From b76337d5bb181c41f3a4cfb51915379587887f68 Mon Sep 17 00:00:00 2001
From: Mateusz Pusz <mateusz.pusz@gmail.com>
Date: Thu, 14 Sep 2023 12:42:45 +0200
Subject: [PATCH] feat: pre-increment and pre-decrement operators now preserve
 value category

---
 src/core/include/mp-units/quantity.h       | 18 ++++++++++--------
 src/core/include/mp-units/quantity_point.h | 18 ++++++++++--------
 2 files changed, 20 insertions(+), 16 deletions(-)

diff --git a/src/core/include/mp-units/quantity.h b/src/core/include/mp-units/quantity.h
index 2e9fab49..c8d1d319 100644
--- a/src/core/include/mp-units/quantity.h
+++ b/src/core/include/mp-units/quantity.h
@@ -210,15 +210,16 @@ public:
     return make_quantity<reference>(-numerical_value_);
   }
 
-  constexpr quantity& operator++()
-    requires requires(rep v) {
+  template<typename Q>
+  friend constexpr decltype(auto) operator++(Q&& q)
+    requires std::derived_from<std::remove_cvref_t<Q>, quantity> && requires(rep v) {
       {
         ++v
       } -> std::same_as<rep&>;
     }
   {
-    ++numerical_value_;
-    return *this;
+    ++q.numerical_value_;
+    return std::forward<Q>(q);
   }
 
   [[nodiscard]] constexpr Quantity auto operator++(int)
@@ -231,15 +232,16 @@ public:
     return make_quantity<reference>(numerical_value_++);
   }
 
-  constexpr quantity& operator--()
-    requires requires(rep v) {
+  template<typename Q>
+  friend constexpr decltype(auto) operator--(Q&& q)
+    requires std::derived_from<std::remove_cvref_t<Q>, quantity> && requires(rep v) {
       {
         --v
       } -> std::same_as<rep&>;
     }
   {
-    --numerical_value_;
-    return *this;
+    --q.numerical_value_;
+    return std::forward<Q>(q);
   }
 
   [[nodiscard]] constexpr Quantity auto operator--(int)
diff --git a/src/core/include/mp-units/quantity_point.h b/src/core/include/mp-units/quantity_point.h
index bcf48acb..91062023 100644
--- a/src/core/include/mp-units/quantity_point.h
+++ b/src/core/include/mp-units/quantity_point.h
@@ -183,11 +183,12 @@ public:
   }
 
   // member unary operators
-  constexpr quantity_point& operator++()
-    requires requires { ++quantity_from_origin_; }
+  template<typename QP>
+  friend constexpr decltype(auto) operator++(QP&& qp)
+    requires std::derived_from<std::remove_cvref_t<QP>, quantity_point> && requires { ++qp.quantity_from_origin_; }
   {
-    ++quantity_from_origin_;
-    return *this;
+    ++qp.quantity_from_origin_;
+    return std::forward<QP>(qp);
   }
 
   [[nodiscard]] constexpr quantity_point operator++(int)
@@ -196,11 +197,12 @@ public:
     return quantity_point(quantity_from_origin_++);
   }
 
-  constexpr quantity_point& operator--()
-    requires requires { --quantity_from_origin_; }
+  template<typename QP>
+  friend constexpr decltype(auto) operator--(QP&& qp)
+    requires std::derived_from<std::remove_cvref_t<QP>, quantity_point> && requires { --qp.quantity_from_origin_; }
   {
-    --quantity_from_origin_;
-    return *this;
+    --qp.quantity_from_origin_;
+    return std::forward<QP>(qp);
   }
 
   [[nodiscard]] constexpr quantity_point operator--(int)