title: Why aren't std::algorithms constexpr and which could be?
body: <p>Why aren't any <code>std::algorithm</code> methods <code>constexpr</code>? If I understand the new C++14 rules correctly, many of these methods could be <code>constexpr</code>. For example, why can't <code>std::find</code> be <code>constexpr</code>?</p>
  
  <pre><code>static constexpr std::array&lt;char, 4&gt; DnaBases {'A', 'C', 'G', 'T'};
  
  constexpr bool is_dna(char b)
  {
      return std::find(std::cbegin(DnaBases), std::cend(DnaBases), b) != std::cend(DnaBases); // why not?
  }
  </code></pre>
  
  <p>Which other <code>std::algorithm</code>s could be <code>constexpr</code>?</p>