[Bug] GraphQL server returns interface type for __typename when UseFieldResolvers is enabled

[magicoder10]

Input

Schema

interface Animal {
    name: String!
}

type Dog implements Animal {
    name: String!
    breed: String!
}

type Cat implements Animal {
    name: String!
    livesLeft: Int!
}

type Query {
    animals: [Animal!]!
}

Query

query {
  animals {
    name
    __typename
  }
}

Output

Without UseFieldResolvers

schema := graphql.MustParseSchema(schemaString, &query{})
	http.Handle("/graphql", &relay.Handler{Schema: schema})
	log.Fatal(http.ListenAndServe(":8080", nil))

{
  "data": {
    "animals": [
      {
        "name": "Buddy",
        "__typename": "Dog"
      },
      {
        "name": "Whiskers",
        "__typename": "Cat"
      }
    ]
  }
}

With UseFieldResolvers

schema := graphql.MustParseSchema(schemaString, &query{}, graphql.UseFieldResolvers())
	http.Handle("/graphql", &relay.Handler{Schema: schema})
	log.Fatal(http.ListenAndServe(":8080", nil))

{
  "data": {
    "animals": [
      {
        "name": "Buddy",
        "__typename": "Animal"
      },
      {
        "name": "Whiskers",
        "__typename": "Animal"
      }
    ]
  }
}

This bug leads to

{
  "errors": [
    {
      "message": "panic occurred: {{\"Dog\" {'\\x05' '\\f'}}} does not implement \"Animal\""
    }
  ]
}

[pavelnikolov]

@magicoder10 thank you for reporting this. Can I, please, have the code for the resolver as well?