From 2b3c9a6afae5e01f099f9a479e4603c540fa7d9b Mon Sep 17 00:00:00 2001
From: Mateusz Pusz <mateusz.pusz@gmail.com>
Date: Wed, 13 Sep 2023 20:06:17 +0200
Subject: [PATCH] feat: `quantity_point` compound assignment now preserves the
 value category

---
 src/core/include/mp-units/quantity_point.h | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/src/core/include/mp-units/quantity_point.h b/src/core/include/mp-units/quantity_point.h
index 1f4a4940a..68dc3c4f9 100644
--- a/src/core/include/mp-units/quantity_point.h
+++ b/src/core/include/mp-units/quantity_point.h
@@ -224,18 +224,22 @@ class quantity_point {
   }
 
   // compound assignment operators
-  constexpr quantity_point& operator+=(const quantity_type& q)
-    requires requires { quantity_from_origin_ += q; }
+  template<typename QP>
+    requires std::derived_from<std::remove_cvref_t<QP>, quantity_point> &&
+             requires(quantity_type q) { quantity_from_origin_ += q; }
+  friend constexpr decltype(auto) operator+=(QP&& qp, const quantity_type& q)
   {
-    quantity_from_origin_ += q;
-    return *this;
+    qp.quantity_from_origin_ += q;
+    return std::forward<QP>(qp);
   }
 
-  constexpr quantity_point& operator-=(const quantity_type& q)
-    requires requires { quantity_from_origin_ -= q; }
+  template<typename QP>
+    requires std::derived_from<std::remove_cvref_t<QP>, quantity_point> &&
+             requires(quantity_type q) { quantity_from_origin_ -= q; }
+  friend constexpr decltype(auto) operator-=(QP&& qp, const quantity_type& q)
   {
-    quantity_from_origin_ -= q;
-    return *this;
+    qp.quantity_from_origin_ -= q;
+    return std::forward<QP>(qp);
   }
 
 private: