How to properly type check nested records allowing partial subtrees with TypeScript?

Question

Please have a look at the following demo.
interface Data defines a schema for nested data.
function check shall validate whether a given partial subtree of this Data structure is fine and throw a compile-time error if not (hopefully with a more or less detailed and understandable error message and not just "... is not assignable to type 'never").

interface Data {
  namespace1: {
    keyA: string,
    keyB: string
  },

  namespace2: {
    keyC: string,
    keyD: string
  }
}

// This function's only purpose is to perform a compile-time check
// whether the given partial data is valid.
// Returns the first and only argument in case of success,
// otherwise a compile-time error will occur.
function check<??>(
  partialData: ????
): ?????? {
  return partialData
}

// Example 1 => okay
const validPartialData1 = check({
  namespace1: {
    keyB: 'b'
  }
})

// Example 2 => okay
const validPartialData2 = check({
  namespace1: {
    keyB: 'b'
  },

  namespace2: {
    keyC: 'c'
  }
})

// Example 3 => okay
const validPartialData3 = check({})

// Example 4 => compile-time error!
const invalidPartialData1 = check({
  namespace1: {
    keyC: 'c'
  }
})

// Example 5 => compile-time error!
const invalidPartialData2 = check({
  xyz: {
    keyA: 'a'
  }
})

Answer 1

You don't need the check function. Use optional field directly.

interface Data {
  namespace1?: {
    keyA?: string,
    keyB?: string
  },

  namespace2?: {
    keyC?: string,
    keyD?: string
  }
}

const validPartialData1:Data = {
  namespace1: {
    keyB: 'b'
  }
}

See playground

If you don't want to change Data type. You can define another PartialData

type NestPartial<T> = {
    [P in keyof T]?: NestPartial<T[P]>;
}
type PartialData = NestPartial<Data>

const validPartialData1: PartialData = {
    namespace1: {
        keyB: 'b'
    }
}

See playground