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

7 files changed, 35 insertions, 57 deletions

diff --git a/testsuite/tests/deriving/should_fail/T4846.stderr b/testsuite/tests/deriving/should_fail/T4846.stderr
index b0ebb9edfb..5a7339bf21 100644
--- a/testsuite/tests/deriving/should_fail/T4846.stderr
+++ b/testsuite/tests/deriving/should_fail/T4846.stderr
@@ -1,10 +1,7 @@
 
-T4846.hs:29:1:
+T4846.hs:29:1: error:
     Couldn't match type ‘Bool’ with ‘BOOL’
-    arising from trying to show that the representations of
-      ‘Expr Bool’ and
-      ‘Expr BOOL’ are the same
-    Relevant role signatures: type role Expr nominal
+      arising from a use of ‘coerce’
     In the expression: coerce (mkExpr :: Expr Bool) :: Expr BOOL
     In an equation for ‘mkExpr’:
         mkExpr = coerce (mkExpr :: Expr Bool) :: Expr BOOL
diff --git a/testsuite/tests/deriving/should_fail/T5498.stderr b/testsuite/tests/deriving/should_fail/T5498.stderr
index ac91aaa4d0..ce87ef1867 100644
--- a/testsuite/tests/deriving/should_fail/T5498.stderr
+++ b/testsuite/tests/deriving/should_fail/T5498.stderr
@@ -1,11 +1,10 @@
 
-T5498.hs:30:39:
+T5498.hs:30:39: error:
     Couldn't match representation of type ‘c a’
                              with that of ‘c (Down a)’
-    arising from the coercion of the method ‘intIso’
-      from type ‘forall (c :: * -> *). c a -> c Int’
-        to type ‘forall (c :: * -> *). c (Down a) -> c Int’
-    Relevant role signatures: type role Down representational
+      arising from the coercion of the method ‘intIso’
+        from type ‘forall (c :: * -> *). c a -> c Int’
+          to type ‘forall (c :: * -> *). c (Down a) -> c Int’
     NB: We cannot know what roles the parameters to ‘c’ have;
       we must assume that the role is nominal
     When deriving the instance for (IntIso (Down a))
diff --git a/testsuite/tests/deriving/should_fail/T6147.stderr b/testsuite/tests/deriving/should_fail/T6147.stderr
index 7f851a656b..a7bd1ab477 100644
--- a/testsuite/tests/deriving/should_fail/T6147.stderr
+++ b/testsuite/tests/deriving/should_fail/T6147.stderr
@@ -1,7 +1,6 @@
 
-T6147.hs:13:32:
+T6147.hs:13:32: error:
     Couldn't match type ‘Int’ with ‘Foo’
-    arising from the coercion of the method ‘foo’
-      from type ‘Int -> T Int’ to type ‘Foo -> T Foo’
-    Relevant role signatures: type role T nominal
+      arising from the coercion of the method ‘foo’
+        from type ‘Int -> T Int’ to type ‘Foo -> T Foo’
     When deriving the instance for (C Foo)
diff --git a/testsuite/tests/deriving/should_fail/T7148.stderr b/testsuite/tests/deriving/should_fail/T7148.stderr
index ba3a88b6fe..84c8205fb2 100644
--- a/testsuite/tests/deriving/should_fail/T7148.stderr
+++ b/testsuite/tests/deriving/should_fail/T7148.stderr
@@ -1,20 +1,14 @@
 
-T7148.hs:27:40:
+T7148.hs:27:40: error:
     Occurs check: cannot construct the infinite type: b ~ Tagged a b
-    arising from the coercion of the method ‘iso2’
-      from type ‘forall b. SameType b () -> SameType b b’
-        to type ‘forall b. SameType b () -> SameType b (Tagged a b)’
-    Relevant role signatures:
-      type role Tagged phantom representational
-      type role SameType nominal nominal
+      arising from the coercion of the method ‘iso2’
+        from type ‘forall b. SameType b () -> SameType b b’
+          to type ‘forall b. SameType b () -> SameType b (Tagged a b)’
     When deriving the instance for (IsoUnit (Tagged a b))
 
-T7148.hs:27:40:
+T7148.hs:27:40: error:
     Occurs check: cannot construct the infinite type: b ~ Tagged a b
-    arising from the coercion of the method ‘iso1’
-      from type ‘forall b. SameType () b -> SameType b b’
-        to type ‘forall b. SameType () b -> SameType (Tagged a b) b’
-    Relevant role signatures:
-      type role Tagged phantom representational
-      type role SameType nominal nominal
+      arising from the coercion of the method ‘iso1’
+        from type ‘forall b. SameType () b -> SameType b b’
+          to type ‘forall b. SameType () b -> SameType (Tagged a b) b’
     When deriving the instance for (IsoUnit (Tagged a b))
diff --git a/testsuite/tests/deriving/should_fail/T7148a.stderr b/testsuite/tests/deriving/should_fail/T7148a.stderr
index 4edb968702..8dd23aa034 100644
--- a/testsuite/tests/deriving/should_fail/T7148a.stderr
+++ b/testsuite/tests/deriving/should_fail/T7148a.stderr
@@ -1,14 +1,10 @@
 
-T7148a.hs:19:50:
+T7148a.hs:19:50: error:
     Couldn't match representation of type ‘b’ with that of ‘Result a b’
-      ‘b’ is a rigid type variable bound by
-          the type forall b1. Proxy b1 -> a -> Result a b1 at T7148a.hs:19:50
-    arising from the coercion of the method ‘coerce’
-      from type ‘forall b. Proxy b -> a -> Result a b’
-        to type ‘forall b.
-                 Proxy b -> IS_NO_LONGER a -> Result (IS_NO_LONGER a) b’
-    Relevant role signatures:
-      type role IS_NO_LONGER representational
-      type role Result nominal nominal
-      type role Proxy phantom
+      arising from the coercion of the method ‘coerce’
+        from type ‘forall b. Proxy b -> a -> Result a b’
+          to type ‘forall b.
+                   Proxy b -> IS_NO_LONGER a -> Result (IS_NO_LONGER a) b’
+    ‘b’ is a rigid type variable bound by
+        the type forall b1. Proxy b1 -> a -> Result a b1 at T7148a.hs:19:50
     When deriving the instance for (Convert (IS_NO_LONGER a))
diff --git a/testsuite/tests/deriving/should_fail/T8851.stderr b/testsuite/tests/deriving/should_fail/T8851.stderr
index 0a2b384bd1..dd4a614785 100644
--- a/testsuite/tests/deriving/should_fail/T8851.stderr
+++ b/testsuite/tests/deriving/should_fail/T8851.stderr
@@ -1,16 +1,11 @@
 
-T8851.hs:24:12:
+T8851.hs:24:12: error:
     Couldn't match type ‘Parser’ with ‘MyParser’
-    arising from the coercion of the method ‘notFollowedBy’
-      from type ‘forall a.
-                 (Monad Parser, Show a) =>
-                 Parser a -> Parser ()’
-        to type ‘forall a.
-                 (Monad MyParser, Show a) =>
-                 MyParser a -> MyParser ()’
-    Relevant role signatures:
-      type role Monad nominal
-      type role Show nominal
-      type role MyParser phantom
-      type role Parser phantom
+      arising from the coercion of the method ‘notFollowedBy’
+        from type ‘forall a.
+                   (Monad Parser, Show a) =>
+                   Parser a -> Parser ()’
+          to type ‘forall a.
+                   (Monad MyParser, Show a) =>
+                   MyParser a -> MyParser ()’
     When deriving the instance for (Parsing MyParser)
diff --git a/testsuite/tests/deriving/should_fail/T8984.stderr b/testsuite/tests/deriving/should_fail/T8984.stderr
index 6606d66f63..1cdc425293 100644
--- a/testsuite/tests/deriving/should_fail/T8984.stderr
+++ b/testsuite/tests/deriving/should_fail/T8984.stderr
@@ -1,11 +1,9 @@
 
-T8984.hs:7:46:
+T8984.hs:7:46: error:
     Couldn't match representation of type ‘cat a (N cat a Int)’
                              with that of ‘cat a (cat a Int)’
-    arising from the coercion of the method ‘app’
-      from type ‘cat a (cat a Int)’ to type ‘N cat a (N cat a Int)’
-    Relevant role signatures:
-      type role N representational nominal nominal
+      arising from the coercion of the method ‘app’
+        from type ‘cat a (cat a Int)’ to type ‘N cat a (N cat a Int)’
     NB: We cannot know what roles the parameters to ‘cat a’ have;
       we must assume that the role is nominal
     When deriving the instance for (C (N cat a))