title: Why b not be constructed to `BigInt<8>` to plus a?
body: <p>I have a class </p>
  
  <pre><code>template &lt;int n&gt; BigInt
  </code></pre>
  
  <p>In this class have method constructor </p>
  
  <pre><code>template&lt;int m&gt; BigInt(const BigInt&lt;m&gt;&amp; b) 
  </code></pre>
  
  <p>and </p>
  
  <pre><code>operator + (const BigInt&amp; b)
  </code></pre>
  
  <p>When I use operator+, example:</p>
  
  <pre><code>BigInt&lt;8&gt; a;
  BigInt&lt;6&gt; b;
  BigInt&lt;8&gt; c = a+b;
  </code></pre>
  
  <p>Why b not be constructed to <code>BigInt&lt;8&gt;</code> to plus a?</p>