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

3 files changed, 58 insertions, 0 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, [''])