From 86b917781699f1538e08a45269b9bb1afc2bef7a Mon Sep 17 00:00:00 2001
From: Mateusz Pusz <mateusz.pusz@gmail.com>
Date: Thu, 28 May 2020 20:19:05 +0200
Subject: [PATCH] Specialize common_type to satisfy comparison concepts
 (resolves #113)

---
 src/include/units/bits/common_quantity.h | 10 ++++++++++
 test/unit_test/static/quantity_test.cpp  | 10 ++++++++++
 2 files changed, 20 insertions(+)

diff --git a/src/include/units/bits/common_quantity.h b/src/include/units/bits/common_quantity.h
index 0a1b4da0..6e9c9640 100644
--- a/src/include/units/bits/common_quantity.h
+++ b/src/include/units/bits/common_quantity.h
@@ -64,3 +64,13 @@ template<Quantity Q1, Quantity Q2, Scalar Rep = std::common_type_t<typename Q1::
 using common_quantity = detail::common_quantity_impl<Q1, Q2, Rep>::type;
 
 }  // namespace units
+
+namespace std {
+
+template<units::Quantity Q1, units::Quantity Q2>
+  requires units::equivalent_dim<typename Q1::dimension, typename Q2::dimension>
+struct common_type<Q1, Q2> {
+  using type = units::common_quantity<Q1, Q2>;
+};
+
+}
\ No newline at end of file
diff --git a/test/unit_test/static/quantity_test.cpp b/test/unit_test/static/quantity_test.cpp
index 9618b0f4..1c0e4cb4 100644
--- a/test/unit_test/static/quantity_test.cpp
+++ b/test/unit_test/static/quantity_test.cpp
@@ -25,6 +25,7 @@
 #include "units/physical/si/frequency.h"
 #include "units/physical/si/speed.h"
 #include "units/physical/si/volume.h"
+#include "units/physical/us/length.h"
 #include <chrono>
 #include <utility>
 
@@ -222,6 +223,15 @@ static_assert(
 static_assert(std::is_same_v<common_quantity<length<kilometre, long long>, length<millimetre, double>>,
                              length<millimetre, double>>);
 
+// common_type
+
+using namespace units::physical::us::literals;
+
+static_assert(std::equality_comparable<decltype(1q_m)>);
+static_assert(std::equality_comparable_with<decltype(1q_m), decltype(1q_cm)>);
+static_assert(0q_m == 0q_ft_us);
+static_assert(std::equality_comparable_with<decltype(1q_m), decltype(1q_ft_us)>);
+
 // quantity_cast
 
 static_assert(std::is_same_v<decltype(quantity_cast<scaled_unit<ratio<1>, metre>>(2q_km))::unit, metre>);