Why is the transaction_safe function not recognized by std :: is_function when compiling with GCC 7, libstdc ++ and -fgnu-tm?

Question

Is the compilation failure lower due to a defect in libstdc ++, or is this behavior consistent with TS Transactional Memory ( n4514 )?

#include <type_traits> static_assert(std::is_function_v<void() transaction_safe>, ""); int main() {}