title: Convert main args to span<std::string_view> without ranges
body: <p>This question has been <a href="https://stackoverflow.com/q/62218832/498437">answered before</a> by using ranges, but I'm using C++17 with a backport of <code>std::span</code> - so I have spans but not ranges.</p>
  <p>Consider this simple function:</p>
  <pre><code>std::span&lt;std::string_view&gt; cmdline_args_to_span(int argc, const char* argv[])
  {
      return std::span&lt;const char*&gt;(argv, argc);
  }
  </code></pre>
  <p>I <em>thought</em> the converting constructor of <code>span</code> would kick in due to the <code>const char*</code> constructor of <code>std::string_view</code> but it isn't.  And neither is:</p>
  <pre><code>std::span&lt;std::string_view&gt; cmdline_args_to_span(int argc, const char* argv[])
  {
      auto c_args = std::span&lt;const char*&gt;(argv, argc);
      return std::span&lt;std::string_view&gt;(c_args);
  }
  </code></pre>
  <p>Or:</p>
  <pre><code>std::span&lt;std::string_view&gt; cmdline_args_to_span(int argc, const char* argv[])
  {
      auto c_args = std::span&lt;const char*&gt;(argv, argc);
      return std::span&lt;std::string_view&gt;(c_args.begin(), c_args.end());
  }
  </code></pre>
  <p><a href="https://godbolt.org/z/6qs9x6vxv" rel="nofollow noreferrer">All of these fail</a> on both Clang and gcc (using the 'real' <code>std::span</code> but it's the same for my backported version).</p>
  <p>I must be doing something stupid, but I can't see what - can anyone help?</p>