A raft of small changes associated with -XConstrainedClassMethods

See Trac #7854.  Specifically:

* Major clean up and simplification of check_op in checkValidClass;
  specifically
     - use checkValidType on the entire method-selector type to detect
       ambiguity
     - put a specific test for -XConstrainedClassMethods

* Make -XConstrainedClassMethods be implied by -XMultiParamTypeClasses
  (a bit ad-hoc but see #7854), and document in the user manual.

* Do the checkAmbiguity test just once in TcValidity.checkValidType,
  rather than repeatedly at every level. See Note [When to call checkAmbiguity]

* Add -XAllowAmbiguousTypes in GHC.IP, since 'ip' really is ambiguous.
  (It's a rather magic function.)

* Improve location info for check_op in checkValidClass

* Update quite a few tests, which had genuinely-ambiguous class
  method signatures.  Some I fixed by making them unambiguous; some
  by adding -XAllowAmbiguousTypes

3 files changed, 22 insertions, 20 deletions

diff --git a/testsuite/tests/polykinds/T8566.hs b/testsuite/tests/polykinds/T8566.hs
index ee5892ce25..248febb586 100644
--- a/testsuite/tests/polykinds/T8566.hs
+++ b/testsuite/tests/polykinds/T8566.hs
@@ -6,6 +6,7 @@
 {-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE UndecidableInstances #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE AllowAmbiguousTypes #-}  -- 'c' is ambiguous
 
 module T8566 where
 
diff --git a/testsuite/tests/polykinds/T8566.stderr b/testsuite/tests/polykinds/T8566.stderr
index 096f058791..168e890404 100644
--- a/testsuite/tests/polykinds/T8566.stderr
+++ b/testsuite/tests/polykinds/T8566.stderr
@@ -1,19 +1,19 @@
-
-T8566.hs:31:9:
-    Could not deduce (C ('AA (t (I a ps)) as) ps fs0)
-      arising from a use of ‘c’
-    from the context: C ('AA (t (I a ps)) as) ps fs
-      bound by the instance declaration at T8566.hs:29:10-67
-    or from: 'AA t (a : as) ~ 'AA t1 as1
-      bound by a pattern with constructor:
-                 A :: forall (r :: [*]) (k :: BOX) (t :: k) (as :: [U *]).
-                      I ('AA t as) r,
-               in an equation for ‘c’
-      at T8566.hs:31:5
-    The type variable ‘fs0’ is ambiguous
-    Relevant bindings include
-      c :: I ('AA t (a : as)) ps -> I ('AA t (a : as)) ps
-        (bound at T8566.hs:31:3)
-    In the expression: c undefined
-    In an equation for ‘c’: c A = c undefined
-    In the instance declaration for ‘C ('AA t (a : as)) ps fs’
+

+T8566.hs:32:9:

+    Could not deduce (C ('AA (t (I a ps)) as) ps fs0)

+      arising from a use of ‘c’

+    from the context: C ('AA (t (I a ps)) as) ps fs

+      bound by the instance declaration at T8566.hs:30:10-67

+    or from: 'AA t (a : as) ~ 'AA t1 as1

+      bound by a pattern with constructor:

+                 A :: forall (r :: [*]) (k :: BOX) (t :: k) (as :: [U *]).

+                      I ('AA t as) r,

+               in an equation for ‘c’

+      at T8566.hs:32:5

+    The type variable ‘fs0’ is ambiguous

+    Relevant bindings include

+      c :: I ('AA t (a : as)) ps -> I ('AA t (a : as)) ps

+        (bound at T8566.hs:32:3)

+    In the expression: c undefined

+    In an equation for ‘c’: c A = c undefined

+    In the instance declaration for ‘C ('AA t (a : as)) ps fs’

diff --git a/testsuite/tests/polykinds/T9200.hs b/testsuite/tests/polykinds/T9200.hs
index ca050661a2..740536a516 100644
--- a/testsuite/tests/polykinds/T9200.hs
+++ b/testsuite/tests/polykinds/T9200.hs
@@ -2,12 +2,13 @@
              TypeFamilies #-}
 
 module T9200 where
+import Data.Proxy
 
 ------
 -- test CUSK on classes
 
 class C (f :: k) (a :: k2) where
-  c_meth :: D a => ()
+  c_meth :: D a => Proxy f -> Proxy a -> ()
   
 class C () a => D a