Alignment not respected in containers for over-aligned enumeration types

[mclow]

Bugzilla Link	42396
Resolution	INVALID
Resolved on	Jun 25, 2019 15:41
Version	unspecified
OS	All
Attachments	Reproducer
CC	@dwblaikie,@mclow,@zygoloid

Extended Description

Inspired by: https://stackoverflow.com/questions/56760874/discrepancy-in-c-between-over-aligned-struct-and-enum-within-container

Given an enumeration type with a specified alignment, the elements in a vector of such a type should be aligned correctly. This appears not to be the case - and can lead to UBSAN going off.

In the reproducer, I also check the size of a 16-element array of these two types.
Clang says they're different sizes - that suggests to me that there's a clang problem here.

The alignment in the containers bit could be related to #23008 . (might even be the same bug)

[mclow]

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

[mclow]

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.