11spd derailleur with 10spd cassette and shifter?

I can't remember. There is a chart online of pull ratios that covers it. One way 10-11 or 11-10 needed a small shim under the derailleur clamp to make up the difference but then it worked fine. 

Works fine for me. Deore 10 speed shifter, Deore 11 speed mech and Microshift 10 speed cassette

Yep, I run 11-speed SLX mechs with both 10-speed Saint and SLX shifters, all good.

Yup, XT 11 spd mech with Saint shifter and a 10spd cheapo Shimano cassette here.

Shimano Deore M5100 11sp mech, 11-48 Microshift 10sp cassette and 10sp XT shifter on one of my bikes.

Some of the older mechs and largest tooth count cassettes (over 42t) don't play as nice!

Zee shifter, microshift 11-48 cassette Deore 5100 mech here... Works a treat, cost peanuts. 

Also go the same mech and cassette operated by an old 11 speed gripshift on my HT, that might be a step too far for some though... 

This thread is making me wish I'd kept my old XTR 11sp shifters, to pair up with those Microshift 11-48 cassettes.

And if 11sp Shimano mechs work with 10sp, then 12sp ones must too (as they work with 11sp shifters)?

Hmm, I must be wrong then or thinking about a different mismatched set up. Thought it was a small difference between the shift ratios and a cog pitch difference of c0.2mm, so if it's right in the middle of the cassette, then it's 1mm out at either end. Maybe that's not enough to be noticeable then. Or the slight difference in shift ratio accounts for that...?

And if 11sp Shimano mechs work with 10sp, then 12sp ones must too (as they work with 11sp shifters)?

Apparently so. Same cable pull since dynasis. 

Not that Shimano publish any actual numbers on cable pull or cassette spacing. 

Can confirm.. shimano deore 12 speed Mech also works fine with shimano 10-speed 11-46 speed cassette.. and 10 speed shifter..

i dont think i would want to go smaller than 45-46 on the largest cog though with a normal 12 speed mech..

the b screw / top jockey wheel is at about its limit with the 46..

(the normal 12 speed mechs are designed around a 51T cassettes cog... so smaller than 45-46 will mean the jockey wheel sits a bit too far away from the cassette and shifting isnt then quite as good in the lower gears..)

I’ve had a SRAM Rival 10 speed brifter work fine with a Rival 11 speed mech and SRAM 11-36 10 speed cassette. 

IME these are all OK:

SRAM 11 speed shifter, 10 speed cassette, 10 speed shimano mech

SRAM 11 speed shifter, 10 speed cassette, 11 speed shimano mech

Shimano 10 speed shifter, 10 speed cassette, 11 speed shimano mech

SRAM 11 speed shifter, 10 speed cassette, 12 speed shimano mech

SRAM 11 speed shifter, 11 speed cassette (42t max), 10 speed shimano mech

SRAM 11 speed shifter, 11 speed cassette, 12 speed shimano mech

These are all MTB shifters and mechs.