Test Trac #6015

testsuite/tests/polykinds/T6015.hs

+{-# LANGUAGE DataKinds, MultiParamTypeClasses, FunctionalDependencies,
+             PolyKinds, UndecidableInstances, ScopedTypeVariables #-}
+
+module T6015 where
+
+data Proxy a = Proxy
+
+class Value a t | a -> t where value :: Proxy a -> t
+
+instance Value True Bool where value _ = True
+
+instance Value a t => Value (Just a) (Maybe t)
+    where value _ = Just (value (Proxy :: Proxy a))
+
+test = value (Proxy :: Proxy (Just True))

testsuite/tests/polykinds/all.T

@@ -45,3 +45,4 @@ test('T6020a', normal, compile, [''])
 test('T6044', normal, compile, [''])
 test('T6054', normal, run_command, ['$MAKE -s --no-print-directory T6054'])
 test('T6081', normal, compile, [''])
+test('T6015', normal, compile, [''])