Expose "set object key" in a utility type?

[mkscrg]

Flow understands when a new key is added to an object:

const o = { foo: 'hello' };
o.bar = 42;
console.log(o.bar); // all good! Flow knows about the key `bar`

But (AFAIK) it's impossible to write a well-typed generic function for setting a key. It might work if we exposed a $SetKey utility:

function setProp<O: {}, K: string, V>(o: O, k: K, v: V): $SetKey<O, K, V> {
  return { ...o, [k]: v };
}

Thoughts? Am I missing something, and this is actually possible today?

Background

I'd like to define types for a binary parsing library with syntax like

const p = new Parser()
  .int16("id")
  .nullTermString("name");

const b = Buffer.from([ 0x04, 0xD2, 0x46, 0x6C, 0x6F, 0x77, 0x00 ]);
console.log(p.parse(b)); // { id: 1234, name: 'Flow' }

But this requires using string literals as keys in object types, e.g.

declare class Parser<T: {}> {
  int16<K: string>(k: K): Parser<$SetKey<T, K, number>>;
  nullTermString<K: string>(k: K): Parser<$SetKey<T, K, string>>;
}

[madbence]

@mkscrg O & { [K]: V } should work, but right now K is always string, even if you pass a string literal to setProp, so it becomes O & { [string: V } which is not very useful 😿

maybe (?) see #2639 #4175 #4748