Test Trac #11552

testsuite/tests/typecheck/should_compile/T11552.hs

+{-# LANGUAGE InstanceSigs        #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+
+module T11552 where
+
+newtype MaybeT m a =
+  MaybeT { runMaybeT :: m (Maybe a) }
+
+instance (Functor m) => Functor (MaybeT m) where
+  fmap f (MaybeT ma) =
+    MaybeT $ (fmap . fmap) f ma
+
+instance forall f . (Applicative f) => Applicative (MaybeT f) where
+  pure :: a -> MaybeT f a
+  pure x = MaybeT (pure (pure x))
+
+  (<*>) :: forall a b . Applicative f => MaybeT f (a -> b) -> MaybeT f a -> MaybeT f b
+  (MaybeT fab) <*> (MaybeT mma) =
+   let fab' :: f (Maybe (a -> b))
+       fab' = fab
+   in MaybeT $ undefined

testsuite/tests/typecheck/should_compile/all.T

@@ -504,3 +504,4 @@ test('T11397', normal, compile, [''])
 test('T11458', normal, compile, [''])
 test('T11516', expect_broken(11516), compile, [''])
 test('T11524', normal, compile, [''])
+test('T11552', normal, compile, [''])