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TypeScript's opaque type.
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| type USD = number & { readonly __brand: unique symbol } | |
| type EUR = number & { readonly __brand: unique symbol } | |
| type Currency = USD | EUR; | |
| function addCurrency<T extends Currency>(a: T, b: T): T { | |
| return (a + b) as T; | |
| } | |
| addCurrency(5 as USD, 6 as USD); | |
| addCurrency(5 as EUR, 7 as USD); // Compilation error. |
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