Use C2x [[fallthrough]] when supported by LLVM/clang [v9.16]
requested to merge 3371-check-for-__attribute__-fallthrough-support-is-sometimes-incorrect-v9_16 into v9_16
Clang added support for the gcc-style fallthrough attribute (i.e. attribute((fallthrough))) in version 10. However, __has_attribute(fallthrough) will return 1 in C mode in older versions, even though they only support the C++11 fallthrough attribute. At best, the unsupported attribute is simply ignored; at worst, it causes errors.
The C2x fallthrough attribute has the advantages of being supported in the broadest range of clang versions (added in version 9) and being easy to check for support. Use C2x [[fallthrough]] attribute if possible, and fall back to not using an attribute for clang versions that don't have it.
Courtesy of Joshua Root
(cherry picked from commit 14c8d438)
Closes #3371 (closed)
Edited by Ondřej Surý