From 4484945c538b26da6881f83f664235988e471d40 Mon Sep 17 00:00:00 2001
From: Jonathan Wakely <jwakely@redhat.com>
Date: Tue, 19 Nov 2024 23:38:19 +0000
Subject: [PATCH] libstdc++: Fix std::unordered_set::emplace optimization
 [PR117686]

The __is_key_type specialization that matches a pair<key_type, T>
argument is intended for std::unordered_map, not for
std::unordered_set<std::pair<K,T>>.

This uses a pair<const Args&...> as the template argument for
__is_key_type, so that it won't match a set's key_type.

libstdc++-v3/ChangeLog:

	PR libstdc++/117686
	* include/bits/hashtable.h (_Hashtable::_M_emplace_uniq):
	Adjust usage of __is_key_type to avoid false positive.
	* testsuite/23_containers/unordered_set/insert/117686.cc:
	New test.
---
 libstdc++-v3/include/bits/hashtable.h            |  4 ++--
 .../23_containers/unordered_set/insert/117686.cc | 16 ++++++++++++++++
 2 files changed, 18 insertions(+), 2 deletions(-)
 create mode 100644 libstdc++-v3/testsuite/23_containers/unordered_set/insert/117686.cc

diff --git a/libstdc++-v3/include/bits/hashtable.h b/libstdc++-v3/include/bits/hashtable.h
index a704816573a..b8bd8c2f418 100644
--- a/libstdc++-v3/include/bits/hashtable.h
+++ b/libstdc++-v3/include/bits/hashtable.h
@@ -2286,9 +2286,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  }
 	else if constexpr (sizeof...(_Args) == 2)
 	  {
-	    pair<const _Args&...> __refs(__args...);
-	    if constexpr (__is_key_type<pair<_Args...>>)
+	    if constexpr (__is_key_type<pair<const _Args&...>>)
 	      {
+		pair<const _Args&...> __refs(__args...);
 		const auto& __key = _ExtractKey{}(__refs);
 		__kp = std::__addressof(__key);
 	      }
diff --git a/libstdc++-v3/testsuite/23_containers/unordered_set/insert/117686.cc b/libstdc++-v3/testsuite/23_containers/unordered_set/insert/117686.cc
new file mode 100644
index 00000000000..3baac16c2b4
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/unordered_set/insert/117686.cc
@@ -0,0 +1,16 @@
+// { dg-do compile { target c++11 } }
+// Bug 117686 - error in unordered_set::emplace
+
+#include <unordered_set>
+#include <utility>
+
+struct H {
+  std::size_t operator()(const std::pair<int, int>&) const { return 0; }
+};
+
+void
+test_117686()
+{
+  std::unordered_set<std::pair<int, int>, H> s;
+  s.emplace(1, 2);
+}