[Bug c++/83690] New: [8 regression] spurious unused variable warings for variables used only in static_assert

[sss@li-snyder.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83690

            Bug ID: 83690
           Summary: [8 regression] spurious unused variable warings for
                    variables used only in static_assert
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sss@li-snyder.org
  Target Milestone: ---

As of 

gcc version 8.0.0 20180102 (experimental) (GCC) 

gcc gives an unused-but-set warning for variables that are used only in a 
static_assert.

For example, with this source:

--- x.cc -------------------------------------
void foo()
{
  constexpr bool foo = true;
  static_assert(foo, "foo");
}
----------------------------------------------

we get a warning:

$ cc1plus -std=c++17 x.cc -quiet -Wall
x.cc: In function ‘void foo()’:
x.cc:3:18: warning: variable ‘foo’ set but not used [-Wunused-but-set-variable]
   constexpr bool foo = true;
                  ^~~

We do not get this warning with gcc 7.2.1.