[Bug c/79667] New: spurious -Wunused-variable on a local array of struct declared unused

["msebor at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79667

            Bug ID: 79667
           Summary: spurious -Wunused-variable on a local array of struct
                    declared unused
           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: ---

Both the C and C++ front ends issue a -Wunused-variable warning for the
declaration of an unused local array of a struct declared unused.  According to
the documentation, attribute unused should suppress such warnings:

  When attached to a type (including a union or a struct), this attribute means
that variables of that type are meant to appear possibly unused. GCC does not
produce a warning for any variables of that type, even if the variable appears
to do nothing.

I'm guessing this may be because the type of the variable GCC warns on is
derived from the type and not of the type itself.  For arrays, I think this is
overly subtle and GCC should avoid diagnosing them just like it does
non-arrays.  This is also what recent versions of Clang and Intel ICC do. 
(Both Clang and ICC used to diagnose arrays just like GCC does now but stopped
some time ago.)

$ (set -x && cat y.C && for lang in c c++; do gcc -S -Wall -Wextra -Wunused
-Wpedantic -x$lang y.C; done)
+ cat y.C
struct __attribute__ ((unused)) A { int i; };

void f (void)
{
  struct A a;      // no warning, ok
}

void g (void)
{ 
  struct A a[1];   // warning, bug
}
+ for lang in c c++
+ /build/gcc-git/gcc/xgcc -B /build/gcc-git/gcc -S -Wall -Wextra -Wunused
-Wpedantic -xc y.C
y.C: In function ‘g’:
y.C:10:12: warning: unused variable ‘a’ [-Wunused-variable]
   struct A a[1];   // warning, bug
            ^
+ for lang in c c++
+ /build/gcc-git/gcc/xgcc -B /build/gcc-git/gcc -S -Wall -Wextra -Wunused
-Wpedantic -xc++ y.C
y.C: In function ‘void g()’:
y.C:10:12: warning: unused variable ‘a’ [-Wunused-variable]
   struct A a[1];   // warning, bug
            ^