[llvm-bugs] [Bug 40831] New: bad code with hand-crafted offsetof

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https://bugs.llvm.org/show_bug.cgi?id=40831

            Bug ID: 40831
           Summary: bad code with hand-crafted offsetof
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: stsp2 at yandex.ru
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

As offsetof doesn't work with template arguments,
I have to create the one by hands:
---
#include <cstddef>
#include <iostream>

class L {};
template <typename T, L (T::*M)[0]>
struct offset_of {
    constexpr operator size_t() const {
        return (std::uintptr_t)&(((T*)nullptr)->*M);
    }
};
template <typename T, L (T::*M)[0]>
struct B {
    char aa[10];
    static const int off = offset_of<T, M>();
};

struct A {
    char a;
    L _mark[0];
    B<A, &A::_mark> b;
};

int main()
{
    A a;
    std::cout << "size " << sizeof(A) << " off " << a.b.off << std::endl;
    return 0;
}
---

This example works perfectly with gcc:
size 11 off 1

But behaves badly when compiled with clang:
size 1 off 1
(and sometimes even segfaults, it seems)

Besides fixing the bug, I wonder if it would be
possible to somehow fix offsetof() to allow the
use of templated args? Otherwise people have to
apply the horrible hacks like this.

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