[committed] libstdc++: Fix std::unordered_set::emplace optimization [PR117686]

[Jonathan Wakely]

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.
---

Tested powerpc64le-linux. Pushed to trunk.

 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);
+}
-- 
2.47.0