title: template method "=0" in the parameter list
body: <p>I saw code like the following:</p>
  
  <pre><code>using EnableIfIntegral = typename std::enable_if&lt;
      std::is_integral&lt;T&gt;::value || std::is_enum&lt;T&gt;::value, int&gt;::type;
  
  template &lt;typename T, EnableIfIntegral&lt;T&gt; = 0&gt;
  constexpr int Seconds(T n) {
    return time_internal::FromInt64(n, std::ratio&lt;1&gt;{});
  }
  
  template &lt;std::intmax_t N&gt;
  constexpr int FromInt64(int64_t v, std::ratio&lt;1, N&gt;) {
    // Do Something
  }
  
  </code></pre>
  
  <p>I understand what a template function is. Why in the template parameter list, does it have <code>SomeClass&lt;T&gt; = 0</code>? I know <code>T</code> is the template parameter.</p>
  
  <p>Why is <code>std::ratio&lt;1, N&gt;</code> a parameter? </p>