util/env_posix.cc [835:871]:
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template <typename EnvType>
class SingletonEnv {
 public:
  SingletonEnv() {
#if !defined(NDEBUG)
    env_initialized_.store(true, std::memory_order::memory_order_relaxed);
#endif  // !defined(NDEBUG)
    static_assert(sizeof(env_storage_) >= sizeof(EnvType),
                  "env_storage_ will not fit the Env");
    static_assert(alignof(decltype(env_storage_)) >= alignof(EnvType),
                  "env_storage_ does not meet the Env's alignment needs");
    new (&env_storage_) EnvType();
  }
  ~SingletonEnv() = default;

  SingletonEnv(const SingletonEnv&) = delete;
  SingletonEnv& operator=(const SingletonEnv&) = delete;

  Env* env() { return reinterpret_cast<Env*>(&env_storage_); }

  static void AssertEnvNotInitialized() {
#if !defined(NDEBUG)
    assert(!env_initialized_.load(std::memory_order::memory_order_relaxed));
#endif  // !defined(NDEBUG)
  }

 private:
  typename std::aligned_storage<sizeof(EnvType), alignof(EnvType)>::type
      env_storage_;
#if !defined(NDEBUG)
  static std::atomic<bool> env_initialized_;
#endif  // !defined(NDEBUG)
};

#if !defined(NDEBUG)
template <typename EnvType>
std::atomic<bool> SingletonEnv<EnvType>::env_initialized_;
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util/env_windows.cc [743:779]:
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template <typename EnvType>
class SingletonEnv {
 public:
  SingletonEnv() {
#if !defined(NDEBUG)
    env_initialized_.store(true, std::memory_order::memory_order_relaxed);
#endif  // !defined(NDEBUG)
    static_assert(sizeof(env_storage_) >= sizeof(EnvType),
                  "env_storage_ will not fit the Env");
    static_assert(alignof(decltype(env_storage_)) >= alignof(EnvType),
                  "env_storage_ does not meet the Env's alignment needs");
    new (&env_storage_) EnvType();
  }
  ~SingletonEnv() = default;

  SingletonEnv(const SingletonEnv&) = delete;
  SingletonEnv& operator=(const SingletonEnv&) = delete;

  Env* env() { return reinterpret_cast<Env*>(&env_storage_); }

  static void AssertEnvNotInitialized() {
#if !defined(NDEBUG)
    assert(!env_initialized_.load(std::memory_order::memory_order_relaxed));
#endif  // !defined(NDEBUG)
  }

 private:
  typename std::aligned_storage<sizeof(EnvType), alignof(EnvType)>::type
      env_storage_;
#if !defined(NDEBUG)
  static std::atomic<bool> env_initialized_;
#endif  // !defined(NDEBUG)
};

#if !defined(NDEBUG)
template <typename EnvType>
std::atomic<bool> SingletonEnv<EnvType>::env_initialized_;
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