title: Use of string_view for map lookup
body: <p>The following code fails to build on recent compilers (g++-5.3, clang++-3.7).</p>
  
  <pre><code>#include &lt;map&gt;
  #include &lt;functional&gt;
  #include &lt;experimental/string_view&gt;
  
  void f()
  {
      using namespace std;
      using namespace std::experimental;
      map&lt;string, int&gt; m;
      string s = "foo";
      string_view sv(s);
      m.find(sv);
  }
  </code></pre>
  
  <p>Error returned by clang : </p>
  
  <pre><code>error: no matching member function for call to 'find'
      m.find(sv);
      ~~^~~~
  </code></pre>
  
  <p>But shouldn't <code>find</code> be able to use comparable types ? 
  Cppreference mentions the following overload : </p>
  
  <p><code>template&lt; class K &gt; iterator find( const K&amp; x );</code> </p>
  
  <p>The same error happens with <code>boost::string_ref</code>.</p>