Improve error messages from functional dependencies

Reponding to Trac #9612:

 * Track the CtOrigin of a Derived equality, arising from a
   functional dependency

 * And report it clearly in the error stream

This relies on a previous commit, in which I stop dropping Derived
insolubles on the floor.

1 files changed, 20 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_fail/T9612.hs b/testsuite/tests/typecheck/should_fail/T9612.hs
new file mode 100644
index 0000000000..a332c47b04
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T9612.hs
@@ -0,0 +1,20 @@
+{-# LANGUAGE FlexibleInstances, FunctionalDependencies, MultiParamTypeClasses #-}
+module T9612 where
+import Data.Monoid
+import Control.Monad.Trans.Writer.Lazy( Writer, WriterT )
+import Data.Functor.Identity( Identity )
+
+class (Monoid w, Monad m) => MonadWriter w m | m -> w where
+    writer :: (a,w) -> m a
+    tell   :: w -> m ()
+    listen :: m a -> m (a, w)
+    pass   :: m (a, w -> w) -> m a
+
+f ::(Eq a) => a -> (Int, a) -> Writer [(Int, a)] (Int, a)
+f y (n,x) {- | y == x    = return (n+1, x)
+             | otherwise = -}
+   = do tell (n,x)
+        return (1,y)
+
+
+instance (Monoid w, Monad m) => MonadWriter w (WriterT w m) where