Thanks for your help! I successfully sent my report. Here's the number: FB22882385

That's the thing. This code should compile, but it doesn't. It compiles with Apple clang v17, but the latest clang (v21) makes things different... Problem probably hides in latest STL library.

Sorry for not creating the project appropriately! Here's the link: https://disk.yandex.ru/d/gDzkLLdlOrZQAg

Thanks for an answer! Here's a test project: #include <unordered_map> #include <system_error> #include <string_view> #include <functional> #include <expected> int main(int argc, const char *argv[]) { std::unordered_map<int, std::pair<int, std::function<std::expected<std::string_view, std::error_code>(void)>>> var{ {24, {42, [] -> std::expected<std::string_view, std::error_code> { return "Hello, World!"; } }} }; const auto &it{var.find(24)}; if (it == var.end()) [[unlikely]] return 1; return 0; } Flags for Apple clang v21.0.0: -03, -std=c++26 If you need more details, feel free to ask me about them. I'll try to reply as soon as I can