Ensure additional properties are present if one property is present

I want to build a type that requires certain properties to be present in case another property is present.

To do so I thought of using an intersection of union types. The union types are either a combination of some property and it's requirements or an empty type.

The following code does not work because it doesn't error out although the desc property is missing:

export type A = {
  a: string;
  desc: string;
};

export type B = {
  b: string;
} & (A | {});

const v: B = {
  b: "",
  a: "123"
};

Replacing {} by Record<string, never> results in errors because all properties are now of type never.

export type A = {
  a: string;
  desc: string;
};

export type B = {
  b: string;
} & (A | Record<string, never>);

const v: B = {
  b: "",
  a: "123"
};

Is there a way to make this concept work? Or maybe a better way?

Your approach is conceptually correct - you want an union of A and a type that has no common keys with A.

Unfortunately sth akin to (A | {}) won't work - TS types allow extra properties.

TS has excess property check for object literals, but it does not work {} unions.

const foo: {} = {a: 1}

ts-eslint even has a rule preventing its use:

Don't use `{}` as a type. `{}` actually means "any non-nullish value".
- If you want a type meaning "any object", you probably want `Record<string, unknown>` instead.
- If you want a type meaning "any value", you probably want `unknown` instead.
- If you want a type meaning "empty object", you probably want `Record<string, never>` 

A | Record<string, never> works fine to alone, but, as you noticed, does not work as a part of an intersection with {b: string;} - Record forces b to be never.

You can use following type that expresses A or sth having no keys of A

export type A = {
  a: string;
  desc: string;
};

type NoneOf<T> = {
    [K in keyof T]?: never
}

export type B = {
  b: string;
} & (A | NoneOf<A>);

const v: B = {
  b: "",
  a: "123"
};

There is related request for Exact Types #12936 - unfortunately not implemented for years.