Symbol modifiers in TypeScript

symbols.ts

declare const NegativeKey: unique symbol;
declare const PositiveKey: unique symbol;

type Negative<T extends number> = T & { [NegativeKey]?: true };
type Positive<T extends number> = T & { [PositiveKey]?: true };

type IsNegative<T> = keyof NonNullable<T> extends keyof Omit<NonNullable<T>, typeof NegativeKey> ? false : true;
type IsPositive<T> = keyof NonNullable<T> extends keyof Omit<NonNullable<T>, typeof PositiveKey> ? false : true;

type ToNumber<T extends number> = T extends infer N & { [NegativeKey]?: true }
  ? N
  : T extends infer P & { [PositiveKey]?: true }
  ? P
  : T;

type A = Negative<-3>;
type B = Positive<4>;

type C = IsNegative<A>;
type D = IsPositive<A>;

type E = ToNumber<A>;
type F = ToNumber<B>;