"Index signature missing" error when using Interface vs Inline Type Definition

[brwarner]

Bug Report

🕗 Version & Regression Information

Typescript version 4.4.4 (also tested on the nightly v4.6.0-dev.20211105)

⏯ Playground Link

Playground link with relevant code

💻 Code

declare const $CombinedState: unique symbol

// Declare combined state as an union of a symbol index and the template parameter S
type CombinedStateWorks<S> = {
    readonly [$CombinedState]?: undefined
} & S;

// Try to assign a combined state to a Record<string, unknown> ... success! no compile errors
{
  const combined: CombinedStateWorks<{a: number}> = { a: 2 };
  const record: Record<string, unknown> = combined;
}

// Declare combined state as an union of the interface "EmptyObject" (identical to the inline type above) and the template parameter S
interface EmptyObject {
    readonly [$CombinedState]?: undefined
}
type CombinedStateFails<S> = EmptyObject & S;

// Try to assign a combined state to a Record<string, unknown> ... fail!
{
  const combined: CombinedStateFails<{a: number}> = { a: 2 };
  // ERROR RIGHT HERE
  const record: Record<string, unknown> = combined;
}

🙁 Actual behavior

Despite CombinedStateFails and CombinedStateWorks being the same type specification (except one uses an interface, one an inline type definition), one fails to case to a Record<string, unknown> and the other succeeds.

🙂 Expected behavior

Both cases should have the same compilation result (success).

This is a problem because recent releases of Redux have refactored their CombinedState type from the first example here to the second, breaking some of my code. I'm not sure what type besides Record<string, unknown> I can use as my parameter type now.

[MartinJohns]

type and interface are not interchangeably. This is working as intended. See #42825 (comment).

[brwarner]

Thanks. Guess I'll need to find a different way to do this.