XOR is expressible in TypeScript

[kshahkshah]

Describe the bug

// The members data and errors MUST NOT coexist in the same document.
// ⛔️ NOT EXPRESSIBLE IN TYPESCRIPT
// If a document does not contain a top-level data key,
//    the included member MUST NOT be present either.
// ⛔️ NOT EXPRESSIBLE IN TYPESCRIPT

is not true any longer

To Reproduce

const foo: Document = {
  errors: [{
    status: 200
  }],
  data: {
    type: 'foo',
    id: '123',
    attributes: {
      foo: 'bar',
      baz: 'biz'
    }
  },
  included: [],
  meta: {
    foo: 'bar'
  }
}

is valid and should not be

Additional context

type Without<T, U> = { [P in Exclude<keyof T, keyof U>]?: never };
type XOR<T, U> = (T | U) extends object ? (Without<T, U> & U) | (Without<U, T> & T) : T | U;

interface DocWithDataAndOptionalMeta extends DocWithData{
  meta?: object;
}
interface DocWithErrorsAndOptionalMeta extends DocWithErrors {
  meta?: object;
}

type JSONAPIDocument = XOR<DocWithErrorsAndOptionalMeta, DocWithDataAndOptionalMeta>;

I believe gets you what we want. I can open a PR, was just not certain about how to handle meta. It seems fine to have meta included in a top level response w/errors present or with a 'normal' doc

[kshahkshah]

Without and XOR lifted from: microsoft/TypeScript#14094 (comment)

[mike-north]

This is great! I'll need us to do some performance testing since this increases the complexity of all JSONAPI resource types (i.e., "document with data") significantly.