<utility>: Remove self swap check from pair.

stl/inc/utility

@@ -441,21 +441,17 @@ struct pair { // store a pair of values
     _CONSTEXPR20 void swap(pair& _Right) noexcept(
         _Is_nothrow_swappable<_Ty1>::value && _Is_nothrow_swappable<_Ty2>::value) {
         using _STD swap;
-        if (this != _STD addressof(_Right)) {
-            swap(first, _Right.first); // intentional ADL
-            swap(second, _Right.second); // intentional ADL
-        }
+        swap(first, _Right.first); // intentional ADL
+        swap(second, _Right.second); // intentional ADL
     }
 
 #if _HAS_CXX23
     template <int = 0> // see GH-3013
     constexpr void swap(const pair& _Right) const
         noexcept(is_nothrow_swappable_v<const _Ty1> && is_nothrow_swappable_v<const _Ty2>) {
         using _STD swap;
-        if (this != _STD addressof(_Right)) {
-            swap(first, _Right.first); // intentional ADL
-            swap(second, _Right.second); // intentional ADL
-        }
+        swap(first, _Right.first); // intentional ADL
+        swap(second, _Right.second); // intentional ADL
     }
 #endif // _HAS_CXX23
 

tests/std/test.lst

@@ -243,6 +243,7 @@ tests\GH_004201_chrono_formatter
 tests\GH_004275_seeking_fancy_iterators
 tests\GH_004388_unordered_meow_operator_equal
 tests\GH_004477_mdspan_warning_5246
+tests\GH_004597_self_swap
 tests\GH_004618_mixed_operator_usage_keeps_statistical_properties
 tests\GH_004618_normal_distribution_avoids_resets
 tests\LWG2381_num_get_floating_point

tests/std/tests/GH_004597_self_swap/env.lst

@@ -0,0 +1,4 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+RUNALL_INCLUDE ..\usual_matrix.lst

tests/std/tests/GH_004597_self_swap/test.cpp

@@ -0,0 +1,90 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <cassert>
+#include <utility>
+
+struct swap_counter {
+    unsigned int* pcnt_ = nullptr;
+
+    friend _CONSTEXPR20 void swap(swap_counter& lhs, swap_counter& rhs) noexcept {
+        assert(lhs.pcnt_ != nullptr);
+        assert(rhs.pcnt_ != nullptr);
+        std::swap(lhs.pcnt_, rhs.pcnt_);
+        ++*lhs.pcnt_;
+        ++*rhs.pcnt_;
+    }
+
+    _CONSTEXPR20 bool operator==(unsigned int x) const {
+        assert(pcnt_ != nullptr);
+        return *pcnt_ == x;
+    }
+};
+
+
+// Test GH-4597 "<utility>: Side effects in self-swaps of pair are skipped"
+_CONSTEXPR20 bool test_gh_4597() {
+    {
+        unsigned int cnt{};
+        std::pair<swap_counter, int> pr{swap_counter{&cnt}, 10};
+        pr.swap(pr);
+        assert(cnt == 2u);
+        assert(pr.first == 2u);
+        assert(pr.second == 10);
+    }
+
+    {
+        unsigned int cnt{};
+        std::pair<swap_counter, int> p1{swap_counter{&cnt}, 10};
+        std::pair<swap_counter, int> p2{swap_counter{&cnt}, 11};
+        p1.swap(p2);
+        assert(cnt == 2u);
+        assert(p1.first == 2u);
+        assert(p1.second == 11);
+        assert(p2.first == 2u);
+        assert(p2.second == 10);
+    }
+
+    {
+        unsigned int c1{};
+        unsigned int c2{2};
+        std::pair<swap_counter, int> p1{swap_counter{&c1}, 11};
+        std::pair<swap_counter, int> p2{swap_counter{&c2}, 13};
+        p1.swap(p2);
+        assert(c1 == 1u);
+        assert(c2 == 3u);
+        assert(p1.first == 3u);
+        assert(p1.second == 13);
+        assert(p2.first == 1u);
+        assert(p2.second == 11);
+    }
+
+#if _HAS_CXX23
+    {
+        unsigned int c1{};
+        unsigned int c2{2};
+        int i1 = 11;
+        int i2 = 13;
+        swap_counter s1{&c1};
+        swap_counter s2{&c2};
+        const std::pair<swap_counter&, int&> p1{s1, i1};
+        const std::pair<swap_counter&, int&> p2{s2, i2};
+        p1.swap(p2);
+        assert(c1 == 1u);
+        assert(c2 == 3u);
+        assert(p1.first == 3u);
+        assert(p1.second == 13);
+        assert(p2.first == 1u);
+        assert(p2.second == 11);
+    }
+#endif // _HAS_CXX23
+
+    return true;
+}
+
+int main() {
+#if _HAS_CXX20
+    static_assert(test_gh_4597());
+#endif // _HAS_CXX20
+    assert(test_gh_4597());
+}