In JS, @type tag doesn't apply generic type to initialiser

[awerlogus]

TypeScript Version: 4.2.0-dev.20201219

Search Terms: template type lost js

Code

/** @template T @typedef {<T1 extends T>(data: T1) => T1} Test */

/** @type {Test<number>} */
const test = data => data

Expected behavior:
data has type T1 extends number

Actual behavior:
data has any type

[sandersn]

Note that if you call the function, the type, and the errors are correct:

test(1) // T1=1
test('hi') // error, and T1=number (the default)

It's just not applying the type to the initialiser. Might be a bug in contextual typing?

[sandersn]

This behaviour is the intended effect of #26236, which says it's trying to avoid circular return type errors. But the bug it fixes only mentions typescript.

I'm going to turn off the erasure to any and see what tests fail.

[sandersn]

No tests fail; #26236 never tested the type tag erasure change and it doesn't look correct to me. I'll remove it and send a PR.