Add test for Trac #4160

testsuite/tests/ghc-regress/indexed-types/should_compile/T4160.hs

+{-# LANGUAGE FlexibleContexts, MultiParamTypeClasses, TypeFamilies #-}
+module Foo where
+
+data P f g r = f r :*: g r
+type family TrieMapT (f :: * -> *) :: * -> (* -> *) -> * -> *
+newtype PMap m1 (m2 :: * -> (* -> *) -> * -> *) k (a :: * -> *) ix = PMap (m1 k (m2 k a) ix)
+type instance TrieMapT (P f g) = PMap (TrieMapT f) (TrieMapT g)
+
+class TrieKeyT f m where
+	unionT :: (TrieMapT f ~ m) => (f k -> a ix -> a ix -> a ix) ->
+		m k a ix -> m k a ix -> m k a ix
+	sizeT :: (TrieMapT f ~ m) => m k a ix -> Int
+
+instance (TrieKeyT f m1, TrieKeyT g m2) => TrieKeyT (P f g) (PMap m1 m2) where
+	unionT f (PMap m1) (PMap m2) = PMap (uT  (\ a -> unionT (\ b -> f (a :*: b))) m1 m2)
+		where uT = unionT
+        sizeT = error "urk"
+

testsuite/tests/ghc-regress/indexed-types/should_compile/all.T

@@ -139,6 +139,7 @@ test('T3220', if_compiler_lt('ghc', '6.11', expect_fail), compile, [''])
 test('T3590', normal, compile, [''])
 test('CoTest3', if_compiler_lt('ghc', '6.13', expect_fail), compile, [''])
 test('Roman1', if_compiler_lt('ghc', '6.13', expect_fail), compile, [''])
+test('T4160', normal, compile, [''])
 test('IndTypesPerf',
      extra_clean(['IndTypesPerf.o', 'IndTypesPerf.hi',
                   'IndTypesPerfMerge.o', 'IndTypesPerfMerge.hi']),