From c2e91d41c4708fe6c3930778f010951a072afeff Mon Sep 17 00:00:00 2001
From: Mateusz Pusz <mateusz.pusz@gmail.com>
Date: Mon, 5 Jun 2023 11:09:02 +0200
Subject: [PATCH] fix: commutativity of a `common_unit()` fixed

---
 src/core/include/mp-units/bits/external/type_name.h | 13 +++++++++++++
 src/core/include/mp-units/unit.h                    |  5 ++++-
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/src/core/include/mp-units/bits/external/type_name.h b/src/core/include/mp-units/bits/external/type_name.h
index 9a74759f..e8fb007f 100644
--- a/src/core/include/mp-units/bits/external/type_name.h
+++ b/src/core/include/mp-units/bits/external/type_name.h
@@ -25,3 +25,16 @@ template<typename T>
   name.remove_suffix(suffix.size());
   return name;
 }
+
+template<typename T1, typename T2>
+[[nodiscard]] consteval auto better_type_name(T1 v1, T2 v2)
+{
+  if constexpr (type_name<T1>().size() < type_name<T2>().size())
+    return v1;
+  else if constexpr (type_name<T2>().size() < type_name<T1>().size())
+    return v2;
+  else if constexpr (type_name<T1>() < type_name<T2>())
+    return v1;
+  else
+    return v2;
+}
diff --git a/src/core/include/mp-units/unit.h b/src/core/include/mp-units/unit.h
index 63ddbef8..0052f12b 100644
--- a/src/core/include/mp-units/unit.h
+++ b/src/core/include/mp-units/unit.h
@@ -629,8 +629,11 @@ template<Unit U1, Unit U2>
   if constexpr (U1{} == U2{}) {
     if constexpr (std::derived_from<U1, U2>)
       return u1;
-    else
+    else if constexpr (std::derived_from<U2, U1>)
       return u2;
+    else
+      // TODO Check if there is a better choice here
+      return better_type_name(u1, u2);
   } else {
     constexpr auto canonical_lhs = detail::get_canonical_unit(U1{});
     constexpr auto canonical_rhs = detail::get_canonical_unit(U2{});