use C++20 concepts without guarding them with the SEASTAR_CONCEPT() macro.

[tchaikov]

hi folks,

now that we are using C++20, can we just use C++20 concepts without guarding them with the SEASTAR_CONCEPT() macro. IIUC, SEASTAR_CONCEPT() is created to be backward compatible with C++17. as Seastar wants to be compatible with -2 C++ standard, and the latest C++ standard is C++20, so it has to use this macro. but this does not apply to Redpanda, am i right?

[BenPope]

I'm not against it, and I'm sure we have some which are unguarded already.

[tchaikov]

@dotnwat , thank you Noah for your insights. but i am not sure if i am 100% following you. the macro is defined like

#if defined(__cpp_concepts) && __cpp_concepts >= 201907 && \
    defined(__cpp_lib_concepts) && __cpp_lib_concepts >= 201907

#define SEASTAR_CONCEPT(x...) x
#define SEASTAR_NO_CONCEPT(x...)

#else

#define SEASTAR_CONCEPT(x...)
#define SEASTAR_NO_CONCEPT(x...) x

#endif

so as long as the concepts C++ feature is supported by the C++ compiler and if we are using C++20, it should be safe to drop this guard. if we would run into such an issue after dropping it, we would have had already suffered from it now. also, per https://libcxx.llvm.org/Status/Cxx20.html (search for "Standard Library Concepts"), libc++ developers consider libc++ shipped along with clang-13 includes the standard library concepts already. so i think we should be safe then, am i right?

@BenPope thank you for your help! will probably start with some clean up changes first.

[tchaikov]

i just came across dd87246

this macro is useful within the context of an IDE, which may not yet understand concepts.

this scares me. do you guys happen to know if some of us are using IDEs which do not understand concepts? i assumed that most of us are using lsp now.

[dotnwat]

at this point if the macro can be dropped and the tree builds, then I think the entire macro can be removed.

so as long as the concepts C++ feature is supported by the C++ compiler and if we are using C++20, i

what I mean is that there are two parts to being able to use concepts in practice. there is the language-level support, which we now have. but then there is all of the pre-defined concepts that are implemented or not implemented in the whatever standard library we are using. we had been using concepts with c++20 enabled, but libc++ didn't have implementations of all of the concepts.

so the concern was only that perhaps the macro hadn't been defined, and we had some of these bits hidden. but in any case, removing the macro and trying to compile seems like the easiest path.

[emaxerrno]

@tchaikov - that was a LONG time ago. Duarte was using the eclipse CDT for c++ dev. I actually think Avi maintained some fork of it at some point. afaik, most people that use IDE moved to vscode w/ lsp and clangd

like @dotnwat mention, we can remove that file

[tchaikov]

thank you Noah and Alex! just created #4919.