typescript-inferring.ts

class A { a: "a" = "a"; }
class B { b: "b" = "b"; }

interface X {
    a: Y<A>;
    b: Y<B>;
}
interface Y<T> { t: T; }
type Z<T> = { [key in "a"]: Y<T> };

function w<T>(z: Z<T>): T {
    return z.a.t;
}

const z1: X = null as any;

// The type of z2 should be inferred to A (but not).
const z2 = w(z1); // z2: any
const z3 = w<A>(z1); // z3: A