The "External Unused" rule lacks complex examples

[glen-84]

There are two examples of @provides usage that are not covered by the spec for the "External Unused" rule:

Example 1

Sub-selections:

type Review {
  id: ID!
  product: Product @provides(fields: "sku variation { size }")
}

type Product @key(fields: "sku variation { id }") {
  sku: String! @external
  variation: ProductVariation!
  name: String!
}

type ProductVariation {
  id: String!
  size: String! @external
}

The spec algorithm fails here, since it expects a @provides directive on a field returning ProductVariation.

Example 2

Fragments:

type Review {
  id: ID!
  # The @provides directive tells us that this source schema can supply different
  # fields depending on which concrete type of Product is returned.
  product: Product
    @provides(
      fields: """
      ... on Book { author }
      ... on Clothing { size }
      """
    )
}

interface Product @key(fields: "id") {
  id: ID!
}

type Book implements Product {
  id: ID!
  title: String!
  author: String! @external
}

type Clothing implements Product {
  id: ID!
  name: String!
  size: String! @external
}

type Query {
  reviews: [Review!]!
}

The spec doesn't include an example like this, which might be useful, even though the algorithm doesn't really define how "references field in @provides" is actually implemented.