[Bug c++/79585] New: spurious -Wunused-variable on a pointer with attribute unused in function template

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79585

            Bug ID: 79585
           Summary: spurious -Wunused-variable on a pointer with attribute
                    unused in function template
           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: ---

While testing a patch for bug 79548 I came across the following inconsistency. 
The program is expected to compile with no warnings.

$ (set -x && cat t.C && for lang in c c++; do gcc -O2 -S -Wunused -x$lang t.C;
done)
+ cat t.C
void f ()
{
  int i  __attribute__ ((unused)) = 0;
  int __attribute__ ((unused)) j = 0;

  int* p  __attribute__ ((unused)) = 0;
  int* __attribute__ ((unused)) q = 0;
}

#if __cplusplus

template <class T>
void g ()
{
  T a  __attribute__ ((unused)) = 0;
  T __attribute__ ((unused)) b = 0;

  T* p  __attribute__ ((unused)) = 0;
  T* __attribute__ ((unused)) q = 0;

}

template void g<int>();

#endif
+ for lang in c c++
+ /build/gcc-git/gcc/xgcc -B /build/gcc-git/gcc -O2 -S -Wunused -xc t.C
+ for lang in c c++
+ /build/gcc-git/gcc/xgcc -B /build/gcc-git/gcc -O2 -S -Wunused -xc++ t.C
t.C: In instantiation of ‘void g() [with T = int]’:
t.C:23:22:   required from here
t.C:19:31: warning: unused variable ‘q’ [-Wunused-variable]
   T* __attribute__ ((unused)) q = 0;   // bogus warning
                               ^