C++20 concept constraints causing compilation scenarios with std::variant and std::visit
I'm trying to debug I'm a bit lost with I've been working on this all day and I'm performance testing and I've looked through the documentation and I'm still confused about I tried several approaches but none seem to work... I'm trying to use C++20 concepts to constrain a template function that operates on `std::variant` types. The goal is to ensure that the variant only contains types that can be printed using `std::ostream`. However, I'm working with a compilation behavior that I need to seem to resolve. Here’s the simplified version of my code: ```cpp #include <iostream> #include <variant> #include <type_traits> #include <concepts> template <typename T> concept Streamable = requires (std::ostream &os, T t) { os << t; }; template <typename... Ts> void printVariant(const std::variant<Ts...>& var) { std::visit([](const auto& value) { std::cout << value << '\n'; }, var); } int main() { std::variant<int, std::string> myVar = "Hello"; printVariant(myVar); return 0; } ``` When I compile this code with `g++` version 10.2.0, I get the following behavior: ``` behavior: no matching function for call to ‘printVariant(std::variant<int, std::string>&)’ ``` I tried to add a constraint to the `printVariant` function template to check if `T` satisfies the `Streamable` concept, but when I do that, I get: ``` behavior: template argument deduction failed ``` I want to ensure that the `printVariant` function can only be called with variants of types that are streamable. I’ve attempted to define the function like this: ```cpp template <typename... Ts> requires (Streamable<Ts> && ...) void printVariant(const std::variant<Ts...>& var) { // Implementation remains the same } ``` But it still doesn't compile. How do I correctly constrain the `printVariant` function so that it only accepts `std::variant` types containing streamable types? Any advice on resolving this scenario would be greatly appreciated! I'd really appreciate any guidance on this. Any help would be greatly appreciated! Thanks in advance! I've been using C++ for about a year now. I've been using C++ for about a year now. I'm working with C++ in a Docker container on Linux. Am I missing something obvious? My team is using C++ for this desktop app. Is there a better approach?