From 9c7d3b0f950326ebb8112e4f68021170c915af70 Mon Sep 17 00:00:00 2001
From: Mateusz Pusz <mateusz.pusz@gmail.com>
Date: Fri, 25 Oct 2024 12:07:06 +0200
Subject: [PATCH] fix: `operator*(M, U u)` fixed for `U` being `scaled_unit`

---
 src/core/include/mp-units/framework/unit.h |  7 ++++++-
 test/static/unit_test.cpp                  | 10 ++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/src/core/include/mp-units/framework/unit.h b/src/core/include/mp-units/framework/unit.h
index 3624e243..c8d99a9a 100644
--- a/src/core/include/mp-units/framework/unit.h
+++ b/src/core/include/mp-units/framework/unit.h
@@ -157,7 +157,12 @@ struct unit_interface {
   {
     if constexpr (std::is_same_v<M, std::remove_const_t<decltype(mp_units::mag<1>)>>)
       return u;
-    else
+    else if constexpr (is_specialization_of_scaled_unit<U>) {
+      if constexpr (M{} * U::mag == mag<1>)
+        return U::reference_unit;
+      else
+        return scaled_unit<M{} * U::mag, std::remove_const_t<decltype(U::reference_unit)>>{};
+    } else
       return scaled_unit<M{}, U>{};
   }
 
diff --git a/test/static/unit_test.cpp b/test/static/unit_test.cpp
index 2269805e..e42a96df 100644
--- a/test/static/unit_test.cpp
+++ b/test/static/unit_test.cpp
@@ -253,6 +253,16 @@ static_assert(is_of_type<m_2, scaled_unit<mag<2>, metre_>>);
 static_assert(is_of_type<get_canonical_unit(m_2).reference_unit, metre_>);
 static_assert(get_canonical_unit(m_2).mag == mag<2>);
 
+constexpr auto m_3 = mag_ratio<1, 2> * m_2;
+static_assert(is_of_type<m_3, metre_>);
+static_assert(is_of_type<get_canonical_unit(m_3).reference_unit, metre_>);
+static_assert(get_canonical_unit(m_3).mag == mag<1>);
+
+constexpr auto m_4 = mag_ratio<1, 2> * (mag<4> * metre);
+static_assert(is_of_type<m_4, scaled_unit<mag<2>, metre_>>);
+static_assert(is_of_type<get_canonical_unit(m_4).reference_unit, metre_>);
+static_assert(get_canonical_unit(m_4).mag == mag<2>);
+
 constexpr auto km_2 = mag<2> * kilometre;
 static_assert(is_of_type<km_2, scaled_unit<mag<2>, kilo_<metre_>>>);
 static_assert(is_of_type<get_canonical_unit(km_2).reference_unit, metre_>);