[YoutubeDL] Raise syntax error for format selection expressions with multiple + operators (closes #27803)

test/test_YoutubeDL.py

@@ -464,6 +464,7 @@ class TestFormatSelection(unittest.TestCase):
         assert_syntax_error('+bestaudio')
         assert_syntax_error('bestvideo+')
         assert_syntax_error('/')
+        assert_syntax_error('bestvideo+bestvideo+bestaudio')
 
     def test_format_filtering(self):
         formats = [

youtube_dl/YoutubeDL.py

@@ -1226,6 +1226,8 @@ class YoutubeDL(object):
                         group = _parse_format_selection(tokens, inside_group=True)
                         current_selector = FormatSelector(GROUP, group, [])
                     elif string == '+':
+                        if inside_merge:
+                            raise syntax_error('Unexpected "+"', start)
                         video_selector = current_selector
                         audio_selector = _parse_format_selection(tokens, inside_merge=True)
                         if not video_selector or not audio_selector: