$ formatExportItemError

            (ppr export_item)

            "attempts to export a default class declaration that is not visible here"

       -> mkSimpleDecorated $

            hsep [ quotes (ppr $ greName gre)

     TcRnExportedParentChildMismatch parent_name ty_thing child parent_names

       -> mkSimpleDecorated $

            text "The type constructor" <+> quotes (ppr parent_name)

       -> diagnosticRelatedLocations msg'

     TcRnBindingNameConflict _ locs

       -> NE.tail locs

     _ ->

       []

                  overloadedrecflds/should_fail/DuplicateExports

                  patsyn/should_compile/T11959

   -}

   {-| TcRnDuplicateNamedDefaultExport is a warning (controlled by -Wduplicate-exports)

       that occurs when a named default declaration appears in an export list

       more than once.

   -}

   {-| TcRnExportedParentChildMismatch is an error that occurs when an export is

       bundled with a parent that it does not belong to

 import GHC.Types.Name.Reader

 import GHC.Types.Hint

 import Control.Monad ( when )

 import qualified Data.List.NonEmpty as NE

 import Data.Traversable   ( for )

      = ExportAccum {

          expacc_exp_occs :: ExportOccMap,

            -- ^ Tracks exported occurrence names

            -- ^ Tracks exported named default declarations

          expacc_mods :: UniqMap ModuleName [Name],

            -- ^ Tracks (re-)exported module names

   where f' acc x

           = fromMaybe (acc, Nothing) <$> attemptM (f acc x)

         -- Tracks what a particular exported OccName

         --   in an export list refers to, and which item

         --   it came from.  It's illegal to export two distinct things

             ; traceRn "efa" (ppr mod $$ ppr all_gres)

             ; addUsedGREs ExportDeprecationWarnings all_gres

                           -- This check_occs not only finds conflicts

                           -- between this item and others, but also

                           -- internally within this item.  That is, if

             ; traceRn "efa" (ppr mod $$ ppr all_gres)

             ; addUsedGREs ExportDeprecationWarnings all_gres

                           -- This check_occs not only finds conflicts

                           -- between this item and others, but also

                           -- internally within this item.  That is, if

                    name = greName gre

                checkThLocalNameNoLift (ieLWrappedUserRdrName l name)

                (export_warn_spans', dont_warn_export', warn_txt_rn)

                  <- process_warning export_warn_spans

                                     dont_warn_export

                        Just cls_dflts -> do

                          let cls = cd_class cls_dflts

                          case lookupNameEnv exp_dflts (className cls) of

                              addDiagnostic $

                              return (Nothing, occs, exp_dflts)

                            Nothing ->

                    _ -> do

                     return (Just avail, occs', exp_dflts)

                checkThLocalNameNoLift (ieLWrappedUserRdrName l name)

                    all_names = map greName all_gres

                checkThLocalNameNoLift (ieLWrappedUserRdrName l name)

                (export_warn_spans', dont_warn_export', warn_txt_rn)

                  <- process_warning export_warn_spans

                                     dont_warn_export

                    all_names = map greName all_gres

                checkThLocalNameNoLift (ieLWrappedUserRdrName l name)

                (export_warn_spans', dont_warn_export', warn_txt_rn)

                  <- process_warning export_warn_spans

                                     dont_warn_export

 -- | Insert the given 'GlobalRdrElt's into the 'ExportOccMap', checking that

 -- each of the given 'GlobalRdrElt's does not appear multiple times in

 -- the 'ExportOccMap', as per Note [Exporting duplicate declarations].

   -- 'gres' are the entities specified by 'ie'

   = do { drf <- xoptM LangExt.DuplicateRecordFields

        ; foldlM (check drf) occs gres }

   where

     -- Check for distinct children exported with the same OccName (an error) or

     -- for duplicate exports of the same child (a warning).

             | drf_enabled || not (isFieldOcc child_occ)

             -> return occs'

             | otherwise

                              $ lookupFieldsOccEnv occs (occNameFS child_occ)

                   ; case flds of { [] -> return occs'; clash1:clashes ->

                   ; return occs } } }

             | child == child' -- Duplicate export of a single Name: a warning.

                   ; return occs }

             | otherwise       -- Same OccName but different Name: an error.

             ->  do { global_env <- getGlobalRdrEnv

                    ; return occs }

       where

         child = greName gre

     -- Try to insert a child into the map, returning Left if there is something

     -- already exported with the same OccName.

     try_insert occs child

       = case lookupOccEnv occs occ of

           Just x  -> Left x

       where

         occ = greOccName child

 T25857.hs:4:33: warning: [GHC-31584] [-Wduplicate-exports (in -Wdefault)]

     The named default declaration for ‘IsString’ is exported by ‘default IsString’ and ‘default IsString’

 test('default-fail08', normal, compile_fail, [''])

 test('T25775', normal, compile_fail, [''])

 test('T25825', normal, compile, [''])

 test('T25206', [extra_files(['T25206_helper.hs'])], multimod_compile, ['T25206', ''])

 test('T25858', normal, compile_and_run, [''])

 test('T25858v1', [extra_files(['T25858v1_helper.hs'])], multimod_compile_and_run, ['T25858v1', ''])

 DuplicateExports.hs:6:29: error: [GHC-47854] [-Wduplicate-exports (in -Wdefault), Werror=duplicate-exports]

     ‘foo’ is exported by ‘foo’ and ‘T(foo, bar)’

      multimod_compile_fail, ['T14953', ''])

 test('T26480', extra_files(['T26480_aux1.hs', 'T26480_aux2.hs']), multimod_compile_fail, ['T26480', '-v0'])

 test('T26480b', normal, compile_fail, [''])

 test('T17420', [extra_files(['T17420A.hs'])], multimod_compile_fail,

      ['T17420', ''])

 test('T17469', [extra_files(['T17469A.hs'])], multimod_compile_fail,

     Multiple declarations of ‘derp’

     Declared at: T7164.hs:5:5

                  T7164.hs:8:1

 test('T6148b', normal, compile_fail, [''])

 test('T6148c', normal, compile_fail, [''])

 test('T6148d', normal, compile_fail, [''])

 test('T7338', normal, compile_fail, [''])

 test('T7338a', normal, compile_fail, [''])

 test('T7454', normal, compile, [''])