Physical quantities with branded types. Example with monetary values.

money.ts

type Branded<P, B> = P & { readonly _brand: B }
type PhysicalQuantity<Q, U> = Branded<number, Q> & U

type Currency<C> = PhysicalQuantity<'Currency', C>

type EUR_ = { readonly EUR: unique symbol }
type EUR = Currency<EUR_>

type USD_ = { readonly USD: unique symbol }
type USD = Currency<USD_>

type Money = EUR | USD

function mkMoney<M extends Money>(n: number): M {
  return n as any
}

function add<M extends Money>(a: M, b: M): M {
  return mkMoney<M>(a + b)
}

function convertTo<MO extends Money>(rate: number): <MI extends Money>(m: MI) => MO {
  return (m) => mkMoney<MO>(m * rate)
}

const someEur = mkMoney<EUR>(3)
const someUsd = mkMoney<USD>(6)

add(someEur, someEur)
add(someUsd, someEur) // Error: Argument of type 'PhysicalQuantity<"Currency", EUR_>' is not assignable to parameter of type 'PhysicalQuantity<"Currency", USD_>'.
add(1, someUsd) // Error: Argument of type '1' is not assignable to parameter of type 'Money'.

convertTo<USD>(1.11)(someEur)