5 Quantities and units library [quantities]

5.5 Representation [qty.rep]

5.5.2 Traits [qty.rep.traits]

5.5.2.2 Quantity character [qty.char.traits]

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template<typename T> constexpr bool disable_scalar = false; template<typename T> constexpr bool disable_complex = false; template<typename T> constexpr bool disable_vector = false;
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Some quantities are defined as having a numerical value (IEC 60050, 112-01-29) of a specific set (IEC 60050, 102-01-02).
The representation concepts use these traits to help determine the sets T represents.
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Remarks: Pursuant to N4971, [namespace.std] ([spec.ext]), users may specialize these templates for cv-unqualified program-defined types.
Such specializations shall be usable in constant expressions (N4971, [expr.const]) and have type const bool.
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[Note 1: 
These templates prevent use of representation types with the library that satisfy but do not in fact model their corresponding concept.
— end note]