From 98c15a235653f6bd21fcd9816fe4fc7a9c9f5015 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Johel=20Ernesto=20Guerrero=20Pe=C3=B1a?=
 <johelegp@gmail.com>
Date: Mon, 18 Jan 2021 15:55:55 -0400
Subject: [PATCH] fix: quantity_cast between different, equivalent dimensions

---
 src/include/units/quantity_cast.h       | 2 +-
 test/unit_test/static/quantity_test.cpp | 8 +++++++-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/src/include/units/quantity_cast.h b/src/include/units/quantity_cast.h
index d98eb0a8..582cfb38 100644
--- a/src/include/units/quantity_cast.h
+++ b/src/include/units/quantity_cast.h
@@ -145,7 +145,7 @@ template<Dimension ToD, typename D, typename U, typename Rep>
   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);
+  return quantity_cast<quantity<ToD, downcast_unit<ToD, U::ratio>, Rep>>(q);
 }
 
 /**
diff --git a/test/unit_test/static/quantity_test.cpp b/test/unit_test/static/quantity_test.cpp
index bb2820c5..be52bc9a 100644
--- a/test/unit_test/static/quantity_test.cpp
+++ b/test/unit_test/static/quantity_test.cpp
@@ -37,7 +37,11 @@
 namespace {
 
 using namespace units;
-using namespace units::physical::si;
+namespace si = physical::si;
+using namespace si;
+using namespace unit_constants;
+
+constexpr auto cgs_cm = cgs::unit_constants::cm;
 
 
 //////////////////////////////
@@ -685,6 +689,8 @@ static_assert(quantity_cast<kilometre>(2000_q_m).count() == 2);
 static_assert(quantity_cast<int>(1.23_q_m).count() == 1);
 static_assert(quantity_cast<dim_speed, kilometre_per_hour>(2000.0_q_m / 3600.0_q_s).count() == 2);
 
+static_assert(quantity_cast<dim_length>(1 * cgs_cm) == 1 * cm);
+
 static_assert(is_same_v<decltype(quantity_cast<litre>(2_q_dm3)), volume<litre, std::int64_t>>);
 static_assert(!is_same_v<decltype(quantity_cast<litre>(2_q_dm3)), volume<cubic_decimetre, std::int64_t>>);