Abstract Spreads in Abstract Scope

Union or interfaces fragments can be used within each other. As long as there exists at least_one_object type that exists in the intersection of the possible types of the scope and the spread, the spread is considered valid.

So for example

fragment unionWithInterface on Pet {
  ...dogOrHumanFragment
}

fragment dogOrHumanFragment on DogOrHuman {
  ... on Dog {
    barkVolume
  }
}

is consider valid becauseDogimplements interfacePetand is a member ofDogOrHuman.

However

fragment nonIntersectingInterfaces on Pet {
  ...sentientFragment
}

fragment sentientFragment on Sentient {
  name
}

is not valid because there exists no type that implements bothPetandSentient.