In JS, @template with @typedef is not equivalent to a generic type alias

[sandersn]

/**
 * @template T
 * @typedef {[T, T]} TPair
 */
/** @type {TPair<number>} */
var tp = [1,2]

TPair doesn't correctly create a generic type alias.

Expected behavior:

TPair's typedef is equivalent to type TPair<T> = [T, T]

Actual behavior:

TPair's typedef is equivalent to type T; type TPair = [T, T] (where T is unique and inexpressible). So @type {TPair} is legal, but nothing can be assigned to it, and @type {TPair<number>} says that TPair is not generic.

[sandersn]

Note that correctly supporting this may mean breaking old code, since this previously made C generic:

/** @template T
 * @typedef {T} D
 */
class C {
}

And fixing this bug should assign T to the typedef, not the class.

[DanielRosenwasser]

Isn't this a duplicate of #23385 which is supposed to be fixed?

[sandersn]

This is the first repro in #23385, which covered both template tags on classes and on typedefs. Notably, this bug has not come up in real code yet, only in our discussions and webpack’s discussions of style guidelines.

[sandersn]

Basically, there were two bugs to fix, so I went for the one that Sean Larkin (@TheLarkInn) was waiting on. 😸

[TheLarkInn]

Thanks for submitting this, I wasn't 100% sure it was a bug but got distracted. I'm glad I was right so yay we can fix it! 😍😂

[sandersn]

Fix is up at #23947