import * as React from 'react'

interface A {
    b: string
}

interface B extends A {
    c: string
}

interface Props {
    cb(val: A): void
}

const Comp: React.SFC<Props> = () => null

function cb(val: B) {
    console.log(val.c)
}

const a = <Comp cb={cb}/> // no error

let f: Props['cb'] 
let f2: (val: A) => void

f = cb // no error

f2 = cb // error