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diff --git a/testsuite/tests/indexed-types/should_compile/T2639.hs b/testsuite/tests/indexed-types/should_compile/T2639.hs
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+{-# LANGUAGE TypeFamilies, EmptyDataDecls #-}
+
+module T2639 where
+
+data Eps
+
+data family Work a v
+data instance Work Eps v = Eps v
+
+type family Dual a
+type instance Dual Eps = Eps
+
+class Connect s where
+    connect :: (Dual s ~ c, Dual c ~ s) => Work s a -> Work c b -> (a,b)
+
+instance Connect Eps where
+    connect (Eps a) (Eps b) = (a,b)