refactor: 💥 ! U::is_named removed from the unit types and replaced with NamedUnit concept

src/core/include/units/bits/basic_concepts.h

@@ -96,9 +96,18 @@ void to_base_scaled_unit(const volatile scaled_unit<R, U>*);
 template<typename T>
 concept Unit = requires(T* t) { detail::to_base_scaled_unit(t); };
 
+namespace detail {
+
+template<typename>
+inline constexpr bool is_named = false;
+
+}
+
+template<typename T>
+concept NamedUnit = Unit<T> && detail::is_named<T>;
+
 // BaseDimension
-template<basic_fixed_string Symbol, Unit U>
-  requires U::is_named
+template<basic_fixed_string Symbol, NamedUnit U>
 struct base_dimension;
 
 namespace detail {

src/core/include/units/bits/unit_text.h

@@ -110,7 +110,7 @@ template<Exponent Exp>
 constexpr auto exponent_list_with_named_units(Exp)
 {
   using dim = TYPENAME Exp::dimension;
-  if constexpr (dimension_unit<dim>::is_named) {
+  if constexpr (NamedUnit<dimension_unit<dim>>) {
     return exponent_list<Exp>();
   } else {
     using recipe = TYPENAME dim::recipe;