TypeError annotations for AllCTRender

servant/src/Servant/API/ContentTypes.hs

@@ -98,6 +98,10 @@ import           Web.FormUrlEncoded               (FromForm, ToForm,
 import           Prelude                          ()
 import           Prelude.Compat
 
+#if MIN_VERSION_base(4,9,0)
+import qualified GHC.TypeLits                     as TL
+#endif
+
 -- * Provided content types
 data JSON deriving Typeable
 data PlainText deriving Typeable
@@ -180,6 +184,12 @@ instance OVERLAPPABLE_
             amrs = allMimeRender pctyps val
             lkup = fmap (\(a,b) -> (a, (fromStrict $ M.renderHeader a, b))) amrs
 
+#if MIN_VERSION_base(4,9,0)
+instance TL.TypeError (TL.Text "No instance for (), use NoContent instead.")
+  => AllCTRender '[] () where
+  handleAcceptH _ _ _ = error "unreachable"
+#endif
+
 --------------------------------------------------------------------------
 -- * Unrender
 

stack.yaml

@@ -15,4 +15,4 @@ extra-deps:
 - hspec-core-2.3.2
 - hspec-wai-0.8.0
 - hspec-expectations-0.8.2
-resolver: lts-6.27
+resolver: lts-7.15