title: (Why) are std::chrono literals lvalues?
body: <p>Basically the example says it all: assigning to a std::chrono literal - does not give an error message.
  I tried on gcc, clang and vc, (c++17) all these compilers accept it.
  Is this intended behaviour?</p>
  <pre><code>#include &lt;chrono&gt;
  using namespace std::chrono_literals;
  
  int main()
  {
      10ms  += 1ms;       // would expect error here
      12ms = 10ms;        // and here
      // 3 = 4;           // like here
      return 0;
  }
  </code></pre>