Validate the subscription root

…not to be an introspection field.

src/Language/GraphQL/Validate/Rules.hs

@@ -137,25 +137,28 @@ singleFieldSubscriptionsRule :: forall m. Rule m
 singleFieldSubscriptionsRule = OperationDefinitionRule $ \case
     Full.OperationDefinition Full.Subscription name' _ _ rootFields location' -> do
         groupedFieldSet <- evalStateT (collectFields rootFields) HashSet.empty
-        case HashSet.size groupedFieldSet of
-            1 -> lift mempty
-            _
-                | Just name <- name' -> pure $ Error
-                    { message = concat
-                        [ "Subscription \""
-                        , Text.unpack name
-                        , "\" must select only one top level field."
-                        ]
-                    , locations = [location']
-                    }
-                | otherwise -> pure $ Error
-                    { message = errorMessage
-                    , locations = [location']
-                    }
+        case HashSet.toList groupedFieldSet of
+            [rootName]
+                | Text.isPrefixOf "__" rootName -> makeError location' name'
+                    "exactly one top level field, which must not be an introspection field."
+                | otherwise -> lift mempty
+            [] -> makeError location' name' "exactly one top level field."
+            _ -> makeError location' name' "only one top level field."
     _ -> lift mempty
   where
-    errorMessage =
-        "Anonymous Subscription must select only one top level field."
+    makeError location' (Just operationName) errorLine = pure $ Error
+        { message = concat
+            [ "Subscription \""
+            , Text.unpack operationName
+            , "\" must select "
+            , errorLine
+            ]
+        , locations = [location']
+        }
+    makeError location' Nothing errorLine = pure $ Error
+        { message = "Anonymous Subscription must select " <> errorLine
+        , locations = [location']
+        }
     collectFields = foldM forEach HashSet.empty
     forEach accumulator = \case
         Full.FieldSelection fieldSelection -> forField accumulator fieldSelection