Demi Lovato, Wilmer Valderrama Go on Lunch Date

Demi Lovato has what appears to be an unbreakable bond with former bf Wilmer Valderrama, because they were hanging together over the weekend.

We got this pic of Demi and Wilmer chowing down for a late lunch Saturday in the San Fernando Valley at Sol Y Luna.

As you know ... the 2 dated for 6 years but broke up in June, 2016.

It's interesting ... Demi released a documentary last October called "Simply Complicated," which talked about the breakup and wondered if she made the right decision. She said her heart is always with him.

Eyewitnesses said they were super friendly with each other, laughing as they ate guacamole which was made for them at the table.