[Bug c/80400] New: missing -Wattributes on a invalid attribute packed on a typedef

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80400

            Bug ID: 80400
           Summary: missing -Wattributes on a invalid attribute packed on
                    a typedef
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

The description of attribute packed in the GCC manual states that:

You may only specify the packed attribute attribute on the definition of an
enum, struct or union, not on a typedef that does not also define the
enumerated type, structure or union. 

In most cases, GCC (in C mode) also issues a warning when the attribute is
specified on a typedef where it is ignored.  However, when the attribute
appears after the struct keyword in the typedef GCC fails to issue the expected
warning.  G++ has no such problem.

  typedef struct __attribute__ ((packed)) S S1;   // missing warning in C

$ (set -x && cat a.c && for lang in c c++; do gcc -S -Wall -Wextra -Wpedantic
-Wattributes -Wignored-attributes -x$lang a.c; done)
+ cat a.c
struct S { int i; char c; };

typedef __attribute__ ((packed)) struct S S0;
typedef struct __attribute__ ((packed)) S S1;   // missing warning in C
typedef struct S __attribute__ ((packed)) S2;
typedef struct S S3 __attribute__ ((packed));

#if __cplusplus
#  define A(e) static_assert(e, #e)
#else
#  define A(e) _Static_assert(e, #e)
#endif

A (sizeof (S0) == 1);
A (sizeof (S1) == 1);
A (sizeof (S2) == 1);
A (sizeof (S3) == 1);

+ for lang in c c++
+ gcc -S -Wall -Wextra -Wpedantic -Wattributes -Wignored-attributes -xc a.c
a.c:3:41: warning: ‘packed’ attribute ignored [-Wattributes]
 typedef __attribute__ ((packed)) struct S S0;
                                         ^
a.c:5:16: warning: ‘packed’ attribute ignored [-Wattributes]
 typedef struct S __attribute__ ((packed)) S2;
                ^
a.c:6:16: warning: ‘packed’ attribute ignored [-Wattributes]
 typedef struct S S3 __attribute__ ((packed));
                ^
a.c:11:16: error: static assertion failed: "sizeof (S0) == 1"
 #  define A(e) _Static_assert(e, #e)
                ^
a.c:14:1: note: in expansion of macro ‘A’
 A (sizeof (S0) == 1);
 ^
a.c:11:16: error: static assertion failed: "sizeof (S1) == 1"
 #  define A(e) _Static_assert(e, #e)
                ^
a.c:15:1: note: in expansion of macro ‘A’
 A (sizeof (S1) == 1);
 ^
a.c:11:16: error: static assertion failed: "sizeof (S2) == 1"
 #  define A(e) _Static_assert(e, #e)
                ^
a.c:16:1: note: in expansion of macro ‘A’
 A (sizeof (S2) == 1);
 ^
a.c:11:16: error: static assertion failed: "sizeof (S3) == 1"
 #  define A(e) _Static_assert(e, #e)
                ^
a.c:17:1: note: in expansion of macro ‘A’
 A (sizeof (S3) == 1);
 ^
+ for lang in c c++
+ gcc -S -Wall -Wextra -Wpedantic -Wattributes -Wignored-attributes -xc++ a.c
a.c:3:43: warning: ‘packed’ attribute ignored [-Wattributes]
 typedef __attribute__ ((packed)) struct S S0;
                                           ^~
a.c:4:41: warning: attributes ignored on elaborated-type-specifier that is not
a forward declaration [-Wattributes]
 typedef struct __attribute__ ((packed)) S S1;
                                         ^
a.c:5:43: warning: ‘packed’ attribute ignored [-Wattributes]
 typedef struct S __attribute__ ((packed)) S2;
                                           ^~
a.c:6:44: warning: ‘packed’ attribute ignored [-Wattributes]
 typedef struct S S3 __attribute__ ((packed));
                                            ^
a.c:9:16: error: static assertion failed: sizeof (S0) == 1
 #  define A(e) static_assert(e, #e)
                ^
a.c:14:1: note: in expansion of macro ‘A’
 A (sizeof (S0) == 1);
 ^
a.c:9:16: error: static assertion failed: sizeof (S1) == 1
 #  define A(e) static_assert(e, #e)
                ^
a.c:15:1: note: in expansion of macro ‘A’
 A (sizeof (S1) == 1);
 ^
a.c:9:16: error: static assertion failed: sizeof (S2) == 1
 #  define A(e) static_assert(e, #e)
                ^
a.c:16:1: note: in expansion of macro ‘A’
 A (sizeof (S2) == 1);
 ^
a.c:9:16: error: static assertion failed: sizeof (S3) == 1
 #  define A(e) static_assert(e, #e)
                ^
a.c:17:1: note: in expansion of macro ‘A’
 A (sizeof (S3) == 1);
 ^