Mel Gibson -- Mexican Dinner Date with World's Richest Spawn

Mel Gibson went to dinner with the son of the RICHEST MAN IN THE WORLD last night ... in Mexico ... and no one was abducted. Hooray!

Gibson's dinner date was Carlos Slim Domit, eldest son of Carlos Slim Helu ... whose reported net worth is $73 BILLION. (Bill Gates is a close second with $67 billion).

Mel and Carlos Jr. were photographed on their way out of a fancy restaurant called Casa Anis in an upscale neighborhood of Mexico City called Polanco.

It's unclear how they know each other or what they talked about at dinner -- but we hear the restaurant has great crab cakes. So, there's that ...