[Bug c/84108] [8 Regression] incorrect -Wattributes warning for packed/aligned conflict on struct members

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84108

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2018-01-29
      Known to work|                            |5.1.0, 6.3.0, 7.2.0
            Summary|incorrect -Wattributes      |[8 Regression] incorrect
                   |warning for packed/aligned  |-Wattributes warning for
                   |conflict                    |packed/aligned conflict on
                   |                            |struct members
     Ever confirmed|0                           |1
      Known to fail|                            |8.0

--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
Confirmed.  Looks like I missed that the two attributes are treated differently
when used with struct members than with standalone objects.  Here's a test case
showing the differences and the expected/bogus warnings:

$ cat pr84108.c && gcc -O2 -S -Wall pr84108.c
#define A(e) _Static_assert (e, #e)

int i2 __attribute__((packed, aligned (2)));   // -Wattributes
int j2 __attribute__((aligned (2), packed));   // -Wattributes
int i1 __attribute__((aligned (2)));           // okay (reduces alignment)

A (_Alignof (i2) == 2);
A (_Alignof (j2) == 2);
A (_Alignof (i1) == 2); 

struct {
  int i2 __attribute__((packed, aligned (2)));   // bogus -Wattributes in GCC 8
  int j2 __attribute__((aligned (2), packed));   // bogus -Wattributes in GCC 8
  int i1 __attribute__((aligned (2)));           // ignored, no warning
} s;

A (_Alignof (s.i2) == 2);
A (_Alignof (s.j2) == 2);
A (_Alignof (s.i1) == 4);

pr84108.c:3:1: warning: ‘packed’ attribute ignored [-Wattributes]
 int i2 __attribute__((packed, aligned (2)));   // -Wattributes
 ^~~
pr84108.c:4:1: warning: ‘packed’ attribute ignored [-Wattributes]
 int j2 __attribute__((aligned (2), packed));   // -Wattributes
 ^~~
pr84108.c:12:3: warning: ignoring attribute ‘aligned’ because it conflicts with
attribute ‘packed’ [-Wattributes]
   int i2 __attribute__((packed, aligned (2)));   // bogus -Wattributes in GCC
8
   ^~~
pr84108.c:13:3: warning: ignoring attribute ‘packed’ because it conflicts with
attribute ‘aligned’ [-Wattributes]
   int j2 __attribute__((aligned (2), packed));   // bogus -Wattributes in GCC
8
   ^~~