@@ -98,6 +98,10 @@ import Web.FormUrlEncoded (FromForm, ToForm,
9898import Prelude ()
9999import Prelude.Compat
100100
101+#if MIN_VERSION_base(4,9,0)
102+import qualified GHC.TypeLits as TL
103+#endif
104+
101105-- * Provided content types
102106data JSON deriving Typeable
103107data PlainText deriving Typeable
@@ -180,6 +184,12 @@ instance OVERLAPPABLE_
180184 amrs = allMimeRender pctyps val
181185 lkup = fmap (\(a,b) -> (a, (fromStrict $ M.renderHeader a, b))) amrs
182186
187+#if MIN_VERSION_base(4,9,0)
188+instance TL.TypeError (TL.Text "No instance for (), use NoContent instead.")
189+ => AllCTRender '[] () where
190+ handleAcceptH _ _ _ = error "unreachable"
191+#endif
192+
183193--------------------------------------------------------------------------
184194-- * Unrender
185195