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diff --git a/testsuite/tests/indexed-types/should_compile/T19336.hs b/testsuite/tests/indexed-types/should_compile/T19336.hs
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+{-# LANGUAGE MultiParamTypeClasses, FlexibleContexts, FlexibleInstances
+           , DataKinds, NoMonomorphismRestriction, UndecidableInstances
+           , TypeFamilies #-}
+
+{-# OPTIONS_GHC -Wall #-}
+
+module T19336 where
+
+import GHC.TypeLits
+
+class X a b where
+  convert :: a -> b
+
+instance X Int String where
+  convert = show
+
+instance X String String where
+  convert = id
+
+instance {-# OVERLAPPABLE #-} TypeError ('Text "Oops") => X a b where
+  convert = error "unreachable"
+
+type family F a where
+  F String = String
+  F Int    = String
+
+convert_f :: X a (F a) => a -> a -> F a
+convert_f _ = convert
+
+----------
+
+class Poly a where
+  poly :: a
+
+instance Poly String where
+  poly = "hi"
+
+instance Poly Int where
+  poly = 2
+
+----------
+
+oops = convert_f poly