narrowing doesn't work when spreading from ternary operator

[DetachHead]

Bug Report

🔎 Search Terms

narrow spread ternary

🕗 Version & Regression Information

v4.5.0-dev.20210907

⏯ Playground Link

Playground link with relevant code

💻 Code

interface Foo {
  type: "foo"
  foo: string
}

interface Bar {
  type: "bar"
  bar: number
}

declare const thing: Foo|Bar

const otherThing: Foo|Bar = { //error: Type '"foo" | "bar"' is not assignable to type '"foo"'.
  type: thing.type,
  ...(thing.type === "foo" ? { foo: thing.foo } : { bar: thing.bar })
}

🙁 Actual behavior

error: Type '"foo" | "bar"' is not assignable to type '"foo"'.

🙂 Expected behavior

no error, since it's not possible for the object to not match either the Foo or Bar types

[MartinJohns]

Related: #19513

[andrewbranch]

Reminds me a bit of #30581, but more difficult to tell that we’re in that kind of scenario. It seems extremely difficult for us to do better here.