title: Header `execution` and `std::reduce` not found
body: <p>I am trying to get this snippet to compile</p>
  
  <pre><code>#include &lt;vector&gt;
  #include &lt;numeric&gt;
  #include &lt;execution&gt;
  
  double result = std::reduce(std::execution::par, v.begin(), v.end());
  </code></pre>
  
  <p>I tried these compilers:</p>
  
  <pre><code>Apple LLVM version 8.1.0 (clang-802.0.42)
  
  clang version 3.8.0-2ubuntu4 (tags/RELEASE_380/final)
  
  g++ (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609
  </code></pre>
  
  <p>All three give me <code>'execution' file not found</code></p>
  
  <p>respectively <code>error: no member named 'reduce' in namespace 'std'
      auto result = std::reduce(v.begin(), v.end());</code></p>
  
  <p>for this snippet</p>
  
  <pre><code>#include&lt;numeric&gt;
  #include&lt;vector&gt;
  
  int main(int argc, char *argv[])
  {
      std::vector&lt;double&gt; v(10, 1);
  
      auto result = std::reduce(v.begin(), v.end());
      return 0;
  }
  </code></pre>
  
  <p>I guess my compilers are too old? But <a href="http://en.cppreference.com/w/cpp/algorithm/reduce" rel="noreferrer">on cppreference</a> it does not say which compiler version is requiered minimum and also I do not see any newer versions for clang or gcc in the repo.</p>