Forbid variables to be parents in import lists.

In the long discussion on #11432, it was decided that when a type
constructor is parsed as a variable ((--.->) is one example) then in
order to export the type constructor then the user should be required to
use the ExplicitNamespaces keyword.

This was implemented in quite an indirect manner in the renamer. It
is much more direct to enforce this in the parser at the expense of
slighty worse error messages. Further to this, the check in the
renamer was actually slightly wrong. If the variable was in scope
then no error was raised, this was causing panics, see #12026 for an
example.

Reviewers: austin, bgamari

Subscribers: davean, skvadrik, thomie

Differential Revision: https://phabricator.haskell.org/D2181

GHC Trac Issues: #12026

1 files changed, 14 insertions, 4 deletions

diff --git a/compiler/parser/RdrHsSyn.hs b/compiler/parser/RdrHsSyn.hs
index 28ee4f07a0..c3f1d53d65 100644
--- a/compiler/parser/RdrHsSyn.hs
+++ b/compiler/parser/RdrHsSyn.hs
@@ -1387,10 +1387,10 @@ mkModuleImpExp n@(L l name) subs =
   case subs of
     ImpExpAbs
       | isVarNameSpace (rdrNameSpace name) -> return $ IEVar  n
-      | otherwise                          -> return $ IEThingAbs  (L l name)
-    ImpExpAll                              -> return $ IEThingAll  (L l name)
+      | otherwise                          -> IEThingAbs . L l <$> nameT
+    ImpExpAll                              -> IEThingAll . L l <$> nameT
     ImpExpList xs                          ->
-      return $ IEThingWith (L l name) NoIEWildcard xs []
+      (\newName -> IEThingWith (L l newName) NoIEWildcard xs []) <$> nameT
     ImpExpAllWith xs                       ->
       do allowed <- extension patternSynonymsEnabled
          if allowed
@@ -1399,9 +1399,19 @@ mkModuleImpExp n@(L l name) subs =
                 pos   = maybe NoIEWildcard IEWildcard
                           (findIndex isNothing withs)
                 ies   = [L l n | L l (Just n) <- xs]
-            in return (IEThingWith (L l name) pos ies [])
+            in (\newName -> IEThingWith (L l newName) pos ies []) <$> nameT
           else parseErrorSDoc l
             (text "Illegal export form (use PatternSynonyms to enable)")
+  where
+    nameT =
+      if isVarNameSpace (rdrNameSpace name)
+        then parseErrorSDoc l
+              (text "Expecting a type constructor but found a variable."
+              $$ if isSymOcc $ rdrNameOcc name
+                   then text "If" <+> quotes (ppr name) <+> text "is a type constructor"
+                   else empty
+              <+> text "then enable ExplicitNamespaces and use the 'type' keyword.")
+        else return $ name
 
 mkTypeImpExp :: Located RdrName   -- TcCls or Var name space
              -> P (Located RdrName)