ts reset for .filter is not working in all cases

[xiduzo]

I am not sure if any of this would even be possible.

Given the following array

const myArray = [1, undefined, 10, undefined, undefined, 0]
          // ^? (number | undefined)[]

I can do the following filter using Boolean

const myFilteredArray = myArray.filter(Boolean)
          // ^? number[]

But when filtering using any other method it does not give back the correct type

const myFilteredArray = myArray.filter(Number.isNaN)
          // ^? (number | undefined)[]

// Below might be impossible, but would be sick if ts can do this
function myCustomFilter(input?: number) {
   return input !== undefined
}

const myFilteredArray = myArray.filter(myCustomFilter)
          // ^? (number | undefined)[]

[mattpocock]

This will be fixed upstream in TS 5.5