diff --git a/doc/DESIGN.md b/doc/DESIGN.md
index 51959d42..a016d618 100644
--- a/doc/DESIGN.md
+++ b/doc/DESIGN.md
@@ -54,8 +54,7 @@ derived from `units::dimension` class template:
 template<typename T>
 concept Dimension =
     std::is_empty_v<T> &&
-    detail::is_dimension<typename T::base_type> &&  // exposition only
-    DerivedFrom<T, typename T::base_type>;
+    detail::is_dimension<upcast_from<T>>; // exposition only
 ```
 
 #### `Exponents`
@@ -199,8 +198,7 @@ derived from `units::unit` class template:
 template<typename T>
 concept Unit =
     std::is_empty_v<T> &&
-    detail::is_unit<typename T::base_type> &&  // exposition only
-    DerivedFrom<T, typename T::base_type>;
+    detail::is_unit<upcast_from<T>>;  // exposition only
 ```
 
 ### `Quantities`
@@ -344,31 +342,33 @@ and
 
 are not arguably much easier to understand thus provide better user experience.
 
-Upcasting capability is provided through dedicated `upcasting_traits` and by `base_type` member
-type in `upcast_base` class template.
+Upcasting capability is provided through dedicated `upcasting_traits`, a few helper aliases and by
+`base_type` member type in `upcast_base` class template.
 
 ```cpp
+template<Upcastable T>
+using upcast_from = typename T::base_type;
+
 template<typename T>
-struct upcasting_traits : std::type_identity<T> {};
+using upcast_to = std::type_identity<T>;
+
+template<typename T>
+struct upcasting_traits : upcast_to<T> {};
 
 template<typename T>
 using upcasting_traits_t = typename upcasting_traits<T>::type;
 ```
 
+With that the upcasting functionality is enabled by:
+
 ```cpp
 struct dimension_length : make_dimension_t<exp<base_dim_length, 1>> {};
-
-template<>
-struct upcasting_traits<typename dimension_length::base_type> :
-    std::type_identity<dimension_length> {};
+template<> struct upcasting_traits<upcast_from<dimension_length>> : upcast_to<dimension_length> {};
 ```
 
 ```cpp
 struct kilometer : unit<dimension_length, std::kilo> {};
-
-template<>
-struct upcasting_traits<typename kilometer::base_type> :
-    std::type_identity<kilometer> {};
+template<> struct upcasting_traits<upcast_from<kilometer>> : upcast_to<kilometer> {};
 ```
 
 
@@ -379,14 +379,14 @@ In order to extend the library with custom dimensions the user has to:
 
 ```cpp
 struct dimension_velocity : make_dimension_t<exp<base_dim_length, 1>, exp<base_dim_time, -1>> {};
-template<> struct upcasting_traits<typename dimension_velocity::base_type> : std::type_identity<dimension_velocity> {};
+template<> struct upcasting_traits<upcast_from<dimension_velocity>> : upcast_to<dimension_velocity> {};
 ``` 
 
 2. Define the base unit (`std::ratio<1>`) and secondary ones and provide upcasting traits for them via:
 
 ```cpp
 struct meter_per_second : unit<dimension_velocity, std::ratio<1>> {};
-template<> struct upcasting_traits<typename meter_per_second::base_type> : std::type_identity<meter_per_second> {};
+template<> struct upcasting_traits<upcast_from<meter_per_second>> : upcast_to<meter_per_second> {};
 ``` 
 
 3. Define a concept that will match a new dimension:
@@ -528,3 +528,12 @@ Additionally, it should make the error logs even shorter thus easier to understa
 
 18. Should we standardize accompany tools (`type_list` operations, `static_sign`, `static_abs`,
     `static_gcd`, `common_ratio`)? 
+     
+19. Do we need to support fractional exponents (i.e. `dimension<exp<"length", 2, 3>>` as 2/3)? 
+
+20. implicit conversion of quantity<Unit,Y> to quantity<Unit,Z> is allowed if Y and Z are implicitly convertible.
+    assignment between quantity<Unit,Y> and quantity<Unit,Z> is allowed if Y and Z are implicitly convertible.
+
+21. explicit conversion between quantity<Unit1,Y> and quantity<Unit2,Z> is allowed if Unit1 and Unit2 have the same dimensions and if Y and Z are implicitly convertible.
+    implicit conversion between quantity<Unit1,Y> and quantity<Unit2,Z> is allowed if Unit1 reduces to exactly the same combination of base units as Unit2 and if Y and Z are convertible.
+
diff --git a/src/include/units/bits/tools.h b/src/include/units/bits/tools.h
index eaaa183d..c4b4edda 100644
--- a/src/include/units/bits/tools.h
+++ b/src/include/units/bits/tools.h
@@ -110,20 +110,30 @@ namespace units {
   template<Ratio Ratio1, Ratio Ratio2>
   using common_ratio_t = typename common_ratio<Ratio1, Ratio2>::type;
 
-  // upcast_base
+  // upcasting
 
   template<typename BaseType>
   struct upcast_base {
     using base_type = BaseType;
   };
 
-  // upcasting_traits
+  template<typename T>
+  concept bool Upcastable =
+      requires {
+        typename T::base_type;
+      } &&
+      std::experimental::ranges::DerivedFrom<T, upcast_base<typename T::base_type>>;
+
+  template<Upcastable T>
+  using upcast_from = typename T::base_type;
 
   template<typename T>
-  struct upcasting_traits : std::type_identity<T> {};
+  using upcast_to = std::type_identity<T>;
+
+  template<typename T>
+  struct upcasting_traits : upcast_to<T> {};
 
   template<typename T>
   using upcasting_traits_t = typename upcasting_traits<T>::type;
 
-
 }  // namespace units
diff --git a/src/include/units/dimension.h b/src/include/units/dimension.h
index 152a25ad..29791b49 100644
--- a/src/include/units/dimension.h
+++ b/src/include/units/dimension.h
@@ -101,8 +101,7 @@ namespace units {
   template<typename T>
   concept bool Dimension =
       std::is_empty_v<T> &&
-      detail::is_dimension<typename T::base_type> &&
-      std::experimental::ranges::DerivedFrom<T, typename T::base_type>;
+      detail::is_dimension<upcast_from<T>>;
 
 
   // dim_invert
diff --git a/src/include/units/si/frequency.h b/src/include/units/si/frequency.h
index 433f905f..d54f0ac8 100644
--- a/src/include/units/si/frequency.h
+++ b/src/include/units/si/frequency.h
@@ -28,25 +28,25 @@
 namespace units {
 
   struct dimension_frequency : make_dimension_t<exp<base_dim_time, -1>> {};
-  template<> struct upcasting_traits<typename dimension_frequency::base_type> : std::type_identity<dimension_frequency> {};
+  template<> struct upcasting_traits<upcast_from<dimension_frequency>> : upcast_to<dimension_frequency> {};
 
   struct millihertz : unit<dimension_frequency, std::milli> {};
-  template<> struct upcasting_traits<typename millihertz::base_type> : std::type_identity<millihertz> {};
+  template<> struct upcasting_traits<upcast_from<millihertz>> : upcast_to<millihertz> {};
 
   struct hertz : unit<dimension_frequency, std::ratio<1>> {};
-  template<> struct upcasting_traits<typename hertz::base_type> : std::type_identity<hertz> {};
+  template<> struct upcasting_traits<upcast_from<hertz>> : upcast_to<hertz> {};
 
   struct kilohertz : unit<dimension_frequency, std::kilo> {};
-  template<> struct upcasting_traits<typename kilohertz::base_type> : std::type_identity<kilohertz> {};
+  template<> struct upcasting_traits<upcast_from<kilohertz>> : upcast_to<kilohertz> {};
 
   struct megahertz : unit<dimension_frequency, std::mega> {};
-  template<> struct upcasting_traits<typename megahertz::base_type> : std::type_identity<megahertz> {};
+  template<> struct upcasting_traits<upcast_from<megahertz>> : upcast_to<megahertz> {};
 
   struct gigahertz : unit<dimension_frequency, std::giga> {};
-  template<> struct upcasting_traits<typename gigahertz::base_type> : std::type_identity<gigahertz> {};
+  template<> struct upcasting_traits<upcast_from<gigahertz>> : upcast_to<gigahertz> {};
 
   struct terahertz : unit<dimension_frequency, std::tera> {};
-  template<> struct upcasting_traits<typename terahertz::base_type> : std::type_identity<terahertz> {};
+  template<> struct upcasting_traits<upcast_from<terahertz>> : upcast_to<terahertz> {};
 
   template<Unit U = hertz, Number Rep = double>
   using frequency = quantity<dimension_frequency, U, Rep>;
diff --git a/src/include/units/si/length.h b/src/include/units/si/length.h
index 2a6ae42f..9381110f 100644
--- a/src/include/units/si/length.h
+++ b/src/include/units/si/length.h
@@ -29,33 +29,33 @@ namespace units {
 
   // dimension
   struct dimension_length : make_dimension_t<exp<base_dim_length, 1>> {};
-  template<> struct upcasting_traits<typename dimension_length::base_type> : std::type_identity<dimension_length> {};
+  template<> struct upcasting_traits<upcast_from<dimension_length>> : upcast_to<dimension_length> {};
 
   // SI units
   struct millimeter : unit<dimension_length, std::milli> {};
-  template<> struct upcasting_traits<typename millimeter::base_type> : std::type_identity<millimeter> {};
+  template<> struct upcasting_traits<upcast_from<millimeter>> : upcast_to<millimeter> {};
 
   struct centimeter : unit<dimension_length, std::ratio<1, 100>> {};
-  template<> struct upcasting_traits<typename centimeter::base_type> : std::type_identity<centimeter> {};
+  template<> struct upcasting_traits<upcast_from<centimeter>> : upcast_to<centimeter> {};
 
   struct meter : unit<dimension_length, std::ratio<1>> {};
-  template<> struct upcasting_traits<typename meter::base_type> : std::type_identity<meter> {};
+  template<> struct upcasting_traits<upcast_from<meter>> : upcast_to<meter> {};
 
   struct kilometer : unit<dimension_length, std::kilo> {};
-  template<> struct upcasting_traits<typename kilometer::base_type> : std::type_identity<kilometer> {};
+  template<> struct upcasting_traits<upcast_from<kilometer>> : upcast_to<kilometer> {};
 
   // US customary units
   struct yard : unit<dimension_length, std::ratio<9'144, 10'000>> {};
-  template<> struct upcasting_traits<typename yard::base_type> : std::type_identity<yard> {};
+  template<> struct upcasting_traits<upcast_from<yard>> : upcast_to<yard> {};
 
   struct foot : unit<dimension_length, std::ratio_multiply<std::ratio<1, 3>, yard::ratio>> {};
-  template<> struct upcasting_traits<typename foot::base_type> : std::type_identity<foot> {};
+  template<> struct upcasting_traits<upcast_from<foot>> : upcast_to<foot> {};
 
   struct inch : unit<dimension_length, std::ratio_multiply<std::ratio<1, 12>, foot::ratio>> {};
-  template<> struct upcasting_traits<typename inch::base_type> : std::type_identity<inch> {};
+  template<> struct upcasting_traits<upcast_from<inch>> : upcast_to<inch> {};
 
   struct mile : unit<dimension_length, std::ratio_multiply<std::ratio<1'760>, yard::ratio>> {};
-  template<> struct upcasting_traits<typename mile::base_type> : std::type_identity<mile> {};
+  template<> struct upcasting_traits<upcast_from<mile>> : upcast_to<mile> {};
 
   // length
   template<Unit U = meter, Number Rep = double>
diff --git a/src/include/units/si/time.h b/src/include/units/si/time.h
index 29605c05..3d412cd6 100644
--- a/src/include/units/si/time.h
+++ b/src/include/units/si/time.h
@@ -28,25 +28,25 @@
 namespace units {
 
   struct dimension_time : make_dimension_t<exp<base_dim_time, 1>> {};
-  template<> struct upcasting_traits<typename dimension_time::base_type> : std::type_identity<dimension_time> {};
+  template<> struct upcasting_traits<upcast_from<dimension_time>> : upcast_to<dimension_time> {};
 
   struct nanosecond : unit<dimension_time, std::nano> {};
-  template<> struct upcasting_traits<typename nanosecond::base_type> : std::type_identity<nanosecond> {};
+  template<> struct upcasting_traits<upcast_from<nanosecond>> : upcast_to<nanosecond> {};
 
   struct microsecond : unit<dimension_time, std::micro> {};
-  template<> struct upcasting_traits<typename microsecond::base_type> : std::type_identity<microsecond> {};
+  template<> struct upcasting_traits<upcast_from<microsecond>> : upcast_to<microsecond> {};
 
   struct millisecond : unit<dimension_time, std::milli> {};
-  template<> struct upcasting_traits<typename millisecond::base_type> : std::type_identity<millisecond> {};
+  template<> struct upcasting_traits<upcast_from<millisecond>> : upcast_to<millisecond> {};
 
   struct second : unit<dimension_time, std::ratio<1>> {};
-  template<> struct upcasting_traits<typename second::base_type> : std::type_identity<second> {};
+  template<> struct upcasting_traits<upcast_from<second>> : upcast_to<second> {};
 
   struct minute : unit<dimension_time, std::ratio<60>> {};
-  template<> struct upcasting_traits<typename minute::base_type> : std::type_identity<minute> {};
+  template<> struct upcasting_traits<upcast_from<minute>> : upcast_to<minute> {};
 
   struct hour : unit<dimension_time, std::ratio<3600>> {};
-  template<> struct upcasting_traits<typename hour::base_type> : std::type_identity<hour> {};
+  template<> struct upcasting_traits<upcast_from<hour>> : upcast_to<hour> {};
 
   template<Unit U = second, Number Rep = double>
   using time = quantity<dimension_time, U, Rep>;
diff --git a/src/include/units/si/velocity.h b/src/include/units/si/velocity.h
index 60137874..8233bc8e 100644
--- a/src/include/units/si/velocity.h
+++ b/src/include/units/si/velocity.h
@@ -29,16 +29,16 @@
 namespace units {
 
   struct dimension_velocity : make_dimension_t<exp<base_dim_length, 1>, exp<base_dim_time, -1>> {};
-  template<> struct upcasting_traits<typename dimension_velocity::base_type> : std::type_identity<dimension_velocity> {};
+  template<> struct upcasting_traits<upcast_from<dimension_velocity>> : upcast_to<dimension_velocity> {};
 
   struct meter_per_second : unit<dimension_velocity, std::ratio<1>> {};
-  template<> struct upcasting_traits<typename meter_per_second::base_type> : std::type_identity<meter_per_second> {};
+  template<> struct upcasting_traits<upcast_from<meter_per_second>> : upcast_to<meter_per_second> {};
 
   struct kilometer_per_hour : unit<dimension_velocity, std::ratio_divide<kilometer::ratio, hour::ratio>> {};
-  template<> struct upcasting_traits<typename kilometer_per_hour::base_type> : std::type_identity<kilometer_per_hour> {};
+  template<> struct upcasting_traits<upcast_from<kilometer_per_hour>> : upcast_to<kilometer_per_hour> {};
 
   struct mile_per_hour : unit<dimension_velocity, std::ratio_divide<mile::ratio, hour::ratio>> {};
-  template<> struct upcasting_traits<typename mile_per_hour::base_type> : std::type_identity<mile_per_hour> {};
+  template<> struct upcasting_traits<upcast_from<mile_per_hour>> : upcast_to<mile_per_hour> {};
 
   template<Unit U = meter_per_second, Number Rep = double>
   using velocity = quantity<dimension_velocity, U, Rep>;
diff --git a/src/include/units/unit.h b/src/include/units/unit.h
index 148e922e..628a28ab 100644
--- a/src/include/units/unit.h
+++ b/src/include/units/unit.h
@@ -49,7 +49,6 @@ namespace units {
   template<typename T>
   concept bool Unit =
       std::is_empty_v<T> &&
-      detail::is_unit<typename T::base_type> &&
-      std::experimental::ranges::DerivedFrom<T, typename T::base_type>;
+      detail::is_unit<upcast_from<T>>;
 
 }  // namespace units