Describe the Bug

Codegen chokes on FragmentSpread operators in the wrong locations. e.g.:

fragment PartialBlogPost on BlogPost {
  title
}

query OperationName {
  interface {
    id
    ... on BlogPost {
      ...PartialBlogPost
    }
    ... on Image {
      url
    }
  }
}

hits an AssertionError 😢

We hjt the same error in the following special case:

fragment AnimalProjection on Animal {
  age
}

query OperationName {
  __typename
  union {
    ... on Animal {
      ...AnimalProjection
    }
    ... on Person {
      name
    }
  }
}

At least for python, the rule has typically been that fragments (when included via a Spread operator) become a shared class so the client doesn't generate multiple versions of this same object. So there is a special case in the above where if we have a FragmentSpread with no common fields (other than __typename), then we probably should continue to re-use the existing type.

System Information

• Operating system: Linux (Ubuntu)
• Strawberry version (if applicable): HEAD -- in the unit-test suite.

Additional Context