Test Trac #5751

3 files changed, 42 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_run/T5751.hs b/testsuite/tests/typecheck/should_run/T5751.hs
new file mode 100644
index 0000000000..f620d8fb63
--- /dev/null
+++ b/testsuite/tests/typecheck/should_run/T5751.hs
@@ -0,0 +1,38 @@
+{-# LANGUAGE FlexibleContexts, FlexibleInstances, GeneralizedNewtypeDeriving, OverlappingInstances, UndecidableInstances #-}
+module Main where
+
+class (Monad m) => MonadIO m where
+    -- | Lift a computation from the 'IO' monad.
+    liftIO :: IO a -> m a
+
+instance MonadIO IO where
+    liftIO = id
+
+class XMLGenerator m where
+    genElement :: (Maybe String, String) -> m ()
+
+newtype IdentityT m a = IdentityT { runIdentityT :: m a }
+    deriving (Monad, MonadIO)
+
+instance (MonadIO m) => (XMLGenerator (IdentityT m)) where
+    genElement _ = liftIO $ putStrLn "in genElement"
+
+main :: IO ()
+main = 
+    do runIdentityT web
+       putStrLn "done."
+
+class (Widgets x) => MonadRender x
+class (XMLGenerator m)  => Widgets m
+-- instance Widgets (IdentityT IO) -- if you uncomment this, it will work
+instance MonadRender m => Widgets m
+instance MonadRender (IdentityT IO)
+
+web :: ( MonadIO m
+       , Widgets m
+       , XMLGenerator m
+       ) => m ()
+web =
+    do liftIO $ putStrLn "before"
+       genElement (Nothing, "p")
+       return ()
diff --git a/testsuite/tests/typecheck/should_run/T5751.stdout b/testsuite/tests/typecheck/should_run/T5751.stdout
new file mode 100644
index 0000000000..0686f3cb49
--- /dev/null
+++ b/testsuite/tests/typecheck/should_run/T5751.stdout
@@ -0,0 +1,3 @@
+before
+in genElement
+done.
diff --git a/testsuite/tests/typecheck/should_run/all.T b/testsuite/tests/typecheck/should_run/all.T
index 709bb32cbc..ec1fcf65f1 100755
--- a/testsuite/tests/typecheck/should_run/all.T
+++ b/testsuite/tests/typecheck/should_run/all.T
@@ -97,3 +97,4 @@ test('T5573b', compose(omit_ways(['ghci']),only_compiler_types(['ghc'])), compil
 test('T7023', normal, compile_and_run, [''])
 test('T7126', normal, compile_and_run, [''])
 test('T6117', expect_broken(6117), compile_and_run, [''])
+test('T5751', normal, compile_and_run, [''])