From 8e1539bf2257b66693baef79b8d67da132a7539c Mon Sep 17 00:00:00 2001
From: "Ralph J. Steinhagen" <r.steinhagen@gsi.de>
Date: Sun, 27 Jun 2021 13:42:55 +0200
Subject: [PATCH] added inheritance unit-test and made is_quantity<T> more
 specific

---
 src/core/include/units/bits/basic_concepts.h |  2 +-
 src/core/include/units/quantity.h            | 11 +++++++++++
 test/unit_test/static/quantity_test.cpp      | 12 ++++++++++++
 3 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/src/core/include/units/bits/basic_concepts.h b/src/core/include/units/bits/basic_concepts.h
index 94425df1..4ec56115 100644
--- a/src/core/include/units/bits/basic_concepts.h
+++ b/src/core/include/units/bits/basic_concepts.h
@@ -269,7 +269,7 @@ concept Reference = detail::is_reference<T>;
 namespace detail {
 
 template<typename T>
-inline constexpr bool is_quantity = requires { typename T::dimension;  typename T::unit;  typename T::rep;};
+inline constexpr bool is_quantity = false;
 
 template<typename T>
 inline constexpr bool is_quantity_point = false;
diff --git a/src/core/include/units/quantity.h b/src/core/include/units/quantity.h
index 03140500..7956cd11 100644
--- a/src/core/include/units/quantity.h
+++ b/src/core/include/units/quantity.h
@@ -460,6 +460,17 @@ namespace detail {
 template<typename D, typename U, typename Rep>
 inline constexpr bool is_quantity<quantity<D, U, Rep>> = true;
 
+template<typename T>
+requires units::is_derived_from_specialization_of<T, units::quantity> &&
+        requires {
+    typename T::dimension;
+    typename T::unit;
+    typename T::rep;
+    requires Dimension<typename T::dimension>;
+    requires Unit<typename T::unit>;
+    requires Representation<typename T::rep>; }
+inline constexpr bool is_quantity<T> = true;
+
 }  // namespace detail
 
 }  // namespace units
diff --git a/test/unit_test/static/quantity_test.cpp b/test/unit_test/static/quantity_test.cpp
index de56d789..c0b458f6 100644
--- a/test/unit_test/static/quantity_test.cpp
+++ b/test/unit_test/static/quantity_test.cpp
@@ -234,6 +234,18 @@ static_assert(length<kilometre, int>(2_q_km).number() == 2);
 static_assert(length<metre, int>(2_q_km).number() == 2000);
 static_assert(length<kilometre>(1500_q_m).number() == 1.5);
 
+///////////////////////////////////////
+// derived quantities
+///////////////////////////////////////
+
+template<Representation Rep, units::Quantity Q, const units::basic_fixed_string additional_nttp_argument>
+struct derived_quantity : quantity<typename Q::dimension, typename Q::unit, Rep> {
+    using dimension = typename Q::dimension;
+    using unit = typename Q::unit;
+    using rep = Rep;
+};
+
+static_assert(units::detail::is_quantity<derived_quantity<double, si::length<metre>, "NTTP type description">>);
 
 /////////
 // CTAD