[youtube] `/live` URLs should raise error if channel is not live

Fixes: https://github.com/ytdl-org/youtube-dl/issues/29090

README.md

@@ -128,7 +128,7 @@ Some of yt-dlp's default options are different from that of youtube-dl and youtu
 * `playlist_index` behaves differently when used with options like `--playlist-reverse` and `--playlist-items`. See [#302](https://github.com/yt-dlp/yt-dlp/issues/302) for details. You can use `--compat-options playlist-index` if you want to keep the earlier behavior
 * The output of `-F` is listed in a new format. Use `--compat-options list-formats` to revert this
 * Youtube live chat (if available) is considered as a subtitle. Use `--sub-langs all,-live_chat` to download all subtitles except live chat. You can also use `--compat-options no-live-chat` to prevent live chat from downloading
-* Youtube channel URLs are automatically redirected to `/video`. Either append a `/featured` to the URL or use `--compat-options no-youtube-channel-redirect` to download only the videos in the home page
+* Youtube channel URLs are automatically redirected to `/video`. Append a `/featured` to the URL to download only the videos in the home page. If the channel does not have a videos tab, we try to download the equivalent `UU` playlist instead. Also, `/live` URLs raise an error if there are no live videos instead of silently downloading the entire channel. You may use `--compat-options no-youtube-channel-redirect` to revert all these redirections
 * Unavailable videos are also listed for youtube playlists. Use `--compat-options no-youtube-unavailable-videos` to remove this
 
 For ease of use, a few more compat options are available:

yt_dlp/extractor/youtube.py

@@ -2854,6 +2854,10 @@ class YoutubeTabIE(YoutubeBaseInfoExtractor):
     }, {
         'url': 'https://www.youtube.com/c/CommanderVideoHq/live',
         'only_matching': True,
+    }, {
+        'note': 'A channel that is not live. Should raise error',
+        'url': 'https://www.youtube.com/user/numberphile/live',
+        'only_matching': True,
     }, {
         'url': 'https://www.youtube.com/feed/trending',
         'only_matching': True,
@@ -3710,8 +3714,11 @@ class YoutubeTabIE(YoutubeBaseInfoExtractor):
         if tabs:
             selected_tab = self._extract_selected_tab(tabs)
             tab_name = selected_tab.get('title', '')
-            if (mobj['tab'] == '/videos' and tab_name.lower() != mobj['tab'][1:]
+            if 'no-youtube-channel-redirect' not in compat_opts:
-                    and 'no-youtube-channel-redirect' not in compat_opts):
+                if mobj['tab'] == '/live':
+                    # Live tab should have redirected to the video
+                    raise ExtractorError('The channel is not currently live', expected=True)
+                if mobj['tab'] == '/videos' and tab_name.lower() != mobj['tab'][1:]:
                     if not mobj['not_channel'] and item_id[:2] == 'UC':
                         # Topic channels don't have /videos. Use the equivalent playlist instead
                         self.report_warning('The URL does not have a %s tab. Trying to redirect to playlist UU%s instead' % (mobj['tab'][1:], item_id[2:]))
@@ -3749,6 +3756,7 @@ class YoutubeTabIE(YoutubeBaseInfoExtractor):
             data, lambda x: x['currentVideoEndpoint']['watchEndpoint']['videoId'],
             compat_str) or video_id
         if video_id:
+            if mobj['tab'] != '/live':  # live tab is expected to redirect to video
                 self.report_warning('Unable to recognize playlist. Downloading just video %s' % video_id)
             return self.url_result(video_id, ie=YoutubeIE.ie_key(), video_id=video_id)