这个C++模板类代码是如何工作的

我正在尝试将Google Test（gtest）代码移植到VxWorks 5.5。严重的缺点是开发环境Tornado 2.2使用了古老的GCC编译器2.96版本。

在分析代码时，我在gtest.h中找到了部分代码，我不理解！这个C++模板类是如何工作的？

// ImplicitlyConvertible<From, To>::value is a compile-time bool
// constant that's true iff type From can be implicitly converted to
// type To.
template <typename From, typename To>
class ImplicitlyConvertible {
 private:
  // We need the following helper functions only for their types.
  // They have no implementations.
  // MakeFrom() is an expression whose type is From.  We cannot simply
  // use From(), as the type From may not have a public default
  // constructor.
  static From MakeFrom();
  // These two functions are overloaded.  Given an expression
  // Helper(x), the compiler will pick the first version if x can be
  // implicitly converted to type To; otherwise it will pick the
  // second version.
  //
  // The first version returns a value of size 1, and the second
  // version returns a value of size 2.  Therefore, by checking the
  // size of Helper(x), which can be done at compile time, we can tell
  // which version of Helper() is used, and hence whether x can be
  // implicitly converted to type To.
  static char Helper(To);
  static char (&Helper(...))[2];  // NOLINT
  // We have to put the 'public' section after the 'private' section,
  // or MSVC refuses to compile the code.
 public:
  // MSVC warns about implicitly converting from double to int for
  // possible loss of data, so we need to temporarily disable the
  // warning.
#ifdef _MSC_VER
# pragma warning(push)          // Saves the current warning state.
# pragma warning(disable:4244)  // Temporarily disables warning 4244.
  static const bool value =
      sizeof(Helper(ImplicitlyConvertible::MakeFrom())) == 1;
# pragma warning(pop)           // Restores the warning state.
#elif defined(__BORLANDC__)
  // C++Builder cannot use member overload resolution during template
  // instantiation.  The simplest workaround is to use its C++0x type traits
  // functions (C++Builder 2009 and above only).
  static const bool value = __is_convertible(From, To);
#else
  static const bool value =
      sizeof(Helper(ImplicitlyConvertible::MakeFrom())) == 1;
#endif  // _MSV_VER
};

创建此类的对象时，如果模板类型From可隐式转换为模板类型To，则名称为value的布尔变量应包含答案。为了得到答案，使用了两个私有函数MakeFrom()和Helper()。但这两个函数只在这里声明，我找不到它们的定义。如果没有其他内容，则此实现不应链接。

我也不理解以下的语法

static char (&Helper(...))[2];

当然，这段代码编译得很好（在Microsoft Visual C++7.1或更新版本或GCC 3.4或更新版本下），谷歌的员工非常清楚他们在做什么。

请开导我！不理解这个代码会让我发疯！：）