feat: users are now allowed to inherit their ow types from absolute point origins

src/core/include/mp-units/bits/quantity_point_concepts.h

@@ -30,7 +30,7 @@
 
 namespace mp_units {
 
-template<QuantitySpec auto Q>
+template<typename Derived, QuantitySpec auto QS>
 struct absolute_point_origin;
 
 namespace detail {
@@ -41,11 +41,11 @@ inline constexpr bool is_quantity_point = false;
 template<typename T>
 inline constexpr bool is_specialization_of_absolute_point_origin = false;
 
-template<auto Q>
-inline constexpr bool is_specialization_of_absolute_point_origin<absolute_point_origin<Q>> = true;
+template<typename D, auto Q>
+inline constexpr bool is_specialization_of_absolute_point_origin<absolute_point_origin<D, Q>> = true;
 
-template<auto Q>
-void to_base_specialization_of_absolute_point_origin(const volatile absolute_point_origin<Q>*);
+template<typename D, auto Q>
+void to_base_specialization_of_absolute_point_origin(const volatile absolute_point_origin<D, Q>*);
 
 template<typename T>
 inline constexpr bool is_derived_from_specialization_of_absolute_point_origin =
@@ -123,18 +123,16 @@ template<typename T>
 inline constexpr bool is_quantity_point<T> = true;
 
 template<PointOrigin PO1, PointOrigin PO2>
-[[nodiscard]] consteval bool same_absolute_point_origins(PO1, PO2)
+[[nodiscard]] consteval bool same_absolute_point_origins(PO1 po1, PO2 po2)
 {
-  if constexpr (is_same_v<PO1, PO2>)
-    return true;
-  else if constexpr (is_derived_from_specialization_of_relative_point_origin<PO1> &&
-                     is_derived_from_specialization_of_relative_point_origin<PO2>)
-    return is_same_v<std::remove_const_t<decltype(PO1::absolute_point_origin)>,
-                     std::remove_const_t<decltype(PO2::absolute_point_origin)>>;
-  else if constexpr (is_derived_from_specialization_of_relative_point_origin<PO1>)
-    return is_same_v<std::remove_const_t<decltype(PO1::absolute_point_origin)>, PO2>;
-  else if constexpr (is_derived_from_specialization_of_relative_point_origin<PO2>)
-    return is_same_v<PO1, std::remove_const_t<decltype(PO2::absolute_point_origin)>>;
+  if constexpr (AbsolutePointOrigin<PO1> && AbsolutePointOrigin<PO2>)
+    return po1 == po2;
+  else if constexpr (RelativePointOrigin<PO1> && RelativePointOrigin<PO2>)
+    return po1.absolute_point_origin == po2.absolute_point_origin;
+  else if constexpr (RelativePointOrigin<PO1>)
+    return po1.absolute_point_origin == po2;
+  else if constexpr (RelativePointOrigin<PO2>)
+    return po1 == po2.absolute_point_origin;
   else
     return false;
 }

src/core/include/mp-units/quantity_point.h

@@ -29,9 +29,25 @@
 
 namespace mp_units {
 
-template<QuantitySpec auto QS>
+namespace detail {
+
+template<typename T>
+  requires requires {
+    {
+      T::zero()
+    } -> std::equality_comparable_with<T>;
+  }
+[[nodiscard]] constexpr bool is_eq_zero(T v)
+{
+  return v == T::zero();
+}
+
+}  // namespace detail
+
+template<typename Derived, QuantitySpec auto QS>
 struct absolute_point_origin {
   static constexpr QuantitySpec auto quantity_spec = QS;
+  using _type_ = absolute_point_origin;
 };
 
 template<QuantityPoint auto QP>
@@ -47,14 +63,30 @@ struct relative_point_origin {
   static constexpr PointOrigin auto absolute_point_origin = QP.absolute_point_origin;
 };
 
+template<PointOrigin PO1, PointOrigin PO2>
+[[nodiscard]] consteval bool operator==(PO1 po1, PO2 po2)
+{
+  if constexpr (detail::AbsolutePointOrigin<PO1> && detail::AbsolutePointOrigin<PO2>)
+    return is_same_v<typename PO1::_type_, typename PO2::_type_>;
+  else if constexpr (detail::RelativePointOrigin<PO1> && detail::RelativePointOrigin<PO2>)
+    return PO1::quantity_point == PO2::quantity_point;
+  else if constexpr (detail::RelativePointOrigin<PO1>)
+    return detail::same_absolute_point_origins(po1, po2) &&
+           detail::is_eq_zero(PO1::quantity_point.quantity_from(PO1::quantity_point.absolute_point_origin));
+  else if constexpr (detail::RelativePointOrigin<PO2>)
+    return detail::same_absolute_point_origins(po1, po2) &&
+           detail::is_eq_zero(PO2::quantity_point.quantity_from(PO2::quantity_point.absolute_point_origin));
+}
+
 namespace detail {
 
-[[nodiscard]] consteval PointOrigin auto get_absolute_point_origin(PointOrigin auto po)
+template<PointOrigin PO>
+[[nodiscard]] consteval PointOrigin auto get_absolute_point_origin(PO po)
 {
-  if constexpr (requires { po.quantity_point.absolute_point_origin; })
-    return po.quantity_point.absolute_point_origin;
-  else
+  if constexpr (AbsolutePointOrigin<PO>)
     return po;
+  else
+    return po.quantity_point.absolute_point_origin;
 }
 
 }  // namespace detail

src/systems/si/include/mp-units/systems/si/point_origins.h

@@ -28,8 +28,11 @@
 namespace mp_units::si {
 
 // clang-format off
-inline constexpr struct absolute_zero : absolute_point_origin<isq::thermodynamic_temperature> {} absolute_zero;
+inline constexpr struct absolute_zero : absolute_point_origin<absolute_zero, isq::thermodynamic_temperature> {} absolute_zero;
+inline constexpr struct zeroth_kelvin : decltype(absolute_zero) {} zeroth_kelvin;
+
 inline constexpr struct ice_point : relative_point_origin<absolute_zero + 273.15 * kelvin> {} ice_point;
+inline constexpr struct zeroth_degree_Celsius : decltype(ice_point) {} zeroth_degree_Celsius;
 // clang-format on
 
 }  // namespace mp_units::si

src/systems/usc/include/mp-units/systems/usc/usc.h

@@ -112,7 +112,7 @@ inline constexpr struct inch_of_mercury : named_unit<"inHg", mag<ratio(3'386'389
 // https://en.wikipedia.org/wiki/United_States_customary_units#Temperature
 inline constexpr struct degree_Fahrenheit : named_unit<basic_symbol_text{"°F", "`F"}, mag<ratio{5, 9}> * si::degree_Celsius> {} degree_Fahrenheit;
 
-inline constexpr struct zero_Fahrenheit : relative_point_origin<si::ice_point - 32 * degree_Fahrenheit> {} zero_Fahrenheit;
+inline constexpr struct zero_Fahrenheit : relative_point_origin<si::zeroth_degree_Celsius - 32 * degree_Fahrenheit> {} zero_Fahrenheit;
 // clang-format on
 
 namespace unit_symbols {

src/utility/include/mp-units/chrono.h

@@ -79,7 +79,7 @@ struct quantity_like_traits<std::chrono::duration<Rep, Period>> {
 };
 
 template<typename C>
-struct chrono_point_origin_ : absolute_point_origin<isq::time> {
+struct chrono_point_origin_ : absolute_point_origin<chrono_point_origin_<C>, isq::time> {
   using clock = C;
 };
 template<typename C>