[PATCH] libstdc++: Value-initialize hashtable helper objects

libstdc++15 and 16 do not allow to construct an unordered set with a
hasher whose ctor is explicit.

This is an issue with Clang, or with GCC -Wsystem-headers. See
https://godbolt.org/z/EGzKbjh84

Fix this by using value-init, similar to commit
f8f0193b5b83f6e85d65015e79c803295baf5166. However, that likely
regressed again in commit 689d4abc0b836aaf1c8ecd38091dde6b14373c60?

patch is attached and copied later.

I'd say the unit test could be expanded by adding
// { dg-additional-options "-Werror -Wsystem-headers" }
, but I wasn't sure if this is recommended.

From d94f4ef24d90d81d6e5dd6e9807957f398d7160e Mon Sep 17 00:00:00 2001
From: MarcoFalke <[email protected]>
Date: Sun, 26 Jul 2026 14:30:58 +0000
Subject: [PATCH] libstdc++: Value-initialize hashtable helper objects

libstdc++-v3/ChangeLog:

    * include/bits/hashtable_policy.h (_Hashtable_ebo_helper):
    Value-initialize the stored object.
    * testsuite/23_containers/unordered_set/cons/explicit.cc: New test.
---
 libstdc++-v3/include/bits/hashtable_policy.h    |  4 ++--
 .../unordered_set/cons/explicit.cc              | 17 +++++++++++++++++
 2 files changed, 19 insertions(+), 2 deletions(-)
 create mode 100644
libstdc++-v3/testsuite/23_containers/unordered_set/cons/explicit.cc

diff --git a/libstdc++-v3/include/bits/hashtable_policy.h
b/libstdc++-v3/include/bits/hashtable_policy.h
index 6e4b365d592..1f1bf0bb111 100644
--- a/libstdc++-v3/include/bits/hashtable_policy.h
+++ b/libstdc++-v3/include/bits/hashtable_policy.h
@@ -1033,7 +1033,7 @@ namespace __detail
        bool __use_ebo = !__is_final(_Tp) && __is_empty(_Tp)>
     struct _Hashtable_ebo_helper
     {
-      [[__no_unique_address__]] _Tp _M_obj;
+      [[__no_unique_address__]] _Tp _M_obj{};
     };

 #if ! _GLIBCXX_INLINE_VERSION
@@ -1042,7 +1042,7 @@ namespace __detail
   template<typename _Tp>
     struct _Hashtable_ebo_helper<_Tp, false>
     {
-      _Tp _M_obj;
+      _Tp _M_obj{};
     };
 #endif

diff --git a/libstdc++-v3/testsuite/23_containers/unordered_set/cons/explicit.cc
b/libstdc++-v3/testsuite/23_containers/unordered_set/cons/explicit.cc
new file mode 100644
index 00000000000..9a358e6debd
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/unordered_set/cons/explicit.cc
@@ -0,0 +1,17 @@
+// { dg-do compile { target c++11 } }
+
+#include <unordered_set>
+
+struct Hash
+{
+  explicit Hash(bool = false) { }
+
+  std::size_t operator()(int value) const
+  { return value; }
+};
+
+void
+test01()
+{
+  std::unordered_set<int, Hash> set;
+}
-- 
2.53.0

From d94f4ef24d90d81d6e5dd6e9807957f398d7160e Mon Sep 17 00:00:00 2001
From: MarcoFalke <[email protected]>
Date: Sun, 26 Jul 2026 14:30:58 +0000
Subject: [PATCH] libstdc++: Value-initialize hashtable helper objects

libstdc++-v3/ChangeLog:

	* include/bits/hashtable_policy.h (_Hashtable_ebo_helper):
	Value-initialize the stored object.
	* testsuite/23_containers/unordered_set/cons/explicit.cc: New test.
---
 libstdc++-v3/include/bits/hashtable_policy.h    |  4 ++--
 .../unordered_set/cons/explicit.cc              | 17 +++++++++++++++++
 2 files changed, 19 insertions(+), 2 deletions(-)
 create mode 100644 libstdc++-v3/testsuite/23_containers/unordered_set/cons/explicit.cc

diff --git a/libstdc++-v3/include/bits/hashtable_policy.h b/libstdc++-v3/include/bits/hashtable_policy.h
index 6e4b365d592..1f1bf0bb111 100644
--- a/libstdc++-v3/include/bits/hashtable_policy.h
+++ b/libstdc++-v3/include/bits/hashtable_policy.h
@@ -1033,7 +1033,7 @@ namespace __detail
 	   bool __use_ebo = !__is_final(_Tp) && __is_empty(_Tp)>
     struct _Hashtable_ebo_helper
     {
-      [[__no_unique_address__]] _Tp _M_obj;
+      [[__no_unique_address__]] _Tp _M_obj{};
     };
 
 #if ! _GLIBCXX_INLINE_VERSION
@@ -1042,7 +1042,7 @@ namespace __detail
   template<typename _Tp>
     struct _Hashtable_ebo_helper<_Tp, false>
     {
-      _Tp _M_obj;
+      _Tp _M_obj{};
     };
 #endif
 
diff --git a/libstdc++-v3/testsuite/23_containers/unordered_set/cons/explicit.cc b/libstdc++-v3/testsuite/23_containers/unordered_set/cons/explicit.cc
new file mode 100644
index 00000000000..9a358e6debd
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/unordered_set/cons/explicit.cc
@@ -0,0 +1,17 @@
+// { dg-do compile { target c++11 } }
+
+#include <unordered_set>
+
+struct Hash
+{
+  explicit Hash(bool = false) { }
+
+  std::size_t operator()(int value) const
+  { return value; }
+};
+
+void
+test01()
+{
+  std::unordered_set<int, Hash> set;
+}
-- 
2.53.0