From d195e2b3bfec59bcafb7dd4c547480da4585f5d8 Mon Sep 17 00:00:00 2001
From: Mateusz Pusz <mateusz.pusz@gmail.com>
Date: Fri, 3 Nov 2023 21:18:57 +0100
Subject: [PATCH] fix: equality for dimensions fixed to provide transitivity

---
 src/core/include/mp-units/dimension.h    | 19 ++++++++++++++++---
 test/unit_test/static/dimension_test.cpp |  6 ++++--
 2 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/src/core/include/mp-units/dimension.h b/src/core/include/mp-units/dimension.h
index f04d7363..14ce1898 100644
--- a/src/core/include/mp-units/dimension.h
+++ b/src/core/include/mp-units/dimension.h
@@ -150,10 +150,23 @@ template<Dimension Lhs, Dimension Rhs>
                                                                                                                 Rhs{});
 }
 
-template<Dimension Lhs, Dimension Rhs>
-[[nodiscard]] consteval bool operator==(Lhs, Rhs)
+namespace detail {
+
+template<auto Symbol>
+[[nodiscard]] consteval std::true_type derived_from_the_same_base_dimension(const base_dimension<Symbol>&,
+                                                                            const base_dimension<Symbol>&)
 {
-  return std::derived_from<Lhs, Rhs> || std::derived_from<Rhs, Lhs>;
+  return {};
+}
+
+[[nodiscard]] consteval std::false_type derived_from_the_same_base_dimension(...) { return {}; }
+
+}  // namespace detail
+
+template<Dimension Lhs, Dimension Rhs>
+[[nodiscard]] consteval bool operator==(Lhs lhs, Rhs rhs)
+{
+  return is_same_v<Lhs, Rhs> || detail::derived_from_the_same_base_dimension(lhs, rhs);
 }
 
 [[nodiscard]] consteval Dimension auto inverse(Dimension auto d) { return dimension_one / d; }
diff --git a/test/unit_test/static/dimension_test.cpp b/test/unit_test/static/dimension_test.cpp
index 08eacb47..808620d7 100644
--- a/test/unit_test/static/dimension_test.cpp
+++ b/test/unit_test/static/dimension_test.cpp
@@ -38,7 +38,8 @@ inline constexpr struct length_ : base_dimension<"L"> {} length;
 inline constexpr struct mass_ : base_dimension<"M"> {} mass;
 inline constexpr struct time_ : base_dimension<"T"> {} time;
 
-inline constexpr struct my_length_ : decltype(length) {} my_length;
+inline constexpr struct my_length1_ : decltype(length) {} my_length1;
+inline constexpr struct my_length2_ : decltype(length) {} my_length2;
 
 QUANTITY_SPEC_(q_time, time);
 inline constexpr struct second_ : named_unit<"s", kind_of<q_time>> {} second;
@@ -170,7 +171,8 @@ static_assert(invalid_operations<time>);
 
 // comparisons of the same dimensions
 static_assert(length == length);
-static_assert(length == my_length);
+static_assert(length == my_length1);
+static_assert(my_length1 == my_length2);
 static_assert(speed == speed);
 
 // comparisons of equivalent dimensions (named vs unnamed/derived)