    {-
    (c) The University of Glasgow 2006-2012
    (c) The GRASP Project, Glasgow University, 1992-2002

    Various types used during typechecking, please see TcRnMonad as well for
    operations on these types. You probably want to import it, instead of this
    module.

    All the monads exported here are built on top of the same IOEnv monad. The
    monad functions like a Reader monad in the way it passes the environment
    around. This is done to allow the environment to be manipulated in a stack
    like fashion when entering expressions... ect.

    For state that is global and should be returned at the end (e.g not part
    of the stack mechanism), you should use an TcRef (= IORef) to store them.
    -}

    {-# LANGUAGE CPP, ExistentialQuantification #-}

    module TcRnTypes(
            TcRnIf, TcRn, TcM, RnM, IfM, IfL, IfG, -- The monad is opaque outside this module
            TcRef,

            -- The environment types
            Env(..),
            TcGblEnv(..), TcLclEnv(..),
            IfGblEnv(..), IfLclEnv(..),

            -- Ranamer types
            ErrCtxt, RecFieldEnv(..),
            ImportAvails(..), emptyImportAvails, plusImportAvails,
            WhereFrom(..), mkModDeps,

            -- Typechecker types
            TcTypeEnv, TcIdBinder(..), TcTyThing(..), PromotionErr(..),
            pprTcTyThingCategory, pprPECategory,

            -- Desugaring types
            DsM, DsLclEnv(..), DsGblEnv(..), PArrBuiltin(..),
            DsMetaEnv, DsMetaVal(..),

            -- Template Haskell
            ThStage(..), PendingStuff(..), topStage, topAnnStage, topSpliceStage,
            ThLevel, impLevel, outerLevel, thLevel,

            -- Arrows
            ArrowCtxt(..),

            -- Canonical constraints
            Xi, Ct(..), Cts, emptyCts, andCts, andManyCts, pprCts,
            singleCt, listToCts, ctsElts, consCts, snocCts, extendCtsList,
            isEmptyCts, isCTyEqCan, isCFunEqCan,
            isCDictCan_Maybe, isCFunEqCan_maybe,
            isCIrredEvCan, isCNonCanonical, isWantedCt, isDerivedCt,
            isGivenCt, isHoleCt, isTypedHoleCt, isPartialTypeSigCt,
            ctEvidence, ctLoc, ctPred, ctFlavour, ctEqRel,
            mkNonCanonical, mkNonCanonicalCt,
            ctEvPred, ctEvLoc, ctEvEqRel,
            ctEvTerm, ctEvCoercion, ctEvId, ctEvCheckDepth,

            WantedConstraints(..), insolubleWC, emptyWC, isEmptyWC,
            andWC, unionsWC, addSimples, addImplics, mkSimpleWC, addInsols,
            dropDerivedWC,

            Implication(..),
            SubGoalCounter(..),
            SubGoalDepth, initialSubGoalDepth, maxSubGoalDepth,
            bumpSubGoalDepth, subGoalCounterValue, subGoalDepthExceeded,
            CtLoc(..), ctLocSpan, ctLocEnv, ctLocOrigin,
            ctLocDepth, bumpCtLocDepth,
            setCtLocOrigin, setCtLocEnv, setCtLocSpan,
            CtOrigin(..), pprCtOrigin,
            pushErrCtxt, pushErrCtxtSameOrigin,

            SkolemInfo(..),

            CtEvidence(..),
            mkGivenLoc,
            isWanted, isGiven, isDerived,
            ctEvRole,

            -- Constraint solver plugins
            TcPlugin(..), TcPluginResult(..), TcPluginSolver,
            TcPluginM, runTcPluginM, unsafeTcPluginTcM,

            CtFlavour(..), ctEvFlavour,

            -- Pretty printing
            pprEvVarTheta,
            pprEvVars, pprEvVarWithType,
            pprArising, pprArisingAt,

            -- Misc other types
            TcId, TcIdSet, HoleSort(..)

      ) where

    #include "HsVersions.h"

    import HsSyn
    import CoreSyn
    import HscTypes
    import TcEvidence
    import Type
    import CoAxiom  ( Role )
    import Class    ( Class )
    import TyCon    ( TyCon )
    import ConLike  ( ConLike(..) )
    import DataCon  ( DataCon, dataConUserType, dataConOrigArgTys )
    import PatSyn   ( PatSyn, patSynType )
    import TysWiredIn ( coercibleClass )
    import TcType
    import Annotations
    import InstEnv
    import FamInstEnv
    import IOEnv
    import RdrName
    import Name
    import NameEnv
    import NameSet
    import Avail
    import Var
    import VarEnv
    import Module
    import SrcLoc
    import VarSet
    import ErrUtils
    import UniqFM
    import UniqSupply
    import BasicTypes
    import Bag
    import DynFlags
    import Outputable
    import ListSetOps
    import FastString
    import GHC.Fingerprint

    import Data.Set (Set)
    import Control.Monad (ap, liftM)

    #ifdef GHCI
    import Data.Map      ( Map )
    import Data.Dynamic  ( Dynamic )
    import Data.Typeable ( TypeRep )

    import qualified Language.Haskell.TH as TH
    #endif

    {-
    ************************************************************************
    *                                                                      *
                   Standard monad definition for TcRn
        All the combinators for the monad can be found in TcRnMonad
    *                                                                      *
    ************************************************************************

    The monad itself has to be defined here, because it is mentioned by ErrCtxt
    -}

    type TcRnIf a b = IOEnv (Env a b)
    type TcRn       = TcRnIf TcGblEnv TcLclEnv    -- Type inference
    type IfM lcl    = TcRnIf IfGblEnv lcl         -- Iface stuff
    type IfG        = IfM ()                      --    Top level
    type IfL        = IfM IfLclEnv                --    Nested
    type DsM        = TcRnIf DsGblEnv DsLclEnv    -- Desugaring

    -- TcRn is the type-checking and renaming monad: the main monad that
    -- most type-checking takes place in.  The global environment is
    -- 'TcGblEnv', which tracks all of the top-level type-checking
    -- information we've accumulated while checking a module, while the
    -- local environment is 'TcLclEnv', which tracks local information as
    -- we move inside expressions.

    -- | Historical "renaming monad" (now it's just 'TcRn').
    type RnM  = TcRn

    -- | Historical "type-checking monad" (now it's just 'TcRn').
    type TcM  = TcRn

    -- We 'stack' these envs through the Reader like monad infastructure
    -- as we move into an expression (although the change is focused in
    -- the lcl type).
    data Env gbl lcl
      = Env {
            env_top  :: HscEnv,  -- Top-level stuff that never changes
                                 -- Includes all info about imported things

            env_us   :: {-# UNPACK #-} !(IORef UniqSupply),
                                 -- Unique supply for local varibles

            env_gbl  :: gbl,     -- Info about things defined at the top level
                                 -- of the module being compiled

            env_lcl  :: lcl      -- Nested stuff; changes as we go into
        }

    instance ContainsDynFlags (Env gbl lcl) where
        extractDynFlags env = hsc_dflags (env_top env)
        replaceDynFlags env dflags
            = env {env_top = replaceDynFlags (env_top env) dflags}

    instance ContainsModule gbl => ContainsModule (Env gbl lcl) where
        extractModule env = extractModule (env_gbl env)

    {-
    ************************************************************************
    *                                                                      *
                    The interface environments
                  Used when dealing with IfaceDecls
    *                                                                      *
    ************************************************************************
    -}

    data IfGblEnv
      = IfGblEnv {
            -- The type environment for the module being compiled,
            -- in case the interface refers back to it via a reference that
            -- was originally a hi-boot file.
            -- We need the module name so we can test when it's appropriate
            -- to look in this env.
            if_rec_types :: Maybe (Module, IfG TypeEnv)
                    -- Allows a read effect, so it can be in a mutable
                    -- variable; c.f. handling the external package type env
                    -- Nothing => interactive stuff, no loops possible
        }

    data IfLclEnv
      = IfLclEnv {
            -- The module for the current IfaceDecl
            -- So if we see   f = \x -> x
            -- it means M.f = \x -> x, where M is the if_mod
            if_mod :: Module,

            -- The field is used only for error reporting
            -- if (say) there's a Lint error in it
            if_loc :: SDoc,
                    -- Where the interface came from:
                    --      .hi file, or GHCi state, or ext core
                    -- plus which bit is currently being examined

            if_tv_env  :: UniqFM TyVar,     -- Nested tyvar bindings
                                            -- (and coercions)
            if_id_env  :: UniqFM Id         -- Nested id binding
        }

    {-
    ************************************************************************
    *                                                                      *
                    Desugarer monad
    *                                                                      *
    ************************************************************************

    Now the mondo monad magic (yes, @DsM@ is a silly name)---carry around
    a @UniqueSupply@ and some annotations, which
    presumably include source-file location information:
    -}

    -- If '-XParallelArrays' is given, the desugarer populates this table with the corresponding
    -- variables found in 'Data.Array.Parallel'.
    --
    data PArrBuiltin
            = PArrBuiltin
            { lengthPVar         :: Var     -- ^ lengthP
            , replicatePVar      :: Var     -- ^ replicateP
            , singletonPVar      :: Var     -- ^ singletonP
            , mapPVar            :: Var     -- ^ mapP
            , filterPVar         :: Var     -- ^ filterP
            , zipPVar            :: Var     -- ^ zipP
            , crossMapPVar       :: Var     -- ^ crossMapP
            , indexPVar          :: Var     -- ^ (!:)
            , emptyPVar          :: Var     -- ^ emptyP
            , appPVar            :: Var     -- ^ (+:+)
            , enumFromToPVar     :: Var     -- ^ enumFromToP
            , enumFromThenToPVar :: Var     -- ^ enumFromThenToP
            }

    data DsGblEnv
            = DsGblEnv
            { ds_mod          :: Module             -- For SCC profiling
            , ds_fam_inst_env :: FamInstEnv         -- Like tcg_fam_inst_env
            , ds_unqual  :: PrintUnqualified
            , ds_msgs    :: IORef Messages          -- Warning messages
            , ds_if_env  :: (IfGblEnv, IfLclEnv)    -- Used for looking up global,
                                                    -- possibly-imported things
            , ds_dph_env :: GlobalRdrEnv            -- exported entities of 'Data.Array.Parallel.Prim'
                                                    -- iff '-fvectorise' flag was given as well as
                                                    -- exported entities of 'Data.Array.Parallel' iff
                                                    -- '-XParallelArrays' was given; otherwise, empty
            , ds_parr_bi :: PArrBuiltin             -- desugarar names for '-XParallelArrays'
            , ds_static_binds :: IORef [(Fingerprint, (Id,CoreExpr))]
              -- ^ Bindings resulted from floating static forms
            }

    instance ContainsModule DsGblEnv where
        extractModule = ds_mod

    data DsLclEnv = DsLclEnv {
            dsl_meta    :: DsMetaEnv,        -- Template Haskell bindings
            dsl_loc     :: SrcSpan           -- to put in pattern-matching error msgs
         }

    -- Inside [| |] brackets, the desugarer looks
    -- up variables in the DsMetaEnv
    type DsMetaEnv = NameEnv DsMetaVal

    data DsMetaVal
       = DsBound Id         -- Bound by a pattern inside the [| |].
                            -- Will be dynamically alpha renamed.
                            -- The Id has type THSyntax.Var

       | DsSplice (HsExpr Id) -- These bindings are introduced by
                              -- the PendingSplices on a HsBracketOut

    {-
    ************************************************************************
    *                                                                      *
                    Global typechecker environment
    *                                                                      *
    ************************************************************************
    -}

    -- | 'TcGblEnv' describes the top-level of the module at the
    -- point at which the typechecker is finished work.
    -- It is this structure that is handed on to the desugarer
    -- For state that needs to be updated during the typechecking
    -- phase and returned at end, use a 'TcRef' (= 'IORef').
    data TcGblEnv
      = TcGblEnv {
            tcg_mod     :: Module,         -- ^ Module being compiled
            tcg_src     :: HscSource,
              -- ^ What kind of module (regular Haskell, hs-boot, ext-core)
            tcg_sig_of  :: Maybe Module,
              -- ^ Are we being compiled as a signature of an implementation?
            tcg_impl_rdr_env :: Maybe GlobalRdrEnv,
              -- ^ Environment used only during -sig-of for resolving top level
              -- bindings.  See Note [Signature parameters in TcGblEnv and DynFlags]

            tcg_rdr_env :: GlobalRdrEnv,   -- ^ Top level envt; used during renaming
            tcg_default :: Maybe [Type],
              -- ^ Types used for defaulting. @Nothing@ => no @default@ decl

            tcg_fix_env   :: FixityEnv,     -- ^ Just for things in this module
            tcg_field_env :: RecFieldEnv,   -- ^ Just for things in this module
                                            -- See Note [The interactive package] in HscTypes

            tcg_type_env :: TypeEnv,
              -- ^ Global type env for the module we are compiling now.  All
              -- TyCons and Classes (for this module) end up in here right away,
              -- along with their derived constructors, selectors.
              --
              -- (Ids defined in this module start in the local envt, though they
              --  move to the global envt during zonking)
              --
              -- NB: for what "things in this module" means, see
              -- Note [The interactive package] in HscTypes

            tcg_type_env_var :: TcRef TypeEnv,
                    -- Used only to initialise the interface-file
                    -- typechecker in initIfaceTcRn, so that it can see stuff
                    -- bound in this module when dealing with hi-boot recursions
                    -- Updated at intervals (e.g. after dealing with types and classes)

            tcg_inst_env     :: InstEnv,
              -- ^ Instance envt for all /home-package/ modules;
              -- Includes the dfuns in tcg_insts
            tcg_fam_inst_env :: FamInstEnv, -- ^ Ditto for family instances
            tcg_ann_env      :: AnnEnv,     -- ^ And for annotations

            tcg_visible_orphan_mods :: ModuleSet,
              -- ^ The set of orphan modules which transitively reachable from
              -- direct imports.  We use this to figure out if an orphan instance
              -- in the global InstEnv should be considered visible.
              -- See Note [Instance lookup and orphan instances] in InstEnv

                    -- Now a bunch of things about this module that are simply
                    -- accumulated, but never consulted until the end.
                    -- Nevertheless, it's convenient to accumulate them along
                    -- with the rest of the info from this module.
            tcg_exports :: [AvailInfo],     -- ^ What is exported
            tcg_imports :: ImportAvails,
              -- ^ Information about what was imported from where, including
              -- things bound in this module. Also store Safe Haskell info
              -- here about transative trusted packaage requirements.

            tcg_dus :: DefUses,   -- ^ What is defined in this module and what is used.
            tcg_used_rdrnames :: TcRef (Set RdrName),
              -- See Note [Tracking unused binding and imports]

            tcg_keep :: TcRef NameSet,
              -- ^ Locally-defined top-level names to keep alive.
              --
              -- "Keep alive" means give them an Exported flag, so that the
              -- simplifier does not discard them as dead code, and so that they
              -- are exposed in the interface file (but not to export to the
              -- user).
              --
              -- Some things, like dict-fun Ids and default-method Ids are "born"
              -- with the Exported flag on, for exactly the above reason, but some
              -- we only discover as we go.  Specifically:
              --
              --   * The to/from functions for generic data types
              --
              --   * Top-level variables appearing free in the RHS of an orphan
              --     rule
              --
              --   * Top-level variables appearing free in a TH bracket

            tcg_th_used :: TcRef Bool,
              -- ^ @True@ <=> Template Haskell syntax used.
              --
              -- We need this so that we can generate a dependency on the
              -- Template Haskell package, because the desugarer is going
              -- to emit loads of references to TH symbols.  The reference
              -- is implicit rather than explicit, so we have to zap a
              -- mutable variable.

            tcg_th_splice_used :: TcRef Bool,
              -- ^ @True@ <=> A Template Haskell splice was used.
              --
              -- Splices disable recompilation avoidance (see #481)

            tcg_dfun_n  :: TcRef OccSet,
              -- ^ Allows us to choose unique DFun names.

            -- The next fields accumulate the payload of the module
            -- The binds, rules and foreign-decl fields are collected
            -- initially in un-zonked form and are finally zonked in tcRnSrcDecls

            tcg_rn_exports :: Maybe [Located (IE Name)],
            tcg_rn_imports :: [LImportDecl Name],
                    -- Keep the renamed imports regardless.  They are not
                    -- voluminous and are needed if you want to report unused imports

            tcg_rn_decls :: Maybe (HsGroup Name),
              -- ^ Renamed decls, maybe.  @Nothing@ <=> Don't retain renamed
              -- decls.

            tcg_dependent_files :: TcRef [FilePath], -- ^ dependencies from addDependentFile

    #ifdef GHCI
            tcg_th_topdecls :: TcRef [LHsDecl RdrName],
            -- ^ Top-level declarations from addTopDecls

            tcg_th_topnames :: TcRef NameSet,
            -- ^ Exact names bound in top-level declarations in tcg_th_topdecls

            tcg_th_modfinalizers :: TcRef [TH.Q ()],
            -- ^ Template Haskell module finalizers

            tcg_th_state :: TcRef (Map TypeRep Dynamic),
            -- ^ Template Haskell state
    #endif /* GHCI */

            tcg_ev_binds  :: Bag EvBind,        -- Top-level evidence bindings

            -- Things defined in this module, or (in GHCi)
            -- in the declarations for a single GHCi command.
            -- For the latter, see Note [The interactive package] in HscTypes
            tcg_binds     :: LHsBinds Id,       -- Value bindings in this module
            tcg_sigs      :: NameSet,           -- ...Top-level names that *lack* a signature
            tcg_imp_specs :: [LTcSpecPrag],     -- ...SPECIALISE prags for imported Ids
            tcg_warns     :: Warnings,          -- ...Warnings and deprecations
            tcg_anns      :: [Annotation],      -- ...Annotations
            tcg_tcs       :: [TyCon],           -- ...TyCons and Classes
            tcg_insts     :: [ClsInst],         -- ...Instances
            tcg_fam_insts :: [FamInst],         -- ...Family instances
            tcg_rules     :: [LRuleDecl Id],    -- ...Rules
            tcg_fords     :: [LForeignDecl Id], -- ...Foreign import & exports
            tcg_vects     :: [LVectDecl Id],    -- ...Vectorisation declarations
            tcg_patsyns   :: [PatSyn],          -- ...Pattern synonyms

            tcg_doc_hdr   :: Maybe LHsDocString, -- ^ Maybe Haddock header docs
            tcg_hpc       :: AnyHpcUsage,        -- ^ @True@ if any part of the
                                                 --  prog uses hpc instrumentation.

            tcg_main      :: Maybe Name,         -- ^ The Name of the main
                                                 -- function, if this module is
                                                 -- the main module.
            tcg_safeInfer :: TcRef Bool,         -- Has the typechecker
                                                 -- inferred this module
                                                 -- as -XSafe (Safe Haskell)

            -- | A list of user-defined plugins for the constraint solver.
            tcg_tc_plugins :: [TcPluginSolver],

            tcg_static_wc :: TcRef WantedConstraints
              -- ^ Wanted constraints of static forms.
        }

    -- Note [Signature parameters in TcGblEnv and DynFlags]
    -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    -- When compiling signature files, we need to know which implementation
    -- we've actually linked against the signature.  There are three seemingly
    -- redundant places where this information is stored: in DynFlags, there
    -- is sigOf, and in TcGblEnv, there is tcg_sig_of and tcg_impl_rdr_env.
    -- Here's the difference between each of them:
    --
    -- * DynFlags.sigOf is global per invocation of GHC.  If we are compiling
    --   with --make, there may be multiple signature files being compiled; in
    --   which case this parameter is a map from local module name to implementing
    --   Module.
    --
    -- * HscEnv.tcg_sig_of is global per the compilation of a single file, so
    --   it is simply the result of looking up tcg_mod in the DynFlags.sigOf
    --   parameter.  It's setup in TcRnMonad.initTc.  This prevents us
    --   from having to repeatedly do a lookup in DynFlags.sigOf.
    --
    -- * HscEnv.tcg_impl_rdr_env is a RdrEnv that lets us look up names
    --   according to the sig-of module.  It's setup in TcRnDriver.tcRnSignature.
    --   Here is an example showing why we need this map:
    --
    --  module A where
    --      a = True
    --
    --  module ASig where
    --      import B
    --      a :: Bool
    --
    --  module B where
    --      b = False
    --
    -- When we compile ASig --sig-of main:A, the default
    -- global RdrEnv (tcg_rdr_env) has an entry for b, but not for a
    -- (we never imported A).  So we have to look in a different environment
    -- to actually get the original name.
    --
    -- By the way, why do we need to do the lookup; can't we just use A:a
    -- as the name directly?  Well, if A is reexporting the entity from another
    -- module, then the original name needs to be the real original name:
    --
    --  module C where
    --      a = True
    --
    --  module A(a) where
    --      import C

    instance ContainsModule TcGblEnv where
        extractModule env = tcg_mod env

    data RecFieldEnv
      = RecFields (NameEnv [Name])  -- Maps a constructor name *in this module*
                                    -- to the fields for that constructor
                  NameSet           -- Set of all fields declared *in this module*;
                                    -- used to suppress name-shadowing complaints
                                    -- when using record wild cards
                                    -- E.g.  let fld = e in C {..}
            -- This is used when dealing with ".." notation in record
            -- construction and pattern matching.
            -- The FieldEnv deals *only* with constructors defined in *this*
            -- module.  For imported modules, we get the same info from the
            -- TypeEnv

    {-
    Note [Tracking unused binding and imports]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We gather two sorts of usage information
     * tcg_dus (defs/uses)
          Records *defined* Names (local, top-level)
              and *used*    Names (local or imported)

          Used (a) to report "defined but not used"
                   (see RnNames.reportUnusedNames)
               (b) to generate version-tracking usage info in interface
                   files (see MkIface.mkUsedNames)
       This usage info is mainly gathered by the renamer's
       gathering of free-variables

     * tcg_used_rdrnames
          Records used *imported* (not locally-defined) RdrNames
          Used only to report unused import declarations
          Notice that they are RdrNames, not Names, so we can
          tell whether the reference was qualified or unqualified, which
          is esssential in deciding whether a particular import decl
          is unnecessary.  This info isn't present in Names.

    ************************************************************************
    *                                                                      *
                    The local typechecker environment
    *                                                                      *
    ************************************************************************

    Note [The Global-Env/Local-Env story]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    During type checking, we keep in the tcg_type_env
            * All types and classes
            * All Ids derived from types and classes (constructors, selectors)

    At the end of type checking, we zonk the local bindings,
    and as we do so we add to the tcg_type_env
            * Locally defined top-level Ids

    Why?  Because they are now Ids not TcIds.  This final GlobalEnv is
            a) fed back (via the knot) to typechecking the
               unfoldings of interface signatures
            b) used in the ModDetails of this module
    -}

    data TcLclEnv           -- Changes as we move inside an expression
                            -- Discarded after typecheck/rename; not passed on to desugarer
      = TcLclEnv {
            tcl_loc        :: RealSrcSpan,     -- Source span
            tcl_ctxt       :: [ErrCtxt],       -- Error context, innermost on top
            tcl_tclvl      :: TcLevel,         -- Birthplace for new unification variables

            tcl_th_ctxt    :: ThStage,         -- Template Haskell context
            tcl_th_bndrs   :: ThBindEnv,       -- Binding level of in-scope Names
                                               -- defined in this module (not imported)

            tcl_arrow_ctxt :: ArrowCtxt,       -- Arrow-notation context

            tcl_rdr :: LocalRdrEnv,         -- Local name envt
                    -- Maintained during renaming, of course, but also during
                    -- type checking, solely so that when renaming a Template-Haskell
                    -- splice we have the right environment for the renamer.
                    --
                    --   Does *not* include global name envt; may shadow it
                    --   Includes both ordinary variables and type variables;
                    --   they are kept distinct because tyvar have a different
                    --   occurrence contructor (Name.TvOcc)
                    -- We still need the unsullied global name env so that
                    --   we can look up record field names

            tcl_env  :: TcTypeEnv,    -- The local type environment:
                                      -- Ids and TyVars defined in this module

            tcl_bndrs :: [TcIdBinder],   -- Stack of locally-bound Ids, innermost on top
                                         -- Used only for error reporting

            tcl_tidy :: TidyEnv,      -- Used for tidying types; contains all
                                      -- in-scope type variables (but not term variables)

            tcl_tyvars :: TcRef TcTyVarSet, -- The "global tyvars"
                            -- Namely, the in-scope TyVars bound in tcl_env,
                            -- plus the tyvars mentioned in the types of Ids bound
                            -- in tcl_lenv.
                            -- Why mutable? see notes with tcGetGlobalTyVars

            tcl_lie  :: TcRef WantedConstraints,    -- Place to accumulate type constraints
            tcl_errs :: TcRef Messages              -- Place to accumulate errors
        }

    type TcTypeEnv = NameEnv TcTyThing

    type ThBindEnv = NameEnv (TopLevelFlag, ThLevel)
       -- Domain = all Ids bound in this module (ie not imported)
       -- The TopLevelFlag tells if the binding is syntactically top level.
       -- We need to know this, because the cross-stage persistence story allows
       -- cross-stage at arbitrary types if the Id is bound at top level.
       --
       -- Nota bene: a ThLevel of 'outerLevel' is *not* the same as being
       -- bound at top level!  See Note [Template Haskell levels] in TcSplice

    data TcIdBinder
      = TcIdBndr
           TcId
           TopLevelFlag    -- Tells whether the bindind is syntactically top-level
                           -- (The monomorphic Ids for a recursive group count
                           --  as not-top-level for this purpose.)

    {- Note [Given Insts]
       ~~~~~~~~~~~~~~~~~~
    Because of GADTs, we have to pass inwards the Insts provided by type signatures
    and existential contexts. Consider
            data T a where { T1 :: b -> b -> T [b] }
            f :: Eq a => T a -> Bool
            f (T1 x y) = [x]==[y]

    The constructor T1 binds an existential variable 'b', and we need Eq [b].
    Well, we have it, because Eq a refines to Eq [b], but we can only spot that if we
    pass it inwards.

    -}

    -- | Type alias for 'IORef'; the convention is we'll use this for mutable
    -- bits of data in 'TcGblEnv' which are updated during typechecking and
    -- returned at the end.
    type TcRef a     = IORef a
    -- ToDo: when should I refer to it as a 'TcId' instead of an 'Id'?
    type TcId        = Id
    type TcIdSet     = IdSet

    ---------------------------
    -- Template Haskell stages and levels
    ---------------------------

    data ThStage    -- See Note [Template Haskell state diagram] in TcSplice
      = Splice      -- Inside a top-level splice splice
                    -- This code will be run *at compile time*;
                    --   the result replaces the splice
                    -- Binding level = 0
          Bool      -- True if in a typed splice, False otherwise

      | Comp        -- Ordinary Haskell code
                    -- Binding level = 1

      | Brack                       -- Inside brackets
          ThStage                   --   Enclosing stage
          PendingStuff

    data PendingStuff
      = RnPendingUntyped              -- Renaming the inside of an *untyped* bracket
          (TcRef [PendingRnSplice])   -- Pending splices in here

      | RnPendingTyped                -- Renaming the inside of a *typed* bracket

      | TcPending                     -- Typechecking the inside of a typed bracket
          (TcRef [PendingTcSplice])   --   Accumulate pending splices here
          (TcRef WantedConstraints)   --     and type constraints here

    topStage, topAnnStage, topSpliceStage :: ThStage
    topStage       = Comp
    topAnnStage    = Splice False
    topSpliceStage = Splice False

    instance Outputable ThStage where
       ppr (Splice _)  = text "Splice"
       ppr Comp        = text "Comp"
       ppr (Brack s _) = text "Brack" <> parens (ppr s)

    type ThLevel = Int
        -- NB: see Note [Template Haskell levels] in TcSplice
        -- Incremented when going inside a bracket,
        -- decremented when going inside a splice
        -- NB: ThLevel is one greater than the 'n' in Fig 2 of the
        --     original "Template meta-programming for Haskell" paper

    impLevel, outerLevel :: ThLevel
    impLevel = 0    -- Imported things; they can be used inside a top level splice
    outerLevel = 1  -- Things defined outside brackets

    thLevel :: ThStage -> ThLevel
    thLevel (Splice _)  = 0
    thLevel Comp        = 1
    thLevel (Brack s _) = thLevel s + 1

    ---------------------------
    -- Arrow-notation context
    ---------------------------

    {- Note [Escaping the arrow scope]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In arrow notation, a variable bound by a proc (or enclosed let/kappa)
    is not in scope to the left of an arrow tail (-<) or the head of (|..|).
    For example

            proc x -> (e1 -< e2)

    Here, x is not in scope in e1, but it is in scope in e2.  This can get
    a bit complicated:

            let x = 3 in
            proc y -> (proc z -> e1) -< e2

    Here, x and z are in scope in e1, but y is not.

    We implement this by
    recording the environment when passing a proc (using newArrowScope),
    and returning to that (using escapeArrowScope) on the left of -< and the
    head of (|..|).

    All this can be dealt with by the *renamer*. But the type checker needs
    to be involved too.  Example (arrowfail001)
      class Foo a where foo :: a -> ()
      data Bar = forall a. Foo a => Bar a
      get :: Bar -> ()
      get = proc x -> case x of Bar a -> foo -< a
    Here the call of 'foo' gives rise to a (Foo a) constraint that should not
    be captured by the pattern match on 'Bar'.  Rather it should join the
    constraints from further out.  So we must capture the constraint bag
    from further out in the ArrowCtxt that we push inwards.
    -}

    data ArrowCtxt   -- Note [Escaping the arrow scope]
      = NoArrowCtxt
      | ArrowCtxt LocalRdrEnv (TcRef WantedConstraints)

    ---------------------------
    -- TcTyThing
    ---------------------------

    data TcTyThing
      = AGlobal TyThing             -- Used only in the return type of a lookup

      | ATcId   {           -- Ids defined in this module; may not be fully zonked
            tct_id     :: TcId,
            tct_closed :: TopLevelFlag }   -- See Note [Bindings with closed types]

      | ATyVar  Name TcTyVar        -- The type variable to which the lexically scoped type
                                    -- variable is bound. We only need the Name
                                    -- for error-message purposes; it is the corresponding
                                    -- Name in the domain of the envt

      | AThing  TcKind   -- Used temporarily, during kind checking, for the
                         -- tycons and clases in this recursive group
                         -- Can be a mono-kind or a poly-kind; in TcTyClsDcls see
                         -- Note [Type checking recursive type and class declarations]

      | APromotionErr PromotionErr

    data PromotionErr
      = TyConPE          -- TyCon used in a kind before we are ready
                         --     data T :: T -> * where ...
      | ClassPE          -- Ditto Class

      | FamDataConPE     -- Data constructor for a data family
                         -- See Note [AFamDataCon: not promoting data family constructors] in TcRnDriver

      | RecDataConPE     -- Data constructor in a recursive loop
                         -- See Note [ARecDataCon: recusion and promoting data constructors] in TcTyClsDecls
      | NoDataKinds      -- -XDataKinds not enabled

    instance Outputable TcTyThing where     -- Debugging only
       ppr (AGlobal g)      = pprTyThing g
       ppr elt@(ATcId {})   = text "Identifier" <>
                              brackets (ppr (tct_id elt) <> dcolon
                                     <> ppr (varType (tct_id elt)) <> comma
                                     <+> ppr (tct_closed elt))
       ppr (ATyVar n tv)    = text "Type variable" <+> quotes (ppr n) <+> equals <+> ppr tv
       ppr (AThing k)       = text "AThing" <+> ppr k
       ppr (APromotionErr err) = text "APromotionErr" <+> ppr err

    instance Outputable PromotionErr where
      ppr ClassPE      = text "ClassPE"
      ppr TyConPE      = text "TyConPE"
      ppr FamDataConPE = text "FamDataConPE"
      ppr RecDataConPE = text "RecDataConPE"
      ppr NoDataKinds  = text "NoDataKinds"

    pprTcTyThingCategory :: TcTyThing -> SDoc
    pprTcTyThingCategory (AGlobal thing)    = pprTyThingCategory thing
    pprTcTyThingCategory (ATyVar {})        = ptext (sLit "Type variable")
    pprTcTyThingCategory (ATcId {})         = ptext (sLit "Local identifier")
    pprTcTyThingCategory (AThing {})        = ptext (sLit "Kinded thing")
    pprTcTyThingCategory (APromotionErr pe) = pprPECategory pe

    pprPECategory :: PromotionErr -> SDoc
    pprPECategory ClassPE      = ptext (sLit "Class")
    pprPECategory TyConPE      = ptext (sLit "Type constructor")
    pprPECategory FamDataConPE = ptext (sLit "Data constructor")
    pprPECategory RecDataConPE = ptext (sLit "Data constructor")
    pprPECategory NoDataKinds  = ptext (sLit "Data constructor")

    {-
    Note [Bindings with closed types]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider

      f x = let g ys = map not ys
            in ...

    Can we generalise 'g' under the OutsideIn algorithm?  Yes,
    because all g's free variables are top-level; that is they themselves
    have no free type variables, and it is the type variables in the
    environment that makes things tricky for OutsideIn generalisation.

    Definition:

       A variable is "closed", and has tct_closed set to TopLevel,
          iff
       a) all its free variables are imported, or are themselves closed
       b) generalisation is not restricted by the monomorphism restriction

    Under OutsideIn we are free to generalise a closed let-binding.
    This is an extension compared to the JFP paper on OutsideIn, which
    used "top-level" as a proxy for "closed".  (It's not a good proxy
    anyway -- the MR can make a top-level binding with a free type
    variable.)

    Note that:
      * A top-level binding may not be closed, if it suffer from the MR

      * A nested binding may be closed (eg 'g' in the example we started with)
        Indeed, that's the point; whether a function is defined at top level
        or nested is orthogonal to the question of whether or not it is closed

      * A binding may be non-closed because it mentions a lexically scoped
        *type variable*  Eg
            f :: forall a. blah
            f x = let g y = ...(y::a)...
    -}

    type ErrCtxt = (Bool, TidyEnv -> TcM (TidyEnv, MsgDoc))
            -- Monadic so that we have a chance
            -- to deal with bound type variables just before error
            -- message construction

            -- Bool:  True <=> this is a landmark context; do not
            --                 discard it when trimming for display

    {-
    ************************************************************************
    *                                                                      *
            Operations over ImportAvails
    *                                                                      *
    ************************************************************************
    -}

    -- | 'ImportAvails' summarises what was imported from where, irrespective of
    -- whether the imported things are actually used or not.  It is used:
    --
    --  * when processing the export list,
    --
    --  * when constructing usage info for the interface file,
    --
    --  * to identify the list of directly imported modules for initialisation
    --    purposes and for optimised overlap checking of family instances,
    --
    --  * when figuring out what things are really unused
    --
    data ImportAvails
       = ImportAvails {
            imp_mods :: ImportedMods,
              --      = ModuleEnv [(ModuleName, Bool, SrcSpan, Bool)],
              -- ^ Domain is all directly-imported modules
              -- The 'ModuleName' is what the module was imported as, e.g. in
              -- @
              --     import Foo as Bar
              -- @
              -- it is @Bar@.
              --
              -- The 'Bool' means:
              --
              --  - @True@ => import was @import Foo ()@
              --
              --  - @False@ => import was some other form
              --
              -- Used
              --
              --   (a) to help construct the usage information in the interface
              --       file; if we import something we need to recompile if the
              --       export version changes
              --
              --   (b) to specify what child modules to initialise
              --
              -- We need a full ModuleEnv rather than a ModuleNameEnv here,
              -- because we might be importing modules of the same name from
              -- different packages. (currently not the case, but might be in the
              -- future).

            imp_dep_mods :: ModuleNameEnv (ModuleName, IsBootInterface),
              -- ^ Home-package modules needed by the module being compiled
              --
              -- It doesn't matter whether any of these dependencies
              -- are actually /used/ when compiling the module; they
              -- are listed if they are below it at all.  For
              -- example, suppose M imports A which imports X.  Then
              -- compiling M might not need to consult X.hi, but X
              -- is still listed in M's dependencies.

            imp_dep_pkgs :: [PackageKey],
              -- ^ Packages needed by the module being compiled, whether directly,
              -- or via other modules in this package, or via modules imported
              -- from other packages.

            imp_trust_pkgs :: [PackageKey],
              -- ^ This is strictly a subset of imp_dep_pkgs and records the
              -- packages the current module needs to trust for Safe Haskell
              -- compilation to succeed. A package is required to be trusted if
              -- we are dependent on a trustworthy module in that package.
              -- While perhaps making imp_dep_pkgs a tuple of (PackageKey, Bool)
              -- where True for the bool indicates the package is required to be
              -- trusted is the more logical  design, doing so complicates a lot
              -- of code not concerned with Safe Haskell.
              -- See Note [RnNames . Tracking Trust Transitively]

            imp_trust_own_pkg :: Bool,
              -- ^ Do we require that our own package is trusted?
              -- This is to handle efficiently the case where a Safe module imports
              -- a Trustworthy module that resides in the same package as it.
              -- See Note [RnNames . Trust Own Package]

            imp_orphs :: [Module],
              -- ^ Orphan modules below us in the import tree (and maybe including
              -- us for imported modules)

            imp_finsts :: [Module]
              -- ^ Family instance modules below us in the import tree (and maybe
              -- including us for imported modules)
          }

    mkModDeps :: [(ModuleName, IsBootInterface)]
              -> ModuleNameEnv (ModuleName, IsBootInterface)
    mkModDeps deps = foldl add emptyUFM deps
                   where
                     add env elt@(m,_) = addToUFM env m elt

    emptyImportAvails :: ImportAvails
    emptyImportAvails = ImportAvails { imp_mods          = emptyModuleEnv,
                                       imp_dep_mods      = emptyUFM,
                                       imp_dep_pkgs      = [],
                                       imp_trust_pkgs    = [],
                                       imp_trust_own_pkg = False,
                                       imp_orphs         = [],
                                       imp_finsts        = [] }

    -- | Union two ImportAvails
    --
    -- This function is a key part of Import handling, basically
    -- for each import we create a separate ImportAvails structure
    -- and then union them all together with this function.
    plusImportAvails ::  ImportAvails ->  ImportAvails ->  ImportAvails
    plusImportAvails
      (ImportAvails { imp_mods = mods1,
                      imp_dep_mods = dmods1, imp_dep_pkgs = dpkgs1,
                      imp_trust_pkgs = tpkgs1, imp_trust_own_pkg = tself1,
                      imp_orphs = orphs1, imp_finsts = finsts1 })
      (ImportAvails { imp_mods = mods2,
                      imp_dep_mods = dmods2, imp_dep_pkgs = dpkgs2,
                      imp_trust_pkgs = tpkgs2, imp_trust_own_pkg = tself2,
                      imp_orphs = orphs2, imp_finsts = finsts2 })
      = ImportAvails { imp_mods          = plusModuleEnv_C (++) mods1 mods2,
                       imp_dep_mods      = plusUFM_C plus_mod_dep dmods1 dmods2,
                       imp_dep_pkgs      = dpkgs1 `unionLists` dpkgs2,
                       imp_trust_pkgs    = tpkgs1 `unionLists` tpkgs2,
                       imp_trust_own_pkg = tself1 || tself2,
                       imp_orphs         = orphs1 `unionLists` orphs2,
                       imp_finsts        = finsts1 `unionLists` finsts2 }
      where
        plus_mod_dep (m1, boot1) (m2, boot2)
            = WARN( not (m1 == m2), (ppr m1 <+> ppr m2) $$ (ppr boot1 <+> ppr boot2) )
                    -- Check mod-names match
              (m1, boot1 && boot2) -- If either side can "see" a non-hi-boot interface, use that

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Where from}
    *                                                                      *
    ************************************************************************

    The @WhereFrom@ type controls where the renamer looks for an interface file
    -}

    data WhereFrom
      = ImportByUser IsBootInterface        -- Ordinary user import (perhaps {-# SOURCE #-})
      | ImportBySystem                      -- Non user import.
      | ImportByPlugin                      -- Importing a plugin;
                                            -- See Note [Care with plugin imports] in LoadIface

    instance Outputable WhereFrom where
      ppr (ImportByUser is_boot) | is_boot     = ptext (sLit "{- SOURCE -}")
                                 | otherwise   = empty
      ppr ImportBySystem                       = ptext (sLit "{- SYSTEM -}")
      ppr ImportByPlugin                       = ptext (sLit "{- PLUGIN -}")

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

    -- The syntax of xi types:
    -- xi ::= a | T xis | xis -> xis | ... | forall a. tau
    -- Two important notes:
    --      (i) No type families, unless we are under a ForAll
    --      (ii) Note that xi types can contain unexpanded type synonyms;
    --           however, the (transitive) expansions of those type synonyms
    --           will not contain any type functions, unless we are under a ForAll.
    -- We enforce the structure of Xi types when we flatten (TcCanonical)

    type Xi = Type       -- In many comments, "xi" ranges over Xi

    type Cts = Bag Ct

    data Ct
      -- Atomic canonical constraints
      = CDictCan {  -- e.g.  Num xi
          cc_ev     :: CtEvidence, -- See Note [Ct/evidence invariant]
          cc_class  :: Class,
          cc_tyargs :: [Xi]        -- cc_tyargs are function-free, hence Xi
        }

      | CIrredEvCan {  -- These stand for yet-unusable predicates
          cc_ev :: CtEvidence   -- See Note [Ct/evidence invariant]
            -- The ctev_pred of the evidence is
            -- of form   (tv xi1 xi2 ... xin)
            --      or   (tv1 ~ ty2)   where the CTyEqCan  kind invariant fails
            --      or   (F tys ~ ty)  where the CFunEqCan kind invariant fails
            -- See Note [CIrredEvCan constraints]
        }

      | CTyEqCan {  -- tv ~ rhs
           -- Invariants:
           --   * See Note [Applying the inert substitution] in TcFlatten
           --   * tv not in tvs(rhs)   (occurs check)
           --   * If tv is a TauTv, then rhs has no foralls
           --       (this avoids substituting a forall for the tyvar in other types)
           --   * typeKind ty `subKind` typeKind tv
           --       See Note [Kind orientation for CTyEqCan]
           --   * rhs is not necessarily function-free,
           --       but it has no top-level function.
           --     E.g. a ~ [F b]  is fine
           --     but  a ~ F b    is not
           --   * If the equality is representational, rhs has no top-level newtype
           --     See Note [No top-level newtypes on RHS of representational
           --     equalities] in TcCanonical
           --   * If rhs is also a tv, then it is oriented to give best chance of
           --     unification happening; eg if rhs is touchable then lhs is too
          cc_ev     :: CtEvidence, -- See Note [Ct/evidence invariant]
          cc_tyvar  :: TcTyVar,
          cc_rhs    :: TcType,     -- Not necessarily function-free (hence not Xi)
                                   -- See invariants above
          cc_eq_rel :: EqRel
        }

      | CFunEqCan {  -- F xis ~ fsk
           -- Invariants:
           --   * isTypeFamilyTyCon cc_fun
           --   * typeKind (F xis) = tyVarKind fsk
           --   * always Nominal role
           --   * always Given or Wanted, never Derived
          cc_ev     :: CtEvidence,  -- See Note [Ct/evidence invariant]
          cc_fun    :: TyCon,       -- A type function

          cc_tyargs :: [Xi],        -- cc_tyargs are function-free (hence Xi)
            -- Either under-saturated or exactly saturated
            --    *never* over-saturated (because if so
            --    we should have decomposed)

          cc_fsk    :: TcTyVar  -- [Given]  always a FlatSkol skolem
                                -- [Wanted] always a FlatMetaTv unification variable
            -- See Note [The flattening story] in TcFlatten
        }

      | CNonCanonical {        -- See Note [NonCanonical Semantics]
          cc_ev  :: CtEvidence
        }

      | CHoleCan {             -- Treated as an "insoluble" constraint
                               -- See Note [Insoluble constraints]
          cc_ev   :: CtEvidence,
          cc_occ  :: OccName,   -- The name of this hole
          cc_hole :: HoleSort   -- The sort of this hole (expr, type, ...)
        }

    -- | Used to indicate which sort of hole we have.
    data HoleSort = ExprHole  -- ^ A hole in an expression (TypedHoles)
                  | TypeHole  -- ^ A hole in a type (PartialTypeSignatures)

    {-
    Note [Kind orientation for CTyEqCan]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Given an equality (t:* ~ s:Open), we can't solve it by updating t:=s,
    ragardless of how touchable 't' is, because the kinds don't work.

    Instead we absolutely must re-orient it. Reason: if that gets into the
    inert set we'll start replacing t's by s's, and that might make a
    kind-correct type into a kind error.  After re-orienting,
    we may be able to solve by updating s:=t.

    Hence in a CTyEqCan, (t:k1 ~ xi:k2) we require that k2 is a subkind of k1.

    If the two have incompatible kinds, we just don't use a CTyEqCan at all.
    See Note [Equalities with incompatible kinds] in TcCanonical

    We can't require *equal* kinds, because
         * wanted constraints don't necessarily have identical kinds
                   eg   alpha::? ~ Int
         * a solved wanted constraint becomes a given

    Note [Kind orientation for CFunEqCan]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    For (F xis ~ rhs) we require that kind(lhs) is a subkind of kind(rhs).
    This really only maters when rhs is an Open type variable (since only type
    variables have Open kinds):
       F ty ~ (a:Open)
    which can happen, say, from
          f :: F a b
          f = undefined   -- The a:Open comes from instantiating 'undefined'

    Note that the kind invariant is maintained by rewriting.
    Eg wanted1 rewrites wanted2; if both were compatible kinds before,
       wanted2 will be afterwards.  Similarly givens.

    Caveat:
      - Givens from higher-rank, such as:
              type family T b :: * -> * -> *
              type instance T Bool = (->)

              f :: forall a. ((T a ~ (->)) => ...) -> a -> ...
              flop = f (...) True
         Whereas we would be able to apply the type instance, we would not be able to
         use the given (T Bool ~ (->)) in the body of 'flop'

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

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

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

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

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

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

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

    ctEvidence :: Ct -> CtEvidence
    ctEvidence = cc_ev

    ctLoc :: Ct -> CtLoc
    ctLoc = ctEvLoc . ctEvidence

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

    -- | Get the flavour of the given 'Ct'
    ctFlavour :: Ct -> CtFlavour
    ctFlavour = ctEvFlavour . ctEvidence

    -- | Get the equality relation for the given 'Ct'
    ctEqRel :: Ct -> EqRel
    ctEqRel = ctEvEqRel . ctEvidence

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

    {-
    Note [Dropping derived constraints]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In general we discard derived constraints at the end of constraint solving;
    see dropDerivedWC.  For example
     * If we have an unsolved (Ord a), we don't want to complain about
       an unsolved (Eq a) as well.

    But we keep Derived *insoluble* constraints because they indicate a solid,
    comprehensible error.  Particularly:

     * Insolubles Givens indicate unreachable code

     * Insoluble kind equalities (e.g. [D] * ~ (* -> *)) may arise from
       a type equality a ~ Int#, say

     * Insoluble derived wanted equalities (e.g. [D] Int ~ Bool) may
       arise from functional dependency interactions.  We are careful
       to keep a good CtOrigin on such constraints (FunDepOrigin1, FunDepOrigin2)
       so that we can produce a good error message (Trac #9612)

    Since we leave these Derived constraints in the residual WantedConstraints,
    we must filter them out when we re-process the WantedConstraint,
    in TcSimplify.solve_wanteds.

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

    isWantedCt :: Ct -> Bool
    isWantedCt = isWanted . cc_ev

    isGivenCt :: Ct -> Bool
    isGivenCt = isGiven . cc_ev

    isDerivedCt :: Ct -> Bool
    isDerivedCt = isDerived . cc_ev

    isCTyEqCan :: Ct -> Bool
    isCTyEqCan (CTyEqCan {})  = True
    isCTyEqCan (CFunEqCan {}) = False
    isCTyEqCan _              = False

    isCDictCan_Maybe :: Ct -> Maybe Class
    isCDictCan_Maybe (CDictCan {cc_class = cls })  = Just cls
    isCDictCan_Maybe _              = Nothing

    isCIrredEvCan :: Ct -> Bool
    isCIrredEvCan (CIrredEvCan {}) = True
    isCIrredEvCan _                = False

    isCFunEqCan_maybe :: Ct -> Maybe (TyCon, [Type])
    isCFunEqCan_maybe (CFunEqCan { cc_fun = tc, cc_tyargs = xis }) = Just (tc, xis)
    isCFunEqCan_maybe _ = Nothing

    isCFunEqCan :: Ct -> Bool
    isCFunEqCan (CFunEqCan {}) = True
    isCFunEqCan _ = False

    isCNonCanonical :: Ct -> Bool
    isCNonCanonical (CNonCanonical {}) = True
    isCNonCanonical _ = False

    isHoleCt:: Ct -> Bool
    isHoleCt (CHoleCan {}) = True
    isHoleCt _ = False

    isTypedHoleCt :: Ct -> Bool
    isTypedHoleCt (CHoleCan { cc_hole = ExprHole }) = True
    isTypedHoleCt _ = False

    isPartialTypeSigCt :: Ct -> Bool
    isPartialTypeSigCt (CHoleCan { cc_hole = TypeHole }) = True
    isPartialTypeSigCt _ = False

    instance Outputable Ct where
      ppr ct = ppr (cc_ev ct) <+> parens (text ct_sort)
             where ct_sort = case ct of
                               CTyEqCan {}      -> "CTyEqCan"
                               CFunEqCan {}     -> "CFunEqCan"
                               CNonCanonical {} -> "CNonCanonical"
                               CDictCan {}      -> "CDictCan"
                               CIrredEvCan {}   -> "CIrredEvCan"
                               CHoleCan {}      -> "CHoleCan"

    singleCt :: Ct -> Cts
    singleCt = unitBag

    andCts :: Cts -> Cts -> Cts
    andCts = unionBags

    listToCts :: [Ct] -> Cts
    listToCts = listToBag

    ctsElts :: Cts -> [Ct]
    ctsElts = bagToList

    consCts :: Ct -> Cts -> Cts
    consCts = consBag

    snocCts :: Cts -> Ct -> Cts
    snocCts = snocBag

    extendCtsList :: Cts -> [Ct] -> Cts
    extendCtsList cts xs | null xs   = cts
                         | otherwise = cts `unionBags` listToBag xs

    andManyCts :: [Cts] -> Cts
    andManyCts = unionManyBags

    emptyCts :: Cts
    emptyCts = emptyBag

    isEmptyCts :: Cts -> Bool
    isEmptyCts = isEmptyBag

    pprCts :: Cts -> SDoc
    pprCts cts = vcat (map ppr (bagToList cts))

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

    data WantedConstraints
      = WC { wc_simple :: Cts              -- Unsolved constraints, all wanted
           , wc_impl   :: Bag Implication
           , wc_insol  :: Cts              -- Insoluble constraints, can be
                                           -- wanted, given, or derived
                                           -- See Note [Insoluble constraints]
        }

    emptyWC :: WantedConstraints
    emptyWC = WC { wc_simple = emptyBag, wc_impl = emptyBag, wc_insol = emptyBag }

    mkSimpleWC :: [Ct] -> WantedConstraints
    mkSimpleWC cts
      = WC { wc_simple = listToBag cts, wc_impl = emptyBag, wc_insol = emptyBag }

    isEmptyWC :: WantedConstraints -> Bool
    isEmptyWC (WC { wc_simple = f, wc_impl = i, wc_insol = n })
      = isEmptyBag f && isEmptyBag i && isEmptyBag n

    insolubleWC :: WantedConstraints -> Bool
    -- True if there are any insoluble constraints in the wanted bag. Ignore
    -- constraints arising from PartialTypeSignatures to solve as much of the
    -- constraints as possible before reporting the holes.
    insolubleWC wc = not (isEmptyBag (filterBag (not . isPartialTypeSigCt)
                                      (wc_insol wc)))
                   || anyBag ic_insol (wc_impl wc)

    andWC :: WantedConstraints -> WantedConstraints -> WantedConstraints
    andWC (WC { wc_simple = f1, wc_impl = i1, wc_insol = n1 })
          (WC { wc_simple = f2, wc_impl = i2, wc_insol = n2 })
      = WC { wc_simple = f1 `unionBags` f2
           , wc_impl   = i1 `unionBags` i2
           , wc_insol  = n1 `unionBags` n2 }

    unionsWC :: [WantedConstraints] -> WantedConstraints
    unionsWC = foldr andWC emptyWC

    addSimples :: WantedConstraints -> Bag Ct -> WantedConstraints
    addSimples wc cts
      = wc { wc_simple = wc_simple wc `unionBags` cts }

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

    addInsols :: WantedConstraints -> Bag Ct -> WantedConstraints
    addInsols wc cts
      = wc { wc_insol = wc_insol wc `unionBags` cts }

    instance Outputable WantedConstraints where
      ppr (WC {wc_simple = s, wc_impl = i, wc_insol = n})
       = ptext (sLit "WC") <+> braces (vcat
            [ ppr_bag (ptext (sLit "wc_simple")) s
            , ppr_bag (ptext (sLit "wc_insol")) n
            , ppr_bag (ptext (sLit "wc_impl")) i ])

    ppr_bag :: Outputable a => SDoc -> Bag a -> SDoc
    ppr_bag doc bag
     | isEmptyBag bag = empty
     | otherwise      = hang (doc <+> equals)
                           2 (foldrBag (($$) . ppr) empty bag)

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

    data Implication
      = Implic {
          ic_tclvl :: TcLevel, -- TcLevel: unification variables
                                    -- free in the environment

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

          ic_given  :: [EvVar],      -- Given evidence variables
                                     --   (order does not matter)
                                     -- See Invariant (GivenInv) in TcType

          ic_no_eqs :: Bool,         -- True  <=> ic_givens have no equalities, for sure
                                     -- False <=> ic_givens might have equalities

          ic_env   :: TcLclEnv,      -- Gives the source location and error context
                                     -- for the implicatdion, and hence for all the
                                     -- given evidence variables

          ic_wanted :: WantedConstraints,  -- The wanted
          ic_insol  :: Bool,               -- True iff insolubleWC ic_wanted is true

          ic_binds  :: EvBindsVar   -- Points to the place to fill in the
                                    -- abstraction and bindings
        }

    instance Outputable Implication where
      ppr (Implic { ic_tclvl = tclvl, ic_skols = skols
                  , ic_given = given, ic_no_eqs = no_eqs
                  , ic_wanted = wanted, ic_insol = insol
                  , ic_binds = binds, ic_info = info })
       = hang (ptext (sLit "Implic") <+> lbrace)
            2 (sep [ ptext (sLit "TcLevel =") <+> ppr tclvl
                   , ptext (sLit "Skolems =") <+> pprTvBndrs skols
                   , ptext (sLit "No-eqs =") <+> ppr no_eqs
                   , ptext (sLit "Insol =") <+> ppr insol
                   , hang (ptext (sLit "Given ="))  2 (pprEvVars given)
                   , hang (ptext (sLit "Wanted =")) 2 (ppr wanted)
                   , ptext (sLit "Binds =") <+> ppr binds
                   , pprSkolInfo info ] <+> rbrace)

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

    Note [Skolems in an implication]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The skolems in an implication are not there to perform a skolem escape
    check.  That happens because all the environment variables are in the
    untouchables, and therefore cannot be unified with anything at all,
    let alone the skolems.

    Instead, ic_skols is used only when considering floating a constraint
    outside the implication in TcSimplify.floatEqualities or
    TcSimplify.approximateImplications

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

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

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

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

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

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

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

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

    pprEvVarTheta :: [EvVar] -> SDoc
    pprEvVarTheta ev_vars = pprTheta (map evVarPred ev_vars)

    pprEvVarWithType :: EvVar -> SDoc
    pprEvVarWithType v = ppr v <+> dcolon <+> pprType (evVarPred v)

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

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

    data CtEvidence
      = CtGiven { ctev_pred :: TcPredType      -- See Note [Ct/evidence invariant]
                , ctev_evtm :: EvTerm          -- See Note [Evidence field of CtEvidence]
                , ctev_loc  :: CtLoc }
        -- Truly given, not depending on subgoals
        -- NB: Spontaneous unifications belong here

      | CtWanted { ctev_pred :: TcPredType     -- See Note [Ct/evidence invariant]
                 , ctev_evar :: EvVar          -- See Note [Evidence field of CtEvidence]
                 , ctev_loc  :: CtLoc }
        -- Wanted goal

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

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

    ctEvLoc :: CtEvidence -> CtLoc
    ctEvLoc = ctev_loc

    -- | Get the equality relation relevant for a 'CtEvidence'
    ctEvEqRel :: CtEvidence -> EqRel
    ctEvEqRel = predTypeEqRel . ctEvPred

    -- | Get the role relevant for a 'CtEvidence'
    ctEvRole :: CtEvidence -> Role
    ctEvRole = eqRelRole . ctEvEqRel

    ctEvTerm :: CtEvidence -> EvTerm
    ctEvTerm (CtGiven   { ctev_evtm = tm }) = tm
    ctEvTerm (CtWanted  { ctev_evar = ev }) = EvId ev
    ctEvTerm ctev@(CtDerived {}) = pprPanic "ctEvTerm: derived constraint cannot have id"
                                          (ppr ctev)

    ctEvCoercion :: CtEvidence -> TcCoercion
    -- ctEvCoercion ev = evTermCoercion (ctEvTerm ev)
    ctEvCoercion (CtGiven   { ctev_evtm = tm }) = evTermCoercion tm
    ctEvCoercion (CtWanted  { ctev_evar = v })  = mkTcCoVarCo v
    ctEvCoercion ctev@(CtDerived {}) = pprPanic "ctEvCoercion: derived constraint cannot have id"
                                          (ppr ctev)

    ctEvId :: CtEvidence -> TcId
    ctEvId (CtWanted  { ctev_evar = ev }) = ev
    ctEvId ctev = pprPanic "ctEvId:" (ppr ctev)

    instance Outputable CtEvidence where
      ppr fl = case fl of
                 CtGiven {}   -> ptext (sLit "[G]") <+> ppr (ctev_evtm fl) <+> ppr_pty
                 CtWanted {}  -> ptext (sLit "[W]") <+> ppr (ctev_evar fl) <+> ppr_pty
                 CtDerived {} -> ptext (sLit "[D]") <+> text "_" <+> ppr_pty
             where ppr_pty = dcolon <+> ppr (ctEvPred fl)

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

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

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

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

    Just an enum type that tracks whether a constraint is wanted, derived,
    or given, when we need to separate that info from the constraint itself.

    -}

    data CtFlavour = Given | Wanted | Derived
      deriving Eq

    instance Outputable CtFlavour where
      ppr Given   = text "[G]"
      ppr Wanted  = text "[W]"
      ppr Derived = text "[D]"

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

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

    Note [SubGoalDepth]
    ~~~~~~~~~~~~~~~~~~~
    The 'SubGoalCounter' takes care of stopping the constraint solver from looping.
    Because of the different use-cases of regular constaints and type function
    applications, there are two independent counters. Therefore, this datatype is
    abstract. See Note [WorkList]

    Each counter starts at zero and increases.

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

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

      The flag -fcontext-stack=n (not very well named!) fixes the maximium
      level.

    * The "type function reduction counter" does the same thing when resolving
    * qualities involving type functions. Example:
      Assume we have a wanted at depth 7:
        [W] d{7} : F () ~ a
      If thre is an type function equation "F () = Int", this would be rewritten to
        [W] d{8} : Int ~ a
      and remembered as having depth 8.

      Again, without UndecidableInstances, this counter is bounded, but without it
      can resolve things ad infinitum. Hence there is a maximum level. But we use a
      different maximum, as we expect possibly many more type function reductions
      in sensible programs than type class constraints.

      The flag -ftype-function-depth=n fixes the maximium level.
    -}

    data SubGoalCounter = CountConstraints | CountTyFunApps

    data SubGoalDepth  -- See Note [SubGoalDepth]
       = SubGoalDepth
             {-# UNPACK #-} !Int      -- Dictionary constraints
             {-# UNPACK #-} !Int      -- Type function reductions
      deriving (Eq, Ord)

    instance Outputable SubGoalDepth where
     ppr (SubGoalDepth c f) =  angleBrackets $
            char 'C' <> colon <> int c <> comma <>
            char 'F' <> colon <> int f

    initialSubGoalDepth :: SubGoalDepth
    initialSubGoalDepth = SubGoalDepth 0 0

    maxSubGoalDepth :: DynFlags -> SubGoalDepth
    maxSubGoalDepth dflags = SubGoalDepth (ctxtStkDepth dflags) (tyFunStkDepth dflags)

    bumpSubGoalDepth :: SubGoalCounter -> SubGoalDepth -> SubGoalDepth
    bumpSubGoalDepth CountConstraints (SubGoalDepth c f) = SubGoalDepth (c+1) f
    bumpSubGoalDepth CountTyFunApps   (SubGoalDepth c f) = SubGoalDepth c (f+1)

    subGoalCounterValue :: SubGoalCounter -> SubGoalDepth -> Int
    subGoalCounterValue CountConstraints (SubGoalDepth c _) = c
    subGoalCounterValue CountTyFunApps   (SubGoalDepth _ f) = f

    subGoalDepthExceeded :: SubGoalDepth -> SubGoalDepth -> Maybe SubGoalCounter
    subGoalDepthExceeded (SubGoalDepth mc mf) (SubGoalDepth c f)
            | c > mc    = Just CountConstraints
            | f > mf    = Just CountTyFunApps
            | otherwise = Nothing

    -- | Checks whether the evidence can be used to solve a goal with the given minimum depth
    -- See Note [Preventing recursive dictionaries]
    ctEvCheckDepth :: Class -> CtLoc -> CtEvidence -> Bool
    ctEvCheckDepth cls target ev
      | isWanted ev
      , cls == coercibleClass  -- The restriction applies only to Coercible
      = ctLocDepth target <= ctLocDepth (ctEvLoc ev)
      | otherwise = True

    {-
    Note [Preventing recursive dictionaries]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    NB: this will go away when we start treating Coercible as an equality.

    We have some classes where it is not very useful to build recursive
    dictionaries (Coercible, at the moment). So we need the constraint solver to
    prevent that. We conservatively ensure this property using the subgoal depth of
    the constraints: When solving a Coercible constraint at depth d, we do not
    consider evidence from a depth <= d as suitable.

    Therefore we need to record the minimum depth allowed to solve a CtWanted. This
    is done in the SubGoalDepth field of CtWanted. Most code now uses mkCtWanted,
    which initializes it to initialSubGoalDepth (i.e. 0); but when requesting a
    Coercible instance (requestCoercible in TcInteract), we bump the current depth
    by one and use that.

    There are two spots where wanted contraints attempted to be solved
    using existing constraints: lookupInertDict and lookupSolvedDict in
    TcSMonad.  Both use ctEvCheckDepth to make the check. That function
    ensures that a Given constraint can always be used to solve a goal
    (i.e. they are at depth infinity, for our purposes)

    ************************************************************************
    *                                                                      *
                CtLoc
    *                                                                      *
    ************************************************************************

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

    data CtLoc = CtLoc { ctl_origin :: CtOrigin
                       , ctl_env    :: TcLclEnv
                       , ctl_depth  :: !SubGoalDepth }
      -- The TcLclEnv includes particularly
      --    source location:  tcl_loc   :: RealSrcSpan
      --    context:          tcl_ctxt  :: [ErrCtxt]
      --    binder stack:     tcl_bndrs :: [TcIdBinders]
      --    level:            tcl_tclvl :: TcLevel

    mkGivenLoc :: TcLevel -> SkolemInfo -> TcLclEnv -> CtLoc
    mkGivenLoc tclvl skol_info env
      = CtLoc { ctl_origin = GivenOrigin skol_info
              , ctl_env    = env { tcl_tclvl = tclvl }
              , ctl_depth  = initialSubGoalDepth }

    ctLocEnv :: CtLoc -> TcLclEnv
    ctLocEnv = ctl_env

    ctLocDepth :: CtLoc -> SubGoalDepth
    ctLocDepth = ctl_depth

    ctLocOrigin :: CtLoc -> CtOrigin
    ctLocOrigin = ctl_origin

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

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

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

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

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

    pushErrCtxt :: CtOrigin -> ErrCtxt -> CtLoc -> CtLoc
    pushErrCtxt o err loc@(CtLoc { ctl_env = lcl })
      = loc { ctl_origin = o, ctl_env = lcl { tcl_ctxt = err : tcl_ctxt lcl } }

    pushErrCtxtSameOrigin :: ErrCtxt -> CtLoc -> CtLoc
    -- Just add information w/o updating the origin!
    pushErrCtxtSameOrigin err loc@(CtLoc { ctl_env = lcl })
      = loc { ctl_env = lcl { tcl_ctxt = err : tcl_ctxt lcl } }

    pprArising :: CtOrigin -> SDoc
    -- Used for the main, top-level error message
    -- We've done special processing for TypeEq and FunDep origins
    pprArising (TypeEqOrigin {}) = empty
    pprArising orig              = pprCtOrigin orig

    pprArisingAt :: CtLoc -> SDoc
    pprArisingAt (CtLoc { ctl_origin = o, ctl_env = lcl})
      = sep [ pprCtOrigin o
            , text "at" <+> ppr (tcl_loc lcl)]

    {-
    ************************************************************************
    *                                                                      *
                    SkolemInfo
    *                                                                      *
    ************************************************************************
    -}

    -- SkolemInfo gives the origin of *given* constraints
    --   a) type variables are skolemised
    --   b) an implication constraint is generated
    data SkolemInfo
      = SigSkol UserTypeCtxt        -- A skolem that is created by instantiating
                Type                -- a programmer-supplied type signature
                                    -- Location of the binding site is on the TyVar

            -- The rest are for non-scoped skolems
      | ClsSkol Class       -- Bound at a class decl
      | InstSkol            -- Bound at an instance decl
      | DataSkol            -- Bound at a data type declaration
      | FamInstSkol         -- Bound at a family instance decl
      | PatSkol             -- An existential type variable bound by a pattern for
          ConLike           -- a data constructor with an existential type.
          (HsMatchContext Name)
                 -- e.g.   data T = forall a. Eq a => MkT a
                 --        f (MkT x) = ...
                 -- The pattern MkT x will allocate an existential type
                 -- variable for 'a'.

      | ArrowSkol           -- An arrow form (see TcArrows)

      | IPSkol [HsIPName]   -- Binding site of an implicit parameter

      | RuleSkol RuleName   -- The LHS of a RULE

      | InferSkol [(Name,TcType)]
                            -- We have inferred a type for these (mutually-recursivive)
                            -- polymorphic Ids, and are now checking that their RHS
                            -- constraints are satisfied.

      | BracketSkol         -- Template Haskell bracket

      | UnifyForAllSkol     -- We are unifying two for-all types
           [TcTyVar]        -- The instantiated skolem variables
           TcType           -- The instantiated type *inside* the forall

      | UnkSkol             -- Unhelpful info (until I improve it)

    instance Outputable SkolemInfo where
      ppr = pprSkolInfo

    pprSkolInfo :: SkolemInfo -> SDoc
    -- Complete the sentence "is a rigid type variable bound by..."
    pprSkolInfo (SigSkol (FunSigCtxt f) ty)
                                = hang (ptext (sLit "the type signature for"))
                                     2 (pprPrefixOcc f <+> dcolon <+> ppr ty)
    pprSkolInfo (SigSkol cx ty) = hang (pprUserTypeCtxt cx <> colon)
                                     2 (ppr ty)
    pprSkolInfo (IPSkol ips)    = ptext (sLit "the implicit-parameter binding") <> plural ips <+> ptext (sLit "for")
                                  <+> pprWithCommas ppr ips
    pprSkolInfo (ClsSkol cls)   = ptext (sLit "the class declaration for") <+> quotes (ppr cls)
    pprSkolInfo InstSkol        = ptext (sLit "the instance declaration")
    pprSkolInfo DataSkol        = ptext (sLit "the data type declaration")
    pprSkolInfo FamInstSkol     = ptext (sLit "the family instance declaration")
    pprSkolInfo BracketSkol     = ptext (sLit "a Template Haskell bracket")
    pprSkolInfo (RuleSkol name) = ptext (sLit "the RULE") <+> doubleQuotes (ftext name)
    pprSkolInfo ArrowSkol       = ptext (sLit "the arrow form")
    pprSkolInfo (PatSkol cl mc) = case cl of
        RealDataCon dc -> sep [ ptext (sLit "a pattern with constructor")
                              , nest 2 $ ppr dc <+> dcolon
                                <+> pprType (dataConUserType dc) <> comma
                                -- pprType prints forall's regardless of -fprint-explict-foralls
                                -- which is what we want here, since we might be saying
                                -- type variable 't' is bound by ...
                              , ptext (sLit "in") <+> pprMatchContext mc ]
        PatSynCon ps -> sep [ ptext (sLit "a pattern with pattern synonym")
                            , nest 2 $ ppr ps <+> dcolon
                              <+> pprType (patSynType ps) <> comma
                            , ptext (sLit "in") <+> pprMatchContext mc ]
    pprSkolInfo (InferSkol ids) = sep [ ptext (sLit "the inferred type of")
                                      , vcat [ ppr name <+> dcolon <+> ppr ty
                                             | (name,ty) <- ids ]]
    pprSkolInfo (UnifyForAllSkol tvs ty) = ptext (sLit "the type") <+> ppr (mkForAllTys tvs ty)

    -- UnkSkol
    -- For type variables the others are dealt with by pprSkolTvBinding.
    -- For Insts, these cases should not happen
    pprSkolInfo UnkSkol = WARN( True, text "pprSkolInfo: UnkSkol" ) ptext (sLit "UnkSkol")

    {-
    ************************************************************************
    *                                                                      *
                CtOrigin
    *                                                                      *
    ************************************************************************
    -}

    data CtOrigin
      = GivenOrigin SkolemInfo

      -- All the others are for *wanted* constraints
      | OccurrenceOf Name           -- Occurrence of an overloaded identifier
      | AppOrigin                   -- An application of some kind

      | SpecPragOrigin Name         -- Specialisation pragma for identifier

      | TypeEqOrigin { uo_actual   :: TcType
                     , uo_expected :: TcType }
      | KindEqOrigin
          TcType TcType             -- A kind equality arising from unifying these two types
          CtOrigin                  -- originally arising from this
      | CoercibleOrigin TcType TcType  -- a Coercible constraint

      | IPOccOrigin  HsIPName       -- Occurrence of an implicit parameter

      | LiteralOrigin (HsOverLit Name)      -- Occurrence of a literal
      | NegateOrigin                        -- Occurrence of syntactic negation

      | ArithSeqOrigin (ArithSeqInfo Name) -- [x..], [x..y] etc
      | PArrSeqOrigin  (ArithSeqInfo Name) -- [:x..y:] and [:x,y..z:]
      | SectionOrigin
      | TupleOrigin                        -- (..,..)
      | ExprSigOrigin       -- e :: ty
      | PatSigOrigin        -- p :: ty
      | PatOrigin           -- Instantiating a polytyped pattern at a constructor
      | RecordUpdOrigin
      | ViewPatOrigin

      | ScOrigin            -- Typechecking superclasses of an instance declaration
      | DerivOrigin         -- Typechecking deriving
      | DerivOriginDC DataCon Int
                            -- Checking constraints arising from this data con and field index
      | DerivOriginCoerce Id Type Type
                            -- DerivOriginCoerce id ty1 ty2: Trying to coerce class method `id` from
                            -- `ty1` to `ty2`.
      | StandAloneDerivOrigin -- Typechecking stand-alone deriving
      | DefaultOrigin       -- Typechecking a default decl
      | DoOrigin            -- Arising from a do expression
      | MCompOrigin         -- Arising from a monad comprehension
      | IfOrigin            -- Arising from an if statement
      | ProcOrigin          -- Arising from a proc expression
      | AnnOrigin           -- An annotation

      | FunDepOrigin1       -- A functional dependency from combining
            PredType CtLoc      -- This constraint arising from ...
            PredType CtLoc      -- and this constraint arising from ...

      | FunDepOrigin2       -- A functional dependency from combining
            PredType CtOrigin   -- This constraint arising from ...
            PredType SrcSpan    -- and this instance
            -- We only need a CtOrigin on the first, because the location
            -- is pinned on the entire error message

      | HoleOrigin
      | UnboundOccurrenceOf RdrName
      | ListOrigin          -- An overloaded list
      | StaticOrigin        -- A static form

    ctoHerald :: SDoc
    ctoHerald = ptext (sLit "arising from")

    pprCtOrigin :: CtOrigin -> SDoc

    pprCtOrigin (GivenOrigin sk) = ctoHerald <+> ppr sk

    pprCtOrigin (FunDepOrigin1 pred1 loc1 pred2 loc2)
      = hang (ctoHerald <+> ptext (sLit "a functional dependency between constraints:"))
           2 (vcat [ hang (quotes (ppr pred1)) 2 (pprArisingAt loc1)
                   , hang (quotes (ppr pred2)) 2 (pprArisingAt loc2) ])

    pprCtOrigin (FunDepOrigin2 pred1 orig1 pred2 loc2)
      = hang (ctoHerald <+> ptext (sLit "a functional dependency between:"))
           2 (vcat [ hang (ptext (sLit "constraint") <+> quotes (ppr pred1))
                        2 (pprArising orig1 )
                   , hang (ptext (sLit "instance") <+> quotes (ppr pred2))
                        2 (ptext (sLit "at") <+> ppr loc2) ])

    pprCtOrigin (KindEqOrigin t1 t2 _)
      = hang (ctoHerald <+> ptext (sLit "a kind equality arising from"))
           2 (sep [ppr t1, char '~', ppr t2])

    pprCtOrigin (UnboundOccurrenceOf name)
      = ctoHerald <+> ptext (sLit "an undeclared identifier") <+> quotes (ppr name)

    pprCtOrigin (DerivOriginDC dc n)
      = hang (ctoHerald <+> ptext (sLit "the") <+> speakNth n
              <+> ptext (sLit "field of") <+> quotes (ppr dc))
           2 (parens (ptext (sLit "type") <+> quotes (ppr ty)))
      where
        ty = dataConOrigArgTys dc !! (n-1)

    pprCtOrigin (DerivOriginCoerce meth ty1 ty2)
      = hang (ctoHerald <+> ptext (sLit "the coercion of the method") <+> quotes (ppr meth))
           2 (sep [ text "from type" <+> quotes (ppr ty1)
                  , nest 2 $ text "to type" <+> quotes (ppr ty2) ])

    pprCtOrigin (CoercibleOrigin ty1 ty2)
      = hang (ctoHerald <+> text "trying to show that the representations of")
           2 (quotes (ppr ty1) <+> text "and" $$
              quotes (ppr ty2) <+> text "are the same")

    pprCtOrigin simple_origin
      = ctoHerald <+> pprCtO simple_origin

    ----------------
    pprCtO :: CtOrigin -> SDoc  -- Ones that are short one-liners
    pprCtO (OccurrenceOf name)   = hsep [ptext (sLit "a use of"), quotes (ppr name)]
    pprCtO AppOrigin             = ptext (sLit "an application")
    pprCtO (SpecPragOrigin name) = hsep [ptext (sLit "a specialisation pragma for"), quotes (ppr name)]
    pprCtO (IPOccOrigin name)    = hsep [ptext (sLit "a use of implicit parameter"), quotes (ppr name)]
    pprCtO RecordUpdOrigin       = ptext (sLit "a record update")
    pprCtO ExprSigOrigin         = ptext (sLit "an expression type signature")
    pprCtO PatSigOrigin          = ptext (sLit "a pattern type signature")
    pprCtO PatOrigin             = ptext (sLit "a pattern")
    pprCtO ViewPatOrigin         = ptext (sLit "a view pattern")
    pprCtO IfOrigin              = ptext (sLit "an if statement")
    pprCtO (LiteralOrigin lit)   = hsep [ptext (sLit "the literal"), quotes (ppr lit)]
    pprCtO (ArithSeqOrigin seq)  = hsep [ptext (sLit "the arithmetic sequence"), quotes (ppr seq)]
    pprCtO (PArrSeqOrigin seq)   = hsep [ptext (sLit "the parallel array sequence"), quotes (ppr seq)]
    pprCtO SectionOrigin         = ptext (sLit "an operator section")
    pprCtO TupleOrigin           = ptext (sLit "a tuple")
    pprCtO NegateOrigin          = ptext (sLit "a use of syntactic negation")
    pprCtO ScOrigin              = ptext (sLit "the superclasses of an instance declaration")
    pprCtO DerivOrigin           = ptext (sLit "the 'deriving' clause of a data type declaration")
    pprCtO StandAloneDerivOrigin = ptext (sLit "a 'deriving' declaration")
    pprCtO DefaultOrigin         = ptext (sLit "a 'default' declaration")
    pprCtO DoOrigin              = ptext (sLit "a do statement")
    pprCtO MCompOrigin           = ptext (sLit "a statement in a monad comprehension")
    pprCtO ProcOrigin            = ptext (sLit "a proc expression")
    pprCtO (TypeEqOrigin t1 t2)  = ptext (sLit "a type equality") <+> sep [ppr t1, char '~', ppr t2]
    pprCtO AnnOrigin             = ptext (sLit "an annotation")
    pprCtO HoleOrigin            = ptext (sLit "a use of") <+> quotes (ptext $ sLit "_")
    pprCtO ListOrigin            = ptext (sLit "an overloaded list")
    pprCtO StaticOrigin          = ptext (sLit "a static form")
    pprCtO _                     = panic "pprCtOrigin"

    {-
    Constraint Solver Plugins
    -------------------------
    -}

    type TcPluginSolver = [Ct]    -- given
                       -> [Ct]    -- derived
                       -> [Ct]    -- wanted
                       -> TcPluginM TcPluginResult

    newtype TcPluginM a = TcPluginM (TcM a)

    instance Functor     TcPluginM where
      fmap = liftM

    instance Applicative TcPluginM where
      pure  = return
      (<*>) = ap

    instance Monad TcPluginM where
      return x = TcPluginM (return x)
      fail x   = TcPluginM (fail x)
      TcPluginM m >>= k =
        TcPluginM (do a <- m
                      let TcPluginM m1 = k a
                      m1)

    runTcPluginM :: TcPluginM a -> TcM a
    runTcPluginM (TcPluginM m) = m

    -- | This function provides an escape for direct access to
    -- the 'TcM` monad.  It should not be used lightly, and
    -- the provided 'TcPluginM' API should be favoured instead.
    unsafeTcPluginTcM :: TcM a -> TcPluginM a
    unsafeTcPluginTcM = TcPluginM

    data TcPlugin = forall s. TcPlugin
      { tcPluginInit  :: TcPluginM s
        -- ^ Initialize plugin, when entering type-checker.

      , tcPluginSolve :: s -> TcPluginSolver
        -- ^ Solve some constraints.
        -- TODO: WRITE MORE DETAILS ON HOW THIS WORKS.

      , tcPluginStop  :: s -> TcPluginM ()
       -- ^ Clean up after the plugin, when exiting the type-checker.
      }

    data TcPluginResult
      = TcPluginContradiction [Ct]
        -- ^ The plugin found a contradiction.
        -- The returned constraints are removed from the inert set,
        -- and recorded as insoluable.

      | TcPluginOk [(EvTerm,Ct)] [Ct]
        -- ^ The first field is for constraints that were solved.
        -- These are removed from the inert set,
        -- and the evidence for them is recorded.
        -- The second field contains new work, that should be processed by
        -- the constraint solver.