checkJs can't correctly determine type of generic function parameter

TypeScript Version: 3.4.5

jsdoc
checkJs
template
function
parameter

/**
 * @template T
 * @param {(left: T, right: T) => boolean} equals
 * @returns {(xs: T[]) => T[]}
 */
const uniq = equals => xs => {
	/** @type {T[]} */
	const seen = []
	const out = []
	const len = xs.length
	let j = 0
	for(let i = 0; i < len; i++) {
		const item = xs[i]		
		if(!seen.some(s => equals(s, item))) {			
			seen.push(item)
			out[j++] = item
	   }
	}

	return out;
}

uniq((x, y) => x == y)([1, 3, 3, 7])

Expected behavior:
Typechecks fine

Actual behavior:
Type 'number' is not assignable to type 'T

Playground Link:
https://www.typescriptlang.org/play/#src=%2F**%0D%0A%20*%20%40template%20T%0D%0A%20*%20%40param%20%7B(left%3A%20T%2C%20right%3A%20T)%20%3D%3E%20boolean%7D%20equals%0D%0A%20*%20%40returns%20%7B(xs%3A%20T%5B%5D)%20%3D%3E%20T%5B%5D%7D%0D%0A%20*%2F%0D%0Aconst%20uniq%20%3D%20equals%20%3D%3E%20xs%20%3D%3E%20%7B%0D%0A%09%2F**%20%40type%20%7BT%5B%5D%7D%20*%2F%0D%0A%09const%20seen%20%3D%20%5B%5D%0D%0A%09const%20out%20%3D%20%5B%5D%0D%0A%09const%20len%20%3D%20xs.length%0D%0A%09let%20j%20%3D%200%0D%0A%09for(let%20i%20%3D%200%3B%20i%20%3C%20len%3B%20i%2B%2B)%20%7B%0D%0A%09%09const%20item%20%3D%20xs%5Bi%5D%09%09%0D%0A%09%09if(!seen.some(s%20%3D%3E%20equals(s%2C%20item)))%20%7B%09%09%09%0D%0A%09%09%09seen.push(item)%0D%0A%09%09%09out%5Bj%2B%2B%5D%20%3D%20item%0D%0A%09%20%20%20%7D%0D%0A%09%7D%0D%0A%0D%0A%09return%20out%3B%0D%0A%7D%0D%0A%0D%0Auniq((x%2C%20y)%20%3D%3E%20x%20%3D%3D%20y)(%5B1%2C%203%2C%203%2C%207%5D)

Related Issues:
This seems related, but I don't think it's it

https://github.com/Microsoft/TypeScript/issues/26883