[Bug c/83559] New: -Wsuggest-attribute=const conflicts with -Wattributes warning about const attribute on function returning void

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83559

            Bug ID: 83559
           Summary: -Wsuggest-attribute=const conflicts with -Wattributes
                    warning about const attribute on function returning
                    void
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: egallager at gcc dot gnu.org
                CC: msebor at gcc dot gnu.org
  Target Milestone: ---

Created attachment 42955
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=42955&action=edit
testcase

Say I have this source and compile it: 

$ cat suggest_attribute_const.c
struct frame_info;

void
show_stack_frame(struct frame_info *fi)
{
  (void)fi;
  return;
}

void __attribute__((const))
show_stack_frame_2(struct frame_info *fi)
{
  (void)fi;
  return;
}
$ /usr/local/bin/gcc -c -Wattributes -Wsuggest-attribute=const -O2
suggest_attribute_const.c
suggest_attribute_const.c:12:1: warning: 'const' attribute on function
returning 'void' [-Wattributes]
 {
 ^
suggest_attribute_const.c: In function 'show_stack_frame':
suggest_attribute_const.c:4:1: warning: function might be candidate for
attribute 'const' [-Wsuggest-attribute=const]
 show_stack_frame(struct frame_info *fi)
 ^~~~~~~~~~~~~~~~
$

The 2 functions are basically the same, except the 2nd one has attribute const
added to silence the warning from the first one. Except that now provokes a new
warning from -Wattributes, which, to silence, requires removing the attribute,
which brings the original -Wsuggest-attribute=const back. This conflict between
warnings should be resolved.