Improve ambiguous-occurrence error message

Trac #15487 correctly reported that the qualification
of a Name in an ambiguous-occurrence error message was
wrong.  This patch fixes it.

It's easily done, in RnUtils.addNameClashErrRn

The problem was that in complaining about M.x we must
enusre that 'M' part is the same as that used in
pprNameProvenance.

1 files changed, 32 insertions, 8 deletions

diff --git a/compiler/rename/RnUtils.hs b/compiler/rename/RnUtils.hs
index 99272c2943..0451e288be 100644
--- a/compiler/rename/RnUtils.hs
+++ b/compiler/rename/RnUtils.hs
@@ -295,16 +295,40 @@ addNameClashErrRn rdr_name gres
                -- If there are two or more *local* defns, we'll have reported
   = return ()  -- that already, and we don't want an error cascade
   | otherwise
-  = addErr (vcat [text "Ambiguous occurrence" <+> quotes (ppr rdr_name),
-                  text "It could refer to" <+> vcat (msg1 : msgs)])
+  = addErr (vcat [ text "Ambiguous occurrence" <+> quotes (ppr rdr_name)
+                 , text "It could refer to"
+                 , nest 3 (vcat (msg1 : msgs)) ])
   where
     (np1:nps) = gres
-    msg1 = ptext  (sLit "either") <+> mk_ref np1
-    msgs = [text "    or" <+> mk_ref np | np <- nps]
-    mk_ref gre = sep [nom <> comma, pprNameProvenance gre]
-      where nom = case gre_par gre of
-                    FldParent { par_lbl = Just lbl } -> text "the field" <+> quotes (ppr lbl)
-                    _                                -> quotes (ppr (gre_name gre))
+    msg1 =  text "either" <+> ppr_gre np1
+    msgs = [text "    or" <+> ppr_gre np | np <- nps]
+    ppr_gre gre = sep [ pp_gre_name gre <> comma
+                      , pprNameProvenance gre]
+
+    -- When printing the name, take care to qualify it in the same
+    -- way as the provenance reported by pprNameProvenance, namely
+    -- the head of 'gre_imp'.  Otherwise we get confusing reports like
+    --   Ambiguous occurrence ‘null’
+    --   It could refer to either ‘T15487a.null’,
+    --                            imported from ‘Prelude’ at T15487.hs:1:8-13
+    --                     or ...
+    -- See Trac #15487
+    pp_gre_name gre@(GRE { gre_name = name, gre_par = parent
+                         , gre_lcl = lcl, gre_imp = iss })
+      | FldParent { par_lbl = Just lbl } <- parent
+      = text "the field" <+> quotes (ppr lbl)
+      | otherwise
+      = quotes (pp_qual <> dot <> ppr (nameOccName name))
+      where
+        pp_qual | lcl
+                = ppr (nameModule name)
+                | imp : _ <- iss  -- This 'imp' is the one that
+                                  -- pprNameProvenance chooses
+                , ImpDeclSpec { is_as = mod } <- is_decl imp
+                = ppr mod
+                | otherwise
+                = pprPanic "addNameClassErrRn" (ppr gre $$ ppr iss)
+                  -- Invariant: either 'lcl' is True or 'iss' is non-empty
 
 shadowedNameWarn :: OccName -> [SDoc] -> SDoc
 shadowedNameWarn occ shadowed_locs