GHC/Tc/Types.hs

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

    -}

    {-# LANGUAGE CPP, DeriveFunctor, ExistentialQuantification, GeneralizedNewtypeDeriving,
                 ViewPatterns #-}

    -- | Various types used during typechecking.
    --
    -- Please see "GHC.Tc.Utils.Monad" 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... etc.
    --
    -- For state that is global and should be returned at the end (e.g not part
    -- of the stack mechanism), you should use a TcRef (= IORef) to store them.
    module GHC.Tc.Types(
            TcRnIf, TcRn, TcM, RnM, IfM, IfL, IfG, -- The monad is opaque outside this module
            TcRef,

            -- The environment types
            Env(..),
            TcGblEnv(..), TcLclEnv(..),
            setLclEnvTcLevel, getLclEnvTcLevel,
            setLclEnvLoc, getLclEnvLoc,
            IfGblEnv(..), IfLclEnv(..),
            tcVisibleOrphanMods,

            -- Frontend types (shouldn't really be here)
            FrontendResult(..),

            -- Renamer types
            ErrCtxt, RecFieldEnv, pushErrCtxt, pushErrCtxtSameOrigin,
            ImportAvails(..), emptyImportAvails, plusImportAvails,
            WhereFrom(..), mkModDeps, modDepsElts,

            -- Typechecker types
            TcTypeEnv, TcBinderStack, TcBinder(..),
            TcTyThing(..), PromotionErr(..),
            IdBindingInfo(..), ClosedTypeId, RhsNames,
            IsGroupClosed(..),
            SelfBootInfo(..),
            pprTcTyThingCategory, pprPECategory, CompleteMatch(..),

            -- Desugaring types
            DsM, DsLclEnv(..), DsGblEnv(..),
            DsMetaEnv, DsMetaVal(..), CompleteMatchMap,
            mkCompleteMatchMap, extendCompleteMatchMap,

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

            -- Arrows
            ArrowCtxt(..),

            -- TcSigInfo
            TcSigFun, TcSigInfo(..), TcIdSigInfo(..),
            TcIdSigInst(..), TcPatSynInfo(..),
            isPartialSig, hasCompleteSig,

            -- Misc other types
            TcId, TcIdSet,
            NameShape(..),
            removeBindingShadowing,
            getPlatform,

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

            -- Role annotations
            RoleAnnotEnv, emptyRoleAnnotEnv, mkRoleAnnotEnv,
            lookupRoleAnnot, getRoleAnnots,

            -- Linting
            lintGblEnv
      ) where

    #include "HsVersions.h"

    import GHC.Prelude
    import GHC.Platform

    import GHC.Hs
    import GHC.Driver.Types
    import GHC.Tc.Types.Evidence
    import GHC.Core.Type
    import GHC.Core.TyCon  ( TyCon, tyConKind )
    import GHC.Core.PatSyn ( PatSyn )
    import GHC.Core.Lint   ( lintAxioms )
    import GHC.Types.Id         ( idType, idName )
    import GHC.Types.FieldLabel ( FieldLabel )
    import GHC.Core.UsageEnv
    import GHC.Tc.Utils.TcType
    import GHC.Tc.Types.Constraint
    import GHC.Tc.Types.Origin
    import GHC.Types.Annotations
    import GHC.Core.InstEnv
    import GHC.Core.FamInstEnv
    import {-# SOURCE #-} GHC.HsToCore.PmCheck.Types (Deltas)
    import GHC.Data.IOEnv
    import GHC.Types.Name.Reader
    import GHC.Types.Name
    import GHC.Types.Name.Env
    import GHC.Types.Name.Set
    import GHC.Types.Avail
    import GHC.Types.Var
    import GHC.Types.Var.Env
    import GHC.Unit
    import GHC.Types.SrcLoc
    import GHC.Types.Var.Set
    import GHC.Utils.Error
    import GHC.Types.Unique.FM
    import GHC.Types.Basic
    import GHC.Data.Bag
    import GHC.Driver.Session
    import GHC.Utils.Outputable
    import GHC.Data.List.SetOps
    import GHC.Utils.Fingerprint
    import GHC.Utils.Misc
    import GHC.Builtin.Names ( isUnboundName )
    import GHC.Types.CostCentre.State

    import Control.Monad (ap)
    import Data.Set      ( Set )
    import qualified Data.Set as S

    import Data.List ( sort )
    import Data.Map ( Map )
    import Data.Dynamic  ( Dynamic )
    import Data.Typeable ( TypeRep )
    import Data.Maybe    ( mapMaybe )
    import GHCi.Message
    import GHCi.RemoteTypes

    import {-# SOURCE #-} GHC.Tc.Errors.Hole.FitTypes ( HoleFitPlugin )

    import qualified Language.Haskell.TH as TH

    -- | A 'NameShape' is a substitution on 'Name's that can be used
    -- to refine the identities of a hole while we are renaming interfaces
    -- (see "GHC.Iface.Rename").  Specifically, a 'NameShape' for
    -- 'ns_module_name' @A@, defines a mapping from @{A.T}@
    -- (for some 'OccName' @T@) to some arbitrary other 'Name'.
    --
    -- The most intruiging thing about a 'NameShape', however, is
    -- how it's constructed.  A 'NameShape' is *implied* by the
    -- exported 'AvailInfo's of the implementor of an interface:
    -- if an implementor of signature @\<H>@ exports @M.T@, you implicitly
    -- define a substitution from @{H.T}@ to @M.T@.  So a 'NameShape'
    -- is computed from the list of 'AvailInfo's that are exported
    -- by the implementation of a module, or successively merged
    -- together by the export lists of signatures which are joining
    -- together.
    --
    -- It's not the most obvious way to go about doing this, but it
    -- does seem to work!
    --
    -- NB: Can't boot this and put it in NameShape because then we
    -- start pulling in too many DynFlags things.
    data NameShape = NameShape {
            ns_mod_name :: ModuleName,
            ns_exports :: [AvailInfo],
            ns_map :: OccEnv Name
        }

    {-
    ************************************************************************
    *                                                                      *
                   Standard monad definition for TcRn
        All the combinators for the monad can be found in GHC.Tc.Utils.Monad
    *                                                                      *
    ************************************************************************

    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 infrastructure
    -- 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
                                 -- BangPattern is to fix leak, see #15111

            env_um   :: !Char,   -- Mask for Uniques

            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)

    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 {
            -- Some information about where this environment came from;
            -- useful for debugging.
            if_doc :: SDoc,
            -- 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.
            -- See Note [Tying the knot] in GHC.IfaceToCore
            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
            -- NB: This is a semantic module, see
            -- Note [Identity versus semantic module]
            if_mod :: Module,

            -- Whether or not the IfaceDecl came from a boot
            -- file or not; we'll use this to choose between
            -- NoUnfolding and BootUnfolding
            if_boot :: IsBootInterface,

            -- 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_nsubst :: Maybe NameShape,

            -- This field is used to make sure "implicit" declarations
            -- (anything that cannot be exported in mi_exports) get
            -- wired up correctly in typecheckIfacesForMerging.  Most
            -- of the time it's @Nothing@.  See Note [Resolving never-exported Names]
            -- in GHC.IfaceToCore.
            if_implicits_env :: Maybe TypeEnv,

            if_tv_env  :: FastStringEnv TyVar,     -- Nested tyvar bindings
            if_id_env  :: FastStringEnv 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:
    -}

    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_complete_matches :: CompleteMatchMap
               -- Additional complete pattern matches
            , ds_cc_st   :: IORef CostCentreState
               -- Tracking indices for cost centre annotations
            }

    instance ContainsModule DsGblEnv where
        extractModule = ds_mod

    data DsLclEnv = DsLclEnv {
            dsl_meta    :: DsMetaEnv,        -- Template Haskell bindings
            dsl_loc     :: RealSrcSpan,      -- To put in pattern-matching error msgs

            -- See Note [Note [Type and Term Equality Propagation] in "GHC.HsToCore.PmCheck"
            -- The set of reaching values Deltas is augmented as we walk inwards,
            -- refined through each pattern match in turn
            dsl_deltas  :: Deltas
         }

    -- 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 GhcTc) -- These bindings are introduced by
                                 -- the PendingSplices on a HsBracketOut

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

    -- | 'FrontendResult' describes the result of running the frontend of a Haskell
    -- module. Currently one always gets a 'FrontendTypecheck', since running the
    -- frontend involves typechecking a program. hs-sig merges are not handled here.
    --
    -- This data type really should be in GHC.Driver.Types, but it needs
    -- to have a TcGblEnv which is only defined here.
    data FrontendResult
            = FrontendTypecheck TcGblEnv

    -- Note [Identity versus semantic module]
    -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    -- When typechecking an hsig file, it is convenient to keep track
    -- of two different "this module" identifiers:
    --
    --      - The IDENTITY module is simply thisPackage + the module
    --        name; i.e. it uniquely *identifies* the interface file
    --        we're compiling.  For example, p[A=<A>]:A is an
    --        identity module identifying the requirement named A
    --        from library p.
    --
    --      - The SEMANTIC module, which is the actual module that
    --        this signature is intended to represent (e.g. if
    --        we have a identity module p[A=base:Data.IORef]:A,
    --        then the semantic module is base:Data.IORef)
    --
    -- Which one should you use?
    --
    --      - In the desugarer and later phases of compilation,
    --        identity and semantic modules coincide, since we never compile
    --        signatures (we just generate blank object files for
    --        hsig files.)
    --
    --        A corrolary of this is that the following invariant holds at any point
    --        past desugaring,
    --
    --            if I have a Module, this_mod, in hand representing the module
    --            currently being compiled,
    --            then moduleUnit this_mod == thisPackage dflags
    --
    --      - For any code involving Names, we want semantic modules.
    --        See lookupIfaceTop in GHC.Iface.Env, mkIface and addFingerprints
    --        in GHC.Iface.{Make,Recomp}, and tcLookupGlobal in GHC.Tc.Utils.Env
    --
    --      - When reading interfaces, we want the identity module to
    --        identify the specific interface we want (such interfaces
    --        should never be loaded into the EPS).  However, if a
    --        hole module <A> is requested, we look for A.hi
    --        in the home library we are compiling.  (See GHC.Iface.Load.)
    --        Similarly, in GHC.Rename.Names we check for self-imports using
    --        identity modules, to allow signatures to import their implementor.
    --
    --      - For recompilation avoidance, you want the identity module,
    --        since that will actually say the specific interface you
    --        want to track (and recompile if it changes)

    -- | '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_semantic_mod :: Module,    -- ^ If a signature, the backing module
                -- See also Note [Identity versus semantic module]
            tcg_src     :: HscSource,
              -- ^ What kind of module (regular Haskell, hs-boot, hsig)

            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 "GHC.Driver.Types"

            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 "GHC.Driver.Types"

            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
              -- NB. BangPattern is to fix a leak, see #15111
            tcg_fam_inst_env :: !FamInstEnv, -- ^ Ditto for family instances
              -- NB. BangPattern is to fix a leak, see #15111
            tcg_ann_env      :: AnnEnv,     -- ^ And for annotations

                    -- 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 transitive trusted package requirements.
              --
              -- There are not many uses of this field, so you can grep for
              -- all them.
              --
              -- The ImportAvails records information about the following
              -- things:
              --
              --    1. All of the modules you directly imported (tcRnImports)
              --    2. The orphans (only!) of all imported modules in a GHCi
              --       session (runTcInteractive)
              --    3. The module that instantiated a signature
              --    4. Each of the signatures that merged in
              --
              -- It is used in the following ways:
              --    - imp_orphs is used to determine what orphan modules should be
              --      visible in the context (tcVisibleOrphanMods)
              --    - imp_finsts is used to determine what family instances should
              --      be visible (tcExtendLocalFamInstEnv)
              --    - To resolve the meaning of the export list of a module
              --      (tcRnExports)
              --    - imp_mods is used to compute usage info (mkIfaceTc, deSugar)
              --    - imp_trust_own_pkg is used for Safe Haskell in interfaces
              --      (mkIfaceTc, as well as in "GHC.Driver.Main")
              --    - To create the Dependencies field in interface (mkDependencies)

              -- These three fields track unused bindings and imports
              -- See Note [Tracking unused binding and imports]
            tcg_dus       :: DefUses,
            tcg_used_gres :: TcRef [GlobalRdrElt],
            tcg_keep      :: TcRef NameSet,

            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.

            tcg_merged :: [(Module, Fingerprint)],
              -- ^ The requirements we merged with; we always have to recompile
              -- if any of these changed.

            -- 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 GhcRn), Avails)],
                    -- Nothing <=> no explicit export list
                    -- Is always Nothing if we don't want to retain renamed
                    -- exports.
                    -- If present contains each renamed export list item
                    -- together with its exported names.

            tcg_rn_imports :: [LImportDecl GhcRn],
                    -- Keep the renamed imports regardless.  They are not
                    -- voluminous and are needed if you want to report unused imports

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

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

            tcg_th_topdecls :: TcRef [LHsDecl GhcPs],
            -- ^ Top-level declarations from addTopDecls

            tcg_th_foreign_files :: TcRef [(ForeignSrcLang, FilePath)],
            -- ^ Foreign files emitted from TH.

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

            tcg_th_modfinalizers :: TcRef [(TcLclEnv, ThModFinalizers)],
            -- ^ Template Haskell module finalizers.
            --
            -- They can use particular local environments.

            tcg_th_coreplugins :: TcRef [String],
            -- ^ Core plugins added by Template Haskell code.

            tcg_th_state :: TcRef (Map TypeRep Dynamic),
            tcg_th_remote_state :: TcRef (Maybe (ForeignRef (IORef QState))),
            -- ^ Template Haskell state

            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 GHC.Driver.Types
            tcg_tr_module :: Maybe Id,   -- Id for $trModule :: GHC.Unit.Module
                                                 -- for which every module has a top-level defn
                                                 -- except in GHCi in which case we have Nothing
            tcg_binds     :: LHsBinds GhcTc,     -- 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 GhcTc],  -- ...Rules
            tcg_fords     :: [LForeignDecl GhcTc], -- ...Foreign import & exports
            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.
               -- NB. BangPattern is to fix a leak, see #15111

            tcg_self_boot :: SelfBootInfo,       -- ^ Whether this module has a
                                                 -- corresponding hi-boot file

            tcg_main      :: Maybe Name,         -- ^ The Name of the main
                                                 -- function, if this module is
                                                 -- the main module.

            tcg_safeInfer :: TcRef (Bool, WarningMessages),
            -- ^ Has the typechecker inferred this module as -XSafe (Safe Haskell)
            -- See Note [Safe Haskell Overlapping Instances Implementation],
            -- although this is used for more than just that failure case.

            tcg_tc_plugins :: [TcPluginSolver],
            -- ^ A list of user-defined plugins for the constraint solver.
            tcg_hf_plugins :: [HoleFitPlugin],
            -- ^ A list of user-defined plugins for hole fit suggestions.

            tcg_top_loc :: RealSrcSpan,
            -- ^ The RealSrcSpan this module came from

            tcg_static_wc :: TcRef WantedConstraints,
              -- ^ Wanted constraints of static forms.
            -- See Note [Constraints in static forms].
            tcg_complete_matches :: [CompleteMatch],

            -- ^ Tracking indices for cost centre annotations
            tcg_cc_st   :: TcRef CostCentreState
        }

    -- NB: topModIdentity, not topModSemantic!
    -- Definition sites of orphan identities will be identity modules, not semantic
    -- modules.

    -- Note [Constraints in static forms]
    -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    --
    -- When a static form produces constraints like
    --
    -- f :: StaticPtr (Bool -> String)
    -- f = static show
    --
    -- we collect them in tcg_static_wc and resolve them at the end
    -- of type checking. They need to be resolved separately because
    -- we don't want to resolve them in the context of the enclosing
    -- expression. Consider
    --
    -- g :: Show a => StaticPtr (a -> String)
    -- g = static show
    --
    -- If the @Show a0@ constraint that the body of the static form produces was
    -- resolved in the context of the enclosing expression, then the body of the
    -- static form wouldn't be closed because the Show dictionary would come from
    -- g's context instead of coming from the top level.

    tcVisibleOrphanMods :: TcGblEnv -> ModuleSet
    tcVisibleOrphanMods tcg_env
        = mkModuleSet (tcg_mod tcg_env : imp_orphs (tcg_imports tcg_env))

    instance ContainsModule TcGblEnv where
        extractModule env = tcg_semantic_mod env

    type RecFieldEnv = NameEnv [FieldLabel]
            -- Maps a constructor name *in this module*
            -- to the fields for that constructor.
            -- 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

    data SelfBootInfo
      = NoSelfBoot    -- No corresponding hi-boot file
      | SelfBoot
           { sb_mds :: ModDetails   -- There was a hi-boot file,
           , sb_tcs :: NameSet }    -- defining these TyCons,
    -- What is sb_tcs used for?  See Note [Extra dependencies from .hs-boot files]
    -- in GHC.Rename.Module

    {- Note [Tracking unused binding and imports]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We gather three sorts of usage information

     * tcg_dus :: DefUses (defs/uses)
          Records what is defined in this module and what is used.

          Records *defined* Names (local, top-level)
              and *used*    Names (local or imported)

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

     * tcg_used_gres :: TcRef [GlobalRdrElt]
          Records occurrences of imported entities.

          Used only to report unused import declarations

          Records each *occurrence* an *imported* (not locally-defined) entity.
          The occurrence is recorded by keeping a GlobalRdrElt for it.
          These is not the GRE that is in the GlobalRdrEnv; rather it
          is recorded *after* the filtering done by pickGREs.  So it reflect
          /how that occurrence is in scope/.   See Note [GRE filtering] in
          RdrName.

      * tcg_keep :: TcRef NameSet
          Records names of the type constructors, data constructors, and Ids that
          are used by the constraint solver.

          The typechecker may use find that some imported or
          locally-defined things are used, even though they
          do not appear to be mentioned in the source code:

          (a) The to/from functions for generic data types

          (b) Top-level variables appearing free in the RHS of an
              orphan rule

          (c) Top-level variables appearing free in a TH bracket
              See Note [Keeping things alive for Template Haskell]
              in GHC.Rename.Splice

          (d) The data constructor of a newtype that is used
              to solve a Coercible instance (e.g. #10347). Example
                  module T10347 (N, mkN) where
                    import Data.Coerce
                    newtype N a = MkN Int
                    mkN :: Int -> N a
                    mkN = coerce

              Then we wish to record `MkN` as used, since it is (morally)
              used to perform the coercion in `mkN`. To do so, the
              Coercible solver updates tcg_keep's TcRef whenever it
              encounters a use of `coerce` that crosses newtype boundaries.

          The tcg_keep field is used in two distinct ways:

          * Desugar.addExportFlagsAndRules.  Where things like (a-c) are locally
            defined, we should 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).

          * GHC.Rename.Names.reportUnusedNames.  Where newtype data constructors
            like (d) are imported, we don't want to report them as unused.

    ************************************************************************
    *                                                                      *
                    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_in_gen_code :: Bool,           -- See Note [Rebindable syntax and HsExpansion]
            tcl_tclvl      :: TcLevel,

            tcl_th_ctxt    :: ThStage,         -- Template Haskell context
            tcl_th_bndrs   :: ThBindEnv,       -- and binder info
                -- The ThBindEnv records the TH binding level of in-scope Names
                -- defined in this module (not imported)
                -- We can't put this info in the TypeEnv because it's needed
                -- (and extended) in the renamer, for untyed splices

            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 constructor (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_usage :: TcRef UsageEnv, -- Required multiplicity of bindings is accumulated here.

            tcl_bndrs :: TcBinderStack,   -- Used for reporting relevant bindings,
                                          -- and for tidying types

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

    setLclEnvTcLevel :: TcLclEnv -> TcLevel -> TcLclEnv
    setLclEnvTcLevel env lvl = env { tcl_tclvl = lvl }

    getLclEnvTcLevel :: TcLclEnv -> TcLevel
    getLclEnvTcLevel = tcl_tclvl

    setLclEnvLoc :: TcLclEnv -> RealSrcSpan -> TcLclEnv
    setLclEnvLoc env loc = env { tcl_loc = loc }

    getLclEnvLoc :: TcLclEnv -> RealSrcSpan
    getLclEnvLoc = tcl_loc

    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

    -- These are here to avoid module loops: one might expect them
    -- in GHC.Tc.Types.Constraint, but they refer to ErrCtxt which refers to TcM.
    -- Easier to just keep these definitions here, alongside TcM.
    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 } }

    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 GHC.Tc.Gen.Splice

    {- 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

    ---------------------------
    -- The TcBinderStack
    ---------------------------

    type TcBinderStack = [TcBinder]
       -- This is a stack of locally-bound ids and tyvars,
       --   innermost on top
       -- Used only in error reporting (relevantBindings in TcError),
       --   and in tidying
       -- We can't use the tcl_env type environment, because it doesn't
       --   keep track of the nesting order

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

      | TcIdBndr_ExpType  -- Variant that allows the type to be specified as
                          -- an ExpType
           Name
           ExpType
           TopLevelFlag

      | TcTvBndr          -- e.g.   case x of P (y::a) -> blah
           Name           -- We bind the lexical name "a" to the type of y,
           TyVar          -- which might be an utterly different (perhaps
                          -- existential) tyvar

    instance Outputable TcBinder where
       ppr (TcIdBndr id top_lvl)           = ppr id <> brackets (ppr top_lvl)
       ppr (TcIdBndr_ExpType id _ top_lvl) = ppr id <> brackets (ppr top_lvl)
       ppr (TcTvBndr name tv)              = ppr name <+> ppr tv

    instance HasOccName TcBinder where
        occName (TcIdBndr id _)             = occName (idName id)
        occName (TcIdBndr_ExpType name _ _) = occName name
        occName (TcTvBndr name _)           = occName name

    -- fixes #12177
    -- Builds up a list of bindings whose OccName has not been seen before
    -- i.e., If    ys  = removeBindingShadowing xs
    -- then
    --  - ys is obtained from xs by deleting some elements
    --  - ys has no duplicate OccNames
    --  - The first duplicated OccName in xs is retained in ys
    -- Overloaded so that it can be used for both GlobalRdrElt in typed-hole
    -- substitutions and TcBinder when looking for relevant bindings.
    removeBindingShadowing :: HasOccName a => [a] -> [a]
    removeBindingShadowing bindings = reverse $ fst $ foldl
        (\(bindingAcc, seenNames) binding ->
        if occName binding `elemOccSet` seenNames -- if we've seen it
            then (bindingAcc, seenNames)              -- skip it
            else (binding:bindingAcc, extendOccSet seenNames (occName binding)))
        ([], emptyOccSet) bindings

    -- | Get target platform
    getPlatform :: TcM Platform
    getPlatform = targetPlatform <$> getDynFlags

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

    data SpliceType = Typed | Untyped

    data ThStage    -- See Note [Template Haskell state diagram]
                    -- and Note [Template Haskell levels] in GHC.Tc.Gen.Splice
        -- Start at:   Comp
        -- At bracket: wrap current stage in Brack
        -- At splice:  currently Brack: return to previous stage
        --             currently Comp/Splice: compile and run
      = Splice SpliceType -- Inside a top-level splice
                          -- This code will be run *at compile time*;
                          --   the result replaces the splice
                          -- Binding level = 0

      | RunSplice (TcRef [ForeignRef (TH.Q ())])
          -- Set when running a splice, i.e. NOT when renaming or typechecking the
          -- Haskell code for the splice. See Note [RunSplice ThLevel].
          --
          -- Contains a list of mod finalizers collected while executing the splice.
          --
          -- 'addModFinalizer' inserts finalizers here, and from here they are taken
          -- to construct an @HsSpliced@ annotation for untyped splices. See Note
          -- [Delaying modFinalizers in untyped splices] in GHC.Rename.Splice.
          --
          -- For typed splices, the typechecker takes finalizers from here and
          -- inserts them in the list of finalizers in the global environment.
          --
          -- See Note [Collecting modFinalizers in typed splices] in "GHC.Tc.Gen.Splice".

      | 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
          QuoteWrapper                -- A type variable and evidence variable
                                      -- for the overall monad of
                                      -- the bracket. Splices are checked
                                      -- against this monad. The evidence
                                      -- variable is used for desugaring
                                      -- `lift`.

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

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

    type ThLevel = Int
        -- NB: see Note [Template Haskell levels] in GHC.Tc.Gen.Splice
        -- 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
    thLevel (RunSplice _) = panic "thLevel: called when running a splice"
                            -- See Note [RunSplice ThLevel].

    {- Node [RunSplice ThLevel]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The 'RunSplice' stage is set when executing a splice, and only when running a
    splice. In particular it is not set when the splice is renamed or typechecked.

    'RunSplice' is needed to provide a reference where 'addModFinalizer' can insert
    the finalizer (see Note [Delaying modFinalizers in untyped splices]), and
    'addModFinalizer' runs when doing Q things. Therefore, It doesn't make sense to
    set 'RunSplice' when renaming or typechecking the splice, where 'Splice',
    'Brack' or 'Comp' are used instead.

    -}

    ---------------------------
    -- 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
    ---------------------------

    -- | A typecheckable thing available in a local context.  Could be
    -- 'AGlobal' 'TyThing', but also lexically scoped variables, etc.
    -- See "GHC.Tc.Utils.Env" for how to retrieve a 'TyThing' given a 'Name'.
    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_info :: IdBindingInfo   -- See Note [Meaning of IdBindingInfo]
          }

      | ATyVar  Name TcTyVar   -- See Note [Type variables in the type environment]

      | ATcTyCon TyCon   -- Used temporarily, during kind checking, for the
                         -- tycons and clases in this recursive group
                         -- The TyCon is always a TcTyCon.  Its kind
                         -- 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 GHC.Tc.Utils.Env.
      | ConstrainedDataConPE PredType
                         -- Data constructor with a non-equality context
                         -- See Note [Don't promote data constructors with
                         --           non-equality contexts] in GHC.Tc.Gen.HsType
      | PatSynPE         -- Pattern synonyms
                         -- See Note [Don't promote pattern synonyms] in GHC.Tc.Utils.Env

      | RecDataConPE     -- Data constructor in a recursive loop
                         -- See Note [Recursion and promoting data constructors] in GHC.Tc.TyCl
      | NoDataKindsTC    -- -XDataKinds not enabled (for a tycon)
      | NoDataKindsDC    -- -XDataKinds not enabled (for a datacon)

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

    -- | IdBindingInfo describes how an Id is bound.
    --
    -- It is used for the following purposes:
    -- a) for static forms in 'GHC.Tc.Gen.Expr.checkClosedInStaticForm' and
    -- b) to figure out when a nested binding can be generalised,
    --    in 'GHC.Tc.Gen.Bind.decideGeneralisationPlan'.
    --
    data IdBindingInfo -- See Note [Meaning of IdBindingInfo and ClosedTypeId]
        = NotLetBound
        | ClosedLet
        | NonClosedLet
             RhsNames        -- Used for (static e) checks only
             ClosedTypeId    -- Used for generalisation checks
                             -- and for (static e) checks

    -- | IsGroupClosed describes a group of mutually-recursive bindings
    data IsGroupClosed
      = IsGroupClosed
          (NameEnv RhsNames)  -- Free var info for the RHS of each binding in the goup
                              -- Used only for (static e) checks

          ClosedTypeId        -- True <=> all the free vars of the group are
                              --          imported or ClosedLet or
                              --          NonClosedLet with ClosedTypeId=True.
                              --          In particular, no tyvars, no NotLetBound

    type RhsNames = NameSet   -- Names of variables, mentioned on the RHS of
                              -- a definition, that are not Global or ClosedLet

    type ClosedTypeId = Bool
      -- See Note [Meaning of IdBindingInfo and ClosedTypeId]

    {- Note [Meaning of IdBindingInfo]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    NotLetBound means that
      the Id is not let-bound (e.g. it is bound in a
      lambda-abstraction or in a case pattern)

    ClosedLet means that
       - The Id is let-bound,
       - Any free term variables are also Global or ClosedLet
       - Its type has no free variables (NB: a top-level binding subject
         to the MR might have free vars in its type)
       These ClosedLets can definitely be floated to top level; and we
       may need to do so for static forms.

       Property:   ClosedLet
                 is equivalent to
                   NonClosedLet emptyNameSet True

    (NonClosedLet (fvs::RhsNames) (cl::ClosedTypeId)) means that
       - The Id is let-bound

       - The fvs::RhsNames contains the free names of the RHS,
         excluding Global and ClosedLet ones.

       - For the ClosedTypeId field see Note [Bindings with closed types]

    For (static e) to be valid, we need for every 'x' free in 'e',
    that x's binding is floatable to the top level.  Specifically:
       * x's RhsNames must be empty
       * x's type has no free variables
    See Note [Grand plan for static forms] in "GHC.Iface.Tidy.StaticPtrTable".
    This test is made in GHC.Tc.Gen.Expr.checkClosedInStaticForm.
    Actually knowing x's RhsNames (rather than just its emptiness
    or otherwise) is just so we can produce better error messages

    Note [Bindings with closed types: ClosedTypeId]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    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.

    Here's the invariant:
       If an Id has ClosedTypeId=True (in its IdBindingInfo), then
       the Id's type is /definitely/ closed (has no free type variables).
       Specifically,
           a) The Id's actual type is closed (has no free tyvars)
           b) Either the Id has a (closed) user-supplied type signature
              or all its free variables are Global/ClosedLet
                 or NonClosedLet with ClosedTypeId=True.
              In particular, none are NotLetBound.

    Why is (b) needed?   Consider
        \x. (x :: Int, let y = x+1 in ...)
    Initially x::alpha.  If we happen to typecheck the 'let' before the
    (x::Int), y's type will have a free tyvar; but if the other way round
    it won't.  So we treat any let-bound variable with a free
    non-let-bound variable as not ClosedTypeId, regardless of what the
    free vars of its type actually are.

    But if it has a signature, all is well:
       \x. ...(let { y::Int; y = x+1 } in
               let { v = y+2 } in ...)...
    Here the signature on 'v' makes 'y' a ClosedTypeId, so we can
    generalise 'v'.

    Note that:

      * A top-level binding may not have ClosedTypeId=True, if it suffers
        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)...

    Under OutsideIn we are free to generalise an Id all of whose free
    variables have ClosedTypeId=True (or imported).  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 [Type variables in the type environment]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The type environment has a binding for each lexically-scoped
    type variable that is in scope.  For example

      f :: forall a. a -> a
      f x = (x :: a)

      g1 :: [a] -> a
      g1 (ys :: [b]) = head ys :: b

      g2 :: [Int] -> Int
      g2 (ys :: [c]) = head ys :: c

    * The forall'd variable 'a' in the signature scopes over f's RHS.

    * The pattern-bound type variable 'b' in 'g1' scopes over g1's
      RHS; note that it is bound to a skolem 'a' which is not itself
      lexically in scope.

    * The pattern-bound type variable 'c' in 'g2' is bound to
      Int; that is, pattern-bound type variables can stand for
      arbitrary types. (see
        GHC proposal #128 "Allow ScopedTypeVariables to refer to types"
        https://github.com/ghc-proposals/ghc-proposals/pull/128,
      and the paper
        "Type variables in patterns", Haskell Symposium 2018.

    This is implemented by the constructor
       ATyVar Name TcTyVar
    in the type environment.

    * The Name is the name of the original, lexically scoped type
      variable

    * The TcTyVar is sometimes a skolem (like in 'f'), and sometimes
      a unification variable (like in 'g1', 'g2').  We never zonk the
      type environment so in the latter case it always stays as a
      unification variable, although that variable may be later
      unified with a type (such as Int in 'g2').
    -}

    instance Outputable IdBindingInfo where
      ppr NotLetBound = text "NotLetBound"
      ppr ClosedLet = text "TopLevelLet"
      ppr (NonClosedLet fvs closed_type) =
        text "TopLevelLet" <+> ppr fvs <+> ppr closed_type

    instance Outputable PromotionErr where
      ppr ClassPE                     = text "ClassPE"
      ppr TyConPE                     = text "TyConPE"
      ppr PatSynPE                    = text "PatSynPE"
      ppr FamDataConPE                = text "FamDataConPE"
      ppr (ConstrainedDataConPE pred) = text "ConstrainedDataConPE"
                                          <+> parens (ppr pred)
      ppr RecDataConPE                = text "RecDataConPE"
      ppr NoDataKindsTC               = text "NoDataKindsTC"
      ppr NoDataKindsDC               = text "NoDataKindsDC"

    pprTcTyThingCategory :: TcTyThing -> SDoc
    pprTcTyThingCategory (AGlobal thing)    = pprTyThingCategory thing
    pprTcTyThingCategory (ATyVar {})        = text "Type variable"
    pprTcTyThingCategory (ATcId {})         = text "Local identifier"
    pprTcTyThingCategory (ATcTyCon {})     = text "Local tycon"
    pprTcTyThingCategory (APromotionErr pe) = pprPECategory pe

    pprPECategory :: PromotionErr -> SDoc
    pprPECategory ClassPE                = text "Class"
    pprPECategory TyConPE                = text "Type constructor"
    pprPECategory PatSynPE               = text "Pattern synonym"
    pprPECategory FamDataConPE           = text "Data constructor"
    pprPECategory ConstrainedDataConPE{} = text "Data constructor"
    pprPECategory RecDataConPE           = text "Data constructor"
    pprPECategory NoDataKindsTC          = text "Type constructor"
    pprPECategory NoDataKindsDC          = text "Data constructor"

    {-
    ************************************************************************
    *                                                                      *
            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 [ImportedModsVal],
              -- ^ Domain is all directly-imported modules
              --
              -- See the documentation on ImportedModsVal in "GHC.Driver.Types" for the
              -- meaning of the fields.
              --
              -- 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 ModuleNameWithIsBoot,
              -- ^ 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 :: Set UnitId,
              -- ^ 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 :: Set UnitId,
              -- ^ 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 (UnitId, 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 [Tracking Trust Transitively] in "GHC.Rename.Names"

            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 [Trust Own Package] in "GHC.Rename.Names"

            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 :: [ModuleNameWithIsBoot]
              -> ModuleNameEnv ModuleNameWithIsBoot
    mkModDeps deps = foldl' add emptyUFM deps
      where
        add env elt = addToUFM env (gwib_mod elt) elt

    modDepsElts
      :: ModuleNameEnv ModuleNameWithIsBoot
      -> [ModuleNameWithIsBoot]
    modDepsElts = sort . nonDetEltsUFM
      -- It's OK to use nonDetEltsUFM here because sorting by module names
      -- restores determinism

    emptyImportAvails :: ImportAvails
    emptyImportAvails = ImportAvails { imp_mods          = emptyModuleEnv,
                                       imp_dep_mods      = emptyUFM,
                                       imp_dep_pkgs      = S.empty,
                                       imp_trust_pkgs    = S.empty,
                                       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 `S.union` dpkgs2,
                       imp_trust_pkgs    = tpkgs1 `S.union` tpkgs2,
                       imp_trust_own_pkg = tself1 || tself2,
                       imp_orphs         = orphs1 `unionLists` orphs2,
                       imp_finsts        = finsts1 `unionLists` finsts2 }
      where
        plus_mod_dep r1@(GWIB { gwib_mod = m1, gwib_isBoot = boot1 })
                     r2@(GWIB {gwib_mod = m2, gwib_isBoot = boot2})
          | ASSERT2( m1 == m2, (ppr m1 <+> ppr m2) $$ (ppr (boot1 == IsBoot) <+> ppr (boot2 == IsBoot)))
            boot1 == IsBoot = r2
          | otherwise = r1
          -- If either side can "see" a non-hi-boot interface, use that
          -- Reusing existing tuples saves 10% of allocations on test
          -- perf/compiler/MultiLayerModules

    {-
    ************************************************************************
    *                                                                      *
    \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 GHC.Iface.Load

    instance Outputable WhereFrom where
      ppr (ImportByUser IsBoot)                = text "{- SOURCE -}"
      ppr (ImportByUser NotBoot)               = empty
      ppr ImportBySystem                       = text "{- SYSTEM -}"
      ppr ImportByPlugin                       = text "{- PLUGIN -}"

    {- *********************************************************************
    *                                                                      *
                    Type signatures
    *                                                                      *
    ********************************************************************* -}

    -- These data types need to be here only because
    -- GHC.Tc.Solver uses them, and GHC.Tc.Solver is fairly
    -- low down in the module hierarchy

    type TcSigFun  = Name -> Maybe TcSigInfo

    data TcSigInfo = TcIdSig     TcIdSigInfo
                   | TcPatSynSig TcPatSynInfo

    data TcIdSigInfo   -- See Note [Complete and partial type signatures]
      = CompleteSig    -- A complete signature with no wildcards,
                       -- so the complete polymorphic type is known.
          { sig_bndr :: TcId          -- The polymorphic Id with that type

          , sig_ctxt :: UserTypeCtxt  -- In the case of type-class default methods,
                                      -- the Name in the FunSigCtxt is not the same
                                      -- as the TcId; the former is 'op', while the
                                      -- latter is '$dmop' or some such

          , sig_loc  :: SrcSpan       -- Location of the type signature
          }

      | PartialSig     -- A partial type signature (i.e. includes one or more
                       -- wildcards). In this case it doesn't make sense to give
                       -- the polymorphic Id, because we are going to /infer/ its
                       -- type, so we can't make the polymorphic Id ab-initio
          { psig_name  :: Name   -- Name of the function; used when report wildcards
          , psig_hs_ty :: LHsSigWcType GhcRn  -- The original partial signature in
                                              -- HsSyn form
          , sig_ctxt   :: UserTypeCtxt
          , sig_loc    :: SrcSpan            -- Location of the type signature
          }

    {- Note [Complete and partial type signatures]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    A type signature is partial when it contains one or more wildcards
    (= type holes).  The wildcard can either be:
    * A (type) wildcard occurring in sig_theta or sig_tau. These are
      stored in sig_wcs.
          f :: Bool -> _
          g :: Eq _a => _a -> _a -> Bool
    * Or an extra-constraints wildcard, stored in sig_cts:
          h :: (Num a, _) => a -> a

    A type signature is a complete type signature when there are no
    wildcards in the type signature, i.e. iff sig_wcs is empty and
    sig_extra_cts is Nothing.
    -}

    data TcIdSigInst
      = TISI { sig_inst_sig :: TcIdSigInfo

             , sig_inst_skols :: [(Name, InvisTVBinder)]
                   -- Instantiated type and kind variables, TyVarTvs
                   -- The Name is the Name that the renamer chose;
                   --   but the TcTyVar may come from instantiating
                   --   the type and hence have a different unique.
                   -- No need to keep track of whether they are truly lexically
                   --   scoped because the renamer has named them uniquely
                   -- See Note [Binding scoped type variables] in GHC.Tc.Gen.Sig
                   --
                   -- NB: The order of sig_inst_skols is irrelevant
                   --     for a CompleteSig, but for a PartialSig see
                   --     Note [Quantified variables in partial type signatures]

             , sig_inst_theta  :: TcThetaType
                   -- Instantiated theta.  In the case of a
                   -- PartialSig, sig_theta does not include
                   -- the extra-constraints wildcard

             , sig_inst_tau :: TcSigmaType   -- Instantiated tau
                   -- See Note [sig_inst_tau may be polymorphic]

             -- Relevant for partial signature only
             , sig_inst_wcs   :: [(Name, TcTyVar)]
                   -- Like sig_inst_skols, but for /named/ wildcards (_a etc).
                   -- The named wildcards scope over the binding, and hence
                   -- their Names may appear in type signatures in the binding

             , sig_inst_wcx   :: Maybe TcType
                   -- Extra-constraints wildcard to fill in, if any
                   -- If this exists, it is surely of the form (meta_tv |> co)
                   -- (where the co might be reflexive). This is filled in
                   -- only from the return value of GHC.Tc.Gen.HsType.tcAnonWildCardOcc
             }

    {- Note [sig_inst_tau may be polymorphic]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Note that "sig_inst_tau" might actually be a polymorphic type,
    if the original function had a signature like
       forall a. Eq a => forall b. Ord b => ....
    But that's ok: tcMatchesFun (called by tcRhs) can deal with that
    It happens, too!  See Note [Polymorphic methods] in GHC.Tc.TyCl.Class.

    Note [Quantified variables in partial type signatures]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
       f :: forall a b. _ -> a -> _ -> b
       f (x,y) p q = q

    Then we expect f's final type to be
      f :: forall {x,y}. forall a b. (x,y) -> a -> b -> b

    Note that x,y are Inferred, and can't be use for visible type
    application (VTA).  But a,b are Specified, and remain Specified
    in the final type, so we can use VTA for them.  (Exception: if
    it turns out that a's kind mentions b we need to reorder them
    with scopedSort.)

    The sig_inst_skols of the TISI from a partial signature records
    that original order, and is used to get the variables of f's
    final type in the correct order.

    Note [Wildcards in partial signatures]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The wildcards in psig_wcs may stand for a type mentioning
    the universally-quantified tyvars of psig_ty

    E.g.  f :: forall a. _ -> a
          f x = x
    We get sig_inst_skols = [a]
           sig_inst_tau   = _22 -> a
           sig_inst_wcs   = [_22]
    and _22 in the end is unified with the type 'a'

    Moreover the kind of a wildcard in sig_inst_wcs may mention
    the universally-quantified tyvars sig_inst_skols
    e.g.   f :: t a -> t _
    Here we get
       sig_inst_skols = [k:*, (t::k ->*), (a::k)]
       sig_inst_tau   = t a -> t _22
       sig_inst_wcs   = [ _22::k ]
    -}

    data TcPatSynInfo
      = TPSI {
            patsig_name           :: Name,
            patsig_implicit_bndrs :: [InvisTVBinder], -- Implicitly-bound kind vars (Inferred) and
                                                      -- implicitly-bound type vars (Specified)
              -- See Note [The pattern-synonym signature splitting rule] in GHC.Tc.TyCl.PatSyn
            patsig_univ_bndrs     :: [InvisTVBinder], -- Bound by explicit user forall
            patsig_req            :: TcThetaType,
            patsig_ex_bndrs       :: [InvisTVBinder], -- Bound by explicit user forall
            patsig_prov           :: TcThetaType,
            patsig_body_ty        :: TcSigmaType
        }

    instance Outputable TcSigInfo where
      ppr (TcIdSig     idsi) = ppr idsi
      ppr (TcPatSynSig tpsi) = text "TcPatSynInfo" <+> ppr tpsi

    instance Outputable TcIdSigInfo where
        ppr (CompleteSig { sig_bndr = bndr })
            = ppr bndr <+> dcolon <+> ppr (idType bndr)
        ppr (PartialSig { psig_name = name, psig_hs_ty = hs_ty })
            = text "psig" <+> ppr name <+> dcolon <+> ppr hs_ty

    instance Outputable TcIdSigInst where
        ppr (TISI { sig_inst_sig = sig, sig_inst_skols = skols
                  , sig_inst_theta = theta, sig_inst_tau = tau })
            = hang (ppr sig) 2 (vcat [ ppr skols, ppr theta <+> darrow <+> ppr tau ])

    instance Outputable TcPatSynInfo where
        ppr (TPSI{ patsig_name = name}) = ppr name

    isPartialSig :: TcIdSigInst -> Bool
    isPartialSig (TISI { sig_inst_sig = PartialSig {} }) = True
    isPartialSig _                                       = False

    -- | No signature or a partial signature
    hasCompleteSig :: TcSigFun -> Name -> Bool
    hasCompleteSig sig_fn name
      = case sig_fn name of
          Just (TcIdSig (CompleteSig {})) -> True
          _                               -> False

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

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

    newtype TcPluginM a = TcPluginM (EvBindsVar -> TcM a) deriving (Functor)

    instance Applicative TcPluginM where
      pure x = TcPluginM (const $ pure x)
      (<*>) = ap

    instance Monad TcPluginM where
      TcPluginM m >>= k =
        TcPluginM (\ ev -> do a <- m ev
                              runTcPluginM (k a) ev)

    instance MonadFail TcPluginM where
      fail x   = TcPluginM (const $ fail x)

    runTcPluginM :: TcPluginM a -> EvBindsVar -> 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 . const

    -- | Access the 'EvBindsVar' carried by the 'TcPluginM' during
    -- constraint solving.  Returns 'Nothing' if invoked during
    -- 'tcPluginInit' or 'tcPluginStop'.
    getEvBindsTcPluginM :: TcPluginM EvBindsVar
    getEvBindsTcPluginM = TcPluginM return

    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 insoluble.

      | 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.

    {- *********************************************************************
    *                                                                      *
                            Role annotations
    *                                                                      *
    ********************************************************************* -}

    type RoleAnnotEnv = NameEnv (LRoleAnnotDecl GhcRn)

    mkRoleAnnotEnv :: [LRoleAnnotDecl GhcRn] -> RoleAnnotEnv
    mkRoleAnnotEnv role_annot_decls
     = mkNameEnv [ (name, ra_decl)
                 | ra_decl <- role_annot_decls
                 , let name = roleAnnotDeclName (unLoc ra_decl)
                 , not (isUnboundName name) ]
           -- Some of the role annots will be unbound;
           -- we don't wish to include these

    emptyRoleAnnotEnv :: RoleAnnotEnv
    emptyRoleAnnotEnv = emptyNameEnv

    lookupRoleAnnot :: RoleAnnotEnv -> Name -> Maybe (LRoleAnnotDecl GhcRn)
    lookupRoleAnnot = lookupNameEnv

    getRoleAnnots :: [Name] -> RoleAnnotEnv -> [LRoleAnnotDecl GhcRn]
    getRoleAnnots bndrs role_env
      = mapMaybe (lookupRoleAnnot role_env) bndrs

    {- *********************************************************************
    *                                                                      *
                      Linting a TcGblEnv
    *                                                                      *
    ********************************************************************* -}

    -- | Check the 'TcGblEnv' for consistency. Currently, only checks
    -- axioms, but should check other aspects, too.
    lintGblEnv :: DynFlags -> TcGblEnv -> (Bag SDoc, Bag SDoc)
    lintGblEnv dflags tcg_env = lintAxioms dflags axioms
      where
        axioms = typeEnvCoAxioms (tcg_type_env tcg_env)