Type parameter constrained to an array is not indexable by `${number}`

🔎 Search Terms

parameter constraint array index indexable number template literal

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.4.0-dev.20231218#code/CYUwxgNghgTiAEAzArgOzAFwJYHtXwxAGcMBGAHgEF4QAPQ1YI+NAa1RwHdUBtAXQB8AClgwAXPEoBKCZR6pkAWwBGIGHwDcAKFCRYCFOmx4CxDACYqNeiEbM2Hbv2GjZMyTwAGAEgDeClTUAX09NIA

💻 Code

declare function test1<A extends unknown[]>(arr: A): A[number];
declare function test2<A extends unknown[]>(arr: A): A[`${number}`];

🙁 Actual behavior

test2 is an error

🙂 Expected behavior

I expect both to be roughly identical - both should not error here.

Additional information about the issue

I think that this should be OK since https://github.com/microsoft/TypeScript/pull/48837:

• Numeric index signatures apply when the index type is ${number}. For example `Foo[][`${number}`] resolves to Foo instead of being an error. This is consistent with our existing rule that index signatures apply when the index type is a numeric string literal. For example Foo[]['0'] already resolves to Foo.

Reproduce the error using the provided TypeScript Playground link and the test1/test2 declarations. Read the behavior described in PR #48837, then trace template-literal indexed access handling and add a regression test for an array-constrained type parameter. Done means test2 is accepted consistently with test1.