implementation of std::begin in gcc stl (why two overloads?) [duplicate]

This question already has an answer here:

• What is the purpose of the const overloads of std::begin and end? 1 answer

In:

C:\Program Files\mingw-w64\x86_64-6.3.0-posix-seh-rt_v5-rev1\mingw64\lib\gcc\x86_64-w64-mingw32\6.3.0\include\c++\bits\range_access.h

there is following implementation of std::begin, there are two functions:

  /**
   *  @brief  Return an iterator pointing to the first element of
   *          the container.
   *  @param  __cont  Container.
   */
  template<typename _Container>
    inline auto
    begin(_Container& __cont) -> decltype(__cont.begin())
    { return __cont.begin(); }

  /**
   *  @brief  Return an iterator pointing to the first element of
   *          the const container.
   *  @param  __cont  Container.
   */
  template<typename _Container>
    inline auto
    begin(const _Container& __cont) -> decltype(__cont.begin())
    { return __cont.begin(); }

my question is why the second overload is actually needed?

Here is some test code I used to find whether I have missed something: http://ift.tt/2j22rzK