There are some more common tags with identical values, i.e. AlbumArtist, Composer and Lyrics - any reason you kept these?

In D15825#334087, @bruns wrote:

There are some more common tags with identical values, i.e. AlbumArtist, Composer and Lyrics - any reason you kept these?

These are the tags directly supported by taglib (e.g. tag->artist() ) which don't require manual reading of tags. These are only the supported tags for the mime types tested in D15833

In D15825#338218, @astippich wrote:

In D15825#334087, @bruns wrote:

There are some more common tags with identical values, i.e. AlbumArtist, Composer and Lyrics - any reason you kept these?

These are the tags directly supported by taglib (e.g. tag->artist() ) which don't require manual reading of tags. These are only the supported tags for the mime types tested in D15833

Sorry, I don't get it yet ...

Is there anything preventing this to work:

SimpleExtractionResult result(testFilePath(fileName), mimeType);
plugin.extract(&result);
QCOMPARE(result.properties().value(Property::AlbumArtist), QVariant(QStringLiteral("Album Artist")));

As far as I can see, this is identical code for all tests, and supported by all formats.

Or are these not supported by some of the four new formats added in D15833?

Sorry, I should have written this more clearly, but you got it right. It's for the new formats. For the most common tags, taglib directly provides the implementation for all its supported tag formats and one can call e.g. tag->artist(). For the others, one has to implement support manually, e.g. query the tag type and then do tag->find("Album Artist") and then read it.
So for any new formats we get the ones ones which are implemented by taglib for free. The others have to be manually added and thus are not implemented for the new formats.
I will update the summary a little bit.