Merge branch 'downcasting_2.0'

2025-08-04 20:54:28 +02:00 · 2020-09-09 20:31:03 +02:00
parent dcc2d24c8e 72ed03fbf9
commit e367afed71
22 changed files with 300 additions and 145 deletions
--- a/conanfile.py
+++ b/conanfile.py
@@ -48,6 +48,12 @@ class UnitsConan(ConanFile):
        "fmt/7.0.3",
        "ms-gsl/3.1.0"
    )
+    options = {
+        "downcast": ["off", "on", "auto"]
+    }
+    default_options = {
+        "downcast": "auto"
+    }
    exports = ["LICENSE.md"]
    exports_sources = ["docs/*", "src/*", "test/*", "cmake/*", "example/*","CMakeLists.txt"]
    # scm = {
@@ -77,6 +83,13 @@ class UnitsConan(ConanFile):

    def _configure_cmake(self, folder="src"):
        cmake = CMake(self)
+        if self.options.downcast_dispatch_mode == "off":
+            cmake.definitions["UNITS_DOWNCAST"] = 0
+        elif self.options.downcast_dispatch_mode == "on":
+            cmake.definitions["UNITS_DOWNCAST"] = 1
+        elif self.options.downcast_dispatch_mode == "auto":
+            cmake.definitions["UNITS_DOWNCAST"] = 2
+
        if self._run_tests:
            # developer's mode (unit tests, examples, documentation, restrictive compilation warnings, ...)
            cmake.configure()
--- a/docs/design/quantity.rst
+++ b/docs/design/quantity.rst
@@ -23,12 +23,7 @@ a few additional member types and functions::
    };

    template<typename D1, typename U1, typename Rep1, typename D2, typename U2, typename Rep2>
-      requires detail::basic_arithmetic<Rep1, Rep2> && equivalent_dim<D1, dim_invert<D2>>
-    [[nodiscard]] constexpr ScalableNumber auto operator*(const quantity<D1, U1, Rep1>& lhs,
-                                                          const quantity<D2, U2, Rep2>& rhs);
-
-    template<typename D1, typename U1, typename Rep1, typename D2, typename U2, typename Rep2>
-      requires detail::basic_arithmetic<Rep1, Rep2> && (!equivalent_dim<D1, dim_invert<D2>>)
+      requires detail::basic_arithmetic<Rep1, Rep2> && (!equivalent<D1, dim_invert<D2>>)
    [[nodiscard]] constexpr Quantity auto operator*(const quantity<D1, U1, Rep1>& lhs,
                                                    const quantity<D2, U2, Rep2>& rhs);

@@ -38,17 +33,12 @@ a few additional member types and functions::
                                                    const quantity<D, U, Rep>& q);

    template<typename D1, typename U1, typename Rep1, typename D2, typename U2, typename Rep2>
-      requires detail::basic_arithmetic<Rep1, Rep2> && equivalent_dim<D1, D2>
-    [[nodiscard]] constexpr ScalableNumber auto operator/(const quantity<D1, U1, Rep1>& lhs,
-                                                          const quantity<D2, U2, Rep2>& rhs);
-
-    template<typename D1, typename U1, typename Rep1, typename D2, typename U2, typename Rep2>
-      requires detail::basic_arithmetic<Rep1, Rep2> && (!equivalent_dim<D1, D2>)
+      requires detail::basic_arithmetic<Rep1, Rep2> && (!equivalent<D1, D2>)
    [[nodiscard]] constexpr Quantity auto operator/(const quantity<D1, U1, Rep1>& lhs,
                                                    const quantity<D2, U2, Rep2>& rhs);

 Additional functions provide the support for operations that result in a
-different dimension type than those of their arguments. ``equivalent_dim``
+different dimension type than those of their arguments. ``equivalent``
 constraint requires two dimensions to be either the same or have convertible
 units of base dimension (with the same reference unit).

--- a/example/alternative_namespaces/conversion_factor.cpp
+++ b/example/alternative_namespaces/conversion_factor.cpp
@@ -28,7 +28,7 @@
 namespace {

 template<units::Quantity Target, units::Quantity Source>
-  requires units::equivalent_dim<typename Source::dimension, typename Target::dimension>
+  requires units::equivalent<typename Source::dimension, typename Target::dimension>
 inline constexpr std::common_type_t<typename Target::rep, typename Source::rep> conversion_factor(Target, Source)
 {
  // get quantities looking like inputs but with Q::rep that doesn't have narrowing conversion
--- a/example/conversion_factor.cpp
+++ b/example/conversion_factor.cpp
@@ -27,7 +27,7 @@
 namespace {

 template<units::Quantity Target, units::Quantity Source>
-  requires units::equivalent_dim<typename Source::dimension, typename Target::dimension>
+  requires units::equivalent<typename Source::dimension, typename Target::dimension>
 inline constexpr std::common_type_t<typename Target::rep, typename Source::rep> conversion_factor(Target, Source)
 {
  // get quantities looking like inputs but with Q::rep that doesn't have narrowing conversion
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -27,6 +27,9 @@ project(mp-units
    LANGUAGES CXX
 )

+set(DOWNCAST_MODE AUTO CACHE STRING "Select downcasting mode")
+set_property(CACHE DOWNCAST_MODE PROPERTY STRINGS AUTO ON OFF)
+
 # set path to custom cmake modules
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/../cmake")

@@ -63,6 +66,7 @@ target_include_directories(mp-units
        $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
        $<INSTALL_INTERFACE:include>
 )
+
 if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
    target_link_libraries(mp-units
        INTERFACE
@@ -80,6 +84,17 @@ elseif(CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
    )
 endif()

+if(DOWNCAST_MODE STREQUAL "AUTO")
+    message(STATUS "Configuring DOWNCAST_MODE=AUTOMATIC")
+    target_compile_definitions(mp-units INTERFACE DOWNCAST_MODE=2)
+elseif(DOWNCAST_MODE)
+    message(STATUS "Configuring DOWNCAST_MODE=ON")
+    target_compile_definitions(mp-units INTERFACE DOWNCAST_MODE=1)
+else()
+    message(STATUS "Configuring DOWNCAST_MODE=OFF")
+    target_compile_definitions(mp-units INTERFACE DOWNCAST_MODE=0)
+endif()
+
 add_library(mp-units::mp-units ALIAS mp-units)

 # installation info
--- a/src/include/units/bits/common_quantity.h
+++ b/src/include/units/bits/common_quantity.h
@@ -23,6 +23,8 @@
 #pragma once

 #include <units/bits/dimension_op.h>
+#include <units/bits/equivalent.h>
+#include <units/quantity_cast.h>

 namespace units {

@@ -55,9 +57,12 @@ struct common_quantity_impl<quantity<D1, U1, Rep1>, quantity<D2, U2, Rep2>, Rep>

 template<typename D1, typename U1, typename Rep1, typename D2, typename U2, typename Rep2, typename Rep>
 struct common_quantity_impl<quantity<D1, U1, Rep1>, quantity<D2, U2, Rep2>, Rep> {
+  using dimension = conditional<is_specialization_of<D1, unknown_dimension>, D2, D1>;
  static constexpr ratio r1 = D1::base_units_ratio * U1::ratio;
  static constexpr ratio r2 = D2::base_units_ratio * U2::ratio;
-  using type = quantity<D1, downcast_unit<D1, common_ratio(r1, r2)>, Rep>;
+  static constexpr ratio cr = common_ratio(r1, r2);
+  using unit = downcast_unit<dimension, cr / dimension::base_units_ratio>;
+  using type = quantity<dimension, unit, Rep>;
 };

 template<typename D, typename U, typename Rep>
@@ -66,7 +71,7 @@ quantity_point<D, U, Rep> common_quantity_point_impl(quantity<D, U, Rep>);
 }  // namespace detail

 template<Quantity Q1, Quantity Q2, ScalableNumber Rep = std::common_type_t<typename Q1::rep, typename Q2::rep>>
-  requires equivalent_dim<typename Q1::dimension, typename Q2::dimension>
+  requires equivalent<typename Q1::dimension, typename Q2::dimension>
 using common_quantity = TYPENAME detail::common_quantity_impl<Q1, Q2, Rep>::type;

 template<QuantityPoint QP1, QuantityPoint QP2>
@@ -79,7 +84,7 @@ using common_quantity_point = decltype(
 namespace std {

 template<units::Quantity Q1, units::Quantity Q2>
-  requires units::equivalent_dim<typename Q1::dimension, typename Q2::dimension>
+  requires units::equivalent<typename Q1::dimension, typename Q2::dimension>
 struct common_type<Q1, Q2> {
  using type = units::common_quantity<Q1, Q2>;
 };
--- a/src/include/units/bits/dimension_op.h
+++ b/src/include/units/bits/dimension_op.h
@@ -27,40 +27,6 @@

 namespace units {

-// equivalent_dim
-namespace detail {
-
-template<BaseDimension D1, BaseDimension D2>
-using equivalent_base_dim = std::conjunction<std::bool_constant<D1::symbol == D2::symbol>,
-                                             same_unit_reference<typename D1::base_unit, typename D2::base_unit>>;
-
-template<Dimension D1, Dimension D2>
-struct equivalent_dim_impl : std::false_type {};
-
-template<BaseDimension D1, BaseDimension D2>
-struct equivalent_dim_impl<D1, D2> : std::disjunction<is_same<D1, D2>, equivalent_base_dim<D1, D2>> {};
-
-template<Exponent E1, Exponent E2>
-struct equivalent_exp : std::false_type {};
-
-template<BaseDimension Dim1, std::intmax_t Num, std::intmax_t Den, BaseDimension Dim2>
-struct equivalent_exp<exponent<Dim1, Num, Den>, exponent<Dim2, Num, Den>> : equivalent_dim_impl<Dim1, Dim2> {};
-
-template<DerivedDimension D1, DerivedDimension D2>
-struct equivalent_derived_dim : std::false_type {};
-
-template<typename... Es1, typename... Es2>
-  requires (sizeof...(Es1) == sizeof...(Es2))
-struct equivalent_derived_dim<derived_dimension_base<Es1...>, derived_dimension_base<Es2...>> : std::conjunction<equivalent_exp<Es1, Es2>...> {};
-
-template<DerivedDimension D1, DerivedDimension D2>
-struct equivalent_dim_impl<D1, D2> : std::disjunction<is_same<D1, D2>, equivalent_derived_dim<downcast_base_t<D1>, downcast_base_t<D2>>> {};
-
-} // namespace detail
-
-template<Dimension D1, Dimension D2>
-inline constexpr bool equivalent_dim = detail::equivalent_dim_impl<D1, D2>::value;
-
 /**
 * @brief Unknown dimension
 * 
--- a/src/include/units/bits/equivalent.h
+++ b/src/include/units/bits/equivalent.h
@@ -0,0 +1,98 @@
+// The MIT License (MIT)
+//
+// Copyright (c) 2018 Mateusz Pusz
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+#pragma once
+
+#include <units/concepts.h>
+#include <units/unit.h>
+
+namespace units {
+
+namespace detail {
+
+template<typename T, typename U>
+struct equivalent_impl : std::false_type {
+};
+
+// units
+
+template<Unit U1, Unit U2>
+struct equivalent_impl<U1, U2> : std::disjunction<std::is_same<U1, U2>, std::is_base_of<U1, U2>, std::is_base_of<U2, U1>> {};
+
+
+// dimensions
+
+template<BaseDimension D1, BaseDimension D2>
+struct equivalent_base_dim :
+    std::conjunction<std::bool_constant<D1::symbol == D2::symbol>,
+                     same_unit_reference<typename D1::base_unit, typename D2::base_unit>> {
+};
+
+template<BaseDimension D1, BaseDimension D2>
+struct equivalent_impl<D1, D2> : std::disjunction<std::is_same<D1, D2>, equivalent_base_dim<D1, D2>> {
+};
+
+template<Exponent E1, Exponent E2>
+struct equivalent_exp : std::false_type {
+};
+
+template<BaseDimension Dim1, std::intmax_t Num, std::intmax_t Den, BaseDimension Dim2>
+struct equivalent_exp<exponent<Dim1, Num, Den>, exponent<Dim2, Num, Den>> : equivalent_impl<Dim1, Dim2> {
+};
+
+template<DerivedDimension D1, DerivedDimension D2>
+struct equivalent_derived_dim : std::false_type {
+};
+
+template<typename... Es1, typename... Es2>
+  requires(sizeof...(Es1) == sizeof...(Es2))
+struct equivalent_derived_dim<derived_dimension_base<Es1...>, derived_dimension_base<Es2...>> :
+    std::conjunction<equivalent_exp<Es1, Es2>...> {
+};
+
+template<DerivedDimension D1, DerivedDimension D2>
+struct equivalent_impl<D1, D2> :
+    std::disjunction<std::is_same<D1, D2>, std::is_base_of<D1, D2>, std::is_base_of<D2, D1>,
+                     equivalent_derived_dim<downcast_base_t<D1>, downcast_base_t<D2>>> {
+};
+
+
+// additionally accounts for unknown dimensions
+template<Unit U1, Dimension D1, Unit U2, Dimension D2>
+struct equivalent_unit : std::disjunction<equivalent_impl<U1, U2>,
+                                          std::bool_constant<U1::ratio / dimension_unit<D1>::ratio == U2::ratio / dimension_unit<D2>::ratio>> {};
+
+// quantities and quantity points
+
+template<typename Q1, typename Q2>
+  requires (Quantity<Q1> && Quantity<Q2>) || (QuantityPoint<Q1> && QuantityPoint<Q2>)
+struct equivalent_impl<Q1, Q2> : std::disjunction<std::is_same<Q1, Q2>,
+                                                  std::conjunction<equivalent_impl<typename Q1::dimension, typename Q2::dimension>,
+                                                                   equivalent_unit<typename Q1::unit, typename Q1::dimension,
+                                                                                   typename Q2::unit, typename Q2::dimension>>> {};
+
+}  // namespace detail
+
+template<typename T, typename U>
+inline constexpr bool equivalent = detail::equivalent_impl<T, U>::value;
+
+}  // namespace units
--- a/src/include/units/bits/external/downcasting.h
+++ b/src/include/units/bits/external/downcasting.h
@@ -25,44 +25,75 @@
 #include <units/bits/external/hacks.h>
 #include <type_traits>

+#ifdef DOWNCAST_MODE
+#if DOWNCAST_MODE < 0 || DOWNCAST_MODE > 2
+#error "Invalid DOWNCAST_MODE value"
+#endif
+#else
+#define DOWNCAST_MODE 2
+#endif
+
 namespace units {

 template<typename BaseType>
 struct downcast_base {
  using downcast_base_type = BaseType;
  friend auto downcast_guide(downcast_base);
+  friend auto downcast_poison_pill(downcast_base);
 };

 template<typename T>
 concept Downcastable =
-  requires {
-    typename T::downcast_base_type;
-  } &&
+  requires { typename T::downcast_base_type; } &&
  std::derived_from<T, downcast_base<typename T::downcast_base_type>>;

-template<typename Target, Downcastable T>
-struct downcast_child : T {
-  friend auto downcast_guide(typename downcast_child::downcast_base /* base */) { return Target(); }
-};
-
-namespace detail {
-
 template<typename T>
-concept has_downcast =
+concept has_downcast_guide =
  requires {
    downcast_guide(std::declval<downcast_base<T>>());
  };

+template<typename T>
+concept has_downcast_poison_pill =
+  requires {
+    downcast_poison_pill(std::declval<downcast_base<T>>());
+  };
+
+template<typename Target, Downcastable T>
+struct downcast_child : T {
+  friend auto downcast_guide(typename T::downcast_base)
+  { return Target(); }
+};
+
+template<Downcastable T>
+struct downcast_poison : T {
+  friend auto downcast_poison_pill(typename T::downcast_base)
+  { return true; }
+};
+
+enum class downcast_mode {
+  off = 0,         // no downcasting at all
+  on = 1,          // downcasting always forced -> compile-time errors in case of duplicated definitions
+  automatic = 2    // downcasting automatically enabled if no collisions are present
+};
+
+template<typename Target, Downcastable T, downcast_mode mode = static_cast<downcast_mode>(DOWNCAST_MODE)>
+struct downcast_dispatch : std::conditional_t<mode == downcast_mode::off, T,
+                                              std::conditional_t<mode == downcast_mode::automatic && has_downcast_guide<T>,
+                                                                 downcast_poison<T>, downcast_child<Target, T>>> {};
+
+namespace detail {
+
 template<typename T>
 constexpr auto downcast_impl()
 {
-  if constexpr (has_downcast<T>)
+  if constexpr(has_downcast_guide<T> && !has_downcast_poison_pill<T>)
    return decltype(downcast_guide(std::declval<downcast_base<T>>()))();
  else
    return T();
 }

-}  // namespace detail
+}

 template<Downcastable T>
 using downcast = decltype(detail::downcast_impl<T>());
--- a/src/include/units/derived_dimension.h
+++ b/src/include/units/derived_dimension.h
@@ -78,7 +78,7 @@ using make_dimension = TYPENAME to_derived_dimension_base<typename dim_consolida
 * @tparam Es the list of exponents of ingredient dimensions
 */
 template<typename Child, Unit U, Exponent... Es>
-struct derived_dimension : downcast_child<Child, typename detail::make_dimension<Es...>> {
+struct derived_dimension : downcast_dispatch<Child, typename detail::make_dimension<Es...>> {
  using recipe = exponent_list<Es...>;
  using coherent_unit = U;
  static constexpr ratio base_units_ratio = detail::base_units_ratio(typename derived_dimension::exponents());
--- a/src/include/units/prefix.h
+++ b/src/include/units/prefix.h
@@ -71,7 +71,7 @@ struct prefix_base : downcast_base<prefix_base<PF, R>> {
 */
 template<typename Child, PrefixFamily PF, basic_symbol_text Symbol, ratio R>
  requires (!std::same_as<PF, no_prefix>)
-struct prefix : downcast_child<Child, detail::prefix_base<PF, R>> {
+struct prefix : downcast_dispatch<Child, detail::prefix_base<PF, R>, downcast_mode::on> {
  static constexpr auto symbol = Symbol;
 };

--- a/src/include/units/quantity.h
+++ b/src/include/units/quantity.h
@@ -76,7 +76,7 @@ public:
  constexpr explicit(!(std::is_same_v<dimension, dim_one> && std::is_same_v<unit, one>)) quantity(const Value& v) : value_{static_cast<rep>(v)} {}

  template<Quantity Q2>
-    requires equivalent_dim<D, typename Q2::dimension> &&
+    requires equivalent<D, typename Q2::dimension> &&
             detail::safe_convertible<typename Q2::rep, rep> &&
             detail::safe_divisible<rep, Q2, quantity>
  constexpr quantity(const Q2& q) : value_{quantity_cast<quantity>(q).count()} {}
@@ -314,7 +314,7 @@ public:
  }

  template<typename D2, typename U2, typename Rep2>
-    requires equivalent_dim<D, D2> &&
+    requires equivalent<D, D2> &&
             std::three_way_comparable_with<Rep, Rep2>
  [[nodiscard]] friend constexpr auto operator<=>(const quantity& lhs, const quantity<D2, U2, Rep2>& rhs)
  {
@@ -323,7 +323,7 @@ public:
  }

  template<typename D2, typename U2, typename Rep2>
-    requires equivalent_dim<D, D2> &&
+    requires equivalent<D, D2> &&
             std::equality_comparable_with<Rep, Rep2>
  [[nodiscard]] friend constexpr bool operator==(const quantity& lhs, const quantity<D2, U2, Rep2>& rhs)
  {
--- a/src/include/units/quantity_cast.h
+++ b/src/include/units/quantity_cast.h
@@ -59,7 +59,7 @@ constexpr auto quantity_ratio(const quantity<D, U, Rep>&)

 // QuantityOf
 template<typename T, typename Dim>
-concept QuantityOf = Quantity<T> && Dimension<Dim> && equivalent_dim<typename T::dimension, Dim>;
+concept QuantityOf = Quantity<T> && Dimension<Dim> && equivalent<typename T::dimension, Dim>;

 // quantity_cast
 namespace detail {
@@ -337,7 +337,7 @@ template<Quantity To, typename D, typename U, typename Rep>
 * @tparam ToD a dimension type to use for a target quantity
 */
 template<Dimension ToD, typename D, typename U, typename Rep>
-  requires equivalent_dim<ToD, D>
+  requires equivalent<ToD, D>
 [[nodiscard]] constexpr auto quantity_cast(const quantity<D, U, Rep>& q)
 {
  return quantity_cast<quantity<ToD, dimension_unit<ToD>, Rep>>(q);
--- a/src/include/units/unit.h
+++ b/src/include/units/unit.h
@@ -74,7 +74,7 @@ struct same_unit_reference : is_same<typename U1::reference, typename U2::refere
 * @tparam Child inherited class type used by the downcasting facility (CRTP Idiom)
 */
 template<typename Child>
-struct unit : downcast_child<Child, scaled_unit<ratio(1), Child>> {
+struct unit : downcast_dispatch<Child, scaled_unit<ratio(1), Child>> {
  static constexpr bool is_named = false;
  using prefix_family = no_prefix;
 };
@@ -92,7 +92,7 @@ struct unit : downcast_child<Child, scaled_unit<ratio(1), Child>> {
 * @tparam PF no_prefix or a type of prefix family
 */
 template<typename Child, basic_symbol_text Symbol, PrefixFamily PF>
-struct named_unit : downcast_child<Child, scaled_unit<ratio(1), Child>> {
+struct named_unit : downcast_dispatch<Child, scaled_unit<ratio(1), Child>> {
  static constexpr bool is_named = true;
  static constexpr auto symbol = Symbol;
  using prefix_family = PF;
@@ -114,7 +114,7 @@ struct named_unit : downcast_child<Child, scaled_unit<ratio(1), Child>> {
 */
 template<typename Child, basic_symbol_text Symbol, PrefixFamily PF, ratio R, Unit U>
  requires UnitRatio<R>
-struct named_scaled_unit : downcast_child<Child, scaled_unit<R * U::ratio, typename U::reference>> {
+struct named_scaled_unit : downcast_dispatch<Child, scaled_unit<R * U::ratio, typename U::reference>> {
  static constexpr bool is_named = true;
  static constexpr auto symbol = Symbol;
  using prefix_family = PF;
@@ -133,7 +133,7 @@ struct named_scaled_unit : downcast_child<Child, scaled_unit<R * U::ratio, typen
 */
 template<typename Child, Prefix P, Unit U>
  requires U::is_named && std::same_as<typename P::prefix_family, typename U::prefix_family>
-struct prefixed_unit : downcast_child<Child, scaled_unit<P::ratio * U::ratio, typename U::reference>> {
+struct prefixed_unit : downcast_dispatch<Child, scaled_unit<P::ratio * U::ratio, typename U::reference>> {
  static constexpr bool is_named = true;
  static constexpr auto symbol = P::symbol + U::symbol;
  using prefix_family = no_prefix;
@@ -155,7 +155,7 @@ struct prefixed_unit : downcast_child<Child, scaled_unit<P::ratio * U::ratio, ty
 template<typename Child, DerivedDimension Dim, Unit U, Unit... URest>
  requires detail::same_scaled_units<typename Dim::recipe, U, URest...> &&
           (U::is_named && (URest::is_named && ... && true))
-struct deduced_unit : downcast_child<Child, detail::deduced_unit<Dim, U, URest...>> {
+struct deduced_unit : downcast_dispatch<Child, detail::deduced_unit<Dim, U, URest...>> {
  static constexpr bool is_named = false;
  static constexpr auto symbol = detail::deduced_symbol_text<Dim, U, URest...>();
  using prefix_family = no_prefix;
@@ -179,7 +179,7 @@ template<typename Child, DerivedDimension Dim, Unit U, Unit... URest>
  requires detail::same_scaled_units<typename Dim::recipe, U, URest...> &&
           (U::is_named && (URest::is_named && ... && true))
 // TODO - 'noble' is placeholder to sort of mean can pass its name on to other deduced units
-struct noble_deduced_unit : downcast_child<Child, detail::deduced_unit<Dim, U, URest...>> {
+struct noble_deduced_unit : downcast_dispatch<Child, detail::deduced_unit<Dim, U, URest...>> {
  static constexpr bool is_named = true;
  static constexpr auto symbol = detail::deduced_symbol_text<Dim, U, URest...>();
  using prefix_family = no_prefix;
@@ -203,7 +203,7 @@ struct noble_deduced_unit : downcast_child<Child, detail::deduced_unit<Dim, U, U
 */
 template<typename Child, DerivedDimension Dim, basic_symbol_text Symbol, PrefixFamily PF, Unit U, Unit... URest>
  requires detail::same_scaled_units<typename Dim::recipe, U, URest...>
-struct named_deduced_unit : downcast_child<Child, detail::deduced_unit<Dim, U, URest...>> {
+struct named_deduced_unit : downcast_dispatch<Child, detail::deduced_unit<Dim, U, URest...>> {
  static constexpr bool is_named = true;
  static constexpr auto symbol = Symbol;
  using prefix_family = PF;
--- a/test/unit_test/static/cgs_test.cpp
+++ b/test/unit_test/static/cgs_test.cpp
@@ -50,6 +50,8 @@ static_assert(centimetre::symbol == "cm");

 // speed

+static_assert((10_q_cm / 5_q_s).count() == 2);
+static_assert((2_q_cm_per_s).count() == 2);
 static_assert(10_q_cm / 5_q_s == 2_q_cm_per_s);
 static_assert(10_q_cm / 2_q_cm_per_s == 5_q_s);
 static_assert(10_q_cm == 2_q_cm_per_s * 5_q_s);
@@ -59,7 +61,11 @@ static_assert(detail::unit_text<dim_speed, centimetre_per_second>() == "cm/s");
 // area
 static_assert(centimetre::ratio / dimension_unit<dim_length>::ratio == ratio(1));

+static_assert((1_q_cm * 1_q_cm).count() == 1);
+static_assert((1_q_cm2).count() == 1);
 static_assert(1_q_cm * 1_q_cm == 1_q_cm2);
+static_assert(100_q_cm * 100_q_cm == area<physical::si::square_metre>(1));
+static_assert(100_q_cm * 100_q_cm == length<physical::si::metre>(1) * length<physical::si::metre>(1));
 static_assert(100_q_cm2 / 10_q_cm == 10_q_cm);

 static_assert(detail::unit_text<dim_area, square_centimetre>() == basic_symbol_text("cm²", "cm^2"));
--- a/test/unit_test/static/custom_unit_test.cpp
+++ b/test/unit_test/static/custom_unit_test.cpp
@@ -49,11 +49,14 @@ using amplitude_spectral_density = quantity<dim_amplitude_spectral_density, U, R

 namespace {

-static_assert(is_same_v<dimension_sqrt<dim_power_spectral_density>, dim_amplitude_spectral_density>);
-static_assert(is_same_v<dimension_pow<dim_amplitude_spectral_density, 2>, dim_power_spectral_density>);
+template<typename T, typename U>
+inline constexpr bool compare = DOWNCAST_MODE != 0 ? is_same_v<T, U> : (is_same_v<T, U> || units::equivalent<T, U>);

-static_assert(is_same_v<decltype(pow<2>(amplitude_spectral_density<volt_per_sqrt_hertz>(4))), decltype(power_spectral_density<volt2_per_hertz>(16))>);
-static_assert(is_same_v<decltype(sqrt(power_spectral_density<volt2_per_hertz>(16))), decltype(amplitude_spectral_density<volt_per_sqrt_hertz>(4))>);
+static_assert(compare<dimension_sqrt<dim_power_spectral_density>, dim_amplitude_spectral_density>);
+static_assert(compare<dimension_pow<dim_amplitude_spectral_density, 2>, dim_power_spectral_density>);
+
+static_assert(compare<decltype(pow<2>(amplitude_spectral_density<volt_per_sqrt_hertz>(4))), decltype(power_spectral_density<volt2_per_hertz>(16))>);
+static_assert(compare<decltype(sqrt(power_spectral_density<volt2_per_hertz>(16))), decltype(amplitude_spectral_density<volt_per_sqrt_hertz>(4))>);

 }

@@ -63,6 +66,5 @@ struct kilogram_per_second : unit<kilogram_per_second> {};
 struct dim_mass_rate : derived_dimension<dim_mass_rate, kilogram_per_second, units::exponent<dim_mass, 1>, units::exponent<dim_time, -1>> {};
 struct kilogram_per_hour : deduced_unit<kilogram_per_hour, dim_mass_rate, kilogram, hour> {};
 constexpr auto a = 1_q_kg / 1_q_h;
-static_assert(is_same_v<decltype(a)::unit, kilogram_per_hour>);

 }
--- a/test/unit_test/static/math_test.cpp
+++ b/test/unit_test/static/math_test.cpp
@@ -20,6 +20,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.

+#include "units/math.h"
+#include "units/physical/international/area.h"
 #include "units/physical/si/area.h"
 #include "units/physical/si/speed.h"
 #include "units/physical/international/area.h"
@@ -27,18 +29,20 @@

 namespace {

-  using namespace units;
-  using namespace units::physical::si::literals;
-  using namespace units::physical::international::literals;
+using namespace units;
+using namespace units::physical::si::literals;
+using namespace units::physical::international::literals;

-  static_assert(is_same_v<decltype(pow<0>(2_q_m)), std::int64_t>);
-  static_assert(is_same_v<decltype(pow<1>(2_q_m)), decltype(2_q_m)>);
-  static_assert(is_same_v<decltype(pow<2>(2_q_m)), decltype(4_q_m2)>);
-  static_assert(is_same_v<decltype(pow<2>(2_q_km)), decltype(4_q_km2)>);
-  static_assert(is_same_v<decltype(pow<2>(2_q_ft)), decltype(4_q_ft2)>);
-  static_assert(is_same_v<decltype(sqrt(4_q_m2)), decltype(2_q_m)>);
-  static_assert(is_same_v<decltype(sqrt(4_q_km2)), decltype(2_q_km)>);
-  static_assert(is_same_v<decltype(sqrt(4_q_ft2)), decltype(2_q_ft)>);
+template<typename T, typename U>
+inline constexpr bool compare = DOWNCAST_MODE != 0 ? is_same_v<T, U> : (is_same_v<T, U> || units::equivalent<T, U>);

+static_assert(compare<decltype(pow<0>(2_q_m)), std::int64_t>);
+static_assert(compare<decltype(pow<1>(2_q_m)), decltype(2_q_m)>);
+static_assert(compare<decltype(pow<2>(2_q_m)), decltype(4_q_m2)>);
+static_assert(compare<decltype(pow<2>(2_q_km)), decltype(4_q_km2)>);
+static_assert(compare<decltype(pow<2>(2_q_ft)), decltype(4_q_ft2)>);
+static_assert(compare<decltype(sqrt(4_q_m2)), decltype(2_q_m)>);
+static_assert(compare<decltype(sqrt(4_q_km2)), decltype(2_q_km)>);
+static_assert(compare<decltype(sqrt(4_q_ft2)), decltype(2_q_ft)>);

 }  // namespace
--- a/test/unit_test/static/quantity_point_test.cpp
+++ b/test/unit_test/static/quantity_point_test.cpp
@@ -35,6 +35,9 @@ namespace {
 using namespace units;
 using namespace units::physical::si;

+template<typename T, typename U>
+inline constexpr bool compare = DOWNCAST_MODE != 0 ? std::is_same_v<T, U> : (std::is_same_v<T, U> || units::equivalent<T, U>);
+
 // class invariants

 template<typename DimLength>
@@ -119,23 +122,23 @@ static_assert((quantity_point(2_q_m) -= 1_q_m).relative().count() == 1);

 // non-member arithmetic operators

-static_assert(is_same_v<decltype(quantity_point<dim_length, metre, int>() + length<metre, double>()),
+static_assert(compare<decltype(quantity_point<dim_length, metre, int>() + length<metre, double>()),
                             quantity_point<dim_length, metre, double>>);
-static_assert(is_same_v<decltype(length<metre, int>() + quantity_point<dim_length, metre, double>()),
+static_assert(compare<decltype(length<metre, int>() + quantity_point<dim_length, metre, double>()),
                             quantity_point<dim_length, metre, double>>);
-static_assert(is_same_v<decltype(quantity_point<dim_length, kilometre, int>() + length<metre, double>()),
+static_assert(compare<decltype(quantity_point<dim_length, kilometre, int>() + length<metre, double>()),
                             quantity_point<dim_length, metre, double>>);
-static_assert(is_same_v<decltype(length<kilometre, int>() + quantity_point<dim_length, metre, double>()),
+static_assert(compare<decltype(length<kilometre, int>() + quantity_point<dim_length, metre, double>()),
                             quantity_point<dim_length, metre, double>>);
-static_assert(is_same_v<decltype(quantity_point<dim_length, metre, double>() - length<metre, int>()),
+static_assert(compare<decltype(quantity_point<dim_length, metre, double>() - length<metre, int>()),
                             quantity_point<dim_length, metre, double>>);
-static_assert(is_same_v<decltype(quantity_point<dim_length, kilometre, double>() - length<metre, int>()),
+static_assert(compare<decltype(quantity_point<dim_length, kilometre, double>() - length<metre, int>()),
                             quantity_point<dim_length, metre, double>>);
 static_assert(
-    is_same_v<decltype(quantity_point<dim_length, metre, double>() - quantity_point<dim_length, metre, int>()),
+    compare<decltype(quantity_point<dim_length, metre, double>() - quantity_point<dim_length, metre, int>()),
                   length<metre, double>>);
 static_assert(
-    is_same_v<decltype(quantity_point<dim_length, kilometre, double>() - quantity_point<dim_length, metre, int>()),
+    compare<decltype(quantity_point<dim_length, kilometre, double>() - quantity_point<dim_length, metre, int>()),
                   length<metre, double>>);

 static_assert((1_q_m + km).relative().count() == 1001);
@@ -187,13 +190,13 @@ static_assert(QuantityPoint<quantity_point<dim_length, millimetre, int>>);

 // common_quantity_point

-static_assert(is_same_v<
+static_assert(compare<
              common_quantity_point<quantity_point<dim_length, metre, int>, quantity_point<dim_length, kilometre, int>>,
              quantity_point<dim_length, metre, int>>);
-static_assert(is_same_v<common_quantity_point<quantity_point<dim_length, kilometre, long long>,
+static_assert(compare<common_quantity_point<quantity_point<dim_length, kilometre, long long>,
                                                   quantity_point<dim_length, metre, int>>,
                             quantity_point<dim_length, metre, long long>>);
-static_assert(is_same_v<common_quantity_point<quantity_point<dim_length, kilometre, long long>,
+static_assert(compare<common_quantity_point<quantity_point<dim_length, kilometre, long long>,
                                                   quantity_point<dim_length, millimetre, double>>,
                             quantity_point<dim_length, millimetre, double>>);

@@ -208,8 +211,7 @@ static_assert(std::equality_comparable_with<decltype(quantity_point(1_q_m)), dec

 // quantity_cast

-static_assert(
-    is_same_v<decltype(quantity_point_cast<scaled_unit<ratio(1), metre>>(quantity_point(2_q_km)))::unit, metre>);
+static_assert(compare<decltype(quantity_point_cast<scaled_unit<ratio(1), metre>>(quantity_point(2_q_km)))::unit, metre>);

 static_assert(quantity_point_cast<quantity_point<dim_length, metre, int>>(quantity_point(2_q_km)).relative().count() ==
              2000);
--- a/test/unit_test/static/quantity_test.cpp
+++ b/test/unit_test/static/quantity_test.cpp
@@ -34,6 +34,9 @@ namespace {
 using namespace units;
 using namespace units::physical::si;

+template<typename T, typename U>
+inline constexpr bool compare = DOWNCAST_MODE != 0 ? std::is_same_v<T, U> : (std::is_same_v<T, U> || units::equivalent<T, U>);
+
 // class invariants

 template<typename DimLength>
@@ -147,37 +150,35 @@ static_assert(invalid_compound_assignments<metre>);

 // non-member arithmetic operators

-static_assert(is_same_v<decltype(length<metre, int>() + length<metre, double>()), length<metre, double>>);
-static_assert(is_same_v<decltype(length<metre, int>() + length<metre, double>()), length<metre, double>>);
+static_assert(compare<decltype(length<metre, int>() + length<metre, double>()), length<metre, double>>);
+static_assert(compare<decltype(length<metre, int>() + length<metre, double>()), length<metre, double>>);
+static_assert(compare<decltype(length<kilometre, int>() + length<metre, double>()), length<metre, double>>);
+static_assert(compare<decltype(length<metre, double>() - length<metre, int>()), length<metre, double>>);
+static_assert(compare<decltype(length<kilometre, double>() - length<metre, int>()), length<metre, double>>);
+static_assert(compare<decltype(length<metre, int>() * 1.0), length<metre, double>>);
+static_assert(compare<decltype(1.0 * length<metre, int>()), length<metre, double>>);
 static_assert(
-    is_same_v<decltype(length<kilometre, int>() + length<metre, double>()), length<metre, double>>);
-static_assert(is_same_v<decltype(length<metre, double>() - length<metre, int>()), length<metre, double>>);
+    compare<decltype(speed<metre_per_second, int>() * physical::si::time<second, int>()), length<metre, int>>);
 static_assert(
-    is_same_v<decltype(length<kilometre, double>() - length<metre, int>()), length<metre, double>>);
-static_assert(is_same_v<decltype(length<metre, int>() * 1.0), length<metre, double>>);
-static_assert(is_same_v<decltype(1.0 * length<metre, int>()), length<metre, double>>);
-static_assert(
-    is_same_v<decltype(speed<metre_per_second, int>() * physical::si::time<second, int>()), length<metre, int>>);
-static_assert(
-    is_same_v<decltype(speed<metre_per_second, int>() * physical::si::time<hour, int>()), length<scaled_unit<ratio(36, 1, 2), metre>, int>>);
-static_assert(is_same_v<decltype(length<metre>() * physical::si::time<minute>()),
+    compare<decltype(speed<metre_per_second, int>() * physical::si::time<hour, int>()), length<scaled_unit<ratio(36, 1, 2), metre>, int>>);
+static_assert(compare<decltype(length<metre>() * physical::si::time<minute>()),
              quantity<unknown_dimension<units::exponent<dim_length, 1>, units::exponent<dim_time, 1>>, scaled_unit<ratio(6, 1, 1), unknown_coherent_unit>>>);
-static_assert(is_same_v<decltype(1 / physical::si::time<second, int>()), frequency<hertz, int>>);
-static_assert(is_same_v<decltype(1 / physical::si::time<minute, int>()), frequency<scaled_unit<ratio(1, 6, -1), hertz>, int>>);
-static_assert(is_same_v<decltype(1 / frequency<hertz, int>()), physical::si::time<second, int>>);
-static_assert(is_same_v<decltype(1 / length<kilometre>()),
+static_assert(compare<decltype(1 / physical::si::time<second, int>()), frequency<hertz, int>>);
+static_assert(compare<decltype(1 / physical::si::time<minute, int>()), frequency<scaled_unit<ratio(1, 6, -1), hertz>, int>>);
+static_assert(compare<decltype(1 / frequency<hertz, int>()), physical::si::time<second, int>>);
+static_assert(compare<decltype(1 / length<kilometre>()),
              quantity<unknown_dimension<units::exponent<dim_length, -1>>, scaled_unit<ratio(1, 1, -3), unknown_coherent_unit>>>);
-static_assert(is_same_v<decltype(length<metre, int>() / 1.0), length<metre, double>>);
-static_assert(is_same_v<decltype(length<metre, int>() / length<metre, double>()), dimensionless<one, double>>);
-static_assert(is_same_v<decltype(length<kilometre, int>() / length<metre, double>()), dimensionless<scaled_unit<ratio(1, 1, 3), one>, double>>);
+static_assert(compare<decltype(length<metre, int>() / 1.0), length<metre, double>>);
+static_assert(compare<decltype(length<metre, int>() / length<metre, double>()), dimensionless<one, double>>);
+static_assert(compare<decltype(length<kilometre, int>() / length<metre, double>()), dimensionless<scaled_unit<ratio(1, 1, 3), one>, double>>);
 static_assert(
-    is_same_v<decltype(length<metre, int>() / physical::si::time<second, int>()), speed<metre_per_second, int>>);
+    compare<decltype(length<metre, int>() / physical::si::time<second, int>()), speed<metre_per_second, int>>);
 static_assert(
-    is_same_v<decltype(length<metre>() / physical::si::time<minute>()), speed<scaled_unit<ratio(1, 6, -1), metre_per_second>>>);
-static_assert(is_same_v<decltype(physical::si::time<minute>() / length<metre>()),
+    compare<decltype(length<metre>() / physical::si::time<minute>()), speed<scaled_unit<ratio(1, 6, -1), metre_per_second>>>);
+static_assert(compare<decltype(physical::si::time<minute>() / length<metre>()),
              quantity<unknown_dimension<units::exponent<dim_length, -1>, units::exponent<dim_time, 1>>, scaled_unit<ratio(6 ,1 , 1), unknown_coherent_unit>>>);
-static_assert(is_same_v<decltype(length<metre, int>() % short(1)), length<metre, int>>);
-static_assert(is_same_v<decltype(length<metre, int>() % length<metre, short>(1)), length<metre, int>>);
+static_assert(compare<decltype(length<metre, int>() % short(1)), length<metre, int>>);
+static_assert(compare<decltype(length<metre, int>() % length<metre, short>(1)), length<metre, int>>);

 static_assert((1_q_m + km).count() == 1001);
 static_assert((1_q_m + 1_q_km).count() == 1001);
@@ -251,11 +252,10 @@ static_assert(Quantity<length<millimetre, int>>);

 // common_quantity

-static_assert(is_same_v<common_quantity<length<metre, int>, length<kilometre, int>>, length<metre, int>>);
+static_assert(compare<common_quantity<length<metre, int>, length<kilometre, int>>, length<metre, int>>);
+static_assert(compare<common_quantity<length<kilometre, long long>, length<metre, int>>, length<metre, long long>>);
 static_assert(
-    is_same_v<common_quantity<length<kilometre, long long>, length<metre, int>>, length<metre, long long>>);
-static_assert(is_same_v<common_quantity<length<kilometre, long long>, length<millimetre, double>>,
-                             length<millimetre, double>>);
+    compare<common_quantity<length<kilometre, long long>, length<millimetre, double>>, length<millimetre, double>>);

 // common_type

@@ -272,7 +272,7 @@ static_assert(!std::equality_comparable_with<dimensionless<one, int>, double>);

 // quantity_cast

-static_assert(is_same_v<decltype(quantity_cast<scaled_unit<ratio(1), metre>>(2_q_km))::unit, metre>);
+static_assert(compare<decltype(quantity_cast<scaled_unit<ratio(1), metre>>(2_q_km))::unit, metre>);

 static_assert(quantity_cast<length<metre, int>>(2_q_km).count() == 2000);
 static_assert(quantity_cast<length<kilometre, int>>(2000_q_m).count() == 2);
@@ -339,6 +339,20 @@ static_assert(1_q_km / 1_q_s == 1000_q_m_per_s);
 static_assert(2_q_km_per_h * 2_q_h == 4_q_km);
 static_assert(2_q_km / 2_q_km_per_h == 1_q_h);

-static_assert(is_same_v<decltype(pow<2>(2_q_m)), decltype(4_q_m2)>);
+static_assert(compare<decltype(pow<2>(2_q_m)), decltype(4_q_m2)>);
+
+// downcasting
+
+#if DOWNCAST_MODE == 0
+
+static_assert(std::is_same_v<decltype(10_q_m / 5_q_s), quantity<unknown_dimension<units::exp<dim_length, 1>, units::exp<dim_time, -1>>, scaled_unit<ratio(1), unknown_coherent_unit>, std::int64_t>>);
+static_assert(std::is_same_v<decltype(1_q_mm + 1_q_km), length<scaled_unit<ratio(1, 1, -3), metre>, std::int64_t>>);
+
+#else
+
+static_assert(std::is_same_v<decltype(10_q_m / 5_q_s), speed<metre_per_second, std::int64_t>>);
+static_assert(std::is_same_v<decltype(1_q_mm + 1_q_km), length<millimetre, std::int64_t>>);
+
+#endif

 }  // namespace
--- a/test/unit_test/static/si_cgs_test.cpp
+++ b/test/unit_test/static/si_cgs_test.cpp
@@ -46,6 +46,13 @@ namespace {

 using namespace units::physical;

+static_assert(units::detail::quantity_ratio(si::length<si::metre>(1)) == units::ratio(1));
+static_assert(units::detail::quantity_ratio(cgs::length<cgs::centimetre>(1)) == units::ratio(1, 100));
+static_assert(units::detail::quantity_ratio(si::speed<si::metre_per_second>(1)) == units::ratio(1));
+static_assert(units::detail::quantity_ratio(cgs::speed<cgs::centimetre_per_second>(1)) == units::ratio(1, 100));
+static_assert(units::detail::quantity_ratio(si::force<si::newton>(1)) == units::ratio(1000));   // defined in terms of kilogram that are 1000 * gram
+static_assert(units::detail::quantity_ratio(cgs::force<cgs::dyne>(1)) == units::ratio(1, 100)); // defined in terms of gram so only centimetre ratio counts here
+
 static_assert(cgs::length<cgs::centimetre>(100) == si::length<si::metre>(1));
 static_assert(cgs::mass<cgs::gram>(1'000) == si::mass<si::kilogram>(1));
 static_assert(cgs::time<cgs::second>(1) == si::time<si::second>(1));
--- a/test/unit_test/static/si_test.cpp
+++ b/test/unit_test/static/si_test.cpp
@@ -241,8 +241,6 @@ static_assert(kilogray::symbol == "kGy");

 // speed

-static_assert(is_same_v<decltype(1_q_km / 1_q_s), speed<scaled_unit<ratio(1, 1, 3), metre_per_second>, std::int64_t>>);
-
 static_assert(10_q_m / 5_q_s == 2_q_m_per_s);
 static_assert(10 / 5_q_s * 1_q_m == 2_q_m_per_s);
 static_assert(1_q_km / 1_q_s == 1000_q_m_per_s);
--- a/test/unit_test/static/unit_test.cpp
+++ b/test/unit_test/static/unit_test.cpp
@@ -21,6 +21,7 @@
 // SOFTWARE.

 #include "units/unit.h"
+#include "units/bits/equivalent.h"
 #include "units/physical/si/prefixes.h"

 namespace {
@@ -28,6 +29,9 @@ namespace {
 using namespace units;
 using namespace units::physical;

+template<typename T, typename U>
+inline constexpr bool compare = DOWNCAST_MODE != 0 ? std::is_same_v<T, U> : (std::is_same_v<T, U> || units::equivalent<T, U>);
+
 struct metre : named_unit<metre, "m", si::prefix> {};
 struct centimetre : prefixed_unit<centimetre, si::centi, metre> {};
 struct kilometre : prefixed_unit<kilometre, si::kilo, metre> {};
@@ -49,11 +53,11 @@ struct metre_per_second : unit<metre_per_second> {};
 struct dim_speed : derived_dimension<dim_speed, metre_per_second, units::exponent<dim_length, 1>, units::exponent<dim_time, -1>> {};
 struct kilometre_per_hour : deduced_unit<kilometre_per_hour, dim_speed, kilometre, hour> {};

-static_assert(is_same_v<downcast<scaled_unit<ratio(1), metre>>, metre>);
-static_assert(is_same_v<downcast<scaled_unit<ratio(1, 1, -2), metre>>, centimetre>);
-static_assert(is_same_v<downcast<scaled_unit<ratio(yard::ratio.num, yard::ratio.den, yard::ratio.exp), metre>>, yard>);
-static_assert(is_same_v<downcast<scaled_unit<yard::ratio * ratio(1, 3), metre>>, foot>);
-static_assert(is_same_v<downcast<scaled_unit<kilometre::ratio / hour::ratio, metre_per_second>>, kilometre_per_hour>);
+static_assert(compare<downcast<scaled_unit<ratio(1), metre>>, metre>);
+static_assert(compare<downcast<scaled_unit<ratio(1, 1, -2), metre>>, centimetre>);
+static_assert(compare<downcast<scaled_unit<ratio(yard::ratio.num, yard::ratio.den, yard::ratio.exp), metre>>, yard>);
+static_assert(compare<downcast<scaled_unit<yard::ratio * ratio(1, 3), metre>>, foot>);
+static_assert(compare<downcast<scaled_unit<kilometre::ratio / hour::ratio, metre_per_second>>, kilometre_per_hour>);

 #if COMP_GCC >= 10
 static_assert([]<ratio R>() { return !requires { typename scaled_unit<R, metre>; }; }.template operator()<ratio(-1, 1)>()); // negative unit ratio