Catch SameFileError explicitly in _copytree

If not, it is caught as a shutil.Error for which it is assumed to be a
recursive error, resulting in splitting of the error string as
```
shutil.Error: ['<', 'D', 'i', 'r', 'E', 'n', 't', 'r', 'y', ' ', ...,
               's', 'a', 'm', 'e', ' ', 'f', 'i', 'l', 'e']
```

Author: berland

Lib/shutil.py

@@ -529,9 +529,11 @@ def _copytree(entries, src, dst, symlinks, ignore, copy_function,
             else:
                 # Will raise a SpecialFileError for unsupported file types
                 copy_function(srcobj, dstname)
-        # catch the Error from the recursive copytree so that we can
-        # continue with other files
+        except SameFileError as err:
+            errors.append((srcname, dstname, str(err)))
         except Error as err:
+            # catch the Error from the recursive copytree so that we can
+            # continue with other files
             errors.extend(err.args[0])
         except OSError as why:
             errors.append((srcname, dstname, str(why)))

Lib/test/test_shutil.py

@@ -1101,6 +1101,32 @@ def test_copytree_subdirectory(self):
         rv = shutil.copytree(src_dir, dst_dir)
         self.assertEqual(['pol'], os.listdir(rv))
 
+    def test_copytree_to_itself_gives_sensible_error_message(self):
+        base_dir = self.mkdtemp()
+        src_dir = os.path.join(base_dir, "src")
+        os.makedirs(src_dir)
+        create_file((src_dir, "somefilename"), "somecontent")
+        _assert_are_the_same_file_is_raised(src_dir, src_dir)
+
+    def test_copytree_to_backpointing_symlink_gives_sensible_error_message(self):
+        base_dir = self.mkdtemp()
+        src_dir = os.path.join(base_dir, "src")
+        target_dir = os.path.join(base_dir, "target")
+        os.makedirs(src_dir)
+        os.makedirs(target_dir)
+        some_file = os.path.join(src_dir, "somefilename")
+        create_file(some_file, "somecontent")
+        os.symlink(some_file, os.path.join(target_dir, "somefilename"))
+        _assert_are_the_same_file_is_raised(src_dir, target_dir)
+
+def _assert_are_the_same_file_is_raised(src_dir, target_dir):
+    try:
+        shutil.copytree(src_dir, target_dir, dirs_exist_ok=True)
+        raise Exception("copytree did not raise")
+    except Error as error:
+        assert len(error.args[0]) == 1
+        assert "are the same file" in error.args[0][0][2]
+
 class TestCopy(BaseTest, unittest.TestCase):
 
     ### shutil.copymode