Better type inference of tagged union predicate functions

[ccorcos]

TypeScript Version: 2.4.0 / nightly (2.5.0-dev.201xxxxx)

Code

type X = {t: "x"}
type Y = {t: "y"}
type Z = X | Y

let a: Array<Z>

function predicate(x: Z) {
  return x.t === "x"
}

const result =  a.find(predicate)

Expected behavior:
I want result to have a type of X. Which means I want the predicate function to have a return type of x is X.

Actual behavior:
The result of the predicate is of type Z and result is of type Z

[mhegazy]

Please see #5101 (comment)

[ccorcos]

Hmm. so I need to overload the .find function? Or should I be doing something like this?

function predicate<T extends Z>(x: Z): x is T {
  return x.t === "x"
}

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.