From 2328a26992a2f67775afbabbf2f33af277d1cfda Mon Sep 17 00:00:00 2001
From: Mateusz Pusz <mateusz.pusz@gmail.com>
Date: Sat, 9 Nov 2019 14:42:06 +0000
Subject: [PATCH] Downcasting support added to `quantity_cast` (resolves #26)

---
 src/include/units/quantity.h            | 13 ++++++++-----
 test/unit_test/static/quantity_test.cpp |  3 +++
 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/src/include/units/quantity.h b/src/include/units/quantity.h
index 812f7a57..67d93440 100644
--- a/src/include/units/quantity.h
+++ b/src/include/units/quantity.h
@@ -157,29 +157,32 @@ namespace units {
   }  // namespace detail
 
   template<Quantity To, typename U, typename Rep>
-  [[nodiscard]] constexpr To quantity_cast(const quantity<U, Rep>& q)
+  [[nodiscard]] constexpr auto quantity_cast(const quantity<U, Rep>& q)
       requires same_dim<typename To::dimension, typename U::dimension>
   {
     using c_ratio = ratio_divide<typename U::ratio, typename To::unit::ratio>;
     using c_rep = std::common_type_t<typename To::rep, Rep, intmax_t>;
-    using cast = detail::quantity_cast_impl<To, c_ratio, c_rep, c_ratio::num == 1, c_ratio::den == 1>;
+    using ret_dim = downcast<typename To::unit::dimension>;
+    using ret_unit = downcast<unit<ret_dim, typename To::unit::ratio>>;
+    using ret = quantity<ret_unit, typename To::rep>;
+    using cast = detail::quantity_cast_impl<ret, c_ratio, c_rep, c_ratio::num == 1, c_ratio::den == 1>;
     return cast::cast(q);
   }
 
   template<Unit ToU, QuantityRep ToRep, typename U, typename Rep>
-  [[nodiscard]] constexpr quantity<ToU, ToRep> quantity_cast(const quantity<U, Rep>& q)
+  [[nodiscard]] constexpr auto quantity_cast(const quantity<U, Rep>& q)
   {
     return quantity_cast<quantity<ToU, ToRep>>(q);
   }
 
   template<Unit ToU, typename U, typename Rep>
-  [[nodiscard]] constexpr quantity<ToU, Rep> quantity_cast(const quantity<U, Rep>& q)
+  [[nodiscard]] constexpr auto quantity_cast(const quantity<U, Rep>& q)
   {
     return quantity_cast<quantity<ToU, Rep>>(q);
   }
 
   template<QuantityRep ToRep, typename U, typename Rep>
-  [[nodiscard]] constexpr quantity<U, ToRep> quantity_cast(const quantity<U, Rep>& q)
+  [[nodiscard]] constexpr auto quantity_cast(const quantity<U, Rep>& q)
   {
     return quantity_cast<quantity<U, ToRep>>(q);
   }
diff --git a/test/unit_test/static/quantity_test.cpp b/test/unit_test/static/quantity_test.cpp
index 61e77dbf..532d7769 100644
--- a/test/unit_test/static/quantity_test.cpp
+++ b/test/unit_test/static/quantity_test.cpp
@@ -269,6 +269,9 @@ namespace {
 
   // quantity_cast
 
+  static_assert(std::is_same_v<decltype(quantity_cast<unit<length, ratio<1>>>(2km))::unit, metre>);
+  static_assert(std::is_same_v<decltype(quantity_cast<unit<dimension<units::exp<base_dim_length, 1>>, ratio<1>>>(2km))::unit, metre>);
+
   // static_assert(quantity_cast<int>(2km).count() == 2000);  // should not compile
   static_assert(quantity_cast<quantity<metre, int>>(2km).count() == 2000);
   static_assert(quantity_cast<quantity<kilometre, int>>(2000m).count() == 2);