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+{-# LANGUAGE TypeApplications, ImpredicativeTypes, ScopedTypeVariables,
+             QuantifiedConstraints, StandaloneDeriving, GeneralizedNewtypeDeriving #-}
+
+module T15290 where
+
+import Prelude hiding ( Monad(..) )
+import Data.Coerce ( Coercible, coerce )
+
+class Monad m where
+  join  :: m (m a) -> m a
+
+newtype StateT s m a = StateT { runStateT :: s -> m (s, a) }
+
+newtype IntStateT m a = IntStateT { runIntStateT :: StateT Int m a }
+
+instance Monad m => Monad (StateT s m) where
+  join = error "urk"
+
+instance (Monad m, forall p q. Coercible p q => Coercible (m p) (m q))
+      => Monad (IntStateT m) where
+
+--   Fails with the impredicative instantiation of coerce, succeeds without
+
+-- Impredicative version (fails)
+--    join = coerce
+--          @(forall a. StateT Int m (StateT Int m a) -> StateT Int m a)
+--          @(forall a. IntStateT m (IntStateT m a)   -> IntStateT m a)
+--          join
+
+
+-- Predicative version (succeeds)
+    join = (coerce
+           @(StateT Int m (StateT Int m a) -> StateT Int m a)
+           @(IntStateT m (IntStateT m a)   -> IntStateT m a)
+           join) :: forall a. IntStateT m (IntStateT m a)   -> IntStateT m a