Provide location information for interface errors

tests/Language/GraphQL/ExecuteSpec.hs

@@ -218,7 +218,7 @@ spec =
                         $ parse document "" "{ philosopher { school } }"
                 in actual `shouldBe` expected
 
-            it "gives location information for invalid interfaces" $
+            it "gives location information for non-null unions" $
                 let data'' = Aeson.object
                         [ "philosopher" .= Aeson.object
                             [ "interest" .= Aeson.Null
@@ -234,6 +234,22 @@ spec =
                         $ parse document "" "{ philosopher { interest } }"
                 in actual `shouldBe` expected
 
+            it "gives location information for invalid interfaces" $
+                let data'' = Aeson.object
+                        [ "philosopher" .= Aeson.object
+                            [ "majorWork" .= Aeson.Null
+                            ]
+                        ]
+                    executionErrors = pure $ Error
+                        { message = "Interface value completion failed."
+                        , locations = [Location 1 17]
+                        , path = []
+                        }
+                    expected = Response data'' executionErrors
+                    Right (Right actual) = either (pure . parseError) execute'
+                        $ parse document "" "{ philosopher { majorWork { title } } }"
+                in actual `shouldBe` expected
+
         context "Subscription" $
             it "subscribes" $
                 let data'' = Aeson.object