https://www.typescriptlang.org/play?target=8#code/C4TwDgpgBA6glsAFgaQiAPAFShAHsCAOwBMBnKAewCMArCAY2ABopkd8iypTgAnOQgHMAfFAC8UAEoMKvYumQse-IaIBkUTAG4AUDvoAbAIalyABSO9SEAPK0GwLOwIly1Oo1EBvHVD9QwAFcqAzh6KHoKQmVAxlkACjB+ADcjAgDLYFIALih4gGs0CgAzTQBKAG0AXTKoH38oAF89BqCQsKgjYnk2PBcuZQERAtzkMtyLK1t7RnR4JFQMTBZkYW9fBv9I6OAM3izcgqLS+ZQ0LBXhSqrxKAqAOkekOFJ…

reify.ts

type WithKey<T extends object, K extends string> = Record<K, string> & T;

class ParseObject<T extends object> {
    public constructor(private parts: (keyof T)[]) {
    }

    public add<K extends string>(k: K): ParseObject<WithKey<T, K>> {
        const parts: (keyof WithKey<T, K>)[] = [...this.parts, k];
        return new ParseObject<WithKey<T, K>>(parts);
    }

    static build(): ParseObject<{}> {
        return new ParseObject([]);
    }

    public reify(): T {
        const foo: [keyof T, string][] = this.parts.map((p) => [p, "foo"]);
        const o: T = Object.fromEntries(foo);
        return o;
    }
}

function parse2<T extends string>(xs: T[]): Record<T, string> {
    throw 1;
}

const a: ParseObject<{}> = ParseObject.build();
const b: ParseObject<{foo: string}> = a.add('foo');
const c: ParseObject<{foo: string, bar: string}> = b.add('foo');

https://www.typescriptlang.org/play?#code/PTAEDEEsDsFMCgDGB7aBnALqA1rAngFygDkAgsaALwnkDcSqmoADgIYBOas7RAFAJRUAfKGgBXALYAjblVADhoAIz0U6LADNkyIgCVYKdgBMAPBjzNYyDTnwAaUOcvWWHLuxHUA3gG1ceAF0iNk5uAF96eBBQADlkTRgEDTFoRAxIVFAta2gTAGlQWAAPDFhoIzRQTHYYAHMHADUhXn8iPIcQ9z5BShEG-j0DZGN8hwVe0CbQL3hQOdA1Jh1QfUNTdvkevs9pv3wg11D2CNn59lgMMXZoUGR6MPggA