Discriminated Unions with a common partially optional property

[nmalyschkin]

Bug Report

🔎 Search Terms

Discriminated Unions with a common partially optional property

🕗 Version & Regression Information

• I tried different versions of 3 and 4.

⏯ Playground Link

Playground link with relevant code

💻 Code

interface B {
  a: true;
  b: number;
}
interface C {
  a?: false;
  c: number;
}
type Union = B | C;

const test1: Union = {
  a: false,
  c: 5,
};
const test2: Union = {
  c: 4,
};
const test3: Union = {
  a: true,
  b: 4,
};
const test4: Union = {
  b: 4,
  c: 4,
};

🙁 Actual behavior

test4 is displayed as valid.

🙂 Expected behavior

test4 should not be valid.

[MartinJohns]

test4 is valid. All properties that must exist according to the interface are present, and no incompatible properties exist.

[whzx5byb]

It is unrelated to discriminated unions or optional property. A minimal repo:

interface B {
  b: number;
}
interface C {
  c: number;
}
type Union = B | C;

const test1: B = { b: 4, c: 4 }; // Error as expected, because of excess property check.
const test2: C = { b: 4, c: 4 }; // Error as expected, because of excess property check.
const test3: Union = { b: 4, c: 4 }; // No error.

See #20863
And this SO answer provides an workaround.

type UnionKeys<T> = T extends T ? keyof T : never;
type StrictUnionHelper<T, TAll> = T extends any ? T & Partial<Record<Exclude<UnionKeys<TAll>, keyof T>, undefined>> : never;
type StrictUnion<T> = StrictUnionHelper<T, T>

const test4: StrictUnion<B | C> = { b: 4, c: 4 }; // Error as expected.

[nmalyschkin]

@whzx5byb
Thank you, that worked for me!