TTG: replace Void/NoExtCon with DataConCantHappen

There were two ways to indicate that a TTG constructor is unused in a phase:
`NoExtCon` and `Void`. This unifies the code, and uses the name
'DataConCantHappen', following the discussion at MR 7041.

Updates haddock submodule

14 files changed, 134 insertions, 147 deletions

diff --git a/compiler/GHC/Hs/Binds.hs b/compiler/GHC/Hs/Binds.hs
index f25d28bee9..f5f9b725f4 100644
--- a/compiler/GHC/Hs/Binds.hs
+++ b/compiler/GHC/Hs/Binds.hs
@@ -71,7 +71,7 @@ Global bindings (where clauses)
 type instance XHsValBinds      (GhcPass pL) (GhcPass pR) = EpAnn AnnList
 type instance XHsIPBinds       (GhcPass pL) (GhcPass pR) = EpAnn AnnList
 type instance XEmptyLocalBinds (GhcPass pL) (GhcPass pR) = NoExtField
-type instance XXHsLocalBindsLR (GhcPass pL) (GhcPass pR) = NoExtCon
+type instance XXHsLocalBindsLR (GhcPass pL) (GhcPass pR) = DataConCantHappen
 
 -- ---------------------------------------------------------------------
 -- Deal with ValBindsOut
@@ -99,16 +99,16 @@ type instance XPatBind    GhcTc (GhcPass pR) = Type    -- Type of the GRHSs
 type instance XVarBind    (GhcPass pL) (GhcPass pR) = NoExtField
 type instance XAbsBinds   (GhcPass pL) (GhcPass pR) = NoExtField
 type instance XPatSynBind (GhcPass pL) (GhcPass pR) = NoExtField
-type instance XXHsBindsLR (GhcPass pL) (GhcPass pR) = NoExtCon
+type instance XXHsBindsLR (GhcPass pL) (GhcPass pR) = DataConCantHappen
 
 type instance XABE       (GhcPass p) = NoExtField
-type instance XXABExport (GhcPass p) = NoExtCon
+type instance XXABExport (GhcPass p) = DataConCantHappen
 
 type instance XPSB         (GhcPass idL) GhcPs = EpAnn [AddEpAnn]
 type instance XPSB         (GhcPass idL) GhcRn = NameSet
 type instance XPSB         (GhcPass idL) GhcTc = NameSet
 
-type instance XXPatSynBind (GhcPass idL) (GhcPass idR) = NoExtCon
+type instance XXPatSynBind (GhcPass idL) (GhcPass idR) = DataConCantHappen
 
 {-
 Note [AbsBinds]
@@ -543,7 +543,7 @@ type instance XIPBinds       GhcTc = TcEvBinds -- binds uses of the
                                                -- implicit parameters
 
 
-type instance XXHsIPBinds    (GhcPass p) = NoExtCon
+type instance XXHsIPBinds    (GhcPass p) = DataConCantHappen
 
 isEmptyIPBindsPR :: HsIPBinds (GhcPass p) -> Bool
 isEmptyIPBindsPR (IPBinds _ is) = null is
@@ -552,7 +552,7 @@ isEmptyIPBindsTc :: HsIPBinds GhcTc -> Bool
 isEmptyIPBindsTc (IPBinds ds is) = null is && isEmptyTcEvBinds ds
 
 type instance XCIPBind    (GhcPass p) = EpAnn [AddEpAnn]
-type instance XXIPBind    (GhcPass p) = NoExtCon
+type instance XXIPBind    (GhcPass p) = DataConCantHappen
 
 instance OutputableBndrId p
        => Outputable (HsIPBinds (GhcPass p)) where
@@ -585,10 +585,10 @@ type instance XMinimalSig       (GhcPass p) = EpAnn [AddEpAnn]
 type instance XSCCFunSig        (GhcPass p) = EpAnn [AddEpAnn]
 type instance XCompleteMatchSig (GhcPass p) = EpAnn [AddEpAnn]
 
-type instance XXSig             (GhcPass p) = NoExtCon
+type instance XXSig             (GhcPass p) = DataConCantHappen
 
 type instance XFixitySig  (GhcPass p) = NoExtField
-type instance XXFixitySig (GhcPass p) = NoExtCon
+type instance XXFixitySig (GhcPass p) = DataConCantHappen
 
 data AnnSig
   = AnnSig {
diff --git a/compiler/GHC/Hs/Decls.hs b/compiler/GHC/Hs/Decls.hs
index 3a58ddfce1..40c54b629a 100644
--- a/compiler/GHC/Hs/Decls.hs
+++ b/compiler/GHC/Hs/Decls.hs
@@ -153,7 +153,7 @@ type instance XRuleD      (GhcPass _) = NoExtField
 type instance XSpliceD    (GhcPass _) = NoExtField
 type instance XDocD       (GhcPass _) = NoExtField
 type instance XRoleAnnotD (GhcPass _) = NoExtField
-type instance XXHsDecl    (GhcPass _) = NoExtCon
+type instance XXHsDecl    (GhcPass _) = DataConCantHappen
 
 -- | Partition a list of HsDecls into function/pattern bindings, signatures,
 -- type family declarations, type family instances, and documentation comments.
@@ -188,7 +188,7 @@ partitionBindsAndSigs = go
         _ -> pprPanic "partitionBindsAndSigs" (ppr decl)
 
 type instance XCHsGroup (GhcPass _) = NoExtField
-type instance XXHsGroup (GhcPass _) = NoExtCon
+type instance XXHsGroup (GhcPass _) = DataConCantHappen
 
 
 emptyGroup, emptyRdrGroup, emptyRnGroup :: HsGroup (GhcPass p)
@@ -309,7 +309,7 @@ instance (OutputableBndrId p) => Outputable (HsGroup (GhcPass p)) where
           vcat_mb gap (Just d  : ds) = gap $$ d $$ vcat_mb blankLine ds
 
 type instance XSpliceDecl      (GhcPass _) = NoExtField
-type instance XXSpliceDecl     (GhcPass _) = NoExtCon
+type instance XXSpliceDecl     (GhcPass _) = DataConCantHappen
 
 instance OutputableBndrId p
        => Outputable (SpliceDecl (GhcPass p)) where
@@ -338,10 +338,10 @@ type instance XClassDecl    GhcPs = (EpAnn [AddEpAnn], AnnSortKey, LayoutInfo)
 type instance XClassDecl    GhcRn = NameSet -- FVs
 type instance XClassDecl    GhcTc = NameSet -- FVs
 
-type instance XXTyClDecl    (GhcPass _) = NoExtCon
+type instance XXTyClDecl    (GhcPass _) = DataConCantHappen
 
 type instance XCTyFamInstDecl (GhcPass _) = EpAnn [AddEpAnn]
-type instance XXTyFamInstDecl (GhcPass _) = NoExtCon
+type instance XXTyFamInstDecl (GhcPass _) = DataConCantHappen
 
 -- Dealing with names
 
@@ -464,7 +464,7 @@ instance OutputableBndrId p => Outputable (FunDep (GhcPass p)) where
   ppr = pprFunDep
 
 type instance XCFunDep    (GhcPass _) = EpAnn [AddEpAnn]
-type instance XXFunDep    (GhcPass _) = NoExtCon
+type instance XXFunDep    (GhcPass _) = DataConCantHappen
 
 pprFundeps :: OutputableBndrId p => [FunDep (GhcPass p)] -> SDoc
 pprFundeps []  = empty
@@ -482,7 +482,7 @@ pprFunDep (FunDep _ us vs) = hsep [interppSP us, arrow, interppSP vs]
 ********************************************************************* -}
 
 type instance XCTyClGroup (GhcPass _) = NoExtField
-type instance XXTyClGroup (GhcPass _) = NoExtCon
+type instance XXTyClGroup (GhcPass _) = DataConCantHappen
 
 
 {- *********************************************************************
@@ -495,10 +495,10 @@ type instance XNoSig            (GhcPass _) = NoExtField
 type instance XCKindSig         (GhcPass _) = NoExtField
 
 type instance XTyVarSig         (GhcPass _) = NoExtField
-type instance XXFamilyResultSig (GhcPass _) = NoExtCon
+type instance XXFamilyResultSig (GhcPass _) = DataConCantHappen
 
 type instance XCFamilyDecl    (GhcPass _) = EpAnn [AddEpAnn]
-type instance XXFamilyDecl    (GhcPass _) = NoExtCon
+type instance XXFamilyDecl    (GhcPass _) = DataConCantHappen
 
 
 ------------- Functions over FamilyDecls -----------
@@ -525,7 +525,7 @@ resultVariableName _                = Nothing
 ------------- Pretty printing FamilyDecls -----------
 
 type instance XCInjectivityAnn  (GhcPass _) = EpAnn [AddEpAnn]
-type instance XXInjectivityAnn  (GhcPass _) = NoExtCon
+type instance XXInjectivityAnn  (GhcPass _) = DataConCantHappen
 
 instance OutputableBndrId p
        => Outputable (FamilyDecl (GhcPass p)) where
@@ -569,10 +569,10 @@ instance OutputableBndrId p
 ********************************************************************* -}
 
 type instance XCHsDataDefn    (GhcPass _) = NoExtField
-type instance XXHsDataDefn    (GhcPass _) = NoExtCon
+type instance XXHsDataDefn    (GhcPass _) = DataConCantHappen
 
 type instance XCHsDerivingClause    (GhcPass _) = EpAnn [AddEpAnn]
-type instance XXHsDerivingClause    (GhcPass _) = NoExtCon
+type instance XXHsDerivingClause    (GhcPass _) = DataConCantHappen
 
 instance OutputableBndrId p
        => Outputable (HsDerivingClause (GhcPass p)) where
@@ -592,7 +592,7 @@ instance OutputableBndrId p
 
 type instance XDctSingle (GhcPass _) = NoExtField
 type instance XDctMulti  (GhcPass _) = NoExtField
-type instance XXDerivClauseTys (GhcPass _) = NoExtCon
+type instance XXDerivClauseTys (GhcPass _) = DataConCantHappen
 
 instance OutputableBndrId p => Outputable (DerivClauseTys (GhcPass p)) where
   ppr (DctSingle _ ty) = ppr ty
@@ -602,7 +602,7 @@ type instance XStandaloneKindSig GhcPs = EpAnn [AddEpAnn]
 type instance XStandaloneKindSig GhcRn = NoExtField
 type instance XStandaloneKindSig GhcTc = NoExtField
 
-type instance XXStandaloneKindSig (GhcPass p) = NoExtCon
+type instance XXStandaloneKindSig (GhcPass p) = DataConCantHappen
 
 standaloneKindSigName :: StandaloneKindSig (GhcPass p) -> IdP (GhcPass p)
 standaloneKindSigName (StandaloneKindSig _ lname _) = unLoc lname
@@ -610,7 +610,7 @@ standaloneKindSigName (StandaloneKindSig _ lname _) = unLoc lname
 type instance XConDeclGADT (GhcPass _) = EpAnn [AddEpAnn]
 type instance XConDeclH98  (GhcPass _) = EpAnn [AddEpAnn]
 
-type instance XXConDecl (GhcPass _) = NoExtCon
+type instance XXConDecl (GhcPass _) = DataConCantHappen
 
 getConNames :: ConDecl GhcRn -> [LocatedN Name]
 getConNames ConDeclH98  {con_name  = name}  = [name]
@@ -728,7 +728,7 @@ ppr_con_names = pprWithCommas (pprPrefixOcc . unLoc)
 -}
 
 type instance XCFamEqn    (GhcPass _) r = EpAnn [AddEpAnn]
-type instance XXFamEqn    (GhcPass _) r = NoExtCon
+type instance XXFamEqn    (GhcPass _) r = DataConCantHappen
 
 type instance Anno (FamEqn (GhcPass p) _) = SrcSpanAnnA
 
@@ -738,7 +738,7 @@ type instance XCClsInstDecl    GhcPs = (EpAnn [AddEpAnn], AnnSortKey) -- TODO:AZ
 type instance XCClsInstDecl    GhcRn = NoExtField
 type instance XCClsInstDecl    GhcTc = NoExtField
 
-type instance XXClsInstDecl    (GhcPass _) = NoExtCon
+type instance XXClsInstDecl    (GhcPass _) = DataConCantHappen
 
 ----------------- Instances of all kinds -------------
 
@@ -752,7 +752,7 @@ type instance XTyFamInstD   GhcPs = NoExtField
 type instance XTyFamInstD   GhcRn = NoExtField
 type instance XTyFamInstD   GhcTc = NoExtField
 
-type instance XXInstDecl    (GhcPass _) = NoExtCon
+type instance XXInstDecl    (GhcPass _) = DataConCantHappen
 
 instance OutputableBndrId p
        => Outputable (TyFamInstDecl (GhcPass p)) where
@@ -881,7 +881,7 @@ instDeclDataFamInsts inst_decls
 -}
 
 type instance XCDerivDecl    (GhcPass _) = EpAnn [AddEpAnn]
-type instance XXDerivDecl    (GhcPass _) = NoExtCon
+type instance XXDerivDecl    (GhcPass _) = DataConCantHappen
 
 type instance Anno OverlapMode = SrcSpanAnnP
 
@@ -963,7 +963,7 @@ type instance XCDefaultDecl    GhcPs = EpAnn [AddEpAnn]
 type instance XCDefaultDecl    GhcRn = NoExtField
 type instance XCDefaultDecl    GhcTc = NoExtField
 
-type instance XXDefaultDecl    (GhcPass _) = NoExtCon
+type instance XXDefaultDecl    (GhcPass _) = DataConCantHappen
 
 instance OutputableBndrId p
        => Outputable (DefaultDecl (GhcPass p)) where
@@ -986,7 +986,7 @@ type instance XForeignExport   GhcPs = EpAnn [AddEpAnn]
 type instance XForeignExport   GhcRn = NoExtField
 type instance XForeignExport   GhcTc = Coercion
 
-type instance XXForeignDecl    (GhcPass _) = NoExtCon
+type instance XXForeignDecl    (GhcPass _) = DataConCantHappen
 
 instance OutputableBndrId p
        => Outputable (ForeignDecl (GhcPass p)) where
@@ -1009,13 +1009,13 @@ type instance XCRuleDecls    GhcPs = EpAnn [AddEpAnn]
 type instance XCRuleDecls    GhcRn = NoExtField
 type instance XCRuleDecls    GhcTc = NoExtField
 
-type instance XXRuleDecls    (GhcPass _) = NoExtCon
+type instance XXRuleDecls    (GhcPass _) = DataConCantHappen
 
 type instance XHsRule       GhcPs = EpAnn HsRuleAnn
 type instance XHsRule       GhcRn = HsRuleRn
 type instance XHsRule       GhcTc = HsRuleRn
 
-type instance XXRuleDecl    (GhcPass _) = NoExtCon
+type instance XXRuleDecl    (GhcPass _) = DataConCantHappen
 
 type instance Anno (SourceText, RuleName) = SrcAnn NoEpAnns
 
@@ -1035,7 +1035,7 @@ flattenRuleDecls decls = concatMap (rds_rules . unLoc) decls
 
 type instance XCRuleBndr    (GhcPass _) = EpAnn [AddEpAnn]
 type instance XRuleBndrSig  (GhcPass _) = EpAnn [AddEpAnn]
-type instance XXRuleBndr    (GhcPass _) = NoExtCon
+type instance XXRuleBndr    (GhcPass _) = DataConCantHappen
 
 instance (OutputableBndrId p) => Outputable (RuleDecls (GhcPass p)) where
   ppr (HsRules { rds_src = st
@@ -1076,10 +1076,10 @@ type instance XWarnings      GhcPs = EpAnn [AddEpAnn]
 type instance XWarnings      GhcRn = NoExtField
 type instance XWarnings      GhcTc = NoExtField
 
-type instance XXWarnDecls    (GhcPass _) = NoExtCon
+type instance XXWarnDecls    (GhcPass _) = DataConCantHappen
 
 type instance XWarning      (GhcPass _) = EpAnn [AddEpAnn]
-type instance XXWarnDecl    (GhcPass _) = NoExtCon
+type instance XXWarnDecl    (GhcPass _) = DataConCantHappen
 
 
 instance OutputableBndrId p
@@ -1103,7 +1103,7 @@ instance OutputableBndrId p
 -}
 
 type instance XHsAnnotation (GhcPass _) = EpAnn AnnPragma
-type instance XXAnnDecl     (GhcPass _) = NoExtCon
+type instance XXAnnDecl     (GhcPass _) = DataConCantHappen
 
 instance (OutputableBndrId p) => Outputable (AnnDecl (GhcPass p)) where
     ppr (HsAnnotation _ _ provenance expr)
@@ -1128,7 +1128,7 @@ type instance XCRoleAnnotDecl GhcPs = EpAnn [AddEpAnn]
 type instance XCRoleAnnotDecl GhcRn = NoExtField
 type instance XCRoleAnnotDecl GhcTc = NoExtField
 
-type instance XXRoleAnnotDecl (GhcPass _) = NoExtCon
+type instance XXRoleAnnotDecl (GhcPass _) = DataConCantHappen
 
 type instance Anno (Maybe Role) = SrcAnn NoEpAnns
 
diff --git a/compiler/GHC/Hs/Expr.hs b/compiler/GHC/Hs/Expr.hs
index ad49273464..6228b7d90e 100644
--- a/compiler/GHC/Hs/Expr.hs
+++ b/compiler/GHC/Hs/Expr.hs
@@ -70,7 +70,6 @@ import Data.Data hiding (Fixity(..))
 import qualified Data.Data as Data (Fixity(..))
 import qualified Data.Kind
 import Data.Maybe (isJust)
-import Data.Void  ( Void )
 import Data.Foldable ( toList )
 
 {- *********************************************************************
@@ -190,30 +189,6 @@ type instance PendingTcSplice' (GhcPass _) = PendingTcSplice
 
 -- ---------------------------------------------------------------------
 
-{- Note [Constructor cannot occur]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-Some data constructors can't occur in certain phases; e.g. the output
-of the type checker never has OverLabel. We signal this by
-* setting the extension field to Void
-* using dataConCantHappen in the cases that can't happen
-
-For example:
-
-   type instance XOverLabel GhcTc = Void
-
-   dsExpr :: HsExpr GhcTc -> blah
-   dsExpr (HsOverLabel x _) = dataConCantHappen x
-
-The function dataConCantHappen is defined thus:
-   dataConCantHappen :: Void -> a
-   dataConCantHappen x = case x of {}
-(i.e. identically to Data.Void.absurd, but more helpfully named).
-Remember Void is a type whose only element is bottom.
-
-It would be better to omit the pattern match altogether, but we
-could only do that if the extension field was strict (#18764).
--}
-
 -- API Annotations types
 
 data EpAnnHsCase = EpAnnHsCase
@@ -231,7 +206,7 @@ type instance XVar           (GhcPass _) = NoExtField
 
 -- Record selectors at parse time are HsVar; they convert to HsRecSel
 -- on renaming.
-type instance XRecSel              GhcPs = Void
+type instance XRecSel              GhcPs = DataConCantHappen
 type instance XRecSel              GhcRn = NoExtField
 type instance XRecSel              GhcTc = NoExtField
 
@@ -241,7 +216,7 @@ type instance XLam           (GhcPass _) = NoExtField
 -- Note [Handling overloaded and rebindable constructs]
 type instance XOverLabel     GhcPs = EpAnnCO
 type instance XOverLabel     GhcRn = EpAnnCO
-type instance XOverLabel     GhcTc = Void  -- See Note [Constructor cannot occur]
+type instance XOverLabel     GhcTc = DataConCantHappen
 
 -- ---------------------------------------------------------------------
 
@@ -257,7 +232,7 @@ type instance XUnboundVar    GhcTc = HoleExprRef
 
 type instance XIPVar         GhcPs = EpAnnCO
 type instance XIPVar         GhcRn = EpAnnCO
-type instance XIPVar         GhcTc = Void -- See Note [Constructor cannot occur]
+type instance XIPVar         GhcTc = DataConCantHappen
 type instance XOverLitE      (GhcPass _) = EpAnnCO
 type instance XLitE          (GhcPass _) = EpAnnCO
 
@@ -274,7 +249,7 @@ type instance XAppTypeE      GhcTc = Type
 -- Note [Handling overloaded and rebindable constructs]
 type instance XOpApp         GhcPs = EpAnn [AddEpAnn]
 type instance XOpApp         GhcRn = Fixity
-type instance XOpApp         GhcTc = Void  -- See Note [Constructor cannot occur]
+type instance XOpApp         GhcTc = DataConCantHappen
 
 -- SectionL, SectionR not present in GhcTc pass; see GHC.Rename.Expr
 -- Note [Handling overloaded and rebindable constructs]
@@ -282,8 +257,8 @@ type instance XSectionL      GhcPs = EpAnnCO
 type instance XSectionR      GhcPs = EpAnnCO
 type instance XSectionL      GhcRn = EpAnnCO
 type instance XSectionR      GhcRn = EpAnnCO
-type instance XSectionL      GhcTc = Void  -- See Note [Constructor cannot occur]
-type instance XSectionR      GhcTc = Void  -- See Note [Constructor cannot occur]
+type instance XSectionL      GhcTc = DataConCantHappen
+type instance XSectionR      GhcTc = DataConCantHappen
 
 
 type instance XNegApp        GhcPs = EpAnn [AddEpAnn]
@@ -341,13 +316,13 @@ type instance XRecordUpd     GhcTc = RecordUpdTc
 
 type instance XGetField     GhcPs = EpAnnCO
 type instance XGetField     GhcRn = NoExtField
-type instance XGetField     GhcTc = Void
+type instance XGetField     GhcTc = DataConCantHappen
 -- HsGetField is eliminated by the renamer. See [Handling overloaded
 -- and rebindable constructs].
 
 type instance XProjection     GhcPs = EpAnn AnnProjection
 type instance XProjection     GhcRn = NoExtField
-type instance XProjection     GhcTc = Void
+type instance XProjection     GhcTc = DataConCantHappen
 -- HsProjection is eliminated by the renamer. See [Handling overloaded
 -- and rebindable constructs].
 
@@ -361,14 +336,14 @@ type instance XArithSeq      GhcTc = PostTcExpr
 
 type instance XBracket       GhcPs = EpAnn [AddEpAnn]
 type instance XBracket       GhcRn = EpAnn [AddEpAnn]
-type instance XBracket       GhcTc = Void -- See Note [Constructor cannot occur]
+type instance XBracket       GhcTc = DataConCantHappen
 
-type instance XRnBracketOut  GhcPs = Void -- See Note [Constructor cannot occur]
+type instance XRnBracketOut  GhcPs = DataConCantHappen
 type instance XRnBracketOut  GhcRn = NoExtField
-type instance XRnBracketOut  GhcTc = Void -- See Note [Constructor cannot occur]
+type instance XRnBracketOut  GhcTc = DataConCantHappen
 
-type instance XTcBracketOut  GhcPs = Void -- See Note [Constructor cannot occur]
-type instance XTcBracketOut  GhcRn = Void -- See Note [Constructor cannot occur]
+type instance XTcBracketOut  GhcPs = DataConCantHappen
+type instance XTcBracketOut  GhcRn = DataConCantHappen
 type instance XTcBracketOut  GhcTc = Type -- Type of the TcBracketOut
 
 type instance XSpliceE       (GhcPass _) = EpAnnCO
@@ -414,10 +389,10 @@ data AnnsIf
 -- ---------------------------------------------------------------------
 
 type instance XSCC           (GhcPass _) = EpAnn AnnPragma
-type instance XXPragE        (GhcPass _) = NoExtCon
+type instance XXPragE        (GhcPass _) = DataConCantHappen
 
 type instance XCDotFieldOcc (GhcPass _) = EpAnn AnnFieldLabel
-type instance XXDotFieldOcc (GhcPass _) = NoExtCon
+type instance XXDotFieldOcc (GhcPass _) = DataConCantHappen
 
 type instance XPresent         (GhcPass _) = EpAnn [AddEpAnn]
 
@@ -425,7 +400,7 @@ type instance XMissing         GhcPs = EpAnn EpaLocation
 type instance XMissing         GhcRn = NoExtField
 type instance XMissing         GhcTc = Scaled Type
 
-type instance XXTupArg         (GhcPass _) = NoExtCon
+type instance XXTupArg         (GhcPass _) = DataConCantHappen
 
 tupArgPresent :: HsTupArg (GhcPass p) -> Bool
 tupArgPresent (Present {}) = True
@@ -438,7 +413,7 @@ tupArgPresent (Missing {}) = False
 *                                                                      *
 ********************************************************************* -}
 
-type instance XXExpr GhcPs = NoExtCon
+type instance XXExpr GhcPs = DataConCantHappen
 type instance XXExpr GhcRn = HsExpansion (HsExpr GhcRn) (HsExpr GhcRn)
 type instance XXExpr GhcTc = XXExprGhcTc
 -- HsExpansion: see Note [Rebindable syntax and HsExpansion] below
@@ -1106,8 +1081,8 @@ type instance XCmdDo      GhcTc = Type
 
 type instance XCmdWrap    (GhcPass _) = NoExtField
 
-type instance XXCmd       GhcPs = NoExtCon
-type instance XXCmd       GhcRn = NoExtCon
+type instance XXCmd       GhcPs = DataConCantHappen
+type instance XXCmd       GhcRn = DataConCantHappen
 type instance XXCmd       GhcTc = HsWrap HsCmd
 
 type instance Anno [LocatedA (StmtLR (GhcPass pl) (GhcPass pr) (LocatedA (HsCmd (GhcPass pr))))]
@@ -1126,7 +1101,7 @@ type instance XCmdTop  GhcPs = NoExtField
 type instance XCmdTop  GhcRn = CmdSyntaxTable GhcRn -- See Note [CmdSyntaxTable]
 type instance XCmdTop  GhcTc = CmdTopTc
 
-type instance XXCmdTop (GhcPass _) = NoExtCon
+type instance XXCmdTop (GhcPass _) = DataConCantHappen
 
 instance (OutputableBndrId p) => Outputable (HsCmd (GhcPass p)) where
     ppr cmd = pprCmd cmd
@@ -1249,10 +1224,10 @@ type instance XMG         GhcPs b = NoExtField
 type instance XMG         GhcRn b = NoExtField
 type instance XMG         GhcTc b = MatchGroupTc
 
-type instance XXMatchGroup (GhcPass _) b = NoExtCon
+type instance XXMatchGroup (GhcPass _) b = DataConCantHappen
 
 type instance XCMatch (GhcPass _) b = EpAnn [AddEpAnn]
-type instance XXMatch (GhcPass _) b = NoExtCon
+type instance XXMatch (GhcPass _) b = DataConCantHappen
 
 instance (OutputableBndrId pr, Outputable body)
             => Outputable (Match (GhcPass pr) body) where
@@ -1286,7 +1261,7 @@ hsLMatchPats (L _ (Match { m_pats = pats })) = pats
 -- item. So this can never be used in practice.
 type instance XCGRHSs (GhcPass _) _ = EpAnnComments
 
-type instance XXGRHSs (GhcPass _) _ = NoExtCon
+type instance XXGRHSs (GhcPass _) _ = DataConCantHappen
 
 data GrhsAnn
   = GrhsAnn {
@@ -1298,7 +1273,7 @@ type instance XCGRHS (GhcPass _) _ = EpAnn GrhsAnn
                                    -- Location of matchSeparator
                                    -- TODO:AZ does this belong on the GRHS, or GRHSs?
 
-type instance XXGRHS (GhcPass _) b = NoExtCon
+type instance XXGRHS (GhcPass _) b = DataConCantHappen
 
 pprMatches :: (OutputableBndrId idR, Outputable body)
            => MatchGroup (GhcPass idR) body -> SDoc
@@ -1446,17 +1421,17 @@ type instance XRecStmt         (GhcPass _) GhcPs b = EpAnn AnnList
 type instance XRecStmt         (GhcPass _) GhcRn b = NoExtField
 type instance XRecStmt         (GhcPass _) GhcTc b = RecStmtTc
 
-type instance XXStmtLR         (GhcPass _) (GhcPass _) b = NoExtCon
+type instance XXStmtLR         (GhcPass _) (GhcPass _) b = DataConCantHappen
 
 type instance XParStmtBlock  (GhcPass pL) (GhcPass pR) = NoExtField
-type instance XXParStmtBlock (GhcPass pL) (GhcPass pR) = NoExtCon
+type instance XXParStmtBlock (GhcPass pL) (GhcPass pR) = DataConCantHappen
 
 type instance XApplicativeArgOne GhcPs = NoExtField
 type instance XApplicativeArgOne GhcRn = FailOperator GhcRn
 type instance XApplicativeArgOne GhcTc = FailOperator GhcTc
 
 type instance XApplicativeArgMany (GhcPass _) = NoExtField
-type instance XXApplicativeArg    (GhcPass _) = NoExtCon
+type instance XXApplicativeArg    (GhcPass _) = DataConCantHappen
 
 instance (Outputable (StmtLR (GhcPass idL) (GhcPass idL) (LHsExpr (GhcPass idL))),
           Outputable (XXParStmtBlock (GhcPass idL) (GhcPass idR)))
@@ -1633,8 +1608,8 @@ type instance XTypedSplice   (GhcPass _) = EpAnn [AddEpAnn]
 type instance XUntypedSplice (GhcPass _) = EpAnn [AddEpAnn]
 type instance XQuasiQuote    (GhcPass _) = NoExtField
 type instance XSpliced       (GhcPass _) = NoExtField
-type instance XXSplice       GhcPs       = NoExtCon
-type instance XXSplice       GhcRn       = NoExtCon
+type instance XXSplice       GhcPs       = DataConCantHappen
+type instance XXSplice       GhcRn       = DataConCantHappen
 type instance XXSplice       GhcTc       = HsSplicedT
 
 -- See Note [Running typed splices in the zonker]
@@ -1761,8 +1736,8 @@ pprSplice (HsQuasiQuote _ n q _ s)      = ppr_quasi n q s
 pprSplice (HsSpliced _ _ thing)         = ppr thing
 pprSplice (XSplice x)                   = case ghcPass @p of
 #if __GLASGOW_HASKELL__ < 811
-                                            GhcPs -> noExtCon x
-                                            GhcRn -> noExtCon x
+                                            GhcPs -> dataConCantHappen x
+                                            GhcRn -> dataConCantHappen x
 #endif
                                             GhcTc -> case x of
                                                        HsSplicedT _ -> text "Unevaluated typed splice"
@@ -1784,7 +1759,7 @@ type instance XDecBrG     (GhcPass _) = NoExtField
 type instance XTypBr      (GhcPass _) = NoExtField
 type instance XVarBr      (GhcPass _) = NoExtField
 type instance XTExpBr     (GhcPass _) = NoExtField
-type instance XXBracket   (GhcPass _) = NoExtCon
+type instance XXBracket   (GhcPass _) = DataConCantHappen
 
 instance OutputableBndrId p
           => Outputable (HsBracket (GhcPass p)) where
diff --git a/compiler/GHC/Hs/Extension.hs b/compiler/GHC/Hs/Extension.hs
index ce28e0355d..d58bd9efbc 100644
--- a/compiler/GHC/Hs/Extension.hs
+++ b/compiler/GHC/Hs/Extension.hs
@@ -32,8 +32,6 @@ import GHC.Types.SrcLoc (GenLocated(..), unLoc)
 import GHC.Utils.Panic
 import GHC.Parser.Annotation
 
-import Data.Void
-
 {-
 Note [IsPass]
 ~~~~~~~~~~~~~
@@ -115,12 +113,12 @@ instance MapXRec (GhcPass p) where
 --   wrapXRec = noLocA
 
 {-
-Note [NoExtCon and strict fields]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Note [DataConCantHappen and strict fields]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Currently, any unused TTG extension constructor will generally look like the
 following:
 
-  type instance XXHsDecl (GhcPass _) = NoExtCon
+  type instance XXHsDecl (GhcPass _) = DataConCantHappen
   data HsDecl p
     = ...
     | XHsDecl !(XXHsDecl p)
@@ -132,17 +130,17 @@ the following function which consumes an HsDecl:
 
   ex :: HsDecl GhcPs -> HsDecl GhcRn
   ...
-  ex (XHsDecl nec) = noExtCon nec
+  ex (XHsDecl nec) = dataConCantHappen nec
 
 Because `p` equals GhcPs (i.e., GhcPass 'Parsed), XHsDecl's field has the type
-NoExtCon. But since (1) the field is strict and (2) NoExtCon is an empty data
-type, there is no possible way to reach the right-hand side of the XHsDecl
-case. As a result, the coverage checker concludes that the XHsDecl case is
-inaccessible, so it can be removed.
+DataConCantHappen. But since (1) the field is strict and (2) DataConCantHappen
+is an empty data type, there is no possible way to reach the right-hand side
+of the XHsDecl case. As a result, the coverage checker concludes that
+the XHsDecl case is inaccessible, so it can be removed.
 (See Note [Strict argument type constraints] in GHC.HsToCore.Pmc.Solver for
 more on how this works.)
 
-Bottom line: if you add a TTG extension constructor that uses NoExtCon, make
+Bottom line: if you add a TTG extension constructor that uses DataConCantHappen, make
 sure that any uses of it as a field are strict.
 -}
 
@@ -219,10 +217,6 @@ type OutputableBndrId pass =
   , IsPass pass
   )
 
--- | See Note [Constructor cannot occur]
-dataConCantHappen :: Void -> a
-dataConCantHappen = absurd
-
 -- useful helper functions:
 pprIfPs :: forall p. IsPass p => (p ~ 'Parsed => SDoc) -> SDoc
 pprIfPs pp = case ghcPass @p of GhcPs -> pp
diff --git a/compiler/GHC/Hs/ImpExp.hs b/compiler/GHC/Hs/ImpExp.hs
index 7282b724cf..4bc0fc9d4a 100644
--- a/compiler/GHC/Hs/ImpExp.hs
+++ b/compiler/GHC/Hs/ImpExp.hs
@@ -122,7 +122,7 @@ type instance XCImportDecl  GhcPs = EpAnn EpAnnImportDecl
 type instance XCImportDecl  GhcRn = NoExtField
 type instance XCImportDecl  GhcTc = NoExtField
 
-type instance XXImportDecl  (GhcPass _) = NoExtCon
+type instance XXImportDecl  (GhcPass _) = DataConCantHappen
 
 type instance Anno ModuleName = SrcSpanAnnA
 type instance Anno [LocatedA (IE (GhcPass p))] = SrcSpanAnnL
@@ -303,7 +303,7 @@ type instance XIEModuleContents  GhcTc = NoExtField
 type instance XIEGroup           (GhcPass _) = NoExtField
 type instance XIEDoc             (GhcPass _) = NoExtField
 type instance XIEDocNamed        (GhcPass _) = NoExtField
-type instance XXIE               (GhcPass _) = NoExtCon
+type instance XXIE               (GhcPass _) = DataConCantHappen
 
 type instance Anno (LocatedA (IE (GhcPass p))) = SrcSpanAnnA
 
diff --git a/compiler/GHC/Hs/Lit.hs b/compiler/GHC/Hs/Lit.hs
index 9341827a79..3b9b6948c6 100644
--- a/compiler/GHC/Hs/Lit.hs
+++ b/compiler/GHC/Hs/Lit.hs
@@ -55,7 +55,7 @@ type instance XHsInteger    (GhcPass _) = SourceText
 type instance XHsRat        (GhcPass _) = NoExtField
 type instance XHsFloatPrim  (GhcPass _) = NoExtField
 type instance XHsDoublePrim (GhcPass _) = NoExtField
-type instance XXLit         (GhcPass _) = NoExtCon
+type instance XXLit         (GhcPass _) = DataConCantHappen
 
 data OverLitRn
   = OverLitRn {
@@ -98,7 +98,7 @@ pprXOverLit GhcPs noExt = ppr noExt
 pprXOverLit GhcRn OverLitRn{ ol_from_fun = from_fun } = ppr from_fun
 pprXOverLit GhcTc OverLitTc{ ol_witness = witness } = pprExpr witness
 
-type instance XXOverLit (GhcPass _) = NoExtCon
+type instance XXOverLit (GhcPass _) = DataConCantHappen
 
 overLitType :: HsOverLit GhcTc -> Type
 overLitType (OverLit OverLitTc{ ol_type = ty } _) = ty
diff --git a/compiler/GHC/Hs/Pat.hs b/compiler/GHC/Hs/Pat.hs
index a98070c6a7..43f161f9bd 100644
--- a/compiler/GHC/Hs/Pat.hs
+++ b/compiler/GHC/Hs/Pat.hs
@@ -82,7 +82,6 @@ import GHC.Types.Name (Name)
 import GHC.Driver.Session
 import qualified GHC.LanguageExtensions as LangExt
 import Data.Data
-import Data.Void
 
 
 type instance XWildPat GhcPs = NoExtField
@@ -139,7 +138,7 @@ type instance XViewPat GhcTc = Type
 
 type instance XSplicePat GhcPs = NoExtField
 type instance XSplicePat GhcRn = NoExtField
-type instance XSplicePat GhcTc = Void -- See Note [Constructor cannot occur]
+type instance XSplicePat GhcTc = DataConCantHappen
 
 type instance XLitPat    (GhcPass _) = NoExtField
 
@@ -155,7 +154,7 @@ type instance XSigPat GhcPs = EpAnn [AddEpAnn]
 type instance XSigPat GhcRn = NoExtField
 type instance XSigPat GhcTc = Type
 
-type instance XXPat GhcPs = NoExtCon
+type instance XXPat GhcPs = DataConCantHappen
 type instance XXPat GhcRn = HsPatExpansion (Pat GhcRn) (Pat GhcRn)
   -- Original pattern and its desugaring/expansion.
   -- See Note [Rebindable syntax and HsExpansion].
@@ -356,7 +355,7 @@ pprPat (ConPat { pat_con = con
 
 pprPat (XPat ext) = case ghcPass @p of
 #if __GLASGOW_HASKELL__ < 811
-  GhcPs -> noExtCon ext
+  GhcPs -> dataConCantHappen ext
 #endif
   GhcRn -> case ext of
     HsPatExpanded orig _ -> pprPat orig
@@ -567,7 +566,7 @@ isIrrefutableHsPat' is_strict = goL
 
     go (XPat ext)          = case ghcPass @p of
 #if __GLASGOW_HASKELL__ < 811
-      GhcPs -> noExtCon ext
+      GhcPs -> dataConCantHappen ext
 #endif
       GhcRn -> case ext of
         HsPatExpanded _ pat -> go pat
@@ -630,7 +629,7 @@ patNeedsParens p = go @p
     go (ViewPat {})      = True
     go (XPat ext)        = case ghcPass @q of
 #if __GLASGOW_HASKELL__ < 901
-      GhcPs -> noExtCon ext
+      GhcPs -> dataConCantHappen ext
 #endif
       GhcRn -> case ext of
         HsPatExpanded orig _ -> go orig
diff --git a/compiler/GHC/Hs/Type.hs b/compiler/GHC/Hs/Type.hs
index 0e6d8564ae..2b17df420e 100644
--- a/compiler/GHC/Hs/Type.hs
+++ b/compiler/GHC/Hs/Type.hs
@@ -149,7 +149,7 @@ type instance XHsForAllVis   (GhcPass _) = EpAnnForallTy
 type instance XHsForAllInvis (GhcPass _) = EpAnnForallTy
                                            -- Location of 'forall' and '.'
 
-type instance XXHsForAllTelescope (GhcPass _) = NoExtCon
+type instance XXHsForAllTelescope (GhcPass _) = DataConCantHappen
 
 type EpAnnForallTy = EpAnn (AddEpAnn, AddEpAnn)
   -- ^ Location of 'forall' and '->' for HsForAllVis
@@ -163,7 +163,7 @@ type instance XHsQTvs GhcPs = NoExtField
 type instance XHsQTvs GhcRn = HsQTvsRn
 type instance XHsQTvs GhcTc = HsQTvsRn
 
-type instance XXLHsQTyVars  (GhcPass _) = NoExtCon
+type instance XXLHsQTyVars  (GhcPass _) = DataConCantHappen
 
 mkHsForAllVisTele ::EpAnnForallTy ->
   [LHsTyVarBndr () (GhcPass p)] -> HsForAllTelescope (GhcPass p)
@@ -192,22 +192,22 @@ type instance XHsOuterExplicit GhcPs _    = EpAnnForallTy
 type instance XHsOuterExplicit GhcRn _    = NoExtField
 type instance XHsOuterExplicit GhcTc flag = [VarBndr TyVar flag]
 
-type instance XXHsOuterTyVarBndrs (GhcPass _) = NoExtCon
+type instance XXHsOuterTyVarBndrs (GhcPass _) = DataConCantHappen
 
 type instance XHsWC              GhcPs b = NoExtField
 type instance XHsWC              GhcRn b = [Name]
 type instance XHsWC              GhcTc b = [Name]
 
-type instance XXHsWildCardBndrs (GhcPass _) _ = NoExtCon
+type instance XXHsWildCardBndrs (GhcPass _) _ = DataConCantHappen
 
 type instance XHsPS GhcPs = EpAnn EpaLocation
 type instance XHsPS GhcRn = HsPSRn
 type instance XHsPS GhcTc = HsPSRn
 
-type instance XXHsPatSigType (GhcPass _) = NoExtCon
+type instance XXHsPatSigType (GhcPass _) = DataConCantHappen
 
 type instance XHsSig (GhcPass _) = NoExtField
-type instance XXHsSigType (GhcPass _) = NoExtCon
+type instance XXHsSigType (GhcPass _) = DataConCantHappen
 
 hsSigWcType :: forall p. UnXRec p => LHsSigWcType p -> LHsType p
 hsSigWcType = sig_body . unXRec @p . hswc_body
@@ -262,7 +262,7 @@ mkEmptyWildCardBndrs x = HsWC { hswc_body = x
 type instance XUserTyVar    (GhcPass _) = EpAnn [AddEpAnn]
 type instance XKindedTyVar  (GhcPass _) = EpAnn [AddEpAnn]
 
-type instance XXTyVarBndr   (GhcPass _) = NoExtCon
+type instance XXTyVarBndr   (GhcPass _) = DataConCantHappen
 
 -- | Return the attached flag
 hsTyVarBndrFlag :: HsTyVarBndr flag (GhcPass pass) -> flag
@@ -361,7 +361,7 @@ pprHsArrow (HsLinearArrow _) = lollipop
 pprHsArrow (HsExplicitMult _ p _) = mulArrow (ppr p)
 
 type instance XConDeclField  (GhcPass _) = EpAnn [AddEpAnn]
-type instance XXConDeclField (GhcPass _) = NoExtCon
+type instance XXConDeclField (GhcPass _) = DataConCantHappen
 
 instance OutputableBndrId p
        => Outputable (ConDeclField (GhcPass p)) where
@@ -807,7 +807,7 @@ type instance XCFieldOcc GhcPs = NoExtField
 type instance XCFieldOcc GhcRn = Name
 type instance XCFieldOcc GhcTc = Id
 
-type instance XXFieldOcc (GhcPass _) = NoExtCon
+type instance XXFieldOcc (GhcPass _) = DataConCantHappen
 
 mkFieldOcc :: LocatedN RdrName -> FieldOcc GhcPs
 mkFieldOcc rdr = FieldOcc noExtField rdr
@@ -821,7 +821,7 @@ type instance XAmbiguous GhcPs = NoExtField
 type instance XAmbiguous GhcRn = NoExtField
 type instance XAmbiguous GhcTc = Id
 
-type instance XXAmbiguousFieldOcc (GhcPass _) = NoExtCon
+type instance XXAmbiguousFieldOcc (GhcPass _) = DataConCantHappen
 
 instance Outputable (AmbiguousFieldOcc (GhcPass p)) where
   ppr = ppr . rdrNameAmbiguousFieldOcc
diff --git a/compiler/GHC/Hs/Utils.hs b/compiler/GHC/Hs/Utils.hs
index c32ec443af..d53fc51786 100644
--- a/compiler/GHC/Hs/Utils.hs
+++ b/compiler/GHC/Hs/Utils.hs
@@ -1235,7 +1235,7 @@ class UnXRec p => CollectPass p where
 instance IsPass p => CollectPass (GhcPass p) where
   collectXXPat _ flag ext =
     case ghcPass @p of
-      GhcPs -> noExtCon ext
+      GhcPs -> dataConCantHappen ext
       GhcRn
         | HsPatExpanded _ pat <- ext
         -> collect_pat flag pat
diff --git a/compiler/GHC/Iface/Ext/Ast.hs b/compiler/GHC/Iface/Ext/Ast.hs
index 9c71235a98..0c74db385d 100644
--- a/compiler/GHC/Iface/Ext/Ast.hs
+++ b/compiler/GHC/Iface/Ext/Ast.hs
@@ -1890,7 +1890,7 @@ instance HiePass p => ToHie (LocatedA (HsSplice (GhcPass p))) where
         []
       XSplice x -> case hiePass @p of
 #if __GLASGOW_HASKELL__ < 811
-                     HieRn -> noExtCon x
+                     HieRn -> dataConCantHappen x
 #endif
                      HieTc -> case x of
                                 HsSplicedT _ -> []
diff --git a/compiler/GHC/Rename/Expr.hs b/compiler/GHC/Rename/Expr.hs
index 1e24c1bb3d..c4f12cf243 100644
--- a/compiler/GHC/Rename/Expr.hs
+++ b/compiler/GHC/Rename/Expr.hs
@@ -2190,7 +2190,7 @@ isStrictPattern (L loc pat) =
     SplicePat{}     -> True
     XPat ext        -> case ghcPass @p of
 #if __GLASGOW_HASKELL__ < 811
-      GhcPs -> noExtCon ext
+      GhcPs -> dataConCantHappen ext
 #endif
       GhcRn
         | HsPatExpanded _ p <- ext
diff --git a/compiler/GHC/Tc/Gen/Expr.hs b/compiler/GHC/Tc/Gen/Expr.hs
index 6739e9a375..b6573897e2 100644
--- a/compiler/GHC/Tc/Gen/Expr.hs
+++ b/compiler/GHC/Tc/Gen/Expr.hs
@@ -881,7 +881,7 @@ tcExpr e@(HsRnBracketOut _ brack ps) res_ty = tcUntypedBracket e brack ps res_ty
 tcExpr (HsOverLabel {})    ty = pprPanic "tcExpr:HsOverLabel"  (ppr ty)
 tcExpr (SectionL {})       ty = pprPanic "tcExpr:SectionL"    (ppr ty)
 tcExpr (SectionR {})       ty = pprPanic "tcExpr:SectionR"    (ppr ty)
-tcExpr (HsTcBracketOut {}) ty = pprPanic "tcExpr:HsTcBracketOut"    (ppr ty)
+tcExpr (HsTcBracketOut x _ _ _) _ = dataConCantHappen x
 
 
 {-
diff --git a/compiler/Language/Haskell/Syntax/Extension.hs b/compiler/Language/Haskell/Syntax/Extension.hs
index 8d8eadf135..9c0cc0408b 100644
--- a/compiler/Language/Haskell/Syntax/Extension.hs
+++ b/compiler/Language/Haskell/Syntax/Extension.hs
@@ -65,7 +65,7 @@ See also Note [IsPass] and Note [NoGhcTc] in GHC.Hs.Extension.
 -- | A placeholder type for TTG extension points that are not currently
 -- unused to represent any particular value.
 --
--- This should not be confused with 'NoExtCon', which are found in unused
+-- This should not be confused with 'DataConCantHappen', which are found in unused
 -- extension /constructors/ and therefore should never be inhabited. In
 -- contrast, 'NoExtField' is used in extension /points/ (e.g., as the field of
 -- some constructor), so it must have an inhabitant to construct AST passes
@@ -80,24 +80,43 @@ instance Outputable NoExtField where
 noExtField :: NoExtField
 noExtField = NoExtField
 
--- | Used in TTG extension constructors that have yet to be extended with
--- anything. If an extension constructor has 'NoExtCon' as its field, it is
--- not intended to ever be constructed anywhere, and any function that consumes
--- the extension constructor can eliminate it by way of 'noExtCon'.
---
--- This should not be confused with 'NoExtField', which are found in unused
--- extension /points/ (not /constructors/) and therefore can be inhabited.
+{-
+Note [Constructor cannot occur]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Some data constructors can't occur in certain phases; e.g. the output
+of the type checker never has OverLabel. We signal this by
+* setting the extension field to DataConCantHappen
+* using dataConCantHappen in the cases that can't happen
+
+For example:
+
+   type instance XOverLabel GhcTc = DataConCantHappen
 
--- See also [NoExtCon and strict fields].
-data NoExtCon
+   dsExpr :: HsExpr GhcTc -> blah
+   dsExpr (HsOverLabel x _) = dataConCantHappen x
+
+The function dataConCantHappen is defined thus:
+   dataConCantHappen :: DataConCantHappen -> a
+   dataConCantHappen x = case x of {}
+(i.e. identically to Data.Void.absurd, but more helpfully named).
+Remember DataConCantHappen is a type whose only element is bottom.
+
+This should not be confused with 'NoExtField', which are found in unused
+extension /points/ (not /constructors/) and therefore can be inhabited.
+
+It would be better to omit the pattern match altogether, but we
+can only do that if the extension field was strict (#18764).
+See also [DataConCantHappen and strict fields].
+-}
+data DataConCantHappen
   deriving (Data,Eq,Ord)
 
-instance Outputable NoExtCon where
-  ppr = noExtCon
+instance Outputable DataConCantHappen where
+  ppr = dataConCantHappen
 
--- | Eliminate a 'NoExtCon'. Much like 'Data.Void.absurd'.
-noExtCon :: NoExtCon -> a
-noExtCon x = case x of {}
+-- | Eliminate a 'DataConCantHappen'. See Note [Constructor cannot happen].
+dataConCantHappen :: DataConCantHappen -> a
+dataConCantHappen x = case x of {}
 
 -- | GHC's L prefixed variants wrap their vanilla variant in this type family,
 -- to add 'SrcLoc' info via 'Located'. Other passes than 'GhcPass' not
diff --git a/utils/haddock b/utils/haddock
-Subproject f7bfa0013f2bc3934a63ea7af21fe41a4e91058
+Subproject 8a73a5babd07530326f1ba06bdfe95f49f66b96