title: VHDL can't determine definition of operator "+"
body: <p>When I am trying to compile my code it has an error in the following part:</p>
  
  <pre><code>overflow &lt;= input_a(15) + input_b(15);
  </code></pre>
  
  <p>I had declared the input_a and input_b as 15 bit vectors and the libraries that I am using are :  </p>
  
  <pre><code>library ieee; 
  use ieee.std_logic_1164.all;
  </code></pre>
  
  <p>So the error is :</p>
  
  <pre><code>Error (10327): VHDL error at alu16.vhd(45): can't determine definition of operator ""+"" -- found 0 possible definitions
  </code></pre>
  
  <p>Thanks in advance</p>