refactor: apply comments

src/include/units/bits/external/hacks.h

@@ -31,7 +31,11 @@
 #define COMP_MSVC _MSC_VER
 #endif
 
-#if COMP_CLANG && false
+#if defined(COMP_CLANG) && !defined(UNITS_LIBCXX)
+#define UNITS_LIBCXX 0
+#endif
+
+#if COMP_CLANG && UNITS_LIBCXX
 
 #include <concepts/compare.hpp>
 #include <concepts/concepts.hpp>
@@ -60,7 +64,7 @@ namespace std {
 template<class T>
 concept default_constructible = constructible_from<T>;
 
-#elif COMP_CLANG && false
+#elif COMP_CLANG && UNITS_LIBCXX
 
 // concepts
 using concepts::three_way_comparable;

src/include/units/quantity_cast.h

@@ -290,7 +290,7 @@ template<typename CastSpec, typename K, typename U, typename Rep>
   requires requires { requires is_specialization_of<CastSpec, quantity_kind>;
               requires requires { quantity_cast<typename CastSpec::quantity_type>(qk.common()); }; } ||
            requires { requires Kind<CastSpec>; requires UnitOf<U, typename CastSpec::dimension>; } ||
-           requires { requires requires { quantity_cast<CastSpec>(qk.common()); }; }
+           requires { quantity_cast<CastSpec>(qk.common()); }
 {
   if constexpr (is_specialization_of<CastSpec, quantity_kind>)
     return CastSpec(quantity_cast<typename CastSpec::quantity_type>(qk.common()));
@@ -349,7 +349,7 @@ template<typename CastSpec, typename PK, typename U, typename Rep>
   requires requires { requires is_specialization_of<CastSpec, quantity_point_kind>;
               requires requires { quantity_kind_cast<typename CastSpec::quantity_kind_type>(qpk.relative()); }; } ||
            requires { requires PointKind<CastSpec> && UnitOf<U, typename CastSpec::dimension>; } ||
-           requires { requires requires { quantity_kind_cast<CastSpec>(qpk.relative()); }; }
+           requires { quantity_kind_cast<CastSpec>(qpk.relative()); }
 {
   if constexpr (is_specialization_of<CastSpec, quantity_point_kind>)
     return CastSpec(quantity_kind_cast<typename CastSpec::quantity_kind_type>(qpk.relative()));