Add -fwarn-context-quantification (#4426)

Summary:
This warning (enabled by default) reports places where a context
implicitly binds a type variable, for example

type T a = {-forall m.-} Monad m => a -> m a

Also update Haddock submodule.

Test Plan: validate

Reviewers: hvr, goldfire, simonpj, austin

Reviewed By: austin

Subscribers: simonmar, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D211

GHC Trac Issues: #4426

7 files changed, 66 insertions, 8 deletions

diff --git a/testsuite/tests/rename/should_compile/T4426.hs b/testsuite/tests/rename/should_compile/T4426.hs
new file mode 100644
index 0000000000..b04f59cd03
--- /dev/null
+++ b/testsuite/tests/rename/should_compile/T4426.hs
@@ -0,0 +1,22 @@
+{-# LANGUAGE RankNTypes #-}
+{-# OPTIONS_GHC -fwarn-context-quantification #-}
+
+module T4426 where
+
+type F a = Monad m => a -> m a
+
+data X a = X (Eq b => a -> b)
+
+data Y a = Y { k :: Eq b => a -> b -> c }
+
+f :: forall b. (Monad m => m b) -> b
+f = undefined
+
+type F' a = forall m. Monad m => a -> m a
+
+data X' a = X' (forall b. Eq b => a -> b)
+
+data Y' a = Y' { k' :: forall b c. Eq b => a -> b -> c }
+
+f' :: forall b. (forall m. Monad m => m b) -> b
+f' = undefined
diff --git a/testsuite/tests/rename/should_compile/T4426.stderr b/testsuite/tests/rename/should_compile/T4426.stderr
new file mode 100644
index 0000000000..f4e0c471d6
--- /dev/null
+++ b/testsuite/tests/rename/should_compile/T4426.stderr
@@ -0,0 +1,35 @@
+
+T4426.hs:6:12: Warning:
+    Variable ‘m’ is implicitly quantified due to a context
+    Use explicit forall syntax instead.
+    This will become an error in GHC 7.12.
+    In the type ‘Monad m => a -> m a’
+    In the declaration for type synonym ‘F’
+
+T4426.hs:8:15: Warning:
+    Variable ‘b’ is implicitly quantified due to a context
+    Use explicit forall syntax instead.
+    This will become an error in GHC 7.12.
+    In the type ‘Eq b => a -> b’
+    In the definition of data constructor ‘X’
+
+T4426.hs:10:21: Warning:
+    Variable ‘b’ is implicitly quantified due to a context
+    Use explicit forall syntax instead.
+    This will become an error in GHC 7.12.
+    In the type ‘Eq b => a -> b -> c’
+    In the definition of data constructor ‘Y’
+
+T4426.hs:10:21: Warning:
+    Variable ‘c’ is implicitly quantified due to a context
+    Use explicit forall syntax instead.
+    This will become an error in GHC 7.12.
+    In the type ‘Eq b => a -> b -> c’
+    In the definition of data constructor ‘Y’
+
+T4426.hs:12:17: Warning:
+    Variable ‘m’ is implicitly quantified due to a context
+    Use explicit forall syntax instead.
+    This will become an error in GHC 7.12.
+    In the type ‘Monad m => m b’
+    In the type signature for ‘f’
diff --git a/testsuite/tests/rename/should_compile/all.T b/testsuite/tests/rename/should_compile/all.T
index d104df4910..7185fba5b8 100644
--- a/testsuite/tests/rename/should_compile/all.T
+++ b/testsuite/tests/rename/should_compile/all.T
@@ -217,3 +217,4 @@ test('T7969',
      run_command,
      ['$MAKE -s --no-print-directory T7969'])
 test('T9127', normal, compile, [''])
+test('T4426', normal, compile, [''])
diff --git a/testsuite/tests/th/T7021a.hs b/testsuite/tests/th/T7021a.hs
index bd191336dd..0eadecf4c6 100644
--- a/testsuite/tests/th/T7021a.hs
+++ b/testsuite/tests/th/T7021a.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE ConstraintKinds, TemplateHaskell, PolyKinds, TypeFamilies #-}
+{-# LANGUAGE ConstraintKinds, TemplateHaskell, PolyKinds, TypeFamilies, RankNTypes #-}
 
 module T7021a where
 
@@ -27,5 +27,5 @@ test = do
     reify fooName
     reify bazName
     reify barName
-    [t| (Show a, (Read a, Num a)) => a -> a |]
+    [t| forall a. (Show a, (Read a, Num a)) => a -> a |]
     [| \_ -> 0 |]
diff --git a/testsuite/tests/th/T8807.hs b/testsuite/tests/th/T8807.hs
index 3090123f95..7d21796298 100644
--- a/testsuite/tests/th/T8807.hs
+++ b/testsuite/tests/th/T8807.hs
@@ -1,8 +1,8 @@
-{-# LANGUAGE ConstraintKinds #-}
+{-# LANGUAGE ConstraintKinds, RankNTypes #-}
 
 module T8807 where
 
 import Data.Proxy
 
-foo :: $( [t| a b => Proxy a -> b -> b |] )
-foo = undefined
\ No newline at end of file
+foo :: $( [t| forall a b. a b => Proxy a -> b -> b |] )
+foo = undefined
diff --git a/testsuite/tests/typecheck/should_compile/T3018.hs b/testsuite/tests/typecheck/should_compile/T3018.hs
index 443d73a17a..296185de30 100644
--- a/testsuite/tests/typecheck/should_compile/T3018.hs
+++ b/testsuite/tests/typecheck/should_compile/T3018.hs
@@ -20,7 +20,7 @@ class Sat a where dict :: a
 class Subst_A a t t' where
     subst_A :: (Monad m) => a -> t -> t' -> m t'
 
-data SubstD_A a t t' = SubstD_A {substD_A:: (Monad m) => a -> t -> t' -> m t'}
+data SubstD_A a t t' = SubstD_A {substD_A:: forall m. (Monad m) => a -> t -> t' -> m t'}
 
 -- Allow override dictionary verion with implementation of type class Subst
 instance Subst_A a t t' => Sat (SubstD_A a t t') where
@@ -103,4 +103,4 @@ constraints, and only makes the attempt when it has all the evidence
 in hand. I'm thinking quite a bit about constraint solving at the
 moment and will bear that in mind. But I can't offer you an immediate
 solution. At least I hope I've explained the problem.
--}
\ No newline at end of file
+-}
diff --git a/testsuite/tests/typecheck/should_compile/tc092.hs b/testsuite/tests/typecheck/should_compile/tc092.hs
index b421809b8f..58493c6715 100644
--- a/testsuite/tests/typecheck/should_compile/tc092.hs
+++ b/testsuite/tests/typecheck/should_compile/tc092.hs
@@ -2,7 +2,7 @@
 
 module ShouldSucceed where
 
-data Empty q			=  Empty (Ord a => q a)
+data Empty q			=  Empty (forall a. Ord a => q a)
 q				:: (Ord a) => [a]
 q				=  []
 e0, e1, e2			:: Empty []