Reject all duplicate declarations involving DuplicateRecordFields (fixes #17965)

This fixes a bug that resulted in some programs being accepted that used the same
identifier as a field label and another declaration, depending on the order they
appeared in the source code.

5 files changed, 24 insertions, 6 deletions

diff --git a/compiler/GHC/Rename/Names.hs b/compiler/GHC/Rename/Names.hs
index df39d01adb..cd39ed701d 100644
--- a/compiler/GHC/Rename/Names.hs
+++ b/compiler/GHC/Rename/Names.hs
@@ -638,9 +638,12 @@ extendGlobalRdrEnvRn avails new_fixities
       | otherwise
       = return (extendGlobalRdrEnv env gre)
       where
-        name = gre_name gre
-        occ  = nameOccName name
-        dups = filter isLocalGRE (lookupGlobalRdrEnv env occ)
+        occ  = greOccName gre
+        dups = filter isDupGRE (lookupGlobalRdrEnv env occ)
+        -- Duplicate GREs are those defined locally with the same OccName,
+        -- except cases where *both* GREs are DuplicateRecordFields (#17965).
+        isDupGRE gre' = isLocalGRE gre'
+                && not (isOverloadedRecFldGRE gre && isOverloadedRecFldGRE gre')
 
 
 {- *********************************************************************
@@ -1767,14 +1770,13 @@ addDupDeclErr gres@(gre : _)
   = addErrAt (getSrcSpan (last sorted_names)) $
     -- Report the error at the later location
     vcat [text "Multiple declarations of" <+>
-             quotes (ppr (nameOccName name)),
+             quotes (ppr (greOccName gre)),
              -- NB. print the OccName, not the Name, because the
              -- latter might not be in scope in the RdrEnv and so will
              -- be printed qualified.
           text "Declared at:" <+>
                    vcat (map (ppr . nameSrcLoc) sorted_names)]
   where
-    name = gre_name gre
     sorted_names =
       sortBy (SrcLoc.leftmost_smallest `on` nameSrcSpan)
              (map gre_name gres)
diff --git a/compiler/GHC/Types/Name/Reader.hs b/compiler/GHC/Types/Name/Reader.hs
index 5f9163bb46..c1e675a19e 100644
--- a/compiler/GHC/Types/Name/Reader.hs
+++ b/compiler/GHC/Types/Name/Reader.hs
@@ -57,7 +57,7 @@ module GHC.Types.Name.Reader (
         gresToAvailInfo,
 
         -- ** Global 'RdrName' mapping elements: 'GlobalRdrElt', 'Provenance', 'ImportSpec'
-        GlobalRdrElt(..), isLocalGRE, isRecFldGRE, greLabel,
+        GlobalRdrElt(..), isLocalGRE, isRecFldGRE, isOverloadedRecFldGRE, greLabel,
         unQualOK, qualSpecOK, unQualSpecOK,
         pprNameProvenance,
         Parent(..), greParent_maybe,
@@ -842,6 +842,12 @@ isRecFldGRE :: GlobalRdrElt -> Bool
 isRecFldGRE (GRE {gre_par = FldParent{}}) = True
 isRecFldGRE _                             = False
 
+isOverloadedRecFldGRE :: GlobalRdrElt -> Bool
+-- ^ Is this a record field defined with DuplicateRecordFields?
+-- (See Note [Parents for record fields])
+isOverloadedRecFldGRE (GRE {gre_par = FldParent{par_lbl = Just _}}) = True
+isOverloadedRecFldGRE _                                             = False
+
 -- Returns the field label of this GRE, if it has one
 greLabel :: GlobalRdrElt -> Maybe FieldLabelString
 greLabel (GRE{gre_par = FldParent{par_lbl = Just lbl}}) = Just lbl
diff --git a/testsuite/tests/overloadedrecflds/should_fail/T17965.hs b/testsuite/tests/overloadedrecflds/should_fail/T17965.hs
new file mode 100644
index 0000000000..b723637f23
--- /dev/null
+++ b/testsuite/tests/overloadedrecflds/should_fail/T17965.hs
@@ -0,0 +1,4 @@
+{-# LANGUAGE DuplicateRecordFields #-}
+main = return ()
+newtype Record a = Record { f :: a -> a }
+class C a where f :: a -> a
diff --git a/testsuite/tests/overloadedrecflds/should_fail/T17965.stderr b/testsuite/tests/overloadedrecflds/should_fail/T17965.stderr
new file mode 100644
index 0000000000..446913eda4
--- /dev/null
+++ b/testsuite/tests/overloadedrecflds/should_fail/T17965.stderr
@@ -0,0 +1,5 @@
+
+T17965.hs:4:17: error:
+    Multiple declarations of ‘f’
+    Declared at: T17965.hs:3:29
+                 T17965.hs:4:17
diff --git a/testsuite/tests/overloadedrecflds/should_fail/all.T b/testsuite/tests/overloadedrecflds/should_fail/all.T
index 92ff1f0015..bc3c0650d2 100644
--- a/testsuite/tests/overloadedrecflds/should_fail/all.T
+++ b/testsuite/tests/overloadedrecflds/should_fail/all.T
@@ -32,3 +32,4 @@ test('hasfieldfail03', normal, compile_fail, [''])
 test('T14953', [extra_files(['T14953_A.hs', 'T14953_B.hs'])],
      multimod_compile_fail, ['T14953', ''])
 test('DuplicateExports', normal, compile_fail, [''])
+test('T17965', normal, compile_fail, [''])