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diff --git a/testsuite/tests/gadt/gadt18.hs b/testsuite/tests/gadt/gadt18.hs
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+++ b/testsuite/tests/gadt/gadt18.hs
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+{-# LANGUAGE GADTs #-}
+-- A simple GADT test from Roman
+-- which nevertheless showed up a bug at one stage
+
+module ShouldCompile where
+
+data T a where
+  T1 :: () -> T ()
+  T2 :: T a -> T b -> T (a,b)
+
+class C a where
+  f :: T a -> a
+
+instance C () where
+  f (T1 x) = x
+
+instance (C a, C b) => C (a,b) where
+  f (T2 x y) = (f x, f y)