Improved error message about exported type operators.

There is ambiguty between (1) type constructors and (2) data
constructors in export lists, e.g. '%%' can stand for both of them. This
ambiguity is resolved in favor of (2).

If the exported data constructor is not in scope, but type constructor
with the same name is in scope, GHC should suggest adding 'type' keyword
to resolve ambiguity in favor of (1) and enabling 'TypeOperators'
extension.

The patch only extends the error message.

See Trac #11432.

Test Plan: `make test`

Reviewers: simonpj, bgamari, austin

Reviewed By: simonpj

Subscribers: mpickering, thomie, goldfire, kosmikus

Differential Revision: https://phabricator.haskell.org/D1902

GHC Trac Issues: #11432

6 files changed, 54 insertions, 13 deletions

diff --git a/compiler/rename/RnEnv.hs b/compiler/rename/RnEnv.hs
index 868712b829..5d74d7c94f 100644
--- a/compiler/rename/RnEnv.hs
+++ b/compiler/rename/RnEnv.hs
@@ -13,7 +13,7 @@ module RnEnv (
         lookupLocalOccRn_maybe, lookupInfoOccRn,
         lookupLocalOccThLvl_maybe,
         lookupTypeOccRn, lookupKindOccRn,
-        lookupGlobalOccRn, lookupGlobalOccRn_maybe,
+        lookupGlobalOccRn, lookupGlobalOccRnExport, lookupGlobalOccRn_maybe,
         lookupOccRn_overloaded, lookupGlobalOccRn_overloaded,
         reportUnboundName, unknownNameSuggestions,
         addNameClashErrRn,
@@ -853,6 +853,27 @@ lookupGlobalOccRn rdr_name
            Nothing -> do { traceRn (text "lookupGlobalOccRn" <+> ppr rdr_name)
                          ; unboundName WL_Global rdr_name } }
 
+-- like lookupGlobalOccRn but suggests adding 'type' keyword
+-- to export type constructors mistaken for data constructors
+lookupGlobalOccRnExport :: RdrName -> RnM Name
+lookupGlobalOccRnExport rdr_name
+  = do { mb_name <- lookupGlobalOccRn_maybe rdr_name
+       ; case mb_name of
+           Just n  -> return n
+           Nothing -> do { env <- getGlobalRdrEnv
+                         ; let tycon = setOccNameSpace tcClsName (rdrNameOcc rdr_name)
+                               msg = case lookupOccEnv env tycon of
+                                   Just (gre : _) -> make_msg gre
+                                   _              -> Outputable.empty
+                               make_msg gre = hang
+                                   (hsep [text "Note: use",
+                                       quotes (text "type"),
+                                       text "keyword to export type constructor",
+                                       quotes (ppr (gre_name gre))])
+                                   2 (vcat [pprNameProvenance gre,
+                                       text "(requires TypeOperators extension)"])
+                         ; unboundNameX WL_Global rdr_name msg } }
+
 lookupInfoOccRn :: RdrName -> RnM [Name]
 -- lookupInfoOccRn is intended for use in GHCi's ":info" command
 -- It finds all the GREs that RdrName could mean, not complaining
diff --git a/compiler/rename/RnNames.hs b/compiler/rename/RnNames.hs
index 7f89025872..d8e08e20aa 100644
--- a/compiler/rename/RnNames.hs
+++ b/compiler/rename/RnNames.hs
@@ -1346,7 +1346,7 @@ exports_from_avail (Just (L _ rdr_items)) rdr_env imports this_mod
     lookup_ie_with :: IE RdrName -> Located RdrName -> [Located RdrName]
                    -> RnM (Located Name, [Located Name], [Name], [FieldLabel])
     lookup_ie_with ie (L l rdr) sub_rdrs
-        = do name <- lookupGlobalOccRn rdr
+        = do name <- lookupGlobalOccRnExport rdr
              let gres = findChildren kids_env name
                  mchildren =
                   lookupChildren (map classifyGRE (gres ++ pat_syns)) sub_rdrs
@@ -1366,7 +1366,7 @@ exports_from_avail (Just (L _ rdr_items)) rdr_env imports this_mod
     lookup_ie_all :: IE RdrName -> Located RdrName
                   -> RnM (Located Name, [Name], [FieldLabel])
     lookup_ie_all ie (L l rdr) =
-          do name <- lookupGlobalOccRn rdr
+          do name <- lookupGlobalOccRnExport rdr
              let gres = findChildren kids_env name
                  (non_flds, flds) = classifyGREs gres
              addUsedKids rdr gres
diff --git a/testsuite/tests/module/T11432.hs b/testsuite/tests/module/T11432.hs
new file mode 100644
index 0000000000..408935d4ec
--- /dev/null
+++ b/testsuite/tests/module/T11432.hs
@@ -0,0 +1,9 @@
+{-
+We expect to get a suggestion to add 'type' keyword
+and enable TypeOperators extension.
+-}
+
+{-# LANGUAGE TypeOperators #-}
+module T11432 ((-.->)(..)) where
+
+newtype (f -.-> g) a = Fn { apFn :: f a -> g a }
diff --git a/testsuite/tests/module/T11432.stderr b/testsuite/tests/module/T11432.stderr
new file mode 100644
index 0000000000..bf2a58b0a0
--- /dev/null
+++ b/testsuite/tests/module/T11432.stderr
@@ -0,0 +1,10 @@
+
+T11432.hs:7:16:
+    Not in scope: ‘-.->’
+    Note: use ‘type’ keyword to export type constructor ‘-.->’
+      defined at T11432.hs:9:1
+      (requires TypeOperators extension)
+
+T11432.hs:7:16:
+    The export item ‘(-.->)(..)’
+    attempts to export constructors or class methods that are not visible here
diff --git a/testsuite/tests/module/all.T b/testsuite/tests/module/all.T
index cd1bdac293..e6446fe480 100644
--- a/testsuite/tests/module/all.T
+++ b/testsuite/tests/module/all.T
@@ -347,3 +347,4 @@ test('T9061', normal, compile, [''])
 test('T9997', normal, compile, [''])
 test('T10233', extra_clean(['T01233a.hi', 'T01233a.o']),
      multimod_compile, ['T10233', '-v0'])
+test('T11432', normal, compile_fail, [''])
diff --git a/testsuite/tests/module/mod89.stderr b/testsuite/tests/module/mod89.stderr
index b355f3050b..5b2f422455 100644
--- a/testsuite/tests/module/mod89.stderr
+++ b/testsuite/tests/module/mod89.stderr
@@ -1,10 +1,10 @@
-

-mod89.hs:5:1: warning:

-    The import item ‘map(..)’ suggests that

-    ‘map’ has (in-scope) constructors or class methods,

-    but it has none

-

-mod89.hs:5:1: warning:

-    The import of ‘Prelude’ is redundant

-      except perhaps to import instances from ‘Prelude’

-    To import instances alone, use: import Prelude()

+
+mod89.hs:5:1: warning:
+    The import item ‘map(..)’ suggests that
+    ‘map’ has (in-scope) constructors or class methods,
+    but it has none
+
+mod89.hs:5:1: warning:
+    The import of ‘Prelude’ is redundant
+      except perhaps to import instances from ‘Prelude’
+    To import instances alone, use: import Prelude()