5 Quantities and units library [quantities]

5.4 Reference [qty.ref]

5.4.4 Unit [qty.unit]

5.4.4.5 Types [qty.unit.types]

5.4.4.5.5 Common [qty.common.unit]

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namespace mp_units { template<Unit U1, Unit U2, Unit... Rest> struct common_unit final : decltype(get-common-scaled-unit(U1{}, U2{}, Rest{}...))::base-type { using base-type = common_unit; // exposition only static constexpr auto common-unit = // exposition only get-common-scaled-unit(U1{}, U2{}, Rest{}...); }; }
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common_unit is used by the library to encapsulate a conversion factor between units (IEC 60050, 112-01-33) common to the operands of quantity addition.
[Example 1: 
The result of 1 * km + 1 * mi has a common unit encapsulated by common_unit<mi, km>.
— end example]
A program that instantiates a specialization of common_unit that is not a possible result of the library specifications is ill-formed, no diagnostic required.
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template<Unit U1, Unit U2, Unit... Rest> consteval Unit auto get-common-scaled-unit(U1, U2, Rest... rest) // exposition only requires see below;
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Effects: Equivalent to: constexpr auto res = [] { constexpr auto canonical_lhs = get-canonical-unit(U1{}); constexpr auto canonical_rhs = get-canonical-unit(U2{}); constexpr auto common_mag = common-magnitude(canonical_lhs.mag, canonical_rhs.mag); if constexpr (common_mag == mag<1>) return canonical_lhs.reference_unit; else return scaled_unit<common_mag, decltype(auto(canonical_lhs.reference_unit))>{}; }(); if constexpr (sizeof...(rest) == 0) return res; else return get-common-scaled-unit(res, rest...);
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Remarks: The expression in the requires-clause is equivalent to: (convertible(U1{}, U2{}) && (sizeof...(Rest) == 0 || requires { get-common-scaled-unit(get-common-scaled-unit(u1, u2), rest...); }))