#1492704: Ensure and document backward compatibility of the change

Doc/library/shutil.rst

@@ -70,7 +70,8 @@ Directory and files operations
       Now returns *dst*.
 
    .. versionchanged:: 3.4
-      Raise :exc:`SameFileError` instead of :exc:`Error`.
+      Raise :exc:`SameFileError` instead of :exc:`Error`.  Since the former is
+      a subclass of the latter, this change is backward compatible.
 
 
 .. exception:: SameFileError

Lib/test/test_shutil.py

@@ -1223,6 +1223,8 @@ class TestShutil(unittest.TestCase):
         src_file = os.path.join(src_dir, 'foo')
         write_file(src_file, 'foo')
         self.assertRaises(SameFileError, shutil.copyfile, src_file, src_file)
+        # But Error should work too, to stay backward compatible.
+        self.assertRaises(Error, shutil.copyfile, src_file, src_file)
 
     def test_copytree_return_value(self):
         # copytree returns its destination path.