build failure on clang 19: deleted operator '=' on atomic #2530
Description
after fixing #2529, there is another build failure with clang 19. the same failure happens on macos and linux.
souffle> /tmp/nix-build-souffle-2.4.1.drv-0/source/src/interpreter/Index.h:417:14: error: overload resolution selected deleted operator '='
souffle> 417 | data = src.data;
souffle> | ~~~~ ^ ~~~~~~~~
souffle> /nix/store/cvl21fnjrwqcw59vkvhfdfl5w4d0xfaw-libcxx-19.1.6-dev/include/c++/v1/__atomic/atomic.h:58:11: note: candidate function has been explicitly deleted
souffle> 58 | atomic& operator=(const atomic&) = delete;
souffle> | ^
souffle> /nix/store/cvl21fnjrwqcw59vkvhfdfl5w4d0xfaw-libcxx-19.1.6-dev/include/c++/v1/__atomic/atomic.h:59:11: note: candidate function has been explicitly deleted
souffle> 59 | atomic& operator=(const atomic&) volatile = delete;
souffle> | ^
souffle> /nix/store/cvl21fnjrwqcw59vkvhfdfl5w4d0xfaw-libcxx-19.1.6-dev/include/c++/v1/__atomic/atomic.h:49:29: note: candidate function
souffle> 49 | _LIBCPP_HIDE_FROM_ABI _Tp operator=(_Tp __d) volatile _NOEXCEPT {
souffle> | ^
souffle> /nix/store/cvl21fnjrwqcw59vkvhfdfl5w4d0xfaw-libcxx-19.1.6-dev/include/c++/v1/__atomic/atomic.h:53:29: note: candidate function
souffle> 53 | _LIBCPP_HIDE_FROM_ABI _Tp operator=(_Tp __d) _NOEXCEPT {
souffle> | ^souffle/src/interpreter/Index.h
Lines 422 to 424 in 040a962
it looks like the code as written wants to implicitly load/store the atomic by coercing it to a bool, but the compiler is falling back to copy assignment for atomics which is deleted. it looks like the implicit load was deprecated (removed?) in C++ 20: https://en.cppreference.com/w/cpp/atomic/atomic/operator_T
to fix this, it might be possible to add explicit .load() and .store() calls. or, a std::atomic_flag could be used instead. i haven't tried any patches since i'm not familiar with the code.