How do you add validation (constraints?) on a paragraph? Compare two fields How does one go about adding validation (contraints?) on a paragraph?

I need to compare two entity reference fields on submission. If one field has a certain entity referenced, the other cannot have certain entity references.

I'd be interested in restricting the autocomplete options as well, but that simply won't be enough as I need to ensure that they user doesn't manually enter another entity value.
You simply inherit from the the default EntityReferenceWidget and you will call it like MyParagraphEntityReference widget. You can here both change the way how the autocomplation works and also how you validate the entered data. 

This is page may help you.