From e6eb9085039119be4994dd8ef262be80515d1de3 Mon Sep 17 00:00:00 2001
From: Mateusz Pusz <mateusz.pusz@gmail.com>
Date: Mon, 3 Apr 2023 17:44:34 +0200
Subject: [PATCH] feat: convertible quantity kinds can now be compared

---
 src/core/include/mp_units/quantity.h      | 6 ++++--
 src/core/include/mp_units/quantity_spec.h | 5 ++++-
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/src/core/include/mp_units/quantity.h b/src/core/include/mp_units/quantity.h
index f33e47b8..9ec4659a 100644
--- a/src/core/include/mp_units/quantity.h
+++ b/src/core/include/mp_units/quantity.h
@@ -509,7 +509,8 @@ template<Quantity Q1, Quantity Q2>
 }
 
 template<Quantity Q1, Quantity Q2>
-  requires(get_kind(Q1::quantity_spec) == get_kind(Q2::quantity_spec)) &&
+  requires(implicitly_convertible_to(get_kind(Q1::quantity_spec), get_kind(Q2::quantity_spec)) ||
+           implicitly_convertible_to(get_kind(Q2::quantity_spec), get_kind(Q1::quantity_spec))) &&
           std::three_way_comparable_with<typename Q1::rep, typename Q2::rep>
 [[nodiscard]] constexpr auto operator<=>(const Q1& lhs, const Q2& rhs)
 {
@@ -518,7 +519,8 @@ template<Quantity Q1, Quantity Q2>
 }
 
 template<Quantity Q1, Quantity Q2>
-  requires(get_kind(Q1::quantity_spec) == get_kind(Q2::quantity_spec)) &&
+  requires(implicitly_convertible_to(get_kind(Q1::quantity_spec), get_kind(Q2::quantity_spec)) ||
+           implicitly_convertible_to(get_kind(Q2::quantity_spec), get_kind(Q1::quantity_spec))) &&
           std::equality_comparable_with<typename Q1::rep, typename Q2::rep>
 [[nodiscard]] constexpr bool operator==(const Q1& lhs, const Q2& rhs)
 {
diff --git a/src/core/include/mp_units/quantity_spec.h b/src/core/include/mp_units/quantity_spec.h
index 81c6a7d6..f29ba8ed 100644
--- a/src/core/include/mp_units/quantity_spec.h
+++ b/src/core/include/mp_units/quantity_spec.h
@@ -782,12 +782,15 @@ template<QuantitySpec Q>
 
 template<QuantitySpec Q1, QuantitySpec Q2>
 [[nodiscard]] consteval QuantitySpec auto common_quantity_spec(Q1 q1, Q2 q2)
-  requires(get_kind(q1) == get_kind(q2))
+  requires(implicitly_convertible_to(get_kind(q1), get_kind(q2)) ||
+           implicitly_convertible_to(get_kind(q2), get_kind(q1)))
 {
   if constexpr (q1 == q2)
     return q1;
   else if constexpr (detail::have_common_base(q1, q2))
     return detail::get_common_base(q1, q2);
+  else if constexpr (implicitly_convertible_to(get_kind(q1), get_kind(q2)))
+    return get_kind(q2);
   else
     return get_kind(q1);
 }