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export function invariant(condition: unknown, message: string | (() => string)): asserts condition {
  if (condition) return;
  const msg = typeof message === 'function' ? message() : message;
  throw new Error(`Invariant failed: ${msg}`);
}

function processUser(u: { id: number; email: string | null }) {
  invariant(u.email, 'user must have an email');
  // u.email is narrowed to `string` here — TS understands `asserts condition`
  sendEmail(u.email);
}