TypeScript

// https://github.com/microsoft/TypeScript/issues/23182#issuecomment-583330532
type NeverAlternative<T, P, N> = [T] extends [never] ? P : N;

type Union<T, U> = NeverAlternative<T, U, T | U>;

class C<T = never> {
    frob<U>(obj: Record<keyof U, any>): C<Union<T, keyof U>> {
        return new C<Union<T, keyof U>>();
    }

    get(key: T) {

    }
}

// x : C<never>
const x = new C();

// y : C<'a' | 'b'>
const y = x.frob({ 'a': 1, 'b': 2 });

y.get('a');

// z : C<'a' | 'b' | 'bleh'>
const z = y.frob({ bleh: 32 })