<mp-units/framework/quantity_spec.h>template<
typename Self,
typename FwdQ,
Quantity Q = std::remove_cvref_t<FwdQ>>
requires (mp_units::explicitly_convertible(Q::quantity_spec, Self{})) &&
requires { typename quantity<make_reference(Self{}, Q::unit), typename Q::rep>; }
[[nodiscard]]
constexpr
Quantity auto
operator()(this
Self,
FwdQ&& q);