On Tue, May 9, 2023, 17:18 Lénárd Szolnoki via Std-Discussion <
std-discussion_at_[hidden]> wrote:

> Hi,
>
> Consider the following code:
>
> #include <type_traits>
>
> struct A {};
> struct B : A { char ch; };
>
> struct C : A {
> B b;
> };
>
> static_assert(std::is_standard_layout_v<C>);
> static_assert(sizeof(C) == 2);
>
> In my reading of https://eel.is/c++draft/class.prop#3 C is standard-
> layout. GCC with libstdc++ agrees.
>
> However the offset of a `b` subobject within an object of C is 1
> instead of 0 as compiled by both gcc and clang, making it definitely
> not pointer-interconvertible with the parent object, even though it's
> required to be:
>
> https://eel.is/c++draft/basic.compound#4.3
>
> Note that there are two subobjects of type A, requiring different
> addresses, which makes implementing this tricky, if not impossible.
>
> Anyway, https://eel.is/c++draft/class.prop#note-2 suggests that Cis not
> intended to be standard-layout, however C satisfies the corresponding
> normative wording https://eel.is/c++draft/class.prop#3.7:
>
> * M(C): https://eel.is/c++draft/class.prop#3.7.2 applies, M(C) is the
> union of {B} and M(B)
> * M(B) same applies, M(B) is union of {char} and M(char)
> * M(char): empty https://eel.is/c++draft/class.prop#3.7.5
>
> Therefore M(C) is {B, char}, none of which is a base of C.
>
> Is this a defect, or did I make a mistake somewhere in my reasoning?
>

Yes, this looks new. We got the current wording in
https://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1672 but see
also https://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1813

TBH, it looks like this has been broken since C++11 (when we first allowed
standard layout class types to have bases).
https://timsong-cpp.github.io/cppwp/n3337/class#7

Fixing it will be slightly tricky; it's only the early (zero-offset) bases
of the first data member that (need to) conflict. What do clang and msvc do
in that case?

Do you want to file via https://github.com/cplusplus/CWG/issues ?


> Cheers,
> Lénárd
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