Ccompilation error fixed on gcc-9

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

@@ -77,11 +77,17 @@ namespace std {
   using concepts::default_constructible;
   using concepts::derived_from;
   using concepts::equality_comparable_with;
+  // using concepts::floating_point;
+  using concepts::integral;
   using concepts::regular;
   using concepts::same_as;
   using concepts::totally_ordered;
   using concepts::totally_ordered_with;
 
+  // missing in Range-v3
+  template<class T>
+  concept floating_point = std::is_floating_point_v<T>;
+
   template<class F, class... Args>
   concept invocable =
   requires(F&& f, Args&&... args) {

src/include/units/random.h

@@ -24,7 +24,6 @@
 
 #include <random>
 #include <functional>
-#include <concepts>
 #include <units/concepts.h>
 
 namespace units {