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.

7 files changed, 79 insertions, 25 deletions

diff --git a/testsuite/tests/typecheck/should_fail/FDsFromGivens.stderr b/testsuite/tests/typecheck/should_fail/FDsFromGivens.stderr
index 56d3006260..f3320d0d8e 100644
--- a/testsuite/tests/typecheck/should_fail/FDsFromGivens.stderr
+++ b/testsuite/tests/typecheck/should_fail/FDsFromGivens.stderr
@@ -1,10 +1,12 @@
-

-FDsFromGivens.hs:21:15:

-    Could not deduce (C Char [a]) arising from a use of ‘f’

-    from the context (C Char Char)

-      bound by a pattern with constructor

-                 KCC :: C Char Char => () -> KCC,

-               in an equation for ‘bar’

-      at FDsFromGivens.hs:21:6-10

-    In the expression: f

-    In an equation for ‘bar’: bar (KCC _) = f

+
+FDsFromGivens.hs:21:15:
+    Couldn't match type ‘Char’ with ‘[a0]’
+    arising from a functional dependency between constraints:
+      ‘C Char [a0]’ arising from a use of ‘f’ at FDsFromGivens.hs:21:15
+      ‘C Char Char’
+        arising from a pattern with constructor
+                       KCC :: C Char Char => () -> KCC,
+                     in an equation for ‘bar’
+        at FDsFromGivens.hs:21:6-10
+    In the expression: f
+    In an equation for ‘bar’: bar (KCC _) = f
diff --git a/testsuite/tests/typecheck/should_fail/T5236.stderr b/testsuite/tests/typecheck/should_fail/T5236.stderr
index 557a0413c9..8a723bab9b 100644
--- a/testsuite/tests/typecheck/should_fail/T5236.stderr
+++ b/testsuite/tests/typecheck/should_fail/T5236.stderr
@@ -1,5 +1,10 @@
-

-T5236.hs:17:5:

-    No instance for (Id A B) arising from a use of ‘loop’

-    In the expression: loop

-    In an equation for ‘f’: f = loop

+
+T5236.hs:13:9:
+    Couldn't match type ‘A’ with ‘B’
+    arising from a functional dependency between:
+      constraint ‘Id A B’
+        arising from the type signature for loop :: Id A B => Bool
+      instance ‘Id A A’ at T5236.hs:10:10-15
+    In the ambiguity check for: Id A B => Bool
+    To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
+    In the type signature for ‘loop’: loop :: Id A B => Bool
diff --git a/testsuite/tests/typecheck/should_fail/T5978.stderr b/testsuite/tests/typecheck/should_fail/T5978.stderr
index db6b8f355e..263e68ba2d 100644
--- a/testsuite/tests/typecheck/should_fail/T5978.stderr
+++ b/testsuite/tests/typecheck/should_fail/T5978.stderr
@@ -1,5 +1,8 @@
-

-T5978.hs:22:11:

-    No instance for (C Double Char) arising from a use of ‘polyBar’

-    In the expression: polyBar id monoFoo

-    In an equation for ‘monoBar’: monoBar = polyBar id monoFoo

+
+T5978.hs:22:11:
+    Couldn't match type ‘Bool’ with ‘Char’
+    arising from a functional dependency between:
+      constraint ‘C Double Char’ arising from a use of ‘polyBar’
+      instance ‘C Double Bool’ at T5978.hs:8:10-22
+    In the expression: polyBar id monoFoo
+    In an equation for ‘monoBar’: monoBar = polyBar id monoFoo
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
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) }
diff --git a/testsuite/tests/typecheck/should_fail/all.T b/testsuite/tests/typecheck/should_fail/all.T
index 4f001f5ab7..431a9ba767 100644
--- a/testsuite/tests/typecheck/should_fail/all.T
+++ b/testsuite/tests/typecheck/should_fail/all.T
@@ -334,3 +334,4 @@ test('T9196', normal, compile_fail, [''])
 test('T9305', normal, compile_fail, [''])
 test('T9323', normal, compile_fail, [''])
 test('T9415', normal, compile_fail, [''])
+test('T9612', normal, compile_fail, [''])
diff --git a/testsuite/tests/typecheck/should_fail/tcfail143.stderr b/testsuite/tests/typecheck/should_fail/tcfail143.stderr
index 394fa43c4e..b36d7a8b37 100644
--- a/testsuite/tests/typecheck/should_fail/tcfail143.stderr
+++ b/testsuite/tests/typecheck/should_fail/tcfail143.stderr
@@ -1,5 +1,8 @@
-

-tcfail143.hs:29:9:

-    No instance for (MinMax (S Z) Z Z Z) arising from a use of ‘extend’

-    In the expression: n1 `extend` n0

-    In an equation for ‘t2’: t2 = n1 `extend` n0

+
+tcfail143.hs:29:9:
+    Couldn't match type ‘S Z’ with ‘Z’
+    arising from a functional dependency between:
+      constraint ‘MinMax (S Z) Z Z Z’ arising from a use of ‘extend’
+      instance ‘MinMax a Z Z a’ at tcfail143.hs:11:10-23
+    In the expression: n1 `extend` n0
+    In an equation for ‘t2’: t2 = n1 `extend` n0