title: Eigen log and exp functions type casting error
body: <p>I am working with Eigen in order to obtain the exponential and the logarithm of a matrix. I do not understand why the following code gives me a compilation error. The code is:</p>
  
  <pre><code>Eigen::Matrix4d speedT = Eigen::Matrix4d::Identity();
  //some other operations with speedT
  Eigen::Matrix4d v = speedT.log();
  </code></pre>
  
  <p>The error is:
  Error   17  error C2440: 'type cast' : cannot convert from 'const Eigen::MatrixExponentialReturnValue' to 'Eigen::Matrix4d'</p>
  
  <p>Error   16  error C2027: use of undefined type 'Eigen::MatrixExponentialReturnValue'</p>
  
  <p>For the exponential function (exp) the same issue appears. I have tried different variations of it also make a type cast (Eigen::Matrix4d) speedT.log(); however the result was the same...
  The version of eigen is  Eigen 3.2.8</p>