export type Assign<T, U> = Omit<T, keyof U> & U;

class Foo<T> {
    constructor(public t: T) {}

    assign(u: {x: 1}): Foo<Assign<T, {x: 1}>> {
        const v: Assign<T, {x: 1}> = Object.assign({}, this.t, u);
        return new Foo(v);
        // ^?
        // Type 'Foo<Assign<Assign<T, { x: 1; }>, { x: 1; }>>' is not assignable to type 'Foo<Assign<T, { x: 1; }>>'.
        // wat
    }
}