Bach BWV 812 Allemande : bar 3 - any suggested fingerings to play the descending D6 - A5 while holding G5 - starting from the 2nd beat
The suggested fingering with 4 on D6 suggests to shorten the full minim to switch the thumb from G5 to A5.
The edition is Henle.
Yes, that's definitely the idea. (You know, I've played this edition for 30 years and never noticed that the suggested fingering is technically impossible. But no one is going to miss the last beat of the upper g, particularly since the bass voice has another g.)
TL;DR I would play the semiquaver D5 – A4 passage 1 - 4 - 3 - 2 by using my left-hand thumb on the minim G4, swapping in my right-hand thumb between the C# - B
I would play the minim (half-note) G4 with my left hand thumb. When the right hand reaches B4, the right-hand thumb can take over the G4 from the left hand. This allows G4 to be granted its full length. (I actually swap thumbs between the C#5 and the B4.)
This use of the thumb(s) allows the semiquavers D5 – A4 to be played 1 - 4 - 3 - 2.
It also requires a change in the left hand compared to the fingering given in the Henle. Rather than LH 1 on beat 3, I use finger 4. Once the RH thumb is swapped in on the G4, I can adjust my LH fingering to prepare for the upcoming descent (and ornament, if included).
The danger is that the left hand thumb will produce an accented G4, so care must be taken to keep the inner voice intact.
Here is the suggestion, notated:
Shortening the G does work, but as an alternative you might want to try 3-2-4-3 on D-C#-B-A. 4 on the B is only slightly more awkward than on the D.
G5 is assigned the right-hand thumb twice in a row, implying that you should not shift your right-hand thumb from G5 to A5 and shorten the minim.
C♯6 is implied to be assigned to the right-hand middle finger, but assigning B5 the right-hand ring or middle finger both seem plausible. Assigning B5 the right-hand pointer finger is more likely to result in an awkward-sounding accent on Beat 3.75's A5 due to B5 probably needing to be slightly clipped (or going for an awkward right-hand middle finger crossover for A5).
Therefore, 4-3-4-3 and 4-3-3-2 for that D6-A5 both seem plausible.
Since Beat 4.0 of the right hand of that measure is assigned the pinky finger, 4-3-4-3 is actually the most plausible fingering of the two options.
I tried it out - I would play 3-2-3-2 on the D-C#-B-A (on the piano, and probably the harpsichord, but not the organ). This naturally enforces a slurring of the notes that I find appropriate - contrary to some traditions, I find that most of Baroque and Classical era music is frequently played far too legato on the piano. Keep in mind that, on the harpsichord which is probably the instrument Bach wrote these suites for (though he would've been fine with playing it on other keyboard instruments as well), fine gradations of loudness are not available and articulation is the primary means of expression.
