Type-safe updaters: non-assignability of unions

[Malien]

I seem to have encountered an issue with the new typesafe updaters. Here's the basic repro: https://github.com/Malien/typesafe-relay-updaters-repro

Obligatory "using typescript, not flow" mention

Assume we have a simple Todo app with todo groupings. Like this:

• Make coffee
• Touch grass
• Buy groceries
  • Tomatoes
  • Bread
• Work out

Let us have the following gql schema:

type Query {
    todos: [Todo!]!
}

interface Node {
    id: ID!
}

union Todo = TodoItem | TodoGroup

type TodoItem implements Node {
    id: ID!
    title: String!
    completed: Boolean!
}

type TodoGroup implements Node {
    id: ID!
    title: String!
    todos: [TodoItem!]!
}

type Mutation {
  createTodo(title: String!): TodoItem!
}

fragment TodoList_assignable_todo on Todo @assignable {
  __typename
}

given the query:

query TodoListQuery {
  todos {
    ...Todo_todo # Some component that renders the todo items/groups
    ...TodoList_assignable_todo
  }
}

and the mutation

mutation TodoListAddTodoMutation($title: String!) {
  createTodo(title: $title) {
    ...Todo_todo
    ...TodoList_assignable_todo
  }
}

the update of

function updater(store, response) {
  const newTodo = response?.createTodo;
  if (!newTodo) return;

  const { updatableData } = store.readUpdatableQuery<TodoListUpdateQuery>(
    graphql`
      query TodoListUpdateQuery @updatable {
        todos {
          ...TodoList_assignable_todo
        }
      }
    `,
    {},
  );

  updatableData.todos = [...todos, newTodo];
}

fails to typecheck, since the generated types from TodoListAddTodoMutation is:

export type TodoListAddTodoMutation$data = {
  readonly createTodo: {
    readonly __id: string;
    readonly __isTodoList_assignable_todo?: "TodoItem";
    readonly " $fragmentSpreads": FragmentRefs<"TodoList_assignable_todo" | "Todo_todo">;
  };
};

__isTodoList_assignable_todo is optional.

This seems to be related to the sections of the docs "where is guaranteed to implement an interface"/"where is not guaranteed to implement an interface", yet this seems like a missed case.

TodoItem (the return type of createTodo) is guaranteed to """implement""" union Todo. Yet this requires additional type-guard function (aka. validators). In the end the new typesafe updaters code is a lot more cumbersome to deal with, in comparison to the old unsafe variant. Not to mention all of the new concepts of assignability and updatability being thrown back at you.

[Malien]

I'm exploring possibilities of opening a PR. Would need some time to familiarise myself with the compiler. Luckily compilers and Rust are not foreign to me