@endecotp : /Applications/Xcode-16.0/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.0.sdk/usr/include/c++/v1/variant:495:12: runtime error: call to function decltype(auto) std::__1::__variant_detail::__visitation::__base::__dispatcher<0ul, 0ul>::__dispatch[abi:de180100]<void std::__1::__variant_detail::__ctor<std::__1::__variant_detail::__traits<std::__1::monostate, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>>::__generic_construct[abi:de180100]<std::__1::__variant_detail::__copy_constructor<std::__1::__variant_detail::__traits<std::__1::monostate, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>, (std::__1::__variant_detail::_Trait)1> const&>(std::__1::__variant_detail::__ctor<std::__1::__variant_detail::__traits<std::__1::monostate, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>>&, std::__1::__variant_detail::__copy_constructor<std::__1::__variant_detail::__traits<std::__1::monostate, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>, (std::__1::__variant_detail::_Trait)1> const&)::'lambda'(std::__1::__variant_detail::__copy_constructor<std::__1::__variant_detail::__traits<std::__1::monostate, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>, (std::__1::__variant_detail::_Trait)1> const&, auto&&)&&, std::__1::__variant_detail::__base<(std::__1::__variant_detail::_Trait)1, std::__1::monostate, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>&, std::__1::__variant_detail::__base<(std::__1::__variant_detail::_Trait)1, std::__1::monostate, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>> const&>(std::__1::__variant_detail::__copy_constructor<std::__1::__variant_detail::__traits<std::__1::monostate, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>, (std::__1::__variant_detail::_Trait)1> const&, std::__1::__variant_detail::__base<(std::__1::__variant_detail::_Trait)1, std::__1::monostate, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>&, std::__1::__variant_detail::__base<(std::__1::__variant_detail::_Trait)1, std::__1::monostate, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>> const&) through pointer to incorrect function type 'void (*)((lambda at /Applications/Xcode-16.0/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.0.sdk/usr/include/c++/v1/variant:814:11) &&, std::__variant_detail::__base<std::__variant_detail::_Trait::_Available, std::monostate, std::string> &, const std::__variant_detail::__base<std::__variant_detail::_Trait::_Available, std::monostate, std::string> &)' variant:532: note: decltype(auto) std::__1::__variant_detail::__visitation::__base::__dispatcher<0ul, 0ul>::__dispatch[abi:de180100]<void std::__1::__variant_detail::__ctor<std::__1::__variant_detail::__traits<std::__1::monostate, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>>::__generic_construct[abi:de180100]<std::__1::__variant_detail::__copy_constructor<std::__1::__variant_detail::__traits<std::__1::monostate, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>, (std::__1::__variant_detail::_Trait)1> const&>(std::__1::__variant_detail::__ctor<std::__1::__variant_detail::__traits<std::__1::monostate, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>>&, std::__1::__variant_detail::__copy_constructor<std::__1::__variant_detail::__traits<std::__1::monostate, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>, (std::__1::__variant_detail::_Trait)1> const&)::'lambda'(std::__1::__variant_detail::__copy_constructor<std::__1::__variant_detail::__traits<std::__1::monostate, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>, (std::__1::__variant_detail::_Trait)1> const&, auto&&)&&, std::__1::__variant_detail::__base<(std::__1::__variant_detail::_Trait)1, std::__1::monostate, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>&, std::__1::__variant_detail::__base<(std::__1::__variant_detail::_Trait)1, std::__1::monostate, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>> const&>(std::__1::__variant_detail::__copy_constructor<std::__1::__variant_detail::__traits<std::__1::monostate, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>, (std::__1::__variant_detail::_Trait)1> const&, std::__1::__variant_detail::__base<(std::__1::__variant_detail::_Trait)1, std::__1::monostate, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>&, std::__1::__variant_detail::__base<(std::__1::__variant_detail::_Trait)1, std::__1::monostate, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>> const&) defined here SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /Applications/Xcode-16.0/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.0.sdk/usr/include/c++/v1/variant:495:12 This error appears only on Xcode 16*

Thanks for looking into this. I've done more testing and found that the issue is broader than originally shown. The problem occurs with complex types in the variant, regardless of monostate. This minimal case triggers the error: class Item { public: std::variant<std::string> value; }; typedef std::forward_list<Item> ItemList; class Test { public: void addItem(const Item& item); ItemList items; }; While the remaining code and compilation steps are the same.