Use of string_view for map lookup
<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>