From fe099b5ad1109b86633716a396eca5fc9d5f31d0 Mon Sep 17 00:00:00 2001
From: Mateusz Pusz <mateusz.pusz@gmail.com>
Date: Thu, 21 Dec 2023 12:25:09 +0100
Subject: [PATCH] feat: propagation of point origins for scaled units added

---
 src/core/include/mp-units/unit.h              | 14 +++++++++++++-
 test/unit_test/static/quantity_point_test.cpp | 10 ++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/src/core/include/mp-units/unit.h b/src/core/include/mp-units/unit.h
index 90ecc0ac..f03257b4 100644
--- a/src/core/include/mp-units/unit.h
+++ b/src/core/include/mp-units/unit.h
@@ -40,6 +40,18 @@
 
 namespace mp_units {
 
+namespace detail {
+
+template<Unit U, bool = requires { U::point_origin; }>
+struct propagate_point_origin {};
+
+template<Unit U>
+struct propagate_point_origin<U, true> {
+  static constexpr auto point_origin = U::point_origin;
+};
+
+}  // namespace detail
+
 /**
  * @brief Unit being a scaled version of another unit
  *
@@ -50,7 +62,7 @@ namespace mp_units {
  *       instantiate this type automatically based on the unit arithmetic equation provided by the user.
  */
 template<Magnitude auto M, Unit U>
-struct scaled_unit {
+struct scaled_unit : detail::propagate_point_origin<U> {
   static constexpr MP_UNITS_CONSTRAINED_AUTO_WORKAROUND(Magnitude) auto mag = M;
   static constexpr U reference_unit{};
 };
diff --git a/test/unit_test/static/quantity_point_test.cpp b/test/unit_test/static/quantity_point_test.cpp
index 2e4dfc85..ac0e6b9a 100644
--- a/test/unit_test/static/quantity_point_test.cpp
+++ b/test/unit_test/static/quantity_point_test.cpp
@@ -106,6 +106,16 @@ static_assert(relative_po<absolute_po<isq::length> + isq::height(42 * m)>.quanti
 static_assert(relative_po<absolute_po<kind_of<isq::length>> + isq::height(42 * m)>.quantity_spec == isq::height);
 static_assert(relative_po<absolute_po<isq::height> + 42 * m>.quantity_spec == isq::height);
 
+inline constexpr struct my_kelvin : named_unit<"my_K", mag<10> * si::kelvin> {
+} my_kelvin;
+
+static_assert(zeroth_point_origin(si::kelvin) == si::absolute_zero);
+static_assert(zeroth_point_origin(si::milli<si::kelvin>) == si::absolute_zero);
+static_assert(zeroth_point_origin(mag<10> * si::kelvin) == si::absolute_zero);
+static_assert(zeroth_point_origin(my_kelvin) == si::absolute_zero);
+
+static_assert(zeroth_point_origin(si::degree_Celsius) == si::ice_point);
+static_assert(zeroth_point_origin(mag<10> * si::degree_Celsius) == si::ice_point);
 
 /////////////////////
 // class invariants