packed struct - strange field offset

[markfirmware]

This program displays 4 for the offset of b, where I am expecting it to be 16.

Linux localhost 4.9.0-9-amd64 #1 SMP Debian 4.9.168-1 (2019-04-12) x86_64 GNU/Linux
0.4.0+381c6a38

const std = @import("std");

const S = packed struct {
    a: u32,
    pad: [3]u32,
    b: u32
};

pub fn main() anyerror!void {
    var offset: usize = @byteOffsetOf(S, "b");
    std.debug.warn("offset {}\n", offset);
}

[FireFox317]

Related: #2627

[SamTebbs33]

Yeah this is a duplicate of the issue linked by @timonkruiper I found a little while ago, where the total size of the struct is also wrong.