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.stderr b/testsuite/tests/typecheck/should_fail/T9612.stderr
new file mode 100644
index 0000000000..823fee112c
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T9612.stderr
@@ -0,0 +1,20 @@
+
+T9612.hs:16:9:
+    Couldn't match type ‘[(Int, a)]’ with ‘(Int, a)’
+    arising from a functional dependency between:
+      constraint ‘MonadWriter (Int, a) (WriterT [(Int, a)] Identity)’
+        arising from a use of ‘tell’
+      instance ‘MonadWriter w (WriterT w m)’ at T9612.hs:20:10-59
+    Relevant bindings include
+      x :: a (bound at T9612.hs:14:8)
+      y :: a (bound at T9612.hs:14:3)
+      f :: a -> (Int, a) -> Writer [(Int, a)] (Int, a)
+        (bound at T9612.hs:14:1)
+    In a stmt of a 'do' block: tell (n, x)
+    In the expression:
+      do { tell (n, x);
+           return (1, y) }
+    In an equation for ‘f’:
+        f y (n, x)
+          = do { tell (n, x);
+                 return (1, y) }