Use TTG empty extensions to remove some `error`'s

None of these error cases should ever have been reachable, so this is
just a matter of leveraging the type system to assert this.

  * Use the `NoExtCon` and `noExtCon` to handle case matches for no
    extension constructors, instead of throwing an `error`.

  * Use the extension field of `HsSpliceTy` to ensure that this variant
    of `HsType` cannot exist in an `HsType DocNameI`.

6 files changed, 21 insertions, 26 deletions

diff --git a/haddock-api/src/Haddock/Backends/LaTeX.hs b/haddock-api/src/Haddock/Backends/LaTeX.hs
index 63b12a14..d52c136f 100644
--- a/haddock-api/src/Haddock/Backends/LaTeX.hs
+++ b/haddock-api/src/Haddock/Backends/LaTeX.hs
@@ -39,6 +39,7 @@ import Data.Char
 import Control.Monad
 import Data.Maybe
 import Data.List            ( sort )
+import Data.Void            ( absurd )
 import Prelude hiding ((<>))
 
 import Haddock.Doc (combineDocumentation)
@@ -530,7 +531,7 @@ ppTyVars unicode = map (ppHsTyVarBndr unicode . unLoc)
 
 
 tyvarNames :: LHsQTyVars DocNameI -> [Name]
-tyvarNames = map (getName . hsLTyVarNameI) . hsQTvExplicit
+tyvarNames = map (getName . hsTyVarBndrName . unLoc) . hsQTvExplicit
 
 
 declWithDoc :: LaTeX -> Maybe LaTeX -> LaTeX
@@ -1080,7 +1081,7 @@ ppr_mono_ty (HsSumTy _ tys) u       = sumParens (map (ppLType u) tys)
 ppr_mono_ty (HsKindSig _ ty kind) u = ppr_mono_lty ty u <+> dcolon u <+> ppLKind u kind
 ppr_mono_ty (HsListTy _ ty)       u = brackets (ppr_mono_lty ty u)
 ppr_mono_ty (HsIParamTy _ (L _ n) ty) u = ppIPName n <+> dcolon u <+> ppr_mono_lty ty u
-ppr_mono_ty (HsSpliceTy {})     _ = error "ppr_mono_ty HsSpliceTy"
+ppr_mono_ty (HsSpliceTy v _)    _ = absurd v
 ppr_mono_ty (HsRecTy {})        _ = text "{..}"
 ppr_mono_ty (XHsType (NHsCoreTy {}))  _ = error "ppr_mono_ty HsCoreTy"
 ppr_mono_ty (HsExplicitListTy _ IsPromoted tys) u = Pretty.quote $ brackets $ hsep $ punctuate comma $ map (ppLType u) tys
diff --git a/haddock-api/src/Haddock/Backends/Xhtml/Decl.hs b/haddock-api/src/Haddock/Backends/Xhtml/Decl.hs
index b450dc94..25669ca7 100644
--- a/haddock-api/src/Haddock/Backends/Xhtml/Decl.hs
+++ b/haddock-api/src/Haddock/Backends/Xhtml/Decl.hs
@@ -32,6 +32,7 @@ import Haddock.Doc (combineDocumentation)
 import           Data.List             ( intersperse, sort )
 import qualified Data.Map as Map
 import           Data.Maybe
+import           Data.Void             ( absurd )
 import           Text.XHtml hiding     ( name, title, p, quote )
 
 import BasicTypes (PromotionFlag(..), isPromoted)
@@ -1215,7 +1216,7 @@ ppr_mono_ty (HsKindSig _ ty kind) u q e =
 ppr_mono_ty (HsListTy _ ty)       u q _ = brackets (ppr_mono_lty ty u q HideEmptyContexts)
 ppr_mono_ty (HsIParamTy _ (L _ n) ty) u q _ =
   ppIPName n <+> dcolon u <+> ppr_mono_lty ty u q HideEmptyContexts
-ppr_mono_ty (HsSpliceTy {})     _ _ _ = error "ppr_mono_ty HsSpliceTy"
+ppr_mono_ty (HsSpliceTy v _) _ _ _ = absurd v
 ppr_mono_ty (HsRecTy {})        _ _ _ = toHtml "{..}"
        -- Can now legally occur in ConDeclGADT, the output here is to provide a
        -- placeholder in the signature, which is followed by the field
diff --git a/haddock-api/src/Haddock/GhcUtils.hs b/haddock-api/src/Haddock/GhcUtils.hs
index 77d6ec39..f600997a 100644
--- a/haddock-api/src/Haddock/GhcUtils.hs
+++ b/haddock-api/src/Haddock/GhcUtils.hs
@@ -165,18 +165,17 @@ nubByName f ns = go emptyNameSet ns
       where
         y = f x
 
+
 -- ---------------------------------------------------------------------
 
 -- These functions are duplicated from the GHC API, as they must be
 -- instantiated at DocNameI instead of (GhcPass _).
 
-hsTyVarNameI :: HsTyVarBndr DocNameI -> DocName
-hsTyVarNameI (UserTyVar _ (L _ n))     = n
-hsTyVarNameI (KindedTyVar _ (L _ n) _) = n
-hsTyVarNameI (XTyVarBndr nec) = noExtCon nec
-
-hsLTyVarNameI :: LHsTyVarBndr DocNameI -> DocName
-hsLTyVarNameI = hsTyVarNameI . unLoc
+-- | Like 'hsTyVarName' from GHC API, but not instantiated at (GhcPass _)
+hsTyVarBndrName :: (XXTyVarBndr n ~ NoExtCon) => HsTyVarBndr n -> IdP n
+hsTyVarBndrName (UserTyVar _ name) = unLoc name
+hsTyVarBndrName (KindedTyVar _ (L _ name) _) = name
+hsTyVarBndrName (XTyVarBndr nec) = noExtCon nec
 
 getConNamesI :: ConDecl DocNameI -> [Located DocName]
 getConNamesI ConDeclH98  {con_name  = name}  = [name]
@@ -311,7 +310,7 @@ restrictDataDefn names defn@(HsDataDefn { dd_ND = new_or_data, dd_cons = cons })
       []    -> defn { dd_ND = DataType, dd_cons = [] }
       [con] -> defn { dd_cons = [con] }
       _ -> error "Should not happen"
-restrictDataDefn _ (XHsDataDefn _) = error "restrictDataDefn"
+restrictDataDefn _ (XHsDataDefn nec) = noExtCon nec
 
 restrictCons :: [Name] -> [LConDecl GhcRn] -> [LConDecl GhcRn]
 restrictCons names decls = [ L p d | L p (Just d) <- map (fmap keep) decls ]
diff --git a/haddock-api/src/Haddock/Interface/Rename.hs b/haddock-api/src/Haddock/Interface/Rename.hs
index 0b122b07..ce3878b8 100644
--- a/haddock-api/src/Haddock/Interface/Rename.hs
+++ b/haddock-api/src/Haddock/Interface/Rename.hs
@@ -313,7 +313,7 @@ renameLTyVarBndr (L loc (KindedTyVar x (L lv n) kind))
   = do { n' <- rename n
        ; kind' <- renameLKind kind
        ; return (L loc (KindedTyVar x (L lv n') kind')) }
-renameLTyVarBndr (L _ (XTyVarBndr _ )) = error "haddock:renameLTyVarBndr"
+renameLTyVarBndr (L _ (XTyVarBndr nec)) = noExtCon nec
 
 renameLContext :: Located [LHsType GhcRn] -> RnM (Located [LHsType DocNameI])
 renameLContext (L loc context) = do
@@ -512,7 +512,7 @@ renameLFieldOcc :: LFieldOcc GhcRn -> RnM (LFieldOcc DocNameI)
 renameLFieldOcc (L l (FieldOcc sel lbl)) = do
   sel' <- rename sel
   return $ L l (FieldOcc sel' lbl)
-renameLFieldOcc (L _ (XFieldOcc _)) = error "haddock:renameLFieldOcc"
+renameLFieldOcc (L _ (XFieldOcc nec)) = noExtCon nec
 
 renameSig :: Sig GhcRn -> RnM (Sig DocNameI)
 renameSig sig = case sig of
diff --git a/haddock-api/src/Haddock/Interface/Specialize.hs b/haddock-api/src/Haddock/Interface/Specialize.hs
index 03cc1b7e..19b03596 100644
--- a/haddock-api/src/Haddock/Interface/Specialize.hs
+++ b/haddock-api/src/Haddock/Interface/Specialize.hs
@@ -9,6 +9,7 @@ module Haddock.Interface.Specialize
     ) where
 
 
+import Haddock.GhcUtils ( hsTyVarBndrName )
 import Haddock.Syb
 import Haddock.Types
 
@@ -56,13 +57,9 @@ specialize specs = go spec_map0
 -- Again, it is just a convenience function around 'specialize'. Note that
 -- length of type list should be the same as the number of binders.
 specializeTyVarBndrs :: LHsQTyVars GhcRn -> [HsType GhcRn] -> HsType GhcRn -> HsType GhcRn
-specializeTyVarBndrs bndrs typs =
-    specialize $ zip bndrs' typs
+specializeTyVarBndrs bndrs typs = specialize $ zip bndrs' typs
   where
-    bndrs' = map (bname . unLoc) . hsq_explicit $ bndrs
-    bname (UserTyVar _ (L _ name)) = name
-    bname (KindedTyVar _ (L _ name) _) = name
-    bname (XTyVarBndr _) = error "haddock:specializeTyVarBndrs"
+    bndrs' = map (hsTyVarBndrName . unLoc) . hsq_explicit $ bndrs
 
 
 
@@ -212,7 +209,7 @@ freeVariables =
             | getName name `Set.member` ctx -> (Set.empty, ctx)
             | otherwise -> (Set.singleton $ getName name, ctx)
         _ -> (Set.empty, ctx)
-    bndrsNames = Set.fromList . map (getName . tyVarName . unLoc)
+    bndrsNames = Set.fromList . map (getName . hsTyVarBndrName . unLoc)
 
 
 -- | Make given type visually unambiguous.
@@ -295,7 +292,7 @@ renameBinder :: HsTyVarBndr GhcRn -> Rename (IdP GhcRn) (HsTyVarBndr GhcRn)
 renameBinder (UserTyVar x lname) = UserTyVar x <$> located renameName lname
 renameBinder (KindedTyVar x lname lkind) =
   KindedTyVar x <$> located renameName lname <*> located renameType lkind
-renameBinder (XTyVarBndr _) = error "haddock:renameBinder"
+renameBinder (XTyVarBndr nec) = noExtCon nec
 
 -- | Core renaming logic.
 renameName :: (Eq name, SetName name) => name -> Rename name name
@@ -349,7 +346,3 @@ located :: Functor f => (a -> f b) -> Located a -> f (Located b)
 located f (L loc e) = L loc <$> f e
 
 
-tyVarName :: HsTyVarBndr name -> IdP name
-tyVarName (UserTyVar _ name) = unLoc name
-tyVarName (KindedTyVar _ (L _ name) _) = name
-tyVarName (XTyVarBndr _ ) = error "haddock:tyVarName"
diff --git a/haddock-api/src/Haddock/Types.hs b/haddock-api/src/Haddock/Types.hs
index 28e3caed..ec76fb72 100644
--- a/haddock-api/src/Haddock/Types.hs
+++ b/haddock-api/src/Haddock/Types.hs
@@ -35,6 +35,7 @@ import Control.Monad.IO.Class (MonadIO(..))
 import Data.Typeable (Typeable)
 import Data.Map (Map)
 import Data.Data (Data)
+import Data.Void (Void)
 import Documentation.Haddock.Types
 import BasicTypes (Fixity(..), PromotionFlag(..))
 
@@ -713,7 +714,7 @@ type instance XOpTy            DocNameI = NoExtField
 type instance XParTy           DocNameI = NoExtField
 type instance XIParamTy        DocNameI = NoExtField
 type instance XKindSig         DocNameI = NoExtField
-type instance XSpliceTy        DocNameI = NoExtField
+type instance XSpliceTy        DocNameI = Void       -- see `renameHsSpliceTy`
 type instance XDocTy           DocNameI = NoExtField
 type instance XBangTy          DocNameI = NoExtField
 type instance XRecTy           DocNameI = NoExtField