Scalar constraint added to quantity constructor

src/include/units/quantity.h

@@ -191,8 +191,9 @@ namespace units {
     quantity() = default;
     quantity(const quantity&) = default;
 
-    template<std::convertible_to<rep> Rep2>
-        requires treat_as_floating_point<rep> || (!treat_as_floating_point<Rep2>)
+    template<Scalar Rep2>
+        requires std::convertible_to<Rep2, rep> &&
+                 (treat_as_floating_point<rep> || (!treat_as_floating_point<Rep2>))
     constexpr explicit quantity(const Rep2& r) : value_{static_cast<rep>(r)}
     {
     }