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

1 files changed, 9 insertions, 4 deletions

diff --git a/compiler/rename/RnSource.lhs b/compiler/rename/RnSource.lhs
index 2dc71db001..4af8be32fd 100644
--- a/compiler/rename/RnSource.lhs
+++ b/compiler/rename/RnSource.lhs
@@ -1286,8 +1286,13 @@ rnConDecl decl@(ConDecl { con_name = name, con_qvars = tvs
 
          -- With an Explicit forall, check for unused binders
          -- With Implicit, find the mentioned ones, and use them as binders
+         -- With Qualified, do the same as with Implicit, but give a warning
+         --   See Note [Context quantification]
         ; new_tvs <- case expl of
                        Implicit -> return (mkHsQTvs (userHsTyVarBndrs loc free_tvs))
+                       Qualified -> do { warnContextQuantification (docOfHsDocContext doc)
+                                                                   (userHsTyVarBndrs loc free_tvs)
+                                       ; return (mkHsQTvs (userHsTyVarBndrs loc free_tvs)) }
                        Explicit -> do { warnUnusedForAlls (docOfHsDocContext doc) tvs free_tvs
                                       ; return tvs }
 
@@ -1471,10 +1476,10 @@ add gp loc (SpliceD splice@(SpliceDecl _ flag)) ds
   = do { -- We've found a top-level splice.  If it is an *implicit* one
          -- (i.e. a naked top level expression)
          case flag of
-           Explicit -> return ()
-           Implicit -> do { th_on <- xoptM Opt_TemplateHaskell
-                          ; unless th_on $ setSrcSpan loc $
-                            failWith badImplicitSplice }
+           ExplicitSplice -> return ()
+           ImplicitSplice -> do { th_on <- xoptM Opt_TemplateHaskell
+                                ; unless th_on $ setSrcSpan loc $
+                                  failWith badImplicitSplice }
 
        ; return (gp, Just (splice, ds)) }
   where