Substrate

type-alias-invariants and branded primitives ​

An intersection of a phantom brand and a bare primitive is a contract shape under type-alias-invariants:

ts
declare const FORMAT: unique symbol;
type FormatBrandType<TF extends string> = { readonly [FORMAT]: TF };

type EmailBrandType = FormatBrandType<'email'> & string;

The rule reports this alias as a contract and its interface remedy cannot represent an interface that extends string. A boxed interface is the lint-compliant representation:

ts
interface EmailBrandInterface {
  readonly value: string;
  readonly [FORMAT]: 'email';
}

Boxing changes the runtime representation from a primitive to an object. Consumers that require primitive-branded values need to account for this design constraint before enabling the rule.