Language/Haskell/Syntax/Decls.hs

    {-# LANGUAGE CPP #-}
    {-# LANGUAGE ConstraintKinds #-}
    {-# LANGUAGE DeriveDataTypeable #-}
    {-# LANGUAGE FlexibleContexts #-}
    {-# LANGUAGE FlexibleInstances #-}
    {-# LANGUAGE ScopedTypeVariables #-}
    {-# LANGUAGE TypeApplications #-}
    {-# LANGUAGE TypeFamilies #-}
    {-# LANGUAGE UndecidableInstances #-} -- Wrinkle in Note [Trees That Grow]
                                          -- in module Language.Haskell.Syntax.Extension
    {-# LANGUAGE ViewPatterns #-}

    {-
    (c) The University of Glasgow 2006
    (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
    -}

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

    -- See Note [Language.Haskell.Syntax.* Hierarchy] for why not GHC.Hs.*

    -- | Abstract syntax of global declarations.
    --
    -- Definitions for: @SynDecl@ and @ConDecl@, @ClassDecl@,
    -- @InstDecl@, @DefaultDecl@ and @ForeignDecl@.
    module Language.Haskell.Syntax.Decls (
      -- * Toplevel declarations
      HsDecl(..), LHsDecl, HsDataDefn(..), HsDeriving, LHsFunDep, FunDep(..),
      HsDerivingClause(..), LHsDerivingClause, DerivClauseTys(..), LDerivClauseTys,
      NewOrData(..), newOrDataToFlavour,
      StandaloneKindSig(..), LStandaloneKindSig,

      -- ** Class or type declarations
      TyClDecl(..), LTyClDecl, DataDeclRn(..),
      TyClGroup(..),
      tyClGroupTyClDecls, tyClGroupInstDecls, tyClGroupRoleDecls,
      tyClGroupKindSigs,
      isClassDecl, isDataDecl, isSynDecl,
      isFamilyDecl, isTypeFamilyDecl, isDataFamilyDecl,
      isOpenTypeFamilyInfo, isClosedTypeFamilyInfo,
      countTyClDecls,
      tyClDeclTyVars,
      FamilyDecl(..), LFamilyDecl,

      -- ** Instance declarations
      InstDecl(..), LInstDecl, FamilyInfo(..), pprFlavour,
      TyFamInstDecl(..), LTyFamInstDecl,
      TyFamDefltDecl, LTyFamDefltDecl,
      DataFamInstDecl(..), LDataFamInstDecl,
      FamEqn(..), TyFamInstEqn, LTyFamInstEqn, HsTyPats,
      LClsInstDecl, ClsInstDecl(..),

      -- ** Standalone deriving declarations
      DerivDecl(..), LDerivDecl,
      -- ** Deriving strategies
      DerivStrategy(..), LDerivStrategy,
      derivStrategyName,
      -- ** @RULE@ declarations
      LRuleDecls,RuleDecls(..),RuleDecl(..),LRuleDecl,HsRuleRn(..),
      RuleBndr(..),LRuleBndr,
      collectRuleBndrSigTys,
      pprFullRuleName,
      -- ** @default@ declarations
      DefaultDecl(..), LDefaultDecl,
      -- ** Template haskell declaration splice
      SpliceExplicitFlag(..),
      SpliceDecl(..), LSpliceDecl,
      -- ** Foreign function interface declarations
      ForeignDecl(..), LForeignDecl, ForeignImport(..), ForeignExport(..),
      CImportSpec(..),
      -- ** Data-constructor declarations
      ConDecl(..), LConDecl,
      HsConDeclH98Details, HsConDeclGADTDetails(..),
      -- ** Document comments
      DocDecl(..), LDocDecl, docDeclDoc,
      -- ** Deprecations
      WarnDecl(..),  LWarnDecl,
      WarnDecls(..), LWarnDecls,
      -- ** Annotations
      AnnDecl(..), LAnnDecl,
      AnnProvenance(..), annProvenanceName_maybe,
      -- ** Role annotations
      RoleAnnotDecl(..), LRoleAnnotDecl,
      -- ** Injective type families
      FamilyResultSig(..), LFamilyResultSig, InjectivityAnn(..), LInjectivityAnn,

      -- * Grouping
      HsGroup(..), hsGroupInstDecls,
        ) where

    -- friends:
    import GHC.Prelude

    import {-# SOURCE #-} Language.Haskell.Syntax.Expr
      ( HsExpr, HsSplice )
            -- Because Expr imports Decls via HsBracket

    import Language.Haskell.Syntax.Binds
    import Language.Haskell.Syntax.Type
    import GHC.Hs.Doc
    import GHC.Core.TyCon
    import GHC.Types.Basic
    import GHC.Types.ForeignCall
    import Language.Haskell.Syntax.Extension
    import GHC.Types.Name.Set
    import GHC.Types.Fixity

    -- others:
    import GHC.Utils.Outputable
    import GHC.Utils.Misc
    import GHC.Types.SrcLoc
    import GHC.Types.SourceText
    import GHC.Core.Type
    import GHC.Unit.Module.Warnings

    import GHC.Data.Maybe
    import Data.Data        hiding (TyCon,Fixity, Infix)
    import Data.Void

    {-
    ************************************************************************
    *                                                                      *
    \subsection[HsDecl]{Declarations}
    *                                                                      *
    ************************************************************************
    -}

    type LHsDecl p = XRec p (HsDecl p)
            -- ^ When in a list this may have
            --
            --  - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnSemi'
            --

    -- For details on above see note [exact print annotations] in GHC.Parser.Annotation

    -- | A Haskell Declaration
    data HsDecl p
      = TyClD      (XTyClD p)      (TyClDecl p)      -- ^ Type or Class Declaration
      | InstD      (XInstD p)      (InstDecl  p)     -- ^ Instance declaration
      | DerivD     (XDerivD p)     (DerivDecl p)     -- ^ Deriving declaration
      | ValD       (XValD p)       (HsBind p)        -- ^ Value declaration
      | SigD       (XSigD p)       (Sig p)           -- ^ Signature declaration
      | KindSigD   (XKindSigD p)   (StandaloneKindSig p) -- ^ Standalone kind signature
      | DefD       (XDefD p)       (DefaultDecl p)   -- ^ 'default' declaration
      | ForD       (XForD p)       (ForeignDecl p)   -- ^ Foreign declaration
      | WarningD   (XWarningD p)   (WarnDecls p)     -- ^ Warning declaration
      | AnnD       (XAnnD p)       (AnnDecl p)       -- ^ Annotation declaration
      | RuleD      (XRuleD p)      (RuleDecls p)     -- ^ Rule declaration
      | SpliceD    (XSpliceD p)    (SpliceDecl p)    -- ^ Splice declaration
                                                     -- (Includes quasi-quotes)
      | DocD       (XDocD p)       (DocDecl)  -- ^ Documentation comment declaration
      | RoleAnnotD (XRoleAnnotD p) (RoleAnnotDecl p) -- ^Role annotation declaration
      | XHsDecl    !(XXHsDecl p)

    {-
    Note [Top-level fixity signatures in an HsGroup]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    An `HsGroup p` stores every top-level fixity declarations in one of two places:

    1. hs_fixds :: [LFixitySig p]

       This stores fixity signatures for top-level declarations (e.g., functions,
       data constructors, classes, type families, etc.) as well as fixity
       signatures for class methods written outside of the class, as in this
       example:

         infixl 4 `m1`
         class C1 a where
           m1 :: a -> a -> a

    2. hs_tyclds :: [TyClGroup p]

       Each type class can be found in a TyClDecl inside a TyClGroup, and that
       TyClDecl stores the fixity signatures for its methods written inside of the
       class, as in this example:

         class C2 a where
           infixl 4 `m2`
           m2 :: a -> a -> a

    The story for fixity signatures for class methods is made slightly complicated
    by the fact that they can appear both inside and outside of the class itself,
    and both forms of fixity signatures are considered top-level. This matters
    in `GHC.Rename.Module.rnSrcDecls`, which must create a fixity environment out
    of all top-level fixity signatures before doing anything else. Therefore,
    `rnSrcDecls` must be aware of both (1) and (2) above. The
    `hsGroupTopLevelFixitySigs` function is responsible for collecting this
    information from an `HsGroup`.

    One might wonder why we even bother separating top-level fixity signatures
    into two places at all. That is, why not just take the fixity signatures
    from `hs_tyclds` and put them into `hs_fixds` so that they are all in one
    location? This ends up causing problems for `GHC.HsToCore.Quote.repTopDs`,
    which translates each fixity signature in `hs_fixds` and `hs_tyclds` into a
    Template Haskell `Dec`. If there are any duplicate signatures between the two
    fields, this will result in an error (#17608).
    -}

    -- | Haskell Group
    --
    -- A 'HsDecl' is categorised into a 'HsGroup' before being
    -- fed to the renamer.
    data HsGroup p
      = HsGroup {
            hs_ext    :: XCHsGroup p,
            hs_valds  :: HsValBinds p,
            hs_splcds :: [LSpliceDecl p],

            hs_tyclds :: [TyClGroup p],
                    -- A list of mutually-recursive groups;
                    -- This includes `InstDecl`s as well;
                    -- Parser generates a singleton list;
                    -- renamer does dependency analysis

            hs_derivds :: [LDerivDecl p],

            hs_fixds  :: [LFixitySig p],
                    -- A list of fixity signatures defined for top-level
                    -- declarations and class methods (defined outside of the class
                    -- itself).
                    -- See Note [Top-level fixity signatures in an HsGroup]

            hs_defds  :: [LDefaultDecl p],
            hs_fords  :: [LForeignDecl p],
            hs_warnds :: [LWarnDecls p],
            hs_annds  :: [LAnnDecl p],
            hs_ruleds :: [LRuleDecls p],

            hs_docs   :: [LDocDecl p]
        }
      | XHsGroup !(XXHsGroup p)

    hsGroupInstDecls :: HsGroup id -> [LInstDecl id]
    hsGroupInstDecls = (=<<) group_instds . hs_tyclds

    -- | Located Splice Declaration
    type LSpliceDecl pass = XRec pass (SpliceDecl pass)

    -- | Splice Declaration
    data SpliceDecl p
      = SpliceDecl                  -- Top level splice
            (XSpliceDecl p)
            (XRec p (HsSplice p))
            SpliceExplicitFlag
      | XSpliceDecl !(XXSpliceDecl p)

    {-
    ************************************************************************
    *                                                                      *
                Type and class declarations
    *                                                                      *
    ************************************************************************

    Note [The Naming story]
    ~~~~~~~~~~~~~~~~~~~~~~~
    Here is the story about the implicit names that go with type, class,
    and instance decls.  It's a bit tricky, so pay attention!

    "Implicit" (or "system") binders
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      Each data type decl defines
            a worker name for each constructor
            to-T and from-T convertors
      Each class decl defines
            a tycon for the class
            a data constructor for that tycon
            the worker for that constructor
            a selector for each superclass

    All have occurrence names that are derived uniquely from their parent
    declaration.

    None of these get separate definitions in an interface file; they are
    fully defined by the data or class decl.  But they may *occur* in
    interface files, of course.  Any such occurrence must haul in the
    relevant type or class decl.

    Plan of attack:
     - Ensure they "point to" the parent data/class decl
       when loading that decl from an interface file
       (See RnHiFiles.getSysBinders)

     - When typechecking the decl, we build the implicit TyCons and Ids.
       When doing so we look them up in the name cache (GHC.Rename.Env.lookupSysName),
       to ensure correct module and provenance is set

    These are the two places that we have to conjure up the magic derived
    names.  (The actual magic is in GHC.Types.Name.Occurrence.mkWorkerOcc, etc.)

    Default methods
    ~~~~~~~~~~~~~~~
     - Occurrence name is derived uniquely from the method name
       E.g. $dmmax

     - If there is a default method name at all, it's recorded in
       the ClassOpSig (in GHC.Hs.Binds), in the DefMethInfo field.
       (DefMethInfo is defined in GHC.Core.Class)

    Source-code class decls and interface-code class decls are treated subtly
    differently, which has given me a great deal of confusion over the years.
    Here's the deal.  (We distinguish the two cases because source-code decls
    have (Just binds) in the tcdMeths field, whereas interface decls have Nothing.

    In *source-code* class declarations:

     - When parsing, every ClassOpSig gets a DefMeth with a suitable RdrName
       This is done by GHC.Parser.PostProcess.mkClassOpSigDM

     - The renamer renames it to a Name

     - During typechecking, we generate a binding for each $dm for
       which there's a programmer-supplied default method:
            class Foo a where
              op1 :: <type>
              op2 :: <type>
              op1 = ...
       We generate a binding for $dmop1 but not for $dmop2.
       The Class for Foo has a Nothing for op2 and
                             a Just ($dm_op1, VanillaDM) for op1.
       The Name for $dmop2 is simply discarded.

    In *interface-file* class declarations:
      - When parsing, we see if there's an explicit programmer-supplied default method
        because there's an '=' sign to indicate it:
            class Foo a where
              op1 = :: <type>       -- NB the '='
              op2   :: <type>
        We use this info to generate a DefMeth with a suitable RdrName for op1,
        and a NoDefMeth for op2
      - The interface file has a separate definition for $dmop1, with unfolding etc.
      - The renamer renames it to a Name.
      - The renamer treats $dmop1 as a free variable of the declaration, so that
        the binding for $dmop1 will be sucked in.  (See RnHsSyn.tyClDeclFVs)
        This doesn't happen for source code class decls, because they *bind* the default method.

    Dictionary functions
    ~~~~~~~~~~~~~~~~~~~~
    Each instance declaration gives rise to one dictionary function binding.

    The type checker makes up new source-code instance declarations
    (e.g. from 'deriving' or generic default methods --- see
    GHC.Tc.TyCl.Instance.tcInstDecls1).  So we can't generate the names for
    dictionary functions in advance (we don't know how many we need).

    On the other hand for interface-file instance declarations, the decl
    specifies the name of the dictionary function, and it has a binding elsewhere
    in the interface file:
            instance {Eq Int} = dEqInt
            dEqInt :: {Eq Int} <pragma info>

    So again we treat source code and interface file code slightly differently.

    Source code:
      - Source code instance decls have a Nothing in the (Maybe name) field
        (see data InstDecl below)

      - The typechecker makes up a Local name for the dict fun for any source-code
        instance decl, whether it comes from a source-code instance decl, or whether
        the instance decl is derived from some other construct (e.g. 'deriving').

      - The occurrence name it chooses is derived from the instance decl (just for
        documentation really) --- e.g. dNumInt.  Two dict funs may share a common
        occurrence name, but will have different uniques.  E.g.
            instance Foo [Int]  where ...
            instance Foo [Bool] where ...
        These might both be dFooList

      - The CoreTidy phase externalises the name, and ensures the occurrence name is
        unique (this isn't special to dict funs).  So we'd get dFooList and dFooList1.

      - We can take this relaxed approach (changing the occurrence name later)
        because dict fun Ids are not captured in a TyCon or Class (unlike default
        methods, say).  Instead, they are kept separately in the InstEnv.  This
        makes it easy to adjust them after compiling a module.  (Once we've finished
        compiling that module, they don't change any more.)

    Interface file code:
      - The instance decl gives the dict fun name, so the InstDecl has a (Just name)
        in the (Maybe name) field.

      - RnHsSyn.instDeclFVs treats the dict fun name as free in the decl, so that we
        suck in the dfun binding
    -}

    -- | Located Declaration of a Type or Class
    type LTyClDecl pass = XRec pass (TyClDecl pass)

    -- | A type or class declaration.
    data TyClDecl pass
      = -- | @type/data family T :: *->*@
        --
        --  - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnType',
        --             'GHC.Parser.Annotation.AnnData',
        --             'GHC.Parser.Annotation.AnnFamily','GHC.Parser.Annotation.AnnDcolon',
        --             'GHC.Parser.Annotation.AnnWhere','GHC.Parser.Annotation.AnnOpenP',
        --             'GHC.Parser.Annotation.AnnDcolon','GHC.Parser.Annotation.AnnCloseP',
        --             'GHC.Parser.Annotation.AnnEqual','GHC.Parser.Annotation.AnnRarrow',
        --             'GHC.Parser.Annotation.AnnVbar'

        -- For details on above see note [exact print annotations] in GHC.Parser.Annotation
        FamDecl { tcdFExt :: XFamDecl pass, tcdFam :: FamilyDecl pass }

      | -- | @type@ declaration
        --
        --  - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnType',
        --             'GHC.Parser.Annotation.AnnEqual',

        -- For details on above see note [exact print annotations] in GHC.Parser.Annotation
        SynDecl { tcdSExt   :: XSynDecl pass          -- ^ Post renameer, FVs
                , tcdLName  :: LIdP pass              -- ^ Type constructor
                , tcdTyVars :: LHsQTyVars pass        -- ^ Type variables; for an
                                                      -- associated type these
                                                      -- include outer binders
                , tcdFixity :: LexicalFixity          -- ^ Fixity used in the declaration
                , tcdRhs    :: LHsType pass }         -- ^ RHS of type declaration

      | -- | @data@ declaration
        --
        --  - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnData',
        --              'GHC.Parser.Annotation.AnnFamily',
        --              'GHC.Parser.Annotation.AnnNewType',
        --              'GHC.Parser.Annotation.AnnNewType','GHC.Parser.Annotation.AnnDcolon'
        --              'GHC.Parser.Annotation.AnnWhere',

        -- For details on above see note [exact print annotations] in GHC.Parser.Annotation
        DataDecl { tcdDExt     :: XDataDecl pass       -- ^ Post renamer, CUSK flag, FVs
                 , tcdLName    :: LIdP pass             -- ^ Type constructor
                 , tcdTyVars   :: LHsQTyVars pass      -- ^ Type variables
                                  -- See Note [TyVar binders for associated declarations]
                 , tcdFixity   :: LexicalFixity        -- ^ Fixity used in the declaration
                 , tcdDataDefn :: HsDataDefn pass }

      | ClassDecl { tcdCExt    :: XClassDecl pass,         -- ^ Post renamer, FVs
                    tcdCtxt    :: Maybe (LHsContext pass), -- ^ Context...
                    tcdLName   :: LIdP pass,               -- ^ Name of the class
                    tcdTyVars  :: LHsQTyVars pass,         -- ^ Class type variables
                    tcdFixity  :: LexicalFixity, -- ^ Fixity used in the declaration
                    tcdFDs     :: [LHsFunDep pass],         -- ^ Functional deps
                    tcdSigs    :: [LSig pass],              -- ^ Methods' signatures
                    tcdMeths   :: LHsBinds pass,            -- ^ Default methods
                    tcdATs     :: [LFamilyDecl pass],       -- ^ Associated types;
                    tcdATDefs  :: [LTyFamDefltDecl pass],   -- ^ Associated type defaults
                    tcdDocs    :: [LDocDecl pass]           -- ^ Haddock docs
        }
            -- ^ - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnClass',
            --           'GHC.Parser.Annotation.AnnWhere','GHC.Parser.Annotation.AnnOpen',
            --           'GHC.Parser.Annotation.AnnClose'
            --   - The tcdFDs will have 'GHC.Parser.Annotation.AnnVbar',
            --                          'GHC.Parser.Annotation.AnnComma'
            --                          'GHC.Parser.Annotation.AnnRarrow'

            -- For details on above see note [exact print annotations] in GHC.Parser.Annotation
      | XTyClDecl !(XXTyClDecl pass)

    data FunDep pass
      = FunDep (XCFunDep pass)
               [LIdP pass]
               [LIdP pass]
      | XFunDep !(XXFunDep pass)

    type LHsFunDep pass = XRec pass (FunDep pass)

    data DataDeclRn = DataDeclRn
                 { tcdDataCusk :: Bool    -- ^ does this have a CUSK?
                     -- See Note [CUSKs: complete user-supplied kind signatures]
                 , tcdFVs      :: NameSet }
      deriving Data

    {- Note [TyVar binders for associated decls]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    For an /associated/ data, newtype, or type-family decl, the LHsQTyVars
    /includes/ outer binders.  For example
        class T a where
           data D a c
           type F a b :: *
           type F a b = a -> a
    Here the data decl for 'D', and type-family decl for 'F', both include 'a'
    in their LHsQTyVars (tcdTyVars and fdTyVars resp).

    Ditto any implicit binders in the hsq_implicit field of the LHSQTyVars.

    The idea is that the associated type is really a top-level decl in its
    own right.  However we are careful to use the same name 'a', so that
    we can match things up.

    c.f. Note [Associated type tyvar names] in GHC.Core.Class
         Note [Family instance declaration binders]
    -}

    {- Note [Class LayoutInfo]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    The LayoutInfo is used to associate Haddock comments with parts of the declaration.
    Compare the following examples:

        class C a where
          f :: a -> Int
          -- ^ comment on f

        class C a where
          f :: a -> Int
        -- ^ comment on C

    Notice how "comment on f" and "comment on C" differ only by indentation level.
    Thus we have to record the indentation level of the class declarations.

    See also Note [Adding Haddock comments to the syntax tree] in GHC.Parser.PostProcess.Haddock
    -}

    -- Simple classifiers for TyClDecl
    -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    -- | @True@ <=> argument is a @data@\/@newtype@
    -- declaration.
    isDataDecl :: TyClDecl pass -> Bool
    isDataDecl (DataDecl {}) = True
    isDataDecl _other        = False

    -- | type or type instance declaration
    isSynDecl :: TyClDecl pass -> Bool
    isSynDecl (SynDecl {})   = True
    isSynDecl _other        = False

    -- | type class
    isClassDecl :: TyClDecl pass -> Bool
    isClassDecl (ClassDecl {}) = True
    isClassDecl _              = False

    -- | type/data family declaration
    isFamilyDecl :: TyClDecl pass -> Bool
    isFamilyDecl (FamDecl {})  = True
    isFamilyDecl _other        = False

    -- | type family declaration
    isTypeFamilyDecl :: TyClDecl pass -> Bool
    isTypeFamilyDecl (FamDecl _ (FamilyDecl { fdInfo = info })) = case info of
      OpenTypeFamily      -> True
      ClosedTypeFamily {} -> True
      _                   -> False
    isTypeFamilyDecl _ = False

    -- | open type family info
    isOpenTypeFamilyInfo :: FamilyInfo pass -> Bool
    isOpenTypeFamilyInfo OpenTypeFamily = True
    isOpenTypeFamilyInfo _              = False

    -- | closed type family info
    isClosedTypeFamilyInfo :: FamilyInfo pass -> Bool
    isClosedTypeFamilyInfo (ClosedTypeFamily {}) = True
    isClosedTypeFamilyInfo _                     = False

    -- | data family declaration
    isDataFamilyDecl :: TyClDecl pass -> Bool
    isDataFamilyDecl (FamDecl _ (FamilyDecl { fdInfo = DataFamily })) = True
    isDataFamilyDecl _other      = False

    -- Dealing with names

    tyClDeclTyVars :: TyClDecl pass -> LHsQTyVars pass
    tyClDeclTyVars (FamDecl { tcdFam = FamilyDecl { fdTyVars = tvs } }) = tvs
    tyClDeclTyVars d = tcdTyVars d

    countTyClDecls :: [TyClDecl pass] -> (Int, Int, Int, Int, Int)
            -- class, synonym decls, data, newtype, family decls
    countTyClDecls decls
     = (count isClassDecl    decls,
        count isSynDecl      decls,  -- excluding...
        count isDataTy       decls,  -- ...family...
        count isNewTy        decls,  -- ...instances
        count isFamilyDecl   decls)
     where
       isDataTy DataDecl{ tcdDataDefn = HsDataDefn { dd_ND = DataType } } = True
       isDataTy _                                                       = False

       isNewTy DataDecl{ tcdDataDefn = HsDataDefn { dd_ND = NewType } } = True
       isNewTy _                                                      = False

    {- Note [CUSKs: complete user-supplied kind signatures]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We kind-check declarations differently if they have a complete, user-supplied
    kind signature (CUSK). This is because we can safely generalise a CUSKed
    declaration before checking all of the others, supporting polymorphic recursion.
    See https://gitlab.haskell.org/ghc/ghc/wikis/ghc-kinds/kind-inference#proposed-new-strategy
    and #9200 for lots of discussion of how we got here.

    The detection of CUSKs is enabled by the -XCUSKs extension, switched on by default.
    Under -XNoCUSKs, all declarations are treated as if they have no CUSK.
    See https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0036-kind-signatures.rst

    PRINCIPLE:
      a type declaration has a CUSK iff we could produce a separate kind signature
      for it, just like a type signature for a function,
      looking only at the header of the declaration.

    Examples:
      * data T1 (a :: *->*) (b :: *) = ....
        -- Has CUSK; equivalant to   T1 :: (*->*) -> * -> *

     * data T2 a b = ...
       -- No CUSK; we do not want to guess T2 :: * -> * -> *
       -- because the full decl might be   data T a b = MkT (a b)

      * data T3 (a :: k -> *) (b :: *) = ...
        -- CUSK; equivalent to   T3 :: (k -> *) -> * -> *
        -- We lexically generalise over k to get
        --    T3 :: forall k. (k -> *) -> * -> *
        -- The generalisation is here is purely lexical, just like
        --    f3 :: a -> a
        -- means
        --    f3 :: forall a. a -> a

      * data T4 (a :: j k) = ...
         -- CUSK; equivalent to   T4 :: j k -> *
         -- which we lexically generalise to  T4 :: forall j k. j k -> *
         -- and then, if PolyKinds is on, we further generalise to
         --   T4 :: forall kk (j :: kk -> *) (k :: kk). j k -> *
         -- Again this is exactly like what happens as the term level
         -- when you write
         --    f4 :: forall a b. a b -> Int

    NOTE THAT
      * A CUSK does /not/ mean that everything about the kind signature is
        fully specified by the user.  Look at T4 and f4: we had to do kind
        inference to figure out the kind-quantification.  But in both cases
        (T4 and f4) that inference is done looking /only/ at the header of T4
        (or signature for f4), not at the definition thereof.

      * The CUSK completely fixes the kind of the type constructor, forever.

      * The precise rules, for each declaration form, for whether a declaration
        has a CUSK are given in the user manual section "Complete user-supplied
        kind signatures and polymorphic recursion".  But they simply implement
        PRINCIPLE above.

      * Open type families are interesting:
          type family T5 a b :: *
        There simply /is/ no accompanying declaration, so that info is all
        we'll ever get.  So we it has a CUSK by definition, and we default
        any un-fixed kind variables to *.

      * Associated types are a bit tricker:
          class C6 a where
             type family T6 a b :: *
             op :: a Int -> Int
        Here C6 does not have a CUSK (in fact we ultimately discover that
        a :: * -> *).  And hence neither does T6, the associated family,
        because we can't fix its kind until we have settled C6.  Another
        way to say it: unlike a top-level, we /may/ discover more about
        a's kind from C6's definition.

      * A data definition with a top-level :: must explicitly bind all
        kind variables to the right of the ::. See test
        dependent/should_compile/KindLevels, which requires this
        case. (Naturally, any kind variable mentioned before the :: should
        not be bound after it.)

        This last point is much more debatable than the others; see
        #15142 comment:22

        Because this is fiddly to check, there is a field in the DataDeclRn
        structure (included in a DataDecl after the renamer) that stores whether
        or not the declaration has a CUSK.
    -}

    {- *********************************************************************
    *                                                                      *
                             TyClGroup
            Strongly connected components of
          type, class, instance, and role declarations
    *                                                                      *
    ********************************************************************* -}

    {- Note [TyClGroups and dependency analysis]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    A TyClGroup represents a strongly connected components of type/class/instance
    decls, together with the role annotations for the type/class declarations.

    The hs_tyclds :: [TyClGroup] field of a HsGroup is a dependency-order
    sequence of strongly-connected components.

    Invariants
     * The type and class declarations, group_tyclds, may depend on each
       other, or earlier TyClGroups, but not on later ones

     * The role annotations, group_roles, are role-annotations for some or
       all of the types and classes in group_tyclds (only).

     * The instance declarations, group_instds, may (and usually will)
       depend on group_tyclds, or on earlier TyClGroups, but not on later
       ones.

    See Note [Dependency analysis of type, class, and instance decls]
    in GHC.Rename.Module for more info.
    -}

    -- | Type or Class Group
    data TyClGroup pass  -- See Note [TyClGroups and dependency analysis]
      = TyClGroup { group_ext    :: XCTyClGroup pass
                  , group_tyclds :: [LTyClDecl pass]
                  , group_roles  :: [LRoleAnnotDecl pass]
                  , group_kisigs :: [LStandaloneKindSig pass]
                  , group_instds :: [LInstDecl pass] }
      | XTyClGroup !(XXTyClGroup pass)

    tyClGroupTyClDecls :: [TyClGroup pass] -> [LTyClDecl pass]
    tyClGroupTyClDecls = concatMap group_tyclds

    tyClGroupInstDecls :: [TyClGroup pass] -> [LInstDecl pass]
    tyClGroupInstDecls = concatMap group_instds

    tyClGroupRoleDecls :: [TyClGroup pass] -> [LRoleAnnotDecl pass]
    tyClGroupRoleDecls = concatMap group_roles

    tyClGroupKindSigs :: [TyClGroup pass] -> [LStandaloneKindSig pass]
    tyClGroupKindSigs = concatMap group_kisigs

    {- *********************************************************************
    *                                                                      *
                   Data and type family declarations
    *                                                                      *
    ********************************************************************* -}

    {- Note [FamilyResultSig]
    ~~~~~~~~~~~~~~~~~~~~~~~~~

    This data type represents the return signature of a type family.  Possible
    values are:

     * NoSig - the user supplied no return signature:
          type family Id a where ...

     * KindSig - the user supplied the return kind:
          type family Id a :: * where ...

     * TyVarSig - user named the result with a type variable and possibly
       provided a kind signature for that variable:
          type family Id a = r where ...
          type family Id a = (r :: *) where ...

       Naming result of a type family is required if we want to provide
       injectivity annotation for a type family:
          type family Id a = r | r -> a where ...

    See also: Note [Injectivity annotation]

    Note [Injectivity annotation]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    A user can declare a type family to be injective:

       type family Id a = r | r -> a where ...

     * The part after the "|" is called "injectivity annotation".
     * "r -> a" part is called "injectivity condition"; at the moment terms
       "injectivity annotation" and "injectivity condition" are synonymous
       because we only allow a single injectivity condition.
     * "r" is the "LHS of injectivity condition". LHS can only contain the
       variable naming the result of a type family.

     * "a" is the "RHS of injectivity condition". RHS contains space-separated
       type and kind variables representing the arguments of a type
       family. Variables can be omitted if a type family is not injective in
       these arguments. Example:
             type family Foo a b c = d | d -> a c where ...

    Note that:
     (a) naming of type family result is required to provide injectivity
         annotation
     (b) for associated types if the result was named then injectivity annotation
         is mandatory. Otherwise result type variable is indistinguishable from
         associated type default.

    It is possible that in the future this syntax will be extended to support
    more complicated injectivity annotations. For example we could declare that
    if we know the result of Plus and one of its arguments we can determine the
    other argument:

       type family Plus a b = (r :: Nat) | r a -> b, r b -> a where ...

    Here injectivity annotation would consist of two comma-separated injectivity
    conditions.

    See also Note [Injective type families] in GHC.Core.TyCon
    -}

    -- | Located type Family Result Signature
    type LFamilyResultSig pass = XRec pass (FamilyResultSig pass)

    -- | type Family Result Signature
    data FamilyResultSig pass = -- see Note [FamilyResultSig]
        NoSig (XNoSig pass)
      -- ^ - 'GHC.Parser.Annotation.AnnKeywordId' :

      -- For details on above see note [exact print annotations] in GHC.Parser.Annotation

      | KindSig  (XCKindSig pass) (LHsKind pass)
      -- ^ - 'GHC.Parser.Annotation.AnnKeywordId' :
      --             'GHC.Parser.Annotation.AnnOpenP','GHC.Parser.Annotation.AnnDcolon',
      --             'GHC.Parser.Annotation.AnnCloseP'

      -- For details on above see note [exact print annotations] in GHC.Parser.Annotation

      | TyVarSig (XTyVarSig pass) (LHsTyVarBndr () pass)
      -- ^ - 'GHC.Parser.Annotation.AnnKeywordId' :
      --             'GHC.Parser.Annotation.AnnOpenP','GHC.Parser.Annotation.AnnDcolon',
      --             'GHC.Parser.Annotation.AnnCloseP', 'GHC.Parser.Annotation.AnnEqual'
      | XFamilyResultSig !(XXFamilyResultSig pass)

      -- For details on above see note [exact print annotations] in GHC.Parser.Annotation

    -- | Located type Family Declaration
    type LFamilyDecl pass = XRec pass (FamilyDecl pass)

    -- | type Family Declaration
    data FamilyDecl pass = FamilyDecl
      { fdExt            :: XCFamilyDecl pass
      , fdInfo           :: FamilyInfo pass              -- type/data, closed/open
      , fdTopLevel       :: TopLevelFlag                 -- used for printing only
      , fdLName          :: LIdP pass                    -- type constructor
      , fdTyVars         :: LHsQTyVars pass              -- type variables
                           -- See Note [TyVar binders for associated declarations]
      , fdFixity         :: LexicalFixity                -- Fixity used in the declaration
      , fdResultSig      :: LFamilyResultSig pass        -- result signature
      , fdInjectivityAnn :: Maybe (LInjectivityAnn pass) -- optional injectivity ann
      }
      | XFamilyDecl !(XXFamilyDecl pass)
      -- ^ - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnType',
      --             'GHC.Parser.Annotation.AnnData', 'GHC.Parser.Annotation.AnnFamily',
      --             'GHC.Parser.Annotation.AnnWhere', 'GHC.Parser.Annotation.AnnOpenP',
      --             'GHC.Parser.Annotation.AnnDcolon', 'GHC.Parser.Annotation.AnnCloseP',
      --             'GHC.Parser.Annotation.AnnEqual', 'GHC.Parser.Annotation.AnnRarrow',
      --             'GHC.Parser.Annotation.AnnVbar'

      -- For details on above see note [exact print annotations] in GHC.Parser.Annotation

    -- | Located Injectivity Annotation
    type LInjectivityAnn pass = XRec pass (InjectivityAnn pass)

    -- | If the user supplied an injectivity annotation it is represented using
    -- InjectivityAnn. At the moment this is a single injectivity condition - see
    -- Note [Injectivity annotation]. `Located name` stores the LHS of injectivity
    -- condition. `[Located name]` stores the RHS of injectivity condition. Example:
    --
    --   type family Foo a b c = r | r -> a c where ...
    --
    -- This will be represented as "InjectivityAnn `r` [`a`, `c`]"
    data InjectivityAnn pass
      = InjectivityAnn (XCInjectivityAnn pass)
                       (LIdP pass) [LIdP pass]
      -- ^ - 'GHC.Parser.Annotation.AnnKeywordId' :
      --             'GHC.Parser.Annotation.AnnRarrow', 'GHC.Parser.Annotation.AnnVbar'

      -- For details on above see note [exact print annotations] in GHC.Parser.Annotation
      | XInjectivityAnn !(XXInjectivityAnn pass)

    data FamilyInfo pass
      = DataFamily
      | OpenTypeFamily
         -- | 'Nothing' if we're in an hs-boot file and the user
         -- said "type family Foo x where .."
      | ClosedTypeFamily (Maybe [LTyFamInstEqn pass])

    ------------- Pretty printing FamilyDecls -----------

    pprFlavour :: FamilyInfo pass -> SDoc
    pprFlavour DataFamily            = text "data"
    pprFlavour OpenTypeFamily        = text "type"
    pprFlavour (ClosedTypeFamily {}) = text "type"

    instance Outputable (FamilyInfo pass) where
      ppr info = pprFlavour info <+> text "family"

    {- *********************************************************************
    *                                                                      *
                   Data types and data constructors
    *                                                                      *
    ********************************************************************* -}

    -- | Haskell Data type Definition
    data HsDataDefn pass   -- The payload of a data type defn
                           -- Used *both* for vanilla data declarations,
                           --       *and* for data family instances
      = -- | Declares a data type or newtype, giving its constructors
        -- @
        --  data/newtype T a = <constrs>
        --  data/newtype instance T [a] = <constrs>
        -- @
        HsDataDefn { dd_ext    :: XCHsDataDefn pass,
                     dd_ND     :: NewOrData,
                     dd_ctxt   :: Maybe (LHsContext pass), -- ^ Context
                     dd_cType  :: Maybe (XRec pass CType),
                     dd_kindSig:: Maybe (LHsKind pass),
                         -- ^ Optional kind signature.
                         --
                         -- @(Just k)@ for a GADT-style @data@,
                         -- or @data instance@ decl, with explicit kind sig
                         --
                         -- Always @Nothing@ for H98-syntax decls

                     dd_cons   :: [LConDecl pass],
                         -- ^ Data constructors
                         --
                         -- For @data T a = T1 | T2 a@
                         --   the 'LConDecl's all have 'ConDeclH98'.
                         -- For @data T a where { T1 :: T a }@
                         --   the 'LConDecls' all have 'ConDeclGADT'.

                     dd_derivs :: HsDeriving pass  -- ^ Optional 'deriving' clause

                 -- For details on above see note [exact print annotations] in GHC.Parser.Annotation
       }
      | XHsDataDefn !(XXHsDataDefn pass)

    -- | Haskell Deriving clause
    type HsDeriving pass = [LHsDerivingClause pass]
      -- ^ The optional @deriving@ clauses of a data declaration. "Clauses" is
      -- plural because one can specify multiple deriving clauses using the
      -- @-XDerivingStrategies@ language extension.
      --
      -- The list of 'LHsDerivingClause's corresponds to exactly what the user
      -- requested to derive, in order. If no deriving clauses were specified,
      -- the list is empty.

    type LHsDerivingClause pass = XRec pass (HsDerivingClause pass)

    -- | A single @deriving@ clause of a data declaration.
    --
    --  - 'GHC.Parser.Annotation.AnnKeywordId' :
    --       'GHC.Parser.Annotation.AnnDeriving', 'GHC.Parser.Annotation.AnnStock',
    --       'GHC.Parser.Annotation.AnnAnyClass', 'GHC.Parser.Annotation.AnnNewtype',
    --       'GHC.Parser.Annotation.AnnOpen','GHC.Parser.Annotation.AnnClose'
    data HsDerivingClause pass
      -- See Note [Deriving strategies] in GHC.Tc.Deriv
      = HsDerivingClause
        { deriv_clause_ext :: XCHsDerivingClause pass
        , deriv_clause_strategy :: Maybe (LDerivStrategy pass)
          -- ^ The user-specified strategy (if any) to use when deriving
          -- 'deriv_clause_tys'.
        , deriv_clause_tys :: LDerivClauseTys pass
          -- ^ The types to derive.
        }
      | XHsDerivingClause !(XXHsDerivingClause pass)

    type LDerivClauseTys pass = XRec pass (DerivClauseTys pass)

    -- | The types mentioned in a single @deriving@ clause. This can come in two
    -- forms, 'DctSingle' or 'DctMulti', depending on whether the types are
    -- surrounded by enclosing parentheses or not. These parentheses are
    -- semantically different than 'HsParTy'. For example, @deriving ()@ means
    -- \"derive zero classes\" rather than \"derive an instance of the 0-tuple\".
    --
    -- 'DerivClauseTys' use 'LHsSigType' because @deriving@ clauses can mention
    -- type variables that aren't bound by the datatype, e.g.
    --
    -- > data T b = ... deriving (C [a])
    --
    -- should produce a derived instance for @C [a] (T b)@.
    data DerivClauseTys pass
      = -- | A @deriving@ clause with a single type. Moreover, that type can only
        -- be a type constructor without any arguments.
        --
        -- Example: @deriving Eq@
        DctSingle (XDctSingle pass) (LHsSigType pass)

        -- | A @deriving@ clause with a comma-separated list of types, surrounded
        -- by enclosing parentheses.
        --
        -- Example: @deriving (Eq, C a)@
      | DctMulti (XDctMulti pass) [LHsSigType pass]

      | XDerivClauseTys !(XXDerivClauseTys pass)

    -- | Located Standalone Kind Signature
    type LStandaloneKindSig pass = XRec pass (StandaloneKindSig pass)

    data StandaloneKindSig pass
      = StandaloneKindSig (XStandaloneKindSig pass)
          (LIdP pass)           -- Why a single binder? See #16754
          (LHsSigType pass)     -- Why not LHsSigWcType? See Note [Wildcards in standalone kind signatures]
      | XStandaloneKindSig !(XXStandaloneKindSig pass)

    {- Note [Wildcards in standalone kind signatures]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Standalone kind signatures enable polymorphic recursion, and it is unclear how
    to reconcile this with partial type signatures, so we disallow wildcards in
    them.

    We reject wildcards in 'rnStandaloneKindSignature' by returning False for
    'StandaloneKindSigCtx' in 'wildCardsAllowed'.

    The alternative design is to have special treatment for partial standalone kind
    signatures, much like we have special treatment for partial type signatures in
    terms. However, partial standalone kind signatures are not a proper replacement
    for CUSKs, so this would be a separate feature.
    -}

    data NewOrData
      = NewType                     -- ^ @newtype Blah ...@
      | DataType                    -- ^ @data Blah ...@
      deriving( Eq, Data )                -- Needed because Demand derives Eq

    -- | Convert a 'NewOrData' to a 'TyConFlavour'
    newOrDataToFlavour :: NewOrData -> TyConFlavour
    newOrDataToFlavour NewType  = NewtypeFlavour
    newOrDataToFlavour DataType = DataTypeFlavour

    -- | Located data Constructor Declaration
    type LConDecl pass = XRec pass (ConDecl pass)
          -- ^ May have 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnSemi' when
          --   in a GADT constructor list

      -- For details on above see note [exact print annotations] in GHC.Parser.Annotation

    -- |
    --
    -- @
    -- data T b = forall a. Eq a => MkT a b
    --   MkT :: forall b a. Eq a => MkT a b
    --
    -- data T b where
    --      MkT1 :: Int -> T Int
    --
    -- data T = Int `MkT` Int
    --        | MkT2
    --
    -- data T a where
    --      Int `MkT` Int :: T Int
    -- @
    --
    -- - 'GHC.Parser.Annotation.AnnKeywordId's : 'GHC.Parser.Annotation.AnnOpen',
    --            'GHC.Parser.Annotation.AnnDotdot','GHC.Parser.Annotation.AnnCLose',
    --            'GHC.Parser.Annotation.AnnEqual','GHC.Parser.Annotation.AnnVbar',
    --            'GHC.Parser.Annotation.AnnDarrow','GHC.Parser.Annotation.AnnDarrow',
    --            'GHC.Parser.Annotation.AnnForall','GHC.Parser.Annotation.AnnDot'

    -- For details on above see note [exact print annotations] in GHC.Parser.Annotation

    -- | data Constructor Declaration
    data ConDecl pass
      = ConDeclGADT
          { con_g_ext   :: XConDeclGADT pass
          , con_names   :: [LIdP pass]

          -- The following fields describe the type after the '::'
          -- See Note [GADT abstract syntax]
          , con_bndrs   :: XRec pass (HsOuterSigTyVarBndrs pass)
            -- ^ The outermost type variable binders, be they explicit or
            --   implicit.  The 'XRec' is used to anchor exact print
            --   annotations, AnnForall and AnnDot.
          , con_mb_cxt  :: Maybe (LHsContext pass)   -- ^ User-written context (if any)
          , con_g_args  :: HsConDeclGADTDetails pass -- ^ Arguments; never infix
          , con_res_ty  :: LHsType pass              -- ^ Result type

          , con_doc     :: Maybe LHsDocString
              -- ^ A possible Haddock comment.
          }

      | ConDeclH98
          { con_ext     :: XConDeclH98 pass
          , con_name    :: LIdP pass

          , con_forall  :: Bool
                                  -- ^ True <=> explicit user-written forall
                                  --     e.g. data T a = forall b. MkT b (b->a)
                                  --     con_ex_tvs = {b}
                                  -- False => con_ex_tvs is empty
          , con_ex_tvs :: [LHsTyVarBndr Specificity pass] -- ^ Existentials only
          , con_mb_cxt :: Maybe (LHsContext pass)         -- ^ User-written context (if any)
          , con_args   :: HsConDeclH98Details pass        -- ^ Arguments; can be infix

          , con_doc       :: Maybe LHsDocString
              -- ^ A possible Haddock comment.
          }
      | XConDecl !(XXConDecl pass)

    {- Note [GADT abstract syntax]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The types of both forms of GADT constructors are very structured, as they
    must consist of the quantified type variables (if provided), followed by the
    context (if provided), followed by the argument types (if provided), followed
    by the result type. (See "Wrinkle: No nested foralls or contexts" below for
    more discussion on the restrictions imposed here.) As a result, instead of
    storing the type of a GADT constructor as a single LHsType, we split it up
    into its constituent components for easier access.

    There are two broad ways to classify GADT constructors:

    * Record-syntax constructors. For example:

        data T a where
          K :: forall a. Ord a => { x :: [a], ... } -> T a

    * Prefix constructors, which do not use record syntax. For example:

        data T a where
          K :: forall a. Ord a => [a] -> ... -> T a

    This distinction is recorded in the `con_args :: HsConDetails pass`, which
    tracks if we're dealing with a RecCon or PrefixCon. It is easy to distinguish
    the two in the AST since record GADT constructors use HsRecTy. This distinction
    is made in GHC.Parser.PostProcess.mkGadtDecl.

    It is worth elaborating a bit more on the process of splitting the argument
    types of a GADT constructor, since there are some non-obvious details involved.
    While splitting the argument types of a record GADT constructor is easy (they
    are stored in an HsRecTy), splitting the arguments of a prefix GADT constructor
    is trickier. The basic idea is that we must split along the outermost function
    arrows ((->) and (%1 ->)) in the type, which GHC.Hs.Type.splitHsFunType
    accomplishes. But what about type operators? Consider:

      C :: a :*: b -> a :*: b -> a :+: b

    This could parse in many different ways depending on the precedences of each
    type operator. In particular, if (:*:) were to have lower precedence than (->),
    then it could very well parse like this:

      a :*: ((b -> a) :*: ((b -> a) :+: b)))

    This would give the false impression that the whole type is part of one large
    return type, with no arguments. Note that we do not fully resolve the exact
    precedences of each user-defined type operator until the renamer, so this a
    more difficult task for the parser.

    Fortunately, there is no risk of the above happening. GHC's parser gives
    special treatment to function arrows, and as a result, they are always parsed
    with a lower precedence than any other type operator. As a result, the type
    above is actually parsed like this:

      (a :*: b) -> ((a :*: b) -> (a :+: b))

    While we won't know the exact precedences of (:*:) and (:+:) until the renamer,
    all we are concerned about in the parser is identifying the overall shape of
    the argument and result types, which we can accomplish by piggybacking on the
    special treatment given to function arrows. In a future where function arrows
    aren't given special status in the parser, we will likely have to modify
    GHC.Parser.PostProcess.mkHsOpTyPV to preserve this trick.

    -----
    -- Wrinkle: No nested foralls or contexts
    -----

    GADT constructors provide some freedom to change the order of foralls in their
    types (see Note [DataCon user type variable binders] in GHC.Core.DataCon), but
    this freedom is still limited. GADTs still require that all quantification
    occurs "prenex". That is, any explicitly quantified type variables must occur
    at the front of the GADT type, followed by any contexts, followed by the body of
    the GADT type, in precisely that order. For instance:

      data T where
        MkT1 :: forall a b. (Eq a, Eq b) => a -> b -> T
          -- OK
        MkT2 :: forall a. Eq a => forall b. a -> b -> T
          -- Rejected, `forall b` is nested
        MkT3 :: forall a b. Eq a => Eq b => a -> b -> T
          -- Rejected, `Eq b` is nested
        MkT4 :: Int -> forall a. a -> T
          -- Rejected, `forall a` is nested
        MkT5 :: forall a. Int -> Eq a => a -> T
          -- Rejected, `Eq a` is nested
        MkT6 :: (forall a. a -> T)
          -- Rejected, `forall a` is nested due to the surrounding parentheses
        MkT7 :: (Eq a => a -> t)
          -- Rejected, `Eq a` is nested due to the surrounding parentheses

    For the full details, see the "Formal syntax for GADTs" section of the GHC
    User's Guide. GHC enforces that GADT constructors do not have nested `forall`s
    or contexts in two parts:

    1. GHC, in the process of splitting apart a GADT's type,
       extracts out the leading `forall` and context (if they are provided). To
       accomplish this splitting, the renamer uses the
       GHC.Hs.Type.splitLHsGADTPrefixTy function, which is careful not to remove
       parentheses surrounding the leading `forall` or context (as these
       parentheses can be syntactically significant). If the third result returned
       by splitLHsGADTPrefixTy contains any `forall`s or contexts, then they must
       be nested, so they will be rejected.

       Note that this step applies to both prefix and record GADTs alike, as they
       both have syntax which permits `forall`s and contexts. The difference is
       where this step happens:

       * For prefix GADTs, this happens in the renamer (in rnConDecl), as we cannot
         split until after the type operator fixities have been resolved.
       * For record GADTs, this happens in the parser (in mkGadtDecl).
    2. If the GADT type is prefix, the renamer (in the ConDeclGADTPrefixPs case of
       rnConDecl) will then check for nested `forall`s/contexts in the body of a
       prefix GADT type, after it has determined what all of the argument types are.
       This step is necessary to catch examples like MkT4 above, where the nested
       quantification occurs after a visible argument type.
    -}

    -- | The arguments in a Haskell98-style data constructor.
    type HsConDeclH98Details pass
       = HsConDetails Void (HsScaled pass (LBangType pass)) (XRec pass [LConDeclField pass])
    -- The Void argument to HsConDetails here is a reflection of the fact that
    -- type applications are not allowed in data constructor declarations.

    -- | The arguments in a GADT constructor. Unlike Haskell98-style constructors,
    -- GADT constructors cannot be declared with infix syntax. As a result, we do
    -- not use 'HsConDetails' here, as 'InfixCon' would be an unrepresentable
    -- state. (There is a notion of infix GADT constructors for the purposes of
    -- derived Show instances—see Note [Infix GADT constructors] in
    -- GHC.Tc.TyCl—but that is an orthogonal concern.)
    data HsConDeclGADTDetails pass
       = PrefixConGADT [HsScaled pass (LBangType pass)]
       | RecConGADT (XRec pass [LConDeclField pass])

    instance Outputable NewOrData where
      ppr NewType  = text "newtype"
      ppr DataType = text "data"

    {-
    ************************************************************************
    *                                                                      *
                    Instance declarations
    *                                                                      *
    ************************************************************************

    Note [Type family instance declarations in HsSyn]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The data type FamEqn represents one equation of a type family instance.
    Aside from the pass, it is also parameterised over another field, feqn_rhs.
    feqn_rhs is either an HsDataDefn (for data family instances) or an LHsType
    (for type family instances).

    Type family instances also include associated type family default equations.
    That is because a default for a type family looks like this:

      class C a where
        type family F a b :: Type
        type F c d = (c,d)   -- Default instance

    The default declaration is really just a `type instance` declaration, but one
    with particularly simple patterns: they must all be distinct type variables.
    That's because we will instantiate it (in an instance declaration for `C`) if
    we don't give an explicit instance for `F`. Note that the names of the
    variables don't need to match those of the class: it really is like a
    free-standing `type instance` declaration.
    -}

    ----------------- Type synonym family instances -------------

    -- | Located Type Family Instance Equation
    type LTyFamInstEqn pass = XRec pass (TyFamInstEqn pass)
      -- ^ May have 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnSemi'
      --   when in a list

    -- For details on above see note [exact print annotations] in GHC.Parser.Annotation

    -- | Haskell Type Patterns
    type HsTyPats pass = [LHsTypeArg pass]

    {- Note [Family instance declaration binders]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The feqn_pats field of FamEqn (family instance equation) stores the LHS type
    (and kind) patterns. Any type (and kind) variables contained
    in these type patterns are bound in the feqn_bndrs field.
    Note that in particular:

    * The feqn_bndrs *include* any anonymous wildcards.  For example
         type instance F a _ = a
      The feqn_bndrs will be HsOuterImplicit {a, _}.  Remember that each separate
      wildcard '_' gets its own unique.  In this context wildcards behave just like
      an ordinary type variable, only anonymous.

    * The feqn_bndrs *include* type variables that are already in scope

       Eg   class C s t where
              type F t p :: *
            instance C w (a,b) where
              type F (a,b) x = x->a
       The feqn_bndrs of the F decl is HsOuterImplicit {a,b,x}, even though the
       F decl is nested inside the 'instance' decl.

       However after the renamer, the uniques will match up:
            instance C w7 (a8,b9) where
              type F (a8,b9) x10 = x10->a8
       so that we can compare the type pattern in the 'instance' decl and
       in the associated 'type' decl

    c.f. Note [TyVar binders for associated decls]
    -}

    -- | Type Family Instance Equation
    type TyFamInstEqn pass = FamEqn pass (LHsType pass)
                -- Here, the @pats@ are type patterns (with kind and type bndrs).
                -- See Note [Family instance declaration binders]

    -- | Type family default declarations.
    -- A convenient synonym for 'TyFamInstDecl'.
    -- See @Note [Type family instance declarations in HsSyn]@.
    type TyFamDefltDecl = TyFamInstDecl

    -- | Located type family default declarations.
    type LTyFamDefltDecl pass = XRec pass (TyFamDefltDecl pass)

    -- | Located Type Family Instance Declaration
    type LTyFamInstDecl pass = XRec pass (TyFamInstDecl pass)

    -- | Type Family Instance Declaration
    data TyFamInstDecl pass
      = TyFamInstDecl { tfid_xtn :: XCTyFamInstDecl pass
                      , tfid_eqn :: TyFamInstEqn pass }
        -- ^
        --  - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnType',
        --           'GHC.Parser.Annotation.AnnInstance',

        -- For details on above see note [exact print annotations] in GHC.Parser.Annotation
      | XTyFamInstDecl !(XXTyFamInstDecl pass)

    ----------------- Data family instances -------------

    -- | Located Data Family Instance Declaration
    type LDataFamInstDecl pass = XRec pass (DataFamInstDecl pass)

    -- | Data Family Instance Declaration
    newtype DataFamInstDecl pass
      = DataFamInstDecl { dfid_eqn :: FamEqn pass (HsDataDefn pass) }
        -- ^
        --  - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnData',
        --           'GHC.Parser.Annotation.AnnNewType','GHC.Parser.Annotation.AnnInstance',
        --           'GHC.Parser.Annotation.AnnDcolon'
        --           'GHC.Parser.Annotation.AnnWhere','GHC.Parser.Annotation.AnnOpen',
        --           'GHC.Parser.Annotation.AnnClose'

        -- For details on above see note [exact print annotations] in GHC.Parser.Annotation

    ----------------- Family instances (common types) -------------

    -- | Family Equation
    --
    -- One equation in a type family instance declaration, data family instance
    -- declaration, or type family default.
    -- See Note [Type family instance declarations in HsSyn]
    -- See Note [Family instance declaration binders]
    data FamEqn pass rhs
      = FamEqn
           { feqn_ext    :: XCFamEqn pass rhs
           , feqn_tycon  :: LIdP pass
           , feqn_bndrs  :: HsOuterFamEqnTyVarBndrs pass -- ^ Optional quantified type vars
           , feqn_pats   :: HsTyPats pass
           , feqn_fixity :: LexicalFixity -- ^ Fixity used in the declaration
           , feqn_rhs    :: rhs
           }
        -- ^
        --  - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnEqual'
      | XFamEqn !(XXFamEqn pass rhs)

        -- For details on above see note [exact print annotations] in GHC.Parser.Annotation

    ----------------- Class instances -------------

    -- | Located Class Instance Declaration
    type LClsInstDecl pass = XRec pass (ClsInstDecl pass)

    -- | Class Instance Declaration
    data ClsInstDecl pass
      = ClsInstDecl
          { cid_ext     :: XCClsInstDecl pass
          , cid_poly_ty :: LHsSigType pass    -- Context => Class Instance-type
                                              -- Using a polytype means that the renamer conveniently
                                              -- figures out the quantified type variables for us.
          , cid_binds         :: LHsBinds pass       -- Class methods
          , cid_sigs          :: [LSig pass]         -- User-supplied pragmatic info
          , cid_tyfam_insts   :: [LTyFamInstDecl pass]   -- Type family instances
          , cid_datafam_insts :: [LDataFamInstDecl pass] -- Data family instances
          , cid_overlap_mode  :: Maybe (XRec pass OverlapMode)
             -- ^ - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnOpen',
             --                                    'GHC.Parser.Annotation.AnnClose',

            -- For details on above see note [exact print annotations] in GHC.Parser.Annotation
          }
        -- ^
        --  - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnInstance',
        --           'GHC.Parser.Annotation.AnnWhere',
        --           'GHC.Parser.Annotation.AnnOpen','GHC.Parser.Annotation.AnnClose',

        -- For details on above see note [exact print annotations] in GHC.Parser.Annotation
      | XClsInstDecl !(XXClsInstDecl pass)

    ----------------- Instances of all kinds -------------

    -- | Located Instance Declaration
    type LInstDecl pass = XRec pass (InstDecl pass)

    -- | Instance Declaration
    data InstDecl pass  -- Both class and family instances
      = ClsInstD
          { cid_d_ext :: XClsInstD pass
          , cid_inst  :: ClsInstDecl pass }
      | DataFamInstD              -- data family instance
          { dfid_ext  :: XDataFamInstD pass
          , dfid_inst :: DataFamInstDecl pass }
      | TyFamInstD              -- type family instance
          { tfid_ext  :: XTyFamInstD pass
          , tfid_inst :: TyFamInstDecl pass }
      | XInstDecl !(XXInstDecl pass)

    {-
    ************************************************************************
    *                                                                      *
    \subsection[DerivDecl]{A stand-alone instance deriving declaration}
    *                                                                      *
    ************************************************************************
    -}

    -- | Located stand-alone 'deriving instance' declaration
    type LDerivDecl pass = XRec pass (DerivDecl pass)

    -- | Stand-alone 'deriving instance' declaration
    data DerivDecl pass = DerivDecl
            { deriv_ext          :: XCDerivDecl pass
            , deriv_type         :: LHsSigWcType pass
              -- ^ The instance type to derive.
              --
              -- It uses an 'LHsSigWcType' because the context is allowed to be a
              -- single wildcard:
              --
              -- > deriving instance _ => Eq (Foo a)
              --
              -- Which signifies that the context should be inferred.

              -- See Note [Inferring the instance context] in GHC.Tc.Deriv.Infer.

            , deriv_strategy     :: Maybe (LDerivStrategy pass)
            , deriv_overlap_mode :: Maybe (XRec pass OverlapMode)
             -- ^ - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnDeriving',
             --        'GHC.Parser.Annotation.AnnInstance', 'GHC.Parser.Annotation.AnnStock',
             --        'GHC.Parser.Annotation.AnnAnyClass', 'GHC.Parser.Annotation.AnnNewtype',
             --        'GHC.Parser.Annotation.AnnOpen','GHC.Parser.Annotation.AnnClose'

      -- For details on above see note [exact print annotations] in GHC.Parser.Annotation
            }
      | XDerivDecl !(XXDerivDecl pass)

    {-
    ************************************************************************
    *                                                                      *
                    Deriving strategies
    *                                                                      *
    ************************************************************************
    -}

    -- | A 'Located' 'DerivStrategy'.
    type LDerivStrategy pass = XRec pass (DerivStrategy pass)

    -- | Which technique the user explicitly requested when deriving an instance.
    data DerivStrategy pass
      -- See Note [Deriving strategies] in GHC.Tc.Deriv
      = StockStrategy (XStockStrategy pass)
                         -- ^ GHC's \"standard\" strategy, which is to implement a
                         --   custom instance for the data type. This only works
                         --   for certain types that GHC knows about (e.g., 'Eq',
                         --   'Show', 'Functor' when @-XDeriveFunctor@ is enabled,
                         --   etc.)
      | AnyclassStrategy (XAnyClassStrategy pass) -- ^ @-XDeriveAnyClass@
      | NewtypeStrategy  (XNewtypeStrategy pass)  -- ^ @-XGeneralizedNewtypeDeriving@
      | ViaStrategy (XViaStrategy pass)
                         -- ^ @-XDerivingVia@

    -- | A short description of a @DerivStrategy'@.
    derivStrategyName :: DerivStrategy a -> SDoc
    derivStrategyName = text . go
      where
        go StockStrategy    {} = "stock"
        go AnyclassStrategy {} = "anyclass"
        go NewtypeStrategy  {} = "newtype"
        go ViaStrategy      {} = "via"

    {-
    ************************************************************************
    *                                                                      *
    \subsection[DefaultDecl]{A @default@ declaration}
    *                                                                      *
    ************************************************************************

    There can only be one default declaration per module, but it is hard
    for the parser to check that; we pass them all through in the abstract
    syntax, and that restriction must be checked in the front end.
    -}

    -- | Located Default Declaration
    type LDefaultDecl pass = XRec pass (DefaultDecl pass)

    -- | Default Declaration
    data DefaultDecl pass
      = DefaultDecl (XCDefaultDecl pass) [LHsType pass]
            -- ^ - 'GHC.Parser.Annotation.AnnKeywordId's : 'GHC.Parser.Annotation.AnnDefault',
            --          'GHC.Parser.Annotation.AnnOpen','GHC.Parser.Annotation.AnnClose'

            -- For details on above see note [exact print annotations] in GHC.Parser.Annotation
      | XDefaultDecl !(XXDefaultDecl pass)

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Foreign function interface declaration}
    *                                                                      *
    ************************************************************************
    -}

    -- foreign declarations are distinguished as to whether they define or use a
    -- Haskell name
    --
    --  * the Boolean value indicates whether the pre-standard deprecated syntax
    --   has been used

    -- | Located Foreign Declaration
    type LForeignDecl pass = XRec pass (ForeignDecl pass)

    -- | Foreign Declaration
    data ForeignDecl pass
      = ForeignImport
          { fd_i_ext  :: XForeignImport pass   -- Post typechecker, rep_ty ~ sig_ty
          , fd_name   :: LIdP pass             -- defines this name
          , fd_sig_ty :: LHsSigType pass       -- sig_ty
          , fd_fi     :: ForeignImport }

      | ForeignExport
          { fd_e_ext  :: XForeignExport pass   -- Post typechecker, rep_ty ~ sig_ty
          , fd_name   :: LIdP pass             -- uses this name
          , fd_sig_ty :: LHsSigType pass       -- sig_ty
          , fd_fe     :: ForeignExport }
            -- ^
            --  - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnForeign',
            --           'GHC.Parser.Annotation.AnnImport','GHC.Parser.Annotation.AnnExport',
            --           'GHC.Parser.Annotation.AnnDcolon'

            -- For details on above see note [exact print annotations] in GHC.Parser.Annotation
      | XForeignDecl !(XXForeignDecl pass)

    {-
        In both ForeignImport and ForeignExport:
            sig_ty is the type given in the Haskell code
            rep_ty is the representation for this type, i.e. with newtypes
                   coerced away and type functions evaluated.
        Thus if the declaration is valid, then rep_ty will only use types
        such as Int and IO that we know how to make foreign calls with.
    -}

    -- Specification Of an imported external entity in dependence on the calling
    -- convention
    --
    data ForeignImport = -- import of a C entity
                         --
                         --  * the two strings specifying a header file or library
                         --   may be empty, which indicates the absence of a
                         --   header or object specification (both are not used
                         --   in the case of `CWrapper' and when `CFunction'
                         --   has a dynamic target)
                         --
                         --  * the calling convention is irrelevant for code
                         --   generation in the case of `CLabel', but is needed
                         --   for pretty printing
                         --
                         --  * `Safety' is irrelevant for `CLabel' and `CWrapper'
                         --
                         CImport  (Located CCallConv) -- ccall or stdcall
                                  (Located Safety)  -- interruptible, safe or unsafe
                                  (Maybe Header)       -- name of C header
                                  CImportSpec          -- details of the C entity
                                  (Located SourceText) -- original source text for
                                                       -- the C entity
      deriving Data

    -- details of an external C entity
    --
    data CImportSpec = CLabel    CLabelString     -- import address of a C label
                     | CFunction CCallTarget      -- static or dynamic function
                     | CWrapper                   -- wrapper to expose closures
                                                  -- (former f.e.d.)
      deriving Data

    -- specification of an externally exported entity in dependence on the calling
    -- convention
    --
    data ForeignExport = CExport  (Located CExportSpec) -- contains the calling
                                                        -- convention
                                  (Located SourceText)  -- original source text for
                                                        -- the C entity
      deriving Data

    -- pretty printing of foreign declarations
    --

    instance Outputable ForeignImport where
      ppr (CImport  cconv safety mHeader spec (L _ srcText)) =
        ppr cconv <+> ppr safety
          <+> pprWithSourceText srcText (pprCEntity spec "")
        where
          pp_hdr = case mHeader of
                   Nothing -> empty
                   Just (Header _ header) -> ftext header

          pprCEntity (CLabel lbl) _ =
            doubleQuotes $ text "static" <+> pp_hdr <+> char '&' <> ppr lbl
          pprCEntity (CFunction (StaticTarget st _lbl _ isFun)) src =
            if dqNeeded then doubleQuotes ce else empty
              where
                dqNeeded = (take 6 src == "static")
                        || isJust mHeader
                        || not isFun
                        || st /= NoSourceText
                ce =
                      -- We may need to drop leading spaces first
                      (if take 6 src == "static" then text "static" else empty)
                  <+> pp_hdr
                  <+> (if isFun then empty else text "value")
                  <+> (pprWithSourceText st empty)
          pprCEntity (CFunction DynamicTarget) _ =
            doubleQuotes $ text "dynamic"
          pprCEntity CWrapper _ = doubleQuotes $ text "wrapper"

    instance Outputable ForeignExport where
      ppr (CExport  (L _ (CExportStatic _ lbl cconv)) _) =
        ppr cconv <+> char '"' <> ppr lbl <> char '"'

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Rewrite rules}
    *                                                                      *
    ************************************************************************
    -}

    -- | Located Rule Declarations
    type LRuleDecls pass = XRec pass (RuleDecls pass)

      -- Note [Pragma source text] in GHC.Types.SourceText
    -- | Rule Declarations
    data RuleDecls pass = HsRules { rds_ext   :: XCRuleDecls pass
                                  , rds_src   :: SourceText
                                  , rds_rules :: [LRuleDecl pass] }
      | XRuleDecls !(XXRuleDecls pass)

    -- | Located Rule Declaration
    type LRuleDecl pass = XRec pass (RuleDecl pass)

    -- | Rule Declaration
    data RuleDecl pass
      = HsRule -- Source rule
           { rd_ext  :: XHsRule pass
               -- ^ After renamer, free-vars from the LHS and RHS
           , rd_name :: XRec pass (SourceText,RuleName)
               -- ^ Note [Pragma source text] in "GHC.Types.Basic"
           , rd_act  :: Activation
           , rd_tyvs :: Maybe [LHsTyVarBndr () (NoGhcTc pass)]
               -- ^ Forall'd type vars
           , rd_tmvs :: [LRuleBndr pass]
               -- ^ Forall'd term vars, before typechecking; after typechecking
               --    this includes all forall'd vars
           , rd_lhs  :: XRec pass (HsExpr pass)
           , rd_rhs  :: XRec pass (HsExpr pass)
           }
        -- ^
        --  - 'GHC.Parser.Annotation.AnnKeywordId' :
        --           'GHC.Parser.Annotation.AnnOpen','GHC.Parser.Annotation.AnnTilde',
        --           'GHC.Parser.Annotation.AnnVal',
        --           'GHC.Parser.Annotation.AnnClose',
        --           'GHC.Parser.Annotation.AnnForall','GHC.Parser.Annotation.AnnDot',
        --           'GHC.Parser.Annotation.AnnEqual',
      | XRuleDecl !(XXRuleDecl pass)

    data HsRuleRn = HsRuleRn NameSet NameSet -- Free-vars from the LHS and RHS
      deriving Data

    -- | Located Rule Binder
    type LRuleBndr pass = XRec pass (RuleBndr pass)

    -- | Rule Binder
    data RuleBndr pass
      = RuleBndr (XCRuleBndr pass)  (LIdP pass)
      | RuleBndrSig (XRuleBndrSig pass) (LIdP pass) (HsPatSigType pass)
      | XRuleBndr !(XXRuleBndr pass)
            -- ^
            --  - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnOpen',
            --     'GHC.Parser.Annotation.AnnDcolon','GHC.Parser.Annotation.AnnClose'

            -- For details on above see note [exact print annotations] in GHC.Parser.Annotation

    collectRuleBndrSigTys :: [RuleBndr pass] -> [HsPatSigType pass]
    collectRuleBndrSigTys bndrs = [ty | RuleBndrSig _ _ ty <- bndrs]

    pprFullRuleName :: Located (SourceText, RuleName) -> SDoc
    pprFullRuleName (L _ (st, n)) = pprWithSourceText st (doubleQuotes $ ftext n)

    {-
    ************************************************************************
    *                                                                      *
    \subsection[DocDecl]{Document comments}
    *                                                                      *
    ************************************************************************
    -}

    -- | Located Documentation comment Declaration
    type LDocDecl pass = XRec pass (DocDecl)

    -- | Documentation comment Declaration
    data DocDecl
      = DocCommentNext HsDocString
      | DocCommentPrev HsDocString
      | DocCommentNamed String HsDocString
      | DocGroup Int HsDocString
      deriving Data

    -- Okay, I need to reconstruct the document comments, but for now:
    instance Outputable DocDecl where
      ppr _ = text "<document comment>"

    docDeclDoc :: DocDecl -> HsDocString
    docDeclDoc (DocCommentNext d) = d
    docDeclDoc (DocCommentPrev d) = d
    docDeclDoc (DocCommentNamed _ d) = d
    docDeclDoc (DocGroup _ d) = d

    {-
    ************************************************************************
    *                                                                      *
    \subsection[DeprecDecl]{Deprecations}
    *                                                                      *
    ************************************************************************

    We use exported entities for things to deprecate.
    -}

    -- | Located Warning Declarations
    type LWarnDecls pass = XRec pass (WarnDecls pass)

     -- Note [Pragma source text] in GHC.Types.SourceText
    -- | Warning pragma Declarations
    data WarnDecls pass = Warnings { wd_ext      :: XWarnings pass
                                   , wd_src      :: SourceText
                                   , wd_warnings :: [LWarnDecl pass]
                                   }
      | XWarnDecls !(XXWarnDecls pass)

    -- | Located Warning pragma Declaration
    type LWarnDecl pass = XRec pass (WarnDecl pass)

    -- | Warning pragma Declaration
    data WarnDecl pass = Warning (XWarning pass) [LIdP pass] WarningTxt
                       | XWarnDecl !(XXWarnDecl pass)

    {-
    ************************************************************************
    *                                                                      *
    \subsection[AnnDecl]{Annotations}
    *                                                                      *
    ************************************************************************
    -}

    -- | Located Annotation Declaration
    type LAnnDecl pass = XRec pass (AnnDecl pass)

    -- | Annotation Declaration
    data AnnDecl pass = HsAnnotation
                          (XHsAnnotation pass)
                          SourceText -- Note [Pragma source text] in GHC.Types.SourceText
                          (AnnProvenance pass) (XRec pass (HsExpr pass))
          -- ^ - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnOpen',
          --           'GHC.Parser.Annotation.AnnType'
          --           'GHC.Parser.Annotation.AnnModule'
          --           'GHC.Parser.Annotation.AnnClose'

          -- For details on above see note [exact print annotations] in GHC.Parser.Annotation
      | XAnnDecl !(XXAnnDecl pass)

    -- | Annotation Provenance
    data AnnProvenance pass = ValueAnnProvenance (LIdP pass)
                            | TypeAnnProvenance (LIdP pass)
                            | ModuleAnnProvenance
    -- deriving instance Functor     AnnProvenance
    -- deriving instance Foldable    AnnProvenance
    -- deriving instance Traversable AnnProvenance
    -- deriving instance (Data pass) => Data (AnnProvenance pass)

    annProvenanceName_maybe :: forall p. UnXRec p => AnnProvenance p -> Maybe (IdP p)
    annProvenanceName_maybe (ValueAnnProvenance (unXRec @p -> name)) = Just name
    annProvenanceName_maybe (TypeAnnProvenance (unXRec @p -> name))  = Just name
    annProvenanceName_maybe ModuleAnnProvenance                      = Nothing

    {-
    ************************************************************************
    *                                                                      *
    \subsection[RoleAnnot]{Role annotations}
    *                                                                      *
    ************************************************************************
    -}

    -- | Located Role Annotation Declaration
    type LRoleAnnotDecl pass = XRec pass (RoleAnnotDecl pass)

    -- See #8185 for more info about why role annotations are
    -- top-level declarations
    -- | Role Annotation Declaration
    data RoleAnnotDecl pass
      = RoleAnnotDecl (XCRoleAnnotDecl pass)
                      (LIdP pass)              -- type constructor
                      [XRec pass (Maybe Role)] -- optional annotations
          -- ^ - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnType',
          --           'GHC.Parser.Annotation.AnnRole'

          -- For details on above see note [exact print annotations] in GHC.Parser.Annotation
      | XRoleAnnotDecl !(XXRoleAnnotDecl pass)