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diff --git a/testsuite/tests/quantified-constraints/T9123a.hs b/testsuite/tests/quantified-constraints/T9123a.hs
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+++ b/testsuite/tests/quantified-constraints/T9123a.hs
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+{-# LANGUAGE QuantifiedConstraints, PolyKinds, ScopedTypeVariables
+           , StandaloneDeriving, RoleAnnotations, TypeApplications
+           , UndecidableInstances, InstanceSigs
+           , GeneralizedNewtypeDeriving #-}
+
+module T9123a where
+
+import Data.Coerce
+
+class MyMonad m where
+  join :: m (m a) -> m a
+
+newtype StateT s m a = StateT (s -> m (a, s))
+
+type role StateT nominal representational nominal   -- as inferred
+
+instance MyMonad m => MyMonad (StateT s m) where
+  join = error "urk"  -- A good impl exists, but is not
+                      -- of interest for this test case
+
+newtype IntStateT m a = IntStateT (StateT Int m a)
+
+type role IntStateT representational nominal   -- as inferred
+
+instance (MyMonad m, forall p q. Coercible p q => Coercible (m p) (m q))
+               => MyMonad (IntStateT m) where
+  join :: forall a. IntStateT m (IntStateT m a) -> IntStateT m a
+  join = coerce @(StateT Int m (StateT Int m a) -> StateT Int m a)
+                @(IntStateT  m (IntStateT  m a) -> IntStateT  m a)
+                join