From 32a65a74059ca2059d7b1d38782ce0226b1d488c Mon Sep 17 00:00:00 2001
From: Mateusz Pusz <mateusz.pusz@gmail.com>
Date: Wed, 18 Sep 2019 06:36:52 -0600
Subject: [PATCH] Support for derived dimensions in exp added

---
 doc/DESIGN.md                                 |  84 ++++++---
 src/include/units/dimension.h                 | 159 +++++++++++-------
 src/include/units/dimensions/acceleration.h   |   2 +-
 .../units/dimensions/base_dimensions.h        |  14 +-
 src/include/units/dimensions/capacitance.h    |   2 +-
 src/include/units/dimensions/energy.h         |   2 +-
 src/include/units/dimensions/force.h          |   5 +-
 src/include/units/dimensions/power.h          |   2 +-
 src/include/units/dimensions/pressure.h       |   3 +-
 src/include/units/dimensions/voltage.h        |   2 +-
 src/include/units/unit.h                      |  16 +-
 test/unit_test/test_custom_units.cpp          |   2 +-
 test/unit_test/test_dimension.cpp             |  23 ++-
 test/unit_test/test_type_list.cpp             |   4 +-
 14 files changed, 207 insertions(+), 113 deletions(-)

diff --git a/doc/DESIGN.md b/doc/DESIGN.md
index 68b96297..8e3fb2d2 100644
--- a/doc/DESIGN.md
+++ b/doc/DESIGN.md
@@ -74,8 +74,8 @@ constexpr units::Velocity auto avg_speed(units::Length auto d, units::Time auto
 
 ### `Dimensions`
 
-`units::dimension` is a type-list like type that stores an ordered list of exponents of one
-or more base dimensions:
+`units::dimension` represents a derived dimension and is implemented as a type-list like type that
+stores an ordered list of exponents of one or more base dimensions:
 
 ```cpp
 template<Exponent... Es>
@@ -94,28 +94,6 @@ concept Dimension =
 
 #### `Exponents`
 
-`units::exp` provides an information about a single base dimension and its (possibly fractional)
-exponent in a derived dimension:
-
-```cpp
-template<const base_dimension& BaseDimension, int Num, int Den = 1>
-struct exp {
-  static constexpr const base_dimension& dimension = BaseDimension;
-  static constexpr int num = Num;
-  static constexpr int den = Den;
-};
-```
-
-where `BaseDimension` is a unique sortable compile-time value:
-
-```cpp
-struct base_dimension {
-  const char* name;
-};
-constexpr bool operator==(const base_dimension& lhs, const base_dimension& rhs);
-constexpr bool operator<(const base_dimension& lhs, const base_dimension& rhs);
-```
-
 `units::Exponent` concept is satisfied if provided type is an instantiation of `units::exp` class
 template:
 
@@ -125,6 +103,45 @@ concept Exponent =
     detail::is_exp<T>;  // exposition only
 ```
 
+`units::exp` provides an information about a single dimension and its (possibly fractional)
+exponent in a derived dimension.
+
+```cpp
+template<typename Dim, int Num, int Den = 1>
+  requires BaseDimension<Dim> || Dimension<Dim>
+struct exp {
+  using dimension = Dim;
+  static constexpr int num = Num;
+  static constexpr int den = Den;
+};
+```
+
+Both a base dimension and a derived dimension can be provided to `units::exp` class template.
+
+`units::BaseDimension` represents a base dimension and should be implemented as a type with
+a unique compile-time text describing the dimension name:
+
+```cpp
+template<typename T>
+concept BaseDimension = std::is_empty_v<T> &&
+  requires {
+    { T::value } -> std::same_as<const char*>;
+  };
+```
+
+For example here is a list of SI base dimensions:
+
+```cpp
+struct base_dim_length { static constexpr const char* value = "length"; };
+struct base_dim_mass { static constexpr const char* value = "mass"; };
+struct base_dim_time { static constexpr const char* value = "time"; };
+struct base_dim_current { static constexpr const char* value = "current"; };
+struct base_dim_temperature { static constexpr const char* value = "temperature"; };
+struct base_dim_substance { static constexpr const char* value = "substance"; };
+struct base_dim_luminous_intensity { static constexpr const char* value = "luminous intensity"; };
+``` 
+
+
 #### `make_dimension`
 
 Above design of dimensions is created with the ease of use for end users in mind. Compile-time
@@ -169,6 +186,23 @@ contained base dimensions. Beside providing ordering to base dimensions it also
 - aggregate two arguments of the same base dimension but different exponents
 - eliminate two arguments of the same base dimension and with opposite equal exponents
 
+`make_dimension_t` is also able to form a dimension type based not only on base dimensions but
+it can take other derived dimensions as well. So for some more complex dimensions user can
+type either:
+
+```cpp
+struct pressure : make_dimension_t<exp<base_dim_mass, 1>, exp<base_dim_length, -1>, exp<base_dim_time, -2>> {};
+```
+
+or
+
+```cpp
+struct pressure : make_dimension_t<exp<force, 1>, exp<area, -1>> {};
+```
+
+In the second case `make_dimension_t` will extract all derived dimensions into the list of
+exponents of base dimensions. Thanks to that both cases will result with exactly the same base
+class formed only from the exponents of base units. 
 
 #### `merge_dimension`
 
@@ -426,7 +460,7 @@ struct downcast_base {
 };
 
 template<typename T>
-concept bool Downcastable =
+concept Downcastable =
     requires {
       typename T::base_type;
     } &&
diff --git a/src/include/units/dimension.h b/src/include/units/dimension.h
index d78f8177..246c353d 100644
--- a/src/include/units/dimension.h
+++ b/src/include/units/dimension.h
@@ -29,81 +29,49 @@
 
 namespace std::experimental::units {
 
-  struct base_dimension {
-    const char* name;
-  };
+  template<typename T>
+  concept bool BaseDimension = std::is_empty_v<T> &&
+      requires {
+        { T::value } -> std::same_as<const char*>;
+      };
 
-  constexpr bool operator==(const base_dimension& lhs, const base_dimension& rhs)
-  {
-    const char* p1 = lhs.name;
-    const char* p2 = rhs.name;
-    for(; (*p1 != '\0') && (*p2 != '\0'); ++p1, (void)++p2) {
-      if(*p1 != *p2) return false;
-    }
-    return *p1 == *p2;
-  }
+  namespace detail {
 
-  constexpr bool operator<(const base_dimension& lhs, const base_dimension& rhs)
-  {
-    const char* p1 = lhs.name;
-    const char* p2 = rhs.name;
-    for(; (*p1 != '\0') && (*p2 != '\0'); ++p1, (void)++p2) {
-      if(*p1 < *p2) return true;
-      if(*p2 < *p1) return false;
+    template<BaseDimension D1, BaseDimension D2>
+    constexpr bool less()
+    {
+      const char* p1 = D1::value;
+      const char* p2 = D2::value;
+      for(; (*p1 != '\0') && (*p2 != '\0'); ++p1, (void) ++p2) {
+        if(*p1 < *p2) return true;
+        if(*p2 < *p1) return false;
+      }
+      return (*p1 == '\0') && (*p2 != '\0');
     }
-    return (*p1 == '\0') && (*p2 != '\0');
+
   }
 
   // base_dimension_less
 
-  template<const base_dimension& D1, const base_dimension& D2>
-  struct base_dimension_less : std::bool_constant<D1 < D2> {
-  };
-
-  // exp
-
-  template<const base_dimension& BaseDimension, int Num, int Den = 1>
-  struct exp {
-    static constexpr const base_dimension& dimension = BaseDimension;
-    static constexpr int num = Num;
-    static constexpr int den = Den;
+  template<BaseDimension D1, BaseDimension D2>
+  struct base_dimension_less : std::bool_constant<detail::less<D1, D2>()> {
   };
 
   // is_exp
   namespace detail {
+
     template<typename T>
     inline constexpr bool is_exp = false;
 
-    template<const base_dimension& BaseDimension, int Num, int Den>
-    inline constexpr bool is_exp<exp<BaseDimension, Num, Den>> = true;
+    // partial specialization for an exp type provided below
+
   }  // namespace detail
 
   template<typename T>
   concept bool Exponent = detail::is_exp<T>;
 
-  // exp_dim_id_less
-
-  template<Exponent E1, Exponent E2>
-  struct exp_less : base_dimension_less<E1::dimension, E2::dimension> {
-  };
-
-  // exp_invert
-
-  template<Exponent E>
-  struct exp_invert;
-
-  template<const base_dimension& BaseDimension, int Num, int Den>
-  struct exp_invert<exp<BaseDimension, Num, Den>> {
-    using type = exp<BaseDimension, -Num, Den>;
-  };
-
-  template<Exponent E>
-  using exp_invert_t = exp_invert<E>::type;
-
-  // dimension
-
   template<Exponent... Es>
-  struct dimension : downcast_base<dimension<Es...>> {};
+  struct dimension;
 
   // is_dimension
   namespace detail {
@@ -121,6 +89,57 @@ namespace std::experimental::units {
       std::is_empty_v<T> &&
       detail::is_dimension<downcast_from<T>>;
 
+  // exp
+
+  template<typename Dim, int Num, int Den = 1>
+    requires BaseDimension<Dim> || Dimension<Dim>
+  struct exp {
+    using dimension = Dim;
+    static constexpr int num = Num;
+    static constexpr int den = Den;
+  };
+
+  // is_exp
+  namespace detail {
+
+    template<typename Dim, int Num, int Den>
+    inline constexpr bool is_exp<exp<Dim, Num, Den>> = true;
+
+  }  // namespace detail
+
+  // exp_less
+
+  template<Exponent E1, Exponent E2>
+  struct exp_less : base_dimension_less<typename E1::dimension, typename E2::dimension> {
+  };
+
+  // exp_invert
+
+  template<Exponent E>
+  struct exp_invert;
+
+  template<typename Dim, int Num, int Den>
+  struct exp_invert<exp<Dim, Num, Den>> {
+    using type = exp<Dim, -Num, Den>;
+  };
+
+  template<Exponent E>
+  using exp_invert_t = exp_invert<E>::type;
+
+  // exp_multiply
+
+  template<Exponent E, int Num, int Den>
+  struct exp_multiply {
+    using type = exp<typename E::dimension, E::num * Num, E::den * Den>;
+  };
+
+  template<Exponent E, int Num, int Den>
+  using exp_multiply_t = exp_multiply<E, Num, Den>::type;
+
+  // dimension
+
+  template<Exponent... Es>
+  struct dimension : downcast_base<dimension<Es...>> {};
 
   // dim_invert
 
@@ -161,7 +180,7 @@ namespace std::experimental::units {
       using type = conditional<std::is_same_v<rest, dimension<>>, dimension<E1>, type_list_push_front<rest, E1>>;
     };
 
-    template<const base_dimension& D, int Num1, int Den1, int Num2, int Den2, typename... ERest>
+    template<BaseDimension D, int Num1, int Den1, int Num2, int Den2, typename... ERest>
     struct dim_consolidate<dimension<exp<D, Num1, Den1>, exp<D, Num2, Den2>, ERest...>> {
       // todo: provide custom implementation for ratio_add
       using r1 = std::ratio<Num1, Den1>;
@@ -171,11 +190,37 @@ namespace std::experimental::units {
                                dim_consolidate_t<dimension<exp<D, r::num, r::den>, ERest...>>>;
     };
 
+    template<Exponent... Es>
+    struct extract;
+
+    template<Exponent... Es>
+    using extract_t = extract<Es...>::type;
+
+    template<>
+    struct extract<> {
+      using type = dimension<>;
+    };
+
+    template<BaseDimension Dim, int Num, int Den, Exponent... ERest>
+    struct extract<exp<Dim, Num, Den>, ERest...> {
+      using type = type_list_push_front<extract_t<ERest...>, exp<Dim, Num, Den>>;
+    };
+
+    template<Exponent... Es, int Num, int Den, Exponent... ERest>
+    struct extract<exp<dimension<Es...>, Num, Den>, ERest...> {
+      using type = type_list_push_front<extract_t<ERest...>, exp_multiply_t<Es, Num, Den>...>;
+    };
+
+    template<Dimension Dim, int Num, int Den, Exponent... ERest>
+    struct extract<exp<Dim, Num, Den>, ERest...> {
+      using type = extract_t<exp<downcast_from<Dim>, Num, Den>, ERest...>;
+    };
+
   }  // namespace detail
 
   template<Exponent... Es>
   struct make_dimension {
-    using type = detail::dim_consolidate_t<type_list_sort<dimension<Es...>, exp_less>>;
+    using type = detail::dim_consolidate_t<type_list_sort<detail::extract_t<Es...>, exp_less>>;
   };
 
   template<Exponent... Es>
diff --git a/src/include/units/dimensions/acceleration.h b/src/include/units/dimensions/acceleration.h
index 5a3a88a0..dfb8efc5 100644
--- a/src/include/units/dimensions/acceleration.h
+++ b/src/include/units/dimensions/acceleration.h
@@ -26,7 +26,7 @@
 
 namespace std::experimental::units {
 
-  struct acceleration : make_dimension_t<exp<base_dim_length, 1>, exp<base_dim_time, -2>> {};
+  struct acceleration : make_dimension_t<exp<velocity, 1>, exp<base_dim_time, -1>> {};
   template<> struct downcasting_traits<downcast_from<acceleration>> : downcast_to<acceleration> {};
 
   template<typename T>
diff --git a/src/include/units/dimensions/base_dimensions.h b/src/include/units/dimensions/base_dimensions.h
index 6b6db449..732e88e0 100644
--- a/src/include/units/dimensions/base_dimensions.h
+++ b/src/include/units/dimensions/base_dimensions.h
@@ -26,12 +26,12 @@
 
 namespace std::experimental::units {
 
-  inline constexpr base_dimension base_dim_length{"length"};
-  inline constexpr base_dimension base_dim_mass{"mass"};
-  inline constexpr base_dimension base_dim_time{"time"};
-  inline constexpr base_dimension base_dim_current{"current"};
-  inline constexpr base_dimension base_dim_temperature{"temperature"};
-  inline constexpr base_dimension base_dim_substance{"substance"};
-  inline constexpr base_dimension base_dim_luminous_intensity{"luminous intensity"};
+  struct base_dim_length { static constexpr const char* value = "length"; };
+  struct base_dim_mass { static constexpr const char* value = "mass"; };
+  struct base_dim_time { static constexpr const char* value = "time"; };
+  struct base_dim_current { static constexpr const char* value = "current"; };
+  struct base_dim_temperature { static constexpr const char* value = "temperature"; };
+  struct base_dim_substance { static constexpr const char* value = "substance"; };
+  struct base_dim_luminous_intensity { static constexpr const char* value = "luminous intensity"; };
 
 }  // namespace std::experimental::units
diff --git a/src/include/units/dimensions/capacitance.h b/src/include/units/dimensions/capacitance.h
index d1fbfe4f..770cacb9 100644
--- a/src/include/units/dimensions/capacitance.h
+++ b/src/include/units/dimensions/capacitance.h
@@ -28,7 +28,7 @@
 
 namespace std::experimental::units {
 
-  struct capacitance : make_dimension_t<exp<base_dim_mass, -1>, exp<base_dim_length, -2>, exp<base_dim_time, 4>, exp<base_dim_current, 2>> {};
+  struct capacitance : make_dimension_t<exp<electric_charge, 1>, exp<voltage, -1>> {};
   template<> struct downcasting_traits<downcast_from<capacitance>> : downcast_to<capacitance> {};
 
   template<typename T>
diff --git a/src/include/units/dimensions/energy.h b/src/include/units/dimensions/energy.h
index 3cd1ff5d..c1acfca1 100644
--- a/src/include/units/dimensions/energy.h
+++ b/src/include/units/dimensions/energy.h
@@ -28,7 +28,7 @@
 
 namespace std::experimental::units {
 
-  struct energy : make_dimension_t<exp<base_dim_mass, 1>, exp<base_dim_length, 2>, exp<base_dim_time, -2>> {};
+  struct energy : make_dimension_t<exp<force, 1>, exp<length, 1>> {};
   template<> struct downcasting_traits<downcast_from<energy>> : downcast_to<energy> {};
 
   template<typename T>
diff --git a/src/include/units/dimensions/force.h b/src/include/units/dimensions/force.h
index 7f0e24c7..f67ca285 100644
--- a/src/include/units/dimensions/force.h
+++ b/src/include/units/dimensions/force.h
@@ -24,12 +24,11 @@
 
 #include <units/dimensions/base_dimensions.h>
 #include <units/dimensions/mass.h>
-#include <units/dimensions/length.h>
-#include <units/dimensions/time.h>
+#include <units/dimensions/acceleration.h>
 
 namespace std::experimental::units {
 
-  struct force : make_dimension_t<exp<base_dim_mass, 1>, exp<base_dim_length, 1>, exp<base_dim_time, -2>> {};
+  struct force : make_dimension_t<exp<base_dim_mass, 1>, exp<acceleration, 1>> {};
   template<> struct downcasting_traits<downcast_from<force>> : downcast_to<force> {};
 
   template<typename T>
diff --git a/src/include/units/dimensions/power.h b/src/include/units/dimensions/power.h
index 63f39135..ca309ca1 100644
--- a/src/include/units/dimensions/power.h
+++ b/src/include/units/dimensions/power.h
@@ -27,7 +27,7 @@
 
 namespace std::experimental::units {
 
-  struct power : make_dimension_t<exp<base_dim_mass, 1>, exp<base_dim_length, 2>, exp<base_dim_time, -3>> {};
+  struct power : make_dimension_t<exp<energy, 1>, exp<base_dim_time, -1>> {};
   template<> struct downcasting_traits<downcast_from<power>> : downcast_to<power> {};
 
   template<typename T>
diff --git a/src/include/units/dimensions/pressure.h b/src/include/units/dimensions/pressure.h
index d3ec83b5..8c630bc8 100644
--- a/src/include/units/dimensions/pressure.h
+++ b/src/include/units/dimensions/pressure.h
@@ -24,10 +24,11 @@
 
 #include <units/dimensions/base_dimensions.h>
 #include <units/dimensions/force.h>
+#include <units/dimensions/area.h>
 
 namespace std::experimental::units {
 
-  struct pressure : make_dimension_t<exp<base_dim_mass, 1>, exp<base_dim_length, -1>, exp<base_dim_time, -2>> {};
+  struct pressure : make_dimension_t<exp<force, 1>, exp<area, -1>> {};
   template<> struct downcasting_traits<downcast_from<pressure>> : downcast_to<pressure> {};
 
   template<typename T>
diff --git a/src/include/units/dimensions/voltage.h b/src/include/units/dimensions/voltage.h
index 0959dfca..b4bece2c 100644
--- a/src/include/units/dimensions/voltage.h
+++ b/src/include/units/dimensions/voltage.h
@@ -30,7 +30,7 @@
 
 namespace std::experimental::units {
 
-  struct voltage : make_dimension_t<exp<base_dim_mass, 1>, exp<base_dim_length, 2>, exp<base_dim_time, -3>, exp<base_dim_current, -1>> {};
+  struct voltage : make_dimension_t<exp<power, 1>, exp<base_dim_current, -1>> {};
   template<> struct downcasting_traits<downcast_from<voltage>> : downcast_to<voltage> {};
 
   template<typename T>
diff --git a/src/include/units/unit.h b/src/include/units/unit.h
index 15e69e61..bbc342c1 100644
--- a/src/include/units/unit.h
+++ b/src/include/units/unit.h
@@ -63,19 +63,19 @@ namespace std::experimental::units {
     template<typename E, typename... Rest>
     struct get_unit_base_dim<dimension<E, Rest...>> {
       static_assert(sizeof...(Rest) == 0, "Base unit expected");
-      static constexpr const base_dimension& dimension = E::dimension;
+      using dimension = E::dimension;
     };
 
-    template<const base_dimension& BaseDimension, typename... Us>
+    template<BaseDimension BD, typename... Us>
     struct get_ratio {
       using ratio = ::std::experimental::units::ratio<1>;
     };
 
-    template<const base_dimension& BaseDimension, typename U, typename... Rest>
-    struct get_ratio<BaseDimension, U, Rest...> {
-      static constexpr const base_dimension& unit_base_dim = get_unit_base_dim<typename U::dimension::base_type>::dimension;
-      using ratio = conditional<&unit_base_dim == &BaseDimension, typename U::ratio,
-                                typename get_ratio<BaseDimension, Rest...>::ratio>;
+    template<BaseDimension BD, typename U, typename... Rest>
+    struct get_ratio<BD, U, Rest...> {
+      using unit_base_dim = get_unit_base_dim<typename U::dimension::base_type>::dimension;
+      using ratio = conditional<std::is_same_v<unit_base_dim, BD>, typename U::ratio,
+                                typename get_ratio<BD, Rest...>::ratio>;
     };
 
     template<typename Result, int UnitExpNum, int UnitExpDen, typename UnitRatio>
@@ -105,7 +105,7 @@ namespace std::experimental::units {
     template<typename E, typename... Rest, typename... Us>
     struct derived_ratio<dimension<E, Rest...>, Us...> {
       using rest_ratio = derived_ratio<dimension<Rest...>, Us...>::ratio;
-      using e_ratio = get_ratio<E::dimension, Us...>::ratio;
+      using e_ratio = get_ratio<typename E::dimension, Us...>::ratio;
       using ratio = ratio_op<rest_ratio, E::num, E::den, e_ratio>::ratio;
     };
 
diff --git a/test/unit_test/test_custom_units.cpp b/test/unit_test/test_custom_units.cpp
index b8349b89..6ffeab57 100644
--- a/test/unit_test/test_custom_units.cpp
+++ b/test/unit_test/test_custom_units.cpp
@@ -29,7 +29,7 @@ namespace {
 
   using namespace std::experimental;
 
-  inline constexpr units::base_dimension base_dim_digital_information{"digital information"};
+  struct base_dim_digital_information { static constexpr const char* value = "digital information"; };
 
   struct digital_information : units::make_dimension_t<units::exp<base_dim_digital_information, 1>> {};
 
diff --git a/test/unit_test/test_dimension.cpp b/test/unit_test/test_dimension.cpp
index 182c54b5..93d10efa 100644
--- a/test/unit_test/test_dimension.cpp
+++ b/test/unit_test/test_dimension.cpp
@@ -27,16 +27,31 @@ using namespace std::experimental::units;
 
 namespace {
 
-  inline constexpr base_dimension d0{"d0"};
-  inline constexpr base_dimension d1{"d1"};
-  inline constexpr base_dimension d2{"d2"};
-  inline constexpr base_dimension d3{"d3"};
+  struct d0 { static constexpr const char* value = "d0"; };
+  struct d1 { static constexpr const char* value = "d1"; };
+  struct d2 { static constexpr const char* value = "d2"; };
+  struct d3 { static constexpr const char* value = "d3"; };
 
   // exp_invert
 
   static_assert(std::is_same_v<exp_invert_t<exp<d0, 1>>, exp<d0, -1>>);
   static_assert(std::is_same_v<exp_invert_t<exp<d1, -1>>, exp<d1, 1>>);
 
+  // extract
+
+  template<typename T>
+  struct typeinfo;
+
+  static_assert(std::is_same_v<detail::extract_t<>, dimension<>>);
+  static_assert(std::is_same_v<detail::extract_t<exp<d0, 1>>, dimension<exp<d0, 1>>>);
+  static_assert(std::is_same_v<detail::extract_t<exp<d0, 1>, exp<d1, 2>>, dimension<exp<d0, 1>, exp<d1, 2>>>);
+  using dim0 = dimension<>;
+  using dim1 = dimension<exp<d0, 1>>;
+  using dim2 = dimension<exp<d0, 1>, exp<d1, 2>>;
+  static_assert(std::is_same_v<detail::extract_t<exp<dim0, 2>, exp<d0, 1>>, dimension<exp<d0, 1>>>);
+  static_assert(std::is_same_v<detail::extract_t<exp<dim1, 2>, exp<d0, 1>>, dimension<exp<d0, 2>, exp<d0, 1>>>);
+  static_assert(std::is_same_v<detail::extract_t<exp<dim2, -2>, exp<d0, 1>, exp<d1, 2>>, dimension<exp<d0, -2>, exp<d1, -4>, exp<d0, 1>, exp<d1, 2>>>);
+
   // make_dimension
 
   static_assert(std::is_same_v<make_dimension_t<exp<d0, 1>>, dimension<exp<d0, 1>>>);
diff --git a/test/unit_test/test_type_list.cpp b/test/unit_test/test_type_list.cpp
index ccf688a4..e3c8db55 100644
--- a/test/unit_test/test_type_list.cpp
+++ b/test/unit_test/test_type_list.cpp
@@ -82,8 +82,8 @@ namespace {
       std::is_same_v<type_list_split_half<type_list<int, long, double, float>>::second_list, type_list<double, float>>);
 
   // type_list_merge_sorted
-  inline constexpr base_dimension d0{"d0"};
-  inline constexpr base_dimension d1{"d1"};
+  struct d0 { static constexpr const char* value = "d0"; };
+  struct d1 { static constexpr const char* value = "d1"; };
 
   static_assert(std::is_same_v<type_list_merge_sorted<type_list<exp<d0, 1>>, type_list<exp<d1, 1>>, exp_less>,
                                type_list<exp<d0, 1>, exp<d1, 1>>>);