Fix std::set for InvalidatableFile in C++20

[dpgrote]

This is needed by AppleClang 21.0.0 to avoid the compiler error shown below. Since InvalidatableFile is used in a std::set, the compiler now requires the comparison operators to be defined for it. The C++20 standard way of doing this is the spaceship operator, <=>. Needs a comparison operator. With the change in this PR, the compilation (of WarpX) is successful.

In file included from /Users/grote1/packages/openPMD-api/src/IO/ADIOS/ADIOS2IOHandler.cpp:23:
In file included from /Users/grote1/packages/openPMD-api/include/openPMD/IO/ADIOS/ADIOS2IOHandler.hpp:25:
In file included from /Users/grote1/packages/openPMD-api/include/openPMD/IO/ADIOS/ADIOS2Auxiliary.hpp:25:
In file included from /Users/grote1/packages/openPMD-api/include/openPMD/IO/ADIOS/macros.hpp:27:
In file included from /opt/homebrew/include/adios2.h:11:
In file included from /opt/homebrew/include/adios2/common/ADIOSMacros.h:12:
In file included from /opt/homebrew/include/adios2/common/ADIOSTypes.h:16:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/map:602:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__tree:42:
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__utility/lazy_synth_three_way_comparator.h:45:21: error: no matching function for call to object of type 'const std::less<void>'

[ax3l]

@dpgrote does the change I pushed work for you, too?

[dpgrote]

@dpgrote does the change I pushed work for you, too?

Yes, this works, WarpX compiles successfully.

[franzpoeschel]

I am a bit surprised why your build system decided to compile openPMD-api with the C++20 standard. This missing definition should not matter for inclusion in a C++20 project; the openPMD-api itself can still be compiled with C++17.

Fixing this is simple enough though and we will move to C++20 at some point anyway, so we should go ahead with fixing this. The current state of the PR has two little issues. I will later push a commit with a fix, could you give it another test again then?

[ax3l]

I am a bit surprised why your build system decided to compile openPMD-api with the C++20 standard.

Our CMake logic says in openPMD-api that we use C++17 or newer. So openPMd-api itself can be built with newer standards if the compiler supports it.

Additionally, if a downstream project used a newer C++ std then our public headers naturally will be parsed as such, too.

[ax3l]

Looking at InvalidatableFile::operator*() this sorts by name. ok!

[franzpoeschel]

Yup, this does the same. I just wanted to avoid having two distinct implementations.

[ax3l]

Looks good! 👍

@dpgrote does this still compile with XCode & C++20 for you?

Self-implemented