Remove some horrible munging of origins for Coercible

I just didn't think it was buying enough for all the cruft it caused.
We can put some back if people start complaining about poor error
messages. I forget quite how I tripped over this but I got sucked in.

* Lots of tidying up in TcErrors

* Rename pprArisingAt to pprCtLoc, by analogy with pprCtOrigin

* Remove CoercibleOrigin data constructor from CtOrigin

* Make relevantBindings return a Ct with a zonked
  and tidied CtOrigin

* Add to TcRnTypes
      ctOrigin   :: Ct -> CtOrigin
      ctEvOrigin :: CtEvidence -> CtOrigin
      setCtLoc   :: Ct -> CtLoc -> Ct

9 files changed, 29 insertions, 38 deletions

diff --git a/testsuite/tests/typecheck/bug1465/bug1465.stderr b/testsuite/tests/typecheck/bug1465/bug1465.stderr
index 118d070075..1c67068762 100644
--- a/testsuite/tests/typecheck/bug1465/bug1465.stderr
+++ b/testsuite/tests/typecheck/bug1465/bug1465.stderr
@@ -1,8 +1,8 @@
 
-C.hs:6:11:
+C.hs:6:11: error:
     Couldn't match expected type ‘bug1465-1.0:A.T’
                 with actual type ‘A.T’
-    NB: ‘bug1465-1.0:A.T’ is defined in ‘A’ in package ‘bug1465-1.0’
-        ‘A.T’ is defined in ‘A’ in package ‘bug1465-2.0’
+    NB: ‘A.T’ is defined in ‘A’ in package ‘bug1465-2.0’
+        ‘bug1465-1.0:A.T’ is defined in ‘A’ in package ‘bug1465-1.0’
     In the expression: B2.f
     In the expression: [B1.f, B2.f]
diff --git a/testsuite/tests/typecheck/should_compile/FD3.stderr b/testsuite/tests/typecheck/should_compile/FD3.stderr
index 179fda19e6..f4c60451d8 100644
--- a/testsuite/tests/typecheck/should_compile/FD3.stderr
+++ b/testsuite/tests/typecheck/should_compile/FD3.stderr
@@ -1,5 +1,14 @@
 
 FD3.hs:15:15: error:
-    No instance for (MkA (String, a) a) arising from a use of ‘mkA’
+    Couldn't match type ‘a’ with ‘(String, a)’
+      arising from a functional dependency between:
+        constraint ‘MkA (String, a) a’ arising from a use of ‘mkA’
+        instance ‘MkA a1 a1’ at FD3.hs:12:10-16
+    ‘a’ is a rigid type variable bound by
+        the type signature for: translate :: (String, a) -> A a
+        at FD3.hs:14:14
+    Relevant bindings include
+      a :: (String, a) (bound at FD3.hs:15:11)
+      translate :: (String, a) -> A a (bound at FD3.hs:15:1)
     In the expression: mkA a
     In an equation for ‘translate’: translate a = mkA a
diff --git a/testsuite/tests/typecheck/should_fail/T10285.stderr b/testsuite/tests/typecheck/should_fail/T10285.stderr
index 1ed2046278..8a13daeae2 100644
--- a/testsuite/tests/typecheck/should_fail/T10285.stderr
+++ b/testsuite/tests/typecheck/should_fail/T10285.stderr
@@ -1,9 +1,6 @@
 
 T10285.hs:8:17: error:
-    Could not deduce: Coercible a b
-      arising from trying to show that the representations of
-        ‘a’ and
-        ‘b’ are the same
+    Could not deduce: Coercible a b arising from a use of ‘coerce’
     from the context: Coercible (N a) (N b)
       bound by a pattern with constructor:
                  Coercion :: forall (k :: BOX) (a :: k) (b :: k).
diff --git a/testsuite/tests/typecheck/should_fail/T10495.stderr b/testsuite/tests/typecheck/should_fail/T10495.stderr
index 6e92505504..eec1cdb7c0 100644
--- a/testsuite/tests/typecheck/should_fail/T10495.stderr
+++ b/testsuite/tests/typecheck/should_fail/T10495.stderr
@@ -1,6 +1,7 @@
 
 T10495.hs:5:7: error:
     Couldn't match representation of type ‘a0’ with that of ‘b0’
+      arising from a use of ‘coerce’
     Relevant bindings include foo :: a0 -> b0 (bound at T10495.hs:5:1)
     In the expression: coerce
     In an equation for ‘foo’: foo = coerce
diff --git a/testsuite/tests/typecheck/should_fail/T10534.stderr b/testsuite/tests/typecheck/should_fail/T10534.stderr
index ab2cde1cf5..5bb0e40097 100644
--- a/testsuite/tests/typecheck/should_fail/T10534.stderr
+++ b/testsuite/tests/typecheck/should_fail/T10534.stderr
@@ -1,9 +1,6 @@
 
 T10534a.hs:10:9: error:
-    Could not deduce: Coercible a b
-      arising from trying to show that the representations of
-        ‘a’ and
-        ‘b’ are the same
+    Could not deduce: Coercible a b arising from a use of ‘coerce’
     from the context: Coercible (DF a) (DF b)
       bound by the type signature for:
                silly :: Coercible (DF a) (DF b) => a -> b
diff --git a/testsuite/tests/typecheck/should_fail/T5853.stderr b/testsuite/tests/typecheck/should_fail/T5853.stderr
index 1489583154..2d5c90284b 100644
--- a/testsuite/tests/typecheck/should_fail/T5853.stderr
+++ b/testsuite/tests/typecheck/should_fail/T5853.stderr
@@ -1,6 +1,6 @@
 
 T5853.hs:15:52: error:
-    Could not deduce: t2 ~ Subst t1 (Elem t2)
+    Could not deduce: Subst t1 (Elem t2) ~ t2
     from the context: (F t,
                        Subst t (Elem t2) ~ t2,
                        Subst t2 (Elem t) ~ t,
diff --git a/testsuite/tests/typecheck/should_fail/TcCoercibleFail.stderr b/testsuite/tests/typecheck/should_fail/TcCoercibleFail.stderr
index 434d5e34a7..e41d8c1b10 100644
--- a/testsuite/tests/typecheck/should_fail/TcCoercibleFail.stderr
+++ b/testsuite/tests/typecheck/should_fail/TcCoercibleFail.stderr
@@ -1,12 +1,14 @@
 
 TcCoercibleFail.hs:11:8: error:
     Couldn't match representation of type ‘Int’ with that of ‘()’
+      arising from a use of ‘coerce’
     In the expression: coerce
     In the expression: coerce $ one :: ()
     In an equation for ‘foo1’: foo1 = coerce $ one :: ()
 
 TcCoercibleFail.hs:14:8: error:
     Couldn't match representation of type ‘m Int’ with that of ‘m Age’
+      arising from a use of ‘coerce’
     NB: We cannot know what roles the parameters to ‘m’ have;
       we must assume that the role is nominal
     Relevant bindings include
@@ -16,18 +18,14 @@ TcCoercibleFail.hs:14:8: error:
     In an equation for ‘foo2’: foo2 = coerce $ (return one :: m Int)
 
 TcCoercibleFail.hs:16:8: error:
-    Couldn't match type ‘Int’ with ‘Age’
-    arising from trying to show that the representations of
-      ‘Map Int ()’ and
-      ‘Map Age ()’ are the same
-    Relevant role signatures: type role Map nominal representational
+    Couldn't match type ‘Int’ with ‘Age’ arising from a use of ‘coerce’
     In the expression: coerce
     In the expression: coerce $ Map one () :: Map Age ()
     In an equation for ‘foo3’: foo3 = coerce $ Map one () :: Map Age ()
 
 TcCoercibleFail.hs:18:8: error:
     Couldn't match representation of type ‘Int’ with that of ‘Down Int’
-    Relevant role signatures: type role Down representational
+      arising from a use of ‘coerce’
     The data constructor ‘Down’ of newtype ‘Down’ is not in scope
     In the expression: coerce
     In the expression: coerce $ one :: Down Int
@@ -35,13 +33,14 @@ TcCoercibleFail.hs:18:8: error:
 
 TcCoercibleFail.hs:21:8: error:
     Couldn't match representation of type ‘Void’ with that of ‘()’
+      arising from a use of ‘coerce’
     In the expression: coerce :: Void -> ()
     In an equation for ‘foo5’: foo5 = coerce :: Void -> ()
 
 TcCoercibleFail.hs:24:9: error:
     Couldn't match representation of type ‘VoidBad ()’
                              with that of ‘()’
-    Relevant role signatures: type role VoidBad phantom
+      arising from a use of ‘coerce’
     In the expression: coerce :: (VoidBad ()) -> ()
     In an equation for ‘foo5'’: foo5' = coerce :: (VoidBad ()) -> ()
 
@@ -60,11 +59,6 @@ TcCoercibleFail.hs:29:8: error:
     Couldn't match representation of type ‘Either
                                              Int (Fix (Either Int))’
                              with that of ‘()’
-    arising from trying to show that the representations of
-      ‘Fix (Either Int)’ and
-      ‘()’ are the same
-    Relevant role signatures:
-      type role Either representational representational
-      type role Fix nominal
+      arising from a use of ‘coerce’
     In the expression: coerce :: Fix (Either Int) -> ()
     In an equation for ‘foo7’: foo7 = coerce :: Fix (Either Int) -> ()
diff --git a/testsuite/tests/typecheck/should_fail/TcCoercibleFail3.stderr b/testsuite/tests/typecheck/should_fail/TcCoercibleFail3.stderr
index f4ebe40bb1..823b4c375d 100644
--- a/testsuite/tests/typecheck/should_fail/TcCoercibleFail3.stderr
+++ b/testsuite/tests/typecheck/should_fail/TcCoercibleFail3.stderr
@@ -1,12 +1,6 @@
 
-TcCoercibleFail3.hs:12:7:
+TcCoercibleFail3.hs:12:7: error:
     Couldn't match representation of type ‘NT1’ with that of ‘NT2’
-    arising from trying to show that the representations of
-      ‘T NT1’ and
-      ‘T NT2’ are the same
-    Relevant role signatures:
-      type role NT2 representational
-      type role NT1 representational
-      type role T representational
+      arising from a use of ‘coerce’
     In the expression: coerce
     In an equation for ‘foo’: foo = coerce
diff --git a/testsuite/tests/typecheck/should_fail/tcfail182.stderr b/testsuite/tests/typecheck/should_fail/tcfail182.stderr
index d223ca5e1a..fa628a4049 100644
--- a/testsuite/tests/typecheck/should_fail/tcfail182.stderr
+++ b/testsuite/tests/typecheck/should_fail/tcfail182.stderr
@@ -1,10 +1,9 @@
 
-tcfail182.hs:9:3:
+tcfail182.hs:9:3: error:
     Couldn't match expected type ‘Prelude.Maybe a’
                 with actual type ‘Maybe t0’
-    NB: ‘Prelude.Maybe’
-          is defined in ‘GHC.Base’ in package ‘base-4.8.2.0’
-        ‘Maybe’ is defined at tcfail182.hs:6:1-18
+    NB: ‘Maybe’ is defined at tcfail182.hs:6:1-18
+        ‘Prelude.Maybe’ is defined in ‘GHC.Base’ in package ‘base-4.8.2.0’
     Relevant bindings include
       f :: Prelude.Maybe a -> Int (bound at tcfail182.hs:9:1)
     In the pattern: Foo