Unit specific concepts introduced

example/example.cpp

@@ -25,11 +25,11 @@
 
 using namespace units;
 
-template<Unit U1, typename Rep1, Unit U2, typename Rep2>
-void foo(velocity<U1, Rep1> v, time<U2, Rep2> t)
+template<typename V, typename T>
+  requires Velocity<V> && Time<T>
+void foo(V v, T t)
 {
   const auto distance = v * t;
-
   std::cout << "A car driving " << v.count() << " km/h in a time of " << t.count() << " minutes will pass "
             << quantity_cast<length<meter, int>>(distance).count() << " meters.\n";
 }

src/include/units/si/frequency.h

@@ -39,6 +39,19 @@ namespace units {
   template<Unit U = hertz, typename Rep = std::intmax_t>
   using frequency = quantity<dimension_frequency, U, Rep>;
 
+  namespace detail {
+
+    template<typename T>
+    struct is_frequency : std::false_type {};
+
+    template<Unit U, typename Rep>
+    struct is_frequency<frequency<U, Rep>> : std::true_type {};
+
+  }
+
+  template<typename T>
+  concept bool Frequency = detail::is_frequency<T>::value;
+
   // ...
 
   namespace literals {

src/include/units/si/length.h

@@ -36,6 +36,19 @@ namespace units {
   template<Unit U = meter, typename Rep = std::intmax_t>
   using length = quantity<dimension_length, U, Rep>;
 
+  namespace detail {
+
+    template<typename T>
+    struct is_length : std::false_type {};
+
+    template<Unit U, typename Rep>
+    struct is_length<length<U, Rep>> : std::true_type {};
+
+  }
+
+  template<typename T>
+  concept bool Length = detail::is_length<T>::value;
+
   namespace literals {
 
     // mm

src/include/units/si/time.h

@@ -39,6 +39,19 @@ namespace units {
   template<Unit U = second, typename Rep = std::intmax_t>
   using time = quantity<dimension_time, U, Rep>;
 
+  namespace detail {
+
+    template<typename T>
+    struct is_time : std::false_type {};
+
+    template<Unit U, typename Rep>
+    struct is_time<time<U, Rep>> : std::true_type {};
+
+  }
+
+  template<typename T>
+  concept bool Time = detail::is_time<T>::value;
+
   // ...
 
   namespace literals {

src/include/units/si/velocity.h

@@ -37,6 +37,19 @@ namespace units {
   template<Unit U = meter_per_second, typename Rep = std::intmax_t>
   using velocity = quantity<dimension_velocity, U, Rep>;
 
+  namespace detail {
+
+    template<typename T>
+    struct is_velocity : std::false_type {};
+
+    template<Unit U, typename Rep>
+    struct is_velocity<velocity<U, Rep>> : std::true_type {};
+
+  }
+
+  template<typename T>
+  concept bool Velocity = detail::is_velocity<T>::value;
+
   // ...
 
   namespace literals {