If Stevie Wonder doesn't sing at your birthday party, did you even have a birthday??? That's what some folks in an L.A. hot spot might be thinking after an impromptu performance.

This video's about as cool as it gets ... the legend was having dinner Thursday night with his son and some other family members at Teleferic in Brentwood, when he busted out his version of "Happy Birthday" -- y'know, the cool one.

We're told everyone in the joint stopped what they were doing to watch and listen to Stevie. You'll notice very few people sang along with him ... it's not like anyone was gonna do it better.

Seems SW and co. were celebrating the birthday of his son's mother-in-law.

Coincidentally, someone at a neighboring table also had a bday, and his pals had just finished singing the lame old traditional version.

Talk about one-upping! Careful where and what ya sing in L.A. -- ya never know when an icon's nearby.