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diff --git a/testsuite/tests/indexed-types/should_fail/T4254b.hs b/testsuite/tests/indexed-types/should_fail/T4254b.hs
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+{-# LANGUAGE TypeFamilies, FunctionalDependencies, RankNTypes, MultiParamTypeClasses #-}
+module T4254b where
+
+class FD a b | a -> b where
+  op :: a -> b;
+  op = undefined
+
+instance FD Int Bool
+
+fails :: forall a b. (a~Int,FD a b) => a -> Bool
+fails  = op
+-- Could fail: no proof that b~Bool
+-- But can also succeed; it's not a *wanted* constraint