T-Pain says Usher summoned him some years back just to casually let him know he had torpedoed the music biz by popularizing autotune ... which T says sent him into depression.

The 'Buy U a Drank' singer recently told the tale on Netflix's new docuseries 'This Is Pop' -- detailing what he claims was a seriously cold-ass encounter in 2013. Watch how T-Pain describes the alleged put-down ... apparently, they were both flying in the same airplane on the way to an award show, when T says he was called to Mr. Raymond's seat. They exchanged pleasantries, and then T-Pain says Usher laid in.

According to T, Usher believed T-Pain had tainted professional singing for guys (like Usher) who use their natural voice, and he supposedly said it with a straight face.

T-Pain says he was shocked and that, before he knew it, he realized he'd fallen into a depression, which he says lasted 4 years -- something he seems to be attributing to the Usher moment itself. It's a wild story.

Of course, people are jumping to T-Pain's defense in light of this, assuring him he not only did NOT ruin music, but might've actually revolutionized it ... considering every other famous rapper/singer these days uses some form of autotune.

And, for argument's sake ... even if you agree with Usher's sentiment, why break it to someone in that manner, and so randomly??? Seems like a jerk move -- on its face, anyway. We'll see if Usher offers a different version of the story.