[Bug c++/105125] New: Bogus and Misleading Warning: Packed attribute is unnecessary.

[Kimon.Hoffmann at lawo dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105125

            Bug ID: 105125
           Summary: Bogus and Misleading Warning: Packed attribute is
                    unnecessary.
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Kimon.Hoffmann at lawo dot com
  Target Milestone: ---

GCC erroneously declares a packed attribute on a struct member as unnecessary,
while it clearly isn't.

Reproducer:

---------- 8< ----------

#include <cstdint>

struct A
{
    std::uint32_t i;
};

struct B
{
    std::uint64_t i;
};

struct C
{
    A a;
    B b;
};

struct D
{
    A a [[gnu::packed]];
    B b;
};

struct E
{
    A a;
    B b [[gnu::packed]];
};

struct F
{
    B b [[gnu::packed]];
    A a;
};

static_assert((sizeof(A) ==  4) && (alignof(A) == 4));
static_assert((sizeof(B) ==  8) && (alignof(B) == 8));
static_assert((sizeof(C) == 16) && (alignof(C) == 8));
static_assert((sizeof(D) == 16) && (alignof(D) == 8));
static_assert((sizeof(E) == 12) && (alignof(E) == 4));
static_assert((sizeof(F) == 12) && (alignof(F) == 4));

---------- >8 ----------

Invocation: cat test.cpp | x86_64-linux-gnu-g++ -xc++ -std=c++17 -Wpacked -c -o
/dev/null -

Observed behavior:
<stdin>:21:7: warning: packed attribute is unnecessary for ‘D::a’
[-Wattributes]
<stdin>:33:7: warning: packed attribute is unnecessary for ‘F::b’
[-Wattributes]

Expected behaviur:
While the warning referring to line 21 is correct, the warning referring to
line 33 isn't, and thus no message is expected.


Please note that this report describes the same behavior as report #59220, but
is more generally applicable, as the aforementioned report uses an explicitly
overaligned type, and might thus give the false impression that overalignment
is a required prerequisite for the issue to.