[Bug c++/84194] fails to pack structs with template members

[kentsangkm at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84194

Kenman Tsang <kentsangkm at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |kentsangkm at gmail dot com

--- Comment #1 from Kenman Tsang <kentsangkm at gmail dot com> ---
Hi, here is another example that the GCC cannot pack a struct correctly.

Refers to the ticket 83732
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83732), this is not only related
to the diagnostic message, but how we pack POD.

Because the A1 is not a C++98 pod ( but it is a POD according to C++11 ), B1
should be able to process the pack (and the result should be sizeof(B1) == 10)

Compared with Clang-4.0 or above, clang can pack this structure correctly


>From the ticket 83732, my best guess is gcc pack a struct only if it fulfill
C++98 pod, but not the pod in general


-----
main.cpp:34:8: warning: ignoring packed attribute because of unpacked non-POD
field ‘A1 B1::b’
     A1 b;
        ^
main.cpp:39:1: error: static assertion failed
 static_assert(sizeof(B1) == 10, "");
 ^~~~~~~~~~~~~
-----
#include <iostream>

struct A1
{
    A1() = default;

    constexpr A1(const int& value) noexcept
    : value(value)
    {
    }

    constexpr A1(int&& value) noexcept
    : value(std::move(value))
    {
    }

    int value;
};
static_assert(std::is_pod<A1>::value,"Pass");
static_assert(sizeof(A1)==4, "Pass");

struct A2
{
    A2() = default;

    int value;
};
static_assert(std::is_pod<A2>::value,"Pass");
static_assert(sizeof(A2)==4, "Pass");

struct B1
{
    char a;
    A1 b;
    char c;
    int d;
} __attribute__((__packed__));
static_assert(std::is_pod<B1>::value,"Pass");
static_assert(sizeof(B1)==10, "This is failed");

struct B2
{
    char a;
    A2 b;
    char c;
    int d;
} __attribute__((__packed__));
static_assert(std::is_pod<B2>::value,"Pass");
static_assert(sizeof(B2)==10, "Pass");

int main()
{
    std::cout << sizeof(A1) << std::endl;
    std::cout << sizeof(A2) << std::endl;
}