[Bug c++/99361] New: "unused variable" warning on a used variable in template lambda

[hewillk at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99361

            Bug ID: 99361
           Summary: "unused variable" warning on a used variable in
                    template lambda
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hewillk at gmail dot com
  Target Milestone: ---

The following code:

auto f() {
  const auto x = 1;
  return [] (auto) { return x; };
}

will emit unused variable warnings with "-std=c++20 -Wall" flags:

<source>: In function 'auto f()':
<source>:2:14: warning: variable 'x' set but not used
[-Wunused-but-set-variable]
    2 |   const auto x = 1;
      |     

godbolt: https://godbolt.org/z/d54j4G