title: IS_X function synthesis
body: <p>I have some problems in synthesizing an entity which employs the function IS_X(). In particular, the message </p>
  
  <blockquote>
    <p>[Error] name IS_X is unknown </p>
  </blockquote>
  
  <p>is shown.<br>
  Is it synthesizable?</p>
  
  <pre><code>library IEEE;
  use IEEE.std_logic_1164.all;
  
  entity FF2 is
      generic(XOUT, XNOUT: std_logic);
      port(   INPUT, CLK: in std_logic;
              OUTPUT, NOUTPUT: out std_logic
      );
  end entity FF2;
  
  architecture behavioral of FF2 is
  
      signal temp : std_logic := '0';
      begin
          FUNC:process(CLK)
          begin
              if(CLK'event and rising_edge(CLK)) then
                  temp &lt;= INPUT;
              end if;
          end process FUNC;
  
          OUTPUT &lt;= XOUT when is_x(temp) else temp;
          NOUTPUT &lt;= XNOUT when is_x(temp) else (not temp);
  
  end architecture behavioral;
  </code></pre>