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Literals that are spread into a const rest are not const-ified #53177
Description
Bug Report
🔎 Search Terms
spread rest literal expression string array const modifier
🕗 Version & Regression Information
- This is the behavior in every version I tried (only relevant to 5.0+)
⏯ Playground Link
Playground link with relevant code
💻 Code
declare function foo<const T extends readonly unknown[]>(...args: T): T;
const res1 = foo(...[1, 'bar']);
// ^? readonly [number, string]
const res2 = foo(..."hello");
// ^? readonly string[]🙁 Actual behavior
When spreading an array expression its elements are not acting like they are in the const context. When spreading a string it's not even inferred as a tuple of strings.
🙂 Expected behavior
I'm not exactly sure what is the expected result here - I can see the argument going both ways. I just think that perhaps this is a little bit underspecified today and that this issue, at the very least, might help to specify the desired behavior for those cases.
Activity
RyanCavanaugh commentedon Mar 9, 2023
Anders Hejlsberg (@ahejlsberg) thoughts?
ahejlsberg commentedon Mar 9, 2023
This is working as intended. Spread elements intentionally don't have contextual types, so the const-ness isn't propagated to the tuple type. It would add a good deal of complexity to compute contextual types for spread expressions, and in many cases it just isn't possible. Furthermore, in real world code, spread expressions are pretty much never literals as that's just a more inconvenient way of writing the elements directly inline.
Andarist commentedon Mar 9, 2023
Thanks for the answer! It’s pretty much what i expected but was a little but unsure about.