narrowing doesn't work when spreading from ternary operator

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

Start with the linked TypeScript Playground and the minimal Foo|Bar example. Trace how the type checker handles narrowing inside a ternary expression used in an object spread. Done means the example type-checks without error while preserving correct checking for incompatible spread branches.