message: [FIX] make tests build with gcc 9.1
diff: @@ -161,8 +161,7 @@ public:
  {
  using S = meta::_t<sentinel_type<I>>;
  using R = decltype(algo_(begin, end, rest...));
  - Algo algo = algo_;
  - auto check_algo = [=](function_ref<void(R)> const & check)
  + auto check_algo = [algo = algo_, begin, end, rest...](function_ref<void(R)> const & check)
  {
  check(algo(begin, end, rest...));
  check(algo(begin, S{base(end)}, rest...));
  @@ -195,8 +194,7 @@ public:
  using S1 = meta::_t<sentinel_type<I1>>;
  using S2 = meta::_t<sentinel_type<I2>>;
  using R = decltype(algo_(begin1, end1, begin2, end2, rest...));
  - Algo algo = algo_;
  - return checker<R>{[=](function_ref<void(R)> const & check)
  + return checker<R>{[algo = algo_, begin1, end1, begin2, end2, rest...](function_ref<void(R)> const & check)
  {
  check(algo(begin1, end1, begin2, end2, rest...));
  check(algo(begin1, S1{base(end1)}, begin2, S2{base(end2)}, rest...));
  