Interface extension with omit produce incoherent results: Omit<C | D, 'a'> !== Omit<C, 'a'> | Omit<D, 'a'>

Bug Report

🔎 Search Terms

wrong set theory omit

🕗 Version & Regression Information

ts-node-dev ver. 1.1.1 (using ts-node ver. 9.1.1, typescript ver. 4.1.3)

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about it
  • ts-node-dev ver. 1.1.1 (using ts-node ver. 9.1.1, typescript ver. 4.1.3)
  • Nightly

⏯ Playground Link

Playground link with relevant code

💻 Code

interface Basic {
  a: string;
  b: number;
}

interface BasicPlusC extends Basic {
  c: string;
}

interface BasicPlusD extends Basic {
  d: string;
}

type BasicPlusCOrD = BasicPlusC | BasicPlusD;

type BasicPlusCOrDMinusA = Omit<BasicPlusCOrD, 'a'>;

const getUnknown = (): BasicPlusCOrDMinusA => (
  Math.random() > .5 ?
    { b: 0, c: ""} // <-- Causes error
  : { b: 0, d: ""}
);

const getUnknownBis = (): Omit<BasicPlusC, 'a'> | Omit<BasicPlusD, 'a'> => (
  Math.random() > .5 ?
    { b: 0, c: ""}
  : { b: 0, d: ""}
);

const main = (): void => {
  getUnknown();
  getUnknownBis();
};

main();

🙁 Actual behavior

Running this code causes the following error:

[INFO] 21:02:26 ts-node-dev ver. 1.1.1 (using ts-node ver. 9.1.1, typescript ver. 4.1.3)
Compilation error in /home/fabio/Projects/aalto/fullstack_open_2020_part9/part_c_patientor/backend/src/delete_me.ts
[ERROR] 21:02:27 ⨯ Unable to compile TypeScript:
src/delete_me.ts:20:13 - error TS2322: Type '{ b: number; c: string; } | { b: number; d: string; }' is not assignable to type 'Pick<BasicPlusCOrD, "b">'.
  Type '{ b: number; c: string; }' is not assignable to type 'Pick<BasicPlusCOrD, "b">'.
    Object literal may only specify known properties, and 'c' does not exist in type 'Pick<BasicPlusCOrD, "b">'.

20     { b: 0, c: ""} // <-- Causes error
               ~~~~~

🙂 Expected behavior

I would have expected both functions, getUnknown and getUnknownBis to work correctly. This is because I'm expecting the typing system to follow the same rules as set theory. In this case I have one function, getUnknown, whose return type structure is ({ a, b, c } | ({ a, b, d, }) \ { a }) which means that the resulting structure should be in the form { b, c } | { b, d }.

In the other function, getUnknownBis, the return type structure is ({ a, b, c } \ { a }) | ({ a, b, d, }) \ { a }) which means that the resulting structure should be again in the form { b, c } | { b, d }.

Hence I would expect both to work in the same way, but it looks like the first function is only expecting to return { b }, instead of { b, c } | { b, d }. This makes me think that the type inference engine is doing { b, c } & { b, d } instead.

リンクされている TypeScript Playground と最小限の union/Omit の例から始め、入力が union の場合に型チェッカーが Omit をどのように評価するかを調査し、明示的な分配形式と比較します。2 つの戻り値の型の例が同等に動作し、報告された代入に対するリグレッションカバレッジがあれば完了です。