Common initial sequence is not respected when alignas is used

[Halalaluyafail3]

The following program demonstrates the issue:

#include<iostream>
#include<type_traits>
int main(){
    struct A{char x;};
    struct alignas(2)B{char x;};
    struct C{A y;char z;};
    struct D{B y;char z;};
    union U{C a;D b;};
    static_assert(std::is_standard_layout_v<C>);
    static_assert(std::is_standard_layout_v<D>);
    static_assert(std::is_layout_compatible_v<C,D>);
    U u;
    u.a.z=1;
    std::cout<<+u.b.z<<'\n';
}

Clang recognizes that C and D are both layout-compatible standard layout classes, therefore all of their members should be part of the common initial sequence. However, Clang does not seem to lay out their respective z members so that they have the same offset. There seems to be two possible solutions here. First, reject that C and D are layout-compatible and possibly also further reject that A and B are layout-compatible. Second, whenever a standard layout class is used as a member it must be aligned to the maximum possible alignment so that any alignment differences do not matter.

See also cplusplus/draft#8787.

[frederick-vs-ja]

Other compilers reject this (Godbolt link).

I think Clang is wrong, but some clarification in the standard might be necessary.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Jay Ghiron (Halalaluyafail3)

Details

The following program demonstrates the issue: ```cpp #include<iostream> #include<type_traits> int main(){ struct A{char x;}; struct alignas(2)B{char x;}; struct C{A y;char z;}; struct D{B y;char z;}; union U{C a;D b;}; static_assert(std::is_standard_layout_v<C>); static_assert(std::is_standard_layout_v<D>); static_assert(std::is_layout_compatible_v<C,D>); U u; u.a.z=1; std::cout<<+u.b.z<<'\n'; } ``` Clang recognizes that C and D are both layout-compatible standard layout classes, therefore all of their members should be part of the common initial sequence. However, Clang does not seem to lay out their respective z members so that they have the same offset. There seems to be two possible solutions here. First, reject that C and D are layout-compatible and possibly also further reject that A and B are layout-compatible. Second, whenever a standard layout class is used as a member it must be aligned to the maximum possible alignment so that any alignment differences do not matter.

See also cplusplus/draft#8787.