interface A {
    a: string
    b: string
}

interface B {
    a: string
    b: string
    c: string
}

function foo(fn: (a: A) => void, a: A) {
    fn(a)
}

function bar(arg: B): void {
    console.log(arg.c)
}

foo(bar, {a: 'a', b: 'b'}) // This works, unfortunately

function baz(arg: string) { }

foo(baz, {a: 'a', b: 'b'}) // This fails correctly