Test Trac #6088

testsuite/tests/indexed-types/should_compile/T6088.hs

+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}  
+{-# LANGUAGE EmptyDataDecls #-}
+
+module T6088 where
+
+class C a
+
+newtype A n = A Int
+
+type family Pos n
+data True
+
+instance (Pos n ~ True) => C (A n)
+
+newtype B n = B (A n) deriving (C)
+  -- This should work, giving
+  -- instance (Pos n ~ True) => C (B n)

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

@@ -188,3 +188,5 @@ test('T5955',
      extra_clean(['T5955.o', 'T5955.hi', 'T5955a.o', 'T5955a.hi']),
      run_command,
      ['$MAKE -s --no-print-directory T5955'])
+
+test('T6088', normal, compile, [''])