exact.ts

// T and E extends checking should be array for primitives support
// See https://mariusschulz.com/blog/conditional-types-in-typescript#distributive-conditional-types
type Exact<T, E> = T[] extends E[]
? E[] extends T[]
    ? T
    : never
: never;

// Primitives
declare function exactStringOrBoolean<T>(input: Exact<T, string | boolean>): void;
declare function stringOrBoolean(input: string | boolean): void;

const value = 'string';

exactStringOrBoolean(value); // Error!
stringOrBoolean(value);

// Object Types
type User = {
    name: string,
    surname: string,
}

declare function safeUserOnly<T>(input: Exact<T, User>): void;
declare function userOnly(input: User): void;

const user = { name: 'John', surname: 'Doe', city: 'New York' };

safeUserOnly(user); // Error!
userOnly(user)