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

25 files changed, 97 insertions, 52 deletions

diff --git a/testsuite/tests/indexed-types/should_compile/T2715.hs b/testsuite/tests/indexed-types/should_compile/T2715.hs
index 0fae15eaf8..c283467b82 100644
--- a/testsuite/tests/indexed-types/should_compile/T2715.hs
+++ b/testsuite/tests/indexed-types/should_compile/T2715.hs
@@ -3,6 +3,8 @@
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE AllowAmbiguousTypes #-}
+    -- The type of 'empty' is indeed ambiguous
 
 module T2715 where
 
@@ -14,9 +16,8 @@ type instance Domain Interval = Interval
 
 type family Value (d :: * -> *) :: *
 
-
 class IDomain d where
-   empty   :: (Ord (Value d), Enum (Value d)) => (Domain d) (Value d)
+   empty   :: (Ord (Value d), Enum (Value d)) => Domain d (Value d)
 
 class (IDomain d1) -- (IDomain d1, IDomain d2, Value d1 ~ Value d2) 
    => IIDomain (d1 :: * -> *) (d2 :: * -> * ) where
@@ -25,7 +26,7 @@ class (IDomain d1) -- (IDomain d1, IDomain d2, Value d1 ~ Value d2)
 
 instance Ord (Value Interval) 
       => IDomain Interval where
-   empty                   = Intv (toEnum 1, toEnum 0)
+   empty = Intv (toEnum 1, toEnum 0)
 
 instance Ord (Value Interval) 
       => IIDomain Interval Interval where
diff --git a/testsuite/tests/indexed-types/should_compile/T4160.hs b/testsuite/tests/indexed-types/should_compile/T4160.hs
index f13aafa103..ee95e8c874 100644
--- a/testsuite/tests/indexed-types/should_compile/T4160.hs
+++ b/testsuite/tests/indexed-types/should_compile/T4160.hs
@@ -1,4 +1,7 @@
 {-# LANGUAGE FlexibleContexts, MultiParamTypeClasses, TypeFamilies #-}
+{-# LANGUAGE AllowAmbiguousTypes #-}
+    -- The type of sizeT is indeed ambiguous
+
 module Foo where
 
 data P f g r = f r :*: g r
diff --git a/testsuite/tests/indexed-types/should_compile/T4200.hs b/testsuite/tests/indexed-types/should_compile/T4200.hs
index feb91e8d8b..a9a1a5bd63 100644
--- a/testsuite/tests/indexed-types/should_compile/T4200.hs
+++ b/testsuite/tests/indexed-types/should_compile/T4200.hs
@@ -5,9 +5,9 @@ module T4200 where
 
 class C a where
   type In a :: *
-  op :: In a -> Int
+  op :: In a -> a -> Int
 
 -- Should be ok; no -XUndecidableInstances required
 instance (In c ~ Int) => C [c] where 
   type In [c] = In c
-  op x = 3
+  op _ _ = 3
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’
diff --git a/testsuite/tests/indexed-types/should_fail/T1900.stderr b/testsuite/tests/indexed-types/should_fail/T1900.stderr
index 0d02a52f11..f986888cfd 100644
--- a/testsuite/tests/indexed-types/should_fail/T1900.stderr
+++ b/testsuite/tests/indexed-types/should_fail/T1900.stderr
@@ -1,12 +1,13 @@
-
-T1900.hs:13:10:
-    Couldn't match type ‘Depend s0’ with ‘Depend s’
-    NB: ‘Depend’ is a type function, and may not be injective
-    The type variable ‘s0’ is ambiguous
-    Expected type: Depend s -> Bool
-      Actual type: Depend s0 -> Bool
-    In the ambiguity check for the type signature for ‘check’:
-      check :: forall s. Bug s => Depend s -> Bool
-    To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
-    In the type signature for ‘check’:
-      check :: (Bug s) => Depend s -> Bool
+

+T1900.hs:7:3:

+    Couldn't match type ‘Depend s0’ with ‘Depend s’

+    NB: ‘Depend’ is a type function, and may not be injective

+    The type variable ‘s0’ is ambiguous

+    Expected type: Depend s -> Depend s

+      Actual type: Depend s0 -> Depend s0

+    In the ambiguity check for the type signature for ‘trans’:

+      trans :: forall s. Bug s => Depend s -> Depend s

+    To defer the ambiguity check to use sites, enable AllowAmbiguousTypes

+    When checking the class method:

+      trans :: forall s. Bug s => Depend s -> Depend s

+    In the class declaration for ‘Bug’

diff --git a/testsuite/tests/indexed-types/should_fail/T2544.hs b/testsuite/tests/indexed-types/should_fail/T2544.hs
index 3653a42317..0e98910055 100644
--- a/testsuite/tests/indexed-types/should_fail/T2544.hs
+++ b/testsuite/tests/indexed-types/should_fail/T2544.hs
@@ -1,4 +1,6 @@
 {-# LANGUAGE TypeOperators, TypeFamilies #-}

+{-# LANGUAGE AllowAmbiguousTypes #-}

+    -- The type of 'empty' is indeed ambiguous

 

 module T2544 where

 

diff --git a/testsuite/tests/indexed-types/should_fail/T2544.stderr b/testsuite/tests/indexed-types/should_fail/T2544.stderr
index 6c5230800c..7a1f564c56 100644
--- a/testsuite/tests/indexed-types/should_fail/T2544.stderr
+++ b/testsuite/tests/indexed-types/should_fail/T2544.stderr
@@ -1,11 +1,11 @@
 

-T2544.hs:15:12:

+T2544.hs:17:12:

     Couldn't match type ‘IxMap l’ with ‘IxMap i0’

     NB: ‘IxMap’ is a type function, and may not be injective

     The type variable ‘i0’ is ambiguous

     Expected type: IxMap (l :|: r) [Int]

       Actual type: BiApp (IxMap i0) (IxMap i1) [Int]

     Relevant bindings include

-      empty :: IxMap (l :|: r) [Int] (bound at T2544.hs:15:4)

+      empty :: IxMap (l :|: r) [Int] (bound at T2544.hs:17:4)

     In the expression: BiApp empty empty

     In an equation for ‘empty’: empty = BiApp empty empty

diff --git a/testsuite/tests/module/all.T b/testsuite/tests/module/all.T
index c86a5f8806..1507ffb0d5 100644
--- a/testsuite/tests/module/all.T
+++ b/testsuite/tests/module/all.T
@@ -57,7 +57,7 @@ test('mod35', normal, compile, [''])
 test('mod36', normal, compile_fail, [''])
 test('mod37', normal, compile, [''])
 test('mod38', normal, compile_fail, [''])
-test('mod39', normal, compile, [''])
+test('mod39', normal, compile_fail, [''])
 test('mod40', normal, compile_fail, [''])
 test('mod41', normal, compile_fail, [''])
 test('mod42', normal, compile_fail, [''])
diff --git a/testsuite/tests/module/mod39.stderr b/testsuite/tests/module/mod39.stderr
new file mode 100644
index 0000000000..3c049f2532
--- /dev/null
+++ b/testsuite/tests/module/mod39.stderr
@@ -0,0 +1,7 @@
+

+mod39.hs:3:17:

+    Constraint ‘Eq a’ in the type of ‘f’

+      constrains only the class type variables

+    Use ConstrainedClassMethods to allow it

+    When checking the class method: f :: forall a. (C a, Eq a) => a

+    In the class declaration for ‘C’

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
 
diff --git a/testsuite/tests/roles/should_compile/Roles3.hs b/testsuite/tests/roles/should_compile/Roles3.hs
index 4c26f0d986..3df74ec8e2 100644
--- a/testsuite/tests/roles/should_compile/Roles3.hs
+++ b/testsuite/tests/roles/should_compile/Roles3.hs
@@ -1,4 +1,5 @@
 {-# LANGUAGE TypeFamilies, MultiParamTypeClasses #-}
+{-# LANGUAGE AllowAmbiguousTypes #-}  -- meth3, meth4 are ambiguous
 
 module Roles3 where
 
diff --git a/testsuite/tests/th/TH_tf2.hs b/testsuite/tests/th/TH_tf2.hs
index 94be291324..399731a7f6 100644
--- a/testsuite/tests/th/TH_tf2.hs
+++ b/testsuite/tests/th/TH_tf2.hs
@@ -1,4 +1,5 @@
 {-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE AllowAmbiguousTypes #-}  -- 'bar' is ambiguous
 
 module TH_tf2 where
 
diff --git a/testsuite/tests/typecheck/should_compile/tc165.hs b/testsuite/tests/typecheck/should_compile/tc165.hs
index ea2fa08ec1..0533c80ca8 100644
--- a/testsuite/tests/typecheck/should_compile/tc165.hs
+++ b/testsuite/tests/typecheck/should_compile/tc165.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE ConstrainedClassMethods #-}
 {-# OPTIONS_GHC -dcore-lint #-}
 
 -- Fails GHC 5.04.2 with -dcore-lint
diff --git a/testsuite/tests/typecheck/should_compile/tc199.hs b/testsuite/tests/typecheck/should_compile/tc199.hs
index d530cfd6d0..dfa2c1f230 100644
--- a/testsuite/tests/typecheck/should_compile/tc199.hs
+++ b/testsuite/tests/typecheck/should_compile/tc199.hs
@@ -1,7 +1,8 @@
-{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE MultiParamTypeClasses, AllowAmbiguousTypes #-}
 
 -- This code defines a default method with a highly dubious type,
 -- because 'v' is not mentioned, and there are no fundeps
+-- Hence needing AllowAmbiguousTypes
 --
 -- However, arguably the instance declaration should be accepted,
 -- beause it's equivalent to 
diff --git a/testsuite/tests/typecheck/should_compile/tc200.hs b/testsuite/tests/typecheck/should_compile/tc200.hs
index bb6a00e1ae..ef799c61d1 100644
--- a/testsuite/tests/typecheck/should_compile/tc200.hs
+++ b/testsuite/tests/typecheck/should_compile/tc200.hs
@@ -1,4 +1,5 @@
 {-# OPTIONS_GHC -w #-}
+{-# LANGUAGE ConstrainedClassMethods #-}  -- Needed for 'baz'
 
 -- A nasty case that crashed GHC 6.4 with a Lint error; 
 -- see Note [Multiple instantiation] in TcExpr
diff --git a/testsuite/tests/typecheck/should_compile/tc235.hs b/testsuite/tests/typecheck/should_compile/tc235.hs
index 55a1a5855d..4973ec1b33 100644
--- a/testsuite/tests/typecheck/should_compile/tc235.hs
+++ b/testsuite/tests/typecheck/should_compile/tc235.hs
@@ -1,6 +1,8 @@
 {-# OPTIONS_GHC -fno-warn-redundant-constraints #-}
 {-# LANGUAGE FlexibleInstances, UndecidableInstances,
              MultiParamTypeClasses, FunctionalDependencies #-}
+{-# LANGUAGE AllowAmbiguousTypes #-}
+    -- 'x' and 'v' are ambiguous
 
 -- Trac #1564
 
diff --git a/testsuite/tests/typecheck/should_compile/tc259.hs b/testsuite/tests/typecheck/should_compile/tc259.hs
index 776bd8416b..6ece4a29ca 100644
--- a/testsuite/tests/typecheck/should_compile/tc259.hs
+++ b/testsuite/tests/typecheck/should_compile/tc259.hs
@@ -1,5 +1,8 @@
 -- Test we don't get a cycle for "phantom" superclasses
 {-# LANGUAGE ConstraintKinds, MultiParamTypeClasses, FlexibleContexts #-}
+{-# LANGUAGE AllowAmbiguousTypes #-}
+    -- 'meth' is ambiguous
+
 module TcOK where
 
 class A cls c where
diff --git a/testsuite/tests/typecheck/should_compile/tc260.hs b/testsuite/tests/typecheck/should_compile/tc260.hs
index 29baeee903..3a4d9b0d8d 100644
--- a/testsuite/tests/typecheck/should_compile/tc260.hs
+++ b/testsuite/tests/typecheck/should_compile/tc260.hs
@@ -1,6 +1,9 @@
 -- Test we don't get a cycle for "phantom" superclasses,
 -- even if the phantomness is behind a type synonym
 {-# LANGUAGE ConstraintKinds, MultiParamTypeClasses, FlexibleContexts #-}
+{-# LANGUAGE AllowAmbiguousTypes #-}
+    -- 'meth' is ambiguous
+
 module TcOK where
 
 class A ctxt c where
diff --git a/testsuite/tests/typecheck/should_fail/all.T b/testsuite/tests/typecheck/should_fail/all.T
index 60c709760e..20eede0f96 100644
--- a/testsuite/tests/typecheck/should_fail/all.T
+++ b/testsuite/tests/typecheck/should_fail/all.T
@@ -133,7 +133,7 @@ test('tcfail146', normal, compile_fail, [''])
 test('tcfail147', normal, compile_fail, [''])
 test('tcfail148', normal, compile_fail, [''])
 test('tcfail149', normal, compile_and_run, [''])
-test('tcfail150', normal, compile, [''])
+test('tcfail150', normal, compile_fail, [''])
 test('tcfail151', normal, compile_fail, [''])
 test('tcfail152', normal, compile_fail, [''])
 test('tcfail153', normal, compile_fail, [''])
diff --git a/testsuite/tests/typecheck/should_fail/tcfail116.stderr b/testsuite/tests/typecheck/should_fail/tcfail116.stderr
index 0136173201..abefc61eb8 100644
--- a/testsuite/tests/typecheck/should_fail/tcfail116.stderr
+++ b/testsuite/tests/typecheck/should_fail/tcfail116.stderr
@@ -1,6 +1,12 @@
-
-tcfail116.hs:5:1:
-    The class method ‘bug’
-    mentions none of the type or kind variables of the class ‘Foo a’
-    When checking the class method: bug :: ()
-    In the class declaration for ‘Foo’
+

+tcfail116.hs:6:5:

+    Could not deduce (Foo a0)

+    from the context: Foo a

+      bound by the type signature for: bug :: Foo a => ()

+      at tcfail116.hs:6:5-13

+    The type variable ‘a0’ is ambiguous

+    In the ambiguity check for the type signature for ‘bug’:

+      bug :: forall a. Foo a => ()

+    To defer the ambiguity check to use sites, enable AllowAmbiguousTypes

+    When checking the class method: bug :: forall a. Foo a => ()

+    In the class declaration for ‘Foo’

diff --git a/testsuite/tests/typecheck/should_fail/tcfail149.hs b/testsuite/tests/typecheck/should_fail/tcfail149.hs
index 2479ed75c8..090db8de22 100644
--- a/testsuite/tests/typecheck/should_fail/tcfail149.hs
+++ b/testsuite/tests/typecheck/should_fail/tcfail149.hs
@@ -1,3 +1,5 @@
+{-# LANGUAGE ConstrainedClassMethods #-}
+
 module Main where
 
 class C a where
diff --git a/testsuite/tests/typecheck/should_fail/tcfail150.stderr b/testsuite/tests/typecheck/should_fail/tcfail150.stderr
index e69de29bb2..c91d404c13 100644
--- a/testsuite/tests/typecheck/should_fail/tcfail150.stderr
+++ b/testsuite/tests/typecheck/should_fail/tcfail150.stderr
@@ -0,0 +1,8 @@
+

+tcfail150.hs:6:3:

+    Constraint ‘Eq a’ in the type of ‘op’

+      constrains only the class type variables

+    Use ConstrainedClassMethods to allow it

+    When checking the class method:

+      op :: forall a. (Foo a, Eq a) => a -> a

+    In the class declaration for ‘Foo’

diff --git a/testsuite/tests/typecheck/should_fail/tcfail151.hs b/testsuite/tests/typecheck/should_fail/tcfail151.hs
index 00578a1eb2..51cf65d5cd 100644
--- a/testsuite/tests/typecheck/should_fail/tcfail151.hs
+++ b/testsuite/tests/typecheck/should_fail/tcfail151.hs
@@ -2,7 +2,7 @@
 module ShouldFail where
 
 class (Show a, Eq a, Monad m) => Name m a where 
-    hashName :: a -> Int 
+    hashName :: a -> m Int 
     newName :: m a 
  
 data Name a => Exp a = MkExp a