bool removed from concepts definitions as gcc-9.1 supports this syntax already

src/include/units/bits/concepts.h

@@ -27,7 +27,7 @@
 namespace units {
 
   template<typename T>
-  concept bool Number = std::regular<T> &&
+  concept Number = std::regular<T> &&
       std::totally_ordered<T> &&
       requires(T a, T b) {
         { a + b } -> T;

src/include/units/bits/downcasting.h

@@ -34,7 +34,7 @@ namespace units {
   };
 
   template<typename T>
-  concept bool Downcastable =
+  concept Downcastable =
       requires {
         typename T::base_type;
       } &&
@@ -48,7 +48,7 @@ namespace units {
   namespace detail {
 
     template<typename T>
-    concept bool has_downcast = requires {
+    concept has_downcast = requires {
         downcast_guide(std::declval<downcast_base<T>>());
     };
 

src/include/units/bits/type_list.h

@@ -37,7 +37,7 @@ namespace units {
   }  // namespace detail
 
   template<typename T>
-  concept bool TypeList = detail::is_type_list<T>;
+  concept TypeList = detail::is_type_list<T>;
 
   // push_front