[llvm-bugs] [Bug 51839] New: GCC and Clang disagree about packed record layout with non-POD fields

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=51839

            Bug ID: 51839
           Summary: GCC and Clang disagree about packed record layout with
                    non-POD fields
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: rnk at google.com
                CC: llvm-bugs at lists.llvm.org, neeilans at live.com,
                    richard-llvm at metafoo.co.uk

Consider:

https://gcc.godbolt.org/z/P4EKvYfr5

struct NonPod {
    NonPod(); // matters
    long long i64;
};
struct __attribute__ ((packed)) Packed {
    char c;
    NonPod a;
};
int theSize = sizeof(Packed);

GCC gives a size of 16, while clang gives a size of 9.

GCC also produces this warning:

warning: ignoring packed attribute because of unpacked non-POD field 'NonPod
Packed::a'
    7 |     NonPod a;
      |            ^


At this point, changing Clang's behavior would probably be a major ABI break.
Presumably this is a no-go for distributors unconcerned with GCC compatibility.
Therefore, it seems we need a LangOpt or something to control this behavior.

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