Type Guards are lost when using currying libraries

[sbking]

TypeScript Version: 2.3.4

TL;DR: When a type guard function is passed to a higher order function with generics, the type guard information is lost and simply turned into boolean.

Code

import { curry } from 'lodash'
// import { curry } from 'ramda' // same behavior

interface Validator<T, V extends T> { (input: T): input is V }
interface Car { speed?: number }
interface FastCar extends Car { speed: number }

const isFastCar = curry(function (
  minSpeed: number, // some configuration setting that needs to be injected
  car: Car
): car is FastCar {
  return car.speed !== undefined && car.speed >= minSpeed
})

function filter<T, V extends T> (validate: Validator<T, V>, list: T[]): V[] {
  const results: V[] = []
  for (const item of list) {
    if (validate(item)) results.push(item)
  }
  return results
}

const cars: Car[] = [{ speed: 30 }, {}, { speed: 60 }, { speed: 45 }, { speed: 0 }, { speed: 75 }]
const fastCars: FastCar[] = filter<Car, FastCar>(isFastCar(55), cars)

Expected behavior:
The isFastCar type guard information (car is FastCar) should be preserved in the curried function, as if the isFastCar function had been manually curried:

const isFastCar = (minSpeed: number) => (car: Car): car is FastCar => {
  return car.speed !== undefined && car.speed >= minSpeed
}

fastCars would end up being [{ speed: 60 }, { speed: 75 }]

Actual behavior:
Error: Argument of type 'CurriedFunction1<Car, boolean> is not assignable to parameter of type 'Validator<Car, FastCar>'. Signature '(): CurriedFunction1<Car, boolean>' must have a type predicate.

The answer may simply be "just manually curry instead of using a library", but in some real cases with longer type assertions and more arguments, manually currying like this becomes unwieldy, and it becomes impossible to supply multiple arguments or use the currying placeholder features that libraries like ramda and lodash provide.

[mhegazy]

The type guard is not maintained by curry. I think you are looking for #10734

[typescript-bot]

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