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

1 files changed, 3 insertions, 3 deletions

diff --git a/testsuite/tests/indexed-types/should_compile/T9582.hs b/testsuite/tests/indexed-types/should_compile/T9582.hs
index f86d723319..d906205834 100644
--- a/testsuite/tests/indexed-types/should_compile/T9582.hs
+++ b/testsuite/tests/indexed-types/should_compile/T9582.hs
@@ -3,12 +3,12 @@ module T9582 where
 
 class C a where
   type T a
-  m :: T a
+  m :: a -> T a
 
 instance C Int where
   type T Int = String
-  m :: String
-  m = "bla"
+  m :: Int -> String
+  m _ = "bla"
 
 -- Method signature does not match class; it should be m :: T Int
 --    In the instance declaration for ‘C Int’