add test for #4808

testsuite/tests/ghc-regress/lib/IO/4808.hs

+import System.IO
+import GHC.IO.Handle
+import GHC.IO.FD as FD
+
+main = do
+  (fd, _) <- FD.openFile "4808.hs" ReadWriteMode
+  hdl <- mkDuplexHandle fd "4808.hs" Nothing nativeNewlineMode
+  hClose hdl
+  (fd2, _) <- FD.openFile "4808.hs" ReadWriteMode
+  print (fdFD fd == fdFD fd2) -- should be True
+  hGetLine hdl >>= print -- should fail with an exception
+

testsuite/tests/ghc-regress/lib/IO/4808.stderr

+4808: 4808.hs: hGetLine: illegal operation (handle is closed)

testsuite/tests/ghc-regress/lib/IO/4808.stdout

+True

testsuite/tests/ghc-regress/lib/IO/all.T

@@ -158,3 +158,5 @@ test('T4113', normal, compile_and_run, [''])
 test('T4144', normal, compile_and_run, [''])
 
 test('encodingerror001', normal, compile_and_run, [''])
+
+test('4808', exit_code(1), compile_and_run, [''])