graphql-code-generator: Generated typescript recursive query types are wrong

Describe the bug

Because codegen relies heavily on Intersection types, typescript does not correctly check query fields which are optional, and allows nulls to go unchecked in code.

An example query:

type Query {
    categories(id: ID!): [Category!]!
}

type Category {
    id: ID!
    name: String!
    children: [Category!]
}

query GetCategories {
    categories(id: 1) {
        id
        name
        children {
            id
            name
            children {
                id
                name
            }
        }
    }
}

To Reproduce

Use the code sandbox:

https://codesandbox.io/s/graphql-codegen-issue-template-563vy

yarn generate will generate the query types for the nested query on Category

test.ts shows how these types provide no protection for null fields in typescript.

We generate some query response data which is a valid structure for the GetCategoriesQuery and then use that data in a function which uses the Category type as if it’s children are not null (should throw an error)

yarn build will show how no type errors are generated from the types.

yarn test will show how a runtime error exists when the .length property is queried on the children field.

Expected result

GetCategoriesQuery should not be compatible with Category as children is not expected to be a null type in Category.

Environment:

• OS: MacOS
• @graphql-codegen/...@1.7.0:
• NodeJS: 10

Additional context

Typescript has issues with intersections being tracked https://github.com/microsoft/TypeScript/issues/33243. Using them for generating these types seems to be the issue here, as manually generating the type without intersections correctly picks up the errors.