[gcc r13-1288] libstdc++: Make std::move_only_function never valueless in std::variant

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:a364488f7a70ebf6c1fd59fed9f6a8a477591087

commit r13-1288-ga364488f7a70ebf6c1fd59fed9f6a8a477591087
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Jun 23 18:42:55 2022 +0100

    libstdc++: Make std::move_only_function never valueless in std::variant
    
    This is not suitable to backport, as it affects the ABI of std::variant
    and so isn't appropriate for a release branch.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/move_only_function.h (_Never_valueless_alt):
            Define partial specialization for std::move_only_function.

Diff:
---
 libstdc++-v3/include/bits/move_only_function.h | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/libstdc++-v3/include/bits/move_only_function.h b/libstdc++-v3/include/bits/move_only_function.h
index f96552a7c89..71d52074978 100644
--- a/libstdc++-v3/include/bits/move_only_function.h
+++ b/libstdc++-v3/include/bits/move_only_function.h
@@ -183,6 +183,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     constexpr bool __is_move_only_function_v<move_only_function<_Tp>> = true;
   /// @endcond
 
+  namespace __detail::__variant
+  {
+    template<typename> struct _Never_valueless_alt; // see <variant>
+
+    // Provide the strong exception-safety guarantee when emplacing a
+    // move_only_function into a variant.
+    template<typename... _Signature>
+      struct _Never_valueless_alt<std::move_only_function<_Signature...>>
+      : true_type
+      { };
+  }  // namespace __detail::__variant
+
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace std