From 944ac56baa7739a79295e5157afd99cb93a3a809 Mon Sep 17 00:00:00 2001
From: Matt Borland <matt@mattborland.com>
Date: Wed, 3 Dec 2025 16:10:25 +0100
Subject: [PATCH] Fix behavior of prefix increment/decrement

---
 include/boost/int128/detail/int128_imp.hpp  | 26 ++++++++-------------
 include/boost/int128/detail/uint128_imp.hpp | 16 ++++++++-----
 2 files changed, 20 insertions(+), 22 deletions(-)

diff --git a/include/boost/int128/detail/int128_imp.hpp b/include/boost/int128/detail/int128_imp.hpp
index ec5eb8f1..4e5fbc0f 100644
--- a/include/boost/int128/detail/int128_imp.hpp
+++ b/include/boost/int128/detail/int128_imp.hpp
@@ -188,11 +188,11 @@ int128_t
 
     // Prefix and postfix increment
     constexpr int128_t& operator++() noexcept;
-    constexpr int128_t& operator++(int) noexcept;
+    constexpr int128_t operator++(int) noexcept;
 
     // Prefix and postfix decrment
     constexpr int128_t& operator--() noexcept;
-    constexpr int128_t& operator--(int) noexcept;
+    constexpr int128_t operator--(int) noexcept;
 
     // Compound Addition
     template <BOOST_INT128_DEFAULTED_INTEGER_CONCEPT>
@@ -2005,14 +2005,11 @@ constexpr int128_t& int128_t::operator++() noexcept
     return *this;
 }
 
-constexpr int128_t& int128_t::operator++(int) noexcept
+constexpr int128_t int128_t::operator++(int) noexcept
 {
-    if (++low == UINT64_C(0))
-    {
-        ++high;
-    }
-
-    return *this;
+    const auto temp {*this};
+    ++(*this);
+    return temp;
 }
 
 //=====================================
@@ -2029,14 +2026,11 @@ constexpr int128_t& int128_t::operator--() noexcept
     return *this;
 }
 
-constexpr int128_t& int128_t::operator--(int) noexcept
+constexpr int128_t int128_t::operator--(int) noexcept
 {
-    if (low-- == UINT64_C(0))
-    {
-        --high;
-    }
-
-    return *this;
+    const auto temp {*this};
+    --(*this);
+    return temp;
 }
 
 //=====================================
diff --git a/include/boost/int128/detail/uint128_imp.hpp b/include/boost/int128/detail/uint128_imp.hpp
index ab44f4d7..09b97611 100644
--- a/include/boost/int128/detail/uint128_imp.hpp
+++ b/include/boost/int128/detail/uint128_imp.hpp
@@ -198,9 +198,9 @@ uint128_t
     #endif // BOOST_INT128_HAS_MSVC_INT128
 
     constexpr uint128_t& operator++() noexcept;
-    constexpr uint128_t& operator++(int) noexcept;
+    constexpr uint128_t operator++(int) noexcept;
     constexpr uint128_t& operator--() noexcept;
-    constexpr uint128_t& operator--(int) noexcept;
+    constexpr uint128_t operator--(int) noexcept;
 
     // Compound Addition
     template <BOOST_INT128_DEFAULTED_INTEGER_CONCEPT>
@@ -2035,9 +2035,11 @@ constexpr uint128_t& uint128_t::operator++() noexcept
     return *this;
 }
 
-constexpr uint128_t& uint128_t::operator++(int) noexcept
+constexpr uint128_t uint128_t::operator++(int) noexcept
 {
-    return ++(*this);
+    const auto temp {*this};
+    ++(*this);
+    return temp;
 }
 
 //=====================================
@@ -2054,9 +2056,11 @@ constexpr uint128_t& uint128_t::operator--() noexcept
     return *this;
 }
 
-constexpr uint128_t& uint128_t::operator--(int) noexcept
+constexpr uint128_t uint128_t::operator--(int) noexcept
 {
-    return --(*this);
+    const auto temp {*this};
+    --(*this);
+    return temp;
 }
 
 //=====================================