Well, I intend to use an SRAM X01 Eagle (12speed) rear derailleur in a 1x11 transmission (with a SRAM X01 "non-Eagle" 11speed trigger, cassette SunRace 11s 11-46, SRAM PC XX1 chain). Will it work perfectly?
Both the SRAM 1x11 and 1x12 drivetrains use the SRAM X-ACTUATION™ technology. This means they both have the same cable pull ratio. I have not tried this but the derailleurs should be interchangeable - keeping in mind the Eagle derailleur will be better suited for larger range cassettes.
Edit: Here is someone who is using an XX1 shifter with an Eagle derailleur: https://endless-sphere.com/forums/viewtopic.php?t=88323
11-50T 11 cog Garbaruk cassette
X01 Eagle Rear Derailleur 12 gear
XX1 X-ACTUATION Trigger Shifter 11 gear
The SRAM 12 speed X01 derailleur has the same cable pull ratio as the SRAM 11 speed XX1 derailleur. This means you can interchange the X01 and XX1 derailleurs between the 11 and 12 gear drivetrains.
My guess on this setup proved right and I get excellent shifting with no backpedal chain drops.
To quote Art's Cyclery:
"Eagle drivetrains are not backwards compatible in any way (the exception being SRAM’s direct mount cranksets, which will still require a new Eagle chainring)"
"SRAM will only make the Eagle chainring in the direct-mount style at first, and we were told it's the only Eagle component compatible with their 1x11 drivetrains. No 11-speed products will work on Eagle."