xo-unit: + .is_natural() method in natural_unit, scaled_unit

include/xo/unit/natural_unit.hpp

@@ -70,6 +70,9 @@ namespace xo {
                 return detail::nu_maker<Int>::make_nu(bpu<Int>(bu, power));
             }
 
+            /** always true.  @see scaled_unit::is_natural **/
+            constexpr bool is_natural() const { return true; }
+
             constexpr std::size_t n_bpu() const { return n_bpu_; }
             constexpr bool is_dimensionless() const { return n_bpu_ == 0; }
 

include/xo/unit/quantity.hpp

@@ -32,6 +32,7 @@ namespace xo {
         template <
             auto NaturalUnit,
             typename Repr = double>
+        requires (NaturalUnit.is_natural())
         class quantity {
         public:
             /** @defgroup quantity-type-traits quantity type traits **/

include/xo/unit/scaled_unit.hpp

@@ -29,7 +29,10 @@ namespace xo {
                                    1.0 / outer_scale_sq_);
             }
 
-            /** @brief true iff scaled unit can be faithfully represented by a @ref natural_unit **/
+            /** @brief true iff scaled unit can be faithfully represented by a @ref natural_unit
+             *
+             * @see natural_unit::is_natural
+             **/
             constexpr bool is_natural() const {
                 return (outer_scale_factor_ == OuterScale(1) && (outer_scale_sq_ == 1.0));
             }