[PATCH] libstdc++: Specialize _Never_valueless_alt for jthread, stop_token and stop_source

[Tomasz Kamiński]

The move constructors for stop_source and stop_token are equivalent to
copying and clearing the raw pointer, as they are wrappers for a
counted-shared state.

For jthread, the move constructor performs a member-wise move of stop_source
and thread. While std::thread could also have a _Never_valueless_alt
specialization due to its inexpensive move (only moving a handle), doing
so now would change the ABI. This patch takes the opportunity to correct
this behavior for jthread, before C++20 API is marked stable.

libstdc++-v3/ChangeLog:

	* include/std/stop_token (__variant::_Never_valueless_alt): Declare.
	(__variant::_Never_valueless_alt<std::stop_token>)
	(__variant::_Never_valueless_alt<std::stop_source>): Define.
	* include/std/thread: (__variant::_Never_valueless_alt): Declare.
	(__variant::_Never_valueless_alt<std::jthread>): Define.
---
 libstdc++-v3/include/std/stop_token | 19 +++++++++++++++++++
 libstdc++-v3/include/std/thread     | 14 ++++++++++++++
 2 files changed, 33 insertions(+)

diff --git a/libstdc++-v3/include/std/stop_token b/libstdc++-v3/include/std/stop_token
index 775ec6aa207..b593daff392 100644
--- a/libstdc++-v3/include/std/stop_token
+++ b/libstdc++-v3/include/std/stop_token
@@ -648,6 +648,25 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _Callback>
     stop_callback(stop_token, _Callback) -> stop_callback<_Callback>;
 
+  /// @cond undocumented
+  namespace __detail::__variant
+  {
+    template<typename> struct _Never_valueless_alt; // see <variant>
+
+    // Provide the strong exception-safety guarantee when emplacing a
+    // stop_token or stop_source into a variant.
+    template<>
+      struct _Never_valueless_alt<std::stop_token>
+      : true_type
+      { };
+
+    template<>
+      struct _Never_valueless_alt<std::stop_source>
+      : true_type
+      { };
+  }  // namespace __detail::__variant
+  /// @endcond
+
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace std
 #endif // __glibcxx_jthread
diff --git a/libstdc++-v3/include/std/thread b/libstdc++-v3/include/std/thread
index 0de08c0bd3e..94ded714e9e 100644
--- a/libstdc++-v3/include/std/thread
+++ b/libstdc++-v3/include/std/thread
@@ -294,6 +294,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     stop_source _M_stop_source;
     thread _M_thread;
   };
+
+  /// @cond undocumented
+  namespace __detail::__variant
+  {
+    template<typename> struct _Never_valueless_alt; // see <variant>
+
+    // Provide the strong exception-safety guarantee when emplacing a
+    // jthread into a variant.
+    template<>
+      struct _Never_valueless_alt<std::jthread>
+      : true_type
+      { };
+  }  // namespace __detail::__variant
+  /// @endcond
 #endif // __cpp_lib_jthread
 
 #ifdef __cpp_lib_formatters // C++ >= 23
-- 
2.50.1