Shimano GRX ST-RX400 shifter (10 speed) compatible with XT RD-M786 (10 speed) Shadow+ rear derailleur

Is the new Shimano GRX ST-RX400 shifter (10 speed) compatible with the old mountain XT RD-M786 (10 speed) Shadow+ rear derailleur?

No, XT 786 uses the 10 speed DynaSys pull ratio and is not compatible with any Shimano road shifters.

Thanks!

No but you can still use an 11 speed 1x derailleur with a Wolftooth Tanpan and it should shift fine with a 10 speed setup. Assuming the goal is to shift a larger cassette than what shimano advertises for the new GRX stuff. 

Thanks DarKris, yes it is to put a larger rear cassette than what GRX is rated for.

What I have discovered is that all 3 GRX groups (RX400, RX600, and RX810) are all based on the Shimano "road" 11 speed derailleurs (just the RX400 is 10 speed but with a pull ratio like Tiagra 4700 which is different than other 10 speed Shimano). 

See: https://productinfo.shimano.com/#/com?acid=C-441&cid=C-432

Also, a RX400 rear derailleur can also be used on any Shimano 11 speed road shifter.