[Bug c++/85963] New: false positive "set but not used" warning [-Wunused-but-set-variable]

[mcortez at airpost dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85963

            Bug ID: 85963
           Summary: false positive "set but not used" warning
                    [-Wunused-but-set-variable]
           Product: gcc
           Version: 8.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mcortez at airpost dot net
  Target Milestone: ---

Related to, but not fixed by:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82728
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82799

Getting an annoying false positive "set but not used" warning when compiling
with -Wall

g++ (GCC) 8.1.1 20180502 (Red Hat 8.1.1-1) on Fedora 28.  gcc was updated to
whatever the latest version is available in Fedora 28 update repos.

Reduced test case:

template<typename T>
struct foo {
  T val, alpha;
  foo() : val(0), alpha(0) {}
  };

template<typename T>
inline void bar(const foo<T>& A, const foo<T>& B, foo<T>& C) {
  const bool use_alpha = true;
  const T        alpha = use_alpha ? (A.alpha * B.alpha) : T(0);

  C.val   = A.val * B.val;
  C.alpha = alpha;
  }


int main() {
  foo<double> A,B,C;

  bar(A,B,C);

  return 0;
  }


g++ gccwarn.cpp -o gccwarn -Wall -O2
gccwarn.cpp: In instantiation of ‘void bar(const foo<T>&, const foo<T>&,
foo<T>&) [with T = double]’:
gccwarn.cpp:21:12:   required from here
gccwarn.cpp:10:14: warning: variable ‘use_alpha’ set but not used
[-Wunused-but-set-variable]
   const bool use_alpha = true;
              ^~~~~~~~~