C++20 no member named 'derived_from' in namespace 'std'

[Tradias]

What version of protobuf and what language are you using?
Version: v28.3
Language: C++

What operating system (Linux, Windows, ...) and version?

Ubuntu 20.04.6, Github runner (see https://github.com/actions/runner-images/blob/ubuntu20/20241117.1/images/ubuntu/Ubuntu2004-Readme.md)

What runtime / compiler are you using (e.g., python version or gcc version)

Clang 10 with -stdlib=libc++

What did you do?

Compile any program that includes src/google/protobuf/port.h.

Additionally, regardless of the compiler, the static_assertion inside google::protobuf::internal::AssertDownCast from src/google/protobuf/port.h will always fail.

What did you expect to see

Compiles cleanly

What did you see instead?

In file included from /home/runner/work/asio-grpc/vcpkg_installed/x64-linux-clang-release/include/google/protobuf/io/coded_stream.h:114:
Error: /home/runner/work/asio-grpc/vcpkg_installed/x64-linux-clang-release/include/google/protobuf/port.h:177:10: error: no member named 'derived_from' in namespace 'std'
    std::derived_from<std::remove_pointer_t<To>,
    ~~~~~^
Error: /home/runner/work/asio-grpc/vcpkg_installed/x64-linux-clang-release/include/google/protobuf/port.h:177:48: error: expected '(' for function-style cast or type construction
    std::derived_from<std::remove_pointer_t<To>,
                      ~~~~~~~~~~~~~~~~~~~~~~~~~^

Anything else we should know about your project / environment

Suggested fixes:

Either keep using C++20

Add the following to the top of src/google/protobuf/port.h

#if __cpp_lib_concepts >= 202002L
#include <concepts>
#endif

Clean up this usage of concepts

protobuf/src/google/protobuf/port.h

Line 184 in 8476dba

static_assert(!requires {

Change

#if defined(__cpp_concepts)
  static_assert(!requires {
    std::derived_from<std::remove_pointer_t<To>,
                      typename std::remove_pointer_t<To>::MessageLite>;
  });

to

#if __cpp_lib_concepts >= 202002L
  static_assert(!std::derived_from<std::remove_pointer_t<To>,
                      typename std::remove_pointer_t<To>::MessageLite>);
#endif

Or switch to older C++ standard

Replace std::derived_from with

template<class Derived, class Base>
struct DerivedFrom : std::integral_constant<bool, (std::is_base_of<Base, Derived>::value &&
    std::is_convertible<const volatile Derived*, const volatile Base*>::value)>
{};

[acozzette]

Thank you for the bug report. We would be happy to review a pull request if you would like to send us one, but if not then we will probably get around to this eventually.

[acozzette]

Unfortunately we do still use this DownCast() function in some internal-only code, so I wasn't able to submit your entire change.

[Tradias]

@acozzette that is surprising. I thought you guys are using C++20 internally. In that case just remove the static_assert seeing that OSS doesn't need it (function is never used) and you don't have C++20.