<mp-units/framework/quantity.h>
template<
RepresentationOf<quantity_spec> ToRep,
/* implementation-defined */ ToU>
requires detail::ValuePreservingConstruction<ToRep, rep> &&
detail::ValuePreservingConversion<unit, rep, ToU{}, ToRep>
[[nodiscard]]
constexpr
QuantityOf<quantity_spec> auto
in(ToU) const;