys mentions no local variables
            -- E.g.  (forall a. b) ~ (forall a. [a])
            -- We should not unify b := [a]!
            ; checkRnEnv env free_tvs2

            -- Occurs check, see Note [Fine-grained unification]
            -- Make sure you include 'kco' (which ty2 does) Trac #14846
            ; occurs <- occursCheck env tv1 free_tvs2

            ; if occurs then maybeApart
                        else extendTvEnv tv1 ty2 }

    occursCheck :: UMEnv -> TyVar -> VarSet -> UM Bool
    occursCheck env tv free_tvs
      | um_unif env
      = do { tsubst <- getTvSubstEnv
           ; return (tv `elemVarSet` niSubstTvSet tsubst free_tvs) }

      | otherwise      -- Matching; no occurs check
      = return False   -- See Note [Self-substitution when matching]

    {-
    %************************************************************************
    %*                                                                      *
                    Binding decisions
    *                                                                      *
    ************************************************************************
    -}

    data BindFlag
      = BindMe      -- A regular type variable

      | Skolem      -- This type variable is a skolem constant
                    -- Don't bind it; it only matches itself
      deriving Eq

    {-
    ************************************************************************
    *                                                                      *
                    Unification monad
    *                                                                      *
    ************************************************************************
    -}

    data UMEnv
      = UMEnv { um_unif :: AmIUnifying

              , um_inj_tf :: Bool
                -- Checking for injectivity?
                -- See (end of) Note [Specification of unification]

              , um_rn_env :: RnEnv2
                -- Renaming InTyVars to OutTyVars; this eliminates
                -- shadowing, and lines up matching foralls on the left
                -- and right

              , um_skols :: TyVarSet
                -- OutTyVars bound by a forall in this unification;
                -- Do not bind these in the substitution!
                -- See the function tvBindFlag

              , um_bind_fun :: TyVar -> BindFlag
                -- User-supplied BindFlag function,
                -- for variables not in um_skols
              }

    data UMState = UMState
                       { um_tv_env   :: TvSubstEnv
                       , um_cv_env   :: CvSubstEnv }

    newtype UM a = UM { unUM :: UMState -> UnifyResultM (UMState, a) }

    instance Functor UM where
          fmap = liftM

    instance Applicative UM where
          pure a = UM (\s -> pure (s, a))
          (<*>)  = ap

    instance Monad UM where
      fail     = MonadFail.fail
      m >>= k  = UM (\state ->
                      do { (state', v) <- unUM m state
                         ; unUM (k v) state' })

    -- need this instance because of a use of 'guard' above
    instance Alternative UM where
      empty     = UM (\_ -> Control.Applicative.empty)
      m1 <|> m2 = UM (\state ->
                      unUM m1 state <|>
                      unUM m2 state)

    instance MonadPlus UM

    instance MonadFail.MonadFail UM where
        fail _   = UM (\_ -> SurelyApart) -- failed pattern match

    initUM :: TvSubstEnv  -- subst to extend
           -> CvSubstEnv
           -> UM a -> UnifyResultM a
    initUM subst_env cv_subst_env um
      = case unUM um state of
          Unifiable (_, subst)  -> Unifiable subst
          MaybeApart (_, subst) -> MaybeApart subst
          SurelyApart           -> SurelyApart
      where
        state = UMState { um_tv_env = subst_env
                        , um_cv_env = cv_subst_env }

    tvBindFlag :: UMEnv -> OutTyVar -> BindFlag
    tvBindFlag env tv
      | tv `elemVarSet` um_skols env = Skolem
      | otherwise                    = um_bind_fun env tv

    getTvSubstEnv :: UM TvSubstEnv
    getTvSubstEnv = UM $ \state -> Unifiable (state, um_tv_env state)

    getCvSubstEnv :: UM CvSubstEnv
    getCvSubstEnv = UM $ \state -> Unifiable (state, um_cv_env state)

    getSubst :: UMEnv -> UM TCvSubst
    getSubst env = do { tv_env <- getTvSubstEnv
                      ; cv_env <- getCvSubstEnv
                      ; let in_scope = rnInScopeSet (um_rn_env env)
                      ; return (mkTCvSubst in_scope (tv_env, cv_env)) }

    extendTvEnv :: TyVar -> Type -> UM ()
    extendTvEnv tv ty = UM $ \state ->
      Unifiable (state { um_tv_env = extendVarEnv (um_tv_env state) tv ty }, ())

    extendCvEnv :: CoVar -> Coercion -> UM ()
    extendCvEnv cv co = UM $ \state ->
      Unifiable (state { um_cv_env = extendVarEnv (um_cv_env state) cv co }, ())

    umRnBndr2 :: UMEnv -> TyCoVar -> TyCoVar -> UMEnv
    umRnBndr2 env v1 v2
      = env { um_rn_env = rn_env', um_skols = um_skols env `extendVarSet` v' }
      where
        (rn_env', v') = rnBndr2_var (um_rn_env env) v1 v2

    checkRnEnv :: UMEnv -> VarSet -> UM ()
    checkRnEnv env varset
      | isEmptyVarSet skol_vars           = return ()
      | varset `disjointVarSet` skol_vars = return ()
      | otherwise                         = maybeApart
                   -- ToDo: why MaybeApart?
                   -- I think SurelyApart would be right
      where
        skol_vars = um_skols env
        -- NB: That isEmptyVarSet guard is a critical optimization;
        -- it means we don't have to calculate the free vars of
        -- the type, often saving quite a bit of allocation.

    -- | Converts any SurelyApart to a MaybeApart
    don'tBeSoSure :: UM () -> UM ()
    don'tBeSoSure um = UM $ \ state ->
      case unUM um state of
        SurelyApart -> MaybeApart (state, ())
        other       -> other

    umRnOccL :: UMEnv -> TyVar -> TyVar
    umRnOccL env v = rnOccL (um_rn_env env) v

    umRnOccR :: UMEnv -> TyVar -> TyVar
    umRnOccR env v = rnOccR (um_rn_env env) v

    umSwapRn :: UMEnv -> UMEnv
    umSwapRn env = env { um_rn_env = rnSwap (um_rn_env env) }

    maybeApart :: UM ()
    maybeApart = UM (\state -> MaybeApart (state, ()))

    surelyApart :: UM a
    surelyApart = UM (\_ -> SurelyApart)

    {-
    %************************************************************************
    %*                                                                      *
                Matching a (lifted) type against a coercion
    %*                                                                      *
    %************************************************************************

    This section defines essentially an inverse to liftCoSubst. It is defined
    here to avoid a dependency from Coercion on this module.

    -}

    data MatchEnv = ME { me_tmpls :: TyVarSet
                       , me_env   :: RnEnv2 }

    -- | 'liftCoMatch' is sort of inverse to 'liftCoSubst'.  In particular, if
    --   @liftCoMatch vars ty co == Just s@, then @listCoSubst s ty == co@,
    --   where @==@ there means that the result of 'liftCoSubst' has the same
    --   type as the original co; but may be different under the hood.
    --   That is, it matches a type against a coercion of the same
    --   "shape", and returns a lifting substitution which could have been
    --   used to produce the given coercion from the given type.
    --   Note that this function is incomplete -- it might return Nothing
    --   when there does indeed exist a possible lifting context.
    --
    -- This function is incomplete in that it doesn't respect the equality
    -- in `eqType`. That is, it's possible that this will succeed for t1 and
    -- fail for t2, even when t1 `eqType` t2. That's because it depends on
    -- there being a very similar structure between the type and the coercion.
    -- This incompleteness shouldn't be all that surprising, especially because
    -- it depends on the structure of the coercion, which is a silly thing to do.
    --
    -- The lifting context produced doesn't have to be exacting in the roles
    -- of the mappings. This is because any use of the lifting context will
    -- also require a desired role. Thus, this algorithm prefers mapping to
    -- nominal coercions where it can do so.
    liftCoMatch :: TyCoVarSet -> Type -> Coercion -> Maybe LiftingContext
    liftCoMatch tmpls ty co
      = do { cenv1 <- ty_co_match menv emptyVarEnv ki ki_co ki_ki_co ki_ki_co
           ; cenv2 <- ty_co_match menv cenv1       ty co
                                  (mkNomReflCo co_lkind) (mkNomReflCo co_rkind)
           ; return (LC (mkEmptyTCvSubst in_scope) cenv2) }
      where
        menv     = ME { me_tmpls = tmpls, me_env = mkRnEnv2 in_scope }
        in_scope = mkInScopeSet (tmpls `unionVarSet` tyCoVarsOfCo co)
        -- Like tcMatchTy, assume all the interesting variables
        -- in ty are in tmpls

        ki       = typeKind ty
        ki_co    = promoteCoercion co
        ki_ki_co = mkNomReflCo liftedTypeKind

        Pair co_lkind co_rkind = coercionKind ki_co

    -- | 'ty_co_match' does all the actual work for 'liftCoMatch'.
    ty_co_match :: MatchEnv   -- ^ ambient helpful info
                -> LiftCoEnv  -- ^ incoming subst
                -> Type       -- ^ ty, type to match
                -> Coercion   -- ^ co, coercion to match against
                -> Coercion   -- ^ :: kind of L type of substed ty ~N L kind of co
                -> Coercion   -- ^ :: kind of R type of substed ty ~N R kind of co
                -> Maybe LiftCoEnv
    ty_co_match menv subst ty co lkco rkco
      | Just ty' <- coreView ty = ty_co_match menv subst ty' co lkco rkco

      -- handle Refl case:
      | tyCoVarsOfType ty `isNotInDomainOf` subst
      , Just (ty', _) <- isReflCo_maybe co
      , ty `eqType` ty'
      = Just subst

      where
        isNotInDomainOf :: VarSet -> VarEnv a -> Bool
        isNotInDomainOf set env
          = noneSet (\v -> elemVarEnv v env) set

        noneSet :: (Var -> Bool) -> VarSet -> Bool
        noneSet f = allVarSet (not . f)

    ty_co_match menv subst ty co lkco rkco
      | CastTy ty' co' <- ty
         -- See Note [Matching in the presence of casts]
      = let empty_subst  = mkEmptyTCvSubst (rnInScopeSet (me_env menv))
            substed_co_l = substCo (liftEnvSubstLeft empty_subst subst)  co'
            substed_co_r = substCo (liftEnvSubstRight empty_subst subst) co'
        in
        ty_co_match menv subst ty' co (substed_co_l `mkTransCo` lkco)
                                      (substed_co_r `mkTransCo` rkco)

      | CoherenceCo co1 co2 <- co
      = ty_co_match menv subst ty co1 (lkco `mkTransCo` mkSymCo co2) rkco

      | SymCo co' <- co
      = swapLiftCoEnv <$> ty_co_match menv (swapLiftCoEnv subst) ty co' rkco lkco

      -- Match a type variable against a non-refl coercion
    ty_co_match menv subst (TyVarTy tv1) co lkco rkco
      | Just co1' <- lookupVarEnv subst tv1' -- tv1' is already bound to co1
      = if eqCoercionX (nukeRnEnvL rn_env) co1' co
        then Just subst
        else Nothing       -- no match since tv1 matches two different coercions

      | tv1' `elemVarSet` me_tmpls menv           -- tv1' is a template var
      = if any (inRnEnvR rn_env) (tyCoVarsOfCoList co)
        then Nothing      -- occurs check failed
        else Just $ extendVarEnv subst tv1' $
                    castCoercionKind co (mkSymCo lkco) (mkSymCo rkco)

      | otherwise
      = Nothing

      where
        rn_env = me_env menv
        tv1' = rnOccL rn_env tv1

      -- just look through SubCo's. We don't really care about roles here.
    ty_co_match menv subst ty (SubCo co) lkco rkco
      = ty_co_match menv subst ty co lkco rkco

    ty_co_match menv subst (AppTy ty1a ty1b) co _lkco _rkco
      | Just (co2, arg2) <- splitAppCo_maybe co     -- c.f. Unify.match on AppTy
      = ty_co_match_app menv subst ty1a [ty1b] co2 [arg2]
    ty_co_match menv subst ty1 (AppCo co2 arg2) _lkco _rkco
      | Just (ty1a, ty1b) <- repSplitAppTy_maybe ty1
           -- yes, the one from Type, not TcType; this is for coercion optimization
      = ty_co_match_app menv subst ty1a [ty1b] co2 [arg2]

    ty_co_match menv subst (TyConApp tc1 tys) (TyConAppCo _ tc2 cos) _lkco _rkco
      = ty_co_match_tc menv subst tc1 tys tc2 cos
    ty_co_match menv subst (FunTy ty1 ty2) co _lkco _rkco
        -- Despite the fact that (->) is polymorphic in four type variables (two
        -- runtime rep and two types), we shouldn't need to explicitly unify the
        -- runtime reps here; unifying the types themselves should be sufficient.
        -- See Note [Representation of function types].
      | Just (tc, [_,_,co1,co2]) <- splitTyConAppCo_maybe co
      , tc == funTyCon
      = let Pair lkcos rkcos = traverse (fmap mkNomReflCo . coercionKind) [co1,co2]
        in ty_co_match_args menv subst [ty1, ty2] [co1, co2] lkcos rkcos

    ty_co_match menv subst (ForAllTy (TvBndr tv1 _) ty1)
                           (ForAllCo tv2 kind_co2 co2)
                           lkco rkco
      = do { subst1 <- ty_co_match menv subst (tyVarKind tv1) kind_co2
                                   ki_ki_co ki_ki_co
           ; let rn_env0 = me_env menv
                 rn_env1 = rnBndr2 rn_env0 tv1 tv2
                 menv'   = menv { me_env = rn_env1 }
           ; ty_co_match menv' subst1 ty1 co2 lkco rkco }
      where
        ki_ki_co = mkNomReflCo liftedTypeKind

    ty_co_match _ subst (CoercionTy {}) _ _ _
      = Just subst -- don't inspect coercions

    ty_co_match menv subst ty co lkco rkco
      | Just co' <- pushRefl co = ty_co_match menv subst ty co' lkco rkco
      | otherwise               = Nothing

    ty_co_match_tc :: MatchEnv -> LiftCoEnv
                   -> TyCon -> [Type]
                   -> TyCon -> [Coercion]
                   -> Maybe LiftCoEnv
    ty_co_match_tc menv subst tc1 tys1 tc2 cos2
      = do { guard (tc1 == tc2)
           ; ty_co_match_args menv subst tys1 cos2 lkcos rkcos }
      where
        Pair lkcos rkcos
          = traverse (fmap mkNomReflCo . coercionKind) cos2

    ty_co_match_app :: MatchEnv -> LiftCoEnv
                    -> Type -> [Type] -> Coercion -> [Coercion]
                    -> Maybe LiftCoEnv
    ty_co_match_app menv subst ty1 ty1args co2 co2args
      | Just (ty1', ty1a) <- repSplitAppTy_maybe ty1
      , Just (co2', co2a) <- splitAppCo_maybe co2
      = ty_co_match_app menv subst ty1' (ty1a : ty1args) co2' (co2a : co2args)

      | otherwise
      = do { subst1 <- ty_co_match menv subst ki1 ki2 ki_ki_co ki_ki_co
           ; let Pair lkco rkco = mkNomReflCo <$> coercionKind ki2
           ; subst2 <- ty_co_match menv subst1 ty1 co2 lkco rkco
           ; let Pair lkcos rkcos = traverse (fmap mkNomReflCo . coercionKind) co2args
           ; ty_co_match_args menv subst2 ty1args co2args lkcos rkcos }
      where
        ki1 = typeKind ty1
        ki2 = promoteCoercion co2
        ki_ki_co = mkNomReflCo liftedTypeKind

    ty_co_match_args :: MatchEnv -> LiftCoEnv -> [Type]
                     -> [Coercion] -> [Coercion] -> [Coercion]
                     -> Maybe LiftCoEnv
    ty_co_match_args _    subst []       []         _ _ = Just subst
    ty_co_match_args menv subst (ty:tys) (arg:args) (lkco:lkcos) (rkco:rkcos)
      = do { subst' <- ty_co_match menv subst ty arg lkco rkco
           ; ty_co_match_args menv subst' tys args lkcos rkcos }
    ty_co_match_args _    _     _        _          _ _ = Nothing

    pushRefl :: Coercion -> Maybe Coercion
    pushRefl (Refl Nominal (AppTy ty1 ty2))
      = Just (AppCo (Refl Nominal ty1) (mkNomReflCo ty2))
    pushRefl (Refl r (FunTy ty1 ty2))
      | Just rep1 <- getRuntimeRep_maybe ty1
      , Just rep2 <- getRuntimeRep_maybe ty2
      = Just (TyConAppCo r funTyCon [ mkReflCo r rep1, mkReflCo r rep2
                                    , mkReflCo r ty1,  mkReflCo r ty2 ])
    pushRefl (Refl r (TyConApp tc tys))
      = Just (TyConAppCo r tc (zipWith mkReflCo (tyConRolesX r tc) tys))
    pushRefl (Refl r (ForAllTy (TvBndr tv _) ty))
      = Just (mkHomoForAllCos_NoRefl [tv] (Refl r ty))
        -- NB: NoRefl variant. Otherwise, we get a loop!
    pushRefl (Refl r (CastTy ty co))  = Just (castCoercionKind (Refl r ty) co co)
    pushRefl _                        = Nothing