Object spread with ternary causes invalid union on property

[karlhorky]

Bug Report

Originally commented over here: #48326 (comment)

🔎 Search Terms

spread, ternary, object spread, union, property

🕗 Version & Regression Information

• This changed between versions 4.1.5 and 4.2.3

⏯ Playground Link

Playground link with relevant code

💻 Code

function fn(h: { a: string; b?: never; c?: never } | { a: string; b: string; c: string }) {}

let data = 'a' as 'a' | undefined;

// ✅ This works in TS
fn(data ? { a: '', b: '', c: data } : { a: '' });

// ❌ TS cannot figure this option out, even though it's also valid
fn({
  a: '',
  ...(data ? { b: '', c: data } : {}),
});

// ✅ This works in TS
fn({
  a: '',
  ...(data ? { b: '', c: data } : {} as {[key: string]: never}),
});

🙁 Actual behavior

b was recognized to be a string | undefined, although it's only possible to be a string

🙂 Expected behavior

b should only be a string

[RyanCavanaugh]

We had to change this behavior in 4.2 to avoid an exponential explosion caused by repeated spreads in the same object.

[RyanCavanaugh]

Thread with more context

[karlhorky]

Ok thanks, closing :)