Apple Clang-17 issues a warning: pack indexing is a C++2c extension

[chenghuaWang]

The compiler I'm using is Apple Clang-17. I have set set(CMAKE_CXX_STANDARD 20) in CMake. However, a warning indicating the use of C++26 features appears during compilation. The README of stdexec states that only the C++20 standard is required:

stdexec requires compiling with C++20 (-std=c++20) but otherwise does not have any dependencies and only requires a sufficiently new compiler:

Could you please advise on how to eliminate this warning?

The warning is shown below:

In file included from /build-osx/_deps/stdexec-src/include/stdexec/execution.hpp:18:
In file included from /build-osx/_deps/stdexec-src/include/stdexec/__detail/__execution_fwd.hpp:19:
/build-osx/_deps/stdexec-src/include/stdexec/__detail/__meta.hpp:1132:26: warning: pack indexing is a C++2c extension [-Wc++26-extensions]
 1132 |       return static_cast<_Ts...[_Np] &&>(__ts...[_Np]);
      |                          ^
/Volumes/D/mllm/build-osx/_deps/stdexec-src/include/stdexec/__detail/__meta.hpp:1132:42: warning: pack indexing is a C++2c extension [-Wc++26-extensions]
 1132 |       return static_cast<_Ts...[_Np] &&>(__ts...[_Np]);
      |                                          ^
2 warnings generated.

[ericniebler]

can you see if #1600 helps?

[chenghuaWang]

Thank you for your reply. It's true that using -Wc++26-extensions can eliminate these warnings. May I ask if this means I'm using C++26 features?

[ericniebler]

yes, it means that even when compiling as C++20, stdexec uses pack indexing if the compiler supports it. recent versions of clang enable pack indexing by default all the way back to C++98 (!!!).