interface X {
    a: string;
    b: string;
}

declare function foo<T = X>(x: keyof T, y: keyof T): T;
declare function bar<T>(x: keyof T, y: keyof T): T;

const a = foo<X>('a', 'b'); // compiles cleanly
const b = foo('a', 'b');    // also clean
const c = bar('a', 'b');    // still clean