Now I'm very confused.

====
struct alignas(16) byte_struct {std::uint8_t value;};
enum alignas(16) byte_enum : std::uint8_t { one=1, two, three };

int main() {
std::cout << "struct: " << sizeof(byte_struct) << std::endl;
std::cout << "enum: " << sizeof(byte_enum ) << std::endl;
std::cout << "01 structs: " << sizeof(byte_struct[1]) << std::endl;
std::cout << "01 enums: " << sizeof(byte_enum [1]) << std::endl;
std::cout << "02 structs: " << sizeof(byte_struct[2]) << std::endl;
std::cout << "02 enums: " << sizeof(byte_enum [2]) << std::endl;
std::cout << "16 structs: " << sizeof(byte_struct[16]) << std::endl;
std::cout << "16 enums: " << sizeof(byte_enum [16]) << std::endl;
}

prints (with a reasonably recent clang):

struct: 16
enum: 1
01 structs: 16
01 enums: 16
02 structs: 32
02 enums: 16
16 structs: 256
16 enums: 16

Richard pointed out (on SO) that this is core issue 2354. Extended alignment and object representation, which was resolved in Kona by removing the ability to put 'alignas' on enumeration types.

Closing as invalid.