5 Quantities and units library [quantities]

5.4 Reference [qty.ref]

5.4.5 Concepts [qty.ref.concepts]

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template<typename T> concept Reference = AssociatedUnit<T> || (is-specialization-of<T, reference>());
A type T that satisfies Reference represents the reference of a quantity (IEC 60050, 112-01-01).
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template<typename T, auto QS> concept ReferenceOf = Reference<T> && QuantitySpecOf<decltype(get_quantity_spec(T{})), QS>;