Inconsistent Behavior with readonly in Template Literal Indexed Access Types

[13OnTheCode]

🔎 Search Terms

"readonly" "template literal" "index signatures"

🕗 Version & Regression Information

v5.4.3

⏯ Playground Link

https://www.typescriptlang.org/play?#code/C4TwDgpgBAogHgQwLZgDbQLxQN4CgpQDacAXFAAYAk2AzsAE4CWAdgOYC+AtMAo6uQF0ydJm3xFSFABYQEAE07URLDoOEMV4+rLkB7ZqhASy5GfMW0NbLjz5qoysQW3z9h4xREXHqoQ6usuOy4uHIQAMaoCNpQ4fp0UMAQdGTwyGgQIUl0hADkXlIIhbkCUFi5AIwATADMuVnJwHlmCoWF3LyoJWVQlbX1uKCQUAAqFT0ACozhANYAPGko6AA0UDMQILoAZrCISxAAfLgA9McEUAB6APy4QA

💻 Code

type Example = {
  [x: `${string}-tail`]: string
  [x: `head-${string}`]: string
  readonly [x: `head-${string}-tail`]: string
  readonly [x: `str-${string}`]: string
}

declare const test: Example

test['str-haha'] = '123' // Expected: error , Actual: error

test['head-haha-tail'] = '123' // Expected: error , Actual: no error

type T1 = Pick<Example, keyof Example>
// {
//    [x: `${string}-tail`]: string;
//    [x: `head-${string}`]: string;
//    [x: `head-${string}-tail`]: string;
//    readonly [x: `str-${string}`]: string;
// }

🙁 Actual behavior

I have encountered an inconsistent behavior when using readonly with template literal indexed access types in TypeScript. Specifically, when a string template index signature extends another string template index signature, attempts to mark it as readonly seem to be ignored in the resulting type when using Pick<Type, keyof Type>.

🙂 Expected behavior

The keyword readonly should be retained, or an error prompt should be given when adding it at the beginning.

Additional information about the issue

No response

[ahejlsberg]

This is working as intended. We consider an element access to be read-only when all applicable index signatures have a readonly modifier. In the example, the string "head-haha-tail" matches three index signatures, two of which are not read-only. Therefore, the assignment is permitted. A similar thing happens when resolving the properties of the Pick<Example, keyof Example> mapped type. For each generated index signature we query the combined read-only-ness of applicable index signatures in the source type, and `head-${string}-tail` matches three, two of which aren't read-only.

[13OnTheCode]

@ahejlsberg I appreciate your response.

Sorry I didn't make it clear before, I understand that the behavior is working as expected, but my confusion is:

The string index head-${string}-tail matches three index signatures, two of which are not read-only. So, when we try to add readonly to head-${string}-tail only, there should be an error prompt, but the actual situation seems to be correct.

[typescript-bot]

This issue has been marked as "Working as Intended" and has seen no recent activity. It has been automatically closed for house-keeping purposes.